The Eastern Los Angeles Basin lies at the intersection of the northwest-vergent Peninsular Ranges (Santa Ana Mountains) and the south-vergent Transverse Ranges (Puente Hills). The complexity of this interaction is due to the batholithic-cored Santa Ana Mountains driven northwestward at 5-6 mm/yr along the Elsinore fault as an oblique indenter through the Cretaceous-Eocene stratigraphy and into the Miocene-Holocene sedimentary basins. The indenter model encompasses all of the secondary geomorphic and tectonic features of the Eastern LA Basin, including the dispersed seismicity and the left-lateral solution for the M5.4 July 29, 2008 Chino Hills earthquake. This model temporally and kinematically explains the right-lateral slip on the Whittier fault, the accelerated uplift and folding within the Eastern Puente and Chino Hills, the northward decrease of oblique, right-lateral, reverse slip on the Chino fault, the uplift and segmentation of Loma Ridge, and the repeated northward offset of Santiago Creek, the uplift of the Coyote Hills, and the folding and uplift of the Anaheim and Peralta Hills. The model requires a previously unrecognized left-lateral shear zone along the western side of the Santa Ana Mountains between the indenter and the sedimentary section, evidence for which lies in a 15 km offset of the Cretaceous Holz Shale contact, a 10 km penetration offset of the Miocene Monterey Formation across the indenter, and several left-laterally deflected river systems. The significance of this model is that while many of the faults and folds created by the collision are small seismic sources, the real significance is that they also pose a myriad of potential surface rupture hazards within this densely urbanized region, a hazard that will challenge our profession's ability to characterize it.

Eldon Gath is the President of Earth Consultants International (ECI), a Past-President of the Association of Engineering Geologists, and a life- long graduate student in many of southern California's colleges (OCC, CSULB, CSULA, UCR, and UCI). He has 30 years of engineering geologic consulting experience, starting as a soil technician and ending as a president; and from Irvine Soils, to Pacific Soils, to Leighton & Associates, and for the last 12 years, ECI. For the past 20 years, he has been studying the neotectonic development of Orange County, the San Joaquin Hills, the Whittier fault, the Puente Hills, and now the Santa Ana Mountains.