Steven C. Cohen

Spatial variations in present‐day deformation, Kenai Peninsula, Alaska, and their implications

From four years of Global Positioning System (GPS) measurements, we find significant spatial variations in present-day deformation between the eastern and western Kenai Peninsula, Alaska. Sites in the eastern Kenai Peninsula and Prince William Sound move to the NNW relative to North America, in the direction of Pacific-North America relative plate motion. Velocities decrease in magnitude from nearly the full plate rate in southern Prince William Sound to about 30 mm/yr at Seward and to about 5 mm/yr near Anchorage. In contrast, sites in the western Kenai Peninsula move to the SW, in a nearly trenchward direction, with a velocity of about 20 mm/yr. The data are consistent with the shallow plate interface offshore and beneath the eastern Kenai and Prince William Sound being completely locked or nearly so, with elastic strain accumulation resulting in rapid motion in the direction of relative plate motion of sites in the overriding plate. The velocities of sites in the western Kenai, along strike to the southwest, are opposite in sign with those predicted from elastic strain accumulation. These data are incompatible with a significant locked region in this segment of the plate boundary. Trenchward velocities are found also for some sites in the Anchorage area. We interpret the trenchward velocities as being caused by a continuing postseismic transient from the 1964 great Alaska earthquake. There may be significant along-strike differences in the long-term behavior of the plate interface between the western and eastern Kenai, based on roughly coincident boundaries in the coseismic slip distribution, cumulative postseismic uplift, present-day plate coupling, and stress field. The present postseismic response appears to generate purely trenchward motion, suggesting a creep process that is purely dip slip. Our observations suggest that postseismic processes after the largest earthquakes can influence patterns of deformation for decades after the event.

Heterodyne detection: phase front alignment, beam spot size, and detector uniformity

We consider the effects of signal and local oscillator phase front misalignment, beam spot sizes, and electric field distributions on heterodyne detection. The signal and local oscillator fields that we consider are various combinations of Airy, Gaussian, and uniform distributions. We show that the values of the beam radii that maximize the heterodyne SNR are sensitive to phase front misalignment and that the degradation with misalignment angle is somewhat less severe for Airy received signals than for uniform. We also prove that for small optical spot sizes and perfect alignment, the optimal ratio of local oscillator Gaussian l/e field radius to signal Airy F number is approximately 0.7lambda. We next consider the effects of nonuniform detector quantum efficiency. Simple examples show that quantum efficiencies averaged over the detector surface give only crude estimates of the sensitivity of a heterodyne system. For accurate estimates full account must be made of the electric field parameters and the detector re ponse at each point on its photosurface.

Numerical models of crustal deformation in seismic zones

Publisher Summary This chapter presents numerical models of crustal deformation in seismic zones. The chapter investigates the features of a simple model of the surface deformation that accompanies faulting on an infinitely long, vertical, strike-slip fault embedded in an elastic half-space. The physical concept of the model presented is the representation of fault slip as a solid-state “dislocation” and a key mathematical feature is the use of Greens functions for solving differential equations. The chapter focuses on the two major “deep” mechanisms of postseismic deformation—viscoelastic flow at depth and fault creep below the seismically locked layer. Because models of postseismic rebound necessarily deal with time dependent processes, they are a natural springboard to considering the entire earthquake cycle. Most of the models reviewed in the chapter are kinematic in the sense that slip is imposed on a fault either to represent actual fault slip or as a mathematical artifact to represent interseismic loading.

The Geoscience Laser Altimetry/Ranging System

The Geoscience Laser Altimetry/Ranging System (GLARS) is a planned highly precise laser distance-measuring system to be used for geoscience measurements requiring extremely accurate geodetic observations from a space platform. The system combines the attributes of a pointable laser ranging system making observations to retroreflectors placed on the ground with those of a nadir-looking laser altimeter making height observations to ground, ice sheet, and oceanic surfaces. In the ranging mode, centimeter-level precise baseline and station coordinate determinations will be made on grids consisting of 100 to 200 targets separated by distances from a few tens of kilometers to about 1000 km. These measurements will be used for studies of seismic zone crustal deformations and tectonic plate motions. Ranging measurements will also be made to a coarser, but globally distributed, array of retroreflectors for both precise geodetic and orbit determination applications. In the altimetric mode, relative height determinations will be obtained with approximately decimeter vertical precision and 70-100-m horizontal resolution. Altimetric profiles consisting of nearly contiguous spots will be available when the system is operated at 40 pulses per second. The height data will be used to study surface topography and roughness, ice sheet and lava flow thickness, and ocean dynamics. Waveform digitization will provide a measure of the vertical extent of topography within each footprint.

Deformation of the Kenai Peninsula, Alaska

Crustal deformation on the Kenai Peninsula in southern Alaska has been studied using data obtained from Global Positioning System (GPS) measurements in 1993 and 1995 and leveling observations in 1964, immediately after the Prince William Sound earthquake. This analysis shows that the Kenai Peninsula has experienced as much as ∼900 mm uplift during the past 3 decades and that the uplift forms an ∼125 km wide elongate dome with its major axis trending southwest to northeast following the trend of the major tectonic features of the region. The averaged uplift rate between 1964 and 1995 is as high as 30 mm yr−1, although the current uplift rate may be substantially lower. The GPS measurements cast further doubt on previously suspect tide gauge data for Nikiski, Alaska, which indicated rapid postseismic uplift at this site located in northwest Kenai Peninsula adjacent to Cook Inlet. Examination of the three-dimensional GPS data indicates that the eastern Kenai Peninsula is currently undergoing significant SSE to NNW contraction in response to North American-Pacific Plate convergence. The horizontal velocities are consistent with the predictions of an elastic half-space model for the interseismic deformation. This result, taken in combination with the small changes in uplift between 1993 and 1995, suggests that most of the present deformation is due to steady plate convergence rather than transient postseismic rebound.

Crustal deformation in the southcentral Alaska subduction zone

Abstract The study of crustal deformation in the subduction zone of the 1964 Prince William Sound, Alaska earthquake reveals a temporally and spatially complex pattern of surface motions and interplate coupling. This temporal–spatial pattern provides fundamental information on the elastic and inelastic processes associated with strain energy accumulation and release in the seismic cycle and development of geological structures in southcentral Alaska. Essential data on the crustal deformation comes from seismological, geological and geodetic observations. The most salient observed features are: (1) large along-strike variations in both the coseismic moment release and the postseismic and interseismic surface deformation and (2) a complicated history of postseismic and interseismic deformation characterized by both steady-state deformation and transient motion occurring over several time scales and involving different deformation mechanisms. Numerical models give insight into the relationship between the observed crustal motions and tectonic plate motion, fault zone processes, and the Earths rheology.

Crustal uplift in the south central Alaska subduction zone: New analysis and interpretation of tide gauge observations

We have examined tide gauge measurements of apparent sea level height in south central Alaska to determine the history of crustal uplift subsequent to the 1964 Prince William Sound earthquake. There are spatial and temporal variations in the uplift rate since the 1994 earthquake that depend on the location of the tide gauge relative to the coseismic rupture features. At Seward, on the eastern side of the Kenai Peninsula, we find slow uplift that is consistent with elastic strain accumulation at the locked North American-Pacific Plate boundary. Conversely, at Seldovia and Nikiski, on the western side of the Kenai Peninsula, we find persistent rapid uplift of ∼10 mm yr−1 that may be longterm transient response to the earthquake but that cannot be sustained over the entire several hundred year recurrence interval for a great earthquake. Farther to the southwest, at Kodiak, the rate of uplift is several millimeters per year but 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 Cordova, and to a lesser extent Valdez, there is a mathematically significant time dependence, although the evidence for the time dependence is less compelling than at Kodiak. At Anchorage, there is little evidence of vertical motion since the earthquake. The along-strike spatial variability in the relaxation time of the rates of vertical motion since the 1964 earthquake may be related to variations in the updip coseismic slip during the megathrust event.

The 1964 great Alaska earthquake: present day and cumulative postseismic deformation in the western Kenai Peninsula

Abstract Global Positioning System (GPS), triangulation and leveling data are used to derive models for the present day (last ∼5 years) and the 30-year average postseismic deformation on the Kenai Peninsula, Alaska following the 1964 Alaska earthquake. The two datasets are inverted using a three-dimensional elastic dislocation model to estimate the magnitude and spatial distribution of slip on the North America–Pacific plate interface, allowing us to examine the time dependence of the processes controlling postseismic deformation. We determine that the 30-year average postseismic slip rate beneath the western Kenai Peninsula was about twice as large as the present day slip rate. The observations suggest a time-decaying process, but are not consistent with a single exponentially decaying relaxation process initiated immediately after the 1964 earthquake. We conclude that the postseismic deformation observed on the western Kenai Peninsula cannot be explained in terms of any single time-decaying process. Either multiple processes acting on different timescales or significant spatial propagation of the postseismic deformation, or both, must occur. In the latter case, postseismic deformation would not begin everywhere at the same time and the rate of spatial propagation would affect the timescale inferred for the postseismic processes.

Time-dependent uplift of the Kenai Peninsula and adjacent regions of south central Alaska since the 1964 Prince William Sound earthquake

Leveling, Global Positioning System, very long baseline interferometry, and tide gauge observations indicate that time-dependent uplift has occurred on the Kenai Peninsula and adjacent regions of southern Alaska that subsided coseismically in the 1964 Prince William Sound earthquake. These observations have been critically assessed and used along with seismological and geological information as constraints in various aseismic slip and viscoelastic flow models of crustal deformation. The postseismic uplift rate in most of the region is significantly greater than the expected interseismic average. Some, but not all, of the data indicate that the uplift rate was quite rapid (more than several centimeters per year) in the years immediately following the earthquake and has slowed to a few centimeters per year or less since then. The modeling shows that the uplift can be explained by aseismic slip at depth but not by viscoelastic flow. The slip at depth since 1964 must have a transient as well as a steady component, since the deduced slip is considerably greater than the roughly 1.5 m expected from North American-Pacific plate convergence since the 1964 event. The minimum amount of slip required to fit the observations comes from an elastic model and is about 2.75 m. Viscoelastic models require more slip, since the dominant effect of viscoelastic flow is to produce subsidence in the region that is uplifted. A simple parameterization of the transient component using an amplitude and decay time is adequate to explain the observations. While the parameters deduced from the viscoelastic models have some ambiguity, the viscosity of models that best fit the observations cannot be much less than several times 1019 Pa s when the thickness of the North American plate is taken to be about 90 km.

Uplift of the Kenai Peninsula, Alaska, since the 1964 Prince William Sound earthquake

Using Global Positioning System (GPS) receivers, we reoccupied several leveling benchmarks on the Kenai Peninsula of Alaska which had been surveyed by conventional leveling immediately following the March 27, 1964, Prince William Sound earthquake (Mw = 9.3). By combining the two sets of measurements with a new, high-resolution model of the geoid in the region, we were able to determine the cumulative 1993–1964 postseismic vertical displacement. We find uplift at all of our benchmarks, relative to Seward, Alaska, a point that is stable according to tide gauge data. The maximum uplift of about 1 m occurs near the middle of the peninsula. The region of maximum uplift appears to be shifted northwest relative to the point of maximum coseismic subsidence. If we use tide gauge data at Nikishka and Seward to constrain the vertical motion, then the observed uplift has a trenchward tilt (down to the southeast) as well as an arching component. To explain the observations, we use creep-at-depth models. Most acceptable models require a fault slip of about 2.75 m, although this result is not unique. If the slip has been continuous since the 1964 earthquake, then the average slip rate is nearly 100 mm/yr, twice the plate convergence rate. Comparing the net uplift achieved in 29 years with that observed over 11 years in an adjacent region southeast of Anchorage, Alaska, we conclude that the rate of uplift is decreasing. A further decrease in the uplift rate is expected as the 29-year averaged displacement rate is about twice the plate convergence rate and therefore cannot be sustained over the entire earthquake cycle.