Seismicity, earthquakes and structure along the Alaska-Aleutian and Kamchatka-Kurile Subduction Zones: A review (original) (raw)
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Seismicity and structure of the Kamchatka Subduction Zone
Journal of Geophysical Research: Solid Earth, 1997
The configuration of the Pacific plate subducted beneath the Kamchatka peninsula and the stress distribution in the Kamchatka subduction zone (KSZ) were studied using the catalog of the Kamchatka regional seismic network, focal mechanism solutions estimated from P wave first motions, the formal inversion of long‐period waveforms, and centroid moment tensor solutions. To the south of ∼55°N, the slab shows an approximately constant dip angle of ∼55°. To the north of ∼55°N, the dip of the slab becomes shallower reaching ∼35°. The maximum depth of seismicity, Dm, varies from ∼500 km depth near 50°N to ∼300 km depth at ∼55°N. The volcanic front is almost linear along the main part of the KSZ whereas it is sharply shifted landward to the north of ∼55°N. The variation of Dm is apparently consistent with the standard empirical relation Dm=ƒ(ϕ), where ϕ is the thermal parameter of the subducted slab. To the north of ∼55°N, the slab is offset toward the northwest, and it is sharply deformed i...
Crustal earthquakes in the Cook Inlet and Susitna region of southern Alaska
Tectonophysics, 2018
Several large (M ≥ 6) earthquakes have occurred in the vicinity of Anchorage, Alaska, within the past century. The presence of the underlying subducting Pacific plate makes it difficult to determine the origin of these older earthquakes as either crustal, slab, or the subduction plate interface. We perform a seismological study of historical and modern earthquakes within the Cook Inlet and Susitna region, west of Anchorage. We first estimate hypocenters for historical large earthquakes in order to assess their likelihood of origin as crustal, slab, or plate interface. We then examine modern crustal seismicity to better understand the style of faulting and the location of active structures, including within (and beneath) the Cook Inlet and Susitna basins. We perform double-couple moment tensor inversions using high frequency body waves (1-10 Hz) for small to moderate (M ≥ 2.5) crustal earthquakes (depth ≤ 30 km) occurring from 2007 to 2017. Our misfit function combines both waveforms differences as well as first-motion polarities in order to obtain reliable moment tensor solutions. The three focus regions-Beluga, upper Cook Inlet, and Susitna-exhibit predominantly thrust mechanisms for crustal earthquakes, indicating an overall compressive regime within the crust that is approximately consistent with the direction of plate convergence. Mechanisms within upper Cook Inlet have strike directions aligned with active anticlines previously identified in Cook Inlet from active-source seismic data. Our catalog of moment tensors is helpful for identifying and characterizing subsurface faults from seismic lineaments and from faults inferred from subsurface images from active-source seismic data. subducting beneath Alaska (Plafker et al., 1978; Eberhart-Phillips et al., 2006; Christeson et al., 2010). The subducting Pacific/Yakutat plate is interpreted to be responsible for the extremely shallow angle of subduction (< 5°), far inland, as well as for the noteworthy lack of volcanism in the Susitna basin and Talkeetna Mountains, in a magmatic gap between the Aleutian volcanic arc on the west and the Wrangell volcanic field on the east (Fig. 1a) (Eberhart-Phillips et al., 2006; Rondenay et al., 2010). We focus on a lowlands region marked by the presence of two major sedimentary basins (Figs. 1b and 3): the Cook Inlet basin south of the Castle Mountain fault, and the Susitna basin north of the fault (Fig. 1b).
Journal of Geophysical Research, 2006
1] In southern and central Alaska the subduction and active volcanism of the Aleutian subduction zone give way to a broad plate boundary zone with mountain building and strike-slip faulting, where the Yakutat terrane joins the subducting Pacific plate. The interplay of these tectonic elements can be best understood by considering the entire region in three dimensions. We image three-dimensional seismic velocity using abundant local earthquakes, supplemented by active source data. Crustal low-velocity correlates with basins. The Denali fault zone is a dominant feature with a change in crustal thickness across the fault. A relatively high-velocity subducted slab and a low-velocity mantle wedge are observed, and high V p /V s beneath the active volcanic systems, which indicates focusing of partial melt. North of Cook Inlet, the subducted Yakutat slab is characterized by a thick low-velocity, high-V p /V s crust. High-velocity material above the Yakutat slab may represent a residual older slab, which inhibits vertical flow of Yakutat subduction fluids. Alternate lateral flow allows Yakutat subduction fluids to contribute to Cook Inlet volcanism and the Wrangell volcanic field. The apparent northeast edge of the subducted Yakutat slab is southwest of the Wrangell volcanics, which have adakitic composition consistent with melting of this Yakutat slab edge. In the mantle, the Yakutat slab is subducting with the Pacific plate, while at shallower depths the Yakutat slab overthrusts the shallow Pacific plate along the Transition fault. This region of crustal doubling within the shallow slab is associated with extremely strong plate coupling and the primary asperity of the M w 9.2 great 1964 earthquake.
Unusual earthquakes in the Gulf of Alaska and fragmentation of the Pacific Plate
Geophysical Research Letters, 1988
Two recent M s 7.6 strike-slip which ruptured in an M s 7.9 event in 1958. West of earthquakes in the northern Gulf of Alaska rup-longitude 144øW, motion is accommodated by undertured a composite 250-km-long north-striking zone thrusting and subduction along the Aleutian megain the Pacific plate. These shocks are attributed thrust, which last slipped in the 1964 earthquake. to a combination of enhanced tensional stress in In the intervening region, which has been identhe Pacific plate seaward of and following the tified as a seismic gap (Tobin and Sykes, 1968; great (M w 9.2) Alaska earthquake of 1964, and Kelleher, 1970; Sykes, 1971), the tectonics are compressional stress resulting from collision of complicated by the collision of the Yakutat terthe Yakutat terrane with North America. The rane, which moves with nearly the Pacific plate occurrence of these shocks reflects fragmentation velocity (Plafker et al., 1978; Lahr and Plafker, of the northeast corner of the Pacific plate, 1980; Perez and Jacob, 1980; Bruns, 1983; Plafker, possibly the initial step in establishing a new 1987). Although no great earthquake has occurred plate boundary seaward of the current boundary. within the Yakataga seismic gap since the turn of the century (McCann et al., 1980), two sequences
New insight into the crust and upper mantle structure under Alaska
Polar Science, 2007
To better understand the seismic structure of the subducting Pacific plate under Alaska, we determined the three-dimensional Pwave velocity structure to a depth of approximately 200 km beneath Alaska using 438,146 P-wave arrival times from 10,900 earthquakes. In this study an irregular grid parameterization was adopted to express the velocity structure under Alaska. The number of grid nodes increases from north to south in the study area so that the spacing between grid nodes is approximately the same in the longitude direction. Our results suggest that the subducting Pacific slab under Alaska can be divided into three different parts based on its geometry and velocity structure. The western part has features similar to those in other subduction zones. In the central part a thick low-velocity zone is imaged at the top of the subducting Pacific slab beneath north of the Kenai Peninsula, which is believed to be most likely the oceanic crust plus an overlying serpentinized zone and the coupled Yakutat terrane subducted with the Pacific slab. In the eastern part, significant high-velocity anomalies are visible to 60e90 km depth, suggesting that the Pacific slab has only subducted down to that depth.
A seismotectonic study of the Southeastern Alaska Region
Tectonophysics, 2011
We compare relocations of recent (1973-2005) and historic (1919-1972) earthquakes to geologic and geophysical (gravity, aeromagnetic, and uplift) information to determine the relationship of seismicity to crustal deformation in southeastern Alaska. Our results suggest that along strike changes in the structure of the Pacific plate may control the location of the ends of rupture zones for large earthquakes along the offshore Queen Charlotte fault system in the southern portion of the study area. There is a marked increase in background seismicity in the northern portion of the study area where the Fairweather fault begins to bend toward the northwest and crustal uplift due to glacial unloading exceeds 20 mm/year. Focal mechanisms indicate that thrust and reverse mechanisms predominate in the region of maximum uplift, as might be expected by the decrease in ice sheet thickness. The diffuse nature of seismicity between the Fairweather and Denali faults in the northern study area suggests a complex interaction between plate/microplate interactions and glacial unloading, making it difficult to determine the optimal fault orientation for failure in moderate magnitude (5.5 to 6.5) earthquakes within this region.
Seismic anisotropy and heterogeneity in the Alaska subduction zone
Geophysical Journal International, 2012
We determined P-and S-wave tomography and P-wave anisotropic structure of the Alaska subduction zone using 259 283 P-and 73 817 S-wave arrival times from 7268 local shallow and intermediate-depth earthquakes recorded by more than 400 seismic stations. The results show strong velocity heterogeneities in the crust and upper mantle. Low-velocity anomalies are revealed in the mantle wedge with significant along-arc variations under the active volcanoes. In the mantle wedge, the low-velocity zone extends down to 100-150 km depth under the backarc. The results indicate that H 2 O and fluids brought downwards by the subducting Pacific slab are released to the mantle wedge by dehydration and they are subsequently transported to the surface by the upwelling flow in the mantle wedge. Significant P-wave anisotropic anomalies are revealed under Alaska. The predominant fast velocity direction (FVD) is trench-parallel in the shallow part of the mantle wedge (<90 km depth) and in the subslab mantle, whereas the FVD is trench-normal within the subducting Pacific slab. The trench-parallel FVDs in the mantle wedge and subslab mantle may be caused by 3-D mantle flow that is induced by the complex geometry and strong curvature of the Pacific slab under Alaska. The flat and oblique subduction of the Pacific slab may play a key role in forming the trench-parallel FVD under the slab. The trench-normal FVD in the subducting Pacific slab may reflect the original fossil anisotropy when the Pacific Plate was produced at the mid-ocean ridge.
Deformation across the Alaska-Aleutian Subduction Zone near Kodiak
Geophysical Research Letters, 1999
The Kodiak-Katmai geodetic array, nine monuments distributed along a profile trending northnorthwestward across Kodiak Island and the Alaska Peninsula, was surveyed in 1993, 1995 and 1997 to determine the deformation at the Alaska-Aleutian subduction zone. Velocities on Kodiak island measured relative to the stable North American plate decrease with distance from the Alaska-Aleutian trench (distance range 106 to 250 km), whereas no appreciable deformation was measured on the Alaska Peninsula (distances 250 to 370 km from the trench). The measured deformation is reasonably well predicted by the conventional dislocation representation of subduction with the model parameters determined independently (i.e., not simply by fitting the observations). The deformation of Kodiak Island is in striking contrast to the very minor deformation measured in the similarly situated Shumagin Islands, 450 km southwest of Kodiak along the Alaska-Aleutian trench.