Bemis S.P.,University of Kentucky |
Weldon R.J.,University of Oregon |
Carver G.A.,Carver Geologic Inc.
Lithosphere | Year: 2015
Active transpressional fault systems are typically associated with the development of broad zones of deformation and topographic development; however, the complex geometries typically associated with these systems often make it difficult to isolate the important boundary conditions that control transpressional orogenic growth. The Denali fault system is widely recognized as transpressional due to the presence of the Denali fault, a major, active, right-lateral fault, and subparallel zones of thrust faults and fault-related folding along both the north and south flanks of the Alaska Range. Measured Quaternary and Holocene slip rates exist for the Denali fault system and portions of the adjacent thrust system, but the partitioning of fault slip between contractional and translational components of this transpressional system has not been previously studied in detail. Exploiting the relatively simple geometry of the Denali fault, we analyze the style and distribution of active faulting within the Alaska Range to define patterns of strain accommodation and determine how contractional and translational strain is partitioned across the Denali fault system. As the trace of the Denali fault curves by-70° across central Alaska, the mean strike of the thrust system to the north remains subparallel to the Denali fault, while to the south, the few faults with known or suspected Quaternary offset are oblique to the Denali fault. This relationship suggests that as the Denali fault system accommodates local fault-parallel strike slip, it partitions the residual part of the regional NW-directed plate motion into NW-SE shortening south of the Denali fault and shortening perpendicular to the Denali fault to the north. The degree of slip partitioning is consistent with a balanced slip budget for the two primary faults that contribute displacement to the Denali fault system (the eastern Denali fault and Totschunda fault). The current obliquity of displacement south of the Denali fault is the result of the late Cenozoic development of the Totschunda fault, which provides a more direct connection for the transfer of strain from the Fairweather transform fault to the Denali fault system. The transmitted strain is partitioned into right-lateral slip on the Denali fault and into Denali fault-normal shortening that is accommodated by thrust faulting in the Alaska Range and distributed left-lateral slip faulting within interior Alaska to the north. © 2015 Geological Society of America.
Shennan I.,Durham University |
Barlow N.,Durham University |
Carver G.,Carver Geologic Inc. |
Davies F.,Durham University |
And 2 more authors.
Geology | Year: 2014
Large to great earthquakes and related tsunamis generated on the Alaska megathrust produce major hazards for both the area of rupture and heavily populated coastlines around much of the Pacific Ocean. Recent modeling studies suggest that single-segment ruptures, as well as multi-segment, 1964-type ruptures, can produce great earthquakes, >M8, and significant hazards both in the near field and to distant locations through the generation of tsunamis. We present new paleoseismological data from Kodiak Island and a new analysis of radiocarbon data based on Bayesian age modeling to combine our observations with previous geological, historical, and archaeological investigations. We suggest that, in addition to multi-segment ruptures in A.D. 1964 and 1020-1150 (95% age estimate), a single-segment rupture occurred in 1788, with coseismic land-surface deformation across Kodiak Island and a tsunami that is recorded in historical documents and in sediment sequences, and another, similar rupture of the same Kodiak segment at A.D. 1440-1620. These indicate shorter intervals between ruptures of the Kodiak segment than previously assumed, and more frequent ruptures than for the Prince William Sound segment. © 2014 Geological Society of America.
Nyman D.J.,D.J. Nyman and Associ. |
Carver G.A.,Carver Geologic Inc. |
Honegger D.G.,D.G. Honegger Consulting
NCEE 2014 - 10th U.S. National Conference on Earthquake Engineering: Frontiers of Earthquake Engineering | Year: 2014
Crossings of active tectonic fault zones are important considerations for pipelines, because to accommodate abrupt displacements of the ground surface, a pipeline must deform longitudinally and in flexure at strains well in excess of that experienced under normal service conditions. Thrust faults tend to be the most difficult of pipeline crossings, because fault displacement results in compression across the fault zone and, depending on the angle of crossing, can induce substantial direct compression into the pipeline and intensify pipe bending deformation in the fault rupture zone. The investigation of active faults pertinent to a pipeline alignment begins with office-based studies of available technical literature and data and detailed examination of lidar imagery and stereo aerial photographs followed by field reconnaissance to confirm the presence of active faults, determine their location to the narrowest bounds practicable, and define displacement parameters. Exploratory trenching is often utilized as final verification of fault rupture recurrence and expected displacement from characteristic events. Design of thrust fault crossings requires selection of crossing alignments to minimize induced compression, nonlinear finite element analysis to accommodate high strain (2 to 4 percent) without loss of pressure integrity, specification of trench geometry and backfill that will promote pipeline breakout from its soil embedment, and assurance of overmatching weld strength and weld quality.
Matmon A.,Hebrew University of Jerusalem |
Briner J.P.,State University of New York at Buffalo |
Carver G.,CARVER GEOLOGIC Inc. |
Bierman P.,University of Vermont |
And 2 more authors.
Quaternary Research | Year: 2010
We present 10Be exposure ages from moraines in the Delta River Valley, a reference locality for Pleistocene glaciation in the northern Alaska Range. The ages are from material deposited during the Delta and Donnelly glaciations, which have been correlated with MIS 6 and 2, respectively. 10Be chronology indicates that at least part of the Delta moraine stabilized during MIS 4/3, and that the Donnelly moraine stabilized ~17ka. These ages correlate with other dates from the Alaska Range and other regions in Alaska, suggesting synchronicity across Beringia during pulses of late Pleistocene glaciation. Several sample types were collected: boulders, single clasts, and gravel samples (amalgamated small clasts) from around boulders as well as from surfaces devoid of boulders. Comparing 10Be ages of these sample types reveals the influence of pre/post-depositional processes, including boulder erosion, boulder exhumation, and moraine surface lowering. These processes occur continuously but seem to accelerate during and immediately after successive glacial episodes. The result is a multi-peak age distribution indicating that once a moraine persists through subsequent glaciations the chronological significance of cosmogenic ages derived from samples collected on that moraine diminishes significantly. The absence of Holocene ages implies relatively minor exhumation and/or weathering since 12ka. © 2010 University of Washington.
Bemis S.P.,University of Kentucky |
Carver G.A.,Carver Geologic Inc. |
Koehler R.D.,354 College Road
Geosphere | Year: 2012
The framework of Quaternary faults in Alaska remains poorly constrained. Recent studies in the Alaska Range north of the Denali fault add significantly to the recognition of Quaternary deformation in this active orogen. Faults and folds active during the Quaternary occur over a length of ~500 km along the northern flank of the Alaska Range, extending from Mount McKinley (Denali) eastward to the Tok River valley. These faults exist as a continuous system of active structures, but we divide the system into four regions based on east-west changes in structural style. At the western end, the Kantishna Hills have only two known faults but the highest rate of shallow crustal seismicity. The western northern foothills fold-thrust belt consists of a 50-km-wide zone of subparallel thrust and reverse faults. This broad zone of deformation narrows to the east in a transition zone where the range-bounding fault of the western northern foothills fold-thrust belt terminates and displacement occurs on thrust and/or reverse faults closer to the Denali fault. The eastern northern foothills foldthrust belt is characterized by ~40-km-long thrust fault segments separated across leftsteps by NNE-trending left-lateral faults. Altogether, these faults accommodate much of the topographic growth of the northern flank of the Alaska Range. Recognition of this thrust fault system represents a significant concern in addition to the Denali fault for infrastructure adjacent to and transecting the Alaska Range. Although additional work is required to characterize these faults sufficiently for seismic hazard analysis, the regional extent and structural character should require the consideration of the northern Alaska Range thrust system in regional tectonic models. © 2012 Geological Society of America.
Peterson C.D.,University of Portland |
Carver G.A.,Carver Geologic Inc |
Clague J.J.,Simon Fraser University |
Cruikshank K.M.,University of Portland
Natural Hazards | Year: 2015
Maximum-recorded run-up estimates of six major nearfield paleotsunamis, dating from 0.3 to 2.8 ka, are compiled from reported studies at 12 reliable localities distributed over a north–south distance of 1000 km in the Cascadia subduction zone. The run-up estimates are based on surveyed elevations and positions of terminal sand sheet layers that were deposited by the dated paleotsunamis. Maximum terminal deposit elevations from open-coastal sites range from 3 to 12 m NAVD88. Paired proximal and distal run-up sites at four localities demonstrate landward vertical attenuation gradients (−2.5 to −4.2 m km−1) of decreasing terminal sand deposit elevation with increasing distance inland. An averaged attenuation gradient is reversed (3.0 m km−1) to project paleotsunami run-up elevations to adjacent ocean shorelines. The run-up projections are further adjusted by paleotsunami age and relative sea level curves to estimate shoreline inundation elevations under modern sea level conditions. The tsunami shoreline inundation elevations range from 3 ± 2 to 15 ± 2 m NAVD88, with the largest values occurring along the central Cascadia margin and the smallest values occurring in the eastern Juan de Fuca Strait. Contradictory to some numerical tsunami modeling assumptions, there is no apparent correlation between duration of interseismic strain accumulation or estimated upper-plate elastic flexure and corresponding paleotsunami run-up heights on the central Cascadia margin. The short duration since the last Cascadia megathrust rupture (0.3 ka) cannot be used to imply smaller run-up values for a near-future Cascadia tsunami. Coastal communities should plan for the maximum paleotsunami run-ups as recorded at the nearest reliable run-up localities. © 2015, Springer Science+Business Media Dordrecht.