Archive for March, 2011

Clarkdale M3.7 Earthquake

As the clamor for information about Japan abates I’d like to revisit the earthquake we had in Clarkdale, Arizona last Friday. Generally after an earthquake event in Arizona the Arizona Earthquake Information Center releases a preliminary bulletin to offer basic information about the size and exact location of the event. For smaller instrumentally felt earthquakes, depending on size and available information, a bulletin is not always generated. For example, there was a small (2.8) earthquake in the east Grand Canyon area last weekend, which has no bulletin but is available for review in our eq database. For both the searchable database and bulletin postings please visit the AEIC site:

The magnitude 3.7 event locates just north of the Magnitude 3.6 event the area experienced west of Sedona in January of this year (distance between them at about .5 km is well below the estimated error in these locations). There was one small (M 1.9) event in December of last year that occurred just east of these events. All three of these are an interesting development in the regional seismicity, since we have seen very little historic activity at this specific site.

Below is a short period record from a station NW of Flagstaff. Most of the felt reports that the Earthquake Center received were reports from people inside of buildings in the Cottonwood area. The shaking lasted just a few seconds and no damage was reported. Several people indicated that the sensation felt mostly like a jolt, followed by short duration shaking. At this time no foreshocks or aftershocks have been associated with this event.

The recent earthquakes, not shown on this map would plot directly north of Clarkdake. The TA data, indicated by the pink dots, are from mine blasts south of the epicenter and are not included in our earthquake catalog. The Clarkdale earthquakes would plot about ten kilometers to the north of that.

The catalog is scattered with not too distant events, some of which are loosely associated with known faults in the region. To the southwest there was a larger 4.9 event, discussed last month. That 1976 event showed 40° dipping SW extension likely associated with the Prescott Valley Grabens near Williamson, Arizona. There was some activity in the Big Chino area in the mid and late 90’s. Smaller events with no source mechanism analysis, but from what we know about that fault, associated activity would be NW trending extensional.

Based on trenching done by the AZGS, a recurrence interval is estimated at 20,000-30,000 years and the maximum credible earthquake is between 7.0-7.25. The survey also mapped a new fault, called the Little Chino, late in 2009. That fault displayed active Quaternary (last 1.8 my) structures and a similar trending fault plane which rotates towards the north as you move north along it. Lithostatic stress caused by the elevation gradient between the Basin and Range Province in SW Arizona and the high elevation Colorado Plateau in NE Arizona is one of several proposed explanations for the observed stress orientations in this area. None of these geographically close faults are the likely culprit for the most recent events, and the absence of fault plane solutions for these events makes fault assignment simply guesswork. They do however explain a little bit about local crustal structure in this region of the transition zone.

March 24, 2011 Lisa Linville

Radiation Monitoring in U.S.

The release of nuclear radiation from breached Japanese nuclear facilities has prompted considerable concern in the U.S.  Civil authorities continue to emphasize there is no risk to U.S. citizens or the U.S. food supply.  US FDA is monitoring the situation in Japan to make certain that imported food remains safe.

The Environmental Protection Agency (EPA) has launched a dedicated webpage spotlighting radiation air monitoring:  Japanese Nuclear Emergency: Radiation Monitoring.  The following information stems directly from that website.    ” As of 1:00 p.m. (EDT) on March 21, 2011 EPA’s RadNet radiation air monitors across the U.S. show typical fluctuations in background radiation levels. The levels detected are far below levels of concern. The Nuclear Regulatory Commission has said we do not expect to see radiation at harmful levels reaching the U.S. from damaged Japanese nuclear power plants. More on EPA’s monitoring efforts.

Mike Conway, 21 March 2011

USGS ~ Earthquake-Tectonic Web Resources

Earthquakes are front and center in the global news arena; an M 9.0 event accompanied by tsunami and horrific cascading events will do that.  The US Geological Survey has done a marvelous job of showcasing web resources that inform and educate the public to the nature of earthquakes, the Earth’s interior, and the role of plate tectonics.  USGS Earthquake Hazards Program

For a crash course in the phenomena of seismicity, there is no better place to start.

Palo Verde Questions

In light of the recent events across the Pacific and the large number of inquiries we have received regarding the safety of the Palo Verde Nuclear Plant and earthquakes, we have decided to post a review of the most commonly asked questions.

Is Palo Verde built on any faults?

There are no known Quaternary faults near Paulo Verde. The geology around Palo Verde is southern basin and range style, older, degraded structures that don’t display much activity or indication of active extension. The closest fault is probably near Gila bend, but not large enough to generate events of concerning magnitude. In a previous post we highlighted a USGS resource for viewing known faults in the US ( This GoogleEarth file (KMZ) available through USGS: If you need GoogleEarth here is a link to their download site:

What if there is a large earthquake near the Palo Verde Nuclear Power Plant?

Based on 2008 data, a map of earthquake damage risk in the United States. The highest risk areas are purple, red and orange. Click image to reference full story.

There were historically maybe half a dozen small to moderate (under magnitude 5) earthquakes in the region. Almost all of those events relocated down in or close to the Mexican border during a relocation study that the AEIC did for APS using updated crustal models. The remaining events are too small to be significant.

More importantly, the region near Arizona’s nuclear power plant lacks the tectonic engine to drive seismicity on the scale that we see at subduction boundaries. It’s true that there are exceptions to the relative stability of intraplate regions, like the New Madrid Seismic Zone. These regions have long histories of failed rifts or exhibit evidence of regional stresses and warning of these hazard is available in local crustal structure, current seismicity or GPS strain measurement. So for an area like Wintersburg, where there is no evidence of an active stress regime, the threat posed by earthquakes is very minimal.

What about radiation or earthquakes from other sources?

The tectonically active Californian coast does host several nuclear Plants. They have been built or retrofitted to withstand the maximum credible earthquake threats in their individual locations and must adhere to the operational guidelines set by the NRC. The same will be true for Palo Verde. Note that the crisis in Japan is not a direct result of ground movement. Fundamentally, the breakdown was in the backup system necessary to keep the reactors cool in the event of power loss.

In response to the threat of heightened levels of radiation in Arizona, a joint news release (Arizona Department of Health Services, Arizona Radiation Regulatory Agency, Arizona Division of Emergency Management) was issued yesterday and indicates that there is no threat to state residents. To find regular updates and more specific information regarding health hazards due to radiation in the state please visit

March 16th, 2011, Lisa Linville

Energy distribution for the entire March 11, 2011 Tohoku-Chiho Taiheiyo-Oki earthquake.

Tohoku-Chiho Taiheiyo-Oki Aftershocks

Aftershocks from the Tohoku-Chiho Taiheiyo-Oki earthquake continue to occur long after the main event on March 11. Below is a plot of events associated with the subduction zone off the east coast of Japan between March 9th and 14th, starting with the magnitude 7.2 which marked the beginning of activity. Please click on images to expand.

To see world wide data for the last seven days visit:

Researchers at Harvard, after imaging rupture properties of the main event, suggest that the 7.2 event is not technically a foreshock since it occurred at a different location on the fault, but rather a separate event which could have triggered the main shock.

Energy distribution for the entire March 11, 2011 Tohoku-Chiho Taiheiyo-Oki earthquake

Likewise with the Magnitude 6.8 which immediately followed the 8.9 Earthquake. They provide interesting animations of the ~150 second rupture sequence. Visit their research page at:

Energy from the trench region continues to rattle seismometers in Arizona. Here at the Shermin Mifflin-Smith Seismic Observatory, teleseisms from Japan still cover our displays. The record below is from a short period AISN station north of Flagstaff showing data between March 12-15. To give a general idea of what kind of activity our stations see on a daily basis I have included a capture of unfiltered broadband data from one of our adopted TA stations near the end of February.

Most of the activity you see on the record is from Taiheiyo-Oki aftershocks, with the exception of a local northern Arizona event on the bottom right.

Distant earthquakes as seen from a seismic site east of Phoenix, AZ

Japan Earthquake “felt” by AISN seismometers

AP Photo/Kyodo News)

The fifth largest earthquake in recorded history struck Japan on Friday afternoon. The magnitude 8.9 event occurred at a shallow depth of 24km, 400 km off the east coast of Japan, releasing a devastating amount of energy to the surrounding area. The earthquake was a result of thrust faulting near the subduction zone plate boundry between the westward moving Pacific Plate and the North America Plate (aprox rate of movement is 83 mm/year). The sea-floor displacement caused a tsunami which was up to 30ft high and extended inland about 10 km in the Miyagi Prefecture on Japan’s east coast. The death toll is still being tallied as search and rescue workers and officials attempt to locate the tens of thousands missing. The Tsunami reached the US Pacific coast earlier this morning where little damage was reported. Damage reports from Japan include extensive fires, including an oil refinery and a petrochemical plant in Sendai, train derailments, a burst dam, and the threats associated with nuclear power plant shut down. There were several fore-shocks which happened earlier in the week. Aftershocks continue to pour in and dominate seismic records in the region.

Currently, the most complete source of scientific information available is from the USGS at:

For other damage figures, try the Red Cross:

The AISN was shaken by this event which occurred ~9,000 km away (from Phoenix, 80.66 degrees). The seismogram below is a paper record from a short period sensor at the Flagstaff station on the NAU campus. Shaking here started on our instruments at 11.58pm local time and continued for hours as additional energy reached us from aftershocks.

Seismic waves captured on the paper record from NAU station FLAG

Digital records from short period (CCAZ) and broadband stations (Y14A) both clip at high wave amplitudes even at such a great distance from the hypocenter. Phase picks from stations U15A, X18A, 319A and 113A of the Arizona Integrated Seismic Network were used in the preliminary USGS data for this event.

March 11, 2011 Lisa Linville

AISN station CCAZ, 44 milesNE of Flagstaff, AZ

AISN station Y14A near Wintersburg, AZ

Fate of Mid-Continent Fault Systems

Studying earthquakes in China may help unravel the location of the next large mid-continent earthquake in

USGS Seismic Hazard Map - New Madrid fault zone is in bullseye near map center.

North America.   The Chinese draw on a 2000-year historical record to characterize the nature of mid-continent seismic activity.  And in the past 2000 years they have not once seen two large magnitudes earthquakes occur on the same fault section in mid-continental setting.

This may be good news for the people living near the New Madrid Fault in SE Missouri – a hop, skip, and a bump from Memphis and St. Louis.  In 1811-1812, three or four closely timed, large magnitude (~ 8.0M) earthquakes badly damaged what were then small rural small communities in the area.

According to Seth Stein, seismologist at Northwestern University, high-precision GPS studies (Global Positioning System) show no significant strain in the New Madrid area.  Stein and his colleagues conclude that large magnitude earthquakes along the New Madrid Fault may be at an end, at least for the immediate future.

Stein and his Chinese colleague, Hui Wang, suspect that mid-continent fault systems are mechanically coupled, so that as stress is released along one fault during a big earthquake it accumulates along other fault zones.  The result:  big earthquakes bounce from fault system to fault system leading to long periods of quiescence on major mid-continental faults.

Distributed stress model for mid-continental fault systems (from S. Stein web page)


Next large central US earthquake may not be along New Madrid Fault Lines. Science Daily 02.09.2011

New Madrid and other earthquakes within continents.  Seth Stein

Mike Conway, 5 March 2011

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