Shaking Things Up in Mammoth Lakes: Hundreds of Earthquakes in 24 Hours

Source: http://www.data.scec.org/recenteqs/Maps/119-38.html

These things seem to happen whenever I have just left a place. I was in Mammoth Lakes only a week ago. There have been upwards of a thousand earthquakes in the last 24 hours or so. The biggest have been in the range of magnitude 3 to 3.8, which are big enough to feel. The USGS emphasizes that the quakes are tectonic (fault-related), not volcanic in origin. This is the place where fears of a volcanic eruption caused political controversy in the 1980s and 1990s (which also formed the basis for much of the movie "Dante's Peak").

Here is an announcement from the USGS California Volcano Observatory:

We have been closely tracking an earthquake swarm in California's Long Valley Caldera, which started yesterday at around 4AM PDT (September 25, 2014). The swarm is located 7 miles east of the town of Mammoth Lakes, about a mile north of the airport. From about 4 AM on the September 25th to 11AM on September 26th there have been more than 500 earthquakes of magnitude M1.0 and above, including 8 earthquakes between M3.0 and M3.8, which were felt locally. This is one of several earthquake swarms that have occurred in the caldera this year. Despite the several felt earthquakes, this is still rather modest activity compared with the much more energetic swarms occurring in the 1980s and 1990s. We do not see any evidence for anomalous ground deformation associated with the swarm at this time. Part of the Long Valley Caldera, known as the "resurgent dome," has been uplifting at a rate of about an inch per year since late 2011, and this remains unchanged. Caldera uplift has occurred sporadically for the last few decades. The uplift rate observed since 2011 is small compared to rates observed in the 1980s and 1990s. The earthquakes themselves are small, brittle-failure (rock breaking) events. Such events are sometimes called "tectonic." The earthquakes do not result from the underground movement of magma. We can distinguish between brittle-failure earthquakes and those resulting from magma movement by the characteristics of the seismic waveforms. 

Earthquake Swarm in Southern California

An earthquake swarm is shaking up part of Southern California along the San Jacinto fault near the town of Anza. According to the U.S. Geological Survey, three moderate quakes occurred in quick succession at about 9:56 AM, with magnitudes 4.7, 4.7 and 4.6. Many dozens of smaller aftershocks have followed, most in the range of magnitude 1 or 2.

The San Jacinto fault is a major structure in Southern California, an active right lateral strike slip fault that trends parallel to the San Andreas. It has produced damaging quakes in the past, most notably in 1899 and 1918 which caused serious damage in Hemet and San Jacinto. At least 10 historic quakes have exceeded magnitude 6.

The quakes today are not nearly as large (a magnitude 5 quake has only 1/30 the energy of a magnitude 6 event), but all earthquake swarms on active fault systems should be noted carefully. Most of the time nothing else happens, but on occasion these swarms may presage a larger event. If you live in the region, it is a good time to evaluate your earthquake preparedness, storing water, flashlights, food, batteries, first aid kit, and radio, and having a family emergency plan.

For more information on earthquake hazards in California refer to this site from the USGS: http://earthquake.usgs.gov/prepare/ . For a look at the historical record of large earthquakes in Southern California, check here: http://www.data.scec.org/significant/index.html . For an interactive map that provides the latest data on earthquakes in California (and other parts of the world), check here: http://earthquake.usgs.gov/earthquakes/map/. If you felt any of the earthquakes and want to report your experience, check out Did You Feel It?.

Update:
As is often the case, the preliminary data is a bit uncertain, and the latest official count is one magnitude 4.7 quake, which as of 1:30 PM has been followed by over 200 aftershocks.

Source: http://www.consrv.ca.gov/cgs/rghm/psha/Pages/index.aspx

Earthquake Swarm in Southern California

A rather extensive earthquake swarm is shaking up the southernmost part of California near the agricultural town of Brawley. At my last count, there have been two magnitude 5+ tremors, with the largest at 5.5. There have been five magnitude 4 quakes, and around twenty that have exceeded magnitude 3. The quakes are occurring between the south end of the San Andreas system and the San Jacinto fault zone just south of the Salton Sea. Some minor damage is reported in Brawley.

Swarms like this are not unusual for the region. As to whether this means some kind of bigger event is coming, one can say it is possible, but many other swarms have occurred without bigger, more damaging shocks. That said, big quakes have occurred in this region, including the 2010 Sierra El Mayor quake that clocked in at magnitude 7.2. The southern stretch of the San Andreas is also a threat for large quakes, in the range of 7.8 magnitude. This is the kind of event that can cause catastrophic damage and injure or kill thousands of people. We are not immune to quakes...

Even if nothing more comes of this swarm (which is the most likely outcome), the event should remind people that southern California does live under the threat of large damaging earthquakes, and that anyone who lives there owes it to themselves to be prepared. Have water, food, batteries, first aid supplies, and an emergency plan in place. Check out these resources for more information.

Postscript: USGS Field Office in Pasadena released this statement: (below the fold...)

Earthquake Swarm near Pinnacles National Monument in California

Source: U.S. Geological Survey (http://earthquake.usgs.gov/earthquakes/recenteqscanv/FaultMaps/121-37.html)

An earthquake swarm is affecting the San Andreas fault system in the Pinnacles National Monument region of the Central California Coast Ranges. The biggest event thus far is a magnitude 4.6 at a depth of 7.6 km, with other significant quakes measuring 3.2, 3.2, 3.3, 3.6, and 3.3. There have been nearly 100 instrumentally recorded quakes between magnitude 1 and 3. The quakes are taking place in a complex region of the fault system where the Calaveras, Hayward and San Andreas faults merge into a single fault. This area is notable because the San Andreas fault is creeping, rather than sticking, meaning that the fault moves constantly (a few millimeters a year) instead of storing up stress and producing monster quakes like it does both to the north and to the south.

Road damage from fault creep on the San Andreas fault near the earthquake epicenter. This is a 2010 picture; the damage was NOT caused by the earthquakes today

This is not an unusual pattern; flurries of similar earthquakes have occurred in the Pinnacles area in 1951, 1972, 1982, 1995, and 2004. If the pattern holds true, the quakes will decline in number and size over the next few days. There is always a small chance that the activity (including the 3.9 quake in San Leandro earlier this week) is a precursor to a much larger earthquake, such as the magnitude 7 event that took place on the Hayward fault in 1868, which killed two dozen people. A recurrence of the 1868 earthquake would cause serious damage and destruction in the East Bay region.

Events like these are reminders that we Californians are living in earthquake country, and that we need to be prepared for larger catastrophic seismic events. It is a virtual certaintly that the state will be rocked by magnitude 7+ events in the next few decades, and some of these earthquakes will affect urban regions. One should always have emergency supplies available, and a plan for what to do in the event of a major earthquake.

Shutter ridge on the San Andreas fault near the earthquake swarm epicenter. The fault follows the fence line.

Earthquake Activity in Puerto Rico

There has been a fair amount of earthquake activity in the vicinity of Puerto Rico and the Dominican Republic in the last few days, with the largest at magnitude 4.1 (near Monte Plata, Dominican Republic), and more than a dozen at greater than magnitude 3. These quakes are happening several hundred miles east of Haiti, which was struck by the horrific quake in January.

Earthquake swarms like this are not unusual, and probably don't signify a threat of greater quakes to come. We have swarms in California constantly, but they quiet down, and nothing comes of it. If anything, though, the quakes should be a reminder that Haiti is not the only Caribbean Island to experience serious earthquakes. Puerto Rico has a "rich" history of earthquakes, with magnitude 7+ events in 1670, 1787, 1831, 1844, 1846, 1865, 1867, 1875, 1890, 1906, 1918, 1943 and 1946. Science Daily has a good rundown on the geologic environment and associated earthquake risk in the region.

Thanks to Edie for the tip!