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Tracking Seasonal Influences on Stress Changes using Estimates of Anomalous Geodetic Strains

Bill E. Holt, Meredith L. Kraner, Adrian A. Borsa, & Alireza Bahadori

Published August 24, 2016, SCEC Contribution #6959, 2016 SCEC Annual Meeting Poster #165

Quantifying transient tectonic signals continues to reveal new insights into fault behavior and crust/mantle rheology [Segall and Matthews, 1997; McGuire and Segall, 2003; Freed and B├╝rgmann, 2004; Ji and Herring, 2012, 2013; Mavrommatis et al, 2014]. Recent work has shown that transient signals associated with snow, ice, and groundwater loading changes can influence seismicity rates and potentially trigger events [Heki, 2003; Lutrell et al., 2007; Bettinelli et al., 2008; Gonzalez et al., 2012; Amos et al., 2014]. Here we use seasonal signals in addition to other transients to fully quantify the time-dependent transient strain field in southern California [Holt and Shcherbenko, 2013], along with its statistical significance, using the PBO cGPS data.

We use the measured strain changes to resolve stress changes on existing fault structures through time. Our analysis of the strain response to seasonal signals is motivated by the recognition of the importance of seasonal hydrologic loading [Borsa et al., 2014] and the apparent stress changes that it makes on structures [Kraner et al., 2016]. The length scales of hydrologic loading influence stresses at significant seismogenic depths [Argus et al., 2014, Amos et al, 2014; Borsa et al., 2014]. Thus stress changes associated with seasonal loading are not merely surficial, but should affect the entire seismogenic layer if seasonal signals have wavelengths of order tens of kilometers. Thermoelastic strains [Prawirodirdjo et al., 2006; Ben Zion and Allam, 2013] may also have an important impact on horizontal seasonal strain and stress changes. In the case of thermoelastic forcing, calculations show that if the near surface is coupled with the underlying elastic layer, the depth of penetration of strain is proportional to the length scales of the anomaly (tens of kilometers) [Prawirodirdjo et al., 2006].

Additionally, we further develop a transient analysis method [Holt and Shcherbenko, 2013] to incorporate seasonal signals and investigate the influence on coulomb stress changes on faults and model seismicity rate changes. We quantify uncertainties, characterize the strain/stress effects on existing structures caused by seasonal anomalies, and investigate their influence on seismicity rate models. Here we mainly present results obtained to date for transient seasonal strains in the regions surrounding the South Napa earthquake.

Key Words
transients, seasonal, hydrologic

Holt, B. E., Kraner, M. L., Borsa, A. A., & Bahadori, A. (2016, 08). Tracking Seasonal Influences on Stress Changes using Estimates of Anomalous Geodetic Strains. Poster Presentation at 2016 SCEC Annual Meeting.

Related Projects & Working Groups
Tectonic Geodesy