Implications of deformation following the 2002 Denali, Alaska, earthquake for postseismic relaxation processes and lithospheric rheology (original) (raw)
Related papers
Bulletin of the Seismological Society of America, 2004
Geophysical information, including deep-crustal seismic reflection, magnetotelluric (MT), gravity, and magnetic data, cross the aftershock zone of the 3 November 2002 M w 7.9 Denali fault earthquake. These data and aftershock seismicity, jointly interpreted, reveal the crustal structure of the right-lateral-slip Denali fault and the eastern Alaska Range orogen, as well as the relationship between this structure and seismicity. North of the Denali fault, strong seismic reflections from within the Alaska Range orogen show features that dip as steeply as 25Њ north and extend downward to depths between 20 and 25 km. These reflections reveal crustal structures, probably ductile shear zones, that most likely formed during the Late Cretaceous, but these structures appear to be inactive, having produced little seismicity during the past 20 years. Furthermore, seismic reflections mainly dip north, whereas alignments in aftershock hypocenters dip south. The Denali fault is nonreflective, but modeling of MT, gravity, and magnetic data suggests that the Denali fault dips steeply to vertically. However, in an alternative structural model, the Denali fault is defined by one of the reflection bands that dips to the north and flattens into the middle crust of the Alaska Range orogen. Modeling of MT data indicates a rock body, having low electrical resistivity (Ͼ10 X•m), that lies mainly at depths greater than 10 km, directly beneath aftershocks of the Denali fault earthquake. The maximum depth of aftershocks along the Denali fault is 10 km. This shallow depth may arise from a higher-than-normal geothermal gradient. Alternatively, the low electrical resistivity of deep rocks along the Denali fault may be associated with fluids that have weakened the lower crust and helped determine the depth extent of the aftershock zone.
Stress-dependent power-law flow in the upper mantle following the 2002 Denali, Alaska, earthquake
Earth and Planetary Science Letters, 2006
Far-field continuous Global Positioning System (GPS) time-series data following the 2002 M7.9 Denali, Alaska earthquake imply that mantle viscoelastic rheology is stress-dependent. A linear viscous mantle cannot explain fast early displacement rates at the surface that rapidly decay with time, whereas a power-law rheology where strain rate is proportional to stress raised to the power of 3.5 ± 0.5 provides decay rates and spatial patterns in agreement with observations. This is consistent with laboratory measurements for hot, wet olivine, implying a hydrated mantle and a relatively thin (60-km-thick) lithosphere beneath south-central Alaska. These results suggest that the viscous strength of the lithosphere varies both spatially and temporally, and that effective viscosities inferred from different loading events or observational time-periods can differ by up to several orders of magnitude. Thus, the very conditions that enable the inference of rheologic strength-transient loading and unloading events-significantly alter the effective viscosity.
Plateau subduction, intraslab seismicity, and the Denali (Alaska) volcanic gap
Geology, 2017
Tectonic tremors in Alaska (USA) are associated with subduction of the Yakutat plateau, but their origins are unclear due to lack of depth constraints. We have processed tremor recordings to extract low-frequency earthquakes (LFEs), and generated a set of six LFE waveform templates via iterative network matched filtering and stacking. The timing of impulsive P (compressional) wave and S (shear) wave arrivals on template waveforms places LFEs at 40-58 km depth, near the upper envelope of intraslab seismicity and immediately updip of increased levels of intraslab seismicity. S waves at near-epicentral distances display polarities consistent with shear slip on the plate boundary. We compare characteristics of LFEs, seismicity, and tectonic structures in central Alaska with those in warm subduction zones, and propose a new model for the region's unusual intraslab seismicity and the enigmatic Denali volcanic gap (i.e., an area of no volcanism where expected). We argue that fluids in the Yakutat plate are confined to its upper crust, and that shallow subduction leads to hydromechanical conditions at the slab interface in central Alaska akin to those in warm subduction zones where similar LFEs and tremor occur. These conditions lead to fluid expulsion at shallow depths, explaining strike-parallel alignment of tremor occurrence with the Denali volcanic gap. Moreover, the lack of double seismic zone and restriction of deep intraslab seismicity to a persistent low-velocity zone are simple consequences of anhydrous conditions prevailing in the lower crust and upper mantle of the Yakutat plate.
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.
Coseismic deformation of the 2002 Denali Fault earthquake: Insights from GPS measurements
Journal of Geophysical Research, 2006
1] We estimate coseismic displacements from the 2002 M w 7.9 Denali Fault earthquake at 232 GPS sites in Alaska and Canada. Displacements along a N-S profile crossing the fault indicate right-lateral slip on a near-vertical fault with a significant component of vertical motion, north-side up. We invert both GPS displacements and geologic surface offsets for slip on a three-dimensional (3-D) fault model in an elastic half-space. We restrict the motion to right-lateral slip and north-side-up dip slip. Allowing for oblique slip along the Denali and Totschunda faults improves the model fit to the GPS data by about 30%. We see mostly right-lateral strike-slip motion on the Denali and Totschunda faults, but in a few areas we see a significant component of dip slip. The slip model shows increasing slip from west to east along the Denali Fault, with four localized higherslip patches, three near the Trans-Alaska pipeline crossing and a large slip patch corresponding to a M w 7.5 subevent about 40 km west of the Denali-Totschunda junction. Slip of 1-3 m was estimated along the Totschunda Fault with the majority of slip being at shallower than 9 km depth. We have limited resolution on the Susitna Glacier Fault, but the estimated slip along the fault is consistent with a M w 7.2 thrust subevent. Total estimated moment in the Denali Fault earthquake is equivalent to M w 7.89. The estimated slip distribution along the surface is in very good agreement with geological surface offsets, but we find that surface offsets measured on glaciers are biased toward lower values.
Deformation across the rupture zone of the 1964 Alaska earthquake, 1993-1997
Journal of Geophysical Research: Solid Earth, 1998
A linear array of 15 geodetic monuments was installed in 1993 across the rupture zone of the 1964 Alaska earthquake (Mw=9.2). The array extends from Middleton Island (at the edge of the continental shelf and 80 km from the Alaska-Aleutian trench) to north of Palmer, Alaska (380 km from the trench), in the approximate direction of Pacific-North American plate convergence (N15.5øW). The array was surveyed in June 1993, May 1995, and June 1997. The changes between surveys are a measure of the deformation of the continental margin across the subduction zone in southern Alaska. Measured relative to the interior of the North American plate, the horizontal velocities on the outer plate margin are parallel to the direction of plate convergence (N15.5øW) and reach a maximum (58 mm yr-•) about 150 km from the trench. Beyond about 300 km from the trench the observed horizontal velocities are small. A narrow (halfwidth 50 km) zone of significant uplift (10 mm yr 4 maximum) is observed about 300 km from the trench, coinciding roughly with the locus of maximum coseismic subsidence associated with the 1964 Alaska earthquake. Although the deformation is roughly described by the conventional model of deformation at a subduction zone (deformation due to virtual back slip on the main thrust zone at the 55 mm yr-• plate convergence rate), a better fit is given with a 65 mm yr-• virtual back (normal) slip rate. This higher rate is attributed to continued postseismic relaxation. The model does not explain the relatively high uplift rate and low N15.5øW velocity observed at Middleton Island. That anomalous motion is attributed to continued thrusting on postulated upward trending splays from the subduction zone beneath the island.
Tectonics, 1999
An industry-acquired seismic reflection line from the Aleutian Arc to the Alaska Trench was prestack depthmigrated and further constrained with wide-angle ocean bottom hydrophone seismic velocity data and swath mapping. From the seismic reflection image, individual thrust slices, the backstop, and the forearc basin were resolved sufficiently well to balance the section. Tectonic shortening and age estimates provided rates of permanent contraction in five discrete segments over-150 km during the past 3 Myr. About 70% of the plate convergence is measured as contraction across the-40-km-wide Neogene accretionary prism which composes less than 10% of the forearc. Permanent contraction is minimal above the 1964 Alaska earthquake aftershock zone indicating dominently elastic strain. Since elastic strain is poorly recorded in the geological record, it was approximated from the historical record of the 1964 Alaska earthquake. The 1964 coseismic vertical deformation displays an inverse pattern to the permanent strain, consistent with the earthquake cycle concept. Elastic deformation is dominant beneath the shelf and upper slope, rapid permanent deformation is concentrated beneath the lower slope, and a transition between them occurs beneath the middle slope. Eocene time, strata in the Stevenson forearc basin recorded only 1 km of shortening across a 65-km width (documented herein). During the great 1964 Alaska earthquake, the underlying plate boundary slipped up to 20 m, and estimated repeat times for large earthquakes are 66-500 years [Nishenko and