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San Francisco Earthquakes Shake Up Bay Area on Earthquake Preparedness Day

Michael Santo's picture

A pair of Oct. 20 San Francisco Bay Area earthquakes has that area of the country somewhat trembling, though less because of their magnitude, which was "relatively" small, but more because of their proximinity --- not in location, but in terms of date.

The new "San Francisco earthquakes," which they've already been dubbed despite the fact that they were centered in the East Bay of the San Francisco Bay Area, were small. The first earthquake hit at 2:41 p.m. near Berkeley. The United States Geological Survey (USGS) said it was a magnitude-4.0 quake. There were reports of shaking, but no reports of damage.

Later that evening, the U.S. Geological Survey initially reported that a magnitude 4.2 quake had hit near Berkeley at about 8:15 p.m., but that was quickly downgraded to 3.9. Both earthquakes occurred on the Hayward Fault, which runs through the East Bay, and which many experts believe will be the location for the next big San Francisco earthquake.

Ironically, the two "San Francisco earthquakes" occurred on the day that millions of people participated in The Great California Shakeout earthquake preparedness drills.

The Bay Area had, just a few days before, remembered the 22nd anniversary of the Loma Prieta quake. That earthquake was really centered closer to Santa Cruz than San Francisco.

The Loma Prieta earthquake occurred on October 17, 1989, with a 6.9 magnitude rocking the San Francisco Bay Area, killing 63 people, injuring 3,757 more and leaving thousands homeless.

Comparing the magnitudes between the sets of quakes, in both the now-used moment magnitude scale and the Richter scale, an increase of one step on the base-10 logarithmic scale corresponds to a 10**1.5 ≈ 32 times increase in the amount of energy released, and an increase of two steps corresponds to a 10**3 = 1000 times increase in energy.

Parts of Interstate 8800 collapsed (the Cypress Structure, shown above), and a section of the Bay Bridge's upper deck fell to the lower deck. The Marina saw fires, and the quake was broadcast live on national television, as it interrupted the preparations for game three of the World Series between the San Francisco Giants and the Oakland A's.

You can see the pregame video below. The Loma Prieta earthquake starts around 4 minutes in.

Image Source: Wikimedia Commons

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