Hilary Jane Fletcher

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.

New GPS constraints on the motion of the Yakutat Block

Global Positioning System (GPS) measurements were made at Yakutat, on the Yakutat terrane of southern Alaska, to investigate the motion of the Yakutat block with respect to the North American plate and to help constrain motion along the Fairweather fault. The velocity of Yakutat derived from the GPS data is 44.1 +/− 1.9 mm/yr toward N37° +/− 4°W relative to stable North America. The magnitude of this velocity is similar to that of the Pacific plate predicted by NUVEL1A, although there is a significant difference in the azimuth of these two vectors. The motion of Yakutat relative to North America is almost exactly parallel to the strike of the Fairweather fault, suggesting that most deformation inboard of Yakutat is right-lateral strike slip on the Fairweather fault or faults parallel to it, and that significant motion normal to the Fairweather fault occurs offshore of Yakutat. The GPS velocity at Yakutat is also used to help constrain the slip rate and locking depth of the Fairweather fault.

Coseismic slip distribution of the 2002 MW7.9 Denali fault earthquake, Alaska, determined from GPS measurements

[1] On 3 November 2002 an Mw7.9 earthquake occurred in central Alaska. The earthquake ruptured portions of the Susitna Glacier, Denali, and Totschunda faults. Inversion of the GPS-measured displacement field indicates that the event was dominated by a complex, right-lateral strike-slip rupture along the Denali fault. GPS sites closest to the epicenter show the effect of thrust motion on the Susitna Glacier fault. The preferred coseismic slip model, with M w 7.8, indicates relatively low slip on the western part of the rupture and high slip from about 60 km east of the hypocenter extending to the junction of the Denali and Totschunda faults. We find mostly shallow slip from the surface to 15 km depth, but the inversion suggests one large deep slip patch about 110 km east of the hypocenter. Our model predicts surface slip in good agreement with surface geological observations, where model resolution is good.

High interseismic coupling of the Alaska Subduction Zone SW of Kodiak Island inferred from GPS data

We use Global Positioning System (GPS) measurements to make the first geodetic study of the Semidi segment of the Alaska-Aleutian subduction zone. This segment, which sustained an MW 8.2 earthquake in 1938, lies between Kodiak Island where the subduction interface appears to presently be fully locked, and the Shumagin Islands segment where substantial aseismic slip occurs. We invert the GPS station velocity estimates using a nonlinear least squares algorithm to solve for the width of the locked zone, the dip, and the interseismic coupling of a model subduction interface. The data are consistent with a shallow plate interface dipping ∼6°, a locking depth of ∼23 km, and high interseismic coupling of ∼80%.

A determination of source properties of large intraplate earthquakes in Alaska

Historically, large and potentially hazardous earthquakes have occurred within the interior of Alaska. However, most have not been adequately studied using modern methods of waveform modeling. The 22 July 1937, 16 October 1947, and 7 April 1958 earthquakes are three of the largest events known to have occurred within central Alaska (Ms=7.3,Ms=7.2 andMs=7.3, respectively). We analyzed teleseismic body waves to gain information about the focal parameters of these events. In order to deconvolve the source time functions from teleseismic records, we first attempted to improve upon the published focal mechanisms for each event. Synthetic seismograms were computed for different source parameters, using the reflectivity method. A search was completed which compared the hand-digitized data with a suite of synthetic traces covering the complete parameter space of strike, dip, and slip direction. In this way, the focal mechanism showing the maximum correlation between the observed and calculated traces was found. Source time functions, i.e., the moment release as a function of time, were then deconvolved from teleseismic records for the three historical earthquakes, using the focal mechanisms which best fit the data. From these deconvolutions, we also recovered the depth of the events and their seismic moments. The earthquakes were all found to have a shallow foci, with depths of less than 10 km.The 1937 earthquake occurred within a northeast-southwest band of seismicity termed the Salcha seismic zone (SSZ). We confirm the previously published focal mechanism, indicating strike-slip faulting, with one focal plane parallel to the SSZ which was interpreted as the fault plane. Assuming a unilateral fault model and a reasonable rupture velocity of between 2 and 3 km/s, the 21 second rupture duration for this event indicates that all of the 65 km long SSZ may have ruptured during this event. The 1947 event, located to the south of the northwest-southeast trending Fairbanks seismic zone, was found to have a duration of about 11 seconds, thus indicating a rupture length of up to 30 km. The rupture duration of the 1958 earthquake, which occurred near the town of Huslia, approximately 400 km ENE of Fairbanks, was found to be about 9 seconds. This gives a rupture length consistent with the observed damage, an area of 16 km by 64 km.