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A Policy for Rapid Mobilization of USGS OBS (RMOBS)

Potential Sites
Alaska Volcanoes
Continental Borderland
Mendocino Triple Junction
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Obligations of RMOBS Participants
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Continental Borderland Offshore Southern California

Written by Douglas Given, Lucy Jones, Egill Hauksson

The offshore area between the Patton Escarpment (continental slope) and the coast of southern California is known as the Continental Borderland. It is a series of generally northwest trending basins and ridges some of which extend above sea level as the Channel Islands. The ridges and basins are bounded by several major active faults (Figure 1). These are major tectonic features capable of producing damaging earthquakes in close proximity to several metropolitan areas of southern California. The are also potentially tsunamigenic. This structural fabric is part of the transform boundary between the Pacific and North American tectonic plates.    

Map of Continental Borderland

Figure 1. Borderland tectonic structures and seismicity. Dark circles show epicenters of MLgreater than or equal to5.0 earthquakes from 1920 to 1997. Green and red dots indicate good and poorer quality earthquake locations from the Southern California Seismic Network catalog since 1981. (from P. Shearer)

The Southern California Seismic Network (SCSN) is tasked through the National Earthquake Hazards Reduction Program (NEHRP) to monitor earthquake activity in the southern California region. It provides real-time information for emergency response agencies, engineering data for hazard mitigation and land use planning, and state-of-the-art scientific data for research.  The SCSN is a cooperative project of USGS and Caltech. Together with USGS – Menlo Park, U.C. Berkeley, and the California Geological Survey we form the California Integrated Seismic Network (CISN) which is a region of the Advanced National Seismic System (ANSS).

The SCSN is one of the largest and most automated seismic networks in the world. It consists of more than 350 analog and digital seismographic stations that relay both continuous and triggered seismic data back to a central computer facility in the Seismology Laboratory on the Caltech campus in Pasadena, Ca. Data from SCSN is used for a variety of research projects that would be enhanced if OBS data were gathered following offshore and near-shore events. Supplementing the SCSN with OBS sites following and offshore event would have several benefits; 1) more events would be recorded because the detection threshold would be lowered by denser station coverage, 2) location and depth determinations would be better constrained, and 3) magnitude determinations would improve.

There are several earthquake scenarios that could justify mobilization of the RMOBS system.
  • A major earthquake (> M 6.5) in the Borderland (For example, 1927 Lompoc earthquake)
  • A major earthquake (> M 6.5) in a coastal region that could be better recorded with addition instrumentation to the west (For example, the 1933 Long Beach earthquake).
  • Unusual  seismic activity like swarms. For example, the unusually protracted 1986 Oceanside earthquake swarm.
In addition, there may be other situations that are not described there that may warrent deployment. In all cases of post-event response rapid deployment is critical to capturing the post productive parts of earthquake sequences. Following is a list of historic events in the Borderland.

year mo dy hrmn      (N)       (W) 
1800 11 22 2130       33  0.   117 18.00   6.5  San Diego region
1812 12 21 19 0       34 12.00 119 54.00   7.0  Santa Barbara Channel
1862  5 27 20 0       32 42.00 117 12.00   6.0  San Diego region
1883  9  5 1230       34 12.00 119 54.00   6.3 Santa Barbara Channel
1925  6 29 1442       34 18.00 119 48.00   6.3  Santa Barbara
1927 11  4 1350       34 42.00 120 48.00   7.3  SW of Lompoc
1933  3 11  154       33 37.00 117 58.00   6.3  Long Beach
1951 12 26  046       32 48.00 118 18.00   5.9  San Clemente Island
1981  9  4 1550       33 40.00 119  7.00   5.9  N. of Santa Barbara Island

There are many harbors in southern California where vessels could be contracted to deploy the RMOBS instruments including Santa Barbara, Ventura-Oxnard, San Pedro-Long Beach, and San Diego.

Any data recorded by the RMOBS system must be integrated with the routine SCSN dataset to be of value. Parametric data (phase arrivals, amplitudes, etc), waveforms, and station metadata describing the instrument responses will be archived at the Southern California Earthquake Data Center (SCEDC) which will make it available to researched, engineers and the public.


The SCEC Borderland Working Group:Science and Data Collection Objectives, 2002
Evaluation of Tsunami Risk to Southern California Coastal Cities
Mark R. Legg, Jose C. Borrero, and Costas E. Synolakis

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