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: http://www4.nau.edu/geology/aeic/aeic.html.

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