June 28, 2010, 9 am

3.5 Magnitude Earthquake Felt at Golden Gate Bridge

An estimated 3.5 magnitude earthquake struck off the coast of San Francisco. The temblor was recorded at 7:47 a.m. today about 3 miles west-northwest of the San Francisco Zoo and about 6 miles northwest of Daly City, according to the U.S. Geological Survey.

After a very quick initial visual inspection of the Golden Gate Bridge, no damage was noted. As part of standard protocol, Bridge staff are currently conducting an inspection of the Bridge which will be completed within a hour.

According to District Engineer Denis Mulligan, “While it was only a 3.5 magnitude event, due to its proximity the peak rock acceleration was fairly high; it was measured by Bridge instruments at 0.5g. The explanation for the high acceleration is that since the earthquake was so close, we experienced a “near field phenomenon”, known as a “velocity pulse” or “fling”. This fling was the initial strong motion that resulted in the 0.5g measured peak acceleration. The vibrations after the initial pulse were very modest and of extremely short duration.”