Volunteer Monitoring - Greater Pleasanton, CA, Area Seismograph Array
The USGS is requesting volunteers in the greater Pleasanton/Dublin/San Ramon area to consider hosting a seismic station for a period of two to three years. All participation is strictly voluntary. The figure to the right shows the research area from East Bay Hills on the west between San Ramon and Pleasanton to the hills on the east and northeast of San Ramon, Dublin, and Pleasanton encompassing San Ramon, Amador, and Livermore Valleys. The benefits, in addition to the personal satisfaction of knowing one has contributed to the scientific understanding of the area, include a yearly guided tour to the earthquakes that are recorded on the home system when the engineers come through to retrieve the data. 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. The planned date for installation of the new array is August 2013.
For more information on participation in this study please contact:
The type of sensor used in this study is the K2 seismograph which is an accelerometer and data-logger combined into a single, portable package. Photos of typical garage, patio and business installations can be seen below. From left to right the K2 is installed in a garage corner (the tiles are 1 foot square), in a garage under a work bench, on a back patio enclosed in a rubber tub for weather protection, and in a crawl space on a concrete floor. The last two photographs show the GPS antenna installation mounted on wooden fence support or mounted on a garage joist under a wooden/asphalt roof. Click on thumbnail images to see larger versions.
All systems are installed on concrete floors anchored using a concrete epoxy; and are AC powered utilizing a nearby vacant AC power outlet consuming about 3 watts of power or approximately 50 cents per month at PG&E rate of 0.16 cents per kilowatt-hour. A separate GPS device must be installed in order to correctly time stamp the earthquake signal. This is accomplished by running a coaxial cable along the rafters of a garage or along an external house corner to a height that enables the GPS to receive signal from the necessary satellites.
Background Information about Planned Study
Earthquake scientists have observed for some time that the magnitude and duration of shaking from an earthquake varies considerably over short distances. For example, one structure may be damaged by the event and a comparable building nearby sustains little or no damage. Scientists have confirmed with instrumental readings that this variability is real and not a facet of construction quality or age of buildings, though such factors do contribute to damage potential. Seismologists define a term ‘site response’ that quantitatively describes the behavior of individual points of ground when subjected to strong shaking. They have learned that site response is determined by a number of factors including the depth of soil, the shear wave velocity of the material, and the water content of this underlying soil. Sites on bedrock transmit incoming energy at a 1:1 ratio. Sites that are on unconsolidated material, such as soil, exhibit ground motion that is amplified in proportion to the depth and material properties of the underlying soft material. An additional factor in determining site response is the complex interaction of seismic waves that can be ‘trapped’ in sedimentary basins such as the Livermore Valley. These basin waves can be reflected at the basin edges and interact with incoming waves in a manner that causes additional amplification as well as increasing the duration of strong shaking in a particular location.
In an attempt to better understand the distribution of site amplification and basin wave behavior, USGS scientists have been operating a number of seismic arrays in the East San Francisco Bay area since 1999. The sensors in these arrays are strong motion instruments, designed to trigger when signals from a nearby earthquake are detected. Arrays are currently operating in San Lorenzo, San Leandro, Niles and Pleasanton. The sensors in these arrays are located in private homes and businesses. 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 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.
Analyzing data from past arrays in the Santa Clara Valley, USGS scientists have observed basin surface waves from the 2007 M5.6 Alum Rock earthquake and other smaller earthquakes crossing the basin. Resultant research of data collected allowed detailed mapping of ground motions in the Evergreen Basin as shown in figure at right. Similarly, to closely study the basin structure beneath the San Ramon, Amador, and Livermore Valleys and better constrain material properties in this densely populated area, scientists are planning to configure a dense seismograph network in August 2013. Station spacing will be about one kilometer with deployment goal of 30 stations. This array configuration should allow scientists to study in greater detail the site amplification phenomena caused by subsurface structure and seismic wave interactions that can increase the amplitude and duration of shaking during an earthquake. Please see map above of the areal extent of the seismograph array.