Fault strength is a fundamental property of seismogenic zones, and its temporal changes can increase or decrease the likelihood of failure and the ultimate triggering of seismic events. While changes in fault strength have been suggested to explain various phenomena, such as the remote triggering of seismicity, there has been, to our knowledge, no means of actually monitoring this important property in situ. Here we argue that 20 years of observation (1987-2008) of the Parkfield area at the San Andreas Fault have revealed a means of monitoring fault strength. We have identified a long-term change in fault strength most likely induced by the 2004 9.1 Sumatra-Andaman earthquake (SM04) (Taira et al., 2009). The change possessed two manifestations: temporal variations in the properties of seismic scatterers - likely reflecting the stress-induced migration of fluids - and systematic temporal variations in the characteristics of repeating-earthquake sequences that are most consistent with changes in fault strength.
There are three excursions in (). The excursion is coincident with the 1993 Parkfield Aseismic Transient (PAT93). It initiated around 1993 (Figure 2.17b), peaked in the mid 1990s and slowly decayed over about a subsequent 7-year period. The excursion (September 2004) is associated with the 2004 Parkfield earthquake (PK04) (Figure 2.17c) and decayed back to the pre-earthquake level after about 2-3 months. The excursion in () occurred about three months after the 2004 Parkfield earthquake (Figure 2.17c). The magnitude of change in () is comparable to that observed for the other two transients. The increase in () takes place over three months, after which time () decays slowly over a subsequent 1 year period (Figure 2.17c).
There are also changes in repeating-earthquake properties that accompany this excursion in (). Following the 2004 Parkfield earthquake, there is a characteristic increase in recurrence interval as is typically observed postseismically. This trend, however, is interrupted roughly three months after the 2004 Parkfield earthquake. In 6 of the available 13 sequences, there is a systematic reduction in that reaches a minimum about 6 months after the 2004 Parkfield earthquake. To explore this apparent disruption further, we removed the post-seismic effect by the 2004 Parkfield earthquake, assuming Omori's law, ()= where is time after the earthquake and and are constants to be estimated, and computed the residual recurrence interval ()=()/().
Using all available sequences, we find that the log of the residuals increases by roughly a factor of 2 beginning three months after the 2004 Parkfield earthquake. This increased variability suggests an additional perturbation to and a temporal change in the mechanical properties of the fault. An interesting feature of this variability is that there is a positive correlation between and seismic moment (Figure 2.17d). Such a correlation is consistent with a slip-predictable model for earthquake occurrence (Shimazaki and Nakata, 1980), where the stress drop and are both determined by the failure strength of the fault for constant loading rate and constant minimum stress. Assuming rupture area is constant among members of a repeating-earthquake sequence (Nadeau and Johnson, 1998), then fault slip will be proportional to which should in turn be proportional to stress drop. The most dramatic correlation that we found is for sequence K3 (5 km depth) (Figure 2.17d). This sequence also has the least variability in event location, so that the assumption of constant fault area should be the most valid. A reduction in (and ) around 6 months after the 2004 Parkfield earthquake indicates a temporary weakening of the fault.
We have localized the onset time of the excursion in (), utilizing all of the repeating-earthquake sequences that displayed a change in (). We find that the excursion must have initiated between December 21 (sequence K2) and December 26, 2004 (sequence K5) (Figure 2.17e). The most dramatic tectonic event to occur within the 5-day time window is the 26 December 2004 9.1 Sumatra-Andaman earthquake, whose origin time is 7 hours before the end of the interval. The timing strongly suggests that the dynamic stresses from this earthquake, estimated to be about 10 kPa [based on the amplitude of long period surface waves (30 s) which are likely to have the strongest impact on fluid flow], induced fluid flow that caused both a structural change in the fault zone region [i.e. changes in ()] and, through variations in pore pressure, consequent changes in the strength of the fault. It is now well documented that such dynamic stresses are capable of remotely triggering seismicity. The present study suggests that these same dynamic stresses can actually produce long-term (1 year for our study) changes in fault strength.
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Taira, T., P.G. Silver, F. Niu, and R.M. Nadeau, Detecting seismogenic stress evolution and constraining fault zone rheology in the San Andreas Fault following the 2004 Parkfield earthquake, J. Geophys. Res., 113, B03303, doi:10.1029/2007JB005151, 2008.
Taira, T., P.G. Silver, F. Niu, and R.M. Nadeau, Remote triggering of fault-strength changes on the San Andreas Fault at Parkfield, Nature, doi:10.1038/nature08395, 2009 (accepted for publication).
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