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Posts by llinville
A small swarm of earthquakes occurred south of Colorado City between December 12th and 13th. The largest event in the swarm was magnitude 3.1, the smallest was 1.7.
|2011||12||13||36.765||-113.018||13.9||0||43||0||2.8||Colorado City, AZ||UU|
|2011||12||13||36.764||-113.017||8.2||23||36||22||3.1||Colorado City, AZ||UU|
|2011||12||12||36.782||-113.001||2.8||9||44||37||1.7||Colorado City, AZ||UU|
|2011||12||12||36.757||-113.02||7.9||8||3||0||2.2||Colorado City, AZ||UU|
This small swarm occurred between the terminus of the Southern Intermontain Seismic Belt (ISB) and the start of the Northern Arizona Seismic Belt. This region regularly experiences small to moderate sized earthquakes. Earlier this summer there were several quakes of similar magnitude in the same area between the Hurricane and Sevier/Toroweap faults.
The National Earthquake Information Center Reports of the southern ISB:
The ISB in southern Utah coincides with a transition between east-west-directed stretching in the Basin and Range to the west and more stable crust of the Colorado Plateau to the east. Tectonic movement on generally north-trending, east- and west-dipping range- and plateau-bounding normal faults, which results in horizontal extension, characterizes this part of Utah. The Sevier Valley is an area of variable and complex deformation involving significant components of folding and both normal and strike-slip faulting. The most prominent geologically young faults in southwestern Utah are the Hurricane and Sevier faults. The Hurricane fault forms the west-facing Hurricane Cliffs, which define the eastern edge of the Basin and Range within the ISB. Faults in the ISB in southern Utah locally show evidence of displacement younger than 10,000 years, but average recurrence intervals are generally longer than those on faults in the ISB in northern Utah. Recurrence intervals for surface faulting on the most active segments of ISB faults in southern Utah are generally many thousand to tens of thousands of years.
|Lisa LinvilleDecember 17, 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
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)
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 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
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 (http://groundswell.azgs.az.gov/2011/02/05/locating-earthquake-faults). This GoogleEarth file (KMZ) available through USGS: http://earthquake.usgs.gov/hazards/qfaults/google.php. If you need GoogleEarth here is a link to their download site: http://www.google.com/earth/download/ge/agree.html
What if there is a large earthquake near the Palo Verde Nuclear Power Plant?
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 www.azein.gov.
March 16th, 2011, Lisa Linville
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.
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.
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: http://seismology.harvard.edu/research_japan.html.
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.
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.
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
There are many competing factors to consider when choosing the location of a seismic site, not the least of which is safety for the equipment. The Arizona Integrated Seismic Network (AISN) has suffered some heavy losses in the last six months from vandalism. Late in the fall we lost a station west of Phoenix from theft and more recently one of our solar panels down near Yuma was shot and destroyed.
As the AISN enters it’s third year in operation we are finding that costs associated with replacement equipment and servicing due to vandalism far outweigh the costs accrued by regular maintenance; a heartbreak considering this is probably the least productive way to exhaust a very limited budget. The usefulness of a network is based on the availability and quality of data coming from the seismic sites. With only a few people on hand to perform the necessary tasks to maintain a high fidelity network, vandalism like this costs us dearly.
It’s a sad story because everybody looses. A seismometer lifted from a vault is ruined unless powered down and locked appropriately. There isn’t a thriving black market for ancillary seismic equipment, and there’s nothing anyone can do with a busted solar panel. Each station is an important part of our network coverage and the components of each seismic site are most valuable operating as such.
Hopefully the rest of the year is kinder.
February 25, 2011 Lisa Linville
We are always keeping an eye out for science outreach resources for educators in Arizona. Erin DiMaggio of ASU has developed a site worth highlighting called SCINEWS.
The purpose of SCINEWS is to provide middle and high school teachers timely, pre-packaged lessons on a science current event (such as an oil spill, earthquake, or shuttle launch) that are short (~15 min), easy to implement, and align to AZ state standards. Materials might include a slide show, videos, maps, photographs,
or KML files for use in Google Earth. Each current event lesson has an associated PDF document that contains a brief overview of the event and lesson, as well as a map, photo(s), and AZ standards targeted. Although using current events in the classroom is not new, the goal here is to provide simple and short lessons that associate ‘textbook’ concepts with real events in the news while allowing for class discussion.
You can find the website at http://sese.asu.edu/teacher-resources
Each lesson has an associated PDF with an overview of the event and lessons with maps and photos and incorporate ppt slide shows, worksheets, video clips or other relevant multimedia. Resources like this greatly benefit from the feedback from target users. Please take the time to fill out the quick survey after using a lesson.
February 17, 2011 Lisa Linville