More Than 200 Earthquakes Just Rattled California
(Worthy News) – For the past week, the ground near San Ramon, about 25 miles east of San Francisco, has been shaking. And shaking and shaking and shaking. Over 200 small quakes have now rattled the notoriously earthquake-prone region.
Several earthquake swarms have struck the Calaveras since the 1970s, and none of them have triggered a big earthquake. If history is any indication, says Kaven, the swarm is likely to last another two weeks with the total number of quakes going into the hundreds. The longest recorded swarm in the area lasted 42 days.
The particular geology of the Calaveras fault makes it prone to earthquake swarms. The fault is creeping along at a more or less constant rate, and it has many different secondary faults branching perpendicularly from the main Calaveras fault, like twigs coming off a branch. When one of those secondary faults gets critically stressed, it could set off a swarm. The quakes in swarms are small, usually 4.0 or below, because those secondary faults are small; not much energy is stored in them in the first place. [ Source ]