Engineering topics

Cohen, Steven C.

Publications and source records attributed to Cohen, Steven C..

10 records

NASA NTRS2004

Crustal Deformation and the Seismic Cycle across the Kodiak Islands, Alaska

The Kodiak Islands are located approximately 130 to 250 km from the Alaska-Aleutian Trench where the Pacific plate is underthrusting the North American plate at a rate of about 57 mm/yr. The southern extent of the 1964 Prince William Sound (${M-w}$ = 9.2) earthquake rupture occurred offshore and beneath the eastern portion of the Kodiak Islands. Here we report GPS results (1993-2001) from northern Kodiak Island that span the transition between the 1964 uplift region along the eastern coast and the region of coseismic subsidence further inland. The horizontal velocity vectors range from 22.9 $\pm$ 2.2 mm/yr at N26.3$\deg$W $\pm$ 2.5$\deg$, about 150 km from the trench, to 5.9 $\pm$ 1.3 mm/yr at N65.9$\deg$W $\pm$ 6.6$\deg$, about 190 km from the trench. Near the northeastern coast of Kodiak the velocity vector above the shallow, locked main thrust zone is between the orientation of PCFC-NOAM plate motion (N22$/deg$W) and the trench-normal (N3O$\deg$W). Further west, our geodetic results suggest the accumulation of shear strain that will be released eventually as left-lateral motion on upper plate faults such as the Kodiak Island fault. These results are consistent with the hypothesis that the difference between the Pacific-North American plate motion and the orientation of the down going slab would lead to 4-8 mm/yr of left-lateral slip. Short-term geodetic uplift rates range from 2 - 14 mm/yr, with the maximum uplift located near the axis of maximum subsidence during the 1964 earthquake. We evaluated alternate interseismic models for Kodiak to test the importance of various mechanisms responsible for crustal deformation rates. These models are based on the plate interface slip history inferred from earlier modeling of coseismic and post-seismic geodetic results. The horizontal (trench perpendicular) and vertical deformation rates across Kodiak are consistent with a model that includes the viscoelastic response to : (1) a downgoing Pacific plate interface that is locked at shallow depths,(2) coseismic slip in the 1964 and (3) interseismic creep below the seismogenic zone. The change in orientation of the horizontal velocity vector occurs down-dip from the locked main thrust zone. In southern Kodiak, the coseismic slip in the 1964 earthquake was smaller than in the northern Kodiak region; yet, the horizontal, interseismic velocities as a function of distance from the trench are comparable to those in northern Kodiak. Based on the earthquake history prior to, and following the 1964 earthquake, we hypothesize that the plate interface in southern Kodiak slips in more frequent large earthquakes than in northern Kodiak.

Sauber, Jeanne↗

NASA NTRS2003

Crustal Deformation in Southcentral Alaska: The 1964 Prince William Sound Earthquake Subduction Zone

This article, for Advances in Geophysics, is a summary of crustal deformation studies in southcentral Alaska. In 1964, southcentral Alaska was struck by the largest earthquake (moment magnitude 9.2) occurring in historical times in North America and the second largest earthquake occurring in the world during the past century. Conventional and space-based geodetic measurements have revealed a complex temporal-spatial pattern of crustal movement. Numerical models suggest that ongoing convergence between the North America and Pacific Plates, viscoelastic rebound, aseismic creep along the tectonic plate interface, and variable plate coupling all play important roles in controlling both the surface and subsurface movements. The geodetic data sets include tide-gauge observations that in some cases provide records back to the decades preceding the earthquake, leveling data that span a few decades around the earthquake, VLBI data from the late 1980s, and GPS data since the mid-1990s. Geologic data provide additional estimates of vertical movements and a chronology of large seismic events. Some of the important features that are revealed by the ensemble of studies that are reviewed in this paper include: (1) Crustal uplift in the region that subsided by up 2 m at the time of the earthquake is as much as 1 m since the earthquake. In the Turnagain Arm and Kenai Peninsula regions of southcentral Alaska, uplift rates in the immediate aftermath of the earthquake reached 150 mm/yr , but this rapid uplift decayed rapidly after the first few years following the earthquake. (2) At some other locales, notably those away the middle of the coseismic rupture zone, postseismic uplift rates were initially slower but the rates decay over a longer time interval. At Kodiak Island, for example, the uplift rates have been decreasing at a rate of about 7mm/yr per decade. At yet other locations, the uplift rates have shown little time dependence so far, but are thought not to be sustainable throughout the several hundred year recurrence time for great earthquake. The nearly 10 mm/yr uplift rate at Seldovia on the Kenai Peninsula is an example.

Cohen, Steven C.↗

NASA NTRS1999

Viscoelastic Postseismic Rebound to Strike-Slip Earthquakes in Regions of Oblique Plate Convergence

According to the slip partitioning concept, the trench parallel component of relative plate motion in regions of oblique convergence is accommodated by strike-slip faulting in the overriding continental lithosphere. The pattern of postseismic surface deformation due to viscoelastic flow in the lower crust and asthenosphere following a major earthquake on such a fault is modified from that predicted from the conventual elastic layer over viscoelastic halfspace model by the presence of the subducting slab. The predicted effects, such as a partial suppression of the postseismic velocities by 1 cm/yr or more immediately following a moderate to great earthquake, are potentially detectable using contemporary geodetic techniques.

Cohen, Steven C.↗

NASA NTRS1999

Crustal Uplift in the Southcentral Alaska Subduction Zones: A New Analysis and Interpretation of Tide Gauge Observations

We have examined the sea level height tide records at seven tide gauge sites in the region of southcentral Alaska that were affected by the 1964 Prince William Sound earthquake to determine the history of crustal uplift subsequent to the earthquake. There is considerable variation in the behavior depending on the location of the site relative to the 1964 rupture. At Seward, on the eastern side of the Kenai Peninsula we find a slow uplift that is consistent with elastic strain accumulation while at Seldovia and Nikiski on the western side of the Kenai we find a persistent rapid uplift of about 1 cm/yr that most likely represents a long term transient response to the earthquake, but which cannot be sustained over the expected recurrence interval for a great earthquake of several hundred years. Further to the southwest, at Kodiak, we find evidence that the rate of uplift, which is still several mm/yr, has slowed significantly over the past three and a half decades. To the east of the Kenai Peninsula we find subsidence at Cordova and an uncertain behavior at Valdez. At both of these sites there is a mathematically significant time-dependence to the uplift behavior, but the data confirming this time dependence are not as convincing as at Kodiak. At Anchorage, to the north there is little evidence of vertical motion since the earthquake. We compare these long term tide gauge records to recent GPS observations. In general there is reasonable consistency except at Anchorage and Cordova where the GPS measurement indicate somewhat more rapid uplift and subsidence, respectively.

Cohen, Steven C.↗

NASA NTRS1992

Postseismic deformation and stress diffusion due to viscoelasticity and comments on the modified Elsasser model

Finite element techniques have been used to investigate the far-field deformation and stress changes due to asthenospheric viscoelastic relaxation following a dip-slip earthquake. The diffusion of extensional stress toward the subduction zone following a thrust earthquake on land is qualitatively consistent with the modified Elsasser model as proposed by Rydelek and Sacks (1988, 1990) to explain the coupled occurrence of land and sea earthquakes near Japan. However, the magnitude of the diffusing tensional signal is significantly smaller. The nominal model consists of a partially faulted elastic lithosphere overlying a viscoelastic substrate. Other models consider thin channel flow, rupturing of the entire elastic lithosphere, and changes in the depth of faulting. While some of these changes have significant impact on the magnitude and spatial features of the stress and deformation field, the far-field stress remains small. Numerical experiments demonstrate that the assumptions of the modified Elsasser model accentuate uniaxial deformation. When these assumptions are replicated in the finite element calculations, reasonable agreement between the models is achieved.

Cohen, Steven C.↗

NASA NTRS1990

Geoscience Laser Ranging System - Estimated accuracy of geodetic parameters and their dependence on system characteristics

Covariance analysis of the performance of the Geoscience Laser Ranging System (GLRS) indicates that three-dimensional relative positions can be recovered to an accuracy of several millimeters over spatial scales from a few kilometers to several hundred kilometers and over temporal scales as short as several days. The key factors influencing the accuracy are range noise, number of targets and their locations, system pointing capability, dwell time on the targets, orbital geometry, and gravity field uncertainties. Based on the present trade-off studies, GLRS is designed to provide range measurements with 10 mm or better accuracy, fire at a rate of 40 pulses-per-second, point over a cone extending to 50 deg from nadir, and operate with a dwell time on individual targets of 2 s or less. Given a strain rate of 10 to the -14th/s, estimated GLRS accuracy parameters suggest that the deformation can be detected in less than a month if it extends over 100 km and in less than 6 months if it extends over 10 km.

Cohen, Steven C.↗

NASA NTRS1989

Geodynamics applications of spaceborne laser ranging

The Geodynamics Laser Ranging System (GLRS) is a spaceborne laser ranging instrument being developoed by NASA as a facility instrument for the Earth Observing System (EOS). GLRS is to be used to study regional and local crustal movements. It is designed to make highly precise range measurements to retroreflector targets located in geophysically interesting sites. Using a two-color ranging scheme, absolute range accuracies of several mm are expected. Simulations based on this accuracy and the EOS orbital parameters show that length of the intersite baseline between retroreflectors can be determined to several mm accuracy at distances from a few km to several hundred km with several passes of GLRS range data collected over a few-day interval. Short-arc techniques are used to minimize the effects of gravity field and other force model uncertainties. Relative heights can be determined to sub-cm accuracy over comparable distances.

Cohen, Steven C.↗

NASA NTRS1988

Applications of spaceborne laser ranger on EOS

An account is given of the design concept and potential applications in science and engineering of the spaceborne laser ranging and altimeter apparatus employed by the Geodynamics Laser Ranging System; this is scheduled for 1997 launch as part of the multiple-satellite Earth Observing System. In the retrograding mode for geodynamics, the system will use a Nd:YAG laser's green and UV output for distance determination to ground retroreflectors. Engineering applications encompass land management and long-term ground stability studies relevant to nuclear power plant, pipeline, and aqueduct locations.

Degnan, John J.↗

NASA NTRS1987

The Geoscience Laser Altimetry/Ranging System

The Geoscience Laser Altimetry/Ranging System (GLARS), a combined laser ranging and altimetry system capable of subcentimeter position determinations of retroflector targets and subdecimeter profiling of topography, is described. The system uses advanced but currently available state-of-the-art components. Laboratory, field, and numerical experiments have indicated the suitability of GLARS as an instrument for Eos and other space platforms.

Cohen, Steven C.↗

NASA NTRS1987

Spaceborne laser ranging from EOS

High precision laser ranging measurements from an Earth Observing System (EOS) platform would provide data for conducting a variety of basic geoscience investigations. For example, range data to retroreflectors arranged in grids located at tectonic plate boundaries could be used to study the spatial and temporal distribution of the crustal movements associated with stress accumulation and release in an earthquake cycle. The Geodynamics Laser Ranging System (GLRS) is a proposed facility instrument for EOS that can perform the ranging function as well as simple height-measurement altimetry over ice sheets. This instrument is the derivative of several generations of ground-based laser trackers and prototype laboratory systems. The Geoscience Laser Altimetry/Ranging System (GLARS) is an advanced laser ranging system which would provide higher accuracy ranging observations and detailed altimetric mapping of surface height distributions over land and ice sheets.

Cohen, Steven C.↗