Llenos Andrea L.

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Llenos
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Andrea L.
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  • Thesis
    Controls on earthquake rupture and triggering mechanisms in subduction zones
    (Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 2010-06) Llenos, Andrea L.
    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 afterslip 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.
  • Article
    Influence of fore-arc structure on the extent of great subduction zone earthquakes
    (American Geophysical Union, 2007-09-06) Llenos, Andrea L. ; McGuire, Jeffrey J.
    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 fore-arc 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.
  • Article
    Detecting aseismic strain transients from seismicity data
    (American Geophysical Union, 2011-06-17) Llenos, Andrea L. ; McGuire, Jeffrey J.
    Aseismic deformation transients such as fluid flow, magma migration, and slow slip can trigger changes in seismicity rate. We present a method that can detect these seismicity rate variations and utilize these anomalies to constrain the underlying variations in stressing rate. Because ordinary aftershock sequences often obscure changes in the background seismicity caused by aseismic processes, we combine the stochastic Epidemic Type Aftershock Sequence model that describes aftershock sequences well and the physically based rate- and state-dependent friction seismicity model into a single seismicity rate model that models both aftershock activity and changes in background seismicity rate. We implement this model into a data assimilation algorithm that inverts seismicity catalogs to estimate space-time variations in stressing rate. We evaluate the method using a synthetic catalog, and then apply it to a catalog of M ≥ 1.5 events that occurred in the Salton Trough from 1990 to 2009. We validate our stressing rate estimates by comparing them to estimates from a geodetically derived slip model for a large creep event on the Obsidian Buttes fault. The results demonstrate that our approach can identify large aseismic deformation transients in a multidecade long earthquake catalog and roughly constrain the absolute magnitude of the stressing rate transients. Our method can therefore provide a way to detect aseismic transients in regions where geodetic resolution in space or time is poor.