The ubiquity of slow earthquakes (SEQs) on faults (both dip-slip and strike-slip), suggests that they are a fundamental mode of strain release. The fact that they characteristically occur in the transition region between steadily slipping and locked faults may allow us to draw general conclusions about the mechanism by which faults transition between locked and creeping. To date, the San Andreas Fault near San Juan Bautista is the only location where slow earthquakes have occured on an accessible strike-slip fault. The slip in these events was much closer to the surface than in the typical subduction zone events, making it a unique and potentially very effective location to study the mechanics of slow slip.
The 1998 5.0 slow earthquake was immediately preceeded by the 5.1 San Juan Bautista earthquake (Uhrhammer et al., 1999) (Large blue circle in Fig. 28.1). The two events were located in the same region, with the slow earthquake rupturing the portion above 5km (Gwyther et al., 2000). The earthquake and SEQ ruptures were of comparable size and resulted in similar amounts of slip. An interferogram spanning from Aug. 18 1997 to Oct. 12 1998 contains adequate coherent data along the central San Andreas to observe near fault movement (Fig 28.2).
Fig. 28.3 compares surface creep measurments made using InSAR to those from creepmeters. Red points were obtained by averaging phase values every 350 meters along strike and 50 meters away from each side of the fault. Pairs of average phase measurements were differenced and converted to San Andreas parallel strike-slip movement. Error bars are the standard deviations of values in each bin. Though the InSAR and creepmeter data match well, the InSAR data suggest that fault movement is much more variable than would be inferred from creepmeters alone. However, the variability may be the result of a poorly defined fault trace. Figure 28.3 also makes it apparent that the contribution of the slow earthquake to surface slip during the time spanned by the interferogram is small; the difference between the blue and green triangles. The contribution of interseismic creep must therefore be accounted for before slip is attributed to the SEQ. The variability in the InSAR data makes it a requirement that we use interseismic creep rates with similar spatial sampling. We are working on creating interferogram stacks using a patch-work method that will allow us to define average creep rates along the San Andreas.
InSAR is capable of measuring small scale movement such as creep. The interferogram shown here suggests that creep along the Central San Andreas varies considerably along strike. Future modelling of the fault system will benefit from the dense spatial sampling of InSAR. However, the contribution of the 1998 San Juan Bautista slow earthquake to the total range change is very subtle and will require careful removal of the interseismic signal.
Gwyther, R. L., C. H. Thurber, M. T. Gladwin, and M. Mee, Seismic and aseismic observations of the 12th August 1998 San Juan Bautista, California M5.3 earthquake,3rh San Andreas Fault Conference, Stanford University, 209-213, 2000
Johanson, I. A. and R. Bürgmann, An investigation of slow earthquakes on the San Andreas Fault using InSAR,EOS Transactions, AGU, 83(47), 357, 2002
Uhrhammer, R., L. S. Gee, M. H. Murray, D. Dreger and B. Romanowicz, The 5.1 San Juan Bautista, California earthquake of 12 August 1998, Seismological Research Letters, 70(1), 1999.
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