Imaging the Crust of the Los Angeles Region: Forward and Inverse Velocity Models and Reflectivity Migrated from Automatic Line Drawings of Shot Gathers
Abstract
In the Los Angeles Region Seismic Experiment (LARSE), conducted in the 1990's, active and passive seismic data were collected along two transects extending from offshore Los Angeles to the Mojave Desert. We discuss here imaging of the onshore, active-source segments of these transects. One segment, Line 1 (L1), extended through the Puente Hills (the vicinity of the 1987 Whittier Narrows earthquake) and San Gabriel Mts. The other segment, Line 2 (L2), extended through the Santa Monica and Santa Susana Mts (the vicinity of the 1971 San Fernando and 1994 Northridge earthquakes) and Central Transverse Ranges. We have performed forward modeling of first and secondary arrival times and also inverse modeling of first arrival times for comparison. We have also migrated automatic line drawings from shot gathers to avoid the problem of statics in stacking. In the latter method, we focus on features that can be identified in more than one migrated shot gather, 'stacking' these features by superposing the migrated lines. In addition, we focus on features within ~2 s of the first arrivals that have a slope of opposite sign from the first arrivals, in order to (a) mute first arrivals and reverberations and (b) enhance steep, fault-related reflections. For both L1 and L2, velocity features agree well between forward and inverse models (mean difference ~0 km/s; std ~0.3 km/s). On L1, steeply dipping, tabular low-velocity zones (LVZ's) correlate with the Sierra Madre fault system (SMF; ~65 deg dip) and the San Andreas fault system (SAF; steep dip). The LVZ associated with the SAF (Δv ~1 km/s) spans the ~10-km region between the Punchbowl Fault (old branch of the SAF), south of the SAF, and the Llano Fault, a 'flower-structure' reverse fault north of the SAF. Reflectivity includes a gently dipping zone that extends southward and upward from a point at 20-km depth and 7 km north of the surface trace of the SAF. This zone of coherent reflections coincides with the top of the 'San Gabriel Mts bright reflective zone' of Ryberg and Fuis (1998; seen in a trace-envelope stack) and appears to have as many as 3 southward branches: (1) a steep upward branch connecting with steep reflectivity in the hanging wall of the SMF, (2) a gently dipping branch that projects toward the hypocenter of the Whittier Narrows earthquake, and (3) a subhorizontal branch. Reflectivity is largely absent southward of the San Gabriel Mts owing to cultural noise. On L2, a narrow (4-km), shallow LVZ correlates with the SAF. In addition there are low-velocity depressions along the downward projections of the San Gabriel Fault and the Northridge Hills Fault (NHF). The depression along the NHF extends to 10-km depth. Strong reflectivity is concentrated in two oppositely dipping zones that meet ~vertically beneath the SAF in the depth range, 20-30 km. The southern zone extends upward nearly to the 13-km-deep hypocenter of the 1971 San Fernando earthquake, as demonstrated in Fuis et al. (2003, from a low- fold stack), with one upward splay toward the active San Gabriel Fault. Based on relocated aftershocks of the 1971 earthquake, Fuis et al. (2003) interpret a decollement connecting the SAF, 1971 hypocenter, and the surface trace of the NHF. Flower-structure reflective zones are seen north of the SAF.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2004
- Bibcode:
- 2004AGUFM.T54A..02R
- Keywords:
-
- 0935 Seismic methods (3025;
- 7294);
- 8038 Regional crustal structure;
- 8107 Continental neotectonics (8002);
- 8108 Continental tectonics: compressional;
- 8111 Continental tectonics: strike-slip and transform