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Analysis of the December 1998 Santa Cruz Mountains, California, Earthquake Sequence

Vicky A. Gallardo^*, Michael L. Begnaud, Jesse Williams, Karen C. McNally, Debra S. Stakes and Gerald W. Simila

Monterey Bay Aquarium Research Institute
7700 Sandholdt Road
Moss Landing, California 95039
(V.A.G., M.L.B, J.W., D.S.)
Earth Science Department and Institute of Tectonics
University of California
1156 High Street
Santa Cruz, California 95064
(K.C.M., V.A.G.)
Department of Geological Sciences
California Statue University
18111 Nordhoff Street
Northridge, California 91330
(G.W.S.)

Manuscript received 24 October 2003.

We studied a sequence of 150 earthquakes (magnitude [M_L, M_D] < 0.8-4.0) that began in December 1998 in an area of complex faulting in the Santa Cruz Mountains, California. Epicenters plot west of the San Andreas Fault, east of the San Gregorio Fault, and just south of the Zayante Fault. Data for systematic hypocenter relocations and focal mechanisms were acquired from temporary deployment of eight IRIS/PASSCAL digital three-component 1-2 Hz seismographs and Northern California Seismic Network data. Our event relocations indicate this sequence is linked to the Zayante fault zone, along a segment previously considered inactive.

In addition to detecting the 51 events listed by the Northern California Earthquake Data Center, 99 uncataloged events, presumably of lesser magnitude, were also recorded by our instruments. All data were processed using a waveform correlation program and shear-wave rotation for high-resolution P- and S-wave arrival times. Relocations were done using the master event method. Relocated hypocenters group tightly within approximately ±1 km about latitude 37° 04.62', longitude 122° 03.26', and range in depth from 2.1 to 8.0 km with a mean depth of 5.1 km. Epicenters plot in a roughly N60-70° W alignment 3.5 km south of the main branch of the mapped Zayante Fault. A plot of hypocenters in 3D also shows a N60-70° W alignment. Aftershocks occurred in rapid succession, with most of the early events within seconds of each other. Spatiotemporal analysis shows a rapid expansion of the aftershock field.

Focal mechanisms are primarily thrust with small components of lateral slip. Average nodal plane strike is N37° W ± 15°, dipping 54° ± 8° southwest (range of 46-65°), and N82° W ± 17°, dipping 47° ± 14° northeast. We interpret the former as representing the fault plane, which is more consistent with the geology, and the resultant right-lateral strike-slip is consistent with the regional stress regime. The projection of the fault-plane solution dip from the hypocenters to the surface, within the range of error, conforms to the mapped trace of the Zayante Fault, and the sense of motion is also in agreement with the down-to-the-northeast mapped offset. Stress orientation indicates that the fault is favorably oriented to accommodate some convergent strain between the San Andreas and San Gregorio fault zones.