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Wanted: Host Homes for Important Quake Instruments Posted: 23 May 2013 05:00 AM PDT Urban Seismic Studies to Begin in East San Francisco Bay AreaAbout 30 seismic sensors are seeking a place to hang out for up to three years in the greater Pleasanton/Dublin/San Ramon, Calif., area. These sensors will help U.S. Geological Survey scientists conduct the next step in vital research to better understand how earthquakes behave in and around the East San Francisco Bay. USGS scientists have been operating a number of urban seismic arrays in the East San Francisco Bay area since 1999. The sensors in these surveys are strong motion instruments, designed to trigger when signals from a nearby earthquake are detected. Arrays are currently operating in private homes and businesses in San Lorenzo, San Leandro, Niles and Pleasanton. Bay Area residents may be acquainted with USGS NetQuakes – a program to increase the number of seismic stations to augment permanent regional seismograph networks in major cities such as San Francisco. In contrast to NetQuakes stations, the instrumentation in the urban arrays for this current study are not designed to connect to a communication system in order to transmit to the USGS National Earthquake Information Center for real-time earthquake monitoring; rather, triggered data is collected and stored on a local disk and retrieved by scientists during maintenance visits twice a year. The placement of stations in the urban arrays are carefully planned to answer specific seismic hazards research questions. The distance between stations 'tunes' the array to detect particular seismic waves in a manner similar to the way radios detect different stations by changing the frequency of the signal receiver. The pattern or distribution of stations is designed to capture the spatial variability in ground motion or site response. The urban array sensors are set to trigger at specific sensitivities that increase the likelihood that acquired data will be earthquake signals rather than other noise sources. Like the NetQuakes stations, scientists hope to capture earthquake data for events as small as magnitude 2.0. Site response observed from small events can help predict ground motion behavior in larger events. USGS scientists will be in the area during the month of June 2013 to meet with interested hosts and answer any questions they may have prior to deployment of the instruments in August. Interested parties may visit the Volunteer Monitoring website for more detailed information and images about the project as well as contacts.
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| Posted: 22 May 2013 01:36 PM PDT Measuring the Forces Generated by Erosive Debris FlowsDiving deep into the mechanics of bedrock incision caused by debris flows, scientists now have a better understanding of the erosive forces responsible for cutting valleys into mountainous terrain, according to recently published research in the Journal of Geophysical Research-Earth Surface. "Our field-based measurements shed new light on what is happening beneath fast-moving debris flows," said U.S. Geological Survey scientist Jeff Coe. "Debris flows carry large rocks and impacts from those rocks make the flows very efficient at carving bedrock from valleys bottoms." Debris flows are fast-moving landslides that occur in a wide variety of environments throughout the world. They are particularly dangerous to life and property because they move quickly, destroy objects in their paths, and often strike without warning. The new research provides an improved understanding of how mountain valleys are formed and a better idea of the rate of formation given the frequency of debris flows in the current climate. "A possible extension of this work would be to incorporate the results into quantitative landscape evolution models that predict how landforms evolve through time given static, or changing climatic conditions. Improved knowledge of how landforms evolve is useful for hazard assessments and possibly for long-term, land use planning," said Coe. For four years, scientists from the University of Colorado and USGS used specialized instruments installed at the Chalk Cliffs Natural Debris Flow Laboratory near Buena Vista, Colo. to monitor 11 naturally-occurring debris flows. During that time, they found that downward-directed impact forces beneath the flows caused about 30-60 millimeters (1.2 to 2.4 inches) of bedrock erosion. The group observed the mechanisms by which the bedrock was removed by passing debris flows and determined the statistical distribution that best characterized the impact forces. "We also found that a thin layer of sediment shielded the bedrock surface from debris-flow impacts and erosion," said Scott McCoy, lead author from the University of Colorado, now at the Massachusetts Institute of Technology. "Our measurements and statistical analyses provide a foundation for linking impact forces that cause erosion to easily measured debris flow properties." |
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