Bulletin of the Seismological Society of America, Vol. 83, No. 4, pp. 1155-1166, August 1993

THE DISTANCE DEPENDENCE OF REGIONAL PHASE DISCRIMINANTS

BY B. L. N. KENNETT

ABSTRACT

Amplitude ratios of P and S phases recorded at regional distances have been suggested as potential discriminants for the character of seismic sources, because of the differences expected in the radiation pattern of earthquakes and well-contained explosions. The most useful reference phase for P waves beyond 200 km from the source is Pn and for S possible choices are the mantle phase Sn and the crustally guided wave Lg. Each of these phases has a different interaction with the seismic structure of the crust and mantle and such structural effects will impose their own patterns on the radiation charac- teristics from the source. The range dependent component in the Lg/Pn and Sn/Pn ratios have been investigated for a dense set of three-component records covering the distance range to 700 km, for an explosive shot in southern Sweden that formed part of the 1979 Fennolora experiment. A stable measure of amplitudes and amplitude ratios is provided by using the vector resultant of ground motion (the square root of the total energy). However, even for a single source there are significant variations in the amplitude ratios for Lg/Pn and Sn/Pn of a factor of 3 or more as a function of range.

INTRODUCTION

The character of a seismic source imposes a distinctive radiation pat tern on the seismic wavefield tha t can then be modified by propagation phenomena such as conversion at the free surface or at major internal boundaries. A well-contained explosive source is expected to generate P waves directly and the accompanying S-wave radiation observed at distance has to be produced by secondary action such as free-surface conversion. Whereas an earthquake source has an intrinsic shear component and should produce larger S than P-wave energy. Measures of the relative amplitude of P and S waves are therefore attractive for discrimination between explosive and earthquake sources; and at regional distances are often implemented by using the ratios of the amplitudes of different P and S phases. The P and S phases tha t are being compared should have similar propagation characteristics, so tha t amplitude differences can be ascribed to the variation in the radiation at the source. The phases Pn and Sn are expected to have similar propagation paths in the uppermost mantle but will sample only a relatively narrow angular range of radiation from the source. Because Pn is the earliest arrival in the distance zone from 200 to 1000 km it can normally be readily distinguished from background noise, but Sn has to be sought against the background of the P coda and its amplitude can be difficult to measure. The largest S phase to 1000 km is usually the superposi- tion of crustally guided S waves denoted by Lg in discrimination work. How- ever Lg is known to be sensitive to variations in crustal structure and so is most effective as a reference for S waves when the propagation paths from different sources are very similar.

Amplitude ratios for the P and S phases are often computed on a component by component basis but this procedure is susceptible to the influence of lateral

1155

1156 B. L, N. KENNETT

heterogeneity in propagation. A more stable procedure is to use the vector amplitude of seismic displacement or its projection into a horizontal or vertical plane. The behaviour of such P/S amplitude ratios with distance from the source is investigated using record sections of three-component digital records from the Fennolora long range refraction profile in Sweden for an explosive source in shallow water.

VECTORIAL P / S AMPLITUDE RATIOS FOR REGIONAL PHASES

The Fennolora Project in Sweden in the summer of 1979 (see e.g., Stangl 1990) was a long-range refraction profile with three-component recording so that it is possible to study both the P and S components of the wavefield. In this study we use data from shot point B to the south of Sweden which has also been used by Kennett (1993) to look at the propagation processes contributing to the character of the recorded regional phases. In Figure 1, we display a record section for the radial component from 51 three-component sets of traces for the shot B3N at Fennolora shotpoint B, for distance ranges to 675 km from the source. The traces have been filtered using a four-pole Butterworth filter with - 3 db points at 1 and 12 Hz. The trace amplitudes have been corrected for the scaling factors applied during the digitization of the original analogue records and so true amplitude relations are available between components. In the plotting a linear scale factor has been applied in range to emphasize the arrivals at greater ranges. The major phase groups (Pn, Sn, and Lg) used in the subsequent analysis are indicated in Figure 1.

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I . . . . . . . I I

FIG. l . Radia l component record sect ion for s e i s m o m e t e r s across the southern part of Sweden , extracted from three -component data for the shots B 3 N at Fennolora shotpo int B. The s e i s m o g r a m s are plotted wi th true ampl i tudes and a l inear d i sp lay ga in in range.

DISTANCE D E P E N D E N C E OF REGIONAL PHASE DISCRIMINANTS 1 1 5 7

The data quality is generally very good but there are occasional noisy or distorted traces tha t have not been removed from the data set.

Ampli tude Distribution with Distance

For each of the main regional seismic phases we have extracted an amplitude distribution as a function of range by extracting the maximum amplitude found for a particular component or vector amplitude within a time window around the expected arrival time for the phase. The windows are 2 sec long for P waves and 4 sec long for S waves.

One representation of the vectorial pat tern of ground motion is via the three orthogonal components vertical (Z), radial (R), and tangential (T) tha t offer a convenient theoretical interpretat ion for stratified media with the separation of S H waves on the tangential component, and P and S V waves in a vertical plane defined by the vertical and radial components. This formulation is used as the basis of the approximate method of free-surface removal used by Kennett (1993) for the same Fennolora data, which is very effective in the display and analysis of the waveforms of the various phases.

However, a simpler alternative approach is to work directly with the vectorial character of the ground motion and to use vector resul tants as a measure of the amplitude of the wavefield. We have used three different measures of the amplitude: the total vector resul tant of motion,

t = [ Z 2 + R 2 + T 2 ] 1/2,

the vector amplitude in the vertical plane,

v = [ Z 2 + R 2 ] 1/2,

and the vector amplitude projected into the horizontal plane

h = [ R 2 + T 2 ] 1/2.

These three quantities can be easily constructed from the cartesian components of motion. Both the amplitude measures t and h are not dependent on the rotation of the horizontal components. For the Fennolora data the horizontal component seismometers were deployed in the radial and tangential directions but in general the azimuth to the source is not known so tha t there is an advantage in amplitude measures tha t does not require knowledge of azimuth. A three-component energy measure (t 2) was used by Booker and Mitronovas (1964) in a study of amplitude ratios for discriminating between NTS explosions and earthquakes as observed using short-period Benioff ins t ruments of the LRSM network.

Pn. The amplitude distributions for the Pn arrivals are displayed in Figure 2 as a function of range. In the upper panel we show the amplitudes from the three cartesian components and in the lower panel the vector amplitude mea- sures introduced above. For distances less than the crossover near 200 km there may be some influence from Pg (in the form of the reflection ProP from the crust-mantle boundary), but for larger ranges Pn is well separated from the other phases. The trends in the amplitude behavior for Pn can be discerned in

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I I I I I I I I i [ I 200. 300. 400, 500. 600. 700.

Range [km]

FIG. 2. The dependence of the max imum ampli tude of the Pn phase as a function of range. Pn is separated from Pg beyond 200 kin. The upper panel shows the car tes ian components of motion and the lower panel shows a set of vector ampli tude measures (t-total amplitude, v-amplitude in vertical plane, and h-ampl i tude in horizontal plane).

D I S T A N C E D E P E N D E N C E OF R E G I O N A L P H A S E D I S C R I M I N A N T S 1159

the cartesian components but are clearer in the vector representation. As would be expected for an explosive source in water most of the energy lies in the vertical plane but there is significant amplitude in the tangential component. The amplitude of Pn tends to decline with distance but there are some signifi- cant local increases tha t are likely to arise from changes in the near surface structure. There are also broader scale amplitude variations reflecting the complex substructure of the Pn phase arising from the influence of velocity variations in the lithosphere (see e.g., Fuchs and Schultz 1976).

Sn. As can be seen from Figure 1, the emergence of the Sn phase from the Lg phase is less distinct than for the emergence of Pn and as a result the Lg energy dominates to 300 km. As noted by Gajewski et al. (1990) such behavior is common on many refraction profiles, but profiles in Precambrian areas (see e.g. Grad and Luosto, 1987) can show strong Sn at the crossover with Sg. Beyond 300 km the amplitude patterns derived from the Fennolora data shown in Figure 3 do reflect the Sn behavior. There is a significant contribution from horizontal motion-- indeed Kennet t (1993) noted tha t a picking criterion for Sn could be based on an increase in energy in the horizontal plane. The vector measures of amplitude are again less scattered and show more systematic trends. There are indications of structure within the amplitude distribution but these are more muted than for Pn. The Sn amplitudes are generally larger on the transverse component than on the radial despite the fact tha t the source was an explosion in water. This arises, in part at least, from the fact tha t SH waves are amplified by a factor of 2 at the free surface whereas the amplifica- tion due to the interaction of the incident and reflected waves is less than two for SV because of interaction with the P wavefield.

Lg. We have already noted tha t the large amplitude Lg arrivals tend to swamp the smaller Sn phase to 300 km. The phase is built up by a complex interference of multiple S reflections within the crust and as a result the maximum amplitude tends to shift relative to the asymptotic group velocity of close to 3.5 k m / s e c as different contributions become important. The amplitude measures (both cartesian and vectorial) in Figure 4 show a slow decline with distance, interrupted by some noise spikes, which could be modeled via an empirical a t tenuat ion function. The maximum amplitudes encountered at differ- ent distances can be associated with differing propagation processes and may not be suitable for direct comparison. However, when using a single seismogram the large amplitude Lg phase offers a convenient measure of the S field. We note tha t although the source is an explosion in water, the transverse compo- nent of Lg is generally larger than the radial, similar behavior has been noted by Kennet t and Mykkeltveit (1984) for other explosions in the Scandinavian area.

Amplitude ratios as a function of range

The amplitude distributions we have jus t discussed describe the behavior of seismic phases whose propagation characteristics are quite complex and tha t display significant substructure. The influence of the differing trends in the amplitude behavior becomes more pronounced when we take amplitude ratios between Sn and Lg and the reference Pn phase. We recall tha t we have a common explosive source in water for all the traces we have considered, so tha t the behavior of the amplitude ratios with range reflects the influence of struc- ture in the crust and upper mantle.

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, I , I , I , I i I , 200. 300. 400. 500. 600. 700.

R a n g e [ k m ]

FIG. 3. The dependence of the max imum ampli tude of the Sn phase as a function of range. Sn is not well separated from Lg unt i l 300 km. The zone in which Lg contaminat ion is likely is indicated by shading. The configuration of the display follows figure 2.

DISTANCE D E P E N D E N C E OF REGIONAL PHASE DISCRIMINANTS 1161

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FIG. 4. The dependence of the maximum amplitude of the Lg phase as a function of range• The configuration of the display follows Figure 2.

1162 B . L . N . KENNETT

S n / P n ratio. We have used both the cartesian components and the vector amplitude measures to examine the distance dependence of the Sn/Pn ratio. As we have noted above the Sn amplitudes are contaminated by Lg for distances less than 300 km, but the time separation is sufficient for distinct Sn beyond 300 km. In the upper panel in Figure 5 we display a component by component comparison of Sn/Pn amplitude ratios. There is some consistency in the behavior for the vertical and radial component ratios, but the transverse components are not very useful because of the low amplitude of the tangential component for Pn. Even if the vertical component amplitudes for Pn are used as common reference for all the components of motion there is still a large scatter in the behavior of the tangential component for Sn. However, if we make use of the vector measures of amplitude (shown in the lower panel of Fig. 5) the different amplitude ratios are much more consistent and a reasonably clear pat tern emerges for both the total vector amplitude (t) and the vector amplitude in the horizontal plane (h). The ratios of the total vector amplitude between Pn and Sn vary by a factor of 3 between 300 and 700 km as a result of the differences in the nature of the P and S structures in the upper mantle and the propagation paths for the particular phases.

L g / P n ratio. The amplitude ratios for the cartesian components are again quite chaotic (particularly for the tangential component) as can be seen in the upper panel in Figure 6. As for the Sn/Pn case part of the confusion arise from the small tangential component for Pn. The use of a constant reference to the vertical component of Pn generally reduces the scatter but is not as useful as working directly with the ratios of the vector amplitude measures displayed in the lower panel of Figure 6. A more consistent set of features can be found in this display but there are variations of at least a factor of 4 in the amplitude ratio for the same source over the distance range from 150 to 700 km. This arises because of the complex interplay between the influence of structure in the upper mantle on the Pn phase and the changing dominance of different orders of S wave crustal multiples for Lg.

DISCUSSION

The amplitude ratios presented in this paper are derived from a single source recorded along a profile where lateral variations in structure are modest (Stangl, 1990). Nevertheless the ratios tha t are intended to provide a measure of the relative significance of P and S show variations of a factor of 3 or more as a function of range. Although there are variations in the S phases much of the structure in the amplitude ratios with range comes from the way in which the reference phase Pn is built up from an en echelon set of subphases re turned from the upper mantle (Kind, 1974).

Amplitude ratios for P and S phases have a useful role to play as one class of discriminant in a decision making process based on the consensus of a wide range of different measures of source character, but we have to be cognizant of the likely influence of ear th structure on their behavior. Blandford (1980) has advocated the use of the ratio of the maximum amplitude for Pn and Lg on the vertical component. Our results suggests tha t a more stable amplitude ratio would be obtained by comparing vector amplitudes. The variation in the Lg/Pn ratios induced by structural effects may help to explain the considerable scatter in the Lg/Pn ratios observed by Mykkeltveit and Husebye (1980) for earth- quakes and explosions in Central Asia recorded in Norway. Lg is quite sensitive

DISTANCE D E P E N D E N C E OF REGIONAL PHASE DISCRIMINANTS 1163

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Range [km] FIG. 5. The dependence of the amplitude ratio of Sn to Pn as a function of range. The zone in

which Lg contamination is likely is indicated by shading. The upperpanel presents the ratios of the cartesian components of motion and the lower panel the ratios of the vector amplitude measures.

1164 B . L . N . KENNETT

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500. 600. 700.

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200. 300. 400. 500. 600. 700.

Range [ k r n ]

FIG. 6. The dependence of the amplitude ratio of Lg to Pn as a function of range. The upper panel presents the ratios of the cartesian components of motion and the lower panel presents the ratios of the vector amplitude measures.

DISTANCE DEPENDENCE OF REGIONAL PHASE DISCRIMINANTS 1165

to crustal s tructure (see e.g. Bostock and Kennett , 1990, and references therein), and this will tend to enhance the structural component in the Lg//Pn ratios for paths crossing major mountain chains or deep sedimentary basins. The Lg//Pn ratio can be a potential discriminant for explosions and ear thquakes in close proximity, observed at a common set of stations, but cannot be used indiscrimi- nately. Sn is less sensitive to heterogeneity within the crust, and for distances beyond 300 km the Sn//Pn ratio is likely to be a useful adjunct to other measures of source character.

The greater stability of the vectorial est imates of phase amplitude and consequently the P//S ratios is likely to arise because of the influence of three-dimensional s tructure during the propagation process. Bostock and Ken- net t (1990) have made a ray-based s tudy of the effect of three-dimensional crustal s tructure on Lg waves and shown hov~ the polarization of S waves will tend to tilt away from the initial vertical or horizontal planes as the wavefield encounters horizontal velocity gradients. As a result the recorded wavefield is likely to be best represented by S-wave groups whose polarization are not the simple SV and SH forms expected for a stratified medium. There will be similar tilting effects for the polarization of P waves that will no longer lie purely in a vertical plane. Under a rotation of the plane of polarization the amplitude of the vector disturbance associated with each wave type will be invariant and vector ampli tudes can be expected to have enhanced stability compared with amplitudes of individual components.

The total vector ampli tude (t) is readily constructed from any three-compo- nent record and does not require knowledge of the azimuth to the source (no prior rotation to radial and tangential components is required). The projection of the vector amplitude on a horizontal plane (h) similarly jus t depends on the vector resul tant of the two horizontal components. However, the vector ampli- tude in the vertical plane (v) we have used does require the radial component and vertical components of motion and so needs rotation of horizontal compo- nents.

An alternative procedure that could be used to reduce the scatter in the amplitude ratio results for this explosive source would be to average the results for the vertical (Z) and radial (R) components. This procedure would ignore the presence of significant S-wave energy on the tangential component, which although not predicted by simple propagation models is commonly observed for explosive sources at regional distances (Kennett and Mykkeltveit, 1984).

ACKNOWLEDGMENTS

I would like to thank Professor K. Fuchs and the staff of the Geophysical Institute, University of Karlsruhe for the provision of the digital data from the Fennolora shotpoint B.

This work was supported in part by the Advanced Research Projects Agency of the US Depart- ment of Defense under Grant AFOSR-90-0131.

REFERENCES

Blandford, R. R. (1980). Seismic discrimination problems at regional distances, in Identification of Seismic Sources--Earthquake or Underground Explosion, (Husebye, E. S. and Mykkeltveit, S., Editors), Riedel, Dordrecht, 695-740.

Booker A. and W. Mitronovas (1964). An application of statistical discrimination to classify seismic events, Bull. Seism. Soc. Am., 54, 961-971.

Bostock, M. G. and B. L. N. Kennett (1990). The effect of 3-D structure on Lg propagation patterns, Geophys. J. Int. 101, 355-365.

1166 B .L .N . KENNETT

Campillo, M., M. Bouchon, and B. Massinon (1984). Theoretical study of the excitation, spectral characteristics and geometrical attenuation of regional seismic phases, Bull. Seism. Soc. Am. 74, 79-90.

Fuchs, K. and K. Schulz (1976). Tunneling of low-frequency waves through the subcrustal litho- sphere, J. Geophys. 42, 175-190.

Gajewski, D., R. Stangl, K. Fuchs, and K. J. Sandmeier (1990). A new constraint on the composition of the topmost continental mantle--anomalously different depth increases of P and S velocity, Geophys. J. Int. 103, 497 507.

Grad, M. and U. Luosto (1987). Seismic models of the crust of the Baltic shield along the SVEKA profile in Finland, Annales Geophysicae 5B, 639-650.

Kennett, B. L. N. (1989). On the nature of regional seismic phases--I. Phase representations for Pn, Pg, Sn, Lg, Geophys. J. R. Astr. Soc. 98, 447-456.

Kennett, B. L. N. (1993). On the nature of regional seismic phases II. 3-component studies of crustal and mantle phases, Geophys J. Int. submitted.

Kennett, B. L. N. and S. Mykkeltveit (1984). Guided waves in laterally varying media, II: Lg-waves in north-western Europe, Geophys. J. R. Astr. Soc. 79, 257-267.

Kind, R. (1974). Long range propagation of seismic energy in the lower lithosphere, J. Geophys. 40, 189-202.

Mykkeltveit S. and E. S. Husebye (1980). Lg wave propagation in Eurasia, in Identification of Seismic Sources--Earthquake or Underground Explosion, Husebye, E. S. and Mykkeltveit, S., (Editors), Riedel, Dordrecht. 421-451.

Pomeroy, P. W., W. J. Best, and T. J. McEvilly (1982). Test ban treaty verification with regional data - - a review, Bull. Seism. Am. 72, $89-S129.

Stangl, R. (1990). Die Struktur der Lithosphere in Schweden, abgeleitet aus einer gemeinsamen Interpretation der P- and S-Wellen Registrierungen auf dem FENNOLORA-Profil, Doktor der Naturwissenschaften Thesis, University of Karlsruhe.