THREE DIMENSIONAL SEISMO-TECTONIC IMAGING: AN EXAMPLE FROM THE SOUTHERN CALIFORNIA TRANSVERSE RANGES

We present a new technique, 3D Seismo-tectonic Imaging (STI), for imaging active tectonics by combining earthquake and structural data. We test the technique using three independent data sets and then compare results. We find a high degree of concordance between the predicted finite deformation field and related instantaneous deformation surfaces inferred from aftershock and background seismicity data as well as seismic reflection data. The principal results are: • We find three types of earthquake assemblages: 1] Clouds; reflect distributed deformation of internal deformation fields produced by folding and/or faulting. 2] Surfaces; reflect regions of limited slip . 3] Clusters: Regions of localized activity whose origin is not understood. • The 1994 Northridge rupture occurred on a transverse fault linking and including two E-W striking thrust systems, the Pico and Oak Ridge. • The Los Angeles Seismic Zone (LASZ) is a region of thrusting associated with the development of a new mountain front forming south of the present one, the Santa Monica Anticline. The thrust is rooted on a mid-crustal ramp forming beneath the LA basin. Deep crustal reflection and tomographic work of Ryberg and Fuis (1997) provides independent evidence for the presence of the LASZ With regard to Transverse Range tectonics, the data supports both ramp/flat geometry for basement as well as deep, high angle faults and suggests a model that allows compatibility between these apparently disparate elements. A flexural flow deformation model gives a reasonable representation of the first order behavior of the earth’s crust in the Transverse Ranges. We interpret the LASZ to mark the formation of a new mountain front forming by propagation of the sole fault into the Los Angeles basin. This zone has a minimum strike length of 100 kms. If this interpretation is correct then there exists the possibility for a major shallow (10 -15 kms) blind thrust earthquake directly beneath the city of Los Angeles. Finally we believe that STI by linking earthquake data to an empirically based quantitative description of the finite deformation field provides a new and fruitful method for integrating geological and seismological data. The synergistic nature of this approach offers further insights into new and existing data sets and the seismogenic process.