Datensatz

Zusammenfassung

We acquired combined refraction and low-fold reflection data along a north-south-trending profile through the epicentral regions of the 1994 M 6.7 Northridge and 1971 M 6.7 San Fernando earthquakes as part of the Los Angeles Region Seismic Experiment, Phase II (LARSE II). The profile extended from the Santa Monica Mountains to the western Mojave Desert (Antelope Valley). The chief goal of LARSE is to image sedimentary basins and faults in the Los Angeles region to better understand and assess earthquake hazards associated with these geologic features. Conventional CDP seismic data processing, extended by several steps, was used to image the crustal reflective structure below the LARSE II line. We found that stacking offsets of 20-25 km produced the most coherent reflections, especially near the bottoms of sedimentary basins, although such offsets push the limits of the CDP method. Gently to moderately dipping reflections in all regions and at all depths along the profile required migration, which was done in two ways. In the first method of migration, we constructed manual and machine line drawings of the CDP stack and then migrated the line segments in a velocity model obtained from tomographic modeling of the first arrivals. Each line segment was migrated based on its dip ($ heta$) and local velocity $v$ ($sin heta/v$ = ray parameter). First, a travel time field was calculated numerically by solving the eikonal equation (using a finite-difference technique). Next rays with the appropriate ray parameters were traced from the surface to the appropriate traveltimes, and the line segments were plotted at these points. Information on reflection strength and coherency for each line segment was preserved. The second method of migration was standard poststack Kirchhoff migration. This method produced a grainy image with migration artifacts in regions of low fold ($<$10), chiefly at the ends of the profile and in the Santa Susana Mountains. Both migration methods produced strong, bowl-shaped reflections beneath the San Fernando, Santa Clarita, and Antelope Valleys that are interpretable as sedimentary-basin-bottom reflections. In addition, both methods showed weak to moderately strong, dipping zones of reflections at middle to lower crustal depths. One weak zone, beneath the Santa Monica Mts and southern San Fernando Valley, dips $~35^o$ southward from the hypocenter of the Northridge mainshock. A strong zone of reflections beneath the Transverse Ranges dips $~25^o$ northward from the hypocenter of the San Fernando mainshock to the vertical projection of the San Andreas fault. Both of these reflective zones are interpretable as the deep continuations of the Northridge and San Fernando fault systems. The connection between the San Fernando and the San Andreas faults is somewhat similar to the fault geometry seen 70 km east, along the LARSE I line, where the Sierra Madre and Whittier Narrows fault systems appear connected to the San Andreas fault by a highly reflective zone, interpretable as a decollement. The seismic images from LARSE I and II will aid in understanding how deformation is transferred from the San Andreas fault to the thrust faults of the Los Angeles region.