Characterization of Recent Earthquake Sequences and Swarms within the Walker Lane Tectonic Region
AuthorHatch-Ibarra, Rachel Lauren
AdvisorSmith, Kenneth D
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We use a variety of techniques, starting with precise earthquake relocation to characterize the spatio-temporal evolution of sequences and swarms within the Walker Lane tectonic region, investigating fault structure and earthquake interaction. This dissertation consists of 3 chapters as well as appendices (A-C) which contribute additional results. For the 2017 Truckee sequence, we use relocation and source parameter analysis to investigate fault structure and earthquake interaction of small-magnitude earthquakes. Two high-angle, left-lateral strike-slip earthquakes (Mw 3.65 foreshock and Mw 3.85 mainshock), occurred 7 min apart, ∼20 km north of Truckee, California, on 27 June 2017. We relocate 50 out of 52 earthquakes which define a single structure between 5 and 6 km depth, trending ∼N45°E and dipping ∼70°–80° to the northwest, matching well with both moment tensor solutions and first-motion focal mechanisms. We observe average stress drops of ∼5 MPa using P and S waves and spatial variation related to the rupture areas of the foreshock and mainshock. We are able to detect components of directivity toward the northeast for the foreshock (Mw 3.65) and directivity toward the southwest for the mainshock (Mw 3.85), both aligning with the fault plane. Our study of the 2014 Virginia City Swarm reveals complex structural features, including an interplay of both fluid‐driven and aseismic‐driven earthquake migration near Virginia City, NV over ~10 days in January 2014. We relocate 305 events to reveal three separate, well‐defined planar structures. We observe the spatio-temporal characteristics which initially expose migration rates consistent with pore fluid diffusion, outlining a moderately dipping plane. The earthquakes then jump to a vertical plane and migrate at a higher rate; the sequence continues to migrate rapidly onto a third, shallowly dipping plane, consistent with rates observed elsewhere associated with aseismic creep. Focal mechanisms indicate right‐lateral strike slip on the vertical plane and both normal and left‐lateral strike slip movement on the other planes, and the newly imaged structures illuminate the orientation of active faults at depth in the Walker Lane tectonic region.The Nine Mile Ranch sequence began on Dec. 28th, 2016 with a multiplet of three moderate sized earthquakes (Mw 5.6, Mw 5.4, and an Mw 5.5) southwest of Hawthorne, NV, and all within one hour. In this study, we use earthquake relocation to detect active structures and characterize the spatio-temporal evolution and mechanics of the sequence. We combine the results with GPS analyses to investigate characteristics of faulting and tectonics of the Walker Lane. We relocate over 6000 events from the sequence, with structures matching moment tensor solutions and focal mechanisms indicating predominantly right- and left-lateral strike-slip and few normal ruptures. Results reveal orthogonal conjugate structures – one trending ~N57W and dipping to the northeast, and one trending ~N33E, vertical-dipping fault plane at the southern terminus of the NW trending fault plane. Our analysis and Coulomb model shows that the initial event likely occurred on the left-lateral NE fault plane. GPS analyses indicate a transtensional environment, with the coseismic offset of the closest four GPS stations slightly misaligned with the long term average strain tensor. The NMR sequence was likely driven by tectonics, and the multiplet of events may be related to static stress changes and fault readiness. Unmapped faults, fault complexity, and the dip of the faults at depth can assist us in estimating the potential for hazard in other basins within the Walker Lane. Structurally, orthogonal conjugate faulting in this sequence may be due to shear band formation below the brittle crust. These analyses illustrate details for modeling hazard which include fault geometry, local earthquake drivers, source properties and rupture directivity. By documenting these variables consistently, we can establish a baseline of observations for future work. We also gain insights to the variable nature of Walker Lane seismicity, as well as understanding earthquake physics and interaction that are observed throughout the world.