Archive for June, 2011
During the last couple weeks northern Arizona has experienced an unusually high level of seismicity. In addition to the occurrence of a number of events around the state during the month of June, there were two small swarm events over the last ten days.
The first cluster of events occurred between Tusayan, Arizona and Red Butte. That swarm included nine events. The largest magnitude was near 2.8 and occurred over a period of 5 days. The nearest fault system is the northeast-southwest trending Bright Angel fault zone which extends through the Grand Canyon. These events were between 3.5-10 km deep.
The second swarm began on June 21 and continued through this weekend for a total of 13 events over a 5 day period. The largest events were magnitude 2.7 and the smallest around 2.0. Average depth was around 6, though the depth range was between 1.8 and 17.3 km. These event occurred near the Anderson Mesa fault, just south of Flagstaff, Arizona.
Though the Lake Mary/Anderson Mesa fault is known to be an active zone which local seismicity has been linked to in the past, the discrete location of the recent swarm has not experienced any comparable clusters since a group of very small, relatively deep events in October of 1979. The Tusayan/Red Butte group lies just south of the majority of seismicity in the Grand Canyon area. More specific investigation of these swarms will be ongoing. For specific event data please visit the bulletin page of the Arizona Earthquake Information Center Website (http://www.cefns.nau.edu/Orgs/aeic/bulletins.html)
Arizona is not new to swarm events. In 2009 the Halloween swarm produced 120 events over a 1 day period. Additionally, Jeff Lockridge of ASU is researching several recent swarms in Arizona using high density network coverage. We hope to learn more about the character of earthquake swarms in the state through his research.
June 28, 2011
Real-time seismograms are the newest innovation in the USGS stable of online seismic resources. Currently, they display seismograms from two seismic arrays: the National Tsunami Hazard Mitigation Program – with seismometers in Washington, Oregon, Alaska, Hawaii and the Aleutian Islands; and California – from Riverside north to Crescent City at the California-Oregon border.
Selecting one of the redbox station symbols causes a seismogram displaying the past 24-hours of activity to pop-up. Also offered is a 15-day archive of seismograms for each station. An included page, About the Seismograms” offers a primer on how to read a seismogram.
This is really a marvelous tool for following seismic activity in the western U.S. and parts of the east Pacific Basin. But be forewarned, this tool is new and they are still populating the site, meaning
that some stations do not display data yet.
USGS Real Time Seismograms – http://earthquake.usgs.gov/monitoring/helicorders/nca/
Mike Conway (18 June 2011)
The San Andreas fault of the Salton Trough of southeastern California presents a considerable hazard to residents of southern California. A consortium comprising the US Geological Survey, Cal Tech, Virginia Tech and Earthscope recently formed to construct a 3-D geo-seismological structural
model of the Salton Trough crust (see block diagram of Salton Trough). Using deep boreholes (see shothole diagram), researchers will detonate explosives at depth – simulating a 1 -2 M earthquake – and record the resulting seismic wave response with an array of portable seismometers. The result: a 3-D picture of the Salton Trough and a better understanding of the response of sediments in that deep basin to earthquakes.
With this in hand, researchers can formulate an integrated model of fault (earthquake) behavior. This should inform earthquake hazard strategies and reduce mortality and civil disruption during the inevitable large magnitude (7-8 M) earthquakes that occur on the San Andreas fault system.
Mike Conway (12 June 2011)
Yesterday, 8 June 2011, the US Geological Survey (USGS) reported a small magnitude (2.5 M) event at 8:25 a.m. PST on the Nevada-Arizona border, 18 miles east of East Las Vegas. The event was listed on the USGS “Latest Earthquakes in the World” site.
By about 10:00 a.m. that same day, the USGS had pulled down the event. Because the earthquake list page is for events of 2.5 M or greater, I assumed they reevaluated the event and pulled it because it fell below that threshold. Lisa Linville of Northern Arizona University’s Arizona Earthquake Information Center thinks otherwise. She believes it more likely that the event was anthropogenic in nature, i.e., a mine blast from the area. If so the USGS pulled the event so as not to admix anthropogenic and natural seismicity in their earthquake catalog.
Mike Conway (8 June 2011)
Most seismic stations require some amount of routine maintenance to ensure that data quality remains high. Wet weather this spring brought chaos to one of our stations near the North Rim of the Grand Canyon.
We were lucky last year to have a minimal amount of failure for all the components at each site in our network. In all we made 10 service visits between May 2010 and May 2011, most of which were due to equipment failure on relatively minor pieces of hardware. The visit to U15A was the most intensive visit due to the collection of issues the site built up over the winter, while it was inaccessible.
While in the area we hiked over to service our Mount Logan station. This is probably the most remote, but beautiful site in the network.
In between, we tested a few radio shots for upgrade plans to the Northern Arizona Seismic Network branch of the network.
Overall, it was a productive weekend of field work for the Arizona Integrated Seismic Network
Lisa Linville (7 May, 2011)
As part of a recent update for hazard mitigation strategies in Arizona, the Arizona Geological Survey is characterizing seismicity and faulting for each county in the state. By far the most active county, Coconino hosted almost 50% of the activity that has occurred since 1830. Mohave is the second most active county with about 30% of the activity. Together with Yavapai, these three counties account for nearly 90% of all known historic activity in the state.The figure above shows the clustering of events within the Northern Arizona Seismic Belt and some of the known quaternary structures in the area.
Lisa Linville (6 May, 2011)