The Flexible Array (FA) component of the USArray densifies the station coverage already provided by the Transportable Array (TA) and permanent networks. FA experiments are PI driven, supported by the PASSCAL instrument center at New Mexico Tech, and usually last 1-3 years. UC Berkeley is currently involved in two such experiments. The Flexible Array Mendocino Experiment (FAME) with the University of Oregon and Rice University is a dense 79 station network in Northern California and the FlexArray along Cascadia Experiment for Segmentation (FACES) is a 23 station array throughout Oregon and Washington in collaboration with Miami University of Ohio. Both experiments employ Guralp CMG-3T broadband seismometers continuously sampling at 40 samples per second with a flat response to approximately 120 seconds period. The figure displays the station coverage along the US West Coast from just north of San Francisco to Seattle.
The Mendocino Experiment is designed to image the crust and upper mantle in Northern California around the Mendocino Triple Junction (MTJ). The 79 stations with a 25km average spacing constitute one fifth of all broadband instruments available from the PASSCAL instrument center. This array consists of three main parts: a dense line along the coast from Cape Mendocino to Healdsburg, CA, an evenly spaced distribution throughout inland Northern California, and 14 stations inherited from the Sierra Nevada Earthscope Project (SNEP) in the southeast region of the array. The instruments were primarily installed in two surges: the first in July 2007 and the second in October 2007. The experiment will run through the fall of 2009.
The primary goal of FAME is to investigate and further understand the complex proccesses and structures involved at the intersection of the North America, Gorda, and Pacific plates where a subduction zone and two transform boundaries meet. This triple junction is migrating to the north relative to a fixed North America plate and thus provides a rare opportunity to study an unstable triple junction. FAME will improve the understanding of how and why the tectonics change from a transform fault to a subduction zone along the west coast.
Gorda crust and overlying sediment subduct and become incorporated in accretionary terranes from the Cascadia subduction zone. This mass flows into the emerging transform margin and becomes structured into the San Andreas strike-slip system. Simultaneously, slab gap opening south of the Gorda slab causes asthenospheric ascent and decompression melting, which magmatically underplates North America near the San Andreas fault. The reprocessed and inflated lithosphere thickens to create a small Cape Mendocino orogenic plateau. Meanwhile, erosion moves large fractions of the upper crust back to the subduction zone along tectonically controlled north-trending rivers. Nearby, Gorda-Juan de Fuca subduction results in the Cascade volcanoes, which also contribute to continental crust growth, segregation and recycling. The 3-D seismic velocity models of the crust and upper mantle, which will be developed as part of this project, will be included in event and strong ground motion characterization for Northern California and southern Oregon, a site of potentially devastating great earthquakes.
FACES is a long array from the California/Oregon border to Seattle deployed at an average spacing of 40km. The majority of stations are in Oregon due to its relatively sparse coverage from permanent networks. The stations were installed in early November 2007. The availability of cellular phone coverage throughout the region allows deployment of modems on most of the stations for real time telemetry. This is the first FA experiment to employ this technology allowing constant monitoring of the state of the network from anywhere with Internet access.
The recurrence of Episodic Tremor and Slip (ETS) is widely observed, but poorly understood. Recent findings show segmentation in recurrance interval of ETS along the Cascadia Subduction Zone (Brudzinski and Allen, 2007). The region appears to be split into three primary zones of distinct recurrence intervals. The best preliminary correlation appears to be with the geologic terrain of the continental plate. The increased station density due to FACES throughout this region will better illuminate this phenomenon.
A common installation difficulty results from an uneven base upon which the seismometer is set. In these deployments, when possible, self-leveling grout was employed and left to set overnight. This allowed for minimal effort being exerted to level the sensors.
Heat fluctuations can damage both the sensor and the controlling electronics package and temperature changes can become a significant source of noise. To combat these dangers, the sensors are placed in a plastic bag and surrounded with insulating sand. The electronics package is protected by a standard plastic case with reflective material reducing the effect of direct sunlight.
The most exciting development in the current deployments is the utilization of telemetry through cellular phone service. A small cellular modem is installed with the electronics package of the sensor and sends station health information and realtime data back to the BSL through cell phone towers. FACES is currently the largest deployment using this technology.
The authors would like to thank the following for their hard work installing and servicing various stations: Derry Webb, Marcos Alvarez, Lloyd Carothers, Eliana Arias Dotson, Pat Ryan, Lisa Linville, Pallavi Chethan, Kevin Jensen, Chris McMillan, Stefany Sit, Andrew Tran, Summer Ohlendorf, Dan'L Martinez, Valerie Zimmer, Will Levandowski, Amanda Thomas, Heidi Reeg, Nickles Badger, Tom Owens, Eileen Evans, Holly Brown, Joanne Emerson, Ajay Limaye, Rick Lellinger.
This work has been made possible with the resources available through the PASSCAL instrument center at New Mexico Tech.
Funding has come from NSF grants EAR0643392 and EAR0745934 for FAME and EAR0643007 for FACES.
Brudzinski, M. and R.M. Allen, Segmentation in Episodic Tremor and Slip All Along Cascadia, Geology, 907-910, 2007.
Berkeley Seismological Laboratory
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