We examine aftershock detectability thresholds for events in the initial part of the 31 October 2001, ML 5.1 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 within the study region (seven had epicentral distances < 20 km). Of the 608 aftershocks (0 < ML < ∼2.8) in the initial 2 hr of this sequence, the first five aftershocks recorded were only identifiable at stations within 30 km after applying a high-pass filter. Using a cluster (radius ≤ 1.1 km) 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 that detectable aftershocks within the mainshock coda include (1) those over magnitude ∼3 that are within 15 km of the network centroid that occur 12 sec or more into the sequence, and (2) those over magnitude ∼2 that are within 30 km of the centroid of the network that occur 60 sec or more into the sequence. We find a lack of large aftershocks in this sequence. The largest aftershock (ML ∼2.8) is substantially smaller than the mainshock (ML 5.1). We suggest this relatively large-magnitude differential is dictated by a combination of factors that includes complexity of the San Jacinto fault system and the lack of large earthquakes in the region in the past ∼20 years.
Online material: Quicktime movies juxtaposing a 3.2 aftershock in the coda of a 5.1 mainshock.