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  1. Abstract

    Subduction forearcs are subject to seismic hazard from upper plate faults that are often invisible to instrumental monitoring networks. Identifying active faults in forearcs therefore requires integration of geomorphic, geologic, and paleoseismic data. We demonstrate the utility of a combined approach in a densely populated region of Vancouver Island, Canada, by combining remote sensing, historical imagery, field investigations, and shallow geophysical surveys to identify a previously unrecognized active fault, theXEOLXELEK‐Elk Lake fault, in the northern Cascadia forearc, ∼10 km north of the city of Victoria. Lidar‐derived digital terrain models and historical air photos show a ∼2.5‐m‐high scarp along the surface of a Quaternary drumlinoid ridge. Paleoseismic trenching and electrical resistivity tomography surveys across the scarp reveal a single reverse‐slip earthquake produced a fault‐propagation fold above a blind southwest‐dipping fault. Five geologically plausible chronological models of radiocarbon dated charcoal constrain the likely earthquake age to between 4.7 and 2.3 ka. Fault‐propagation fold modeling indicates ∼3.2 m of reverse slip on a blind, 50° southwest‐dipping fault can reproduce the observed deformation. Fault scaling relations suggest aM6.1–7.6 earthquake with a 13 to 73‐km‐long surface rupture and 2.3–3.2 m of dip slip may be responsible for the deformation observed in the paleoseismic trench. An earthquake near this magnitude in Greater Victoria could result in major damage, and our results highlight the importance of augmenting instrumental monitoring networks with remote sensing and field studies to identify and characterize active faults in similarily challenging environments.

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  2. null (Ed.)
    ABSTRACT New paleoseismic trenching indicates late Quaternary oblique right-lateral slip on the Leech River fault, southern Vancouver Island, Canada, and constrains permanent forearc deformation in northern Cascadia. A south-to-north reduction in northward Global Navigation Satellite System velocities and seismicity across the Olympic Mountains, Strait of Juan de Fuca (JDF), and the southern Strait of Georgia, has been used as evidence for permanent north–south crustal shortening via thrust faulting between a northward migrating southern forearc and rigid northern backstop in southwestern Canada. However, previous paleoseismic studies indicating late Quaternary oblique right-lateral slip on west-northwest-striking forearc faults north of the Olympic Mountains and in the southern Strait of Georgia are more consistent with forearc deformation models that invoke oroclinal bending and(or) westward extrusion of the Olympic Mountains. To help evaluate strain further north across the Strait of JDF, we present the results from two new paleoseismic trenches excavated across the Leech River fault. In the easternmost Good Hope trench, we document a vertical fault zone and a broad anticline deforming glacial till. Comparison of till clast orientations in faulted and undeformed glacial till shows evidence for postdeposition faulted till clast rotation, indicating strike-slip shear. The orientation of opening mode fissuring during surface rupture is consistent with right-lateral slip and the published regional SHmax directions. Vertical separation and the formation of scarp-derived colluvium along one fault also indicate a dip-slip component. Radiocarbon charcoal dating within offset glacial till and scarp-derived colluvium suggest a single surface rupturing earthquake at 9.4±3.4  ka. The oblique right-lateral slip sense inferred in the Good Hope trench is consistent with slip kinematics observed on other regional west-northwest-striking faults and indicates that these structures do not accommodate significant north–south shortening via thrust faulting. 
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  3. Abstract

    We use calibrated earthquake relocations to reassess the distribution and kinematics of faulting in the Zagros range, southwestern Iran. This is among the most seismically active fold‐and‐thrust belts globally, but knowledge of its active faulting is hampered by large errors in reported epicenters and controversy over earthquake depths. Mapped coseismic surface faulting is extremely rare, with most seismicity occurring on blind reverse faults buried beneath or within a thick, folded sedimentary cover. Therefore, the distribution of earthquakes provides vital information about the location of active faulting at depth. Using an advanced multievent relocation technique, we relocate ∼2,500 earthquakes across the Zagros mountains spanning the ∼70‐year instrumental record. Relocated events have epicentral uncertainties of 2–5 km; for ∼1,100 of them we also constrain origin time and focal depth, often to better than 5 km. Much of the apparently diffuse catalog seismicity now collapses into discrete trends highlighting major active faults. This reveals several zones of unmapped faulting, including possible conjugate left‐lateral faults in the central Zagros. It also confirms the activity of faults mapped previously on the basis of geomorphology, including oblique (dextral‐normal) faulting in the NW Zagros. We observe a primary difference between the Lurestan arc, where seismicity is focused close to the topographic range front, and the Fars arc, where out‐of‐sequence thrusting is evident over a width of ∼100–200 km. We establish a focal depth range of 4–25 km, confirming earlier suggestions that earthquakes are restricted to the upper crust but nucleate both within and beneath the sedimentary cover.

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  4. Abstract

    Observations of surface deformation within 1–2 km of a surface rupture contain invaluable information about the coseismic behavior of the shallow crust. We investigate the oblique strike‐slip 2016 M7 Kumamoto, Japan, earthquake, which ruptured the Futagawa‐Hinagu Fault. We solve for variable fault slip in an inversion of differential lidar topography, satellite optical image correlation, and Interferometric Synthetic Aperture Radar (InSAR)‐derived surface displacements. The near‐fault differential lidar pose several challenges. The model fault geometry must follow the surface trace at the sub‐kilometer scale. Integration of displacement datasets with different sensitivities to the 3D deformation field and varying spatial distribution permits additional complexity in the inferred slip but introduces ambiguity that requires careful selection of the regularization. We infer a Mwearthquake. The maximum slip of 6.9 m occurred at 4.5‐km depth, suggesting an on‐fault slip deficit in the upper several kilometers of the crust that likely reflects distributed and inelastic deformation within the shallow fault zone.

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  5. Abstract

    The 12 November 2017Mw 7.3 Ezgeleh‐Sarpolzahab earthquake is the largest instrumentally recorded earthquake in the Zagros Simply Folded Belt by a factor of ∼10 in seismic moment. Exploiting local, regional, and teleseismic data and synthetic aperture radar interferometry imagery, we characterize the rupture, its aftershock sequence, background seismicity, and regional tectonics. The mainshock ruptured slowly (∼2 km/s), unilaterally southward, for ∼40 km along an oblique (dextral‐thrust) fault that dips ∼14°E beneath the northwestern Lurestan arc. Slip is confined to basement depths of ∼12–18 km, resolvably beneath the sedimentary cover which is ∼8 km thick in this area. The gentle dip angle and basement location allow for a broad slip area, explaining the large magnitude relative to earthquakes in the main Fars arc of the Zagros, where shallower, steeper faults are limited in rupture extent by weak sedimentary layers. Early aftershocks concentrate around the southern and western edges of the mainshock slip area and therefore cluster in the direction of rupture propagation, implying a contribution from dynamic triggering. A cluster of events ∼100 km to the south near Mandali (Iraq) reactivated the ∼50° dipping Zagros Foredeep Fault. The basement fault responsible for the Ezgeleh‐Sarpolzahab earthquake probably accounts for the ∼1 km elevation contrast between the Lurestan arc and the Kirkuk embayment but is distinct from sections of the Mountain Front Fault that define frontal escarpments elsewhere in the Zagros. It may be related to a seismic interface underlying the central and southern Lurestan arc, and a key concern is whether or not the more extensive regional structure is also seismogenic.

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