INTRODUCTION

The seismic earthquake hazards posed by blind thrust faults in southern California have been reported by a number of investigators [Stein and King, 1984; Stein and Yeats, 1989; Wright, 1991; Crouch and Suppe, 1993; Davis and Namson, 1994; Shaw and Suppe, 1994; Shaw et al., 1994]. Seismic-reflection methods are a valuable and necessary means of imaging the folding associated with blind thrusts, and have provided constraints for balanced cross sections used to map thrust ramps in the subsurface. Both the damaging 1971 San Fernando and 1994 Northridge earthquakes occured on a blind thrusts [U.S. Geological Survey and the Southern California Earthquake Center, 1994].

This report describes shipboard operations on the R/V Maurice Ewing, leg EW-9415, including the acquisition of deep-crustal multichannel seismic-reflection data as part of the Los Angeles Regional Seismic Experiment (LARSE). LARSE is a cooperative study of the crustal structure of southern California involving scientists from the U.S. Geological Survey (USGS), Caltech, the University of Southern California, the University of California Los Angeles, and the Southern California Earthquake Center (SCEC). Seismic-reflection profiling in the greater Los Angeles area, 13-21 October, 1994, used the Ewing's 8470 cu. in. (137.7 liter) air gun array and 160-channel, 4.2-km digital streamer. The Ewing's air gun source was also recorded by 170 temporary land recorders (REFTEKs), the permanent southern California earthquake net, an array of 9 ocean-bottom seismometers deployed along two lines, and two successful sonobuoys. The Ewing fired nearly 24,000 air gun shots during the LARSE work. The Ewing, a 239' (72.8 m) long UNOLS vessel, formerly an industry seismic-reflection vessel, is operated by the Lamont-Doherty Earth Observatory.

The R/V Ewing acquired several deep-crustal seismic-reflection profiles on the continental shelf of the Inner California Borderland, in the vicinity of Los Angeles, stretching from the 32 degrees 38'N to 34 degrees N and from 119 degrees 45'W to 117 degrees 40'W (Figure 1a). At the heart of LARSE was the collection of three long onshore-offshore lines, Lines 1 to 3, which provided air gun signals for land recorders deployed along the projections of these lines. These lines include: (1) Line 1 trending N-S from the center of San Clemente Island through Seal Beach projecting into the 1933 Long Beach earthquake epicenter and the Mojave Desert, (2) Line 2 trending N-S along the western shores of San Clemente and Catalina Islands through Santa Monica projecting through the 1994 Northridge earthquake epicenter, and (3) Line 3 trending NE-SW from northwest of San Nicolas Island through the center of Los Angeles basin [Wright, 1991]. These three lines provide a regional reconnaissance of the crustal structure centered on the Los Angeles Basin: in addition they provide specific information about the crustal structure in the vicinity of two recent, damaging earthquakes in the greater Los Angeles region. Special care was taken to acquire large segments of these lines late at night (2300 L to 0500 L) [all local times given herein are Pacific Standard Time, which is 7 hours behind UTC], causing us to repeat parts of Lines 1 and 2 up to 5 times (Figures 2a and 2b). The planned ship track was altered throughout the cruise in order to maximize the acquisition of onshore-offshore data during late-night hours. Ocean bottom seismometers (OBS's) were deployed only along LARSE Lines 1 and 2.

Shorter transit reflection lines, TR1, TR2, and TR3, connected LARSE Lines 1, 2 and 3 (see Figure 1a and 1b). These transit lines were recorded in an oblique, fan geometry by the temporary and permanent onshore seismic stations. LARSE Line 3 was acquired between Lines 1 and 2 to provide enough time for the array of nine ocean bottom seismometers (OBSs) to be retrieved from along Line 1 and redeployed along Line 2 (Appendix 1). Upon completion of Line 2, the Ewing acquired reflection data along Line TR3 and then repeated the southern half of Line 1. The Ewing then acquired a series of short lines (Lines 4 and 6) crossing the Palos Verde and Newport-Inglewood faults while heading southeasterly towards San Diego, California (Appendix 1). Pre-cruise plans to shoot multichannel seismic (MCS) reflection lines in Santa Barbara Channel were abandoned because of the need to reshoot parts of Lines 1 and 2. No air gun shots were fired within the three-mile state limit for acoustic sources. Companion Open-file Reports describing the ocean-bottom seismometer and onshore recording of the LARSE air gun shots as well as the LARSE land refraction work using large chemical explosions are in preparation.

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