Creepex of Kamchatka shallow earthquakes

209

ISSN 1069-3513, Izvestiya, Physics of the Solid Earth, 2008, Vol. 44, No. 3, pp. 209–225. © Pleiades Publishing, Ltd., 2008.Original Russian Text © S.A. Boldyrev, V.I. Levina, 2008, published in Fizika Zemli, 2008, No. 3, pp. 40–57.

INTRODUCTION

The earthquake value in a source is estimated fromseismic records in terms of the magnitude, seismicclass, and seismic moment. These integral characteris-tics depend on the geometric dimensions of the source,mechanism of the elastic stress release, elastic parame-ters of the deformed medium, and conditions of propa-gation and recording of seismic vibrations. In thiswork, we present results of investigation of the seismicfocal zone (SFZ) lithosphere from dynamic character-istics of Kamchatka earthquake records obtained atregional stations. We assumed that the specificity of thesource zone can be estimated by the value

Cr

=

log

E

P

–

b

log

E

S

–

c

characterizing relative energies of shortperiod transverse and longitudinal waves in the sourceand their variations in space and time. The energy classvalues

K

P

=

log

E

P

and

K

S

=

log

E

S

in joules calculatedfrom Fedotov’s nomograms [Fedotov, 1972] were usedas initial data. The study is based on the experimentalrelation

b

=

∆

K

P

/

∆

K

S

reflecting the effectiveness of radi-ation of

S

waves compared to

P

waves in a certainregion and the value

Cr

, characterizing deviations ofobserved values from theoretical estimates. It is sup-posed that

Cr

variations can be related, via specificproperties of radiation (the source type and orientationof principal stresses), to the structure of the medium inthe source region and its effective rigidity, controllingthe faulting rate and the stress drop. The rigidity of themedium is determined not only by elastic parameters ofthe medium but also by the dimensions and degree of its

fragmentation. By analogy with dilatancy, the latter canvary in time.

A similar problem based on the difference of energyclass estimates

K

S

–

K

P

was posed in [Potapova andFedotov, 1974], where it was shown that there existnonrandom regional variations in the parameter associ-ated with known seismotectonic structures, volcanicareas, and different levels of seismic activity. Potapovaand Fedotov supposed that the use of statistical meth-ods can qualitatively improve results and provide somenumerical estimates. The relative nature of the parame-ter

K

S

–

K

P

can significantly compensate for the imper-fection of energy nomograms; however, the depen-dence of this parameter on the source energy signifi-cantly complicates the problem. A more correctapproach was applied in the study of the creepex

Cr

=

M

S

–

km

b

–

l

[Prozorov and Hadson, 1983], where theauthors used the magnitude

M

S

from surface waves of aperiod

í

≈

20 s and the magnitude

m

b

calculated fromthe intensity of

P

waves at a period

í

≈

1 s. Thus, thevariables

M

S

and

m

b

are a measure of the radiationenergy in the low and high frequency ranges of thespectrum. The proposed relation was found to be a gooddiscriminant of nuclear explosions because of the dif-ference in dimensions of sources, as well as in theirmechanisms and depths. However, of primary impor-tance is the fact that

Cr

in the formula

Cr

=

M

S

–

km

b

–

l

isvirtually independent of the source energy. The spatialdistribution of the creepex, according to the opinion ofKaverina and Prozorov [1994], is associated with thetectonic nature of the source: positive values of

Cr

(ahigh intensity (

M

S

) of long period vibrations) prevail inshallow sources of mid-ocean ridges under conditionsof horizontal extension, and its negative values (due to

Creepex of Kamchatka Shallow Earthquakes

S. A. Boldyrev

a

†

and V. I. Levina

b

a

Schmidt Institute of Physics of the Earth, Russian Academy of Sciences, Bol’shaya Gruzinskaya ul. 10, Moscow, 123995 Russia

b

Kamchatka Branch, Geophysical Service, Russian Academy of Sciences, Petropavlovsk-Kamchatski, Russia

Received March 20, 2007

Abstract

—Results of investigation of the lithosphere in the Kamchatka seismic focal zone from dynamic char-acteristics of earthquake records obtained at regional stations are presented. It is assumed that the specificity ofthe source zone can be estimated by the relation

Cr

=

K

P

–

bK

S

–

c

characterizing relative energies (energyclasses, according to [Fedotov, 1972]) of short period transverse and longitudinal waves in the source. Azi-muthal, spatial, and temporal variations in

Cr

and their relation to focal mechanisms are examined. Spatiotem-poral variations in this parameter are shown to be caused by the influence of variations in the conditions in thesource zone (its substance or process) on the radiation of

P

and

S

waves.

PACS numbers: 91.30.Bi

DOI:

10.1134/S106935130803004X

†

Deceased.

210

IZVESTIYA, PHYSICS OF THE SOLID EARTH

Vol. 44

No. 3

2008

BOLDYREV, LEVINA

the high frequency radiation of

P

waves), at activeocean margins under conditions of subhorizontal com-pressive stresses and a high strength of the lithosphere.However, data of the International Seismological Cen-ter (ISC) for 1976–1990 [Kaverina and Prozorov, 1994]showed that significant regional differences in creepexvalues exist within the Pacific mobile belt. In this con-nection, we should note a somewhat subjective charac-ter of magnitude estimates in world catalogs that candepend on the set of stations chosen for events of differ-ent intensities in different regions. In Italy, such anapproach was used for analysis of short period recordsof regional stations, and the creepex was determined bythe formula

Cr

=

M

d

– 0.605

M

L

– 1.335

, where

M

L

is thelocal magnitude calculated from maximum amplitudesof

P

waves and

M

d

is the magnitude determined fromthe coda length and used as an analogue of the

M

S

value[Panza and Prozorov, 1994]. It was shown that the tec-tonic factor prevails in

Cr

variations: zones of tectoniccompression are characterized by negative creepex val-ues (

P

waves are more intense), whereas

Cr

> 0 intransform fault zones dominated by shear deforma-tions.

Creepex of Kamchatka earthquakes

is calculatedby the relation

Cr

=

M

S

– 1.2

m

b

– 0.84

obtained by theauthors with the use of the 2-D histogram of

M

S

and

m

b

values for 1024 Kamchatka shallow (

H

< 70 km) eventsof 1964–2005 (www.isc.ac.uk). The scheme presentedin Fig. 1 is constructed from average

Cr

values forsources recorded within

25

×

25

-km areas. With aweighted mean standard deviation

σ

of

Cr

≈

0.5

,regional creepex variations exceed 1. In the shelf zone(0 <

X

< 100 km), with the highest seismic activity level,and for rare sources east of the trench,

Cr

values areusually negative; i.e., here the radiation of short period

P

waves is more intense compared to the remaining partof the region. Beneath the continentward slope of thetrench (100 <

X

< 200 km),

Cr

> 0 (a high level of

M

S

).A diagonal band of positive creepex values includingthe aftershock zone of the strong (

K

S

> 14) earthquakeof 1996 in the area of the Karymsky volcanic center(X = –50 km, Y = 350 km) and the continuation of thesubmarine Shatsky Rise beyond the trench axis (X >200 km, Y = 150 km) is traceable against this back-ground. Such variations (Fig. 1) cannot be explained(by analogy with [Kaverina and Prozorov, 1994]) interms of different release patterns of seismotectonicstresses in these areas because, according to data of theHarvard catalog (http://www.globalcmt.org), nearly alloff-Kamchatka shallow sources have thrust-type mech-anisms due to subhorizontal compressive stressesdirected across regional structures. The creepex differ-ences shown in Fig. 1 are most likely accounted for bydifferent properties of the lithosphere and differentthrust directions: beneath the shelf, a continental litho-spheric block with a lower velocity of elastic wavesmoves eastward, whereas, beneath the continentalslope, an oceanic lithospheric block with a higher

velocity of elastic waves moves westward [Boldyrev,2002, 2005].

DATA AND METHODS OF ANALYSISof Cr = KP – bKS – c

This work presents results obtained for the parame-ter Cr = logEP – blogES – c = KP – bKS – c in relation tothe study of regional distinctions of source zones andprocesses in them. We retain here the term “creepex”because it designates the specific functional relationbetween energy parameters in the source. As distinctfrom the canonical representation [Prozorov and Had-son, 1983], the argument in this relation is the energyestimate from S waves KS, which is the main character-istic of regional earthquakes.

The criterion b = ∆KP/∆KS reflects the relative effec-tiveness of the P and S wave radiation in the range 1–10 Hz. In this work, we used data of nearly 30000 shal-low (h < 70 km) earthquakes with KS = 8.5–15 forwhich both KS and KP estimates are available from theregional catalog of Kamchatka and the KomandorskieIslands over 1971–2004. For convenience of calcula-tions and representation of results, coordinates ofsources and recording stations were converted to theorthogonal coordinate system with the origin at (51°N,157°E) rotated through 30° with respect to the merid-ian. In this case, the Y axis coincides with the directionof the main morphostructures and positive X values cor-respond to the position of the SFZ of Kamchatka shal-low earthquakes. The study region coincides with the areaof reliable recording of KS > 8.5 sources (–200 < X <300 km, 0 < Y < 800 km). As was shown in [Boldyrev,2002, 2005], sources of Kamchatka shallow earth-quakes group to form three lineaments parallel to themain morphostructures (to the Y axis). The seismicityattains its maximum levels at ï = 75, 140, and 200 km.These features were taken into account in the choice ofsizes and positions of averaging areas. The positions ofthe deep-sea trench axis (long dashes) and the –3500-misobath (short dashes), which roughly coincides withthe boundary between lithospheric blocks, are shown inschematic maps presented in this work.

The accuracy of a Cr estimate for an individualevent depends on the determination uncertainties of theenergy classes KP and KS that can be caused by bothobjective and subjective factors. The energy classes oflocal earthquakes were determined from records ofKamchatka stations as follows. Maximum values of thevibrational (mass) velocity in groups of P and S waveswere estimated from records of each station i and werethen converted with the use of nomograms [Fedotov,1972] into the classes KP(i) and KS(i). Specific featuresof observation conditions were taken into accountthrough station corrections ∆Ki for each wave type.Then, the arithmetic averages of station determinationsof energy classes, KP1av and KS1av, and the deviationsfrom them ∆Ki = K1av – Ki were found for each event

IZVESTIYA, PHYSICS OF THE SOLID EARTH Vol. 44 No. 3 2008

CREEPEX OF KAMCHATKA SHALLOW EARTHQUAKES 211

from the set of corrected station values. We rejected thevalues of classes obtained at stations where |∆Ki(P, S)|were larger than 0.8 or 1.0 for P or S waves, respec-tively. Average values K2av were calculated for eachwave type from a new set of station data. On the whole,with respect to the convergence of station (corrected)

data, the average KP and KS values recorded at no lessthan five stations had a dispersion of ±0.3. Such a pro-cedure produces the main subjective error of the cata-log: the average KP and KS values, even with the intro-duction of station corrections, are related to the numberand quality of stations that could vary depending on the

00 100 200 300

X, km

100

200

300

400

500

600

700

800

0.5

0.3

0.1

0

–0.1

–0.2

–0.3

–0.5

–0.6

Y, km

Fig. 1. Schematic map showing the distribution of Cr = MS – 1.12mb + 0.84 for shallow earthquakes of 1964–2005 (www.isc.ac.uk).The stars are contemporary volcanoes, the long-dash line is the deep-sea trench axis, the short-dash line is the –3500-m isobath, andthe crosses are epicenters of earthquakes with KS > 14.

212


BOLDYREV, LEVINA

source position and energy. In addition, data withanomalous deviations |K – Ki| > 1.0 K and records ofstations for which KP and KS could not be estimatedbecause of a weak signal did not participate in the cal-culations; therefore, the results of formal statistical pro-cedures are not indisputable. Actually, recording condi-tions and specific features of seismic traces also affectestimates of the energy class. Thus, the problemreduces to the identification of the Cr variation compo-nent related to the source zone, elucidation of variousaspects and features of the parameter variations, deter-mination of the possible nature of these variations, anddevelopment of a monitoring technology for individualsource zones.

The standard statistical Excel programs were usedin the analysis of various types of interrelation betweenthe parameters studied. The formalized approachenables the calculation of parameters characterizingvarious types of dependences virtually for any amountof data and the concurrent estimation of the determi-nateness or reliability coefficient of approximation r2,quantifying the degree of adequacy of a chosen approx-imation to interrelations between variables. In thesearch for the most trustworthy version, actual (Y) andpredicted (y) values of the sought dependence are com-pared using the coefficient r2 = (Σ(y – Y)2 – Σ(y2 –Y2))/Σ(y – Y)2. The smaller the residual sum Σ(y2 – Y2), thecloser r2 to 1, implying closeness between the actual andestimated values. If r2 = 0, the chosen method of approxi-mation is unsuitable for predicting parameter values.

In the catalog of Kamchatka earthquakes over1971–2004, 23 500 sources with KS > 8.5 are located inthe region of reliable recording (–200 < X < 300 km, 0 <Y < 800 km) and have estimates of both KP and KS val-ues determined from records of 1–23 stations. Nearly90% of KP and KS station values were obtained withinthe range of epicentral distances 100–400 km. On aver-age, KS and KP were estimated from records of sevenand four stations, respectively. The number of stationsrecording an individual earthquake (and, accordingly,providing the values of KP and KS) depends on thesource position and energy, as well as on the period ofrecording (by a varying network of stations). We usedtwo variants of calculation of the regression coefficientsKP = bKS + c. In the first variant, the averaging straightline was drawn manually in accordance with the maxi-mum density of points in the 2-D histogram. However,the second, main variant was based on an analyticmethod implemented with the use of Excel standardprograms. The two averaging variants yielded differentvalues of coefficients, but both lines pass through thecenter of the cloud of points. The sums of the creepexvalues calculated for the entire set of earthquakes areclose to zero in both variants, and the values themselvesare linearly interrelated; i.e., they are equal in the regionof zero values and increase at different rates. Therefore,

the anomalies in the resulting maps have the same posi-tions and signs and differ only by the degree of contrast.

According to catalog data for 1971–2004, KP =0.72KS + 1.57 at r2 = 0.68, whereas KP = 0.71KS + 1.68(r2 = 0.77) if the events with the KP and KS estimatesaveraged over data of at least three stations are used(21 200 events). In our case, distinctions between bothvariants are minimal. The parameters of the relation KP =bKS – c and the coefficient r2 are almost the same ineach of the 5-yr intervals. The functional KP = 0.71KS +1.68 accepted for further calculations indicates that, asthe source energy increases, the fraction of S wave radi-ation also increases, the radiation energies of P andS waves coincide (E = 105.2 J) at the level KP = KS = 5.2,and KS estimates for the sources with KS = 10 averageKP = 8.8.

Characteristics of 41 stations of the regional net-work that were used for the analysis are presented in thetable, giving the coordinates (X, Y, H) and the numberof earthquakes for which both estimates of the energyclasses (N) are available. The next columns of the tablepresent the coefficients b and c of the regression KP =bKS – c, as well as average (over the observation period)deviations of station values of energy classes from theiraverages for the set of stations given in the catalog(∆KP = KPi – KP and ∆KS = KSi – KS).

Calculations of the creepex from the catalog datawere performed for sources that are located in the zoneof reliable recording and have both KP and KS estimates.The values of Cr = KP – 0.71KS – 1.68 vary from −2.7to +2.5. Extreme values |Cr| > 1.0 (4%) are uniformlydistributed in time and are mainly concentrated in thenorthern part of the zone studied. Minimum values areconfined to the Komandorskie Islands area (X = 200km, Y = 700 km). For the events that occurred in 25 ×25-km cells (from 3 to 100 sources), the weighted meandispersion of the Cr average is 0.18. However, if earth-quakes with extreme Cr values are excluded, theweighted mean dispersion decreases to 0.12, and Craverages also change significantly in some areas con-fined to the trench and the Komandorskie Islands.These changes are due to data deficiency and distinc-tions of the coefficients of the functional KP = bKS – cfrom the accepted values. Figure 2 shows the schematicdistribution of creepex averages for sources that arosein 1971–2004 within 25 × 25-km cells. Catalog valuesof KP and KS were used. Variations in the Cr averagesare greater than 0.6, which exceeds the weighted meandispersion, implying that they are not random. Eventswith |Cr| > 1.0 and data of cells containing less thanfour sources were excluded from the construction of thescheme shown in Fig. 2.

The Cr distribution calculated from data of theregional catalog (Fig. 2) is similar in many aspects tomagnitude creepex estimates from the sitewww.isc.ac.uk (Fig. 1). Sources in shelf cells (0 < X <



Initial characteristics of stations

Station code X, km Y, km H, km N b c ∆KP ∆KS

ALD –82 –63 1.4 350 0.68 1.83 0.49 0.20

APC –98 191 8900 0.52 3.58 –0.46 –0.86

APH –75 607 3500 0.75 1.18 –0.05 0.03

AVH –27 278 0.9 6750 0.72 1.62 0.47 0.39

BER 15 173 2450 0.64 2.30 0.00 0.00

BKI 239 721 2580 0.67 2.10 –0.13 –0.28

CIR –87 615 1.42 1080 0.72 1.58 0.30 0.32

ES2 –184 528 0.495 577 0.48 4.00 –0.20 –0.62

ESO –183 529 8516 0.56 3.16 –0.44 –0.81

GNL –97 292 1.2 3109 0.76 1.17 0.03 –0.03

GRL –23 186 1.25 2323 0.76 1.31 –0.04 –0.18

KBG 4 696 5054 0.60 2.64 –0.13 –0.10

KBT 13 695 2079 0.65 1.95 –0.40 –0.36

KII –30 376 970 0.69 1.68 0.11 0.23

KLY –94 636 0.1 8145 0.56 3.40 0.18 –0.01

KMN –92 564 1.1 3216 0.66 1.97 –0.19 –0.20

KOZ –129 580 3546 0.67 2.20 0.10 0.06

KPT –106 583 1085 0.72 1.62 –0.47 –0.72

KRI 26 487 8246 0.58 2.90 0.08 0.19

KRK –33 276 756 0.78 1.10 –0.07 –0.25

KRM –36 215 382 0.79 1.17 0.18 –0.20

KRS –103 617 1.2 1505 0.70 1.63 –0.71 –0.79

KRY –32 376 0.9 2570 0.76 1.04 0.37 0.47

KZY –128 582 0.45 2251 0.65 2.20 –0.26 –0.33

LGN –88 609 2.5 657 0.81 0.70 –0.12 –0.25

MIP –85 115 310 0.71 1.68 –0.61 –0.96

MKZ 61 505 2940 0.73 1.00 –0.32 –0.07

NLC 14 290 0.02 2844 0.76 0.89 0.27 0.40

OZR –19 469 675 0.54 3.40 0.22 0.44

PAU –38 39 4576 0.65 2.35 0.02 –0.18

PDK –87 618 0.8 4060 0.61 2.37 –0.09 0.23

PET –17 251 0.1 8672 0.68 1.95 –0.10 –0.14

PTR –17 251 0.1 940 0.75 1.19 –0.56 –0.70

RUS 9 190 0.075 3427 0.72 1.67 0.03 –0.18

SDL –19 284 1.23 2460 0.78 1.07 0.03 –0.09

SMA –23 279 1.235 1071 0.84 0.30 –0.53 –0.62

SML –7 401 1436 0.60 2.91 0.49 0.61

SPN 55 308 0.17 9970 0.63 2.28 0.00 0.16

SRD –153 599 0.8 1544 0.70 2.14 –0.16 –0.67

SVL –91 674 0.9 3430 0.57 3.12 0.35 0.31

TOP –64 250 5381 0.60 2.67 –0.06 –0.15

TUM –68 516 426 0.79 1.16 –0.42 –0.91

UGL –19 275 900 0.78 1.05 0.18 0.16

VDP –94 565 3124 0.72 1.65 0.15 0.11

ZLN –79 608 1.1 3400 0.65 2.30 0.48 0.48

214


BOLDYREV, LEVINA

100 km) are characterized by a higher intensity of Pwaves, which is supported by values Cr < 0 for thestrongest (mb > 4) earthquakes of the region (Fig. 1) andpositive values of Cr = KP – bKS – c for events with KS > 8.5,which is equivalent to mb > 2.5 (Fig. 2). In bothschemes, sources beneath the continental slope (100 <X < 200 km) are characterized by a higher level of the

S wave intensity with a maximum on the continuationof the submarine Shatsky Rise (X > 150 km, Y =150 km). As seen from Fig. 2, a high level of P waveradiation is fixed at the Karymsky volcanic center (X =−50 km, Y = 350 km), being due to aftershocks of the 1996earthquake (KS > 14), and on the oceanward side of thetrench. Anomalous Cr values in the peninsula territoryand to the east of the trench are mostly obtained from

00 100 200 300

X, km

100

200

300

400

500

600

700

800

0.5

0.3

0.1

0

–0.1

–0.3

–0.5

Y, km

–0.2

KS

14 to 16

–100–200

Fig. 2. Scheme of Cr = KP – 0.71KS – 1.68 constructed with the use of data of the 1971–2004 catalog. The stars are volcanoes, andthe crosses are sources with KS > 14.



small amounts of data; however, they group to formfairly vast zones, which may point to their nonrandomcharacter, particularly because data on sources with|Cr| > 1.0 were excluded from the analysis. The anom-alous Cr values and positions of such zones can berelated to the nonuniversal nature of the coefficients ofthe chosen functional Cr = KP – 0.71KS – 1.68.

Analysis of variations in the criterion b =DKP/DKS. Using data of the 2001–2004 catalog, we cal-

culated the relation KP = bKS + c for groups of KS > 8.5earthquakes recorded in the 25 × 25-km cells (no lessthan ten sources). The values of b = ∆KP/∆KS (Fig. 3)vary from 1.2 in the northwest (Klyuchevsky group ofstations) to 0.3 in the trench area in the southeast, whererecords of southern stations of the network (Y < 400 km)play a decisive role in the calculations of KP and KS. Inthe major part of the zone studied, b ≈ 0.7. Extreme val-ues are characteristic of weakly seismic peripheral

00 100 200 300

X, km

100

200

300

400

500

600

700

800

1.0

Y, km

KS

14 to 16

–100–200

0.9

0.8

0.6

0.5

0.4

0.7 PET

SPN

ESO

KLY

KBG

BKI

Fig. 3. Variations in the parameter b of the functional KP = bKS + c (the increment ratio ∆KP/∆KS). The triangles are seismic stations,and the crosses are sources with KS > 14. Codes of the reference stations are indicated.

216


BOLDYREV, LEVINA

zones, such as areas of recent volcanism or areaslocated east of the trench (X > 200 km), where eventswith KS > 10 are rare (i.e., the plot slope is estimated ona shorter base) and the criterion r2 is less than 0.4. Nev-ertheless, these anomalous estimates are objectivebecause they are obtained for five to eight neighboringcells with close values of ∆KP/∆KS. We should particu-larly note a vast area of a high S wave radiation inten-sity (b < 0.5) located south of Avachinsky Bay (X =100–200 km, Y = 50–200 km), where a high density ofsources in a wide KS range is recorded.

The coefficients of the regression KP = bKS + c werecalculated for some stations (see the table). At the reli-ability level of the linear approximation r2 ≈ 0.6, stationvalues bi vary within 0.5–0.8. No unambiguous relationbetween the values bi and station deviations from theenergy classification has been discovered (table). Thereare observed weak tendencies toward an increase in theplot slope increase as the station–SFZ distancedecreases and the altitude increases. The coefficients ofthe relation KP = bKS + c for Kamchatka shallow earth-quakes are interdependent, c = 8.5 – 9.6b (the approxi-mation reliability is r2 = 0.96). A similar dependencewith the same correlation coefficients is obtained foryearly groups of events within a certain area and forfunctionals of individual stations; i.e., the experimentalplots KP = bKS + c for different samples (stations, areasof the region studied, and temporal and azimuthalgroups) intersect near an area centered at KS = 9.6 andKP = 8.5. Therefore, in cells where the plot slopeexceeds its standard value (b > 0.72), negative creepexvalues are obtained for events with KS = 8.5, while Cris positive for the strongest earthquakes of the groupwith KS > 9.6. For example, in a b = 1.0 cell of theKlyuchevsky area, Cr = –0.5 and +0.5 for sourceswith KS = 8.5 and 12, respectively. On the contrary, forsmall slopes of the plot in the south of the research area(b = 0.5), the respective creepex values are +0.3 and−0.6. Naturally, the number of KS = 8.5 events is about3 times larger than the number of KS = 9.5 events and40 times larger than the number of KS = 12 events.Therefore, it is the weakest earthquakes that determinemaps of the creepex distribution. The slope of the recur-rence curve of Kamchatka shallow earthquakes varieslittle within the research area (dlogN/dKS ≈ 0.5); there-fore, taking into account the logarithmic scale of theplot, the creepex value should be affected not only byvariations in the coefficients of the relation KP = bKS – cbut also by the activity level: the higher the level, thestronger the influence of weak events.

Variations in the coefficient b = ∆KP/∆KS providethe most unbiased constraints on specific features of theP and S wave radiation. However, groups of earth-quakes in a wide energy range are required for a reliabledetermination of the slope b (in the Kamchatka region,40 events with KS ≥ 8.5 are, on average, recorded pereach KS = 12 earthquake). The weakest earthquakes

(8.5 ≤ KS < 9.5) account for about a half of the earth-quakes used and actually determine the main patternsof creepex variations. For source groups identifiedaccording to the place or time of earthquake occurrenceand azimuthal features, positive creepex values arecaused by a higher KP level at which the ∆KP/∆KS val-ues are lower than the standard estimate (b < 0.72).Negative Cr values are characteristic of b > 0.72 sourcegroups.

Specific features of recording conditions wereanalyzed from individual station relations KP = bKS – cand maps of their derivatives Cr. We used uncorrectedKP and KS values for 44 stations of the Kamchatka net-work that fixed more than 100 sources with both KP andKS estimates. On the whole, the plots of KP = bKS – c aresimilar for the majority of stations: the slope b =∆KP/∆KS varies from 0.5 to 0.8 with the common pointin the region (KS = 9.6 ± 0.1, KP = 8.5 ± 0.1). For the sixreference stations PET, KLY, SPN, ESO, KBG, andBKI (Fig. 3), located in different geotectonic settings,the parameters of the relation KP = bKS – c are similarand their values are retained at nearly the same level ina series of 5-yr intervals. Most likely, variations in theparameters of the station functionals are due to a ran-dom component, whereas regional distinctions in theradiation conditions of various types of seismic vibra-tions are the main factor controlling the values of Cr =KP – bKS – c.

The station creepex values were calculated for eachof the 44 stations with the use of individual depen-dences Cri = KP – bKS – c. Then the resulting valueswere averaged over sources recorded in 25 × 25-kmcells (at least three events). If extreme values |Cr| > 1.0are excluded, the weighted mean dispersion of averagesis δCri < 0.2. Schematic maps of Cri from data of indi-vidual stations covered only a part of the zone studied;however, the available fragments show a good coinci-dence of positions of maximums and minimums similarto those in Fig. 2, which is likely to reflect variations inradiation conditions. A Cri average and dispersion, aswell as average values of the epicentral distance Ri andsource–station azimuth Azi, were calculated for eachelementary cell and for each individual station. The setof data of different stations obtained within cells did notdisplay any unambiguous dependence of Cri on the epi-central distance Cr = Cr(R). The azimuth variationrange seldom exceeded 100°, so that the azimuthaldependence of Cr could be estimated only in areas ofhigh seismic activity at the center of the research area.In the remaining areas, the extrema of the distributionCr = Cr(Az) were determined formally by polynomialaveraging of the data. The reliability of data approxi-mation by a polynomial of degree 4 is higher comparedto a linear function (r2 = 0.2–0.6 and 0.1–0.3, respec-tively). The feature most stably identified in the plotsCr = Cr(Az) within the available range of azimuths is aminimum of Cr values (the predominance of S waves).



Figure 4 shows the resulting schematic distribution ofCr averages for the complex of station data (with aweighted mean dispersion of an average being 0.12)and the directions of an increase in the P wave radiationlevel that were determined from the azimuthal plotsaveraged by a polynomial of degree 4. The vectorlength in Fig. 4 is proportional to the difference

between the arithmetic mean and the minimum of theplot (∆Cr = Crav – Crmin = 0–0.3). The amplitudes anddirections of minimums in the station dependencesCr = Cr(Az) vary monotonically within the epicentralzone, possibly pointing to a nonrandom character oftheir variations shown in Fig. 4. In the south of theresearch area (Y < 400 km), the amplitude of the azi-

00 100 200 300

X, km

100

200

300

400

500

600

700

800

–0.1

Y, km

–100–200

0.3

0.1

0

–0.2

0.2

Fig. 4. Variations in average values of Cr = KP – bKS – c for the set of stations. The rhombs show the direction and intensity of theazimuthal dependence, and the triangles are regional seismic stations.

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BOLDYREV, LEVINA

muthal dependence ∆Cr is generally higher, being mostpronounced in anomalous cells, first of all, in theShatsky Rise area (X = 100–200 km, Y = 100–200 km).

The distribution of ër averages in Fig. 4 coincideswell with the scheme calculated from data of theregional catalog (Fig. 2). Low-gradient variationswithin ër ≈ 0 ± 0.1 are characteristic of the seismicallyactive zone (0 < X < 200 km). Beneath the continentalslope (X > 100 km), negative creepex values (up to−0.6) are obtained for sources beneath the SE continu-ation of the Shatsky Rise (X = 150 km, Y = 150 km). Inthis area of high seismic activity, creepex values have aminimum dispersion of averages. Positive Cr values (alow level of S wave vibrations) are confined to thecoastal zone, where they attain a maximum (Cr = 0.4)in the Kronotsky volcano area (X = –50 km, Y =350 km). The difference between Cr values within theresearch area attains unity. Epicenters of the strongest(KS ≥ 14) earthquakes of 1964–2005 with are located inregions of both positive and negative Cr values.

Monitoring of creepex variations was conductedin five seismically active areas (Fig. 5) and in a southernfragment of the epicentral zone of the 1997 Kronotskyearthquake (Fig. 6) from data of six reference stations.The annual averages of Cr and their dispersions are pre-sented in both figures. We used data of earthquakesrecorded in 50 × 50-km areas (the coordinates of theircenters and the numbers of events are indicated in Fig. 5 tothe right of the plots). The smooth lines in the plots arecreepex variations in 1971–2004 obtained by polyno-mial averaging, the thick dashed lines show the totalnumbers of recorded events characterizing seismicactivity variations within a given area (the right-handscale), and the thin vertical dashed lines mark the timemoments of the strongest (KS > 14) earthquakes thatoccurred in 1971–2005 (see Fig. 1). An instrumentalepicenter of a KS ≥ 14 earthquake is located within eacharea of Fig. 5 (its occurrence time is marked by achange in the slope of the plot of the number of eventscaused by aftershocks).

The upper plot in Fig. 5 shows variations in thecreepex (Cr = KP – 0.72KS – 1.57, where the KP and KS

values are taken from the catalog) in a Kamchatka Bayarea centered at (X = 75 km, Y = 675 km). The amountof data being small (286), the dispersion of Cr averagesis comparable with the smoothed parameter variations,and one may speak only of a tendency toward adecrease in the Cr value after the local earthquake ofDecember 15, 1971 (KS ≥ 14); beginning from 1986(possibly, after the earthquake of December 1984), thistendency changed to a stable Cr increase. The swarm ofaftershocks of the 1985 earthquake had virtually noeffect on the plot of the number of events. At a large dis-persion of data from sources near the southern termina-tion of Kamchatka (the bottom plot in Fig. 5 for X = 75,Y = 125, n = 233), the variations shown in the plot dis-play a few significant maximums in 1983, 1990, and2000, which are virtually unrelated to the activation of

1993. The Avachinsky Bay area (75, 275; n = 1175) wascharacterized by a stably high level of seismic activity thatwas little affected by aftershocks of the strong earthquakesof 1992 and 2001. The significant Cr maximums of 1974and 1990 do not coincide with the period of seismic acti-vation of this area. The areas located south (75, 475) andnortheast (75, 575) of the Kronotsky Peninsula are charac-terized by low-gradient creepex variations whose ampli-tudes are comparable with the weighted mean dispersion(±0.1). These areas are located at the flanks of the epicen-tral zone of the 1997 Kronotsky earthquake. During thepreceding period (1992–1997), the Cr values at the south-ern flank of the source zone changed from –0.2 to +0.2(the fraction of P waves increased), and the Cr level at thenorthern flank nonsynchronously decreased by nearly thesame value over the same period.

The plots in Fig. 6 show the variations in yearlyaverage creepex values and their dispersions for earth-quakes in the southern part of the source zone of the1997 Kronotsky earthquake (50 < X ≤ 100 km, 450 <Y ≤ 500 km) from records of the six reference stationslocated in different seismotectonic conditions at nearlythe same distances from the source zone (Fig. 3). Thenumber of sources, the corresponding average epicen-tral distances, and the source–station azimuths are indi-cated to the right of each plot, near the station code.Each point plots the Cr value averaged over 1–40 earth-quakes that occurred from July 1 to June 30 of the nextyear; i.e., the Cr level at the point of 1998 is determinedby aftershocks of the Kronotsky earthquake (December1997). The discreteness of data equal to 1 yr allows oneto reliably identify oscillations with a period of morethan 3–5 yr in the plots. The plot for the BKI stationcontains little information because of the scarcity ofdata and a large dispersion. As regards the remainingstations, the plotted variations exceed the dispersion ofaverages, and significant oscillations can be identifiedin the plots even against the background of the polyno-mial averaging. However, notwithstanding the signifi-cance of these variations, they cannot be unambigu-ously related to the time moments of strong regionalearthquakes (vertical dashed lines) except for the1990s, when an increase in Cr values (in the fraction ofP wave radiation) immediately before an earthquake(1995–1997) is traceable in all plots and a characteristicpattern of the “lagoon” type (a decrease in a parameterfollowed by its increase) is recognizable in the plots.The beginning of lagoons can be related to the 1984maximum in the plots of the BKI and KBG stations,located farther to the north, and to maximums after1990 in the plots of other stations. If we choose yearswhen at least ten Cr values (six intervals) were avail-able for each station, the station values will differ fromeach other only by their levels and such differences willbe approximately identical. In fact, this fact confirmsthe existence of the azimuthal factor affecting the radi-ation intensities of P and S waves.

A similar analysis of plots of the reference stationswas performed for a number of other epicentral zones



in order to verify the revealed patterns. In particular, wecompared plots of yearly Cr averages smoothed by themethod of a 3-yr moving average for two areas locatedat the southern and northern flanks of the zone of the1997 Kronotsky earthquake. On the whole, distinctionsare more numerous than coincidences. At the givenlevel of statistical representativeness, one may state thatthere are significant space–time creepex variations;

however, it seems as yet premature to relate them tonucleation of strong earthquakes (KS > 14 or M > 6.5).

Relation of the creepex to focal mechanisms. Theanalysis of global variations in the creepex Cr = MS –kmb – l [Kaverina and Prozorov, 1994] revealed that itsvalues are related to the focal mechanism type. Accord-ing to data of the Harvard catalog (http://www.global-cmt.org), more than 90% of Kamchatka shallow earth-

–0.4

0

1972

Cr

–0.2

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150

50

75; 125(233)

75; 275(1178)

1976 1980 1984 1988 1992 1996 2000 2004

75; 475(853)

75; 575(1092)

75; 675(286)

Fig. 5. Variations in annual averages of Cr (the left-hand scale) calculated from the catalog for epicentral areas 50 × 50 km in size(the epicentral coordinates and the number of sources are indicated to the right). The thick dashed lines show variations in the num-ber of recorded events (the right-hand scale). The thin vertical dashed lines show the occurrence times of earthquakes with KS > 14(their positions are shown by crosses in Fig. 1).

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quakes have mechanisms of nearly the same type thatrelease subhorizontal compressive stresses directedacross regional structures. Below, we compare mecha-nisms of 1100 local sources of 1962–2003 (data of theKamchatka Branch of the Geophysical Service, Rus-sian Academy of Sciences (KB GS RAS)) constructedfrom signs of first arrivals of P waves at stations ofregional and global networks. These data are character-ized by considerable space–time variations in seismo-tectonic stresses. Figure 7 shows the contours of aver-

age dip angles of compression axes (PPL) in the mecha-nisms of sources recorded within 25 × 25-km cells; thearrows in Fig. 7 show azimuths (PAZ) of individualsources. The proposed scheme indicates that cellscontinuing peninsulas of eastern Kamchatka andhaving rather steep slopes of P axes are recognizableagainst the predominant subhorizontal compressivestresses. Moreover, the diversity of orientations ofcompressive stresses is noticeable even within cellsthemselves.

0.4

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KLY (n = 333)

1976 1980 1984 1988 1992 1996 2000 2004 2006

R = 220 kmAz = 320°

SPN (n = 330)R = 170 kmAz = 190°

0.2

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00.2

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ESO (n = 292)R = 255 kmAz = 285°

PET (n = 270)R = 270 kmAz = 205°

KBG (n = 173)R = 220 kmAz = 350°

BKI (n = 68)R = 290 kmAz = 40°

Fig. 6. Variations in annual averages of Cr for sources recorded by individual stations in the area (50< X ≤ 100 km, 450 < Y ≤500 km). Station codes, numbers of sources used, average epicentral distances, and source-to-station azimuths are given to theright. The thin vertical dashed lines show occurrence times of earthquakes with KS > 14 (their epicenters are shown by crossesin Fig. 1).



00 100 200 300

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70

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PPL

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5050

Fig. 7. Data (KB GS RAN) on focal mechanisms of shallow earthquakes of 1962–2003. The triangles are regional seismic stations.The contours show average dip angles of compression axes (PPL) calculated for sources in 25 × 25-km cells, and the arrows showazimuths of P axes in the given coordinate system. The circles are areas where Cr values were compared, and the encircled grayrhombs show directions of P axes of maximum creepex values (see Fig. 8).

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BOLDYREV, LEVINA

The interrelation between the creepex and the direc-tion of compressive stresses (P axes) released insources of the strongest earthquakes was established fornine epicentral areas (shown as circles in Fig. 7) with ahigh density of sources. These areas of a radius of25 km include 25–70 events of the class KS ≥ 11.5(M > 5) for which focal mechanisms were calculated.The station estimates of KP and KS values were chosen

for each earthquake from the database and used for esti-mating the parameters of the functional Cr = KP – bKS – cin the range 11 ≤ KS < 15. Unfortunately, intense signalsare difficult to measure in records of regional stationsand, therefore, energy class estimates were availablemainly from the data of remote stations and earth-quakes that were not overly strong. Nevertheless, eachgroup contained 50–500 station values of Cri calculated

0

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PPLCr

9

8

7

5

6

4

3

1

2

Fig. 8. Dependences of Cr = KP – 0.54KS – 3.6 values (diamonds) versus the P axis azimuth (PAZ) for the earthquake groups in thecircular areas in Fig. 7 (the groups are marked by the same numbers). The thick dashed lines are polynomial approximations of theexperimental data. The triangles in the lower parts of the plots are dip angles of the P axis (PPL, right-hand scale).



from the dependence Cr = KP – 0.54KS – 3.6, whichgeneralizes the data used.

The results of this comparison are presented inFig. 8. The dependence of station creepex values on theazimuth of compressive stresses Cr = Cr(PAZ) withrespect to the Y axis is shown for each epicentral group.The average values for 10° intervals of Cr values andtheir dispersions are plotted. The thick dashed linesaverage experimental values by a polynomial of degree6. The reliability of such an approximation r2 is muchhigher compared to a linear approximation. The averag-ing curves in seven of the nine azimuthal plots (Fig. 8)exhibit a well-expressed periodicity of 180°, and theamplitude of fluctuations exceeds the dispersion of theCr averages. The coincidence of Cr values in oppositedirections (a periodicity of 180°) reflects the physicalessence of stressed systems and provides a basis forrelating Cr variations to changes in seismotectonicstresses. The triangles and the thin averaging lines in thelower parts of the plots presented in Fig. 8 show the dipangles of P axes (the right-hand scale) for the samesources. Subhorizontal compressive stresses (0–30°)prevail in virtually all of the groups. In some groups, PAZ

intervals with predominant dip angles PPL or a tendencytoward their change are recognizable, but an unambigu-ous relation to specific features of Cr variations cannotbe detected. The density of symbols provides an idea ofthe distribution of the earthquakes in use over differentazimuthal intervals. It is remarkable that the revealed180° periodicity is most clearly expressed and the coef-ficient r2 is larger than 0.6 (r2 = 0.3–0.4 for a linearapproximation) in groups 2, 4, 5, and 7, in which data areuniformly represented in the PAZ range.

The directions of P axes having maximum Cr val-ues (an increased radiation level of P waves in therange 1–7 Hz) are shown in Fig. 7 by encircled grayrhombs. The orientations of traces with minimumcreepex values (and, accordingly, weakened intensityof P waves) differ by 90° from those presented in Fig.7. On the whole, the largest creepex values correspondto directions of P axes that are not characteristic ofisland arc systems (along the strike of regional struc-tures, i.e., across the geodynamic scheme). These fea-tures are established for earthquakes that occurred inthe Kamchatka and Aleutian offshore zones. In theband extending along the continental slope of the Kam-chatka trench (100 < X < 200 km), the maximum inten-sity of P waves (Cr) is obtained in the direction of com-pressive stresses that are nearly normal here to thestrike of arc structures. We should note that the revealedpattern is based on fragmentary creepex values ofstrong earthquakes (KS ≥ 11), for which the use of cali-brating nomograms is somewhat arbitrary.

DISCUSSION AND CONCLUSIONS

The recording quality of seismic networks is deter-mined by the accuracy, degree of detail, and homogene-

ity of earthquake data (coordinates, occurrence time,and source energy). Since 1964, the Kamchatkaregional network of stations has recorded local earth-quakes of KS ≥ 8.5 virtually without interruptions withthe use of a unified technique. Since 1971, energies ofnearly 90% of recorded earthquakes (more than 30000)have been estimated from the radiation intensity of Pand S waves. The resulting database is adequate forcontinuing investigations of the parameter Θ = KS – KP

[Potapova and Fedotov, 1974] at a new statistical andmethodological level. Analysis of dynamic characteris-tics of records of Kamchatka earthquakes showed thatvariations in the creepex Cr = KP – bKS – c are due tospecific features of radiation. This conclusion is basedon the analysis of the spatiotemporal creepex distribu-tion obtained from data of world and regional catalogs,coincidence of data of different stations, monotonicareal variations in the parameter, and (primarily) itsrelation to focal mechanisms. The relative character ofCr allows statistical methods to be used for the identifi-cation and estimation of the main factors responsiblefor its variations in a wide dynamic range of seismicvibrations. The study of spatiotemporal, azimuthal, andstatistical characteristics of creepex variations can pro-vide deeper insights into the radiation process in sourcezones, enabling the use of Cr values for the monitoringof this process.

The experimental database changed with the devel-opment of technologies of observations and seismicrecords processing, and this influenced the choice ofmethods of study. The most stable and thereby objec-tive results are obtained for regions of the research area(0 < X < 200 km, 0 < Y < 800 km) characterized by ahigh density and regularity of earthquakes. Creepexvalues vary within –2.5 < Cr < +2.5 for Kamchatkashallow earthquakes of 1971–2004. These variationsinclude a considerable random component (an rmsdeviation of 0.5). Using the proposed sizes of space andtime windows, the mathematical expectation of Cr canbe obtained with the dispersion at a level of 0.12–0.15,whereas variations in the Cr average mapped or shownin time plots exceed 0.5–0.8. The density of sourcesand the scatter of creepex values obtained from data ofglobal (Fig. 1) and regional (Figs. 2, 4) observationsystems allowed us to identify anomalies more than50 km in size or their time variations with periodslonger than 5 yr.

A good coincidence of anomalies determined fromdata of global and regional catalogs, as well as fromrecords of individual stations and their sets, is noted inschematic distributions of the parameters under study.In all cases, the Cr values beneath the continental slopeof the trench (100 < X < 200 km) indicate an increasedlevel of the S wave radiation, whereas the relative inten-sity of P waves sharply increases in earthquake sourcesbeneath the shelf (0 < X < 100 km). The comparison ofdata of global and regional networks suggests that theconditions in the source zone rather than the wave-

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BOLDYREV, LEVINA

length are the major factor. The revealed distinctionscoincide with the tectonic scheme proposed in [Bold-yrev 2002, 2005], according to which a low-velocitylithospheric block beneath the shelf (X < 100 km) over-rides the fault zone plunging under Kamchatka and ahigh-velocity block is thrust in the opposite directionover a fault plunging under the ocean; i.e., all otherthings being equal, sources radiate more intense Swaves in a medium with a high velocity of elasticwaves. Against this background, the configurations ofcontours in the southern half of the research area (Figs.1, 2, 4) are controlled by a vast region of intense S waveradiation. In plan view, this anomaly continues thediagonal structure of the Shatsky Rise into the trencharea (100 < X < 250 km). The anomaly is bounded byhigh gradients of the parameter and is possibly con-nected with a deep fault (or a fault system) in the litho-sphere that extends, intersecting the island-arc struc-tures at an angle of ~45°, from the oceanic swell of thetrench through Avachinsky Bay to the rift zone of cen-tral Kamchatka. Note that a higher intensity of surfacewaves (actually, S waves) is typical of earthquakes withrifting-type sources of mid-ocean ridges.

Rare and scattered sources on the oceanic side of thetrench (150 < X < 250 km) are characterized by a higherradiation level of P waves. Similar data are obtained for thearea of the Klyuchevsky volcano group (X = –100 km, Y =600 km) with shallow sources. Results obtained fromdata of global and regional observation systems differsignificantly in the zone of the Karymsky volcanogroup (X = –50 km, Y = 350 km), where the majority ofevents are aftershocks of the 1996 earthquake with KS > 14.Here, a high level of long period surface waves (Cr > 0;see Fig. 1) is observed concurrently with a lower inten-sity of S waves in the range 1–7 Hz (Figs. 2, 4).

Variations in the creepex of local earthquakes (Fig. 2)are somewhat related to the fact that the coefficients inthe functional Cr = KP – 0.71KS – 1.68 are not the samefor different structures (areas) of the seismogenic litho-sphere. The most significant of these coefficients b =∆KP/∆KS (Fig. 3) reflects a variation in the relative radi-ation intensities of the two wave types. In this work, weused a formalized method for estimating the parametersof the regression KP = bKS – c with the aid of Excel sta-tistical programs, selecting a solution with a maximumreliability level of approximation. In the major part ofthe epicentral zone, b ≈ 0.6–0.8, which is within thedetermination accuracy of this parameter. It is notewor-thy that b values in the shelf zone (X < 100 km) andbeneath Kamchatka are larger than in lithosphericsources of the continental slope with negative creepexvalues. Maximum values of the coefficient (b > 1.0) areobtained for earthquakes that occurred northwest of theKlyuchevsky group of stations. Here, a high radiationlevel of P waves is possibly due to volcanotectonicearthquakes. In the southern part of the research area(100 < X < 200 km, 100< Y < 200 km), minimum b val-

ues (∆KP/∆KS < 0.5) are related to a low efficiency ofP wave radiation.

The coefficients of the relation KP = bKS + c for dif-ferent samples of Kamchatka shallow earthquakes areinterdependent: c = 8.5 – 9.6b (the approximation reli-ability is r2 = 0.96); i.e., the experimental plots KP =bKS + c intersect near the center of the cloud of pointswith KS = 9.6 and KP = 8.5. The plot slopes depend onthe completeness of data in various KS intervals but pri-marily on the ratio KP/KS at the ends of the range (KS > 12).However, on the whole, variations in the slopes ofexperimental plots influence only slightly the Cr distri-bution. Anomalies in the resulting maps retain theirpositions and signs and differ only in the degree of con-trast. Apparently, variations in the coefficient b =∆KP/∆KS provide more unbiased evidence on specificfeatures of the radiation of P and S waves. However,sets of earthquakes in a wide range of energies areneeded for reliable determination of the slope b (inKamchatka, on average, 40 events with KS ≥ 8.5 arerecorded per earthquake of KS = 12). The weakest earth-quakes (8.5 ≤ KS < 9.5) account for about half of theearthquakes used and actually control the main patternsof creepex variations. For sources grouped according tothe place or time of earthquake occurrence or azimuthalindicators, positive creepex values are due to larger KP

values, which can be related to the fact that the∆KP/∆KS values are smaller than the standard slope (b <0.72). A high level of S wave radiation should be pre-dominant for earthquakes with b > 0.72.

The Cr values calculated from the KP and KS valuesof the catalog and the energy class values themselvesare influenced by specific properties of a set of record-ing stations that depend on the recording period andsource position and energy. In most cases, station val-ues of the parameters of the functional KP = bKS + c dif-fer within the accuracy limits. The coincidence ofanomalies in the maps of station creepex values indi-cates that specific features of the medium or process ina source zone are mainly responsible for the formationof these anomalies. The creepex values determined forthe set of stations (Fig. 4) are close to the Cr distribu-tion obtained from the regional catalog (Fig. 2). For themajority of cells, the station creepex values obey a cer-tain azimuthal dependence that, if taken into account,decreases the dispersion of averaging. Mapping ofthese anomalous directions reveals monotonic arealvariations in the azimuth and amplitude of the azi-muthal dependence (Fig. 4). The azimuthal control ofCr is more pronounced in the southern part of theresearch area. In the seismically most active sites (0 <X < 200 km), a higher level of S wave intensity is fixedin the directions along the strike of major regionalstructures. It is noteworthy that, in an anisotropic modelof the lithosphere [Boldyrev, 2005], “fast” velocitiesand an “intense” attenuation of elastic waves take placein the same directions.



The creepex was monitored on the basis of data ofthe regional catalog (Fig. 5) and six reference stations(Fig. 6). The annual creepex averages vary from –0.5 to+0.5 and their weighted mean dispersion is 0.15. Sig-nificant fluctuations of the parameter having a period ofmore than 5 yr indicate that these variations are authen-tic. Extrema in the Cr plots do not coincide with occur-rence times of the strongest events (KS > 14).

The creepex relation to earthquake sources is sup-ported by the correlation dependence between varia-tions in Cr and the stress state (azimuth of the P axis)in earthquake sources (the KB GS RAS catalog of focalmechanisms) and by the 180° periodicity of variationsin Cr = Cr(AzP) (Fig. 8). In the shelf zones of the Kam-chatka and Aleutian arcs, the predominant stress sys-tems (the P axes are directed across regional structures)are realized at a higher level of S wave radiation. Theradiation intensity of high-frequency P waves is stron-ger in sources in which P axes and fault motions are ori-ented along regional structures. In the band extendingalong the continental slope of the Kamchatka trench(100 < X < 200 km), the maximum intensity of P wavesand, accordingly, Cr values (plots 1–4 and 6 in Fig. 8)is observed at compressive stresses nearly normal to thestrike of arc structures. We failed to detect an unambig-uous relation between the P axis dip angle, determiningthe focal mechanism type, and creepex variations. Nev-ertheless, anomalously high levels of S and surfacewaves and smaller values of ∆KP/∆KS are fixed in thearea of the diagonal Shatsky Rise (in the south of theresearch area), where steeper (up to 70°) slopes of Paxes are observed (Fig. 7) and, consequently, the riftingmechanism type, untypical of active ocean margins,prevails.

The analysis performed in this work has revealedsignificant space–time variations in the creepex causedby the radiation conditions of P and S waves. The mostreliable relation is that between the creepex and thevelocity of elastic waves in the lithosphere: sourceswith a high intensity of S waves arise in a high-velocitymedium beneath the continental slope of the trench. Asshown above, such areas are characterized by smallercoefficients of the recurrence slope (i.e., a larger per-centage of events with large KS values). In our opinion,variations in the coefficient b = ∆KP/∆KS play a decisiverole in spatiotemporal and azimuthal variations of thecreepex. Based on this suggestion, a high level of Pwave radiation (Cr > 0) corresponds to the conditions ofa low-velocity lithosphere with increased b values,whereas negative creepex values in the high-velocityoceanic block of the lithosphere (100 < X < 200 km) arecaused by a stronger S wave radiation (b is smaller thanits average value). Time variations in the creepex, its

azimuthal dependences, and its relation to focal mech-anisms are possibly controlled by the directivity ofsource radiation.

This work has demonstrated that the parameter Cr =KP – bKS is promising for the study of earthquakesource zones. Data of sources with KS ≥ 8.5 were usedin this analysis. A decrease in the representativenesslevel to KS = 7.5 increases the number of events bynearly three times, which is beneficial to the study ofself-similarity of the P and S wave radiation process ina wide range of energies and enables more detailedmonitoring of the seismotectonic process. The origin ofanomalous values |Cr| > 1 could not be elucidatedwithin the scope of this work, particularly because thereare known cases where amplitudes of P and S wavesfrom rather strong earthquakes could not be identifiedand measured.

REFERENCES1. S. A. Boldyrev, “The Effect of the Lithospheric Structure

and Properties on the Seismic Field of the KamchatkaRegion,” Fiz. Zemli, No. 6, 5–28 (2002) [Izvestiya,Phys. Solid Earth 38, 447–468 (2002)].

2. S. A. Boldyrev, “Seismic Heterogeneity and SeismicAnisotropy of the Lithosphere of the Seismic Focal Zonein the Kamchatka Region,” Fiz. Zemli, No. 1, 19–35(2005) [Izvestiya, Phys. Solid Earth 41, 17–33 (2005)].

3. A. M. Dziewonski, T. Chou, and J. S. Woodhouse,“Determination of Earthquake Source Parameters fromWaveform Data for Studies of Global and Regional Seis-micity,” J. Geophys. Res. 86 (B4), 2825–2852 (1981).

4. S. A. Fedotov, Energy Classification of Kurile-Kam-chatka Earthquakes and the Problem of Magnitudes(Nauka, Moscow, 1972) [in Russian].

5. A. N. Kaverina and A. G. Prozorov, “Variations in theCreepex as a Function of the Tectonic Structural Typeand Focal Mechanism: A Statistical Analysis,” in Geody-namics and Earthquake Prediction, Vol. 26 of Computa-tional Seismology (Nauka, Moscow, 1994), pp. 85–93[in Russian].

6. G. F. Panza and A. G. Prozorov, “Extension of theCreepex Definition to Magnitudes of Weak Earthquakes:The Italian Region,” in Geodynamics and EarthquakePrediction, Vol. 26 of Computational Seismology(Nauka, Moscow, 1994), pp. 78–84 [in Russian].

7. O. V. Potapova and S. A. Fedotov, “Parameter Θ =logES/logEP from Data on Kamchatka Earthquakes,” inSeismicity, Seismic Prediction, and Upper Mantle Prop-erties: Implications for the Kamchatka Volcanism(Nauka, Novosibirsk, 1974), pp. 133–140 [in Russian].

8. A. G. Prozorov and D. Hadson, “Relation between MLHand mPV as a Function of Regional Conditions and LocalFeatures,” in Magnitude and Energy Classification ofEarthquakes (IFZ AN SSSR, Moscow, 1974), Vol. 2,pp. 208–216 [in Russian].

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Creepex of Kamchatka shallow earthquakes