High-resolution Imaging Of Plate Boundary Fault Zones With Advanced Seismic Location And Tomography Methods

Most earthquakes in the world are associated with active fault zones along major plate boundaries.Parts of faults within the continental and oceanic crusts can be locked to accumulate stress for large earthquakes,while some portions of the faults may creep continuously with small earthquakes or transiently with episodic slow slip events and tectonic tremors.Similarly,earthquakes and episodic tremor and slow slip occur at different depths along the subduction plate interface.Understanding the physical state and mechanical properties that control these fault behaviors are important for deciphering physical mechanisms of regular and slow earthquakes,and improving seismic hazard assessment.This thesis developed three advanced seismic location and tomography methods for improving earthquake and tremor locations and characterizing high-resolution fault-zone velocity structures.Applying these new methods to three different plate boundary fault zones helps to better understand the relationship between variations in fault-zone physical properties and fault behaviors.We developed three seismic body-wave travel time based location and tomography methods.(1)Motivated by improving earthquake and tremor locations,we developed a triple-difference location method that makes use of station-pair and double-pair differential times and can simultaneously improve absolute and relative event locations.(2)Subsequently,by extending triple-difference location method to triple-difference tomography method that can jointly invert event locations and velocity model,we can further improve the event locations and determine high-resolution velocity models both near the source region at depth and outside the source region at shallow depths.(3)We developed a Vp/Vs model consistency constrained double-difference tomography method that can reliably determine high-resolution Vp/Vs model by using S-P times and a Vp/Vs model consistency constraint.We tested the performance of the three methods with synthetic and real datasets.A key uncertainty in seismic hazard estimates for the Pacific Northwest derives from the lack of direct observational or physical constraints on the down dip extent of great earthquake rupture on the Cascadia megathrust.Geodetic observations demonstrate that the plate interface is segmented in the down-dip direction into an interseismically locked zone expected to rupture in future large earthquakes,a region of episodic tremor and slip(ETS)at greater depth that accommodates plate motion without earthquakes,and an intervening transition zone of uncertain behavior in great earthquakes.Here,we image the evolution of physical properties(ratio of compressional to shear wave seismic velocities)related to fluid content of the plate boundary zone across these megathrust segments using a dense onshore-offshore seismic dataset from the southernmost Cascadia subduction zone.Combined with a new resistivity model,our result suggests that both the locked zone and the deeper ETS zone show high fluid content implying a significant porosity,yet the intervening transition region shows an order of magnitude lower porosity.These strong variations are consistent with rock mechanics models that contain a ductile zone between the earthquake rupture and ETS zones.Interpreting the transition zone as a low porosity region governed by a ductile rheology suggests that large ruptures nucleating at shallower depths will have difficulty propagating a significant distance into the transition zone and hence reduce the seismic hazard.The Gofar transform fault(GTF),40° S on the East Pacific Rise,can generate Mw 5.5-6 earthquakes quasiperiodically on certain specific patches that are separated by stationary rupture barriers.Small earthquakes along strike show a clear spatial and temporal evolution.To better understand the cause of the observed behaviors of large and small earthquakes,we determined high-resolution earthquake locations within a period of one year covering the 2008 Mw 6.0(M6)earthquake,as well as Vp,Vs,and Vp/Vs models along the westernmost segment of the GTF,using a well-recorded ocean bottom seismograph dataset and the Vp/Vs model consistency-constrained double-difference tomography method.Our tomographic Vp/Vs model reveals strong structural variations at multiple scales along the fault,which likely control the behaviors of large and small earthquakes.The M6 mainshock is generated within a specific?8-km-long fault patch composed of intact rocks.By contrast,multiple fluid-filled damaged zones on both sides of this asperity are imaged and have varying sizes which are suggested to be critical in their ability of stopping?M6 ruptures.High-resolution earthquake relocations and velocity models also indicate that the occurrence of small earthquakes is also correlated with structural variations.Combined with previous studies,our results further suggest that strong variations in physical properties likely control the fault mechanics and earthquake behavior along the GTF.In the last decade,tectonic tremors were widely discovered in the root of San Andreas fault(SAF)in California and suggested to represent frictional fault slip at depth due to high pore-fluid pressure.However,the source of fluid and how is the fluid distributed at depth are still unclear.Meanwhile,strong variations in along-fault and across-fault tremor characteristics were observed,but their controlling mechanism is also not clear.Here,we imaged the shear wave velocity structure of the central SAF near Parkfield with great resolution at depth by using seismic datasets from deep tremors with the triple-difference tomography method.In addition to imaging fault-zone structures,tremor locations are improved simultaneously.Our result clearly shows that tremor generation zones are associated with ultra-low velocity anomalies,indicating high pore fluid pressure.Clear deep fluid pathways connecting the upper mantle to the upper crust are imaged and may originate from the dehydration of a serpentined mantle wedge in the northeast of the SAF.Variations in pore fluid pressures likely control various tremor characteritics in the middle and lower crust,and fault coupling segmentation in the upper crust.