SMALL-SCALE STRESS HETEROGENEITY IN THE ANZA SEISMIC GAP, SOUTHERN CALIFORNIA

Focal mechanism inversions reveal significant lateral variations in st ress orientations along the Anza segment of the San Jacinto fault zone . The most notable stress anomaly is within the 20-km aseismic (seismi c gap) portion of the fault zone, where sigma1, the maximum compressiv e stress, is nearly horizontal and is oriented at 74-degrees +/- 13-de grees relative to the fault strike. This contrasts with orientations r anging from 62-degrees +/- 11-degrees to 49-degrees +/- 7-degrees alon g the more seismically active portions of the fault zone immediately t o the northwest and southeast of the seismic gap. Regional stress resu lts, found by inverting all focal mechanisms simultaneously, indicate that sigma1 is horizontal and trends north-south, while sigma3 is hori zontal and trends east-west. Approximately, 15 km west of the seismic gap, in the off-fault Cahuilla swarm area, sigma1 and sigma3 solutions are rotated clockwise by about 25-degrees relative to the regional mo del. Roughly, 10 km southeast of the seismic gap near the Buck Ridge f ault, sigma1 and sigma3 are rotated counterclockwise by about 10-degre es relative to the regional solution. Northwest of the seismic gap alo ng the fault zone, sigma3 plunges about 30-degrees from the horizontal , correlating with a local increase in reverse faulting between the Ho t Springs and San Jacinto faults. Southeast of the seismic gap, sigma1 plunges about 45-degrees from the horizontal, correlating with a loca l increase in normal faulting in the trifurcation region of the Buck R idge, Clark, and Coyote Creek faults. We propose a simple mechanical m odel in which a block rotation superimposed on the dominant right-late ral strike-slip motion of the fault zone satisfies the first-order obs ervations of stress orientation, faulting, and horizontal surface stra in. Under this model the Anza seismic gap is the region of zero conver gence between the northeast and southwest sides of the fault, and the fault zone strength within the seismic gap is either comparable to or exceeds the fault zone strength adjacent to the gap.