Haeussler P.J.,4210 University Dr |
Armstrong P.A.,California State University, Fullerton |
Liberty L.M.,Boise State University |
Ferguson K.M.,California State University, Fullerton |
And 3 more authors.
Quaternary Science Reviews | Year: 2014
Megathrust splay faults are a common feature of accretionary prisms and can be important for generating tsunamis during some subduction zone earthquakes. Here we provide new evidence from Alaska that megathrust splay faults have been conduits for focused exhumation in the last 5Ma. In most of central Prince William Sound, published and new low-temperature thermochronology data indicate little to no permanent rock uplift over tens of thousands of earthquake cycles. However, in southern Prince William Sound on Montague Island, apatite (U-Th)/He ages are as young as 1.1Ma indicating focused and rapid rock uplift. Montague Island lies in the hanging wall of the Patton Bay megathrust splay fault system, which ruptured during the 1964 M9.2 earthquake and produced ~9m of vertical uplift. Recent geochronology and thermochronology studies show rapid exhumation within the last 5Ma in a pattern similar to the coseismic uplift in the 1964 earthquake, demonstrating that splay fault slip is a long term (3-5my) phenomena. The region of slower exhumation correlates with rocks that are older and metamorphosed and constitute a mechanically strong backstop. The region of rapid exhumation consists of much younger and weakly metamorphosed rocks, which we infer are mechanically weak. The region of rapid exhumation is separated from the region of slow exhumation by the newly identified Montague Strait Fault. New sparker high-resolution bathymetry, seismic reflection profiles, and a 2012 Mw4.8 earthquake show this feature as a 75-km-long high-angle active normal fault. There are numerous smaller active normal(?) faults in the region between the Montague Strait Fault and the splay faults. We interpret this hanging wall extension as developing between the rapidly uplifting sliver of younger and weaker rocks on Montague Island from the essentially fixed region to the north. Deep seismic reflection profiles show the splay faults root into the subduction megathrust where there is probable underplating. Thus the exhumation and extension in the hanging wall are likely driven by underplating along the megathrust de´collement, thickening in the overriding plate and a change in rheology at the Montague Strait Fault to form a structural backstop. A comparison with other megathrust splay faults around the world shows they have significant variability in their characteristics, and the conditions for their formation are not particularly unique. © 2014.
Babcock E.L.,Geotek Alaska Inc. |
Babcock E.L.,4210 University Dr |
Annan A.P.,Sensors and Software |
Bradford J.H.,Boise State University
Journal of Environmental and Engineering Geophysics | Year: 2016
Conductive cables have always represented a source of noise in ground-penetrating radar (GPR) data. In some instances, commercially available GPR systems use data processing tools to reduce cable noise. Such processes seldom respect the amplitude fidelity of the signal. For the purpose of careful, quantitative amplitude measurements, use of the raw recorded data is a critical starting point for reliable interpretation of results. During subsequent processing, users can compensate for cable effects to varying degrees. To illustrate the issues inherent with cable noise, we show an example where cable movement during data acquisition generates deviations in reflection amplitude up to 19%. We then present the ramifications of these variations for quantitative data analysis. We conclude that precise cable handling can improve data quality and subsequent data interpretation. Our results are particularly pertinent for quantitative analysis and inversion of GPR data where precise amplitude information is crucial.
Hotovec A.J.,University of Washington |
Prejean S.G.,4210 University Dr |
Vidale J.E.,University of Washington |
Gomberg J.,University of Washington
Journal of Volcanology and Geothermal Research | Year: 2013
During the 2009 eruption of Redoubt Volcano, Alaska, gliding harmonic tremor occurred prominently before six nearly consecutive explosions during the second half of the eruptive sequence. The fundamental frequency repeatedly glided upward from <. 1. Hz to as high as 30. Hz in less than 10. min, followed by a relative seismic quiescence of 10 to 60. s immediately prior to explosion. High frequency (5 to 20. Hz) gliding returned during the extrusive phase, and lasted for 20. min to 3. h at a time. Although harmonic tremor is not uncommon at volcanoes, tremor at such high frequencies is a rare observation. These frequencies approach or exceed the plausible upper limits of many models that have been suggested for volcanic tremor. We also analyzed the behavior of a swarm of repeating earthquakes that immediately preceded the first instance of pre-explosion gliding harmonic tremor. We find that these earthquakes share several traits with upward gliding harmonic tremor, and favor the explanation that the gliding harmonic tremor at Redoubt Volcano is created by the superposition of increasingly frequent and regular, repeating stick-slip earthquakes through the Dirac comb effect. © 2012 Elsevier B.V.