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Impact of broadband seismology on the understanding of strong motions

SEISMOLOGICAL LABORATORY CALIFORNIA INSTITUTE OF TECHNOLOGY, PASADENA, CALIFORNIA 91125

Abstract

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 sP_mP. These phases as well as the depth phase sS_mS 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 large 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.

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