San Jacinto Fault Zone - Characteristics - Segments

Segments

A 1995 report by the Working Group on California Earthquake Probabilities identified seven individual segments of the SJFZ. The group consisted of more than three dozen seismologists, including Keiiti Aki and C. Allin Cornell, and was organized by the Southern California Earthquake Center for the USGS and the California Office of Emergency Services. The 1995 paper was the third in a series of reports that was set in motion following the 1992 Landers earthquake in southern California with the intention of updating the data and the approach for calculating the probabilities for large earthquakes along the southern San Andreas and San Jacinto Fault zones. Both these fault zones were grouped together as having adequate paleoseismic data to assign conditional probabilities for future damaging earthquakes.

The original Working Group in 1988 had identified five segments of the fault zone. From north to south, the segments were labeled the San Bernardino Valley, San Jacinto Valley, Anza, Borrego Mountain, and Superstition Hills. The 1995 group then added the Coyote Creek and Superstition Mountain segments, defined the Anza segment to include the Clark and Casa Loma faults, and updated the slip rates for each segment. The three northern sections (San Bernardino, San Jacinto, and Anza) were assigned 12 mm per year of slip and the four remaining sections were given 4 mm of slip, and error rates were half the total estimated slip for each segment (±6 mm and ±2 mm respectively) with the exception of the Anza segment which had slightly exaggerated rates of +7 mm and -5 mm.

Thirty year probabilities for segment-rupturing earthquakes were estimated using three separate models then a preferred weighted result was presented for each segment. While the San Bernardino (37%) and San Jacinto (43%) segments both saw large increases since the 1988 report, due in part to increased estimates for slip rates and decreased estimates for inherent displacement, the Anza segment (17%) was determined to have a decreased probability, based on an increased segment length. The Coyote Creek (18%), Superstition Mountain (9%), and Superstition Hills (2%) segments received first time estimates (none were assigned in 1988) and the Borrego Mountain segment received a more specific value of 6%.