Unit Affiliation: Marine and Polar Geophysics, Lamont-Doherty Earth Observatory (LDEO)
Earth's largest and most destructive earthquakes and tsunamis are generated along subduction megathrusts. The portion of these plate tectonic boundary faults that ruptures in earthquakes is known as the siesmogenic zone. Recent observations of high slip that propagates to the near surface, and new discoveries of anomalously slow slip events, have raised fundamental questions about widely held hypotheses that explain seismogenic zone behavior. In particular, the seismogenic zone of many subduction faults appears to be 'patchy', with some regions that fail suddenly in large earthquakes and others that slide by stable, aseismic creep. Additionally, in certain depth ranges, typically at the shallow and deep fringes of the seismogenic zone, slow slip events and earthquakes with anomalous low frequency energy have been observed at many margins. Current knowledge of the fault zone conditions and processes that cause these different modes of slip is limited, largely because quantitative constraints on in situ conditions in the subsurface are scarce. As a result, the associated earthquake and tsunami hazards are similarly poorly constrained. This project will combine high quality regional geophysical studies from the Aleutian subduction margin with laboratory experimental measurements on relevant rock and sediment, to calibrate the geophysical data and quantify in situ pore fluid pressure and stress along the subduction megathrust. Ultimately by providing quantitative estimates of the subsurface conditions along the plate boundary from the trench through the seismogenic zone, this study will test hypothesized mechanisms for the wide range of earthquake behavior. To accomplish this, the study will integrate laboratory data from modern oceanic sediment and exhumed metapelites with existing, multi-resolutional geophysical data to improve our understanding of in situ conditions and processes along the plate boundary megathrust from the trench to ~30-40 km depth. The project will address the following questions: 1) What are the in situ conditions, materials, and properties along the subduction megathrust that are sensed by low Vp, high Vp/Vs ratio, and high reflectivity? To what extent do seismic data image weak metasedimentary material vs. high-porosity channels or patches at near-lithostatic pressure and low effective stress? 2) How do the properties of the plate boundary change with depth and along-strike, and how do they relate to seismicity and upper plate deformation? These two overarching questions will be addressed by: (1) extracting detailed constraints on geophysical properties of the megathrust (Vp, Vp/Vs, reflectivity) from active and passive source datasets from the trench to depths of ~30-40 km; (2) defining elastic and hydrologic properties of sediment and rock relevant to the in situ subduction interface at the Alaska margin, via lab experiments on IODP cores from offshore Alaska and samples from exhumed fault zones on Kodiak Island; (3) integrating the geophysical and laboratory components to test hypotheses about the roles of material properties and state variables on geophysical signatures along the subduction thrust; and (4) investigating the correlation of megathrust properties with earthquake locations via precise relocation of microseismicity. The work will leverage several existing high-quality geophysical datasets, a suite of already collected samples, and DSDP/IODP cores and data. As a whole, this coordinated study will vastly improve our understanding of the conditions and materials along the megathrust, their relationship to seismicity, and serve as a template for similar studies at other margins.