The LB3D can be used to analyze the seismicty in the Long Beach area. The Southern California Seismic Network (SCSN) has approximately 10 broadband stations in the region including one within the LB3D itself. As shown in Figure 1, the SCSN located 11 events in the Long Beach area during the first 6 months of 2011. All events were between 2 and 2.5 in magnitude.
We applied the STA/LTA picker of Nippress and Reitbrock to each station in the LB3D, and generated over 28 million picks. These were then associated into events using the code by Nippress and Reitbrock. A simple 1D velocity model, estimated from a borehole just north of the LB3D was used. A total of 2% of the picks were associated into 20,716 events. We have no estimate of magnitudes for these events, but they are all likely very small (not felt).
It is clear that a number of these events are artifical (note
for example the stripe of events near the bottom of events shown
in Figure 1b). It is likely that a number of the events are man-made.
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| Figure 1. Seismicity in Long Beach. The left panel (Fig 1a) shows the seismicity located by the SCSN during the first 6 month of 2011. The green dots are the SCSN broadband stations. In all there are over 20,000 events in the right panel. |
One of the interesting aspects of the seismicity is its time variations. The top panels in Figure 2 show the seismicity on 3 different days. The right and center show the variation in number of events that can occur. The right panel shows some seismicity aligns along the Newport-Inglewood Fault.
The lower two panels show the seismicty variation with hour of day and day of week. The strong correlation of these patterns with the rise and fall of cultural noise (i.e. rush hours during the work week). One possible explanation for this is that the increase of noise de-sensitizes the network to earthquake activity, but the explanation is not yet known.
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| The top panels show the seismicity on 3 different days. The bottom panels show the variation in seismicity with time of day, and day of the week. |