6.9 earthquake off Baja California, Mexico

[Shiloh]

Source: http://www.alertnet.org/thenews/newsdesk/N03528903.htm


Strong earthquake hits off northwest Mexico
03 Aug 2009 19:03:15 GMT
Source: Reuters
(Adds detail, changes dateline, pvs WASHINGTON)

TIJUANA, Mexico, Aug 3 (Reuters) - A strong 6.9-magnitude earthquake struck off Baja California in northwestern Mexico on Monday, shaking buildings as far away as San Diego, California, and raising fears of a small tsunami.

The U.S. Geological Survey said the quake struck inside the Sea of Cortez, 76 miles (122 km) from the town of Santa Isabel on the Baja California peninsula and was 6.2 miles (10 km) deep. The figures were revised from its initial estimates.

There were no initial reports of deaths or serious damage. Baja California, home to desert and mountains, is sparsely populated.

The quake was felt strongly in San Diego just across the border from Mexico and City Hall was evacuated, local media said.

"It felt very calm, there was barely a vibration," said hotel receptionist Graciela Alvarado at a motel in the town of Caborca, in the Mexican state of Sonora, across from Baja California.

Local authorities and the USGS warned there was a risk of a small, localized tsunami in the area but no official tsunami warning for the main Pacific coastline.

"There's a very high risk this could occur," said Baja California emergency services director Alfredo Escobedo.

The area attracts U.S. tourists. (Reporting by Lizbeth Diaz in Tijuana and Cyntia Barrera in Mexico City)

[AlaskaDenise]

Re: 6.9 earthquake off Baja California, Mexico

MAP 5.9 2009/08/03 18:40:50 29.330 -113.681 10.0 GULF OF CALIFORNIA
MAP 5.0 2009/08/03 18:33:34 28.931 -113.022 10.0 GULF OF CALIFORNIA
MAP 6.9 2009/08/03 17:59:56 29.066 -112.871 10.0 GULF OF CALIFORNIA
MAP 5.8 2009/08/03 17:55:23 28.976 -112.988 10.3 GULF OF CALIFORNIA

The 6.9 was preceeded by a 5.8 quake only 4 1/2 minutes earlier!

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[Zac]

Re: 6.9 earthquake off Baja California, Mexico

I called a friend in the LA area, but they didn't feel it. Those 4 quakes are close to the san andreas fault so I'm a little nervous. I live within 100 miles of that fault and my parents' house is about 2 miles from it. I wonder if my parents and I could buy earthquake insurance for the next week or two. :-)

[AlaskaDenise]

Re: 6.9 earthquake off Baja California, Mexico

Maybe they'll soon get a mother-in-law apartment, courtesy of Mr. San Andreas. I remember the house in Pt. Reyes Penninsula that had it's sidewalk to the door moved a nice neat 16 feet away. My geology teacher used to point out that Pt. Reyes was actually part of Los Angeles, that had taken a little trip north through the years.

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[Hogweed]

Re: 6.9 earthquake off Baja California, Mexico

From USGS

http://earthquake.usgs.gov/eqcenter/recenteqsus/Quakes/us2009jwbh.php#summary

Tectonic Summary

The Gulf of California earthquakes of August 3, 2009, occurred in the plate boundary region between the North America and Pacific plate. At the latitude of the earthquake, the Pacific plate moves northwest with respect to the North America plate at about 45 mm/y. The plate boundary beneath the Gulf consists of a series of transform faults separated by small spreading centers or pull-apart basins: earthquakes occur as the result of strike-slip faulting and normal faulting. The seismographically recorded radiation pattern of the main shock of August 3, 18:00 UTC, implies that the shock occurred as the result of strike-slip faulting, but the earthquake has not yet been associated with a specific geologically mapped fault. The largest historically recorded shocks from the Gulf of California have had magnitudes of about 7.


----

So it's about the largest shock ever recorded in the area.

[morningperson]

Re: 6.9 earthquake off Baja California, Mexico

Here's a map showing earthquake activiity around the world for the last seven days...

http://earthquake.usgs.gov/eqcenter/recenteqsww/

[peter m]

Re: 6.9 earthquake off Baja California, Mexico

The sea of Cortez appears to be a rifting zone, where ithe fault is spreading apart, kind of like the African Rift valley, and also the Red Sea in middle east. The Cortez undersea rift zone/fault continues into the imperial valley, which sees a lot of seismic activity. That fault continues to run up thru California and merges with the San Andreas fault, but also branches off into numerous other faults(Ca has 1000's of faults of varying length running mostly in a northwest direction. somewhat parallel to the major SA fault)
Imperial valley regularly records some of the largest quakes in CA, so undersea earthquakes in the Sea of cortez no surprize.
the sheer forces of the grinding of the Pacific and North Americn plates against each other , as the PA plates grinds in a NW direction relative to the the NA plate, cause the faults, and are responsible in part for the 400 mile East Sierra mountain wall, and the separation and isolation of 3 SCal mountain ranges.

CA is riddled with faults, but many lie dormant for 100's of yrs, before doing the occasional slip, but slippage rate is measured by geologiic time in 100's, 1000's of years. San Andreas slips a big 8.0 about every every 150 years or so. last big one was the fort Tejon quake in the gorman pass 100 miles north of LA dwtn, which occurred around 1850.

Next big SA fault slippage/quake will occur either tomorrow or next 50 years , likely in Palmdale or San Berdoo desert counties 100 miles north/east of LA

Last two major quakes in immediate La metro region were both 7.0's in the seismicly active east/central San Fernando Valley. One with epicenter near Sylmar(1971) right off the 1-5 fwy, and the second centered near Northridge(1994) or the mountains just north of it(santa susanna mts), Both epicenters were about 40-50 miles from dwtn LA so these quakes did a lot of damage .

[Zac]

Re: 6.9 earthquake off Baja California, Mexico

One other curiosity is the 4 largest magnitude quakes in baja california area were all at a 6.2 mile depth though separated by up to dozens of miles. It would be interesting if there are any seismologist/geologists who would like on comment on that.

[AlaskaDenise]

Re: 6.9 earthquake off Baja California, Mexico

Good observation.

if you look back at other quakes on the same list, similar locations have varying depths - like Puerto Rico.

Who is our inhouse geologist again?

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[AlaskaDenise]

Re: 6.9 earthquake off Baja California, Mexico

the closest depths for near-time earthquakes are:

MAP 3.3 2009/07/28 07:58:10 51.179 -178.438 23.1 ANDREANOF ISLANDS, ALEUTIAN IS., ALASKA
MAP 2.9 2009/07/28 07:52:07 51.134 -178.410 18.8 ANDREANOF ISLANDS, ALEUTIAN IS., ALASKA
MAP 2.9 2009/07/28 07:47:40 51.165 -178.441 22.5 ANDREANOF ISLANDS, ALEUTIAN IS., ALASKA
MAP 3.8 2009/07/28 07:44:50 51.165 -178.441 22.8 ANDREANOF ISLANDS, ALEUTIAN IS., ALASKA

That location is in the plate subduction area off Tanaga volcano.

Shall we speculate about a potential eruption of a mexican volcano in the next few weeks?

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[AlaskaDenise]

Re: 6.9 earthquake off Baja California, Mexico

Tres Virgenes is the closest volcano.

See: http://en.wikipedia.org/wiki/File:Ma..._volcanoes.gif

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[Zac]

Re: 6.9 earthquake off Baja California, Mexico

The first quake that shows at a depth of 10.3 km may actually be the same 10.0 km as the others since the data quality on that one shows as "poorly constrained"


(I couldn't get it copy with the formatting--if anyone knows how to do that without a screen capture, I would be interested.)

[AlaskaDenise]

Re: 6.9 earthquake off Baja California, Mexico

Yes, the first quake depth was +/- 14.4 miles, while the second was +/- 4.4 miles.

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[Snowy Owl]

Re: 6.9 earthquake off Baja California, Mexico

4.4 bugs me

How is it in Eureka up North and south-east of Diego

Snowy

[Zac]

Re: 6.9 earthquake off Baja California, Mexico

I just found this on the USGS site:

Depth is the distance below sea level in kilometers. If the depth of an event is not satisfactorily determined by the data, it is held to a default depth, and the Location Quality shows "depth fixed by location program."

http://neic.usgs.gov/neis/reference/definitions.html

So, the depth is the same because the data wasn't good enough to calculate a good depth number and they just arbitrarily assign 10.0 km (6.2 miles).