SEISMIC AND ASEISMIC OBSERVATIONS OF THE 12TH AUGUST 1998 SAN JUAN BAUTISTA, CALIFORNIA M 5.3 EARTHQUAKE Ross L. Gwyther*, Clifford H. Thurber**, Michael T. Gladwin* and Marie Mee* * C.S.I.R.O. Exp. & Mining, Australia ** Dept. Geology & Geophysics, Uni. of Wisconsin-Madison. ABSTRACT A slow earthquake with a characteristic period of 7 days was observed near San Juan Bautista, California on August 12, 1998 on a nearby borehole tensor strainmeter. This slow slip event, with a magnitude of 5.0 was initiated by a seismic event with magnitude of 5.2 and typical aftershock sequence extending over 8 days. The slip region we model as causing the slow earthquake is immediately adjacent to, and directly above, the aftershock zone delineating the seismic failure surface. These combined strain and seismic observations provide direct evidence of the linkage between slow earthquakes and associated seismicity. The slow earthquake in this region is the third in 6 years, and the failure surfaces for these three slow events, in December 1992, April 1996 and August 1998 are both separate and adjacent. Together the three slow earthquakes have relieved a 25 km region of the fault surface in the transition zone of the San Andreas Fault. INTRODUCTION The San Juan Bautista region is a critical transition zone between the locked and creeping sections of the San Andreas Fault in northern California (Figure 1). Measurements of fault processes in this region are providing insight into stress transfer between the two sections of the fault, and earthquake preparation, initiation, and propagation processes in the region. A small borehole strain array consisting of a Sacks-Evertson Dilatometer at Searle Road (operated by USGS personnel) and a Gladwin Borehole Tensor Strainmeter (BTSM) near San Juan Bautista was installed in 1982-83 and operated continually since that time. The joint 15 year dataset has provided some critical observations of slow earthquakes and long period strain anomalies extending over at least a 5km baseline. Unfortunately, the Searle Road dilatometer failed irreparably in 1998. The strain array lies within a relatively dense section of the U.S. Geological Survey Northern California Seismic Network. We present a combined analysis of data from the two networks to gain insight into the relation between seismic and aseismic processes in this area. DATA OBSERVATIONS On August 12 1998 a magnitude 5.3 earthquake occurred 10km southeast of San Juan Bautista. Relocation of the foreshock and aftershock sequence using precise relative arrival times (Dodge et al., 1995, Rubin et al., 1999) indicates that the two principal foreshocks (M 2.8 and 3.1) occurred within 20 of meters of the main shock (M 5.3), and that the larger aftershocks cluster in a relatively narrow depth range (8 - 10 km). The aftershock sequence defines a region of the fault approximately 6 km by 8 km at a depth of 5 km, which failed seismically during the event (see Figure 3). Historical microearthquakes in this section of the fault have been reolcated accurately and are also included in this figure as small open circles (Rubin, 1999, pers. comm). 3rd San Andreas Fault Conference, pp. 209-213, Stanford University, 2000