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Second morning earthquake surprises East Bay

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Magnitude 3.5 quake hits in almost exactly the same spot as Wednesday

A seismograph registering an earthquake. Photo by Zilzila/Wikicommons

For the second day in a row the East Bay received a rude awakening in the form of an earthquake along the Hayward fault, with a Thursday early a.m. shakeup similar to the seismic surprise just one day prior.

According to the U.S. Geological Survey (USGS), the Thursday quake hit shortly after 6:00 a.m. and measured 3.5 in magnitude. Wednesday’s East Bay quake was a 3.43, meaning that the new tremor was about 1.17 times more powerful.

Disquietingly enough, Thursday’s early morning earthquake happened in almost exactly the same place as Wednesday’s quake, with a probable epicenter near Tunnel Road along the Berkeley/Oakland border, just a few hundred feet from the last one.

Less than an hour later more than 3,500 people told USGS they had felt the quake, with reports from as far away as Sacramento.

Once again nobody reported any injuries. And though it was a bit more powerful than the last one, the USGS scale predicts that significant damage anywhere was unlikely.

The similarity of the two back-to-back incidents does admittedly feel a bit spooky. But as the USGS fact sheet covering quake myths and realities explains, the two incidents could well be linked:

As a general rule, aftershocks represent readjustments in the vicinity of a fault that slipped at the time of the mainshock. The frequency of these aftershocks decreases with time.

If an aftershock is larger than the first earthquake then we call it the mainshock and the previous earthquakes in a sequence become foreshocks. About five to ten percent of earthquakes in California are followed by a larger one within a week and then are considered a foreshock.

It is possible to have two earthquakes of about the same size in a sequence. There is a five percent chance of having the two largest earthquakes in a sequence be within 0.2 units of magnitude, during the first week of a sequence.

Sometimes faults also experience so-called “earthquake swarms,” which the Swiss Seismological service explains are “numerous earthquakes [occurring] locally over an extended period without a clear sequence of foreshocks, main quakes and aftershocks.”

But even two very similar quakes in the same place might also just be chance. Because of the ongoing federal government shutdown, USGS says it is not available for clarifying comments.

Nevertheless, this is as good a time as any to review basic earthquake safety and emergency response and, if feasible, to prepare an earthquake kit and emergency household plan.

After all, time always favors disaster sooner or later.