Archive for February, 2011
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
In the 1950s and 1960s traditional land survey techniques, e.g., differential leveling, revealed widespread land subsidence in the basins of south-central Arizona. The Picacho Basin, situated between Phoenix and Tucson, is a good example; a site in the basin near Eloy experienced more than 15 ft of subsidence between 1952 and 1985. But differential leveling surveys are time-consuming, expensive, and prone to long time gaps between successive surveys.
In 2002, the Arizona Department of Water Resources turned to satellite InSAR data (SAR for synthetic aperture radar, In for interferometry) to monitor ongoing subsidence. InSAR data is capable of measuring cm-scale
changes in elevation. Satellite monitoring offers opportunities to revisit a basin with greater frequency than occurs with traditional survey methods. ADWR’s InSAR program includes time-series data for select valleys in Maricopa, La Paz, Pima, Pinal, Graham, and Cochise Counties.
The InSAR time-series data provides, too, a baseline against which disruption of the basin floor, say due to faulting, can be measured. Arizona hosts a number of faults active in the Holocene (past 10,000 years) and the Pleistocene (2.6 million years to 10,000 years ago). Most of these faults appear to have long repose periods between rupture events. And most, because they are characterized by small- to moderate-magnitude earthquakes, bear little surface expression. But the advent of InSAR data could be instrumental in detecting small deformation associated with these not-so-uncommon geologic structures. Insar is capable, too, of detecting deep, cryptic faults that display a subtle and hard-to-detect surface expression.
Mike Conway, 24 February 2011
Some sources on InSAR and fault detection.
Anticipating Earthquakes — High above Earth where seismic waves never reach, satellites may be able to detect earthquakes before they strike.
I sat in on FEMA’s Earthquake Safety and Mitigation for School Buildings webinar this past Thursday. FEMA’s goal: provide guidance to stakeholders on planning and deploying mitigation strategies for minimizing earthquake risk at America’s schools.
- Understanding earthquake hazards
- Recognizing earthquake vulnerabilities in schools
- Reducing EQ risk
- Implementing Incremental seismic rehabilitation
- Recommended actions
As Bill pointed out, risk of injury or infrastructure damage from earthquakes is considered a low probability — high consequence event; of course, in western U.S. the probability of such an event are somewhat greater. FEMA uses the classic risk formula for quantifying risk; Risk = Hazard * Vulnerability
FEMA favors an incremental seismic rehabilitation approach to mitigating risk. This incremental approach minimizes disruption of school activities while providing opportunities to phase in mitigation strategies as part of building maintenance and upkeep, thus reducing costs.
Bill drew heavily from FEMA’s Risk Management Series, Incremental Seismic Rehabilitation of School Buildings (K-12) Providing Protection to People and Buildings.
There are three parts to the text:
Part A, Critical Decisions for Earthquake Safety in Schools, is for superintendents, board members, business managers, principals, and other policy makers who will decide on allocating resources for earthquake mitigation.
Part B, Managing the Process for Earthquake Risk Reduction in Existing School Buildings, is for school district facility managers, risk managers, and financial managers who will initiate and manage seismic mitigation measures.
Part C, , is for school district facility managers, or those otherwise responsible for facility management, who will implement incremental seismic rehabilitation programs.
Mike Conway, 18 February 2011
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
Joe Cook (AZGS Earth Fissure Program Manager) and I met today with a tour group hosted by the Water Education Foundation of Sacramento, California. The group, which comprised representatives from federal and state agencies, mining and agricultural industries, water suppliers, water district managers, and conservation groups, met us at the south end of the Casa Grande Mountains to discuss the role of ground water pumping on subsidence and earth fissure formation.
We had 30-minutes to introduce the topic and walk the entire crew of 51 along a fissure while pointing out its salient features. The group was energetic and had lots of questions; too many for us to answer in that short time.
One question that surprised me went like this, “Is earth fissure formation accompanied by earthquakes.” (The most widely accepted model of fissure formation has fissures forming at the groundwater table and then propagating upward to the ground surface.) My response was no, fissure propagation from depth is aseismic, after all it is not like a fault where stresses slowly accumulate to the point that rocks suddenly rupture.
But fissures form as tensional stress accumulates during heterogeneous subsidence of the basin floor. Perhaps a tightly sewn seismic net would detect microseisms. Fissures in the north Las Vegas area coincide with Pleistocene faults. It may not be unreasonable to infer coincidence of fissure propagation and small-magnitude seismicity. Rare fissures in Cochise County show as much as a meter of offset across the fissure, so the question of seismicity – or more likely microseismicity – accompanying fissure formation may in some cases be valid.
With that said, I’m unaware of any studies that document a causal relationship between fissure formation and seismicity in basins of the Basin and Range Province.
Mike Conway, 16 February 2011
In 2003, a magnitude 6.5 earthquake occurred near San Simeon Bay, just three miles from the Hearst Castle. A new video by FEMA, QuakeSmart – Mitigation Works for Business, extols the architectural design of the San Simeon castle which sailed through the quake suffering no damage. Julia Morgan, a pioneer female architect and survivor of the 1906 San Francisco earthquake, designed the building. No doubt William Randolph Hearst would be enheartened.
In Nearby El Paso del Robles, just six miles from the epicenter, it was a mixed story. Six months before the earthquake, local businessman Jim Saunders finished a seismic retrofit on his turn-of-the-century building. They stiffened the building by installing ledger beams, trusses, I-beams and by reinforcing structural integrity by tying the interior walls to the outer walls. His building, too, sailed through the quake with no damage. But there the story takes a grim turn as Mr. Saunders describes how partial collapse of a building across the street killed two women.
QuakeSmart is six-minutes long with superior cinematography and compelling interviews.
QUAKESMART ~ http://www.fema.gov/medialibrary/media_records/3566
Mike Conway, 8 February 2011
A question we commonly get following an earthquake large enough to be reported in the news is “What fault did this earthquake occur on?” Earthquakes are, after all, the release of elastic strain energy following brittle failure of the crust. Since tectonic earthquakes are a response to the stress applied to a dynamic crust experiencing pressure from ongoing sources, it makes sense that we try to identify the zones of weakness on which we expect those stresses to be manifested. Unfortunately the answer to this question is rarely straightforward and earthquakes in Arizona are certainly no exception.
Fault plane solutions
An analysis of the fault associated with a given earthquake starts with a focal mechanism generated by observations taken from the seismic record. Modeling and analysis of waveforms from events give us meaningful information about source dynamics. For small to moderate earthquakes in regions with sparse network coverage phase data is often too poor to yield unambiguous moment tensor solutions. For our record of 1216 cataloged events in Arizona, only a small handful have associated fault plane solutions. Availability of data and relevant scientific interest will always be legitimate barriers to a complete understanding of crustal structure, but they are certainly not the only limitations.
Going from source mechanisms to known fault:
Once the orientation and direction of movement that define the fault plane are identified, pinpointing the responsible fault isn’t always as easy as looking at a map. An understanding of historic faults is heterogeneously created from the research of scientists interested in the crustal structure of a certain area. Generally this interest stems from the need for hazard mitigation or known historical events. The USGS for example compiled a map of Quaternary age faults for the western US thought to be associated with events greater than magnitude 6 from thousands of journal articles, maps, theses and other research (http://earthquake.usgs.gov/hazards/qfaults/google.php).
This is a good community resource for understanding the location and names of known faults, however there is likely a level of incompleteness for certain types of faults as well as poorly monitored regions or unpopulated places. Often, as in the case of Christchurch, NZ, little is known about the existence or extent of a fault until large events focus attention on them (NZ article on extent of Greendale fault from last years 7.0 quake: http://www.stuff.co.nz/the-press/news/4586170/Whose-fault-is-it-anyway-The-scientists-remain-divided). Likewise after the earthquake in Haiti last January, scientist blamed the Enriquillo fault. After months of dedicated research and investigation we now know that the earthquake was a result of rupture on a previously unknown parallel fault named the Léogâne fault. These are earthquakes of considerable size. The amount of data collected for them is enormous. The USGS reported 428 phase picks for the Haitian quake. By comparison, an earthquake in the magnitude 2.5 range in California generates on average probably 20 phase picks, in some areas of Arizona or Alaska the number of stations which register that magnitude might be as few as 6.
A lot of our understanding of fault recurrence intervals is built upon observations from the geologic record. Mike Conway’s Lidar post on January 21st is a good example of new tools being used to identify and understand surface faults. Or the surface trenching that was done to help understand the Chino Fault covered in yesterdays anniversary post. These are often our best tools for understanding fault dynamics over the long term. But surface rupture is not a guaranteed part of the earthquake process. Whether an earthquake breaks the crust at the surface or anywhere near it is a function of many variables: the amount of energy released during the event, the depth of the focus, the composition of the overlying material, etc. The deeper the earthquake focus, the larger the earthquake has to be to rupture the surface. It is often the case that we are unable to use ground based observations to verify movement or location for a fault surface and it is often these faults which small to moderate seismicity is related to. A large fraction of global seismicity is not associated with correlative surface rupture because there isn’t any. The 1994 Northridge earthquake (Mw 6.7) occurred on a blind thrust and produced the strongest ground motions ever instrumentally recorded in an urban setting in North America. The Southern California Earthquake Data Center provides an animation for understanding this type of fault (http://www.data.scec.org/chrono_index/pop_blind.html). The amount of data we have for the Northridge fault is a function of the location of the fault and size of the events it tends to generate. It is often the case that these faults go undetected unless they pose a specific threat or generate obvious seismicity.
For each earthquake, realistically what we can do is use the data we collect from an event to characterize the style and orientation of failure in a certain zone, and then use what we know about current and historical structure to tell the best story we can. This is harder for some events than others. For some earthquakes like the February 4th Chino Valley event, there is a likely culprit on hand. This is also true for a good bit of seismicity in the northeastern part of Arizona. Faults are often less discrete than the lines on our maps indicate. When an earthquake is located within the region of a fault but not on top of it- its likely that the fault is better characterized as a zone of weakness than a single, clean line of failure. It is also sometimes the case that earthquakes occur in places that are unexpected. New faults like the Léogâne are being found all the time. The closer we look, the more we find, and the more complete our stories become.
Lisa Linville, 4 February 2011
“Earthquake jars area; no damage”, read the headline in the Prescott Courier on 4 February 1976. The shaking lasted a mere 2 or 3 seconds. No one was injured and there was no significant damage to homes, buildings, or roads, but shop owners in Chino Valley reported canned goods and flower pots falling off shelves, according to the Prescott Courier. One Prescott resident thought the boiler in her neighbor’s home blew. The epicenter – that point on the Earth’s surface where the seismic waves first strike the ground surface – was situated north of Prescott between Paulden and Perkinsville.
The 7.5 M Guatemala earthquake totally eclipsed the Chino Valley event that day.
The Guatemalan quake killed more than 23,000 people, injured 54,000+, and left 200,000 homeless — adobe construction, a style particularly vulnerable to earthquakes, was common throughout the Guatemalan Highlands. The people of Guatemala City, Guatemala’s capital, were particularly hard hit. Large aftershocks in the wake of the main event added to the human toll.
The Chino Valley earthquake occurred at precisely 00:04:58 GMT on 4 February 1976 with a magnitude (ML) of 5.1. The maximum Modified Mercalli intensity, a measure of impact on human society, was VI, characteristic of moderate shaking, falling objects, and felt by all in the vicinity. Shaking was felt over 10,000 sq. miles, from Yuma and Tucson in the south to Flagstaff in the north. From the focal mechanism, seismologists inferred a fault plane trending about 120 degrees or WNW-ESE, dipping 40 degrees SW. Fault motion was normal – footwall up and hangingwall down – consistent with Basin and Range extensional faulting. Depth to the focus – the point of rupture along the fault – was ill-defined and estimated at 6 to 9 miles. Seismometers recorded nearly 200 aftershocks, only a few of which were felt.
Identifying the fault that caused the Chino Valley earthquake was precluded by the absence of observed ground rupture; the seismic network in central Arizona was insufficiently dense for precisely locating small to moderate-sized earthquakes. But a likely suspect was the Big Chino Fault or one of a number of related splay faults.
The Big Chino fault strikes NW-SE and parallels Big Black Mesa for nearly 35 miles northwest of Paulden (see figure to the right). A prominent fault scarp testifies to the youthful nature of this fault system. From studies of the surface fault scarp and from three trenches excavated along the fault, it appears that displacement during a rupture event ranges from 6 to 9 feet. Repose, the period of time between major rupture events, is roughly estimated at 10,000 years to as much as 30,000 years. On the basis of the fault length and displacement of individual rupture events, Phil Pearthree, chief of AZGS’s Environmental Geology section, estimates that the Big Chino fault is capable of yielding 6.5 to 7.1 or 7.2 magnitude earthquakes. The latter is roughly equivalent to the devastating Guatemalan earthquake of 1976.
The Guatemalan earthquake was attributed to rupture of the Motagua Fault, complemented by movement on the Mixto Fault near Guatemala City. The former is a left-lateral, strike-slip fault that transects Guatemala from east to west and forms the plate boundary between the North American and Caribbean Plates. The focus of the 1976 event was pinpointed at a depth of 3 miles. The Motagua Fault is among the world’s most active fault zones and certainly capable of producing moderate to large earthquakes with a frequency on the human scale.
Mike Conway, 3 February 2011