Magnitude-4.5 Banning quake struck the San Gorgonio Pass, where the San Gorgonio thrust fault links the right-lateral San Andreas and San Jacinto faults

Jan 6, 2016:  Quake Insights

Today’s Magnitude-4.5 shock probably slipped the San Gorgonio fault Pass fault, which has a longterm slip rate of 1.4-1.5 mm/yr, making it active but not major. In contrast, the section of the San Andreas fault immediately to the east, known as the southern branch of the San Bernardino Mountain section, slips at a rapid 9.4-9.5 mm/yr and ‘dips’ or is inclined 60° to the north. The last great quake on this portion of the San Andreas struck in the late 1600’s, and given the intervening time period during which stress is being added, the fault is now capable of a M~7.8. The 1812 M~7 shock stuck on the San Andreas to the northwest of today’s shock. The San Jacinto fault to the west slips at 10.4-10.5 mm/yr, and so is also major. For these reasons, the seismic hazard rank for the site is very high, at 85.

The USGS reports that earthquake ruptured at 17-18 km (11 mi) depth. Many of the surrounding quakes over the past month (green dots in the map above) lie at almost this depth. In fact, this is the site of deepest earthquakes along the entire 1,000 km (600 mile) span of  the San Andreas. The focal mechanism and earthquake location are most compatible with thrust slip on the San Gorgonio fault. The quake depth and thrust motion are probably related to the 20° counter-clockwise bend of the San Andreas through the pass, which contorts the fault, uplifts the ranges, and breaks the fault into many sections.

Focal mechanism solutions furnished by the USGS and the Caltech/USGS Southern California Seismic Network. Both show largely thrust motion on an east-west striking fault

The San Gorgonio fault is probably capable of a M~7 earthquake on its own, but could link up to the southern branch of the San Bernardino Mountain section of the San Andreas to produce a far larger shock. The recency of fault slip along the San Gorgonio fault is evident from the scarps (in yellow) along the base of the mountains. The mountains have been uplifted in successive earthquakes. The city of Banning lies in the down-thrown basin that has filled with sediments.

Google Earth image centered on today’s M=4.5 epicenter. The orange lines are traces of the active San Gorgonio fault. The width of the photo is about 8 km (5 mi), and San Gorgonio Pass lies 5 km (3 mi) to the east (right). Faults from the USGS and California Geological Survey.

Ross Stein and Volkan Sevilgen, Temblor

Data from USGS, Caltech/USGS Southern California Seismic Network, and California Geological Survey

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