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A Policy for Rapid Mobilization of USGS OBS (RMOBS)

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Mendocino Triple Junction Offshore Northern California

Written by David Oppenheimer

The Mendocino Triple Junction (MTJ) is one of the most seismically active regions of the San Andreas transform system. Since 1983 the region (Figure 1) has generated about 80 Mgreater than or equal to3.0 quakes each year, and historically the region has experienced major quakes.  This activity is generated in response to ongoing plate motions between the Gorda, North America, and Pacific plates.

Image of seismicity for northern California

Figure 1. Mgreater than or equal to2 in northern California coastal region 1985-2003.  White lines are major roads.  Brown lines are faults active since the Late Quaternary.  Labeled green polygons bound regions described in scenario section of text. G-NA= Gorda-North America, G-P= Gorda-Pacific, and P-NA=Pacific North America.

  • The Gorda plate is converging on the North America plate at about 2.5 to 3 cm/year in the direction N50ºE to N55ºE. Gorda – North America plate subduction process has resulted in major plate subduction quakes (M8+ in 1700, Ms7.1 in 1992) and is responsible for the uplift of the coastal ranges in the region through motion on thrust and reverse faults. 
  • Interaction between the Gorda and Pacific plates occurs as oblique convergence at a rate of approximately 5 cm/year in the direction N115ºE.  Translational motion occurs along the east-west-trending, vertical right-lateral Mendocino transform fault.  The most recent, significant quake to rupture this fault occurred 9/1/1994 and had an Mw 7.1.  The Gorda-Pacific convergence results in internal deformation of the Gorda plate, such as the 11/8/1980 Ms7.2 quake, and more recently the two Ms6.6 aftershocks of the 1992 quake. 
  • The San Andreas fault marks the principal Pacific-North America plate boundary south of the MTJ. The SAF last ruptured in the 1906 M7.7 quake and geophysical evidence indicates that rupture extended within a few 10s of km south of the MTJ. 
The USGS Northern California Seismic Network (NCSN) and the Berkeley Seismological Laboratory of the University of California Berkeley jointly monitor earthquake activity in the MTJ region.  The two institutions operate real-time seismic networks that receive automatically post earthquake information on the Internet within minutes of occurrence. They compute a complete range of seismological products, from earthquake locations, Md, Ml, and Mw, ShakeMaps, first-motion mechanisms, moment tensors, and finite fault estimates through inversion of waveform data. In this region the network consists of analog stations and digital stations with tri-axial broadband and accelerometers that transmit data continuously to either Menlo Park or Berkeley.

There are no islands offshore of the California coast at the MTJ. Thus, earthquake location accuracy rapidly degrades as the quakes occur farther from shore because of the one-sided seismic network station geometry.  Depths of quakes west of 124º40'W (approximately 50km offshore) are unreliable except for the larger (M>3.5) quakes for which moment tensor inversions can resolve the depth.  Likewise, the location accuracy in the direction perpendicular to the coast degrades with distance offshore.  The addition of Ocean Bottom Seismometers (OBS) would greatly improve the ability of the NCSN and BSL to understand the seismotectonics of the MTJ following significant quakes.  Improved understanding of the rupture mechanics of a subduction zone quake would enable seismologists and hydrodynamicists to more realistically forecast onshore shaking levels and tsunamis.  OBS recordings with extended dynamic range and frequency response could possibly reveal the generation mechanics of slow, tsunami quakes.  Near-field recordings of offshore earthquakes would greatly improve depth estimates of aftershock sequences and confirm stress transfer models in a region with significant seismic risk.

There are several earthquake scenarios that could warrant a mobilization of the OBS systems. a) M>6.5 quake on the Gorda-North America subduction zone ("G-NA" polygon), b) M>6.5 quake on the Mendocino fault east of longitude 126ºW ("G-P" polygon), and c) M>6 quake on the SAF north of Point Arena ("P-NA" polygon).   Deployments within 3 days of such an event are highly desirable due to the unpredictable aftershock decay rate. 

There are several harbors along the coast that would provide a base where vessels could be hired to deploy the instruments.  Humboldt Bay (Arcata and Eureka) provides the best facilities in the vicinity of the MTJ. There are harbor facilities north of Humboldt Bay at Trinidad and Crescent City. The principal harbors south of the MTJ, which would be more suitable for responding to quakes on the offshore section of the SAF, are at Shelter Cove and Fort Bragg.

Any data recorded by the OBS system will be integrated with the seismic data collected by the NCSN and BSL real-time systems.  Phase information (arrival times, amplitudes, durations, etc.) will be merged into the catalog to improve the data set.  All parametric data, waveforms, and metadata describing the waveforms will be archived at the Northern California Earthquake Data Center which provides access to the public.

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