Karen R. Felzer
Annual Meeting, Southern California Earthquake Center, 2005.
On June 12, 2005, a M_W 5.2 earthquake occurred on the San Jacinto fault system near the town of Anza and was felt throughout Southern California. Two subsequent events appeared unusual. The first was an elongated cluster of aftershocks along the San Jacinto Fault zone. The mainshock fault length was on the order of several km, as was the length of the most densely clustered part of the aftershock sequence, but a clear scattering of (mostly small) aftershocks also extended from 30 km to the north to at least 20 km to the south of the mainshock hypocenter. This raised early speculation that aftershocks were being triggered by a creep event along the San Jacinto. A creep event at depth has now been observed, with an estimated moment equivalent to that of an M 5.0 (Agnew and Wyatt, 2005). Whether or not this creep event is unusual, and thus whether it could have created an unusual aftershock sequence, is not possible to say as a lack of instrumentation elsewhere has prevented similar observation. Aseismic afterslip at the surface is routinely observed, and aseismic slip below the surface on both the mainshock and surrounding faults was inferred after the Loma Prieta earthquake from GPS measurements (Segall et al. 2000).
An alternative explanation for the elongated sequence is that aftershock sequences are always this long -- we just usually can't tell because we usually don't record at the completeness provided by the densely instrumented Anza seismic array. To test this hypothesis we plot the Anza aftershock sequence with different lower magnitude cutoffs; only when we include magnitudes below the normal completeness threshold does the sequence appear to be long. We also use the Felzer and Brodsky (2005) relationships to simulate what the aftershock sequence of an M_W 5.2 earthquake should look like with very small magnitudes included. The simulations agree well with observation if we restrict most of the aftershocks to the trend of the San Jacinto fault zone.
The other potentially unusual event was the occurrence of a M_W 4.9 earthquake 4 days after the Anza mainshock, 72 km away. There is less than a 2% probability that this earthquake was random chance; over the past five years (2000-2005) there have been only 8 M > 4.9 earthquakes in Southern California. Is it plausible that a M_W 5.2 earthquake could have triggered another earthquake over 70 km away? Static stress changes are negligible at such distances, but dynamic stress changes are not. Using M 5-6 mainshocks from throughout California we demonstrate that triggering can occur out to several hundred kilometers at high statistical significance, corroborating statistics by Ebel and Kafka (2002). On average, at distances over 50 km, within a time period of 7 days, we expect an M 5.2 earthquake to trigger a M > 4 earthquake about 10% of the time and to trigger a M > 5 earthquake about 1% of the time.