Spatial/Temporal interdependence of aftershocks following the 10/31/2001 M5.1 Anza Earthquake

On 10/31/2001, a M5.1 earthquake occurred in the middle of the ANZA network (7 24-bit broadband stations were within 20 km of the epicenter) that spans the San Jacinto fault zone in southern California. A high pass filter (f > 1.0 Hz) was used to identify seismic arrival times of the aftershocks and in turn determine the aftershock locations. In this way, we cataloged 599 events (0< M < 2.5) in the initial 2 hours of this sequence and 4500 aftershocks within the first 2 months, complete to M ≈ 0.0. Here, we study three different temporal/spatial features found in these data. (1) Initially we suspected earthquakes within the region of the mainshock had a bimodal distribution of earthquake magnitudes (peaks at M=0.1 and M=1.5); however, we found this distribution was an artifact of the spatial recording capabilities of small magnitude aftershocks. (2) In the original aftershock locations we found two linear voids in seismicity (trends ∼N45W and ∼N45E) in the primary aftershock cluster forming an X pattern. This is not likely caused by the number of significant digits in the location algorithm because these voids do not follow individual latitude or longitude lines, nor is this likely due to recording inaccuracies because the network coverage of the region is more than optimal. We are investigating other causes of these voids. (3) In the broadband data, we found only one detectable aftershock in the first 2 minutes of the continuous waveforms; yet on the short period records at one of the closest stations, TRO, we can identify an additional event at 15 seconds into the sequence. To quantify our detection capabilities, we estimate when aftershocks of different magnitudes can be identified within the mainshock coda. We are fairly confident that \> M 1.5 events 45 seconds or longer after the mainshock should be detectable, which suggests that the lack of seismicity in the 45 second-2.0 minute range is potentially real. This non-zero lag-time between the mainshock and the first aftershock is inconsistent with the hypothesis that stress changes induced by a mainshock earthquake instantaneously trigger aftershocks.