Evidence for a Recent Dike Injection in Northern California Using Gravity and Seismicity A Senior Thesis in Geosciences

In the year 2000, an anomalous sequence of off-fault seismicity was detected near Lake Pillsbury in northern California. The seismicity was interpreted by Hayes et al. (2005, 2006) to be associated with a dike injection. I completed a gravity survey over the location of the seismicity and used that data in gravity modeling, to test the hypothesis that the seismicity was directly linked to the intrusion of a dike. A symmetrical, negative gravity anomaly of maximum amplitude 3 mGal and approximate width 5 km was detected directly above the location of the seismicity. The results of the gravity modeling suggest that the negative anomaly is generated by two zones where density has been reduced, as a result of rock fracturing open in response to the intrusion of the dike. The two fractured regions used in the gravity models are consistent with seismicity patterns and coulomb stress models and therefore provide additional support for the dike injection hypothesis.