The well-worn phrase "it could be tomorrow, it could be years from now" is about the least comforting phrase there is to any Bay Area resident. But yet another study has come out reinforcing the idea of cyclical periods of seismic activity, and reminding us that yes, we are due for a Big One. Or Big Ones.
As reported in the journal Nature and in the Bulletin of the Seismological Society of America, new research looks at the period from 1600 to the present, using both existing records and geological evidence, and suggests that the Bay Area has seen two periods of violent seismic jolting in that time, with lengthy, 100+-year periods of "seismic quiet" in between. The first period was between 1690 and 1776, during which time a cluster of earthquakes rocked the region, and then things went pretty quiet right up until 1906.
This notion of there being 100-year great quakes, or something thereabouts, is not a new one, but this study is different in that it looks at paleoseismic evidence, i.e. evidence in rock and soil, in order to identify major seismic activity before records were being kept in this region. "The idea is that stress builds up, is released and builds up again," says US Geological Survey geologist David Schwartz, who led the study. That is, of course, something we've heard before.
But also new here is the idea that we could expect another extended period of repeated major jolts, as opposed to just one Big One. Looking at the evidence of that 86-year-period starting in 1690, the region seems to have had six large quakes ranging from 6.6 to 7.8 magnitude, on five different faults. A situation like that, where we had major quakes every five or ten years, would make projects like the new eastern span of the Bay Bridge laughable since that took about 20 years too many to complete.
And for those would prefer the head-in-the-sand approach to this type of news, you'll be comforted to know that another researcher involved in the study is more cautious in giving credence to this paleoseismic stuff.
“Identifying something that looks like a pattern becomes very interesting because this behavior can be extrapolated into the future,” says Yuri Fialko, a geophysicist at the Scripps Institution of Oceanography in La Jolla, California, who was not involved in the study.But Fialko warns the study does not prove definitively that the seismicity is cyclic. Palaeoseismic data can be unreliable or incomplete, as they are based on interpretations of displacement seen in layers of rock and soil. Historical records do not extend back far enough to demonstrate a cyclic trend.
Officially, per the U.S. Geological Survey, this remains the prediction:
There is a 63% chance of one or more magnitude 6.7 or larger quakes striking the Bay Area in the next 30 years.
Nonetheless, now is a good time for yet another nudging reminder that you should be pulling an earthquake kit together, if you haven't already. And you should probably check the dates on any canned goods you might have included in a kit you made 8 years ago, and replace the water. Because 8-year-old bottled water is not drinkable. You may, however, use it to fill your toilet or washing machine should that become necessary, and assuming the city doesn't burn to the ground again within 24 hours.
As we've tried to remind you before, after 1989's Loma Prieta quake, an estimated 12,000 Bay Area residents were displaced permanently or temporarily, and homes that remained intact had no water, electricity, or phone service for days. And we're due for a bigger one than that.
