Report for air-gun data acquired at onshore stations during the 1994 Los Angeles Region Seismic Experiment (LARSE), California

Abstract

This report describes the acquisition and reduction of deep-crustal onshore-offshore seismic refraction/wide-angle reflection data in the Inner California Borderland-Los Angeles metropolitan region, conducted in October 1994 as part of the Los Angeles Regional Seismic Experiment (LARSE). LARSE is a cooperative study of the crustal structure of southern California involving earth scientists from the U.S. Geological Survey (USGS), California Institute of Technology (Caltech), the University of Southern California (USC), the University of California Los Angeles (UCLA), and other institutions of the Southern California Earthquake Center (SCEC). During LARSE, the R/V Ewing's 20-element air gun array, totaling 137.7 liters (8470 cu. in.), was primarily used as a seismic source for wide-angle recording along three main onshore-offshore lines centered on the Los Angeles basin and the epicenters of the 1933 Long Beach and 1994 Northridge earthquakes. The LARSE onshore-offshore lines were each 200-250 km long, with the offshore source lines being between 90 and 150 km long. The 22,128 air gun signals generated by the Ewing were recorded by an array of 174 PASSCAL Reftek recorders deployed at 2 km intervals along all three of the onshore lines and 9 ocean bottom seismometers (OBSs) deployed along two of the lines. The Ewing's 4.2-km, 160-channel, digital streamer was also used to record approximately 1250 km of 40-fold multichannel seismic-reflection data.To enhance the fold of the wide-angle data recorded onshore, mitigating against cultural and wind noise in the Los Angeles basin, the entire ship track was repeated at least once resulting in fewer than about 660 km of unique trackline coverage in the Inner Borderland. Portions of the seismic-reflection lines were repeated up to 6 times. The marine air-gun signals were recorded by portable land recorders which were deployed in continuous recording mode. With the use of accurate GPS timing at both sources and receivers, seismograms were extracted from the continuously recorded data. In this report, we describe the equipment and procedures used to acquire the onshore-offshore seismic refraction/wide angle reflection data, discuss the reduction of the data, and present reduced travel-time seismic sections of the acquired common-station-gather profiles. A more complete description of the marine seismic data collected by the Ewing is available in Brocher et al. (1995) and ten Brink et al. (1996). A third portion of the full LARSE experiment involved onland explosive sources collected by onland portable recorders which is described in Murphy et al. (1996). Local earthquakes which occurred during the acquisition of the air-gun data are described in Okaya et al. (1996). A separate LARSE experiment in 1993 to collect earthquakes over a several week period along Line 1 is described in Kohler et al. (1996).

Lower crustal deformation beneath the central Transverse Ranges, southern California: Results from the Los Angeles Region Seismic Experiment

“…In 1994, the first active source phase of the Los Angeles Region Seismic Experiment was carried out [ Brocher et al , 1995; Okaya et al , 1995; Fuis et al , 1996; Murphy et al , 1996; ten Brink et al , 1996]. Three multichannel seismic (MCS) reflection lines were recorded: two (lines 1 and 2) trending approximately north to northeast and one trending approximately east‐west (line 3) (Figure 1b).…”

“…The onshore‐offshore data set recorded along line 1 consists of the R/V Ewing air gun shots recorded by 84 onshore seismometers [ Okaya et al , 1995]. The R/V Ewing made multiple traverses along each MCS profile.…”

Crustal structure of the Borderland‐Continent Transition Zone of southern California adjacent to Los Angeles

“…The Los Angeles Region Seismic Experiment (LARSE) is a multiphase, multicomponent study of the crustal structure of southern California carried out by the Southern California Earthquake Center and the U.S. Geological Survey [ Fuis et al , 1996]. LARSE includes a passive seismic survey [ Kohler et al , 1996], deep crustal reflection and refraction surveys from onshore and offshore sources with onshore and ocean‐bottom seismometers, and an offshore multichannel seismic survey [ Brocher et al , 1995; Okaya et al , 1996; ten Brink et al , 1996]. In this paper, data from the onshore‐offshore component of LARSE are analyzed.…”