Time Domain Regional Discriminants

AD-A249 949 1I1',

PL-TR-91-2278

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,

Time Domain Regional Discriminants

L. J. Burdick, E. J. Garnero, C. K. Saikia and D. V. Helmberger

Woodvard-Clyde Consultants 566 El Dorado Street. Suite 100 Pasadena, CA 91101

• MAR 10392" 1 November 1991

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Time Domain Regional Discriminants

PE 62101F

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PR 7600 TA 09 WU AZ

L.J.Burdick E.J. Garnero

C.K.Saikia D.V. Helmberger

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13. ABSTRACT (Maximum 200 words)

Most regional discriminants developed to date have been based purely on empirics, and many rely on the characteristics of spectra. At Woodward Clyde, a unique type of time domain discriminant has been developed based on a theoretical understanding of wave propagation and the direct identification of depth phases in P. and Pg. This work has involved original basic research into the nature of the short period Pn waveform and the effect of pPn on it; development of an understanding of the nature of Pn propagation; and inquiries into the relation of pPn and spall. The studies of Pg have involved identification of a frequency band in which the waveform is stable; a forward modeling study of NTS regional data; and a careful breakdown of the wavenumber integration synthetics into the generalized rays that compose them. This work has led to direct identification of depth phase effects in Pg and a preliminary indication of how to detect and discriminate with them. This is the first report on a multi-year effort to complete development of the P discriminants, to extend the technology to Sn and Sg and to test the power of the discriminants on the new data bases from the Soviet Union. Strong efforts to analyze the new data from the Soviet Union have been undertaken including studies of both the older hand digitized analog records and of the digital data from the new IRIS stations. 14. SUBJECT TERMS

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R(e(lional I)iscrimirant,;, P

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TABLE OF CONTENTS

Introduction Recently Developed Waveform Discriminants U pper Mantle Triplications Beneath Asia

26

C(omposition of Regional Pn and Sn Waveforms Recorded in the U.S. and U.S.S.R.

51

References

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INTRODUCTION The time and frequency domains are equivalent displays o. seismic traca, information, though some qualities of the signal are more ea3ily observed in one domain than the other.

The relative

frequency excitation of Lg, for instance, is most easily ,,iewed in the frequency domain, but such waveform qualities as the sequence in which pulses arrive in the wave train or the sharpness of pulse onset are most easily studied in the time domain (vlurphy 1981).

and Bennett, 1982, Blandford,

Because of the tremendous complexity of high frequeijcy regional data, most attempts at

using it for discrimination purposes have involved analysis of the frequency content of the various arrivals either through transforming selected windows or through multiple bandpass filtering.

We

report here on our initial attempts to explore the alternative and to discriminate events using those waveform characteristics most easily observed in the time domain. A second advantage of time domain analysis approaches is that they permit a deeper insight into the physical processes creating a seismic signal's character.

For this reason, they can be more

e3silv used to evaluate the transportabilty of a discriminant to varying geophysical and tectonic regimes. This is an especially important feature in the development of regional discriminants. The most prominent and successful spectral regional discriminants have been empirically developed. This means that they must be redeveloped and reverified ir. e,.ch new area.

As we shall show in

the following, through rigorous time domain analysis such feLtures as regional depth phases can be identified and used to discriminate.

Discriminants based on such simple physical features as

source depth should be transportable anywhere. In work recently completed under the treaty verification program, we have proved that such time domain discriminants do exist.

In analyzing a test discrimination data set from the western

U. S., we have discovered that the onset of Pn is always very similar for explosions and that few earthquakes have this unique waveform character.

This inforin.,tion can be constructed into a

simple discrimination scheme by testing the correlation of observed P. waveform onsets with average waveforms observed from explosions. correlations earthquakes.

High correlations indicate explosions and low

We have also discovered that the regional phase Pg is actually composed

I

of a sequence of sub-arrivals which corresponrd to successively higher orders of reverberation in the crust. In realistic crust models, the depth phases play an important rol" in the w'Aveshapes of

these sub-arrivals. By selecting an appropriate frequency band to analyze, we have been able to accurately model this type of data from explosions in the western United States.

O'er the very

relevant regional distance ranges of 200 to 600 kni, it appears that a discrimination procedure very similar to the one which is known to work for Pn will also be effective for Pg. We are investigating whether similar discriminants can be constructed based on the phases S, and Sg in areas where those phases are prominent arrivals.

RECENTLY DEVELOPED WYAV-EFORM DISCR.IMILNANTS Though the technology for recording broad band seismic data digitally has existed for some time, a good regional net of stations surrounding NTS has only t-een put into place recentlv (Figure I). Several different types of stations and seismometers are i•n the net including DNWWSSN (JAS, AI.Q or ANN40), LLNL broad bands (ELK, MNV, LAC, KNMl by Caltech (PAS) and UCSD (PFO).

and the new Streckeisens installed

Much recent effort at Woodward Clyde has been directed at

developing short period regional discriminants that work on daita from this net.

The situation of

having data available from many sources ,,eiv close to each other which are similar in character .tuch as NIS explosions is a unique one.

l'he timing and locations of the events are known exactly.

This turns any single regional station in the western U. S. into the equivalent of a regional seismic array.

Any of the standard array processing techniques can be used with the role of sources and

stations being reversed. There is some variability in source time history and near source structure, but on the other hand the receiver structure is constant.

We betan our previous work by simply

obtaining suites of records from each of the stations in the net and summing to form stable averages. In so doing, we observed important features in the average Pn and the average Pg waveforms which could each be used to develop time domain discriminants.

Our success to date is summarized

below. The

n.iWaveform Discriminant Our initial success in developing a Pn waveform discriminant was

rooted in past experience with modeling waveforms of teleseismic P waves from nuclear explosions. Figure 2 is taken from Burdick et al. (1984).

It shows observed and synthetic, long and short

period records from the nuclear test, CANNIK IN. Arrows on the left of the figure draw attention to a subtle feature in the short period records which is associated with the arrival of the phase pP. The records where the feature appears are assumed to be along high Q paths ýt* = 0.8 s).

Along

Io% !r Q paths the feature washes out. The long periods are not affected by pP in a clearly visible way.

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Comparison of observed short and long period 1' •a~es with synthetic, seismograms for a range of t° values at teleseismic distances. As t° increases, the interference in the second upswing produced b v pP becomes less apparent.

producedi in the synthetics w-ith an

pP can be observed

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This previous experience with the effects of pP on teleseismic short period P waves provt-d valuable in interpreting regional P,, waxeftrnrs. As it happens, some of the stations in the western U.S. digital net have a band-limited response equivalent to the WWSSN stations in Figure 2. To make a uniform comparison, all of the records its the data base were transformed to short perioc W\VSSN instrument response. When the digital signals from NIS explosions were averaged, it was observed that the average Pn waveform was very s!milar to those on the left of 1-igure 2 although much shorter in period. 3

Average wxaveforms at 5 stations in the digital net are shov. n in Figure

The consistent splitting of th, second upswing is indicated by the arrows. *rhe clear Implication

is that the physics oi the vave piopagation of short period teleseismic P and short period P, is \,er\ comparable. More precisely, the interaction of pP with P must be similar in the two instances. Ihis is not unreasonable in that the apparent ýelocity of Pn is about 8 kms and that ot teleseismic P only increases to about 12 km/s at 300 small.

The associated change in pP timing and amplitude is

The implications for regional discrimination are clear.

Only very shallow sources like

explosions will have depth phases at very short times. Farrhqunke depth phases will be much !ater. To test the performance of this discriminant, we assembied a set of Pn waveforms from small earthquakes near NIS, windowed out the first three seconds of P,, and measured the correlation with the average Pr, waveforms of explosions like those shown in Figu.e 3 was carried our on the explosion data base.

A similar procedure

rhe results from one typical station, JAS are shown

in Figure 4. The explosions are displaed as stars and the earthquakes as crosses. The ieparatiwn Of th-

,'- :!".1' ons is good en,'ugh to warrant more stud, of this

discritiiant.

i he d;scrimination capabilit\ illustrated in Figure 4 only demonstrates that the P, waseform of explosions is stable at JAS and consistently different from earthquake waveforms. the •imilar*t.

However,

of the wa,,cforms in Figure 3 suggests that the shape of the explosion waveform is

consistent from station to station.

That this is indeed the case is shown in Figure 5 where the

average JAS explosion waveform has been correlated with the explosion data base at MNV.

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