Ross S. Stein reports that the mysterious (aka poorly studied) ‘Fontana Trend’ lit up in shallow and widely felt shocks during the past week, putting residents on edge. The swarm lies near the junction of the major Sierra Madre and San Jacinto Faults. About 110 M≥1 earthquakes have struck in the past 10 days along a 5-km-long (3-mi-long) trend between Riverside and Rancho Cucamonga, near Fontana and Glen Avon. The quakes lie only 2-4 km (1.5-2.5 mi) below the ground, and are unusually shallow and so readily felt despite their small size. The largest three shocks during the past month have been just over M=3, and so do no damage. Their focal mechanisms indicate left-lateral faulting (whichever side you are on, the other side moves to the left, as indicated with the grey half-arrows in the map below). The swarm lies just beyond the end of an unnamed fault mapped by the California Geological Survey. The major San Jacinto Fault lies to the east, and the major Sierra Madre Fault lies to the north.
The so-called ‘Fontana Trend’ (Gooding, 2007) of seismicity has persisted for at least 30 years, so far without producing damage. The largest earthquake on the trend in the past 20 years is M=3.8. This might indicate that the trend is caused by slip on a relatively young set of discontinuous fault patches, or that sediments flow into the basin faster than the fault slips, continuously blanketing the fault at the surface. In Upland, about 20 km (13 mi) to the northwest, a M=4.6 shock struck in 1988, and a M=5.2 struck in 1990, on a previously unknown left-lateral fault with a similar trend (Hauksson and Jones, 1991). So, there is no reason to presume that M=3.8 is the upper bound along the Fontana Trend.
While the cause of the swarm is unknown, generally swarms are thought to result from ‘fault creep,’ when faults become unstuck and slide rapidly for some period, perhaps lubricated by the release of fluids. There is an industrial settling pond just to the west of the swarm, and so it’s not impossible that industrial seepage played some role. Dr. Susan Hough at the USGS in Pasadena said to us, “Induced seismicity seems unlikely in this case. I wouldn’t imagine the settling pond affects stress in any significant way, even at shallow depths”. Seismic swarms often precede eruptions, but only rarely precede large earthquakes.
See your risk, is you live in California: https://temblor.net/app/address/5124-cochrane-avenue-oakland-california-94618-united-states
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