Summary of recent events in the region

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The southern California region has generated nearly 50 magnitude 6 or greater earthquakes since 1850 (Ellsworth, 1990). Sixty percent of these moderate to large earthquakes are associated with the San Andreas and San Jacinto fault systems and their continuations into Baja California. It is interesting to note that only seven of these events have significant surface rupture. These events include the 1857 Fort Tejon (Mw = 7.8) along the Cholame and Mojave segments of the San Andreas, the 1940 (Mw = 6.9) and 1979 (Mw = 6.4) on the Imperial Fault, the 1968 (Mw = 6.5) Borrego Mountain and 1987 (Mw = 6.5) Superstition Hills located on the southern San Jacinto fault, and the 1952 (Mw = 7.5) Kern County and the 1992 (Mw = 7.8) Landers which are not directly associated with the San Andreas-San Jacinto fault system. These historical surface ruptures are shown in this figure which also highlights the two major sections without significant surface offsets: the San Bernardino and Coachella Valley segments of the San Andreas fault and the San Bernardino, San Jacinto Valley, Anza, and the Coyote Creek segments of the San Jacinto fault.

The San Jacinto fault zone is one of the most active strike-slip faults in southern California. The long-term slip rate is 1 cm/year, determined from 29 kilometers of offset of geologic formations across the fault in the last 3 million years (Sharp, 1967). Recent measurements of offset sediments in the Anza Valley yield a similar slip rate (Rockwell, et al. 1990). The Anza segment of the San Jacinto fault zone has been identified by Thatcher et al. (1975) as a seismic slip gap for a 6 M 7 earthquake. The study of Sanders and Kanamori (1984) revealed a 15 km element of the estimated seismic gap that has been includely aseismic in modern times. Klinger and Rockwell (1989) trenched the San Jacinto Fault at Hog Lake located in the center of the Anza seismic gap and found evidence for surface rupture from three events since 1210. Additional evidence suggests that these events occurred about 1210, 1530, and 1750.

In 1988, the Working Group on California Earthquake Probabilities (USGS Open File Report 88-398) defined the Anza segment to be the 50 km section between the southern end of the inferred 1899 M=6.4 (Abe, 1988), 1918 M=6.8 (Ellsworth, 1990) rupture just north of Anza and the north end ofthe 1968 Borrego Mountain M=6.8 surface rupture (see Anza seismic slip gap map). They used a slip rate of 11 mm/yr, a recurrence interval of 142 years, and assumed the previous event in this segment was 1892. Based on this information a probability of 0.3 was assigned for a magnitude 7 earthquake in the Anza area in the next 30 years.

Recently, the Southern California Earthquake Center presented its Phase II report which reassesses the results of the 1988 report. Using the results of Klinger and Rockwell (1989) and Rockwell et al. (1990), the Anza segment of the San Jacinto fault zone is considered by the Working Group on California Earthquake Probabilities [1995] to be the entire 90 km long Clark fault with an average repeat time for a magnitude 7.0 to 7.5 to be 250 (+321, -145) years. Because the dimension of the segment increased, the characteristic slip is now 3.0 m (slip map).

The most significant recent information to be developed for the seismic potential of the Anza segment is the 1750 date for the last major earthquake. Using the 142 year recurrence interval of the 1988 report a magnitude 7 earthquake is now 100 years overdue. If one prefers the Phase II report then the characteristic earthquake can be a magnitude 7.5 with the peak in the conditional probability distribution in the year 2000. In either scenario, the characteristic earthquake can generate significant damage in the major population areas of the Los Angeles basin (90-150 km distant), San Diego (90 km), and the San Bernardino Valley (60 km) (map of southern California). In similar situations, significant damage was caused in San Francisco at 120 km distance by the magnitude 6.9 1989 Loma Prieta and in various parts of the Los Angeles basin by the magnitude 6.7 1994 Northridge earthquake over 100 km from the source.


URL: [Last updated: 2015-10-22 (295) 23:07:44 UTC]