Abstract

On 1 October 1969, two damaging earthquakes of Richter magnitude (ML) 5.6 and 5.7 occurred near the city of Santa Rosa followed by at least 200 aftershocks. These are the two largest events to occur in the northern San Francisco Bay area since the great 1906 San Francisco earthquake and its aftershocks. The epicentral area coincides with a 1-km right step-over from the northwest-striking Rodgers Creek fault to the Healdsburg fault zone and a 6-km right step-over to the Maacama fault. The 1969 sequence and its relationship to the Healdsburg and Rodgers Creek faults has taken on greater significance because of the potential for a moment magnitude (Mw) 7 earthquake occurring on the latter, possibly in the next several decades. We have relocated the five largest events in the 1969 sequence using the largest aftershock recorded by a temporary network operated by the U.S. Geological Survey (USGS) as the master event. The locations and focal mechanisms of the principal events, in addition to many of the aftershocks, are consistent with right-lateral strike-slip faulting on the northwest-striking Healdsburg fault. An interesting feature of the aftershocks is that they exhibit a cruciform pattern suggesting possible activity on a northeast-striking cross-fault located within the step-over. The significance of such a cross structure is that it may act as a barrier to northward rupture from a large earthquake on the Rodgers Creek fault. Seismicity since 1969 continues to image an active southern Healdsburg fault surrounding a quiescent rupture zone of the 1969 earthquakes and the possible cross-fault. The Rodgers Creek fault is generally aseismic during this period.

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