It wasn’t holiday presents but a twitching reminder of its potentially destructive presence that the Hayward Fault gave East Bay residents this holiday season, ending—for the moment—with a weak Thursday morning spasm.
The seismic swarm began with a sharp little shake six days before the holiday at 7:12 p.m. on Wednesday the 20th, initially reported as registering 3.67 on the 10-point Richter Scale.
The next shock—a magnitude 3.68—hit at 10:49 p.m. Friday, followed by an imperceptible 1.4 42 minutes later centered a mile further southeast.
A 3.51 jolt at 9:21 a.m. Saturday, located at the same site as the two earlier and stronger quakes. Two more smaller quakes—a 1.6 and a 1.0—followed at 2:23 p.m. Saturday and 12:12 p.m. Sunday.
An April 15 quake measuring 2.8 on the Richter Scale originated in the same location—1.2 miles southeast of California Memorial Stadium, which sits directly over the fault.
According to a report by the UC Berkeley Seismological Laboratory, the recent quakes “are occurring in a pocket of seismicity in the Berkeley-Piedmont border region.”
As of noon Saturday, the area had been the scene of “99 earthquake(s) of magnitude 2.0 or larger and 10 of magnitude 3.0 or larger occurring within a radius of 1 mile of this sequence since 1970.” The three sharpest pre-Christmas temblors this year are “the three largest earthquakes to date.”
University plans to renovate the Memorial Stadium interior and add a gym along its western wall are currently facing legal challenges, along with other projects located adjacent to the fault zone—including an underground parking lot.
One contention in all the suits is that the gym should be prohibited and stadium work limited because of the Alquist-Priolo Act, a state law barring new construction on faults and limiting renovations to have of a structure’s value.
Another project, a transformation of residential Bowles Hall into a corporate learning retreat is also complicated because a corner of the building may be directly over the fault as well.
The strongest quakes all originated at depths of between 2.8 and 6 miles beneath the surface about 1.2 miles southeast of Memorial Stadium, according to reports posted by the U.S. Geological Survey (USGS).
While the small 2.6 shaker that struck at 10:07 Christmas morning also originated on the Hayward Fault, its epicenter lay further to the southeast in the hills above Union City.
And another small jolt—a 1.8 centered on the fault’s northern reaches beneath San Francisco Bay five miles northwest of San Pablo—preceded Wednesday’s first shaker by five hours and 5 minutes.
The latest quake Thursday morning measured 2.8, and originated about eight tenths of a miles further southeast that the epicenter of the strongest quakes.
In a twist, the epicenter for the strongest quakes was located precisely where the UC Berkeley Seismology Laboratory pinpointed the origin of a hypothetical 6.5 earthquake for a 2003 exercise.
According to the university’s estimates, a quake of that intensity at that epicenter would result in more than $5.6 billion in damages.
For more on the simulation, see the university’s web site at http://seismo.berkeley.edu/eqw/q2003/
For up to date information on the recent quakes, see the USGS web site at http://quake.wr.usgs.gov/recenteqs/Maps/San_Francisco.htm. “Events,” as seismologists described them, are posted within moments of their occurrence.
To see where else the quake was felt, click on the first of the two “Did you feel it?” links for a map of reports, and then on the “Statistics” link for a list of reports by ZIP code.