Ellen M. Syracuse

A fundamental component of seismic research is the use of phase arrival times, which are central to event location, Earth model development, and phase identification, as well as derived products. Hence, the accuracy of arrival times is crucial. However, errors in the timing of seismic waveforms and the arrival times based on them may go unidentified by the end user, particularly when seismic data are shared between different organizations. Here, we present a method used to analyze travel‐time residuals for stations in and around Iran to identify time periods that are likely to contain station timing problems. For the 14 stations with the strongest evidence of timing errors lasting one month or longer, timing corrections are proposed to address the problematic time periods. Two additional stations are identified with incorrect locations in the International Registry of Seismograph Stations, and one is found to have erroneously reported arrival times in 2011.

Conventional traveltime seismic tomography methods with Tikhonov regularization (L₂ norm) typically produce smooth models, but these models may be inappropriate when subsurface structure contains discontinuous features, such as faults or fractures, indicating that tomographic models should contain sharp boundaries. For this reason, we develop a double-difference (DD) traveltime tomography method that uses a modified total-variation regularization scheme incorporated with a priori information on interfaces to preserve sharp property contrasts and obtain accurate inversion results. In order to solve the inversion problem, we employ an alternating minimization method to decouple the original DD tomography problem into two separate subproblems: a conventional DD tomography with Tikhonov regularization and a L₂ total-variation inversion. We use the LSQR linear solver to solve the Tikhonov inversion and the split-Bregman iterative method to solve the total-variation inversion. Through our numerical examples, we show that our new DD tomography method yields more accurate results than the conventional DD tomography method at almost the same computational cost.

Joint inversions of seismic data recover models that simultaneously fit multiple constraints while playing upon the strengths of each data type. Here we jointly invert 14 years of local earthquake body wave arrival times from the Alaska Volcano Observatory catalog and Rayleigh wave dispersion curves based upon ambient noise measurements for local V_p, V_s, and hypocentral locations at Akutan and Makushin Volcanoes using a new joint inversion algorithm. The velocity structure and relocated seismicity of both volcanoes are significantly more complex than many other volcanoes studied using similar techniques. Seismicity is distributed among several areas beneath or beyond the flanks of both volcanoes, illuminating a variety of volcanic and tectonic features. The velocity structures of the two volcanoes are exemplified by the presence of narrow high‐V_p features in the near surface, indicating likely current or remnant pathways of magma to the surface. A single broad low‐V_p region beneath each volcano is slightly offset from each summit and centered at approximately 7 km depth, indicating a potential magma chamber, where magma is stored over longer time periods. Differing recovery capabilities of the V_p and V_s data sets indicate that the results of these types of joint inversions must be interpreted carefully.

Dike intrusions both deform Earth's surface and induce propagating earthquake swarms. We develop methods to utilize both deformation and seismicity from brittle, volcano‐tectonic earthquakes to image time‐dependent dike propagation. Dieterich's (1994) seismicity‐rate theory is used to relate dike‐induced stress changes to seismicity rate and is combined with elastic Green's functions relating dike opening to deformation. Different space‐time patterns of seismicity develop if earthquakes occur at the same depth as the dike compared to above/below the dike. In the former, seismicity initiates near the dike's leading edges but shuts off as the dike tips pass and seismogenic volumes fall into stress shadows. In the latter, seismicity continues at a decaying rate after the tips pass. We focus on lateral propagation and develop a nonlinear inversion method that estimates dike length and pressure as a function of time. The method is applied to the 2007 Father's Day intrusion in Kilauea Volcano. Seismicity is concentrated at ∼3 km depth, comparable to geodetic estimates of dike depth, and decays rapidly in time. With lateral propagation only and a vertical dike‐tip line, it is difficult to fit both GPS data and the rapid down‐rift jump in seismicity, suggesting significant vertical propagation. For the events to have occurred below the dike requires a very short aftershock decay time, hence unreasonably high background stressing rate. The rapid decay is better explained if the dike extends somewhat below the seismicity. We suggest that joint inversion is useful for studying the diking process and may allow for improved short‐term eruption forecasts.