Ross S. Stein, Ph.D., Temblor, Inc.
Last night, nearly everyone in the greater Bay area was awakened at 10:30 pm by a Magnitude 4.5 shock near the town of Pleasant Hill, north of Walnut Creek. Over 70,000 people from Santa Rosa to Salinas to Sacramento responded to the USGS Did You Feel It? online questionnaire.
Citation: Ross S. Stein (2019), Magnitude 4.5 Pleasant Hill earthquake felt throughout the S.F. Bay area and into the Central Valley, Temblor, http://doi.org/10.32858/temblor.049
Awakened
As we approach the 30th anniversary of the 1989 M 6.9 Loma Prieta earthquake—a shock 25,000 times more powerful than the M 4.5—the Earth seemed to want us to pause and get prepared. Many people associate earthquakes exclusively with the San Andreas Fault and San Francisco, the San Andreas is actually a collection of faults that extend well into the East Bay, where the risk of magnitude-7 sized shocks is exacerbated by pockets of liquefiable soil.
The quake struck where two major faults don’t quite link up A closer look at the quake reveals that it struck within a broad zone of faulting where two major faults, the Concord-Green Valley to the north, and the Calaveras to the south, approach each other but do not join. They are right-lateral faults that typify the San Andreas system, meaning that whatever side you are on, the other side moves to the right. The two faults are offset from each other, causing the formation of valleys and basins that are filled with young, unconsolidated sediments. When the water table is high and these basins are shaken in large earthquakes, the soil can liquify, turning to a kind of quicksand that can cause buildings to sink or tilt, as happened disastrously in the 2010-2011 Canterbury quakes in New Zealand. Christchurch’s entire downtown business district was destroyed even though the quakes were only M~6.3. Pleasant Hill lies in such a zone of liquefaction susceptibility.
Why don’t the quakes strike on the major faults?
The most striking feature of this region is that very few of the earthquakes over the past 20 years have stuck along the major faults (see map below). Most quakes, instead, appear to occur on secondary faults. The good news is that very large quakes on these secondary faults are unlikely because the faults are too small. But the flip side is that these quakes could load the major faults, which are capable of rupturing in magnitude-7 shocks, which they have done during prehistoric times.
There are perhaps three major elements to this Concord-Calaveras fault relay (or ‘echelon’) zone. The crust between the two faults is pulled apart and so subsides; the crust beneath Mt. Diablo is compressed, which has resulted in a thrust fault; and there appears to be a perpendicular (SW-NE) faulting trend that is left-lateral, marked by the light grey pairs of half-arrows in the map. Given enough time, say several million years, and these faults will link up, but in the meantime—our time—they will produce extensive ‘off-fault’ deformation and seismicity.
Magnitude 4.5 appears to have stuck a fault parallel to the Concord
Based on its ‘focal mechanism’ (geometry and sense of slip) and early aftershocks, the Pleasant Hill M 4.5 shock appears to be right-lateral on a fault parallel to the Concord-Green Valley. That fault is probably isolated from the Concord, and so the likelihood that the M 4.5 will trigger a larger quake on the Concord Fault is diminished. The M 4.5 has a swarm-like character that typifies the region. There was a similar swarm near Danville in February 2018. Why swarms are so common here is a mystery. It might indicate that fluids infiltrate the fault zones, or that there is a particularly slippery rock type at depth. Whatever the cause, its effect can rattle residents because the small quakes just keep coming. So, let’s get ready rather than getting rattled.
References
USGS Earthquake Event Page
https://earthquake.usgs.gov/earthquakes/eventpage/nc73291880/executive
California Geological Survey Online Active Fault Map
http://maps.conservation.ca.gov/cgs/fam/