Authors

Debi Kilb, IGPP/SIO/UCSD dkilb -@- ucsd.edu
Vladislav G. Martynov, IGPP/SIO/UCSD, vladik -@- ucsd.edu
Frank L. Vernon, IGPP/SIO/UCSD, vernon -@- epicenter.ucsd.edu

Project Description
We study the temporal behavior of the initial part of the 31 October 2001 ML 5.1 aftershock sequence in southern California. This sequence occurred directly below the broadband ANZA seismic network, which recorded continuous waveform data at 13 azimuthally well-distributed stations about the study region (7 had epicentral distances < 20 km). Of the 608 aftershocks (0 < ML < ~2.8) in the initial 2-hours of this sequence, the initial 5 aftershocks were only identifiable at stations within 30 km. Using a cluster (radius ≤ 1.1km) of 200 representative aftershocks, we track the maximum seismogram amplitude versus earthquake magnitude. This relationship helps us quantify the visibility of aftershocks within the mainshock coda and assess our detection capabilities. We estimate detectable aftershocks within the mainshock coda include those: (1) over magnitude ~4 that are within 11 km of the network centroid that occur 15 seconds or more into the sequence; and (2) over magnitude ~2 that are within 30 km of the centroid of the network that occur 60 seconds or more into the sequence. There is a lack of large aftershocks in the ANZA 2001 sequence, as the 2.3 magnitude differential between the mainshock (ML=5.1) and largest aftershock (ML~2.8) is well above the expected ~1 value. We suggest the size of the magnitude differential is dictated by a combination of factors that include the mainshock energy release, directivity of rupture, complexity of the fault system, in addition to the elapsed time from the previous large earthquake in the region.