Earthquake Swarm near Pinnacles National Monument in California

Source: U.S. Geological Survey (http://earthquake.usgs.gov/earthquakes/recenteqscanv/FaultMaps/121-37.html)

An earthquake swarm is affecting the San Andreas fault system in the Pinnacles National Monument region of the Central California Coast Ranges. The biggest event thus far is a magnitude 4.6 at a depth of 7.6 km, with other significant quakes measuring 3.2, 3.2, 3.3, 3.6, and 3.3. There have been nearly 100 instrumentally recorded quakes between magnitude 1 and 3. The quakes are taking place in a complex region of the fault system where the Calaveras, Hayward and San Andreas faults merge into a single fault. This area is notable because the San Andreas fault is creeping, rather than sticking, meaning that the fault moves constantly (a few millimeters a year) instead of storing up stress and producing monster quakes like it does both to the north and to the south.

Road damage from fault creep on the San Andreas fault near the earthquake epicenter. This is a 2010 picture; the damage was NOT caused by the earthquakes today

This is not an unusual pattern; flurries of similar earthquakes have occurred in the Pinnacles area in 1951, 1972, 1982, 1995, and 2004. If the pattern holds true, the quakes will decline in number and size over the next few days. There is always a small chance that the activity (including the 3.9 quake in San Leandro earlier this week) is a precursor to a much larger earthquake, such as the magnitude 7 event that took place on the Hayward fault in 1868, which killed two dozen people. A recurrence of the 1868 earthquake would cause serious damage and destruction in the East Bay region.

Events like these are reminders that we Californians are living in earthquake country, and that we need to be prepared for larger catastrophic seismic events. It is a virtual certaintly that the state will be rocked by magnitude 7+ events in the next few decades, and some of these earthquakes will affect urban regions. One should always have emergency supplies available, and a plan for what to do in the event of a major earthquake.

Shutter ridge on the San Andreas fault near the earthquake swarm epicenter. The fault follows the fence line.