Unraveling the San Gorgonio Knot: Numerical and Analog Investigations

University Of Massachusetts Amherst, Amherst MA

Investigators

Abstract

While structurally simple in many parts of California, the active San Andreas Fault within the San Gorgonio Pass, southern California, slips along several, non-parallel, strike-slip, thrust and oblique faults. The present-day complexity of the San Gorgonio Pass region has developed over the past few million years in response to propagation of new active strands, abandonment of old strands and reactivation of other old strands. Many geophysical models of the San Andreas Fault simplify the system as vertical and continuous through the San Gorgonio Pass. Such models cannot capture the present-day deformation because they underestimate the importance of fault topology and interaction with secondary fault structures. To address this problem, this study uses three-dimensional analog and numerical modeling of the evolution of the San Andreas Fault through the so-called San Gorgonio knot to explore the change in mechanical efficiency associated with abandonment and development of new fault strands. By simulating deformation along the southern San Andreas Fault within both analog and numerical models, it will be possible to observe some fault strands become mechanically inefficient and become abandoned in favor of other, more efficient strands. The uplift pattern and slip rates associated with changes in the active fault system configuration are compared to geologic data to constrain the nature of fault evolution. The goals of this study are to 1) constrain the three-dimensional evolution of the San Andreas fault by comparison to available geologic data, 2) evaluate the mechanics of fault evolution with regard to the work budget of the evolving system, 3) resolve discrepancies between different geologic and geodetic calculations of deformation that may owe to different time scales of measurement and 4) test innovative laboratory and numerical techniques towards better interpreting strike-slip tectonics. The results of this study will help constrain the present-day active fault surface configuration of the San Andreas Fault in this southern California by examining the evolution of the fault system over the past half- million years. By understanding how this region of fault complexity developed will lead to better understanding of the present-day slip distribution and seismic hazards for the region. The study uses an innovative combination of analog and numerical models. Consequently, in addition to providing information for seismic hazard analysis in the region, the project allows comparison of analog and numerical models of long-term deformation of fault systems. This combination of analog and numerical models of strike-slip faulting in southern California nicely complements on going efforts by other international workers to compare analog and numerical model results in other types of fault systems. The study also involves students at high schools for the Deaf around the country in research via refinement of existing classroom analog modeling activities and videoconferences between the research team and the classrooms. These activities enhance the students' learning and invigorate the teachers' presentations of lithospheric science in their earth system courses.