MULTIPLE ScS ATTENUATION AND TRAVEL TIMES BENEATH WESTERN NORTH AMERICA BY THORNE LAY AND TERRY C. WALLACE ABSTRACT

Transcription

1 Bulletin of the Seismological Society of America, Vol. 78, No. 6, pp , December 1988 MULTIPLE ScS ATTENUATION AND TRAVEL TIMES BENEATH WESTERN NORTH AMERICA BY THORNE LAY AND TERRY C. WALLACE ABSTRACT Variations in attenuation and travel time of multiple ScS whole-mantle reverberations indicate very strong lateral gradients in upper-mantle properties beneath the tectonically active province of western North America. Whole-mantle attenuation estimates (Qscs) obtained by spectral stacking of phases sampling discrete subregions span the entire range previously determined using a similar procedure for a global set of continental and oceanic Qscs measurements. The relationship between travel time and attenuation anomalies, which has been used to argue for predominantly thermal control on the variations in oceanic environments, is poorly defined within the continental regime. However, a correlation between surface heat flow and Qscs variations in the United States indicates that thermal variations do control attenuation heterogeneity. For paths beneath Mexico, stacking 13 pairs of ScS, phases from the 24 October 1980 Huajuapan, Mexico and the 19 June 1982 El Salvador events yields Qscs _+ 10 in the frequency band 7.8 to 46.9 mhz. Time-domain measurements for the same phases yield a similar estimate of Qs~s = The average SCSn+l -- ScS n JB (Jeffreys-Bullen) travel-time residual (htscs) is +0.9 sec (slower than JB). All of the differential travel-time estimates are corrected for dispersion effects in the passband of the observations, and are appropriate for a reference period of about 30 sec. Multiple ScS arrivals from the 29 April 1965 Puget Sound earthquake recorded by WWSSN, CSN, LRSM and Pasadena stations provide the first determination of whole-mantle attenuation beneath the westernmost United States. Pasadena recordings from three instruments provide ScS, paths traversing the Cascade and Sierra Nevada ranges. Stacking 9 ScS, pairs yields Qscs = (7.8 to 54.7 mhz) and the same phases give htscs = +2.5 sec. Six pairs of ScS, reverberations under the Pacific Northwest give Qs s = (15.6 to 54.7 mhz) and Atscs = +5.5 sec. A combined estimate for the relatively low heat flow region of the Pacific borderlands gives Qscs = (7.8 to 46.9 mhz), comparable to the high Qscs estimates previously determined for South America. On the other hand, 12 SCSn pairs sampling the northern Basin and Range, which has high heat flow, yield a very low Qs~s = 95 -_. 4 (7.8 to 31.3 mhz) with htscs = +5.1 sec. The latter Qscs value is even lower than estimates for young oceanic regions, suggesting extensive partial melting in the low-velocity zone under the northern Basin and Range. INTRODUCTION Lateral variations in upper-mantle properties underlying the North American continent have been intensely studied for several decades, yet major unresolved questions still persist. These concern the spatial variation and frequency dependence of anelastic properties, the depth extent of both elastic and anelastic lateral heterogeneity, and the physical mechanisms responsible for the heterogeneity. It is well established that the principal upper-mantle variations are associated with the surface tectonic history, with there being a strong contrast between the stable Canadian Shield and the surrounding Phanerozoic fold belts. Shear-wave traveltime anomalies provide particularly clear evidence that the Rocky Mountain front and Appalachian Highlands overlie abrupt lateral transitions in upper-mantle 2041

2 2042 THORNE LAY AND TERRY C. WALLACE velocity structure, with higher velocities being located under the shield and stable platform (Doyle and Hales, 1967; Wickens and Buchbinder, 1980; Lay and Helmberger, 1983; Grand and Helmberger, 1984; Grand, 1987). Similarly, seismic amplitudes and attenuation estimates define first-order variations in anelastic properties between the high-q stable platform and the surrounding low-q regions with active Phanerozoic orogenic activity (Der et al., 1975, 1980, 1982; Lay and Helmberger, 1981; Butler, 1984). Yet it is also clear that the upper-mantle variations are more complex than a simple bimodal shield/tectonic distribution. For example, the Basin and Range province has particularly strong attenuation and low shear velocities, even relative to other western regions (Solomon and Toksbz, 1970; Der et al., 1982; Lay and Helmberger, 1983). The North American observations are usually attributed to coupled lateral variations in the upper-mantle low-velocity and low-q zones (Hales et al., 1968; Solomon, 1972; Hales and Herrin, 1972; Helmberger et al., 1985). However, the relationship between travel-time and attenuation anomalies is a very complex one (Lay and Helmberger, 1983; Butler, 1984), so quantification of the underlying physical mechanisms has proceeded slowly. An example of this complexity is provided by the Pacific Borderlands province, where, relative to the adjacent Basin and Range region, short-period P-wave and S-wave amplitude anomalies are not as low (Lay and Helmberger, 1983; Butler, 1984), S-wave travel-time anomalies are similar (Lay, 1983), long-period Q estimates are higher (Solomon and Toks6z, 1970), and short-period Q estimates are comparable (Deret al., 1975). Taken at face value, these observations require complex lateral and frequency-dependent variations of upper-mantle properties. However, improved constraints on the seismically inferred variations are needed before much significance can be attached to interpretations of mantle temperature or compositional variations (Solomon, 1972; Butler, 1984). In this paper, we will explore the lateral variations of upper-mantle properties in western North America using the well-established procedure of analyzing multiple ScS (ScSn) reverberations (Kovach and Anderson, 1964; Sato and Espinosa, 1967; Yoshida and Tsujiura, 1975; Sipkin and Jordan, 1980a,b). Differential measurements of these phases are insensitive to source complexity and sample localized upper-mantle regions, allowing lateral variations to be mapped out. Many previous studies of ScS~ phases have established that there are systematic differences in upper-mantle attenuation and shear velocity structure between stable continental regions and old ocean basins as well as variations with lithospheric age in oceanic environments. Continental regimes, however, have not been extensively sampled, and we lack a thorough understanding of variations within tectonically active regions as well as within stable platforms and shields. We will present whole-mantle attenuation and travel-time estimates for several discrete regions of the tectonic province of North America, including the first such determinations for the western United States, and then appraise the significance of these results in both the continental and global context. DATA AND ANALYSIS PROCEDURE The primary reason for the poor sampling of continental regions with ScSn reverberations is the paucity of suitable source-receiver combinations. The highest quality ScSn phases are generally produced by intermediate or deep focus subduction zone earthquakes, because the relevant time interval for observing these phases is obscured by the short-period surface waves that are strongly excited by shallower

3 MULTIPLE SCS ATTENUATION AND TRAVEL TIMES 2043 sources. It is also important to have favorable focal mechanisms, with one steeply dipping P-wave nodal plane, as well as a good distribution of calibrated stations. As a consequence of these requirements, most continental paths with reliable wholemantle attenuation, Qscs, estimates have been located in South America or southeastern Eurasia (Sipkin and Jordan, 1980b; Chan and Der, 1988), while wholemantle travel-time anomalies for ScS,+I - ScSn (Atscs) are available for only a few additional continental regions (Okal and Anderson, 1975; Sipkin and Jordan, 1976, 1980a). Lay and Wallace (1983; hereinafter Paper 1) provided additional Qscs and ~tscs measurements for continental paths beneath Mexico and Central America by utilizing ultra-long-period Berkeley and Pasadena recordings along with GDSN data from the 19 June 1982 E1 Salvador (Table 1) earthquake. The rapid highfrequency roll-off of the instrument response of the ultra-long-period stations intrinsically reduces the contamination of the records by 20 sec period surface waves, allowing clear ScSn phases to be observed and fruitfully analyzed, despite the relatively shallow depth of the event (52 km). Suitable filtering of the GDSN data also allowed useful ScSn information to be retrieved. Motivated by this success, we searched the ultra-long-period recordings for additional events with high quality ScSnphases traversing continental paths, finding two suitable earthquakes in North America. These are the 24 October 1980 Huajuapan, Mexico event and the 29 April 1965 Puget Sound earthquake (Table 1). Examples of the multiple ScS phases recorded by the ultra-long-period stations at Pasadena and Berkeley for the two new events are shown in Figure 1, while data for the E1 Salvador event are shown in Figure 2 of Paper 1. The traces have all been bandpass-filtered to eliminate digitization and very-long-period noise, but there is clearly favorable signal-to-noise ratio in the 50 to 150 sec period range. The energy in the interval between ScSn arrivals is certainly higher than typically observed for ScSn wavetrains from deeper events sampling oceanic paths (see, for example, Revenaugh and Jordan, 1987); however, this appears to be primarily an unavoidable result of the shallower source depths and the fact that continental paths tend to produce higher coda levels as a result of stronger lithospheric scattering properties (Sipkin and Jordan, 1980b). The three events used all have normal fault mechanisms with a steeply dipping plane trending along the azimuth to Pasadena, and all three occurred deeper than 50 kin, which helps to reduce surface-wave contamination (Table 1). The fault orientations ensure that strong downgoing SH energy contributes to the ScSn phases. The surface reflected energy, sscs,~ arrives within 30 sec of each downgoing phase and hence cannot be identified as discrete arrivals in the ultra-long-period data. Scalloping of the signal spectra resulting from the depth phase interference does reduce the signal-to-noise ratio near 30-sec period, but this is not a major problem since the noise levels are quite high for periods less than 30 sec anyway. TABLE 1 EARTHQUAKE PARAMETERS Date Origin Latitude, Longitude, Mo Depth, Strike Dip Rake Reference Time* N* W* (10~6dyne-cm) km 29 April :28: a 24 October :53: b 19 June :21: b * ISC, a: Langston and Blum, 1977; b: Lay et al., 1988.

4 2044 THORNE LAY AND TERRY C. WALLACE ScS 2 ScS 3 ScS 4 BKS ULP PAS ULP ScS ScS 2 ScS 3 ScS 4 PAS t i ~ sec ~oo FIG. 1. Representative tangential component ScSn recordings for the 24 October 1980 Huajuapan, Mexico event (upper two traces) and the 29 April 1965 Puget Sound earthquake (lower two traces). The data have been passed through a Butterworth bandpass filter with frequency cutoffs of 8 mhz and 25 mhz. ULP indicates recordings from the ultra-long-period seismometers, while denotes the Press Ewing instrument at Pasadena. The Huajuapan event reverberations sample central Mexico, and the Puget Sound event reverberations traverse the Sierra Nevada. Along with the ultra-long-period Pasadena recording of the Puget Sound event, Figure 1 displays the corresponding ScS~ wave train obtained from the Pasadena Press-Ewing instrument. Although the 30 sec pendulum period for this instrument leads to enhanced short-period surface wave contamination, it is clear that useful ScS,~ arrivals for the Puget Sound event can be recovered from similar instrumentation (which is much more abundant than the ultra-long-period seismometers found only at Pasadena and Berkeley). We exploit this fact in both extending the bandwidth of the ScSn analysis at Pasadena and in collecting additional data for this event from WWSSN, CSN (Canadian Seismic Network) and LRSM (Long Range Seismic Monitoring) stations. Table 2 lists the complete set of ScSn waveforms analyzed in this study. The additional seismograms will be presented and discussed later. The precise data processing involved in analyzing all of the ScS,~ signals in this paper follows the general procedures described in Paper 1. For differential traveltime estimation, the start time of the first wave of each pair was placed 10 sec ahead of the theoretical JB time with corrections for source depth, ellipticity of each ScS leg, and receiver and bouncepoint elevations. This reference phase was convolved with an appropriate Futterman attenuation operator with a t* value compatible with a preliminary stacking determination of Qzcs. The filtered trace was aligned with the original trace using the onset of the highest frequency arrival, which had a frequency of 33 to 40 mhz in our data. The start time of the second wave was tentatively placed 20 sec prior to the calculated arrival time with depth, ellipticity, and elevation corrections, and a differential time was determined by cross-correlating the filtered reference trace and the second arrival. Variable duration time

5 MULTIPLE SCS ATTENUATION AND TRAVEL TIMES 2045 TABLE 2 SCS REVERBERATIONS Date Station ScS1 ScS2 ScS~ ScS4 ScS5 29 April 1965 COR X X DUG X X GOL X X JR-AZ X X X NL2-AZ X X X PAS ULP X X X X PAS 3O-9O X X PAS 1-90 X X PHC X X PNT X X X SN-AZ X X X TUC X X YKC X X 24 October 1980 BKS ULP X X X PAS ULP X X X 19 June 1982 ANMO X X BKS ULP X X X PAS ULP X X X x windows were used depending on noise conditions, but usually a 150 sec interval was adopted. The phases were prefiltered with a three-pole Butterworth bandpass filter when high-frequency noise was strong. The dispersion correction was applied because there were visible broadening effects in the waveforms of the sequential ScS~ arrivals. Simply cross-correlating the raw seismograms will lead to differential travel times that tend to be biased slightly too large, typically by up to 1.5 sec, because one is cross-correlating impulsive waveforms that are not the same shape. This would not be a problem if one had perfectly narrow-band data, for the crosscorrelation would directly yield a differential time at the corresponding frequency (Sipkin and Jordan, 1980a); however, our data do have a finite bandwidth and the change in shape bias is measurable. Application of the Futterman operator eliminates this bias, and results in differential travel times that are referenced to the high-frequency end of the passband, which in our case in about 33 to 40 mhz. This allows us to directly compare our differential times. Comparisons with other determinations of differential times must include an assessment of the dispersion effects between different passbands. Having determined the final differential travel times, adjusted start times were used and the signal spectra were computed from the original traces for time windows of 90, 128 or 150 sec. The shorter time windows were used for the noisier WWSSN and CSN observations, and spectral interpolation was performed to enable stacking all of the data using independent frequency points with a separation of 7.8 mhz. A 10 per cent cosine taper was applied to each signal, but no bandpass filtering was used. A noise power spectrum was determined for a corresponding length of signal following ScS or preceding ScS2, ScS3, ScS4 or ScS~. The noise spectra were smoothed with a 9 mhz running average. These spectra were then used in the spectral stacking technique of Jordan and Sipkin (1977), with comparisons being made using the maximum likelihood technique introduced by Nakanishi (1979). Both spectral stacking methods explicitly incorporate the signal-to-noise properties, which is critical to stable Qscsestimation. In order to estimate the actual attenuation

6 2046 THORNE LAY AND TERRY C. WALLACE operator, spreading corrections for a JB earth model were determined and applied to the data, and a variance weighted least-squares regression on the attenuation operator modulus estimates was used to determine Qscs. A time-domain method for estimating Qscs was also used in a few instances where very high signal-to-noise ratio data were available. This procedure will be described in the next section. ANALYSIS OF ScSn PHASES UNDER MEXICO In the initial analysis of the E1 Salvador earthquake data (Paper 1), eight ScS~ arrivals with 15 surface bouncepoints concentrated within Mexico were processed. The observations were taken from the ultra-long-period seismograms at Pasadena and Berkeley and the SRO station ANMO (Table 2). The seven spectral pairs of these phases provided an estimate of Qscs = 147 _+ 27 in the frequency range 8 to 40 mhz using the spectral stacking technique of Jordan and Sipkin (1977), and the five independent spectral ratios gave Qscs = 144 _+ 15 using the method of Nakanishi (1979). The two procedures differ primarily in their noise weighting characteristics, and produce increasingly consistent results as the number of phase pairs included in the stacks goes up. Measurement of the ScSn+l - ScS~ JB residuals for the 7 possible combinations gave an average Atscs sec (positive = slower than JB), for a dominant signal period of about 50 sec. The ultra-long-period recordings from the Huajuapan earthquake provide six additional ScSn phases (Figure 1), which were combined with the data for the E1 Salvador event. Figure 2 shows the corresponding surface reflection points for all of the whole-mantle reverberations from the two events. The majority of the 27 bouncepoints are located within continental Mexico, with many of the western reverberations traversing the Sierra Madre Occidental. The combined spectral stack S AS [] [] ANMO &, [] \\ ~ "-~\ 5\" \= OCTOBER~ 1980 FIG. 2. Map location of the 24 October 1980 Huajuapan and 19 June 1982 El Salvador events along with ScSn surface reflection points for the recordings obtained from stations BKS, PAS and ANMO. The dashed region indicates the Sierra Madre Occidental.

7 MULTIPLE SCS ATTENUATION AND TRAVEL TIMES 2047 for this data is shown in Figure 3, where the 13 pairs of ScSn phases have been processed using the algorithm of Jordan and Sipkin (1977). Only those spectral points in the frequency band 7.8 to 46.9 mhz were included in the variance-weighted regression, which yielded Qscs = Extending the spectral window to 54.7 mhz results in Qscs = 132 ± 14 however, the noise levels are quite high above 50 mhz so this result is not as stable. There is only a mild dispersive effect on the phase in this band, consistent with the stacking results of Jordan and Sipkin (1977) and Sipkin and Jordan (1980b). Note that our differential times should give small phase anomalies in the frequency band 33 to 40 mhz, as observed. Using eight independent spectral ratios in the method of Nakanishi (1979) results in Qzcs = 146 ± 16 for the band 7.8 to 39.1 mhz. The latter procedure is intrinsically less stable due to the smaller number of stacked spectra, but the results are encouragingly similar. There is some concern that these spectral stacking methods may be biased toward low Qscs determinations. This is the contention of Chan and Der (1988) who proposed an alternate time-domain methodology for estimating Qscs. Their procedure involves matching the amplitude decay rate of the observed ScSn wave train at a given station by successive filtering (and correction for geometric spreading) of the first ScSn arrival. This procedure intrinsically emphasizes the dominant period of the time-domain signal, which in turn is controlled by the instrument response characteristics. Objective quantification of the signal-to-noise ratio properties of the data is difficult with such a time-domain approach, and the procedure utilized by Chan and Der (1988) makes no attempt to quantify the phase coherence. Despite these drawbacks, it is clearly desirable to obtain consistent results between frequency-domain and time-domain methods for estimating Qs~s, at least when the data have high signal-to-noise ratios. The ultra-long-period recordings at PAS and BKS for both the Huajuapan and E1 Salvador earthquakes are the highest quality data in this study, so we applied a modified time-domain analysis to the 12 go I I [ I I TT c- "0 "-'0 09 c- ~'-TT ol ] r ; ~ ] J I I I I Frequency (mhz) FIG. 3. Estimates of the modulus and phase of the Qscs attenuation operator for paths beneath Mexico. Error bars are _+ 1~. A variance weighted least-squares regression on the modulus data in the range 7.8 to 46.9 mhz (filled points) gives the Qscs estimate shown. Thirteen pairs of ScS~ phases were used in the stacking, following the procedure of Jordan and Sipkin (1977).

8 2048 THORNE LAY AND TERRY C. WALLACE available ScS2, ScS3 and ScS4 phases. Our variant of the procedure suggested by Chan and Der (1988) involves use of a waveform equalization norm, given by: Nw = fo0 tmax (ScSi(t) - ScSh(t) * F(t*).gjk) 2 dt in which tmax is the time window over which the waveforms are compared (after alignment for the optimal cross-correlation), ScSj(t) and ScSk(t) are the two arrivals being compared (j > k), F(t*) is a Futterman attenuation operator parameterized by the t* value corresponding to Qscs which is convolved (*) with the reference phase, and gik is a scalar geometric spreading correction. Minimization of this norm yields the optimal differential t* between the ScS~ phases. The waveform norm is primarily sensitive to the absolute amplitudes of the dominant-period energy, hence preserving the basic merit of the time-domain procedure of Chan and Der (1988), but the norm also weights the phase coherence of the equalized signals. It should be noted that the spectral techniques that we are using also place significant weight on the absolute amplitude information since geometric spreading corrections are applied and the Qscs regressions involve determining only the spectral slope, with the intercept value being defined to be a ratio of 1 (see Figure 3). The results of applying our time-domain attenuation estimation procedure to the ultra-long-period Pasadena (PAS) recordings are shown in Figure 4. The ScS2 waveforms were equalized to ScSa and ScS4 independently, and ScSa was equalized to ScS4. This procedure was applied to the Berkeley (BKS) signals as well, yielding 12 estimates of differential t* from which Qscs could be determined. The estimates are listed in Table 3. The results for PAS are particularly consistent, with the phase stability being readily apparent in Figure 4. This figure also demonstrates the waveform smoothing for successive ScS~phases that requires a dispersion correction prior to differential travel-time estimation. The BKS results show more variability, but still provide consistent average values. The overall average of is in satisfactory agreement with the results from the two spectral procedures, so we are PAS ULTRA-LONG PERIOD HUAJUAPAN ~ / ~ EL SALVADOR ScS 3 & Oct. 24,1980 Jor, e19,~982 ~ / ~j SEC o ~--~--~ ~o ScS3 ScS2*F i1~.o) ScS4 Sc "%o.u scs 4 F(12.5) FIG. 4. Example of the time-domain estimation of Qzcs for paths under Mexico obtained using ultralong-period recordings at PAS. The reference phases, either ScS2 or ScSa are equalized to ScSa or ScS4 phases by convolving with a Futterman attenuation operator, F(t*), with the indicated differential t*, and multiplying by an appropriate geometric spreading constant. The choice of t* is based on optimization of the wave form norm given in the text. The equalized waveforms are plotted with true relative amplitudes compared to the corresponding observations.

9 MULTIPLE SCS ATTENUATION AND TRAVEL TIMES 2049 TABLE 3 TIME DOMAIN Qs,s ESTIMATES FOR MEXICO Date Station ScS~ Pair Qs~s 24 October 1980 PAS ScS::ScS3 143 ScS2:ScS4 143 ScS3:ScS4 146 BKS ScS2:ScS~ 220 ScS2:ScS4 143 ScS3:ScS~ June 1982 PAS ScS2:ScS3 154 ScS2:ScS4 148 ScS3:ScS4 128 BKS ScS2:ScS3 193 ScS2:ScS4 142 ScS3:ScS4 109 Average 148 _ 9 confident that the frequency-domain analyses are not biased in this instance. We do recognize, however, the limited bandwidth of the ultra-long-period data, and emphasize that all of these Qscs estimates are appropriate for a dominant period of T to 60 sec. Chan and Der (1988) analyzed data with paths from Central America to ANMO, finding Qscs = , which may suggest that attenuation diminishes in northeastern Mexico (see Figure 2); however, the data quality of their shorter-period GDSN recordings is significantly lower than that of the ultra-longperiod recordings used in this study. The final differential travel-time anomalies determined for the reverberations under Mexico are listed in Table 4. The precision of the travel-time anomalies indicated by reporting two significant figures is somewhat optimistic given the difficulty of precise time measurements for the ultra-long-period recordings; however, the cross-correlation procedure does yield general consistency between successive ScSn phases. There is substantial variability in the individual differential times which exceeds the _+1 sec accuracy estimated for the measurements. This suggests substantial velocity variations beneath Mexico, but it is difficult to confidently isolate the regions responsible for each anomaly. Referring back to Figure 2 makes it apparent that the paths traversing the Sierra Madre in northwestern Mexico tend to have faster times. There is no evidence for a systematic variation of Qscs associated with the inferred lateral gradient in shear velocity, but the spatial resolution is poor. The average anomaly for all of the ScSn phases beneath Mexico is +0.9 sec, the significance of which will be discussed later. ANALYSIS OF ScSn PHASES UNDER THE WESTERN UNITED STATES An extensive search was conducted for ScSn recordings from the 29 April 1965 Puget Sound earthquake once it was recognized that Pasadena recorded high quality ScS~ phases for this event (Figure 1). Unfortunately, the ultra-long-period recordings at Berkeley were not usable; however, data were obtained from four WWSSN stations, three CSN stations, and three LRSM stations that were part of the extended Tonto Forest array deployment. The locations of the stations and the associated ScSn surface reflection points are shown in Figure 5. One CSN station, YKC, is located off the northern end of this map. Many additional recordings were digitized, rotated and filtered, but ultimately discarded because of poor signal quality.

10 2050 THORNE LAY AND TERRY C. WALLACE TABLE 4 Ats s MEASURMENTS FOR MEXICO Date Station ScS~ Pair Ats~s, sec 24 October June 1982 BKS ScS3-ScS2-0.3 ScS4-ScS3-0.3 ScS4-ScS2-1.3 (2) PAS ScS3-ScS2-4.1 ScS4-ScS3 6.9 ScS4-ScSz 0.5 (2) ANMO ScS3-ScSz 1.2 BKS ScS~-ScS2 4.0 ScS4-ScS3 1.0 ScS4-ScS2 2.4 (2) PAS ScS3-ScS2 2.0 ScS4-ScS3 0.2 ScS4-ScS2 2.8 (2) Average +0.9 sec. COR ~I PNT ----~@... ~I~'APRIL: " "'.....-"'. ~, :'..... Columbia Plateau ' ~....'". " ~.~.. '. n. ~ ". " :: '*~ ADUG... GOL '. o~_ Colorado '. ~ ] m- c~ ~." Plateau.... '..... :. ANL2 AZ i. "',41tJRAZ. - '" PA~ ': a SNAZ Fro. 5. Map location of the 29 April 1965 Puget Sound event along with ScS~ surface reflection points for the recordings obtained from the stations that are indicated. The bouncepoint symbols are different for the three sets of reverberations sampling the Basin and Range,, the Pacific and the Sierra Nevada,.. Inspection 0f the data made it readily apparent that substantial variations in the relative amplitude of ScS1 and ScS2 were present in the data, so we defined three subgroups of ScSn paths. These are paths in the Pacific Northwest (stations COR, PHC, PNT, YKC), paths traversing just east of the Cascades and along the Sierra Nevada range (station PAS), and paths traversing the Columbia Plateau and

11 MULTIPLE SCS ATTENUATION AND TRAVEL TIMES 2051 northern Basin and Range (stations DUG, GOL, JR-AZ, NL2-AZ, SN-AZ, TUC). The corresponding surface reflection points are distinguished in Figure 5. This data set constitutes the first set of ScSn reverberations in the Western United States and, because the Puget Sound event is unusual in both its mechanism and source depth, it may be the only data set available for some time to come. The data sampling the Sierra Nevada were collected from three Pasadena eastwest horizontal component instruments: the Gilman ultra-long-period, the Press Ewing, and the 1-90 Benioff (in each case the first number is the pendulum period and the second number is the galvonometer period). From the latter two instruments only ScS1 and ScS2 were used in the spectral analysis because of the high noise levels later in the records, whereas ScS~ to ScS5 were obtained from the ultra-long-period instrument. The ScSs arrival was contaminated by longarc overtone arrivals and was consequently down-weighted in the Qscs determination. The shorter-period instruments were included in order to obtain better time resolution and to stabilize the higher-frequency spectra in the stacking algorithms. The ScS~ phases sampling the Pacific Northwest are shown in Figure 6. The traces have all been moderately filtered with a bandpass filter to reduce 20 sec period surface-wave noise levels, but it is clear that both ScS1 and ScS2 have good signal energy. These recordings have clear sscs, arrivals, which are responsible for the second large pulse in each waveform. This arrival is manifested as phase instability near 30 mhz for these data as well as for the Pasadena recordings. The amplitude ratios in Figure 6 exhibit some variability suggestive of rapid Qscs fluctuations in the subregion; however, the number of recordings is too low to further subdivide the data set. Little confidence should be placed in individual Qscs determinations from single wave pairs, since attenuation estimation by either timedomain or frequency-domain procedures should be viewed as a statistical process. The third subset of ScSn phases, which traverse the northern Basin and Range, are displayed in Figure 7. The left hand columns show the WWSSN recordings of PHC YKC &=4.7 =316.8 /~ ScS ScS ScS2 ~ ScS2 COR PNT 2,9 ~ o /I 2, o Sc.S ScS sos2 sos2 0~120 ScS3 FIG. 6. Representative ScS,, phases recorded at stations in the Pacific Northwest. The distance and azimuth of each station from the Puget Sound event are shown. The data have been bandpass filtered in the frequency band 8 to 50 mhz. The traces are also shifted to align on the first peak of each ScSn arrival.

12 2052 THORNE LAY AND TERRY C. WALLACE ScS ScS 2 ScS 2 ScS 3 ScS 4 TUC A A NL2AZ "'17"S %'/ ]~] / ^A "0 / ~ " ~ GOoL A JRAZ ~j V ~ DUG/~ j~ SNAZ~ ~, ,0 oil ~/ ~ \j~" - ~ "8 FIG. 7. ScSn phases from the Puget Sound event recorded by WWSSN and LRSM stations with paths that sample the Basin and Range Province. The three recordings from WWSSN stations (TUC, GOL, DUG) have been filtered with a bandpass filter in the frequency band 8 to 50 mhz. The LRSM recordings are filtered in the band 8 to 25 mhz. ScS 1 and ScS2, while the LRSM data are shown on the right. For the latter stations the ScS1 phases were too large to recover, but the long-period instrument response and the slow recording speed of the LRSM system permitted the ScSa and ScS4 phases to be used. The LRSM data are filtered slightly more than the WWSSN data to better display the long-period phase coherence. In every case except for TUC the components used are the radial components of ground motion, while all other traces used in this study are tangential components. This is because the focal mechanism of the Puget Sound events produces slightly more stable down-going SV radiation at these azimuths. A penalty is paid for this in that the surface-wave noise levels are slightly higher than on the transverse components, resulting in somewhat constricted bandwidth and reduced phase coherence. However, the strong signal energy near the 50 sec period still permits a fairly stable estimate of Qscs. Note that, since the epicentral distances are all quite small (<18 ), the radial ScS, phases are not strongly affected by S to P conversions at the core-mantle boundary or near the free surface. The nature of such energy losses will be to bias our Qscs estimate toward lower values. The spectral stacking technique of Jordan and Sipkin (1977) was applied to each of the three subsets of data as well as to a combined set of phases for the Pacific Borderlands. The estimate of the attenuation operator obtained from the Pasadena recordings is shown at the top of Figure 8. Nine pairs of ScSn phases were included in this stack, with all but two being from the ultra-long-period recording. The phase spectrum shows some instability near 30 mhz, and the modulus estimates exhibit correspondingly large uncertainties. The regression for the frequency band 7.8 to 54.7 mhz yields Qscs = , while the band 7.8 to 46.9 mhz gives Qscs = 236 _ 78. Utilizing only frequencies of 23.4 mhz and less gives even higher values of 377 _ 105. The degradation of signal quality above 30 mhz is the combined effect of the surface reflection interference, the rapid roll-off of the instrument response, and the short-period surface-wave noise levels, all of which are only partially mitigated by the inclusion of the data from the shorter-period instruments. The actual uncertainty in the Qscs estimate is thus somewhat higher than the formal regression uncertainties indicate. The time-domain method described above was applied to these data, yielding generally consistent results in most cases, but with

13 MULTIPLE SCS ATTENUATION AND TRAVEL TIMES 2053 T o 5 "o-1 0 _~-2 --Y/'F i i 1[--,, '- ' ' I,'E, FT~-, T -3 L "~ -2 tqscs=344±88 ~ --_3 / J J I I I J "-TT/l k,,, I o,i "5-3 i i [ ~ J I I Frequency (mhz) Frequency (mhz) FIG. 8. Estimates of the modulus and phase of the Qscs attenuation operator for paths beneath the Sierra Nevada (top row), the Pacific Northwest (middle row) and a combined data set for the Pacific Borderlands (bottom row). The frequency range of the spectral points used in the variance weighted regressions yielding each Qscs estimate are indicated by the filled symbols. 60 poorer phase coherence than for the data sampling Mexico. We thus prefer the frequency-domain results. The six ScSn pairs from the Pacific Northwest yield the relatively stable Qscs estimate shown in the middle of Figure 8. The spectral instability near 30 mhz is again apparent in both the modulus and phase, but the frequency range of 15.6 to 54.7 mhz yields a high Qzcs = 344 _ 88. In deciding upon the spectral band to use in this regression we also considered the results of the maximum likelihood stacking method. The time-domain procedure was applied to these phases as well, with the high Qscs estimate being corroborated, but the high noise levels for these data precluded a comprehensive analysis. While we do feel that the difference in Qscs for the Sierra Nevada and Pacific Northwest subregions is strongly suggested, we recognize the intrinsic limitations of the spectral stacking procedures for these moderate size data sets. Therefore, a conservative estimate of Qscs was made by combining the two sets of phases (excluding YKC) and stacking the 11 highest quality pairs sampling the larger scale Pacific Borderlands region. This yielded the very stable result shown at the bottom of Figure 7, where an estimate of Qscs = 257 _ 20 is found for the 7.8 to 46.9 mhz band. This value clearly represents an average of the results for the two separate subregions. The Qscs estimate for the northern Basin and Range province is shown in Figure 9. In this case, the spectral scalloping, high noise levels, and intrinsically low Qscs conspire to restrict the useful bandwidth to the range 7.8 to 31.3 mhz. A fairly stable estimate of 95 ± 4 is obtained, with good phase coherence for the 12 pairs of ScSn arrivals. Very similar values are obtained using the maximum likelihood

14 / 2054 THORNE LAY AND TERRY C. WALLACE - i - i I r I "o-1 0 "'-2 t'- -3,E-T[._~ i I I i / L t I "-'0 O9 c" a-_rr I i I I Frequency (mhz) FIG. 9. Estimates of the modulus and phase of the Qscs attenuation operator for paths sampling the Basin and Range. A weighted least-squares regression on the modulus data in the range 7.8 to 31.3 mhz gives the Qscs estimate shown. +- method, although the spectral stability between 30 and 40 mhz is much poorer, which is the reason for restricting the regression bandwidth to 31.3 mhz. This is an exceptionally low Qscs estimate, and the limited bandwidth and mediocre signal quality suggest that the uncertainty is probably four to five times larger than the formal estimate of ±4. The time-domain procedure was not used given the high noise levels. The Atscs estimates for the same sets of phases are presented in Table 5. For a few of the higher-frequency stations, absolute ScS~ travel-time anomalies measured directly from the impulsive first arrival are included, while all of the differential times were obtained by cross-correlation. The average anomaly for the Pacific Northwest region is +5.5 sec, corresponding to a one-way travel-time anomaly of sec. This value would be even slower if YKC were omitted. The differential times are internally consistent and compatible with the direct ScS1 measurements. The measurement uncertainties are fairly small, ±0.5 sec or so, due to the good time resolution of these instruments, although the noise levels are high for the later arrivals. The traces were slightly filtered during the cross-correlation procedure, so these times are appropriate for a dominant period of 30 sec. The Pasadena travel times are significantly faster, averaging +2.5 sec. It appears that the ultra-long-period measurements are systematically faster than the and 1-90 values, perhaps due to time resolution problems; however, the noise levels are much higher on the shorter-period records. The Basin and Range paths yield anomalies that are similar to those in the Pacific Northwest, averaging +5.1 sec. In this case, the noise levels were quite high, but the WWSSN times are somewhat more reliable than the measurements from the narrow-band LRSM stations. TUC has the only fast differential time, for a bouncepoint right in the heart of the Basin and Range. While formal uncertainties are particularly difficult to determine for this small data set, we estimate that each subregion mean value is accurate to within - 1 sec.

15 MULTIPLE ScS ATTENUATION AND TRAVEL TIMES TABLE 5 Atscs MEASUREMENTS FOR WESTERN UNITED STATES Date Station ScSn Pair Ats~s, sec 29 April 1965 COR ScS2-ScS1 5.3 PHC ScSI* 4.7 ScS2-ScS1 7.3 PNT ScSI* 7.6 ScS2-ScSI 3.8 ScS3-ScS (2) ScS3-ScS YKC ScSI* 1.0 ScS2-ScS1 1.5 Pacific Northwest Average +5.5 sec PAS ULP ScS2-ScSI 1.9 ScS3-ScS1 4.0 (2) ScS4-ScS1 6.0 (3) ScS3-ScS2 2.4 ScS4-ScS2 4.5 (2) ScS4-ScS3 1.7 PAS ScS2-ScS1 5.0 ScS3-ScS1 5.3 (2) ScS3-ScS2 0.9 PAS 1-90 ScS2-ScS1 5.0 Sierra Nevada Average +2.5 sec DUG ScSI* 9.7 ScS2-ScS1 5.3 GOL ScSI* 6.2 ScS2-ScS~ 6.0 JR-AZ ScS~-ScS2 8.3 ScS4-ScS2 7.0 (2) NL2-AZ ScS3-ScS2 7.5 ScS4-ScS3 8.0 (2) SN-AZ ScS4-ScS2 8.5 (2) TUC ScS2-ScS~ 0.0 Basin and Range Average +5.1 sec * Measurements made on impulsive first arrival DISCUSSION The foregoing analysis has indicated the presence of substantial regional variability in Qscs and AtscS beneath western North America. As in all studies of wholemantle measurements like these, it is difficult to confidently associate all of the heterogeneity with upper-mantle properties, given that the lower mantle is known to have significant localized heterogeneity (Lay, 1983; Grand, 1987). However, it is well-established that the most pronounced mantle heterogeneity is located in the upper 300 km of the mantle, and that these variations are often correlated with surface tectonic history. Thus, we will consider the implications of our results in terms of upper-mantle variations, following most previous investigators of ScSn phases (e.g., Sipkin and Jordan, 1980 a,b; Chan and Der, 1988). First, consider the total range spanned by the Qscs estimates for western North America: from 95 _ 4 in the northern Basin and Range to 344 _ 88 in the Pacific Northwest. This is as large as the range obtained in previous ScS~ spectral stacking investigations of such diverse regions as subduction zones, old oceans, young ocean, and continental regions. Published estimates of Qscs in continental regimes obtained

16 2056 THORNE LAY AND TERRY C. WALLACE by spectral stacking or time-domain methods include values for South America of , (Sipkin and Jordan, 1980b), and (Chan and Der, 1988); for tectonic Eurasia of , 186 _ 73 (Sipkin and Jordan, 1980b), and 180 _ 42 (Chan and Der, 1988); and for Mexico of (Chan and Der, 1988) and 145 _+ 27 (Paper 1). Sipkin and Jordan (1980b) suggest an average continental Qscs = 225. Estimates of Qzcs in old oceanic regions have been given as 155 to 184 (Sipkin and Jordan, 1980b), 226 (Nakanishi, 1979), and 205 _+ 33 (Chan and Der, 1988), while estimates for the east Pacific have lower values of 135 to 142 (Sipkin and Jordan, 1980b) and 138 _ 19 (Chan and Der, 1988). Typical Qscs values for reference earth models derived using free oscillation data are 236 for the SL8 model of Anderson and Hart (1978) and 223 for the PREM model of Dziewonski and Anderson (1981). The Basin and Range estimate obtained in this study is actually lower than the Qscs estimates for the eastern Pacific, although only a few of the bouncepoints in the study of Chan and Der (1988) actually sample the vicinity of the East Pacific rise. This is consistent with the notion that the Basin and Range upper-mantle structure is essentially similar to the end member of a very young oceanic ridge, as suggested by the tectonic history of overriding the northern East Pacific rise, the high heat flow (Lachenbruch and Sass, 1980), and the well-developed low-velocity zone for shear velocity structure (Grand and Helmberger, 1984). The Qscs estimate for Mexico (142 10) is low for a tectonically active continental environment, but is comparable to the old Pacific ocean value of found by Sipkin and Jordan (1980b). The paths along the Sierra Nevada have a Qscs (208 _+ 39) very comparable to the measurements in tectonic Eurasia (northern China). The greater surprise is the high Qscs = 344 _ 88 in the Pacific Northwest, which is more characteristic of the high Qscs values found in regions with major subduction zones such as South America (Sipkin and Jordan, 1980b) and Japan (Nakanishi, 1979). The strong variations in Qscs within tectonic North America are not closely correlated with shear velocity variations. The highest and lowest Qscs estimates are both associated with very slow paths having Ats~s anomalies of more than +5.0 sec. The intermediate Qscs values beneath the Sierra Nevada and Mexico are associated with somewhat faster travel times, so the correlation between travel-time anomalies and attenuation perturbations is not totally destroyed, but it is certainly less pronounced than in oceanic environments (Sipkin and Jordan, 1980b). There are only a few ScSn observations within the core of the North American shield with which to compare the attenuation and travel-time behavior of the tectonically active region. Der et al. (1986) have provided Qscs estimates of 370 to 520 for paths across the Canadian shield for 15 sec period waves, but the data quality is quite low. It is of interest to compare the Atzcs measurements with upper-mantle shear velocity measurements. A one-way JB travel-time anomaly of +2.5 sec is quite consistent with station shear-wave travel-time anomalies in the Basin and Range and Pacific Northwest (Lay, 1983), but it is somewhat less than the +4.7 sec anomaly predicted for the upper-mantle shear-velocity structure, TNA (Grand and Helmberger, 1984), which was proposed for the entire tectonically active region. It is, of course, hard to compare the whole-mantle travel-time anomaly with just an upper-mantle structure, for that particular structure does not have a prescribed lower-mantle complement. Simply extending the TNA structure downward using the JB model (an ad hoc procedure) would predict Ats~z = +9.4 sec. However, Lay (1983) has suggested that deep-mantle velocities under the western United States

17 MULTIPLE SCS ATTENUATION AND TRAVEL TIMES 2057 may be higher than normal JB structure on the basis of ScS-S differential time anomalies, which would tend to reduce the whole-mantle anomaly. Our data do exhibit more than 2.0 sec of variation in one-way travel time through the whole mantle beneath western North America, suggesting significant local perturbations of the TNA upper-mantle structure; however, we are unable to isolate the depth extent of the actual variations. At any rate, these variations are far less than the 6.0 sec variation between the TNA model and the counterpart shield model SNA (Grand and Helmberger, 1984). The interpretation that the ScS~ attenuation and travel-time variations in Western North America are caused by shallow-mantle properties is bolstered by consideration of Figure 10, which is modified from Lachenbruch and Sass (1980). This contour map of regional heat flow in the western United States indicates that the ScSn reverberations sampling the Sierra Nevada region traverse the area of lowest heat flow, while the northern Basin and Range paths (Figure 5) traverse the Battle Mountain high heat flow province. The reverberations in the Pacific Northwest are located within a relatively low heat flow region. It appears that the upper-mantle thermal variations between the Sierra Nevada and the Basin and Range may explain both the attenuation and shear-velocity differences. It is also interesting to note that the multiple ScS paths under Mexico sample a relatively high heat flow region with measured values of 1.8 to 3.0 HFU (Smith et al., 1979). The region of the Sierra Madre Occidental thus has higher heat flow, lower Qscs, and slightly higher mantle velocities than the region under the Sierra Nevada. While we cannot use our data alone to reliably constrain the depth extent of the attenuation heterogeneity beneath western North America, we can make some qualitative assessments. Table 6 provides estimates of the average Q in the upper FIG. 10. Contour map of heat flow in the western United States. The region of high heat flow in the northern Basin and Range (BR) is the Battle Mountain high. The other regions identified are the Sierra Nevada (SN), the Columbia Plateau (CP) and the Colorado Plateau (CP*). Modified from Lachenbruch and Sass (1980).

18 2058 THORNE LAY AND TERRY C. WALLACE Region TABLE 6 UPPER MANTLE Q MODELS PREM.WUS SL8.WUSa SL8.WUSb (0-400) (0-400) (70 300) Mexico Basin and Range Sierra Nevada Pacific Borderlands Pacific Northwest mantle consistent with our regional Qscs determinations for three lower-mantle Q models. In model PREM.WUS the Q structure below 400 km depth is the same as the PREM model (Dziewonski and Anderson, 1981), with the upper 400 km having the indicated constant Q value. Similarly, upper-mantle Q values are given for a model (SL8.WUSa) where the Q structure below 400 km is from the SL8 model (Anderson and Hart, 1978). A third model, SL8.WUSb, has the zone of laterally varying Q confined to the depth range 70 to 300 km, with greater depths having the SL8 Q structure. This model has Q = 120 in the upper 35 km and Q = 50 in the depth range 35 to 70 km, for consistency with Basin and Range surface-wave attenuation measurements (Patton and Taylor, 1984; Hwang and Mitchell, 1987). Of course, all of the absolute values in Table 6 are dependent upon the accuracy of the reference earth models for the lower mantle and the assumption of homogeneity in the lower mantle. The calculations in Table 6 indicate the existence of more than an order of magnitude of upper-mantle Q variation in a thick region of the upper mantle under western North America. The extremely low Q values for the Basin and Range are consistent with the surface-wave analyses of Lee and Solomon (1979), Patton and Taylor (1984), and Hwang and Mitchell (1987), all of which indicate Q values less than 40 in the depth range 80 to 150 km. Low Q persists to greater depths of 250 to 300 km in the models of Lee and Solomon (1979) and Hwang and Mitchell (1987), while Q increases below 100 km depth in the models of Patton and Taylor (1984). The surface-wave data do not give good resolution of the deeper Q structure. Since most surface-wave work has involved paths within the heart of the Basin and Range, additional analysis is needed to establish the upper-mantle component of the Qscs variability that we have observed. The very low Q values and low shear velocities for the Basin and Range strongly suggest the presence of a small component of partial melting (Shankland et al., 1981; Patton and Taylor, 1984; Hwang and Mitchell, 1987). Only a few tenths of a per cent of melting is needed to explain the attenuation behavior, while a greater component is needed to account for the variations in shear velocity. The Pacific Northwest path is puzzling in that the Qzcs is high despite the low velocities, which suggests that either a frequency-dependent effect (due to using higher frequency data for the Qscs determination than in the other regions) or a compositional effect plays a role. This study supports the results of Solomon and ToksSz (1970) who found that long-period body-wave attenuation in the Pacific Borderlands is less pronounced than in the Basin and Range; however, we do not find clear evidence for the strong attenuation beneath the state of Washington that they inferred from observations at the WWSSN station LON. Our results are also consistent with the general

19 MULTIPLE SCS ATTENUATION AND TRAVEL TIMES 2059 inference from the studies of Sipkin and Jordan (1980b) and Chan and Der (1988) regarding the need for regionally varying long-period body-wave attenuation models, rather than simple variable absorption band roll-off models. However, defining the precise frequency dependence of these models requires high-quality broadband data (Taylor et al., 1986). Reconciling the large variation in long-period differential attenuation (Solomon and ToksSz, 1970; Lay and Helmberger, 1981; the studies of ScSn) with the much smaller high-frequency differential attenuation estimates between shield and tectonic provinces (Der et al., 1982; Lay and Helmberger, 1983; Taylor et al., 1986) requires much additional investigation. At this point, it is clear that strong heterogeneity in both seismic velocity and attenuation for the longperiod body-wave passband does exist beneath the western portion of North America. CONCLUSIONS A new data set of multiple ScS phases under the tectonically active region of western Northern America reveals the existence of strong lateral variations in attenuation and shear-velocity structure in the upper mantle. The strongest attenuation, as measured by the whole-mantle attenuation estimate, Qscs, is found beneath the northern Basin and Range, along with very slow shear velocities. The low Qscs = 95 _+ 4 for this high heat flow region suggests the presence of extensive partial melting, and may be representative of upper-mantle properties under young ocean ridges. A much less attenuating region is found along the Pacific Borderlands, particularly in the Pacific Northwest, where Qscs = 344 _+_ 88, despite the latter region having shear velocities comparable to the Basin and Range. An intermediate Qscs = 208 _ 39 is found beneath the Sierra Nevada, along with significantly higher shear velocities. This region has the lowest heat flow in the western U.S. A relatively low Qscs = 142 _ 10 is found below Mexico, with many of the paths sampling the Sierra Madre Occidental, which has significantly higher heat flow than the Sierra Nevada. The overall lateral variations within the continental environment are much more pronounced than those observed for the Pacific Ocean regime away from subduction zones, but this appears to be exacerbated by the unusual tectonic history of western North America in that the continent has overridden much of the East Pacific rise. ACKNOWLEDGMENTS We thank Bruce Bolt for providing excellent copies of the ultra-long-period recordings from Berkeley. Judi Sheridan digitized most of the seismograms used. Kevin Furlong provided helpful insight into heat flow variations. An anonymous reviewer provided some constructive comments. This research was partially supported by the National Science Foundation under Grant EAR Acknowledgment is made to the Donors of the Petroleum Research Fund, administered by the American Chemical Society for the partial support of this research. REFERENCES Anderson, D. L. and R. S. Hart (1978). Q of the earth, J. Geophys. Res. 83, Butler, R. (1984). Azimuth, energy, Q, and temperature: variations on P wave amplitudes in the United States. Rev. Geophys. Space Phys. 22, Chan, W. W. and Z. A. Der (1988). Attenuation of multiple ScS in various parts of the world, Geophys. J. 92, Der., Z. A., W. W. Chan, A. C. Lees, and M. E. Marshall (1986). Models of the frequency dependence of Q in the mantle underlying tectonic areas of North America, Eurasia and Eastern Pacific, Technical Report, TGAL-86-8, Teledyne Geotech, Alexandria, Virginia.

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