The digital array at Anza, California: Processing and initial interpretation of source parameters

Seismicity along the San Jacinto fault suggests that a 40-km-long section near the town of Anza may constitute a gap in the occurrence of ML = 6–7 earthquakes. The potential for a gap-filling shock and the high rate of seismicity at the southern end of the gap (five events of 4.0 ≤ ML ≤ 5.5 since 1970) provided the impetus for deploying a digital seismic array to collect high-quality ground motion recordings of all events 2 ≤ ML ≤ 4.5 (on-scale recording for shocks with magnitudes above ML = 4 can be obtained from an existing array of eight strong motion accelerographs). The Anza site also had the advantage of being in the southern California batholith, which appears to be relatively homogeneous compared to the Franciscan/Gablian contrast of the central San Andreas; we expected that the granitic rocks of the batholith would yield relatively accurate earthquake locations and efficiently propagate high frequencies. The field instrumentation is specifically designed for broadband recording (up to 70 Hz) and high dynamic range (96 dB in the on-site digitizer alone), since both are necessary for determining the rupture history of earthquakes. Both local VHF and microwave digital telemetry transmit the data from the Anza region to San Diego for computer data logging. In the first 30 months of the array's operation, approximately 292 events have been recorded, located, and processed for source parameters. Most events occur in one of five clusters or in a diffuse zone near the Buck Ridge fault. Two of these clusters are located at right-stepping en echelon offsets (Coyote Creek-San Jacinto and San Jacinto-Hot Springs); two others are directly below and about 8 km west of Anza, respectively. The fifth cluster is just to the northeast of the Hot Springs fault. Although event depths are generally between 11 and 14 km, at the southern end of the Hot Springs fault, depths extend to 18 km; these are some of the deepest strike-slip earthquakes on the San Andreas system. We calculate source parameters such as the scalar moment and stress drop for the analysis of high-frequency waves, scaling relations, and earthquake interaction. The largest event recorded thus far had a moment of 4.4 × 1021 dyn cm (M = 3.8) and a stress drop of 55 bars. Both arms and Brune stress drops increase with moment; source radius increases only slowly with moment. The maximum values of both the Brune and arms stress drops increase with depth down to 10 km, remain approximately constant to 14 km, and may decrease below 14 km. The data suggest that stress drops of a group of earthquakes can be related to the strength of the upper crust calculated from frictional and quasi-plastic flow laws, although individual events may not be related. Stress drops on the San Jacinto fault are high compared to those on the central creeping section of the San Andreas, where stress drops are about 10 bars or less. This observation is consistent with the relative rupture area of events of equal ML (5.5) and may be related to the lithologie differences between the two fault segments and the amount of normal stress compared to shear stress.