Variations of Seismicity in the Avachinsky Gulf (Kamchatka, Russia)

Natural Hazards19: 87–96, 1999.© 1999Kluwer Academic Publishers. Printed in the Netherlands.

87

Variations of Seismicity in the AvachinskyGulf (Kamchatka, Russia)

VADIM SALTYKOV, VICTOR CHEBROV and JULIA KUGAENKOKamchatkan Seismological Department, Geophysical Service 9, Piip Av.,Petropavlovsk-Kamchatsky, Russia, 683006

(Received 21 October 1997; in final form 21 April 1999)

Abstract. Variations of seismic mode in the region of the Avachinsky Gulf (Kamchatka, Russia) areconsidered. Observed anomalies (seismic quiescence, the ring seismicity, reduction of the slope of theearthquake recurrence diagram) provide a basis to consider this region as a place of strong earthquakepreparation. The Kamchatka regional catalogues of earthquakes between 1962–1995 were used in theanalysis. A reduced seismicity rate is observed during 10 years in an area of 150 km× 60 km in size.During the last five years, in the vicinity of the area considered, earthquakes withM > 5 occurredthree times more often than the average over thirty years. It is interpreted as ring seismicity. Theblock of 220 km× 220 km in size, including the quiescence zone, is characterized by a continuousdecrease of the recurrence diagram slope, which has reached a minimum value for the last 33 yearsin this region.

Key words: precursor, seismic gap, seismic quiescence, ring seismicity.

1. Introduction

The Kamchatka Peninsula, the northern part of the Kuril–Kamchatka arc, is one ofthe most seismically active places on the Earth. Because of the small populationin this region, earthquakes with magnitude∼8 (1904, 1923, 1959, etc.) have notcaused serious damage. However, Petropavlovsk–Kamchatsky is the largest city inKamchatka and is close to the seismoactive zone. It could suffer in the event of anearby strong earthquake.

A long-term forecast by Fedotov and Chernyshev (1990) suggests that oneof the most probable places for strong earthquakes (M > 7.75) in the Kuril–Kamchatka arc is Avachinsky Gulf (Southern Kamchatka) (Figure 1). The long-term forecast is based on the fact that this zone is a ‘seismic gap’. A seismic gap isunderstood to be a part of a seismoactive area, which is not filled by the sources ofprevious strong earthquakes and is probably the location for the next earthquake(Mogi, 1968). The unraveling of such zones is the basis of Fedotov’s method.However, it should be noted that, while the location is predicted, the time of theevent is still unknown.

The data, according to the literature (e.g., Mogi, 1985), show that the regions ofpreparation of a strong earthquake exhibit variations of seismic mode over a long

88 VARIATIONS OF SEISMICITY IN THE AVACHINSKY GULF (KAMCHATKA, RUSSIA)

Figure 1. Sketch map of Southern Kamchatka, showing epicenters of earthquakes 1962–1995with magnitude 2.0 6 M < 3.0 (1), 3.0 6 M < 4.0 (2), 4.0 6 M < 5.0 (3), 5.06 M < 6.0(4), 6.0 6 M < 7.0 (5), 7.0 6 M (6) and with depth less than 70 km (7) and more than70 km (8). The dotted curve (6) marks the boundary of the region with reliable registration ofearthquakes with magnitudeM = 2.0.

period of time. This justifies the choice of the target region for the present work.The purpose of this work is the detection of changes in seismic mode, which arecharacteristic of the preparation of a strong earthquake in the Avachinsky Gulf. Themodes under examination are seismic quiescence, ring seismicity, reduction of therecurrence diagram slope.

2. Data

Regional Kamchatkan earthquake catalogues from 1962 to 1995 (the entire periodof operation of the Kamchatkan regional network) were used in the analysis ofseismicity (Figure 1). First, catalogues were cleared of aftershocks and swarms ofearthquakes by the method of Molchan and Dmitrieva (1991). The lower powerthreshold of these earthquakes was fixed according to the quality level for theregion under examination. The catalogue is complete for events ofM > 2.6, butfor the Avachinsky Gulf the catalogue is complete for earthquakes ofM = 2.0(Gordeevet al., 1998, Figure 1).

VADIM SALTYKOV ET AL. 89

Figure 2. Left and middle: Central parts of circles (1, 2) with radius of 30 km (left) and 15 km(middle), in which seismic flow is calculated. 1 – corresponds to circles without decrease inseismicity rate, 2 – circles in which the decrease in seismicity rate is observed. The ellipse,approximating a zone of seismic quiescence, is drawn. Right: Set of zones (I–V), in whichseismicity is investigated.

3. Method of Analysis and Results

Seismic quiescence.Seismic quiescence means the phenomena manifested as adecrease in seismic activity over time, compared to the background level for afixed spatial volume before strong earthquakes. Sometimes this is referred to as aseismic gap (Mogi, 1979), but it is necessary to understand, that this nomenclaturedescribes a completely different process than the one described above.

The technique for detection of zones with anomalous seismicity rate (numberof earthquakes per unit time) consists of the following:

(1) The region considered was covered by a set of circles with a radius of 30 km(Figure 2, left);

(2) For each circle the cumulative seismic flow, i.e., the cumulative number ofearthquakes withM > 2.6 as a function of timet , was evaluated;

(3) Circles corresponding to a decrease in seismicity rate (i.e., a decrease in theslope of the cumulative seismic flow diagram) in recent years were identified(and are shown in Figure 2 (left) as solid dots). The shape of the diagram of thecumulative seismic flow in Figure 3 (zones I and II) is typical for these soliddots;

(4) The region showing solid dots (Figure 2, centre) was in turn covered with aset of smaller circles (R = 15 km), and steps 2 and 3 were repeated. Thisprovides a more effective determination of the anomaly. The whole procedurewas followed using earthquakes withM > 2.0. This allowed us to outline an“abnormal zone” delineated by an ellipse in Figure 2 (centre), whose parame-ters are: 75 km and 25 km for half-major and half-minor axes respectively. Ifthis zone is actually a zone of seismic quiescence, then its size corresponds tothe source size of an earthquake of magnitude about 7.5.


Figure 3. Cumulative number of earthquakes (left) and seismicity rate (right) as a function oftime in each zone. Magnitude threshold isM = 2.0 (zone I–IV) orM = 2.6 (zone V).


To derive quantitative parameters for the processes occurring within and at theedge of the anomalous region, seismicity within a set of areas enclosed in con-centric ellipses with half-major/minor axes 50 km/20 km (ellipse I), 75 km/30 km(ellipse II), 100 km/40 km (ellipse III), 125 km/50 km (ellipse IV) and the rectangle51.5◦–53.5◦N and 158◦–161◦E (Figure 2, right), is compared. Zone I is the area ofellipse I, zone II is the area between ellipses I and II, zone III – between ellipses IIand III, zone IV – between ellipses III and IV, and zone V – between the rectangleand ellipse IV. The earthquakes considered were less than 70 km deep. A decreasein seismic rate is observed in zones I and II (Figure 3). Variations are clearer inzone I. In zone III, the seismicity rate does not exhibit an anomaly in recent years(visually), and in zone IV it increases; outside the ellipses (zone V), it is constant.

For an estimation of the confidence on alleged changes in seismicity rate, theZ-test has been applied. The method (Habermann, 1983, 1991) consists of:

(1) At any timet , averaged rates calculated over two intervals (from the beginningof monitoring t0 to instantt ; from instantt to the end of monitoring periodt1) are compared.t spans the whole interval (t0, t1). The parameterAS, whichcharacterizes the distinction of rates, is taken from standard deviateZ-test andis calculated by the formula:

AS(t) = Z = (R1− R2) · (σ 21/n1+ σ 2

2 /n2)−1/2,

whereR1 is the mean rate in period 1 (t0, t) andR2 is the mean rate in period 2(t , t1); σ1 andσ2 are the corresponding standard deviations, andn1 andn2 arethe numbers of samples. The timet at whichAS(t) is maximum separates atime period into two intervals with different rate, with a maximum of reliability.

(2) If the functionAS(t) accepts the maximum valueZ0 at the timet = τ , it isnecessary to find out whether the anomaly, with durationdτ = t1−τ , is unique.For this purpose, the functionLTA(t) = Z over a sliding window of widthdτ(t − dτ , t) and over the rest of the time period, is evaluated; the seismicity ratewithin time windows of durationdτ (R2) is compared to rate of the overallsample, minus the data withindτ (R1). If theZ-values obtained do not exceedZ0, then the identified anomaly, with durationdτ , can be considered as unique.

In the present study, large positiveZ-values (>6) were obtained in the first twozones (Figure 4).dτ for maximumAS decreased from 15 years to 10 years passingfrom zone I to zone II. This can be interpreted as follows: seismic quiescencestarted in zone I and then spread over zone II. No other similar anomalies wereobserved in these zones during the monitoring. As for the amount of change inrate in zone I, decrease was about 70% (Figure 3), that is close to decreases beforestrong earthquakes reported in the literature (Wyss and Habermann, 1988; Wiemerand Wyss, 1994). Apart from quiescence, some increase of seismicity in zones IIIand IV was observed during the last several years (2.5–3.0 yr); but this increase isnot unique, becauseLTA parameter has similar values at other times.


Figure 4. Diagrams of functionsAS(t) (bold line) andLTA(t) (dotted line) for the five zones.The vertical dotted line shows a moment of change in seismicity rate (moment of maximumAS). dτ – duration of seismic quiescence (ifAS > 0) or activation (ifAS < 0).


Ring seismicity.In the case of the existence of a seismic gap, increased seismicactivity is observed at its edges (Mogi, 1985). The activation of background seis-micity in zone IV was pointed out above. In addition, the consideration of strongerearthquakes is of interest.

The location of strong earthquakes was considered; the lower magnitude thres-hold was equal to 5. The following peculiarities were observed (Figure 5):

For earthquakes with depth less than 70 km:

(1) In zones II and III, there is a time period of 6 years (1985–1990) during whichearthquakes withM > 5 did not occur;

(2) In the same zones, a marked increase in the number of earthquakes withM > 5was observed in 1991–1995 as compared to the period 1962–1984; in zoneII, the seismicity rate has increased three times (from 0.26 to 0.8 yr−1) andtwice in zone III (from 0.6 to 1.2 yr−1). It is important to realize that the weakseismicity displayed opposite behaviour: as indicated above, it decreased inzone II and did not vary in zone III;

For earthquakes with depth greater than 70 km:

(3) In zone I, earthquakes withM > 5 did not occur. Temporal anomalies ofseismicity are not observed in the other zones;

In the whole range of depth:

(4) In 1991–1994, events withM > 6, which were not observed earlier, havetaken place in zone IV. These data confirm the hypothesis about ring seismicityformation in the Avachinsky gulf.

The reduction of the earthquake recurrence diagram slopealso indicates thepreparation of a strong earthquake (Smith, 1981).

The earthquake recurrence can be formulated in terms of magnitude (in thiscase the slope of the diagram is designated byb) or in terms of energy class (theslope is designated byγ ): N ∼ 10−γK or N ∼ 10−bM , whereN is the number ofearthquakes with magnitudeM or of energy classK (K = log(E),E is the energyin J ).

At various levels of seismicity (from destruction of model samples to prepa-ration of real earthquakes), the slope exhibits a decrease as breaking approaches(Main and Meredith, 1989; Smith, 1981; Weeks et al., 1978; Zavialov, 1984). Butin making use of this parameter, a problem appears: the accuracy of the slopeσγdepends on the number of earthquakesn: σγ = γ /

√n. Therefore, it is necessary

to use large volumes of space and long intervals of time. In our case to obtainstatistically reliable changes, the slope of the recurrence diagram was calculated forthe region 51.5◦–53.5◦N and 158◦–161◦E, within a sliding time window of width6 years and with steps of 0.5 years (Figure 6). The last five points are obtained forsmaller windows, and this is reflected in increased confidence intervals.

94VA

RIA

TIO

NS

OF

SE

ISM

ICIT

YIN

TH

EAVA

CH

INS

KY

GU

LF

(KA

MC

HA

TK

A,R

US

SIA

)

Figure 5. Seismic energy emitted in zones I–IV in two depth ranges –h 6 70 km (left) andh > 70 km (right) during 1962–95.


Figure 6. Temporal variations of the slope of the recurrence diagram in area 51.5◦–53.5◦N,158◦–161◦E, depth 0–70 km, magnitudeM > 2.6. Points numbered corresponds to timewindows less than 6 years.

The observed continuous decrease of the recurrence diagram slope has no ana-logue within 33 years. Thus, theγ -value reaches its minimum for the whole timeinterval considered.

4. Conclusion

Processes (seismic quiescence, ring seismicity, reduction of the recurrence diagramslope), which testify to the preparation of a strong earthquake (M > 7.5), areobserved presently in the Avachinsky Gulf. Such an earthquake could represent aserious danger to Petropavlovsk–Kamchatsky.

It is difficult to estimate the date of this earthquake, as the dispersion ofresults reported in the literature about time of occurrence of similar precursors isgreat (Rikitake, 1976; Wyss and Habermann, 1988). Apparently, one must speak inyears, rather than in months, since the monitoring of other predictable parameters(geodetic, hydrogeochemical, etc.) gives no cause for alarm.

Acknowledgements

This paper benefited greatly from detailed comments by critical reviewers. Theauthors are particularly thankful to anonymous reviewers, who provided commentsand suggestions that improved the clarity of the paper. Authors would also like tosay special thanks to Dr Vladimir Smirnov, whose computer programs were used.


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Variations of Seismicity in the Avachinsky Gulf (Kamchatka, Russia)