Quasi-static slip-rate shielding by locked and creeping zones as an explanation for small repeating earthquakes at Parkfield

In a recent study of microearthquakes along the Parkfield segment of the San Andreas fault, Nadeau Et al. (1995) have found that much of the seismicity in the region is characterized by quasi-periodic repeating sequences of small earthquakes that are essentially identical in waveform, size and, location. Nadeau and Johnson (1998) interpreted these as repeated slip on a given asperity driven by a steady slip rate of 2.3 cm/yr and concluded that the stress drops needed to be extremely high, of the order of 20 kilobars. We propose another explanation for these small repeating events, namely that an inner asperity is surrounded by a larger creeping zone, which in turn is surrounded by a still larger locked zone. This geometry produces a local slip velocity much less than the overall creep velocity observed on a still larger scale (slip velocity shielding). We have constructed a foam rubber model to illustrate the phenomenon. The time sequences of small events at the asperity, punctuated by large events which rupture the whole block, look very similar to the cumulative moment plots of Nadeau and Johnson. The actual dynamic stress drops are of the same order as for the large events. Thus the results of the model correspond to the observations of Nadeau and Johnson and suggest that the model may be appropriate to explain their observations, without requiring super strong asperities.