Four similar earthquakes in central California

We study four ML ‐ 2.7 earthquakes on the San Andreas Fault in Central California. The first two events occurred within five minutes of each other in November 1978; the two events in January 1979 occurred within a nine hour period. The CALNET (USGS local array) seismograms of these four events display only some general similarity. However, when low ‐ pass filtered below 5 Hz, the four events have nearly identical seismograms. The similarity is even more striking in the pass‐band below 2 Hz. This suggests that all four events are within a radius of no more than a quarter wavelength, or about 200 ‐ 400 m. Although no definitive conclusion can be reached from a study of only four earthquakes, our results strongly suggest that the following hypothesis should be further tested: Small to moderate earthquakes may commonly be much more tightly clustered in both hypocentral location and depth than is suggested by routine locations from local arrays. The physical basis of this clustering is that the earthquakes represent repeated stress release at the same asperity, or stress concentration, along the fault surface. Identification of such asperities might be useful in understanding the sequence of events leading to the initiation of a larger earthquake.