During the four weeks of continuous recording (November 13 - December 11, 1993) stations recorded signals from ~160 teleseismic and ~400 local (Ml > 2.0) events. The data recorded with the sensors is velocity (100 volts/cm/sec) and is easily deconvolved back to earth motion assuming a "general" instrument type. Since the sensore were different, the instrument responses were also different. For arrivals from the northwest, rotation of the horizontal components results in components nearly parallel and perpendicular to the plate boundary marked by the strike of the San Andreas fault. The variation in back azimuths of the teleseismic arrivals has provided good coverage around the receivers, and their mostly steep (<20 degree) incidence angles preclude near-surface interference phenomena. Local events provide arrivals from which to obtain information about the San Andreas fault characteristics.

TELESEISMIC EVENTS

Of the teleseismic events, about 20 display useful P-wave arrivals from which travel times and amplitudes could be determined. These events fall into several distinct back-azimuth ranges with distances between source and receiver ranging from 8 to 90 degrees. The teleseismic events include the December 4, 1993 aftershock (Ms=5.2) in Klamath Falls, Oregon as well as a number of intermediate earthquakes with epicenters in the Aleutian Island, Kamchatka, Japan, Fiji Island, Peru and Chile regions.

P-wave travel-time curves for stations along the array were determined for 17 teleseismic events. These events fell into several distinct back- azimuth ranges with distances between sources and receivers ranging from 30 to 80 degrees. P-wave travel times were determined for each station and corrected for topography and the one-dimensional iasp91 Earth model. Within each back-azimuth range, the resulting travel-time residual curves display consistent patterns with relatively low residuals at the southern San Gabriel Mountain foothills and relatively large residuals across most of the San Gabriel Mountains, including the San Andreas fault. The most drastic differences in residuals, occurring for raypaths from the northwest (Kamchatka and Unimak Island), were about one second. The travel-time residual curves, although displaying small variations for different raypath arrival directions, show almost no lateral spatial shift of maximum or minimum residual along the array, indicating that the source of the large residual is shallow. The patterns of residuals suggest that a sharp change in shallow velocites is required between the Los Angeles Basin and the San Gabriel Mountains over a horizontal distance of less than 10 km. In addition, a relatively shallow, low-velocity anomaly is required to explain the larger San Gabriel Mountain station residuals.

TELESEISMIC RECORD SECTIONS

Profiles of record sections across the array are similar from event to event and the quality of the section depends on the location of the station. In general, stations in the San Gabriel Mountains have the highest signal-to-noise ration, Mojave Desert stations display smaller ratios, and San Gabriel Valley and Los Angeles Basin stations display the lowest ratios. The initial P wave is often the most impulsive of arrivals for the events and is followed by the higher-amplitude crustal phases corresponding to a particular distance range. Because a few of the stations are vertical component only, the initial P wave is the most useful for imaging characteristics across the array. The following record sections contain the vertical velocity signals for events which occurred in:
• Unimak Island Region
• Kamchatka
• Klamath Falls, Oregon
• Fiji Island Region

Seismograms which contain high levels of noise and severe timing problems have been removed from these figures and the largest gaps in these plots correspond to regions in the San Gabriel Valley and the Los Angeles Basin with no stations. The teleseismic seismograms have been bandpass filtered for frequencies between 0.1 and 1.0 Hz, the range in which most of the seismic signal is found. As the larger-distance sections show, the initial P wave is followed by the larger-amplitude pP and sP phases.

The typically quiet mountain stations show subtle variations in P-wave amplitudes across the San Andreas fault; the noisier valley and basin stations are typically too noisy to obtain arrival times. The Oregon earthquake is an example of one of few regional events in which the appearance of the crustal phases changes across the span of the array.

LOCAL EVENTS

According to catalog data supplied by the Southern California Earthquake Center Data Center (SCEC_DC), approximately 400 local events with Ml greater than or equal to 2.0 occurred during LARSE '93.Local events include 1991 Sierra Madre earthquake (Ml=5.8) aftershocks.

LOCAL EVENT RECORD SECTIONS

The following record sections show velocity records for events which occurred in Southern California less than 200 km from the array. The profiles contain all seismograms recorded for each event and have been bandpass filtered for frequencies between 0.1 and 10.0 Hz. The major arrivals in these profiles are the crustal phases Pn followed by Pb, Pg, and their reflections, as well as Sn, Sb, Sg, and their reflections.
• November 22, 1993 (Landers aftershock - Magnitude 2.1)
• November 24, 1993 (Ridgecrest earthquake - Magnitude 2.7)

NOTE: Neither of these figures have been corrected for timing errors; thus, some phases arrive as much as several seconds before or after the same phases in adjacent seismograms.