The San Miguel fault system strikes in the northwest direction within the Peninsular Ranges of Baja California. The study concentrates primarily on the southern sector of the San Miguel fault (SSM) which is about 20 km long and had a large rupture in 1956.
There is a high rate of seismicity along the SSM. In a time period of 23 days, 109 events were located, with duration magnitudes between 1.4 and 2.8. Focal depths are between 5 and 15 km. Epicentral locations show three clusters of seismicity. Two clusters are on the SSM. Another cluster is closer to a normal fault that strikes parallel 8 km to the southwest of SSM. The seismicity is stationary, with an average of 20 events per day. A “b” value of 0.92 ± 0.02 is characteristic for the background seismicity. A larger “b” value of 1.4 ± 0.07 is associated with the cluster at the northwest end of SSM. Those inconsistent “b” values suggest a heterogeneous state of stress.
Most fault plane solutions in the Northern Peninsular Ranges of Baja California indicate strike-slip motions (one normal mechanism is also observed). The greatest principal stress is oriented approximately north-south.