P Signal Complexity Re-examined

Geophys. J. R. astr. SOC. (1973) 33, 195-221.

P Signal Complexity Re-examined A. Douglas, P. D. Marshall, P. G. Gibbs, J. B. Young and C. Blarney

(Received 1973 January 26)

Summary

The early recordings made by short-period seismometer arrays showed clearly that P signals differ very widely in their complexity. Many attempts have been made to explain this complexity and here evidence is presented to show that greater absorption of the direct signal relative to the later arrivals may be the principal explanation of complexity at least for explosion signals. Deconvolution and synthesis of explosion signals is used to demonstrate that multipathing also contributes to complexity.

From a comparison of explosion signals from sites in shield areas with those from sites in or near fold belts it is suggested that fold belts are underlain by regions of much lower Q than shield areas. Thus methods of discriminating between earthquake and explosion signals that are based on the observed differences between explosion signals from sites in shield areas and earthquake signals from foci in fold belts, may not then be measuring difFerences in the two sources but rather differences in Q in the upper mantle in the source regions.

1. Introduction

The complexity of P signals has been much discussed as a possible way of identifying underground explosions particularly by our research group at Blacknest (see, e.g. Thirlaway 1963; Carpenter 1963; Carpenter 1964; UKAEA 1965). Many of the first teleseismic P signals from underground explosions recorded by the UKAEA designed arrays were observed to be simple, consisting of one or two cycles of relatively large amplitude followed by a tail or coda of much lower amplitude. Signals from earthquakes on the other hand were often observed to be complex consisting of a series of arrivals of comparable amplitude spread over several tens of seconds.

Until 1964 no UKAEA designed arrays had recorded a complex explosion signal but in September of 1964 the array at Yellowknife (YKA), Canada recorded a complex signal from an explosion at Novaya Zemlya (NZ) USSR (Thirlaway 1966). All other explosions fired at Novaya Zemlya and recorded at YKA are also complex and similar (complex) signals have been recorded at other stations in N. America. Complex signals have also been recorded from the LONGSHOT explosion particularly at stations along the western side of N. America (Key 1968). Consequently complexity as a method of identifying earthquakes is now little used.

A satisfactory explanation of complexity has still to be found and even to explain in detail the form of the first few seconds of simple explosion signals is sometimes difficult using simple theory (see, e.g. Hasegawa 1971). Recently however evidence has been presented of multipathing in relatively simple P signals (Mack 1969;

195

196 A Douglas et al.

Douglas et aE. 1972b) and Douglas, Marshall & Corbishley (1971) have shown that some features of complex explosion signals can be explained by assuming greater non-elastic absorption of the first arriving P signal relative to the coda. Here we present further evidence of multipathing and of the greater absorption of the first P arrival relative to the coda of complex explosion signals and show that together these processes can explain most of the features of explosion P signals. We also discuss some of the geophysical implications of these explanations of complexity and consider how this affects the prob!em of identifying seismic signals from earthquakes and explosions. We begin however with a brief review of some of the other explanations of complexity that have been put forward in the past 10 years.

2. ExplaPaations of complexity

The &st attempts to explain complexity were mainly concerned with why earthquakes alone appeared to generate complex signals. The possibility that earthquakes are multiple sources was ruled out (at least for earthquakes of say body wave magnitude, m,5.5 or less) because both simple and complex signals were often recorded from a single earthquake (Thirlaway 1966; Douglas 1967) which is unlikely to happen if earthquakes are multiple sources, Also by confining attention to signals recorded at epicentral distances of 30"-90" it was assumed that there is little coIitri- bution to complexity from multiple arrivals due to triplication in the P travel-time curve resulting from changes in the velocity gradient in the upper mantle. Such multiple arrivals should occur only at shorter ranges. Thirlaway (1963) and Carpenter (1963, 1964) suggest that complexity might result from reverberations of the P and S signals in the crustal layers near the earthquake source with conversion of some of the S energy to P. Earthquake signals on this hypothesis should then be more complex than those from explosions because the earthquake source is richer in S energy and because the depth of focus of earthquakes is usually a few tens of kilometres, so p P and sP the surface reflections should be well separated from P compared to the surface reflections from the very shallow explosions. Douglas (1967) extended this idea and suggested that complex signals may be those for which direct P leaves the earthquake source in a direction close to a node in the source radiation pattern: direct P is thus small relative to the later arrivals and the record appears complex. On this hypothesis complexity is the result not of a large amplitude tail but rather a small first arrival and Douglas (1967) found some evidence that where complex and simple signals are recorded from the same earthquake the complex signals do give lower magnitudes than the simple signals, as one would expect if complex signals emerge from the source near a node. The diEculty with these hypotheses is that for any reasonable crustal structure the reflections from discontinuities in the crust are of small amplitude compared to p P and sP so that any record should consist of at most three discrete large amplitude arrivals ( P , p P and sP) rather than the multiple arrivals observed. Douglas et al. (1972a) have made more recent attempts to generate complex signals but again they find it is difficult to reproduce the observed complexity of earthquake seismograms.

These hypotheses, which go some way to explaining the complexity of earthquake signals, cannot however offer a satisfactory explanation of complex explosion signals. To explain the complexity of such signals when recorded by a single seismometer, Key (1968) suggested that most of the coda of the complex signals might be simply noise generated by the direct P signals on striking topographic features near the recording station. Key (1968) was able to demonstrate the presence of such signal generated noise (SGN) using array data and many of the stations that have recorded complex signals are sited in mountainous areas in agreemept with this SGN hypothesis. Also Key (1968) was able to show, using three component sets of seismometers, that some of the complexity in complex explosion signals is apparently locally generated.

P signal complexity reexamined 197

The main drawback to this explanation of complexity is it leaves unexplained the complex explosion signals that have been recorded by seismic array stations; the process of forming the sum of the signals from an array of seismometers tends to cancel out the signal generated noise. The SGN hypothesis also does not offer a satisfactory explanation of the observed negative correlation of magnitude and complexity: as with earthquake signals, the magnitude of an explosion measured from a complex signal is less than that measured from a simple signal (Key 1968).

Greenfield (1971) has suggested that signal generated noise near the explosion source is responsible for complexity: high frequency Rayleigh waves ( N 1 Kz) generated by the explosion are assumed to be converted to P waves on striking topographic irregularities near the source and some of these P waves then foliow a path to the receiving station similar to that of P, thus forming the complex coda. Greenfield (1971) has made a detailed study of complex signals from two Novaya Zemlya (NZ) explosions recorded at the Large Apertllre Seismic Array (LASA) and concludes that the mechanism can account for the complexity. But again this hypothesis does not explain why complex signals have in general lower magnitudes than simple signals from the same explosion. Greenfield (1971) argues that the LASA magnitudes for the two NZ explosions he studied are not low compared to the average magnitude given by the United States Coast and Geodetic Survey (USCGS) but this comparison could be misleading: the individual station magnitudes that contribute to the average may all have come from relatively complex signals with low magnitudes compared to stations that recorded simple signals. Davies (1970) has shown in fact that far explosions at NZ, magnitude and complexity are negatively correlated (as complexity increases magnitude decreases).

Davies (1970) suggests that lateral velocity contrasts in the upper mantle beneath the source increases the geometrical spreading over parts of the wave front of the P signal which effectively casts a shadow at teleseismic distances within which the P signal has anomalously low amplitude. Within this shadow zone Davies (1970) suggests that the recorded signals are likely to be complex because later scattered arrivals for which the lateral velocity contrasts do not cause similar increased geometrical spreading may be large relative to the initial P signal and thus the signal will appear complex. Julian & Davies (1971) and Davies & Julian (1972) have shown theoretically that a steeply dipping lithospheric slab of the type postulated by the hypothesis of plate tectonics will produce such a shadow. The shadow computed for the specific example of the LONGSHOT explosion however does not show a signi- ficant correlation with the observed distribution of anomalously low magnitudes recorded from this explosion.

3. The complexity of explosion signals

(a) Eflects of absorption Douglas et al. (1971) propose an explanation of complexity which is similar to

that of Davies (1970) except that direct P (and p P ) is assumed to be strongly attenuated by a region of relatively low Q (Q is the elastic quality factor) and that the later arrivals that form the coda of the complex signals are in effect scattered signals that have travelled by relatively high Q paths and hence with little attenuation. Con- sequently although the later scattered arrivals are much reduced in amplitude by scattering they are nevertheless of similar amplitude to direct P because the P signal has been much more strongly attenuated than the scattered arrivals. The resulting record thus appears complex. For simple records the P signal is presumed to travel by a high Q path and so is of much larger amplitude than the scattered energy. (Simple signals could also be recorded on low Q paths if the low Q is extensive enough to cut off all high Q paths.)

198 A. Douglas et at.

On this explanation of complexity, there should be less high frequency energy in the direct P arrival of complex signals than in the direct P arrival of simple signals from the same explosion and the coda of complex signals should be richer in high frequency energy than the direct P arrivals (low Q attenuates the high frequency components of a signal more rapidly than the low frequency components). Also, the body wave magnitude (mb) of an explosion as computed from a complex signal should be less than that computed from a simple signal. Douglas et al. (1971) show that the three complex signals they analysed do have these properties: the differences in the frequency content betweeo the direct P and the coda of complex signals is actually visible in the seismograms. Here, to demonstrate these spectral differences more clearly and allow a quantitative measure of these differences to be made, we have used analogue frequency filtering to separate the high and low frequency components. Each signal is filtered into two bands: a low frequency (LF) band of 0.5 Hz-0-75 Hz and a high frequency (HF) band of 1.25 Hz-2-25 Hz. The result of squaring these two filtered outputs is also presented; this process shows up very clearly any differences in the complexity of the HF and LF records. If the absorption hypothesis of complexity is correct, complexity should be frequency dependent: the HF component should be more complex than the LF component. As a quantitative measure of frequency content we use HF/LF, the ratio of the maximum amplitude in the HF component to the maximum amplitude in the LF component of the direct P signal. As a measure of the frequency content of LONGSHOT signals we also use the spectral ratios computed by Lambert et al. (1969). This ratio is defined as follows:

1.0

where A ( f ) is the spectrum (as a function of frequencyf) of the first 2.5-4.5s of the signal.

Two methods are used to measure the complexity. For recordings of explosions fired at NZ and recorded at UKAEA arrays or similar recording systems we use the energy ratio (ER) method which we use routinely to measure complexity. To determine the ER the signal is filtered in a 1-2Hz band, squared and smoothed by an exponential window with a 1.5 s time constant. The ER is the ratio of the area under the smoothed curve in the first 5s of the signal to the area under the curve from 5 to 35 s making allowances for the effects of noise. For simple records this ratio is roughly unity or greater, for complex records the energy ratio is less than 1.0. Note that in what follows all HFJLF and ER measurements for recordings made by UKAEA arrays have been made on the recordings from only one seismometer from the array and not from the array sum. This has been done so that these measurements can then be compared directly with those from non-array stations. As a measure of the complexity of the LONGSHOT signals we use the complexity factor F, computed for these records by the Seismic Data Laboratory (see Lambert et al. 1969). F, is determined by squaring the signal; smoothing with a 3-s integration window, square rooting and taking the ratio of the area under the curve from 5 to 35 s to the areaunder the curve in the first 5 s. For simple signals F, is around 2 or less, for complex signals F, is usually greater than 2.

The complex explosion signals that we analyse here are the Bukhara (USSR) explosion analysed by Douglas ef al. (1971) the LONGSHOT explosion fired at Amchitka Island and three explosions at Novaya Zemlya (NZ).

P signal complexity re-examined 199

(i) Bukhara, USSR. Fig. 1 shows the P signals recorded at arrays at Eskdalemuir (EKA), Scotland and Gauribidanur (GBA) India from an explosion fired near Bukhara USSR on 1968 May 21. The EKA seismogram is a typical simple explosion signal: the direct P wave and what may be the surface reflection p P can be identified. The GBA signal is similar to the EKA signal for the first few seconds, but overall the GBA signal is more complex. It is obvious by eye that the later arrivals P,, and PH2 in the more complex GBA signal have a higher dominant frequency than the first arrivals P and p P and this is codrmed by the filtered and squared records also shown in Fig. 1 . Table 1 lists the HF/LF ratios, the body wave magnitude and the energy ratios for the two signals. Clearly the more complex GBA record has a lower mb and a lower HF/LF ratio than the simple EKA records, as would be expected on the absorption explanation of complexity.

Table 1

Spectral and energy ratios and magnitudes for Bukhara explosion of 1968 May 21 Epicentral distance Energy

Station (de& mb HF/LF ratio

EKA 47.3 5.83 2-5 2.0

GBA 27.4 5.28 1-4 0.25

(ii) LONGSHOT. Fig. 2 shows three signals from the LONGSHOT explosion of the 1965 October 29 recorded in N. America at stations in the Long Range Seismic Measurements (LRSM) network. The signal recorded at KC-MO is a simple signal, the SI-BC signal is an extreme example of a complex explosion signal and the complexity of the YR-CL signal lies between these two extremes. The HF/LF ratios for these and 12 other stations are given in Table 2 together with mb value and F, factor for each station from Lambert et al. (1969). The stations with mb < 6-0 listed in Table 2 are those that recorded LONGSHOT signals classified complex by Key (1968); also listed are four of the LRSM stations that recorded simple signals from LONG SHOT. The correlation of increasing complexity with decreasing HF/LF and mb and the greater high frequency content of the later arrivals in complex signals as compared to the first arrivals are all clearly seen.

Fig. 3 shows the recordings of the Novaya Zemlya explosion of 1967 October 21 as recorded at the array stations at Yellowknife, Canada (YKA) and EKA together with the result of filtering and squaring these records. In addition the recordings of the NZ explosion of 1966 October 27 at a temporary station near Ipswich is shown. The HF/LF ratios, complexities and magnitudes of these three signals are given in Table 3. The mb for the Ipswich record cannot be determined because the recording system was not calibrated. Again these records show the expected correlation of increasing complexity (decreasing ER) with decreasing HF/LF and (for EKA and YKA) decreasing mb and also the later arrivals in the complex records tend to be richer in high frequencies than the first arrival. Strictly the Ipswich records should be compared to the EKA and YKA recordings of the 1966 October 27 explosion. This explosion however overloaded the YKA recording system and the only EKA recording is from an uncalibrated geophone cluster. However all the evidence is that the difference in the source function between the 1966 and 1967 explosions is so small that the differences can be ignored in making HF/LF and ER comparisons. Key (1968) has pointed out that the Ipswich recording is probably of a relatively low magnitude because if the reasonable assumption is

(iii) Novaya Zemlya USSR.

200 A. Douglas et al.

FIG. 1. P signals recorded at GBA and EKA from an underground explosion iired near Bukhara, USSR. Filtered and squared signals also shown. Note high

frequency arrivals PHI and PH2 on GBA records.


Table 2

Spectral ratios, complexities andmagnitudes for LRSM recordings of the LONGSHOT explosion

Station

BH-YK SI-EC PG-BC JP-AT

YR-CL HL2-ID TF-CL KN-UT FN-WV FL-BC TE-GL RK-ON RG-SD KC-MO CR-N B

Epicentral distance

(deg) 24.6 31.8 34.5 37.4 40.1 44.1 45.8 49.1 67-2 33.6 45.4 51-5 49.6 58.3 56.2

in,,* HF/LF

5.74 2 . 0 4.95 0.56 4.75 0.83 5-38 1.64 5-86 0 . 6 5.97 0.85 5.83 1.12 5.84 1 .0 5.86 2.08 5.64 2.38 5.97 3-2 6.16 4 .7 6.26 5 .9 6.47 3-6 6.50 6 - 4

LF/HF

NA NA

11.52 1 . 3 1 NA NA NA 2.99 NA NA NA 0.52 0.27 0.42 0.36

Complexity (F,)* 8.52 8.36 7.93 4.16 2.94 2.74 2.95 2.63 2.14 3.61 2.87 1-22 2.28 1.21 1.66

HF/LF = Maximum amplitude in band 1.25-2.25 Hz/Max amplitude 0.5-0.75 Hz.

LF/HF (from LVAG*) = Integrated amplitudes 0.5-1 .O Hz/integrated amplitudes 1.1-2.0 Hz.

NA = Not Available.

*LVAG Lambert et al. (1969).

made that the absolute amplitudes of PcP recorded at EKA and Ipswich for any given explosion are roughly equal, P is of about equal amplitude to PcP at Ipswich; for EKA recordings of NZ explosions, P is about 7 times larger than PcP.

Key (1968) has suggested that the complexity of the Ipswich records compared to EJSA may be due to signa! generated noise at the Ipswich site. The Ipswich recording site is on a series of sedimentary rocks of Mesozoic and Tertiary age whereas the EKA site is on tightly folded compact Palzeozoic rocks. The NZ record shown in Fig. 3 is the only record available from this Ipswich site and so it is not possible to check whether all signals recorded at this site are complex regardless of azimuth and distance to the source which would suggest a site effect. However two R and D stations in the south of England operated by our laboratory at Blacknest- one station at Blacknest itself (BKN) the other at Wolverton (WOL) 6 km from BKN- are sited on a similar sedimentary series to Ipswich and recordings have been made at BKN and WOL of signals from explosions at several sites. Fig. 4 shows signals recorded at EKA and WOL from the NZ explosion of I971 September 27 and a Kazakh explosion of 1970 June 28. The Kazakh signals although differing in detail at EKA and WOL are not noticeably different in complexity but the NZ recordings are more complex at WOL than EKA. From this we conclude that site contributions to complexity at WOL and BKN (and hence probably at Ipswich which is a geologi- cally similar site) are not important. Note again the familiar correlation of increasing compiexity (decreasing ER) and decreasing HF/LF ratio (Table 3). In this example however the correlation of decreasing mb and increasing complexity is not shown.


LF 0.5-0.75 H.?

HF 1.25-2.25 Hz

(LFP

YR-CL A:40.1'

LF C.5 -0.75 HZ

HF 1.25-2.25 HZ

( L F ) ~

KC- MOA = 58.3'

LF 0.5 -6.75 H t

HF 1325--2.25HZ

( L F ) ~

( H F ~ ~

FIG. 2. P signals recorded at three LRSM stations in North America from the explosion LONGSHOT together with filtered and squared records.

P signal complexity re-examined

Table 3

203

Spectral and energy ratios and magnitudes for three Novaya Zemlya explosions and an explosion in East Kazakh

Explosion site Date

Novaya Zemlya 66 Oct. 27

67 Oct. 21

71 Sept. 27

East Kazakh 70 June 28

Station

Ipswich

YKA EKA

EKA

BKN

WOL

EKA

BKN

WOL

Epicentral distance

(deg) 30

44

29

29

32

32

47

48

48

mb

NA*

5 . 2

6 . 5

6 .8

6 . 7

6 .8

6 .1

6 .3

6 .6

HF/LF

3 .3

5

10

4.0

0.4

1.9

3 .6

4 . 0

8 . 0

Energy ratio

0 . 3

0 . 8

1.5

5.25

0.24

0 .5

1 .3

1 . 5

1.1

*NA Not available.

(b) Effects of multipathing

Several authors have shown that the first few cycles of relatively simple explosion P signals can be predicted adequately by simple theory (see for example Carpenter 1966) but this is not always so (see for example Hasegawa 1971). In this section we try to show that where theory and observation are not in close agreement this could indicate multipathing, i.e. there is duplication or triplication of the travel-time curve so that the recorded signal consists of a series of overlapping arrivals.

Two approaches are used to demonstrate multipathing. One is to deconvolve the recorded signal in an attempt to separate out the various arrivals. The effects of recording system and anelastic absorption are removed from explosion signals using a spiking filter as described by Douglas et al. (1972b). The original reason for doing the deconvolution was to try and separate P and pP so that the depth of firing of the explosion could be estimated. However Douglas et al. (1972b) found that some spiked (deconvolved) signals show not only P and p P but several other arrivals which could be due to multipathing.

The spiking filter is constructed as follows. The required impulse response g(t) of the spiking filter is that which when convolved with f ( t ) gives the best approximation to a unit impulse or spike (in the least squares -sense). f ( t) is the Fourier transform of F(w)I(w) where F ( o ) is the effect of absorption and I(w) the effect of recording system, at angular frequency o. I(o) is known for any given recording system and -F(o) can be written as an amplitude term exp (-ot*/2) and a phase shift. t* is T/QAv where T is the travel time and QAv the effective average Q (elastic quality factor) along the path between source and receiver. The value of t* to be used in deconvolving any given signal will not usually be known so the correct spike filter cannot be constructed. Several spiking filters for different values of t" are therefore used and the resulting spiked seismograms examined to see which values of t* gives the most easily interpreted record. From studies of explosion spectra Carpenter (1966) suggests t* N 1 for many ray paths; Carpenter & Flinn (1965) suggest t" N 0.8. We have used values of t" ranging from 0.4 to 1.0 to construct spiking filters. Robinson (1967) discusses spike filters and has published a computer

204 A. Douglas ef cil.

IPSWICH a = 30°

HF 1.25-2.25 HZ

(LF)'

(HF)'

Y K A A 1440

LF 0 5- 0.75 H z

HF 1.25-2.25 Hz

(LFF

(HFF

EKA = 29"

FIG. 3. P signals recorded at EMA, YKA and a temporary station near Ipswich, south-east England from explosions at Novaya Zernlya together with filtered and

squared records.


E K A n ~ 2 8 . 9 ~

LF 0.5-0.75 Hm

H F 1.25-2.25 HZ

(LFJ2

(HF)'

VIOL 8= 4 8 1 O

HF 1.25-2.25Hz

(LFF

EKA A=47.l0

LF 0.5-0 .75HZ

FIG. 4. P signals recorded at EKA and WOL from an explosion at Novaya Zemlya and an explosion in East Kazakh together with filtered and squared records.

206 A. Douglas ei al.

Table 4

Crustal models used in the computation of seismograms

P-wave velocity S-wave velocity Density Thickness (km s- 1) (km s-1) (gcm-3> (km)

1st layer 3 .6 2 . 0 2.4 2.0 2nd layer 5 .5 3 .2 2 .7 10.0

Half space 8.1 4 .7 3 .3

1st layer 4.80 3.4 2 .7 2 . 0

Half space 7.81 4 .6 3.3

1st layer 6.14 - 2.80 5-3

A. Amchitka Island Crust-Refraction survey (Hasegawa 1971)

3rd layer 6 .6 3.8 3 .0 30.0

B. Nevada Test Site-Granite Crust (Werth & Herbst 1963)

2nd layer 6.15 3.5 2.8 24.6

C. Eskdalemuir, Scotland (Parks 1967)

2nd layer 7.28 - 3-20 19.7 Half space 8.09 - 3.28

Note: Where S wave velocity (8) is not listed it is assumed that ,B = a/.\/3 where a is the P wave velocity.

program (Robinson 1966) to generate them; we used his program in the computations described here.

The second method used to demonstrate multipathing compares observed and predicted signals and the signals computed assuming multipathing. The predicted signal is computed using the methods described by Douglas et al. (1972a) and the multipathed signals computed by adding the predicted signal to itself with a delay. In computing the predicted signal, models have to be assumed; crustal models are available for some test sites and stations and some of those available have been used. A value of t* has also to be chosen and this is more difficult. However by matching computed amplitudes and observed amplitudes for explosions of known yield it is possible to obtain an estimate of t*. The estimation of t* will be discussed in detail elsewhere, here we will simply assume that for the EKA signals from explosions at the test sites at Amchitka Island and Nevada described below, t* = 0.6, and t* = 1.0 respectively.

(i) Amchitka Island, Aleutians. Three explosions have been fired at Amchitka Island: LONGSHOT, MILROW and CANNIKIN. The yields, firing depths and predicted P to p P time (from the uphole velocity of 3-38 km s-l, measured at the LONGSHOT firing site) are given in Table 5(a). The signals recorded from these three explosions at EKA (A = 73.6") and the results of deconvolving these signals are shown in Fig. 5 . The p P - p time measured from these deconvolved signals for these three explosions appears to be much greater than predicted (see Table 5(a)). If P and p P have been correctly identified in the deconvolved record these observed P - p P times imply a velocity in the overburden of about 1.4 km s-l which is so low as to be unacceptable. To explain this discrepancy between the predicted and observed P - p P time Douglas et al. (1972b) (who looked only at LONGSHOT and MILROW) suggested that each recorded signal consists of two overlapping P arrivals separated by about 0.8s. On this hypothesis the time delay measured from the deconvolved signal is not the true P - p P time but the time between the direct P of the k s t arrival and p P of the second arrival.

Tabl

e 5(

a)

P-pP

Ti

mes

for

the

Aleu

tian

Isla

nd e

xplo

sion

s pre

dict

ed from

the

obse

rved

P v

eloc

ity i

n th

e ov

erbu

rden

and

Jiri

ng

dept

h, c

ompa

red

to o

bser

ved P

-pP

Tim

es an

d im

plie

d P

velo

city

in o

verb

urde

n.

P-p

P t

ime@

mea

sure

d fr

om s

pike

d re

cord

Im

plie

d ve

loci

ty

True

firin

g Pr

edic

ted

P-p

P

assu

min

g no

in

Ex

plos

ion

Yie

ld (k

t) de

pth

(km

) tim

e(s)

* m

uitip

athi

ng

over

burd

en (

km s

-l)

LON

GSH

OT

80

0-7

0.41

1.2

1.17

MIL

RO

W

N

1.2

0.71

1.7

1.4

CA

NN

IKTN

N

500

0 1.

8 1.06

2.4

1.5

* P v

eloc

ity in

ove

rbur

den

assu

med

to b

e 3.38 k

m s

- l.

Tabl

e 5(

b)

Yiel

ds, P

-pP

times

and

tim

e del

ays f

or m

ultip

athi

ng, u

sed

to m

odel

the E

KA

sig

nals

from

the A

leut

ian

Isla

nd e

xplo

sions

to

geth

er w

ith th

e ap

pare

nt P

-pP

times

show

n by

thes

e co

mpu

ted

signa

ls.

Tim

e di

ffere

nce

betw

een

Yie

ld (k

t) of

P-pP

tim

e(s)

fir

st a

nd se

cond

arr

ival

s A

ppar

ent P

-pP

tim

e(s)

so

urce

func

tion

inco

rpor

ated

in

used

to c

ompu

te m

ultip

athe

d m

easu

red f

rom

dec

onvo

lved

Ex

plos

ion

used

in m

odel

ling

the

mod

el

sign

als

mul

tipat

hed

sign

als

LON

GSH

OT

80

0.45

0.

94

1.3

MIL

RO

W

300

0.95

0.

94

1.8

CA

NN

IKIN

lo00

1.25*

1-19

2.4

* Lay

ex 1

of A

mch

itka

Isla

nd m

odel

incr

ease

d to

2-5

km t

o al

low

this

P-p

P t

o be

inco

rpor

ated

in th

e m

odel

and

yet

reta

in th

e so

urce

in la

yer 1

.

b

h, s


To demonstrate that this is a possible explanation we have attempted to model the three EJSA signals. Fig. 5 shows the signals computed for the LONGSHOT, MILROW and CANNIKIN explosions both with and without multipathing using the Amchitka Island structure as source layering, EKA structure as receiver layering and t* = 0.6. To obtain the multipathed signal the signal predicted without multipathing is simply added to itself with a delay. The deiays used to produce the multipathed signals were derived by trial and error, aiid are listed in Table 5(b). The predicted multipathed signals (both before and after deconvolution) do clearly show many of the features of the observed signals although agreement is by no means perfect.

The computed signals presented in Fig. 5 represent the best match so far obtained to the observed signals. To obtain these we have had to use slightly different P - p P times than those listed in Table 5(a) predicted using true depth and measured velocity in the overburden. Also the source functions used in modelling MILROW and CANNIKIN are those predicted by the expressions of Haskell (1967) for 300 and 1000 kt explosions in granite respectively. His source functions for yields of 1000 and 5000 kt the approximate yields of MILROW and CANNIKIN respectively give poorer fits between computed and observed signals.

Fig. 6 shows the evidence for multipathing for the path Nevada Test Site (NTS) to EKA. Fig. 6(a) is the recorded signal at EKA from the NTS explosion PILEDRIVER and the deconvolved signals for t* = 0.6 and t* = 1.0. PILEDRIVER with a yield of 55 kt was fired at a depth of 463 m in NTS granite (Springer & Kinnaman 1971) having a P velocity of 4*8kms-' (Werth & Herbst 1963). Fig. 6(b) shows an attempt to model the PILEDRIVER signal using the values of yield, depth of firing and P velocity and t* = 1.0. The deconvolution of this computed signal with t* = 0.6 and t* = 1.0 is also shown. Comparing the observed seismogram (Fig. 6(a)) with the computed seismogram (Fig. 6(b)) shows only poor agreement between the two. In particular the deconvolution of the observed signal using t" = 0.6 shows an apparent P-pP separation of ~ 0 . 7 s implying a velocity in the overburden of 1.3 km s-l which is unacceptably low. This discrepancy between observed and computed signals is similar to that found for the Aleutian Island explosions and can also be explained as the effect of multipathing.

Fig. 6(c) is an attempt to simulate this multipathing by adding the computed signal (Fig. 6(b)) to itself with a delay of 0.4s. Fig. 6(c) also shows the results of deconvolving this multipathed signal. This computed multipathed signal shows better agreement with the observed signal both before and after deconvolution than the signal computed without multipathing.

The agreement with observation is also better than that obtained for the signal (Fig. 6(d)) computed so that the P - p P separation is 0-8s and which thus closely models the apparent P-pP separation shown by the observed signal. Finally comparing this signal (Fig. 6(d)) deconvolved with t* = 1.0 with the signal computed for the same model but without including any effects of attenuation or recording instrument (illustrated below Fig. 6(d)) shows the effectiveness of using a spike filter for deconvolution. If the deconvolution were perfect then the deconvolved signal and the signal computed without the effects of absorption and recording instrument would be identical. The agreement between the two is very good re- membering that spike filtering is only an approximate method of deconvolution.

(ii) Nevada Test Site ( N T S ) .

4. Complex signals from earthquakes

We have yet to complete a detailed study of the complexity of earthquake signals but the first results of this study show that the frequency dependent complexity observed for explosions is also present in earthquake signals. Fig. 7 shows the results of filtering and squaring the signals recorded by the four UKAEA arrays

209

LONGSHOT

Computed signal (:io multipcthing)

P signal complexity re-examined

Seismogram SDiked Record

Observed signal at EVA

0 12s

Computed signal with rnuilipathing

MiLROW

CcmpJted siqn31 (no multipathing)

Observed signal a t EKA

Computed signal with rnultipathing

CANNlKlN

Computed signa! (no multipathing)

Observed signal at E K A

Computed signal wi th multipathing

FIG. 5. Comparison of observed, theoretical and theoretical multipath signal for LONGSHOT, MILROW and CANNIKIN together with spiked signals.

6


Seismogram Spiked record t* '0.6

Spiked record I*'I.O

I Signal without effects of instrument or absorption

FIG. 6. Comparison of observed, theoretical and theoretical multipath signals for PILEDRIVER together with spiked signals. (a) Observed signal a t EKA: (b) Computed signal (no multipathing); (c) Computed signal with multipathing (d)

Computed signal with P-pP time of 0.8 s.

21 1

FIG. 7. P signals from an Andreanof Island earthquake of 1970 December 2 (origin time: 02 34 59-5) as recorded at YKA, WRA, GBA, and EKA together

with filtered and squared records.

212 A. Douglas et ai.

Table 6

Spectral ratios and complexities from LRSM recordings of LONGSHOT and an Andrecnof Island earthquake*

LONGSHOT Station LF/HF

PG-BC 11-52

JP-AT 1.31

KN-UT 2.99

=-ON 0-52

R G S D 0.27

KC-MO 0.42

CR-NB 0.36

* Data from Lambert et al. (1969).

Earthquake LF/HF

6.01

4.54

5.98

1-71

2.87

2.61

1.92

LONGSHOT Fc

7-93

4.16

2.63

1.22

2-28

1.21

1.66

Earthquake FC

8.73

5.92

7.31

3.51

4-61

3.01

3.21

from an earthquake in the Andreanof Islands. Tine two prominent arrivals in the LF signal on the Warramunga, Australia (WRA) record, of amplitude greater than P, 16 and 23 s after P, are probably p P and sP respectively for a depth of about 60 km, in comparison with the depth of 57 km estimated by the USCGS. p P also appears to be present on the GBA record. These examples show that pP and sP are more easily recognized on the low frequency record where the high frequency scattered energy has been filtered off. The low frequency WRA record is the kind of record that one might expect theoretically consisting of only three prominent arrivals P, p P and sP. Incidently, Fig. 7 shows why the complexity of the YKA, GBA and WRA records cannot be due to multiple sources: the EKA record of the event is simple, suggesting only a single source.

Another line of evidence that supports the suggestion that absorption also contributes to the complexity of earthquake as well as explosion signals is the data presented by Lambert et al. (1969) in comparing LONGSHOT signals with signals from a nearby earthquake. Lambert et al. (1969) looked at the value of various discriminants such as complexity and spectral ratios for distingaishing between an explosion (LONG- SHOT) and an Andreanof Island earthquake. Table 6 shows spectral ratios and complexities of LONGSHOT and the Andreanof Island earthquake of the 1965 November 22 taken from Lambert et al. (1969)). The stations listed in the Table 6 being common with this study and that of Lambert et al. (1969). It is clear that although the earthquake is in general more complex and has less high frequency content, the same correlation of increasing complexity with decreasing proportion of high frequency is shown by both the earthquake and the explosion.

Lambert e f al. (1969) do not publish any station magnitudes for the Andreanof Island earthquake so that the correlation of magnitude with complexity for earthquakes cannot be investigated from this data.

From routine analyses we have made of large numbers of earthquake signals recorded by the UKAEA arrays we have found that when both simple and complex signals are recorded from the same event the simple signals have in general the larger magnitudes, as would be expected on the absorption hypothesis of complexity.


FIG. 8. Computed complex P signal from an 80 kt explosion, t* = 1.5 for direct P, t* = 0.5 for later arrivals. Filtered and squared records also shown.


5. The synthesis of complex seismograms

To obtain an estimate of the differences in Q that are required to produce complexity we have attempted a crude synthesis of a complex explosion seismogram. To do this two explosion seismograms have been computed which are identical except that one was computed using t* = 0.5 (high Q) and the other with t* = 1.5 (low Q). The high Q seismogram is then scaled down and added with delays to the low Q seismogram. Fig. 8 shows the high Q and low Q seismograms and a synthesized complex seismogram computed as described above using scale factors of 50-100 and delays of 1-30seconds. Fig. 8 also shows the results of filtering and squaring the complex signal in the same way as the observed seismograms. Comparing these filtered signals with the complex signals from the LONGSHOT explosion shows that the kinds of differences in complexity observed for LONGSHOT require differences in t * of about 1.

6. Some geophysicaI implications

The evidence presented above shows that most of the complexity of explosion signals (and probably of earthquake signals) can be explained as the result of greater attenuation of the fist P arrivals compared to later arrivals. We have also tried to show that some of the simple explosion signals are nevertheless more complex than would be expected on simple theory and have suggested that this could be explained as the effects of multipathing (although obviously we cannot rule out other possi- bilities). If multipathing does take place for the relatively simple signals discussed here then without doubt it contributes to the complexity of complex records.

These explanations of complexity clearly imply structure on the source-receiver path: Julian & Davies (1971) and Davies & Julian (1972) for example have shown that a descending lithospheric plate beneath Amchitka Island will produce multipathing but to check whether or not this may be the effect we are seeing at EKA for explosions fired at Amchitka more detailed studies are required. Similarly to identify all the high Q arrivals in the coda of complex records also requires a detailed study of

/ /

\

/ \

FIG. 9(a). PdPray path and suggested Q model. 9(b) P, Clp, ray path and suggested Q model.


each complex signal and this is not attempted here. We simply suggest some possible ray paths that could contribute to complexity and draw some broad conclusions on Q structure from the signals so far studied.

Douglas et al. (1971) suggest that for the signal from the Bukhara explosion as recorded at GBA (Fig. 1) the principal arrival in the coda (PHI) is of the PdP type (Bolt, O’Neill & Qamar 1968). PHI has a much lower phase velocity (10.8 kms-l) than direct P (12.1 km s- l ) and arrives about 5 s after P which is consistent with reffection from the underside of a discontinuity at a depth of about 500 km. Douglas et al. (1971) also suggest that the subsidiary arrival, PHz (which has about the same phase velocity as P H I ) may be a signal thzt is reflected at the Moho near the explosion source then at the free surface and then fallows the PdP path to the receiver. The proposed ray paths and Q distribution are shown in Fig. 9(a): the PdP path skips over the top of the low Q layer penetrated by P.

An alternative explanation of the arrival pH1 is that it is a diffracted arrival that strikes the top of the low Q layer with grazing incidence, runs along the boundary between the high and low Q and then travels back to the surface so that the ray path has the same form as P,,; we will call this type of ray path P,,dP, to distinguish it from PdP. Mr. J. A. Woodhouse (private communication) hzs made some cal- culations for such a diffracted ray and shown that provided the velocity difference across the boundary is greater than about 2 per cent, a diffracted signal is propagated along the boundary. Woodhouse (1973) also shows that a high Q-low Q boundary at a depth of 535 km would produce a P, dP, arrival at GBA with the correct arrival time and phase velocity and this arrival would have the same amplitude at 1 Hz as direct P provided Q = 450 in the high Q layer and Q = 175 in the low Q layer. As Q values for P down to about 40 have been measured in some areas the Q values required to produce signals of the P, dP, type are not unreasonable. Fig. 9(b) shows the proposed ray paths and Q distribution.

Of course any other discontimity in the upper mantle that is shallower than the depth of maximum penetration of P might produce a prominent arrival depending on the detailed Q structure of the upper mantle. A whole family of PdP- P,, dP,, arrivals between P and P P is thus possible. Similarly any reflector deeper than the maximum depth of penetration of P might produce observable arrivals given that the low Q zone is of limited lateral extent. The arrival times of such reflections will be between P and PcP. Multiple reflections such as the Moho reflected PdP (or P,,dP,) described by Douglas et al. (1971) are also possible. In addition, there are all the possible arrivals, which may well be the most important, that result from lateral changes in the structure of the crust and upper mantle.

To identify which of these ray paths contribute to complexity requires array records for two reasons. First, the velocity and azimuth of each arrival can be estimated so helping with the phase identification. Thus the arrival P I in Fig. 2 might be a PdP (or P,, dP,) phase but without an array to determine the phase velocity there is no way of testing this suggestion. From our routine studies of array records we have been puzzled for some time to note that the velocity and azimuth of the high frequency components of complex signals do differ signilicantly from those of direct P. However this is just what would be expected on the absorption explanation of complexity and we are now looking more closely at the variation in velocity and azimuth of the high frequency component within the coda of complex signals.

Use of an array also allows signal generated noise (and of course ambient noise) to be suppressed so that this noise is not mistaken for an arrival from some reflector in the mantle. Summing the individual outputs of an array of seismometers to enhance high velocity phases and suppress signal generated noise can only reduce (usually by a factor of fit where n is the number of seismometers, Key 1968) and not eliminate signal generated noise completely. Because of this the scattered arrivals of the type observed on complex records are unlikely to be seen on simple records even


if they are present with the same absolute amplitude. The first arrivals in the simp!e signal will often generate noise of amplitude large enough to swamp later arrivals. Only on a complex record where the direct P and hence the signal generated noise due to P is attenuated relative to the later arrivals are these later arrivals visible above the signal generated noise background. Thus the best seismograms for studying the fine details of Earth structure may not be those for which the P signal travelled along a path of high Q but rather one in which P is strongly attenuated.

On the other hand the simple high Q path signals are obviously the best for studying the seismic source. The high frequency part of the signal has not been absorbed and P, p P and sP are more easily identified in the low level coda. The body wave magnitude (mb) of an event is also probably best determined by averaging only the station magnitudes measured on relatively simple signals, excluding complex records in which P may have traversed low Q paths. Average magnitudes that include station magnitudes from both simple and complex records will tend to have a bias that is a function of magnitude. At low magnitudes near the detection threshold only the high Q simple signals will be recorded, the low Q complex signals will be below the detection threshold. At higher magnitudes complex signals that give in general lower mb than simple signals will be included in the magnitude average thus lowering the mb estimate. Measuring nib on simple records will also solve the practical problem of defining what amplitude to measure: this is simply the maximum of the direct P (or pcssibly pP). On complex records with multiple arrivals such a simple definition cannot be applied.

Turning now to the distribution of low Q regions, we can draw some very general conclusions from the patterns of simple and complex explosion signals gefierated by explosions at each different firing site. The data appears to be best explained by assuming that low Q layers underlie some if not all post-Cambrian fold belts at depths down to about 700 km and that these low Q layers are absent under stable slxeld areas. Thus explosions k e d in East Kazakh and test sites in South Algeria both zncient shield areas and recorded by UKAEA arrays (three on shield and one on Lower Palaeozoic rccks) are all the simple high frequency signals expected if the P signal follows a relatively high Q path. Similarly signals from explosions in southern USSR such as the Bukhara explosion for which the ray paths travel north- wards under the shield are again simple. From Bukhara south to GBA however where the ray paths reach depths of only about 700 km and pass under the Himalayas, P is reduced in amplitude, the high frequencies are attenuated and the record is complex. Explosions fired in fold belts such as at NZ (in an extension of the Urals fold be'its) and at the LONGSHOT site (in the Aleutian arc) also show complex signals at stations on great circle paths that lie along fold mountain belts. The complexity is maximum when the stations themselves lie in such fold belts, e.g. Sl-BC and PG-BC paths (Fig. lG(a)). The evidence thus points to low Q under fold belts.

The complex signals recorded at Ipswich, WOL and BKN compared to EKA for explosions fired at NZ may indicate low Q under the Caledonian fold belt in South Norway or under the active basin of deposition under the North Sea. The great circle paths from NZ to EKA, WOL and Ipswich are shown in Fig. lO(b). The path to EKA crosses the fold belt and the Norwegian Sea; to WOL, BKN and Ipswich the path follows the eastern edge of the Caledonian fold belt and passes under the North Sea.

The other major test site, the Nevada Test Site, has not been discussed in detail here because of the four arrays to which we have access to sufficient data on magnetic tape, only EKA is within the 30"-90" window relevant to the study. Examinaton of LRSM (paper) records of signals from NTS explosions shows that they also are relatively complex which is to be expected as the NTS is in a (low Q) fold belt. The association of low Q with fold belts would also explain why earthquake P signals

FIG

. lO(a

). G

reat

cir

cle

path

s fr

om L

ON

GSH

OT

to L

RSM

sta

tions

in N

orth

Am

eric

a. K

ey: 1

, TE

-GL

; 2, B

H-Y

K;

3, F

L-B

C; 4

, RK

-ON

; 5,

FN

-WV

; 6, S

I-B

C; 7

, PG

-BC

; 8, J

P-A

T; 9

, RG

-SD

; 10,

CR

-NB

; 11,

KC

-MO

; 12,

HL

2-ID

; 13,

KN

-UT

; 14

, Y

R-C

L;

15, T

F-C

L.

(b)

Gre

at

circ

le p

aths

fro

m N

OV

AY

A Z

EM

LY

A to

YK

A, E

KA

, WO

L an

d Ip

swic

h.

Key

: 1,

YK

A;

2, E

KA

; 3,

WO

L; 4

, Ip

swic

h.


were often complex: earthquakes take place almost exclusively in fold belts. The distribution of low Q regions proposed above is in general agreement with the

results of other workers. Solomon & Toksoz (1970) show that the mountain ranges of the western United States are underlain by relatively low Q whereas the stable central and eastern United States are regions of relatively high Q. Ward & Toksoz (1971) show that the Baltic Shield area and the stable areas of Central Asia (East Kazakh) are underlain by high Q and again in the western United States there is evidence of low Q (-75). Baranzangi & Isacks (1971) have presented evidence of very low Q (-40) in the island arc regions of Fiji and Tonga. Molnar & Oliver (1969) looked at the distribution of Qs (the elastic quality factor for shear waves) and find that in the uppermost crust and mantle Q, is low in active areas and high in stable shield areas.

We also suggest that a low Q zone may be present beneath old fold belts that are now almost inactive seismically. If low Q layers at depths of several hundred kilometres beneath ancient and inactive fold belts is confirmed this suggests that the crust and upper mantle down to these depths have not moved relative to each other since Palaeozic times a suggestion in conflict with the current hypothesis of rigid (high Q) plates moving over a less rigid (low Q) substratum. We postulate instead a high Q upper mantle in which there are regions of low Q associated with fold belts. Some of the original evidence for plates comes from observations of differences in attenuation of P and S waves in active island arc regions which is interpreted as evidence of high Q plates dipping down into a low Q upper mantle. These observations may be equally well explained by low Q layers within a relatively high Q mantle.

7. ImpIications for identifying earthquake and explosion signals

Our original interest in complexity arose because it seemed to offer an easy way of identifying earthquake signals. The absorption explanation of complexity makes it possible in retrospect to see why the method once appeared to be so good for identifying earthquakes. The first studies of complexity made by UKAEA staff used data from an UKAEA array at Pole Mountain, Wyoming (PMW). The site of this array is near the boundary between the stable (high Q) shield areas of the central United States and the active (low Q ) earthquake areas of the western United States. The first signals recorded at PMW were from the central Asian firing site of East Kazakh (high Q) and the Sahara firing site. The latter is also in a stable area and hence probably high Q. Not surprisingly then explosions from these test sites recorded at PMW are simple. Most of the earthquakes recorded by PMW were from earthquake areas in the north and western Pacific which arrived at the recording station having passed under the active mountain region of the western United States. How much the low Q layers in western United States contributed to the complexity of these earthquake signals and how much is contributed by low Q near the earthquake source is not clear at this stage but obviously there is a strong possibility that PMW was ideally sited to record simple explosion signals and complex earthquake signals and indeed this is what was found. This array station alone appeared to be able to identify 90 per cent of all earthquakes recorded from distances of 30"-90".

Another factor that may have made the complexity method appear so good for identifying earthquakes is that the first firings of underground explosions appear to have been of relatively low yield so that the only stations to observe signals from these explosions would be those that lay on high Q paths from the test site and such stations would obviously record simple signals. Any complex signals from the explosions would have magnitudes below the detection threshold of most stations and so would go unobserved. Thus it appears that these early observations on the differences in the complexity of earthquake and explosion signals were measuring principally differences in the upper mantle structure of the explosion firing sites and


earthquake source areas and not differences between earthquake and explosion sources.

The differences between the upper mantle structure of the explosion firing site and earthquake source areas also seems to be the basis of several other identification criteria or at least is the reason why earthquake and explosion populations are well separated. Part of the reason why mb for explosions is large compared to mb for earthquakes at a given Ms (the so called m, : Ms criterion for identifying earthquakes and exp!osions) is probably because much of the data for explosions is from firing sites in stable shield areas which are high Q 2reas. Ward & Toksoz (1971) have reached similar conclusions. The spectral ratio method of identifying explosions by the greater high frequency content of their P signals compared to earthquakes (Green 1968) probably works best for explosions in Central Asia compared to other test sites for the same reason. Explosions in Central Asia radiate high frequency signals because the region is one of high Q; earthquakes and explosions in low Q areas may contain high frequency energy at source but this is absorbed during propagation. This criterion is yet another expression of the observation that many explosions from stable shield areas are simple whereas many earthquake P signals are complex.

The measurement of complexity could probably be redefined to make the criteria more reliable for identifying earthquakes. Measuring complexity on the low frequency portion of the signal would reduce the effects of the scattered (high Q) arrivals and so any depth phases in the coda would be the principle contributions to complexity. Identifying earthquakes by complexity would then be equivalent to identification by depth of focus.

The best method of identifying explosions and earthquakes thus still seems to be the mb : M , method for even when applied to explosions in low Q areas (NTS) the method seems to work although the separation between earthquake and explosion populations for such low Q areas is small. However there are some events both explosions and earthquakes which although the P signals are detected the surface waves are not seen and the event then cannot be identified on the mb : M , criteria. Such events seem likely to be those which are truly small (as measured by M,) but by chance the paths for P signals to the few detecting stations are paths of high Q; mb is thus anomalously high. Consequently these signals will usually be simple and may be difficult to identify from their P waves unless a surface reflection is visible.

Ho~vever, if the P signals have followed high Q paths deconvolution of the signals should be relatively easy so that P and p P should be clearly seen after deconvolution if the event is an explosion. Deconvolution of earthquslke signals recorded on high Q paths may reveal the source function and if this is sufficiently different from an explosion source function this will provide a method of identification.

Acknowledgments

of signals around a low Q obstacle. We thank Mr J. H. Woodhouse for permission to use his results on the diffraction

UKAEA, Blacknest, Brimpton,

Reading RG7 4RS, Berkshire.

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Documents

P Signal Complexity Re-examined