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).

7.2 Magnitude Earthquake in Baja California: What's the Difference a Foreshock and an Aftershock?

This is follow-up on the 7.2 magnitude earthquake today in Baja California... The fault responsible for the quake is not yet officially determined, but the Laguna Salada fault is a strong candidate, given the pattern of aftershocks running northwest from the epicenter along the fault, and the history and size of quakes in the area (including a 7.2 quake in 1898). The Laguna Salada fault lines up with the Elsinore fault, which has been mostly quiet in historic time, but which has some features suggestive of a fairly active fault system (the Lake Elsinore graben, for instance). I will be very interested in learning whether this quake produced surface offsets

The USGS record for the region records seven earthquakes exceeding magnitude 3 in the week leading up to the quake, including a magnitude 4.2 on March 31st, and a magnitude 4.3 on April 3rd. I don't know how this pattern compares to "normal", but I would expect that these events will be considered foreshocks to the big quake today. Foreshocks can be considered as something like the cracking and popping one hears while bending a stick, just before it snaps. It would be wonderful thing to identify a particular earthquake as being a foreshock in order to give warnings, but the only way we have to tell a foreshock from a main shock from an aftershock is...hindsight. It would be good to keep in mind that a moderate earthquake in California, say, a magnitude 4 or 5, could be a foreshock to a larger impending event. Or not. The chances are pretty low, typically 1 in 20, but it is always a good idea to check on your earthquake preparations when such moderate quakes occur: do you have emergency supplies in place? Water, non-perishable food, first aid? Do you have a family plan for what you will do, and do you have a relative or friend outside the region that everyone knows to contact?

Aftershocks are another matter. They will always occur, there will be a great many of them, and some will be large enough to cause some serious damage. There will be instances when buildings sustain damage in the main quake, but will still be standing afterwards. Residents will often want to go back into the buildings to retrieve their belongings, but emergency workers will often prevent them from doing so. The reason for this is that structural damage is cumulative. The building may have survived the main shock, but just a small amount of additional shaking could very well bring the building down.

One other interesting aspect of today's quake is the heightened seismic activity in the region beyond the Laguna Salada fault zone. Large quakes may very well set off tremors along other nearby fault systems, especially if stresses are already high on the other faults. That appears to be happening today, as a number of small and moderate events are occurring across the region.

Follow up: We can't predict earthquakes, but the folks at XKCD comics note a unique quake warning system....