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Deep sedimentary basins amplify long‐period shaking from seismic waves, increasing the seismic hazard for cities sited on such basins. We perform 3‐D simulations of point source earthquakes distributed around the Seattle and Tacoma basins in Washington State to examine the dependence of basin amplification on source azimuth, depth, and earthquake type. For periods between 1 and 10 s, the pattern of amplification is spatially heterogeneous and differs considerably with the source‐to‐site azimuth. For close‐in earthquakes, the greatest basin amplification occurs toward the farside of the basin and ground motions from crustal earthquakes experience greater amplification than those from more vertically incident, deeper intraplate earthquakes. Love and Rayleigh waves form similar spatial patterns for a given source location, although the magnitude of amplification varies. The source dependence of basin amplification is an important factor for seismic hazard assessment, in both the Seattle and Tacoma basins, and by extension for deep sedimentary basins worldwide. Plain Language Summary: Sedimentary basins amplify the ground shaking due to an earthquake. This increases the seismic hazard for cities located above such basins. Therefore, determining the amount and distribution of shaking amplification is critical to estimating the hazard associated with future earthquakes. We use computer simulations to determine how an earthquake's location, depth, and focal mechanism affect ground shaking for communities above the Seattle and Tacoma sedimentary basins, Washington State. We find that the amount of shaking amplification strongly depends on where the earthquake is located, as well as the earthquake type. This has important implications for seismic safety (e.g., for buildings and infrastructure) located above deep sedimentary basins worldwide. Key Points: Basin amplification within the Seattle and Tacoma basins is strongly dependent on earthquake‐to‐site azimuth, source mechanism, and depthFor close‐in sources, peak long‐period basin amplification is generally focused on the farside of the basin relative to the sourceImpact of source‐dependent basin amplification needs to be considered for estimates of seismic hazard in sedimentary basins worldwide [ABSTRACT FROM AUTHOR]

Abstract: The extent of damage in Washington, DC, from the 2011 Mw 5.8 Mineral, VA, earthquake was surprising for an epicenter 130 km away; U.S. Geological Survey “Did‐You‐Feel‐It” reports suggest that Atlantic Coastal Plain and other unconsolidated sediments amplified ground motions in the city. We measure this amplification relative to bedrock sites using earthquake signals recorded on a temporary seismometer array. The spectral ratios show strong amplification in the 0.7 to 4 Hz frequency range for sites on sediments. This range overlaps with resonant frequencies of buildings in the city as inferred from their heights, suggesting amplification at frequencies to which many buildings are vulnerable to damage. Our results emphasize that local amplification can raise moderate ground motions to damaging levels in stable continental regions, where low attenuation extends shaking levels over wide areas and unconsolidated deposits on crystalline metamorphic or igneous bedrock can result in strong contrasts in near‐surface material properties. [ABSTRACT FROM AUTHOR]

The Yakima folds of central Washington, USA, are prominent anticlines that are the primary tectonic features of the backarc of the northern Cascadia subduction zone. What accounts for their topographic expression and how much strain do they accommodate and over what time period? We investigate Manastash anticline, a north vergent fault propagation fold typical of structures in the fold province. From retrodeformation of line- and area-balanced cross sections, the crust has horizontally shortened by 11% (0.8-0.9 km). The fold, and by inference all other folds in the fold province, formed no earlier than 15.6 Ma as they developed on a landscape that was reset to negligible relief following voluminous outpouring of Grande Ronde Basalt. Deformation is accommodated on two fault sets including west-northwest striking frontal thrust faults and shorter north to northeast striking faults. The frontal thrust fault system is active with late Quaternary scarps at the base of the range front. The fault-cored Manastash anticline terminates to the east at the Naneum anticline and fault; activity on the north trending Naneum structures predates emplacement of the Grande Ronde Basalt. The west trending Yakima folds and west striking thrust faults, the shorter north to northeast striking faults, and the Naneum fault together constitute the tectonic structures that accommodate deformation in the low strain rate environment in the backarc of the Cascadia Subduction Zone. [ABSTRACT FROM AUTHOR]

On 7 May 2015, a M w 4.0 earthquake occurred near Venus, northeast Johnson County, Texas, in an area of the Bend Arch-Fort Worth Basin that reports long-term, high-volume wastewater disposal and that has hosted felt earthquakes since 2009. In the weeks following the M w 4.0 earthquake, we deployed a local seismic network and purchased nearby active-source seismic reflection data to capture additional events, characterize the causative fault, and explore potential links between ongoing industry activity and seismicity. Hypocenter relocations of the resulting local earthquake catalog span ~4-6 km depth and indicate a fault striking ~230 °, dipping to the west, consistent with a nodal plane of the M w 4.0 regional moment tensor. Fault plane solutions indicate normal faulting, with B axes striking parallel to maximum horizontal compressive stress. Seismic reflection data image the reactivated basement fault penetrating the Ordovician disposal layer and Mississippian production layer, but not displacing post-Lower Pennsylvanian units. Template matching at regional seismic stations indicates that low-magnitude earthquakes with similar waveforms began in April 2008, with increasing magnitude over time. Pressure data from five saltwater disposal wells within 5 km of the active fault indicate a disposal formation that is 0.9-4.8 MPa above hydrostatic. We suggest that the injection of 28,000,000 m3 of wastewater between 2006 and 2015 at these wells led to an increase in subsurface pore fluid pressure that contributed to inducing this long-lived earthquake sequence. The 2015 M w 4.0 event represents the largest event in the continuing evolution of slip on the causative fault. [ABSTRACT FROM AUTHOR]

Alleghanian dextral transpression produced multiple strike-slip shear zones forming a contractional mega-duplex in the central and southern Appalachian Piedmont. Structural upwelling along the zones is identifiable as crustal arching on seismic reflection profiles that roughly corresponds to the Piedmont gravity high. The structural upwelling coincides both temporally and spatially with areas of peak Alleghanian plutonism and metamorphism. The compositionally bimodal plutons generally occur in or around the fault zones and are isotopically primitive. Geobarometry of cognate xenocryst inclusions of coexisting plagioclase and pyroxene from a granitoid indicates 8-11 kbar or 31- to 42.5-km depth to proposed source region. This depth roughly corresponds to the pre-erosional location of multiple reflections beneath the arches on seismic profiles that are interpreted as the Moho. The granitoids are proposed to have formed by partial melting of the lowermost crust through heat from mafic intrusions and decompression resulting from crustal arching beneath the strike-slip faults. The granitoids ascended along the strike-slip faults and were emplaced in or near them. The crustal arching and melting processes appear to be important mechanisms in the development of strike-slip orogens. [ABSTRACT FROM AUTHOR]