Two M=1.5 quakes on December 28 in Concord appear to be part of a subtle increase in seismicity rate since the August 2014 M=6.0 Napa mainshock struck. Even without this increase, the seismic hazard rank in Concord is quite high—85. This means that compared to all other populated sites in the U.S., Concord ranks in the 85 percentile. So, only 15% of sites have a higher probability of strong earthquake shaking. This is due chiefly to its proximity to the active Concord-Green Valley fault, which slices through Concord, and lies just 2 miles east of Pleasant Hill and 5 miles east Benicia (thick red line below). Further, Pleasant Hill, and to a lesser extent, Concord, lie in liquefiable sedimentary basins, which can cause buildings to sink or tilt permanently during earthquake shaking (purple shades below).
Shinji Toda and I published a paper this fall in Seismological Research Letters which argues that the Napa earthquake brought parts of the Concord-Green Valley fault closer to failure. This calculation is partly supported by observed increases in the seismicity rate in the two months after the M=6.0 event compared to the 5 years beforehand (yellow-red splotches below). The rate increase in Concord was subtly visible a year ago, and seems to be sustained. The Green Valley fault slips at 6±1 mm/yr, about half of which occurs as creep. Trenching across the fault suggests a roughly 200-yr recurrence interval for M~7 shocks on the fault, and ~400 yr has elapsed time since the last large earthquake). The fault poses a substantial risk, since it bisects the city of Concord, with 200,000 residents.
Earthquakes near the city of Concord, and the Concord-Green Valley fault as a whole, should be carefully watched in the near future.
Ross Stein, Temblor CEO
Data from USGS, Berkeley Digital Seismic Network, California Geological Survey, Toda & Stein (Seismol. Res. Letts., 2015), and Lienkaemper et al (Bull. Seismol. Soc. Amer., 2014).