A Seismic Deployment to Explore the Sierra Madre-Cucamonga Fault Zone Transition
Authors:Jonathan Levario, David Nget, Celia Pazos
Mentor:Jascha Polet, Associate Professor of Geophysics, Cal Poly Pomona
In the Summer of 2013, we conducted a micro-seismicity survey of the Sierra Madre-Cucamonga fault zone in order to better understand the relationship between the local fault systems. This fault zone is an active, North-dipping thrust fault system located at the base of the San Gabriel Mountains. Near the mouth of San Antonio Canyon, the fault bends and transitions from the Sierra Madre Fault to the Cucamonga Fault. Coinciding with this bend is the active San Antonio-San Jose fault system. Because the fault systems do not surface at the transition, little is known about the geometry between them. Three-dimensional structural relationships between these intersecting faults are also unclear. This fault system contributes to seismic hazards that potentially affect the major populated areas of Upland, Rancho Cucamonga, and Claremont. The uncertainties about the relationships between the fault systems must be resolved to constrain estimates of maximum credible earthquakes. We deployed 9 GURALP CMG-6TD Seismometers in the area of the transition region. We recorded the ground motions produced by very small earthquakes (magnitude less than 1). In addition to the portable seismometers, existing, permanent Southern California Seismic Network stations also recorded ground motion data. We will collect arrival times of seismic waves, as recorded by the instruments, and process these times to locate the micro-earthquakes. Knowing the location of these earthquakes will yield a better understanding of the local fault geometry. During the deployment, our seismometers recorded data for a period of two months. Within those two months, they recorded numerous micro-earthquakes as well as two recent M3.7 quakes in the Pomona area and their aftershocks. We will present our preliminary findings from the analysis of the data from our deployment.