Plateau subduction, intraslab seismicity, and the Denali (Alaska) volcanic gap (original) (raw)
Related papers
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.
Geophysical Monograph Series, 2000
We present a review of great earthquakes and seismicity patterns along the Alaska-Aleutian and Kamchatka-Kurile arcs as an overview of one of the longest subduction zone complexes on the planet. Seismicity patterns, double seismic zones and focal mechanism solutions are described and used to illustrate the distribution of stress in the Pacific plate as it collides with North America and Eurasia. Seismicity along the Alaska-Aleutian arc is relatively shallow as compared to the Kamchatka-Kurile arc where the plate is considerably older and thicker prior to entering the sub duction zone. Tomographic inversions of the slab generally show high velocity anomalies where seismicity is high, presumably tracking the cold subducting lithosphere.
Spatial and temporal patterns of nonvolcanic tremor along the southern Cascadia subduction zone
Journal of Geophysical Research, 2010
Episodic tremor and slip (ETS), the spatial and temporal correlation of slow slip events monitored via GPS surface displacements and nonvolcanic tremor (NVT) monitored via seismic signals, is a newly discovered mode of deformation thought to be occurring downdip from the seismogenic zone along several subduction zone megathrusts. To provide overall constraints on the distribution and migration behavior of NVT in southern Cascadia, we apply a semiautomated location algorithm to seismic data available during the EarthScope Transportable Array deployment to detect the most prominent pulses of NVT and invert analyst-refined relative arrival times for source locations. In the processing, we also detect distinct and isolated bursts of energy within the tremor similar to observations of low-frequency earthquakes in southwest Japan. We investigate in detail eight NVT episodes between November 2005 and August 2007 with source locations extending over a 650 km along-strike region from northern California to northern Oregon. We find complex tremor migration patterns with periods of steady migration (4-10 km/d), halting, and frequent along-strike jumps (30-400 km) in activity. The initiation and termination points of laterally continuous tremor activity appear to be repeatable features between NVT episodes which support the hypothesis of segmentation within the ETS zone. The overall distribution of NVT epicenters occur within a narrow band primarily confined by the surface projections of the 30 and 40 km contours of the subducting plate interface. We find as much as 50 km spatial offset from the updip edge of the tremor source zone to the downdip edge of the thermally and geodetically defined transition zone, which may inhibit ETS from triggering earthquakes further updip. Intriguingly, NVT activity is spatially anticorrelated with local seismicity, suggesting the two processes are mutually exclusive. We propose that the transition in frictional behavior coupled with high pore fluid pressures in the ETS zone favor tremor generation instead of regular interplate seismicity and frequent ETS produces a semicontinuous relaxation of strain within the overriding and subducting plates that further inhibit seismogenesis surrounding the ETS source region.
Strain accumulation in the Yakataga Seismic Gap, southern Alaska
Journal of Geophysical Research, 1986
Strain accumulation in the Yakataga seismic gap has been estimated from the deformation of a 60 km x 40 km trilateration network surveyed in 1979-1980, 1982, and 1984. The contraction in the approximate direction (N19øW) of plate convergence is 0.19 + 0.04 #strain/yr, and there is a minor (0.07 + 0.04 ttstrain/yr) orthogonal extension. A significant right-lateral shear (0.09 _+ 0.02 #strain/yr) occurs across a vertical plane striking N7 IøE. A simple dislocation representation of the plate interaction model proposed by Lahr and Plafker for the Yakataga seismic gap quantitatively accounts for the observed N19øW contraction and qualitatively accounts for the right-lateral shear, the two strains being associated with the normal and transverse components, respectively, of the oblique convergence between the Yakutat block and the North American plate. The measurements are consistent with strain accumulation building up to a great thrust earthquake in the Yakataga gap. In addition, there is a suggestion of right-lateral shear strain accumulation across the Contact fault, but the inverval over which data are available is probably too short to prove such accumulation. often the case in very oblique subduction , the transcurrent motion is accommodated on a vertical, strike-slip fault, whereas the convergence is accommodated on a separate thrust. In this case the strike-slip fault is the Fairweather fault and the thrust outcrops at the Transition Zone ), follows the interface between the Yakutat block and the Pacific plate, and finally plunges steeply beneath the North American plate in the vicinity of the Fairweather fault [Lahr and . Then, relative to the North American plate, the Yakutat block moves N36øW, and the relative motion between the two is accommodated by pure strike slip on the Fairweather fault. The convergence between the Pacific plate and the North American plate is accommodated by thrusting of the Yakutat block and North American
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).
Rapid tremor reversals in Cascadia generated by a weakened plate interface
Nature Geoscience, 2011
Slow slip along the plate interface at subduction zones can generate weak seismic tremor in a quasi-periodic process called episodic tremor and slip. This process differs in character from regular earthquake rupture and can release stresses that build up on the deep plate interface. Here we analyse the spatial and temporal evolution of the five largest episodic tremor and slip events between 2004 and 2009 in northern Washington on the Cascadia subduction zone. We find that the events are similar, but not identical because they initiate in different locations and propagate along the plate interface at different average speeds of 7 to 12 km per day. Our analysis reveals that tremor can migrate rapidly back, away from the region where tremor and slip are advancing, through parts of the plate interface that have just ruptured in the past three days. These rapid tremor reversals propagate backwards for tens of kilometres at speeds that are 20 to 40 times faster than the relatively slow, steady advance of episodic tremor and slip. Our observations suggest that once the plate interface is weakened by the initial advance of episodic tremor and slip, it allows stresses to induce slip more easily or fluid pressure waves to migrate back more rapidly, generating rapid tremor reversals. E pisodes of weak seismic radiation (termed tremor) and slow slip occur together in a coupled process that has been detected recently in several subduction zones 1-5 . The episodes are called Episodic Tremor and Slip (ETS) in Cascadia and Slow Slip Events (SSE) in Japan. Reference 6 provides a recent review of Cascadia-wide ETS. In contrast with regular earthquakes, tremor signals exhibit low amplitudes, long durations, and emergent character. In northern Washington, ETS manifests in SSE that last 2-4 weeks, extend over 150 km along strike of the Cascadia subduction zone, and involve a moment release equivalent to a M6.4 to 6.8 earthquake. Recurring every 12-15 months, they are tantalizingly near-periodic in occurrence, compared with the aperiodicity of regular earthquakes.
Geotechnical Reconnaissance of the 2002 Denali Fault, Alaska, Earthquake
Earthquake Spectra, 2004
The 2002 M7.9 Denali fault earthquake resulted in 340 km of ruptures along three separate faults, causing widespread liquefaction in the fluvial deposits of the alpine valleys of the Alaska Range and eastern lowlands of the Tanana River. Areas affected by liquefaction are largely confined to Holocene alluvial deposits, man-made embankments, and backfills. Liquefaction damage, sparse surrounding the fault rupture in the western region, was abundant and severe on the eastern rivers: the Robertson, Slana, Tok, Chisana, Nabesna and Tanana Rivers. Synthetic seismograms from a kinematic source model suggest that the eastern region of the rupture zone had elevated strong-motion levels due to rupture directivity, supporting observations of elevated geotechnical damage. We use augered soil samples and shear-wave velocity profiles made with a portable apparatus for the spectral analysis of surface waves (SASW) to characterize soil properties and stiffness at liquefaction sites and three trans-Alaska pipeline pump station accelerometer locations.
Earthquake Triggering at Alaskan Volcanoes Following the 3 November 2002 Denali Fault Earthquake
The 3 November 2002 M W 7.9 Denali fault earthquake provided an excellent opportunity to investigate triggered earthquakes at Alaskan volcanoes. The Alaska Volcano Observatory operates short-period seismic networks on 24 historically active volcanoes in Alaska, 247–2159 km distant from the mainshock epicenter. We searched for evidence of triggered seismicity by examining the unfiltered wave-forms for all stations in each volcano network for 1 hr after the M W 7.9 arrival time at each network and for significant increases in located earthquakes in the hours after the mainshock. We found compelling evidence for triggering only at the Katmai volcanic cluster (KVC, 720–755 km southwest of the epicenter), where small earthquakes with distinct P and S arrivals appeared within the mainshock coda at one station and a small increase in located earthquakes occurred for several hours after the mainshock. Peak dynamic stresses of 0.1 MPa at Augustine Volcano (560 km southwest of the epicenter) are significantly lower than those recorded in Yellowstone and Utah (3000 km southeast of the epicenter), suggesting that strong directivity effects were at least partly responsible for the lack of triggering at Alaskan volcanoes. We describe other incidents of earthquake-induced triggering in the KVC, and outline a qualitative magnitude/distance-dependent triggering threshold. We argue that triggering results from the perturbation of magmatic-hydrothermal systems in the KVC and suggest that the comparative lack of triggering at other Alaskan volcanoes could be a result of differences in the nature of magmatic-hydrothermal systems.