Impact of Broadband Seismology on the Understanding of Strong Motions

Most analyses of strong motion attenuation assume simple whole-space type geometrical spreading, namely (1 /R ) or its modified form e kR/R. HOWever, broadband data presently becoming available suggests a more complex behavior with substantial crustal effects. Events such as the Sierra Madre event, M = 5.8, triggered the strong motion channels at all of the TERRAscope stations allowing for 0.01-sec sampling of the wavefield. We find that most of the well-defined crustal bodywave arrivals defined and modeled in the 1 to 0.1-hz bandpass also contain high-frequency energy. By comparing the triggered channels with the continuous channels we see that several of the more distant stations triggered on the depth phase SPmP. These phases as well as the depth phase sSmS are obvious in velocity and quite apparent in accelerations. Our best models for Southern California contain a relatively thick low-velocity layer at the surface, roughly 5 km thick with shear velocities below 3 km/sec. This layer or zone, because it appears to vary considerably, controls the wavefield at nearly all frequencies out to about 60 km and yields attenuation decay faster than (1 / R). At larger ranges the lower crustal triplications dominate and the attenuation curve flattens. Adding random scatters to these layered models adds additional complexity but does not alter the basic flat-layer predictions. INTRODUCTION Amplitude decay or at tenuat ion as defined by the strong-motion community has received a great deal of attention in recent years. Earlier strong-motion datasets were t runcated at relatively small distances, typically around 70 km. This seemed to be caused by the prevalent processing method, in which one t runcates the range of interest at the' first strong-motion station that failed to trigger. However, more complete datasets such as those produced from the Loma Prieta earthquake, show strong evidence for a flattening and a possible increase in amplitude near a distance of 100 km Campbell, 1991 and Somerville and Yoshimura 1990. The lat ter s tudy suggested that reflections from the Moho discontinuity was responsible for this effect. Weak-motion observations from aftershocks seem to confirm the "Moho-reflected hypothesis" as reported by McGarr et al. (1991). The relatively thin crustal thickness and relatively large source depth in this region are apparent ly the reasons for the shift in Moho phases to nearer distances. I-Iowever, given the scattered nature of the amplitudes produced by the relatively narrowband conventional strong-motion instruments, it proves difficult to resolve these issues. Fortunately, the recently installed TERRAscope array is providing the ideal data to address the role of the crust in strong-motion generation. In particular, we can now examine the broadband wavefield for relatively strong ear thquakes and their aftershocks along similar paths. Thus, the Sierra Madre earthquake, M = 5.8, has been recorded well enough to examine displacements, velocities, and accelerations 830 B R O A D B A N D S E I S M O L O G Y ON U N D E R S T A N D I N G S T R O N G M O T I O N S 831 with little concern about instrumental distortions. Moreover, we find that the displacement field can be relatively well explained with standard Southern California travel time models, at least at long periods. Such models indicate a cross-over in distance at about 130 km, where ray paths bottoming in the lower crust become the first arrivals. By comparing the observed broadband records with the strong motions, we find that the critical angles identified for l-sec signals correspond with the timing of strong high-frequency arrivals. This paper is primarily concerned with characterizing the seismic paths at these ranges, 20 to 160 km, and their associations with the complete field of motion. The data analyzed in this report were produced by the well-studied Sierra Madre earthquake of 28 June 1991 (Dreger and Helmberger, 1991a; Wald; 1992). The latter study indicates that this event was relatively high-stress drop, and thus an appropriate source for studying the attenuation of the strong-motion field. Figure 1 displays the locations of the existing TERRAscope array stations in which the event proves to be equidistant to four of the stations (Table 1). A comparison of the displacements produced by these stations with corresponding synthetics is given in Figure 2. The waveform data are plotted in absolute time, whereas the synthetics appropriate for a SoCal velocity model A (Table 2) have been delayed by 0.35 sec for alignment with the first arrivals. Arrival times of Pn (mantle headwave), Pm P and S m S (reflection from the Moho), and the depth phase SPmP appropriate for model SoCal are indicated. The phase marked Pn is near the critical angle, so the Moho reflection, Pm P, dominates the associated Sierra Madre Event: 28 June 1991 36