The San Juan Bautista Earthquakes as recorded at Modesto Junior College

We have a simple classroom seismometer installed on the third floor of our Science Community Center at Modesto Junior College. It nicely captured the two earthquakes from San Juan Bautista that took place last night just five minutes apart. In the record above you can see the 3.6 magnitude foreshock on the left side, and the larger 4.2 magnitude quake in the center. This seismogram nicely illustrates the vast difference in the size of the two quakes. In terms of energy release, a magnitude 4 quake is about 32 times more powerful than a magnitude 3. So the 4.2 quake is many times larger than the 3.6 (can a seismologist out there do the calculation?), which is obvious from the graph above. The 4.2 was not just a little bit bigger, it was huge in comparison.

I isolated the magnitude 4.2 and "stretched" it out. One can see from the horizontal scale that the vibrations continued for two or three minutes at our location. The quake would have been a few quick jolts near the epicenter, but the unit is far more sensitive than humans are. The really big quakes can  reverberate for hours.

The unit is an EQ-1 Seismometer from Next Generation Science. It uses a suspended magnet in near contact with an electronic coil. Vibrations are recorded electronically and converted to digital signals on the computer which can be manipulated to bring out details of the individual quakes. In the few months I've had it installed, I've recorded California quakes in the range of magnitude 3.5-6.0 (the Napa quake). It has also captured magnitude 7 and larger events worldwide.

The unit shouldn't really be on the third floor of the building, because it catches all manner of foot traffic, trains and traffic. On the other hand, what a great teaching tool. I've used a signal splitter to put a second monitor in the window with some interpretive notes. I've had any number of people who've stopped by to jump up and down, creating their own little earthquakes (about magnitude -2, I understand).

4.2 Magnitude Earthquake near San Juan Bautista

What else are you going to call a restaurant situated right on the San Andreas?

A 4.2 magnitude earthquake struck at 10:26 PM PST tonight about 3 miles south of the town of San Juan Bautista in the Coast Ranges of Central California (the official USGS report is here). It was preceded by a magnitude 3.6 foreshock at 10:21 PM. It was felt over a fairly large region, but quakes of this size rarely cause damage. The San Andreas is the major active fault present in the region, although the zone includes a few other fault strands, including the Vergeles fault, which is listed on the state map as having been active within the last 700,000 years.

Mission San Juan Bautista and the scarp of the San Andreas fault

Mission San Juan Bautista is quite famously built on a scarp of the San Andreas, and was at the south end of the fault rupture that occurred during the 1906 San Francisco earthquake (magnitude 7.8). To the south, the fault is creeping on a continual basis, as described in some of my recent posts.

There is a very slight chance (5% or less) that this quake is a foreshock to a larger magnitude event, as the San Andreas fault has over a century of accumulated stress. There is no way to say until a larger quake takes place. It is good to have moderate quakes like this once in a while to remind all of us of the necessity of being prepared. The larger quakes, when they come, will cause extreme damage to our infrastructure, meaning power outages lasting for days, disruptions of transportation corridors and communications (no internet!). Always have an emergency kit with extra water, food, radio, flashlight, batteries, and first aid supplies in your house and in your car.

Source: Southern California Earthquake Data Center

I've lived in California for most of my life, but it's been quite a few years since I've felt any (and yes, be careful what one wishes for). I was in Hollister on Saturday waiting to feel a quake, but was out of luck...