Surface faulting associated with the 2016 Mw 7.8 Kaikoura earthquake: complexity of ruptures, 3D structure, geological history and fault source definition
K. R. BerrymanP. VillamorNicola LitchfieldR. M. LangridgeTimothy A. LittleAndrew NicolJack WilliamsJesse KearseSusan EllisD. M. Eberhart-PhillipsMark RattenburyD. TownsendStephen Bannister
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The upper crust in the forearc of the Cascadia subduction zone hosts a complex network of faults that accommodate trench-parallel and trench-normal shortening due to oblique subduction and northward migration of the Oregon/Washington forearc block. Outside of the Seattle area, the seismic potential of major faults, as well as how they connect in a 3-D network, is poorly known. The trench-normal Doty fault, a major, north-dipping forearc fault crosses the I-5 corridor south of the Centralia-Chehalis urban area. Its length, orientation, and hypothesized total offset are comparable to the active Seattle fault, but it is unclear if the Doty fault poses a similar modern seismic hazard. We present preliminary results from the Chehalis Basin project (fieldwork summer 2018). We seek to define the Doty fault’s length, structure, dip, and linkages with smaller, likely transpressive faults to accommodate 3-D crustal deformation. We investigate possible blind faults south of the mapped Doty fault, near the site of a proposed flood-control water retention facility, and present evidence for recent fault activity. A multi-disciplinary approach is critical for regional investigations given dense foliage and glacial cover. Thus, we apply aeromagnetic and ground magnetic data, a regional gravity grid, high-resolution gravity lines, seismicity from a local broadband network, targeted geologic mapping, provenance characterization of Quaternary to Neogene sediments, dating, Lidar interpretation and field reconnaissance of geomorphic features to our research questions. The aeromagnetic data and prior geologic mapping suggest the Doty fault connects to unnamed NNW-striking faults to the west, and our new data will confirm or refute this hypothesis. Initial mapping and aeromagnetic interpretation suggest transpressive faults exist NNE of the Doty fault, which together bound a discrete region of uplift (Lincoln Creek uplift). There is little seismicity in the region recorded by the PNSN regional seismic network, and our PASSCAL array will help confirm the existence or absence of small, local earthquakes that could indicate neotectonic activity.
Forearc
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Reflection
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Seismotectonics
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Abstract. Natural fault networks are geometrically complex systems that evolve through time. The evolution of faults and their off-fault damage pattern are influenced by both dynamic earthquake ruptures and aseismic deformation in the interseismic period. To better understand each of their contributions to faulting we simulate both earthquake rupture dynamics and long-term deformation in a visco-elasto-plastic crust subjected to rate-and-state-dependent friction. The continuum mechanics-based numerical model presented here includes three new features. First, a 2.5-D approximation to incorporate effects of a viscoelastic lower crustal substrate below a finite depth. Second, we introduce a dynamically adaptive (slip-velocity-dependent) measure of fault width to ensure grid size convergence of fault angles for evolving faults. Third, fault localization is facilitated by plastic strain weakening of bulk rate-and-state friction parameters as inspired by laboratory experiments. This allows us to for the first time simulate sequences of episodic fault growth due to earthquakes and aseismic creep. Localized fault growth is simulated for four bulk rheologies ranging from persistent velocity-weakening to velocity-strengthening. Interestingly, in each of these bulk rheologies, faults predominantly localize and grow due to aseismic deformation. Yet, cyclic fault growth at more realistic growth rates is obtained for a bulk rheology that transitions from velocity-strengthening friction to velocity-weakening friction. Fault growth occurs under Riedel and conjugate angles and transitions towards wing cracks. Off-fault deformation, both distributed and localized, is typically formed during dynamic earthquake ruptures. Simulated off-fault deformation structures range from fan-shaped distributed deformation to localized Riedel splay faults. We observe that the fault-normal width of the outer damage zone saturates with increasing fault length due to the finite depth of the seismogenic zone. We also observe that dynamically and statically evolving stress fields from neighboring fault strands affect primary and secondary fault growth and thus that normal stress variations affect earthquake sequences. Finally, we find that the amount of off-fault deformation distinctly depends on the degree of optimality of a fault with respect to the prevailing but dynamically changing stress field. Typically, we simulate off-fault deformation on faults parallel to the loading direction. This produces a 6.5-fold higher off-fault energy dissipation than on an optimally oriented fault, which in turn has a 1.5-fold larger stress drop. The misalignment of the fault with respect to the static stress field thus facilitates off-fault deformation. These results imply that fault geometries bend, individual fault strands interact and that optimal orientations and off-fault deformation vary through space and time. With our work we establish the basis for simulations and analyses of complex evolving fault networks subject to both long-term and short-term dynamics.
Fault gouge
Earthquake rupture
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