Earthquake Research

A recent Nature-Geosciences study:
Deep creep as a cause for the excess seismicity along the San Jacinto fault

Since 1890, the San Jacinto fault in Southern California has been the site of eleven earthquakes of moderate magnitude (6 < M < 7) and tens of thousands of small earthquakes, but none of large magnitude. This activity contrasts sharply with the seismic quiescence of the nearby southern San Andreas fault. Although this fault slips at a rate higher than that associated with the San Jacinto fault—23–27 mm/yr versus 12–22 mm/yr - it has produced very few earthquakes and no moderate or larger events in historical times. Here I use recent seismic and geodetic data to reveal that at depths of 10–17 km within the seismogenic (brittle) crust, the San Jacinto fault is creeping and releasing elastic strain by many small earthquakes. As a result, the accumulation of strain along this fault occurs mostly in its upper 10 km; moderate earthquakes are likely to be sufficient to release such strain. In contrast, the southern San Andreas fault accumulates elastic strain throughout its vertical extent in the seismogenic crust, which will most probably be released by stronger earthquakes.

Figure 1:  Seismicity and geodetically observed crustal movements along the southern SAFS.  Colour dots mark relocated seismic events during the period 1981-20066. Blue dot marks ongoing seismic activity, yellow dots mark three-month long aftershock clusters following the 1987 Elmore Ranch (ER) and Superstition Hill (SH) earthquakes, and green dots mark aftershock clusters following the 1986 Palm Spring (PS), 1992 Joshua Tree (JT), 1992 Landers (L), and 1999 Hector Mine (HM) earthquakes. Red stars mark the location of moderate and strong events (M > 6) known from historic documents and seismic record for the period 1800-2005. Magenta arrows mark a subset of geodetically observed crustal movements across the SSAF and SJF calculated with respect to stable North America. Thin black lines mark the location of major fault segments and thick lines mark the geographic locations of the Northern, Central, and Southern segments of the SJF shown in Figure 2.

Figure 2: Seismic activity along the SJF based on the relocated earthquake catalog of6. (a) Hypocenter location along the northern segment. Orange line marks the locking depth5, which is determined from geodesy. (b) Hypocenter along the central segment. Dash line marks the base of the seismogenic crust, which is tilted northwestward. “A” - Anza Gap. (c) Hypocenter location along the southern segment. (d, e, f) Seismic activity as function of time showing that seismicity occurs regularly along the northern and central segments. The seismicity along the southern segment had a spike in activity following the 1987 Superstition Hill earthquake. Nevertheless, ongoing seismicity occurs mainly along the base of the seismogenic layer at depth of 5-10 km. (g,h, i) Earthquake distribution with depth showing peak activity near the base of the seismogenic layer.

Figure 3: Block diagram showing interseismic strain accumulation mechanisms along the SAF system. Both the SSAF and SJF accumulate elastic strain due to steady slip in the ductile crust, beneath the faults. The SSAF is locked throughout the seismogenic crust and does not generate interseismic earthquakes. The SJF is locked only in the upper 10 km and creep seismically and aseismically above the base of the seismogenic crust, which is tilted northwestward, at depth of 10-17 km. Blue stars represent schematic hypocenter locations. Blue and green dots on the shaded relief plot mark ongoing seismicity and aftershock clusters, respectively, in southern California.

More details at Wdowinski (2009)