Could a great quake strike on the enigmatic San Jacinto fault?

Today’s magnitude-3.5 Moreno Valley, CA, quake near the San Jacinto fault begs the question

The 13.5 km (8 mi) deep right-lateral strike-slip earthquake was widely, though weakly, felt in the cities of Riverside, Redlands, San Bernardino, Hemet and Corona. The event struck on or close to the San Jacinto fault, with a slip rate of about 12 mm/yr (0.5 in/yr) is one of California’s most active faults, and so the seismic hazard rank at this location is high. The same or adjacent section of the fault experienced a M=6.8 shock in 1918. Since that time, the section has accumulated a ‘slip deficit’ of about 1.2 m (4 ft), and so another M~6.8 shock is certainly possible in the near future. But what about larger shocks?

The San Jacinto fault extends southeast from the San Andreas fault, making it difficult to distinguish the slip rate of the San Jacinto from the adjacent section of the San Andreas. But the most recent geologic and geodetic evidence suggests that the San Andreas probably slips at almost twice the rate, at about 21 mm/yr, and so has a doubled rate of moderate to large shocks.

During the past 120 years of historical and instrumental records, no earthquake larger than magnitude-6.8 has struck along any part of the San Jacinto fault, raising the possibility that its maximum magnitude could be capped at about M~7 despite its 135 mi (200 km) length, which would otherwise make magnitude of M~7.5 possible. Bear in mind that this difference is important: A M=7.5 shock is twenty times larger than the M=6.8 in 1918, and a million times larger than today’s M=3.5 quake.

But the San Jacinto is unique in its ‘en echelon’ character. Rather than a continuous fault like most of the San Andreas, the San Jacinto is broken up into a series of ~30 km (20 mi) long sections offset from each other (referred to as ‘echelons’) by as much as 5 km (3 mi). These offsets, if they extend to depth, may prevent—or at least inhibit—a rupture from jumping from one section to another, which would be needed to create a M>7 rupture. Today’s quake struck at the northernmost end of one of these echelons.

While one might dismiss the possibility of a great rupture, in 1992 a series of echelon faults ruptured together to create the powerful M=7.3 Landers, CA, earthquake. Humbled by this surprise, one should not assume that the past century of seismic behavior will represent the next. Recent work by Salisbury et al. (2012) suggests that prehistoric earthquakes along the fault produced slip of up to 4 m (13 ft), which would require M~7.5 earthquakes. They also uncovered a poorly located large earthquake occurred in southern California on 22 November 1800 that might have been a M>7 shock on the San Jacinto fault.

Ross Stein and Volkan Sevilgen, Temblor

Data from USGS, Caltech/Southern California Seismic Network; and  J. B. Salisbury, T. K. Rockwell, T. J. Middleton, and K. W. Hudnut, ‘LiDAR and Field Observations of Slip Distribution for the Most Recent Surface Ruptures along the Central San Jacinto Fault’, Bull. Seismol. Soc. Amer., 102, doi: 10.1785/0120110068 (2012); and Thomas Rockwell, Christopher Loughman, and Paul Merifield, ‘Late Quaternary rate of slip along the San Jacinto Fault Zone near Anza, southern California,’ J. Geophys. Res., doi: 10.1029/JB095iB06p08593 (1990).

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