Structural and Seismologic Investigation of Blind-thrust Earthquake Segmentation, San Joaquin Basin, California

Harvard University, Cambridge MA

Investigators

Abstract

The nature of blind-thrust earthquake segmentation is being examined through integrated geologic and seismologic analyses of the 1982 New Idria (Mw=5.4), 1983 Coalinga (Mw=6.5), and 1985 Kettleman Hills (Mw=6.1) earthquakes, which occurred in the western San Joaquin basin of central California. These earthquakes occurred beneath the New Idria, Coalinga, and Kettleman Hills anticlines, which form an en echelon system of structures that extend south to Lost Hills. Patterns of coseismic uplift and subsurface studies have demonstrated convincingly that these structures are fault-related folds that develop in response to motions on the underlying blind-thrust system. The limits of the rupture areas for the three earthquakes are defined by the distinct en echelon offsets of this fold system, which are thought to overlie geometric segment boundaries in the underlying blind-thrust fault. The folds have excellent surface exposures, and the subsurface structure is defined by a wealth of industry seismic reflection and well log data. Thus, this area presents an excellent opportunity to study processes of blind-thrust earthquake segmentation, insights into which can help to assess earthquake hazards in other regions of active thrusting. The geometry and kinematics of the Lost Hills - Kettleman Hills - Coalinga fold and thrust chain is characterized utilizing seismic reflection profiles, well control, surface geology, remote sensing imagery, and precisely relocated seismicity. The analysis is documenting the occurrence of tear (strike-slip) faults, lateral ramps, and other types of fault linkages at the earthquake segment boundaries. Insights into the fault geometries of these boundaries will translate into helping to develop a better methodology for defining potential segment boundaries on other blind-thrust systems, which often dictate the locations and magnitudes of earthquakes, two key parameters in probabilistic seismic hazard assessment. Moreover, these detailed structural models can provide a well-documented case study for numerical investigations of dynamic rupture propagation and aftershock distribution during blind-thrust events. Additionally, these investigations will provide a seismotectonic model that addresses the rates and temporal variability of fault activity, which can be compared to more regional measurements such as space geodesy and to earthquake activity on adjacent faults systems, including the San Andreas.