Andrea L. Llenos

Influence of fore-arc structure on the extent of great subduction zone earthquakes

[1] Structural features associated with fore-arc basins appear to strongly influence the rupture processes of large subduction zone earthquakes. Recent studies demonstrated that a significant percentage of the global seismic moment release on subduction zone thrust faults is concentrated beneath the gravity lows resulting from fore-arc basins. To better determine the nature of this correlation and to examine its effect on rupture directivity and termination, we estimated the rupture areas of a set of Mw 7.5–8.7 earthquakes that occurred in circum-Pacific subduction zones. We compare synthetic and observed seismograms by measuring frequency-dependent amplitude and arrival time differences of the first orbit Rayleigh waves. At low frequencies, the amplitude anomalies primarily result from the spatial and temporal extent of the rupture. We then invert the amplitude and arrival time measurements to estimate the second moments of the slip distribution which describe the rupture length, width, duration, and propagation velocity of each earthquake. Comparing the rupture areas to the trench-parallel gravity anomaly (TPGA) above each rupture, we find that in 11 of the 15 events considered in this study the TPGA increases between the centroid and the limits of the rupture. Thus local increases in TPGA appear to be related to the physical conditions along the plate interface that favor rupture termination. Owing to the inherently long timescales required for forearc basin formation, the correlation between the TPGA field and rupture termination regions indicates that long-lived material heterogeneity rather than short timescale stress heterogeneities are responsible for arresting most great subduction zone ruptures.

Time-dependent dike propagation from joint inversion of seismicity and deformation data

Dike intrusions both deform Earths 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. Dieterichs (1994) seismicity-rate theory is used to relate dike-induced stress changes to seismicity rate and is combined with elastic Greens 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 dikes 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 Fathers 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.

Characterizing potentially induced earthquake rate changes in the Brawley Seismic Zone, southern California

The Brawley seismic zone (BSZ), in the Salton trough of southern California, has a history of earthquake swarms and geothermal energy exploitation. Some earthquake rate changes may have been induced by fluid extraction and injection activity at local geothermal fields, particularly at the North Brawley Geothermal Field (NBGF) and at the Salton Sea Geothermal Field (SSGF). We explore this issue by examining earthquake rate changes and interevent distance distributions in these fields. In Oklahoma and Arkansas, where considerable wastewater injection occurs, increases in background seismicity rate and aftershock productivity and decreases in interevent distance were indicative of fluid‐injection‐induced seismicity. Here, we test if similar changes occur that may be associated with fluid injection and extraction in geothermal areas. We use stochastic epidemic‐type aftershock sequence models to detect changes in the underlying seismogenic processes, shown by statistically significant changes in the model parameters. The most robust model changes in the SSGF roughly occur when large changes in net fluid production occur, but a similar correlation is not seen in the NBGF. Also, although both background seismicity rate and aftershock productivity increased for fluid‐injection‐induced earthquake rate changes in Oklahoma and Arkansas, the background rate increases significantly in the BSZ only, roughly corresponding with net fluid production rate increases. Moreover, in both fields the interevent spacing does not change significantly during active energy projects. This suggests that, although geothermal field activities in a tectonically active region may not significantly change the physics of earthquake interactions, earthquake rates may still be driven by fluid injection or extraction rates, particularly in the SSGF.

Controls on Earthquake Rupture and Triggering Mechanisms in Subduction Zones

Large earthquake rupture and triggering mechanisms that drive seismicity in subduction zones are investigated in this thesis using a combination of earthquake observations, statistical and physical modeling. A comparison of the rupture characteristics of M 7.5 earthquakes with fore-arc geological structure suggests that long-lived frictional heterogeneities (asperities) are primary controls on the rupture extent of large earthquakes. To determine when and where stress is accumulating on the megathrust that could cause one of these asperities to rupture, this thesis develops a new method to invert earthquake catalogs to detect space-time variations in stressing rate. This algorithm is based on observations that strain transients due to aseismic processes such as fluid flow, slow slip, and afters lip trigger seismicity, often in the form of earthquake swarms. These swarms are modeled with two common approaches for investigating time-dependent driving mechanisms in earthquake catalogs: the stochastic Epidemic Type Aftershock Sequence model [Ogata, 1988] and the physically-based rate-state friction model [Dieterich, 1994]. These approaches are combined into a single model that accounts for both aftershock activity and variations in background seismicity rate due to aseismic processes, which is then implemented in a data assimilation algorithm to invert catalogs for space-time variations in stressing rate. The technique is evaluated with a synthetic test and applied to catalogs from the Salton Trough in southern California and the Hokkaido corner in northeastern Japan. The results demonstrate that the algorithm can successfully identify aseismic transients in a multi-decade earthquake catalog, and may also ultimately be useful for mapping spatial variations in frictional conditions on the plate interface.