The Sierra El Mayor Earthquake

More detailed information is available about the Easter Sunday earthquake in Baja California Courtesy of the Southern California Seismic Network operated by Caltech and US Geological Survey (thanks to Lee at Arizona Geology for the link). The 7.2 magnitude Sierra El Mayor Earthquake appears to have ruptured the surface for a distance of 75 kilometers from the epicenter to the California-Mexico border, with displacements ranging as high as 2-3 meters. An overflight confirmed offsets along 28 kilometers of the Borrego fault, north to the international border. Fault motion was oblique, down to the east with a component of right lateral motion. The team from SCSN took some 1,300 pictures that will be posted soon.

A satellite image shows the dust plume raised by rockslides in the mountains near the epicenter. The link at Gawker.com includes a time lapse sequence of the satellite view, and a link to the video of the immense dust clouds from the ground.

UPDATE: Lee Allison at Arizona Geology has been on top of the latest developments on the Sierra El Mayer quake. He has posted a number of pictures of road damage on Highway 5 south of Mexicali, including this one (photographer Danny Ashcraft):

Note the right lateral offset of the road surface

UPDATE #2: The Southern California Seismic Network is beginning to release some of their 1,500 photographs of the fault ruptures on the Borrego fault and others. An example below!

A Rude A'Shakening: the 7.2 Magnitude Baja California Earthquake

U.S. Geological Survey poster on the April 4 Baja California Earthquake. Click on the image for a medium view. For a larger view, click here

More information is coming out about the nature of yesterday's quake in Baja California. The magnitude 7.2 tremor is the largest event to strike the region in more than 100 years, although events in 1915, 1934, and 1940 were comparable. The first-motion tensor diagrams indicate that the event occurred with primarily right-lateral motion with a slight component of vertical offset as well. This would translate to a fault rupture at the surface in which the opposite side of the fault moves to the observer's right, perhaps several feet, but also rising a few inches, forming a low terrace. We are awaiting confirmation of any kind of ground rupture, which would also confirm that the event took place on the Laguna Salada fault, as is highly suggested by the pattern of the aftershocks.

A large number of aftershocks have shaken the region, and these can be expected to continue for a number of years, although they will decline in intensity and frequency. The quake also appears to have triggered small earthquakes on other faults in the region. There are concerns about how this quake might be related to the highly stressed south end of the San Andreas fault system to the northwest at Salton Sea. The southern section of the San Andreas fault is considered to have a 59% probability of rupturing with a 6.7+ magnitude quake in the next 30 years (See the Uniform California Earthquake Rupture Forecast for details). The quake yesterday may have heightened stresses on the San Andreas, but it also may have released some of it; I await word from the geophysicists who are no doubt analyzing the data right now.

People all across southern California felt the earthquake. A quake has only one magnitude (7.2 in this case), as this is the measure of the total energy released by the fault rupture. People at varying distances from the quake feel a different level of shaking, which is described as the intensity of the quake. Intensity is measured on a 1-12 scale (the Modified Mercalli Scale), and a particular earthquake will have many reported intensities to the extent that the earthquake shaking and ground motion can be mapped.

One more comment, or really a commentary: my internet was out this morning, so I turned on the television to get the latest news on the earthquake. Let's see: a major earthquake, the biggest in a generation, strikes southern and Baja California, affecting millions. And Tiger Woods started playing golf again. Guess which story the cable news networks were spending their time on this morning? Truly pathetic...

Update: On the other hand, this is cool; check out the dust kicked up by the quake. Hat tip to Jeff for the link!

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....

7.2 Magnitude Earthquake in Baja California

A large earthquake (preliminary magnitude 6.9, depth 20 miles) has shaken the delta region of the Colorado River in Baja California. It was felt over a wide region of Southern California and western Arizona. The nearest towns are Guadalupe Victoria in Mexico (16 miles), and San Luis and Gadsen in Arizona. The magnitude currently being reported is equivalent to the Loma Prieta earthquake of 1989 that shook the Santa Cruz and San Francisco Bay regions.

A 5.1 quake has been reported farther north in the Imperial Valley of California moments ago as well. These earthquakes are not on the San Andreas fault, but can be considered related, as a series of northwest trending faults take up the stresses related to the spreading of the Gulf of California. Large quakes have occurred in the region in the past, most notably a 6.4 quake in 1979, and a 7.1 event in 1940 (with nine fatalities).

Update: The magnitude has been upgraded to 7.2 and the depth to 6.2 miles. Changes to the initial magnitude report are not unusual as the U.S. Geological Survey gets more information. The depth of the quake is also tricky, especially when the region is not covered by numerous seismometers. A series of aftershocks are now being reported, ranging as high as magnitude 5.4.

Update #2: There have been a large number (40 or so) of magnitude 3+ aftershocks in the region, most notably lined up to the northwest along the Laguna Salada fault, which lines up with the Elsinore fault in California, and produced a 7.2 quake in 1892. It will be a while before we hear of any ground rupture, but quakes of this size are capable of producing several feet of offset over several tens of miles.