Wells, Nevada Earthquake February 21, 2008 A 6.0 magnitude earthquake strikes the northeastern edge of the Basin and Range

Wells, Nevada Earthquake Basin and Range Why Nevada? The earthquake occurred near the edge of the Basin and Range province. Over the Basin and Range province the continent is actively rifting. Parks and Plates ©2005 Robert J. Lillie

Continental Rift Features Wells, Nevada Earthquake Basin and Range Tectonics Continental Rift Features Hot Asthenosphere Parks and Plates ©2005 Robert J. Lillie

Continental Rift Features Wells, Nevada Earthquake Basin and Range Tectonics Continental Rift Features Parks and Plates ©2005 Robert J. Lillie

Wells, Nevada Earthquake Historical Seismicity near Wells, Nevada

Wells, Nevada Earthquake The earthquake occurred near the center of more than 400 recording seismometers. Text box links to an animation showing the seismic waves sweeping the seismic array. http://eqseis.geosc.psu.edu/~cammon/QA/wellsNV-21Feb2008.html

Wells, Nevada Earthquake Seismic Data The seismograms surrounding the earthquake center don’t look identical. The differences between the seismograms can be used to reconstruct detailed information on the earthquake. Since the earthquake occurred in the middle of a dense grid, these differences are obvious.

Wells, Nevada Earthquake Strainmeters Strainmeters installed in boreholes at about 230 m depth can measure very small strains associated with crustal deformation. Therefore they pick up earthquakes centered far away, as you can see on the next slide. This slide shows a map of borehole strainmeter installations in the EarthScope PBO network, drilling operations, the actual instrument, and schematic diagrams of it. Strainmeters can resolve strain changes of less than one part per billion (1 mm in 1000 km) at short periods, which makes them ideal for capturing short-term transient deformation over time intervals ranging from seconds to months; at longer periods, GPS techniques are more stable, and by using GPS and strainmeters together, EarthScope captures a fuller range of Earth deformation processes. Borehole strainmeters have been used for more than two decades to measure crustal deformation near active faults and volcanoes in the United States, Taiwan, China, Iceland and Japan. Gladwin Tensor Strainmeters (GTSMs), the instruments that are used by PBO, use differential capacitive plate transducers to measure change in the borehole diameter. As the borehole deforms the plates move relative to each other causing a change in capacitance proportional to the change in distance. Demonstrates how the differences in seismograms around the earthquake can be used to reconstruct detailed information of the earthquake.

Wells, Nevada Earthquake Strainmeters Borehole strainmeters in the EarthScope PBO network from Vancouver Island through California recorded strain events associated with the Wells, NV, earthquake. The timing of the onset at the various stations varies by distance, so you can use these data to determine the epicenter of the earthquake.

Wells, Nevada Earthquake Horizontal Displacement The actual source mechanism (USGS) The modeled horizontal displacement for this earthquake is consistent with the extensional tectonics in the Basin and Range. These modeled displacements can now be compared to the displacements measured with GPS data before and after the earthquake. Discrepanices between the modeled displacements and the actual displacements can be used to refine the model. The modeled source mechanism (Hammond and Kreemer, University of Nevada Reno)

 Geodetic Strain Rates in Great Basin Wells, Nevada Earthquake Map of contours of the rate of strain accumulation. The redder the colors the more rapidly the crust is accumulating strain. This strain is generally released in future earthquakes. The map is produced by interpolation of horizontal motions measured at hundreds of points using GPS. The Wells earthquake (star) happened in an area where the GPS measurements indicate the same amount of strain accumulation as within the stable continent. The earthquake occurred in an area with little strain accumulation, which is surprising. The occurrence of the earthquake can give us new insights in the tectonics. Source: C. Kreemer, Nevada Geodetic Laboratory, UNR.

Why are there so many geophysical instruments in the West, anyway? Wells, Nevada Earthquake EarthScope Why are there so many geophysical instruments in the West, anyway? They are a part of EarthScope - a national earth science experiment. EarthScope is: Exploring the structure and evolution of the North American continent. Understanding the processes that cause earthquakes and volcanic eruptions.

What instruments does EarthScope have? Wells, Nevada Earthquake EarthScope What instruments does EarthScope have? Drilling into the San Andreas Fault Portable Seismometers Permanent Seismometers GPS Stations Borehole Strainmeters Long-baseline Laser Strainmeters EarthScope consists of a vast amount of instruments - GPS receivers, strainmeters, seismometers, magnetotelluric sensors and a hole drilled into the San Andreas Fault at Parkfield. 400 portable seismometers and 100 reference seismometers 20 transportable and 7 permanent magnetotelluric sensors A pool of over 2000 seismic stations available for researchers 116 new and 20 existing GPS receivers along plate boundaries 875 permanent GPS receivers in dense clusters along faults and magmatic centers. A pool of ~100 GPS receivers available for researchers. 103 borehole strainmeters A hole drilled into the San Andreas Fault

Wells, Nevada Earthquake EarthScope Partners