Magnitude 4.4 Earthquake in Southern California

There was a magnitude 4.4 earthquake in southern California this morning at 1:35 AM, centered about three miles north of Fontana along the southeastern edge of the San Gabriel Mountains in a complex zone where three major fault systems intersect: the San Andreas, the San Jacinto, and the Cucamonga. Very minor damage was reported.

The picture above was taken relatively close to the epicenter area and shows the incredibly steep and rugged topography produced by the faults that have been active in the region. Cucamonga Peak on the left rises to an elevation of nearly 9,000 feet. The mountains have been pushed up in a very short period of time geologically speaking, only a few million years.

Source: http://www.cisn.org/shakemap/sc/shake/11413954/intensity.html

The focal mechanism suggests right lateral motion, consistent with the expected stresses on the San Jacinto fault.

Source: http://earthquake.usgs.gov/earthquakes/eventpage/ci11413954#scientific

There doesn't seem anything particularly unusual about the quake. As always, this region is traversed by several active faults that are capable of generating large damaging quakes. The San Jacinto has produced nearly a dozen magnitude 6+ quakes in the last 125 years, although most of the epicenters were well to the south of today's quake. The San Andreas fault is the more famous fault with quakes that are considerably larger, but also rarer. A magnitude 7.8 earthquake in 1857 was centered near Fort Tejon along Interstate 5, and around 1680 a very large earthquake ruptured the fault between Salton Sea and the Cajon Pass area. There may have been a serious earthquake here in 1812 as well. The recurrence interval for big quakes has been around 150 years so sufficient stress is currently built up to cause a serious event, so if nothing else, today's minor shaker is a reminder to be prepared for the BIG ONE (we always have to write it in big letters or something). Always keep an extra supply of water and food on hand, carry an emergency kit in your car, and have a family plan in the event that you are separated.

For the official information on the earthquake, check out the USGS page:  http://earthquake.usgs.gov/earthquakes/eventpage/ci11413954#summary

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...)

5.4 Earthquake in Southern California: Not an aftershock this time

A magnitude 5.4 quake has struck in southern California, but in this instance, I don't think this is an aftershock to the Easter Sunday El Mayor-Cucapah quake earlier this year. The quake took place on a different fault system, probably the Coyote Creek or San Jacinto fault. The constant aftershock activity from the El Mayor quake may very well have had a role in destabilizing the fault that moved today. As always, moderate quakes like this serve as a reminder that California is earthquake country, and of all natural disasters, these are the most unexpected. We can't predict them, so we have to be ready for them, and understand the faults and their history the best way we can. I am repeating a description of the San Jacinto fault that I wrote in February after an earthquake swarm in the Redlands area along the same fault system....

From February 19, 2010:

"A lot of people, when they hear of quakes in California, think San Andreas Fault. Many California residents who have lived here all their lives cannot think of the name of another fault in the state, but our landscape is literally crisscrossed by active faults (look at the map above; most of the brown lines are active faults). Most earthquakes in CA happen on faults other than the San Andreas, and this week's swarm is no exception. It appears to be taking place on one what is arguably the most active fault in the state, the San Jacinto fault.

The San Jacinto fault is certainly part of the San Andreas system. It splits off from the San Andreas at the east end of the San Gabriel Mountains, and runs roughly parallel to the San Andreas for 140 miles south into the Imperial Valley. It has the same type of motion, right lateral (features on the opposite side of the fault have been shifted to the observer's right). Since it began moving a few million years ago, something like 15 miles of lateral motion has taken place.

The San Andreas fault is justly famous for several devastating earthquakes, including the 1906 San Francisco event that killed 3,000 people, and the 1989 Loma Prieta quake (the World Series quake). It also produced a magnitude 8 event in southern California in 1857. But most of the time the fault is quiet (in a menacing way; it is storing up stress). Compare this with the history of the San Jacinto system (courtesy of Wikipedia):

1890 - Magnitude 6.5 that occurred in the "San Jacinto or Elsinore Fault region".

1892 - Another magnitude 6.5 occurred in the same region as the 1890 earthquake.

1899 San Jacinto Earthquake - Magnitude 6.4 earthquake destroys San Jacinto and Hemet.

1918 San Jacinto Earthquake - Magnitude 6.9 earthquake strikes the same area that was damaged by an earthquake 19 years earlier, with an epicenter roughly 10 mi NW of the previous earthquake.

1923 North San Jacinto Fault Earthquake - Magnitude 6.3 earthquake damaged the San Bernardino and Redlands area. Last time the fault, which runs under the I-215/I-10 interchange, ruptured in this area.

1937 Terwilliger Valley Earthquake - Magnitude 6.0

1942 Fish Creek Mountains Earthquake - Magnitude 6.3

1954 Arroyo Salada Earthquake - Magnitude 6.2

1968 Borrego Mountain Earthquake - Magnitude 6.5

1987 Superstition Hills Earthquake - Magnitude 6.6 (Note: some consider it to have occurred on a fault completely unrelated to the San Jacinto Fault Zone)

These quakes are considerably smaller than the 1906 event, by a factor of 32 or more (it takes the energy of 32 magnitude six quakes to equal the energy of a single magnitude 7 quake; a magnitude 8 quake is 32 times more powerful than a magnitude 7 quake and more than a thousand times more powerful than a six). But they clearly happen more often . The message is clear: to live in California, we must be prepared not just for the BIG, HUGE ONE, but also lots of lesser BIG ONES."

The picture above is a linear valley along the San Jacinto fault near the mountain town of Idylwild in the San Jacinto Mountains. I was there four days ago. As usual I missed the quake.

The Redlands Earthquake Swarm: California Has Her Faults; Lots of Them!

A swarm of small and moderate earthquakes has been shaking things up in Southern California in and around the town of Redlands. The largest, on February 13, was magnitude 4.1, with five additional events between 3.1 and 3.6 in the days that followed. Well over a hundred smaller quakes have been detected as well. Swarms are interesting and exciting if you live on top of them, but they don't usually signify bigger things; the earth is shifting a bit, and that is often the end of it. On much rarer occasions, they may be a sign of an impending larger event, a foreshadowing disturbance of the earth similar to the cracks and pops a tree branch makes before it breaks (disclaimer: I am NOT making ANY kind of prediction; I don't have any access to data, nor do I have the background to do so, and seismologists know better than to try and make a short-term prediction of any quake). My message for the day is to remind people who live in earthquake country that they should ALWAYS be prepared for a large seismic event.

The view from Santa Rosa Peak in the Peninsular Ranges reveals the parallel valleys of the San Jacinto fault. The perspective is towards the west; the San Andreas fault is some twenty miles to the east of the peak

There is another opportunity here: a lot of people, when they hear of quakes in California, think San Andreas Fault. Many California residents who have lived here all their lives cannot think of the name of another fault in the state, but our landscape is literally crisscrossed by active faults (look at the map above; most of the brown lines are active faults). Most earthquakes in CA happen on faults other than the San Andreas, and this week's swarm is no exception. It appears to be taking place on one what is arguably the most active fault in the state, the San Jacinto fault.

The San Jacinto fault is certainly part of the San Andreas system. It splits off from the San Andreas at the east end of the San Gabriel Mountains, and runs roughly parallel to the San Andreas for 140 miles south into the Imperial Valley. It has the same type of motion, right lateral (features on the opposite side of the fault have been shifted to the observer's right). Since it began moving a few million years ago, something like 15 miles of lateral motion has taken place.

The San Andreas fault is justly famous for several devastating earthquakes, including the 1906 San Francisco event that killed 3,000 people, and the 1989 Loma Prieta quake (the World Series quake). It also produced a magnitude 8 event in southern California in 1857. But most of the time the fault is quiet (in a menacing way; it is storing up stress). Compare this with the history of the San Jacinto system (courtesy of Wikipedia):

1890 - Magnitude 6.5 that occurred in the "San Jacinto or Elsinore Fault region".

1892 - Another magnitude 6.5 occurred in the same region as the 1890 earthquake.

1899 San Jacinto Earthquake - Magnitude 6.4 earthquake destroys San Jacinto and Hemet.

1918 San Jacinto Earthquake - Magnitude 6.9 earthquake strikes the same area that was damaged by an earthquake 19 years earlier, with an epicenter roughly 10 mi NW of the previous earthquake.

1923 North San Jacinto Fault Earthquake - Magnitude 6.3 earthquake damaged the San Bernardino and Redlands area. Last time the fault, which runs under the I-215/I-10 interchange, ruptured in this area.

1937 Terwilliger Valley Earthquake - Magnitude 6.0

1942 Fish Creek Mountains Earthquake - Magnitude 6.3

1954 Arroyo Salada Earthquake - Magnitude 6.2

1968 Borrego Mountain Earthquake - Magnitude 6.5

1987 Superstition Hills Earthquake - Magnitude 6.6 (Note: some consider it to have occurred on a fault completely unrelated to the San Jacinto Fault Zone)

These quakes are considerably smaller than the 1906 event, by a factor of 32 or more (it takes the energy of 32 magnitude six quakes to equal the energy of a single magnitude 7 quake; a magnitude 8 quake is 32 times more powerful than a magnitude 7 quake and more than a thousand times more powerful than a six). But they clearly happen more often . The message is clear: to live in California, we must be prepared not just for the BIG, HUGE ONE, but also lots of lesser BIG ONES.