Archive for May, 2011

A year in the life of the southern San Andreas Fault System

The southern reach of the San Andreas Fault system, which forms the tectonic boundary between the Pacific and North American tectonic plates, extends south through the Salton Trough to the northern tip of the Sea of Cortez.  Each year thousands of earthquakes occur along principal and associated faults, rivaling the most tectonically active areas in the world.

Earthquake Epicenter for southern San Andreas Fault system, April 2010 to May 2011. Magnitude range from 2.5 to 7.2 M.

Most events are small (Table 1), with 68% (1181 earthquakes) ranging from 2.5 to 3.5M  (NEIC Catalog search does not display events less than 2.5 M).  Infrequent large-magnitude events, with a potential to crack irrigation ditches, disrupt roads, and collapse buildings, occur, too.  One such event, the El-Mayor-Cucupah M7.2 event, occurred on the Laguna Salada Fault – along the west flank of the Sierra Cucupah in Baja California, Mexico – at 3:40 p.m. (PDT) on 4 April 2010.  The nearby communities of Mexicali, Mexico, and Calexico, California, reported damage – some of which was extensive.  Fatalities were few and only 100 people were injured.

Following the El Mayor-Cucapah event, aftershocks reportedly occurred at 150 to 200 per day through early May 2010 (Southern California Earthquake Network,

Table 1.   Earthquake events for the southern extent of the San Andreas Fault system cataloged by magnitude.

EQ Magnitude Number of Events

2.5 – 3.5                   1181 events

3.5 – 4.5                    531 events

4.5 — 5.5                    41 events

5.5 – 6.5                       1 event

6.5 – 7.5                       1 event (El Mayor-Cucupah event)

Total events          1755 events


Data from USGS’s National Earthquake Information Center.  Search parameters:   Latitude range –  31.5 to 33.5 N; Longitude range –  116.3 to 114.25 W; Date range –  1 April 2010 to 20 May 2011.

Arizona Earthquake Information from USGS

The USGS Earthquake Hazards Program hosts individual pages on earthquake information for each of the 50 States.  The Arizona page is situated here.

The sorts of things you’ll find there:

Arizona seismic hazard map (source, USGS)

  • Brief overview of earthquake history of Arizona;
  • Quaternary fault information;
  • Links to institutions that provide state or regional earthquake information;
  • Recent seismicity map;
  • Seismic hazard map;
  • Latest earthquakes in Arizona.

For a quick overview it is an excellent resource.

Mike Conway  (22 May 2011)

Citizen Scientists Monitoring Earthquakes in Southern California

In southern California, the US Geological Survey is building an innovative Citizen Scientist program to enhance existing seismic networks.   The NetQuakes program has already distributed 1600 Swiss-build, bread-box size seismometers to residents in southern California.  The inexpensive devices – they cost several thousand dollars –  measure ground acceleration in three directions: north-south, east-west, and up-down.  Captured seismic events (i.e., earthquakes) are then send via internet to USGS offices for analysis.

Citizen scientist participants bolt the devices to their garage or ground floor.  And if something goes wrong with the seismometer, they mail it back to the USGS for a working model; hence the nickname of the device, Netflix seismometer.

NetFlix seismometer - about the size of a loaf of bread.

These inexpensive and easily distributed seismometers may play a role in USGS efforts to establish early-warning seismic alerts in the Western U.S.

To learn more, listen to NPR’s 3-minute broadcast of the story from 10 May; or visit the USGS NetQuakes website.

Mike Conway  11 May 2011

Arizona Earthquake Roundup – Spring-2011

“Arizona seismicity started slow in 2011 but picked up in March, when more than 50% of the events to date occurred.  Additionally, seismometers of the Arizona Integrated Seismic Network (AISN) were inundated by the 9.0 M Tohoku earthquake and its many larger aftershocks.”

Those are the first sentences in Lisa Linville’s excellent summary of seismic activity in Arizona for the first quarter of 2011.  For more details and for some excellent graphics see Lisa’s article, “Earthquakes and Microseisms in Arizona“.

Seismogram of ML 3.7 north of Clarkdale, AZ, which was widely felt in Cottonwood, AZ.

Collapsed adobe homesteads in Sonora, Mexico.

1887 Sonora Earthquake rocks Arizona

Collapsed adobe homesteads in Sonora, Mexico.

Intensity map for 3 May 1887 earthquake.

“On May 3, 1887, a major earthquake shook much of the southwest United States and Mexico, an area of nearly two million square kilometers.”  That’s the lead sentence in Susan Dubois’ and Ann Smith’s historic account of the effects of one the largest historical earthquakes to rock northern Mexico and the southwestern U.S.

Dubois and Smith used newspapers, oral histories, pioneer journals, military reports, and personal correspondence to reconstruct the impact of the earthquake on communities throughout Sonora, Mexico, Arizona, New Mexico and Texas.  See the isoseismic map for a distribution of intensity on the Modified Mercalli Intensity Scale.

The epicenter of the estimated 7.4 M event was about 65 km south of Douglas, Arizona, on the Pitaycachi Fault.  The resulting fault scarp broke the Earth’s surface for about 100 km, with the northern tip exposed just 8 km south of Douglas.  The average vertical offset on the fault was a whopping 3 meters (9 feet); maximum displacement was between 4.5- and 5.1 meters (16 feet)!   It is the largest earthquake to have caused historical damage in Arizona – either as a territory or state.

The community of Tepic, Sonora, about 190 kilometers south of Tombstone, Arizona, was flattened by the event; most homes were destroyed, irrigation ditches broken, and the town plaza and environs “ripped up” by ground fissures.  In northern Sonora, 51 lives were lost.

Fault scarp with 2 meter vertical offset.

Ground shaking in southeastern Arizona lasted 1- to 3-minutes.  Throughout the impacted area, homes were damaged, roads were disrupted, and rock avalanches occurred in nearby mountain ranges.

In 1887, fewer than 90,000 people lived in the Arizona Territory.  Now over five million people inhabit the metropolitan areas of Phoenix and Tucson combined.  While events such as the 7.4 M 1887 earthquake (7.2 M) are rare in the southwestern U.S., excluding California, they do occur.  (Geomorphic evidence points to a recurrence interval of more than 100,000 years on the Pitaycachi Fault).   Nonetheless, civil authorities, geologists, and the broader public must ask, “are we ready?”.


The 1887 Earthquake in San Bernardino Valley, Sonora: Historic Accounts and Intensity Patterns in Arizona, by S.M. DuBois and A.W. Smith, 1980, 112 p, Bureau of Geology and Mineral Technology, Special Paper No.3.  Printed copies are available through the Arizona Geological Survey for $8.75 plus shipping.

The 1887 Sonoran Earthquake:  It wasn’t our fault, by Tom G. McGarvin, 1987,  Fieldnotes, p. 1-2.

Historic Earthquakes, Northern Sonora, Mexico, 1887, May 03, U.S. Geological Survey,

Active Tectonics of Northeastern Sonora, Mexico (Southern Basin and Range Province) and the 3 May 1887 Mw 7.4 Earthquake, by Max Suter and Juan Contreras, 2002, Bulletin of the Seismological Society of America, v. 92, no. 2, p. 581-589.

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