Geology topics

J.T. Freymueller

Publications and source records attributed to J.T. Freymueller.

5 recordsLinked to original sources

USGS2016

Evidence for shallow megathrust slip across the Unalaska seismic gap during the great 1957 Andreanof Islands earthquake, eastern Aleutian Islands, Alaska

We reassess the slip distribution of the 1957 Andreanof Islands earthquake in the eastern part of the aftershock zone where published slip models infer little or no slip. Eyewitness reports, tide gauge data, and geological evidence for 9–23 m tsunami runups imply seafloor deformation offshore Unalaska Island in 1957, in contrast with previous studies that labeled the area a seismic gap. Here, we simulate tsunami dynamics for a suite of deformation models that vary in depth and amount of megathrust slip. Tsunami simulations show that a shallow (5–15 km deep) rupture with ~20 m of slip most closely reproduces the 1957 Dutch Harbor marigram and nearby >18 m runup at Sedanka Island marked by stranded drift logs. Models that place slip >20 km predict waves that arrive too soon. Our results imply that shallow slip on the megathrust in 1957 extended east into an area that presently creeps.

Geophysical Research Letters↗

USGS2009

The postseismic response to the 2002 M 7.9 Denali Fault earthquake: Constraints from InSAR 2003-2005

InSAR is particularly sensitive to vertical displacements, which can be important in distinguishing between mechanisms responsible for the postseismic response to large earthquakes (afterslip, viscoelastic relaxation). We produce maps of the surface displacements resulting from the postseismic response to the 2002 Denali Fault earthquake, using data from the Canadian Radarsat-1 satellite from the periods summer 2003, summer 2004 and summer 2005. A peak-to-trough signal of amplitude 4 cm in the satellite line of sight was observed between summer 2003 and summer 2004. By the period between summer 2004 and summer 2005, the displacement rate had dropped below the threshold required for observation with InSAR over a single year. The InSAR observations show that the principal postseismic relaxation process acted at a depth of ∼50 km, equivalent to the top of the mantle. However, the observations are still incapable of distinguishing between distributed (viscoelastic relaxation) and localized (afterslip) deformation. The imposed coseismic stresses are highest in the lower crust and, assuming a Maxwell rheology, a viscosity ratio of at least 5 between lower crust and upper mantle is required to explain the contrast in behaviour. The lowest misfits are produced by mixed models of viscoelastic relaxation in the mantle and shallow afterslip in the upper crust. Profiles perpendicular to the fault show significant asymmetry, which is consistent with differences in rheological structure across the fault.

Geophysical Journal International↗

USGS2003

The 2002 Denali fault earthquake, Alaska: A large magnitude, slip-partitioned event

The MW (moment magnitude) 7.9 Denali fault earthquake on 3 November 2002 was associated with 340 kilometers of surface rupture and was the largest strike-slip earthquake in North America in almost 150 years. It illuminates earthquake mechanics and hazards of large strike-slip faults. It began with thrusting on the previously unrecognized Susitna Glacier fault, continued with right-slip on the Denali fault, then took a right step and continued with right-slip on the Totschunda fault. There is good correlation between geologically observed and geophysically inferred moment release. The earthquake produced unusually strong distal effects in the rupture propagation direction, including triggered seismicity.

Science↗

USGS2000

Aseismic inflation of Westdahl volcano, Alaska, revealed by satellite radar interferometry

Westdahl volcano, located at the west end of Unimak Island in the central Aleutian volcanic arc, Alaska, is a broad shield that produced moderate-sized eruptions in 1964, 1978-79, and 1991-92. Satellite radar interferometry detected about 17 cm of volcano-wide inflation from September 1993 to October 1998. Multiple independent interferograms reveal that the deformation rate has not been steady; more inflation occurred from 1993 to 1995 than from 1995 to 1998. Numerical modeling indicates that a source located about 9 km beneath the center of the volcano inflated by about 0.05 km3 from 1993 to 1998. On the basis of the timing and volume of recent eruptions at Westdahl and the fact that it has been inflating for more than 5 years, the next eruption can be expected within the next several years.

Geophysical Research Letters↗

USGS1998

Synthetic aperture radar interferometry coherence analysis over Katmai volcano group, Alaska

The feasibility of measuring volcanic deformation or monitoring deformation of active volcanoes using space-borne synthetic aperture radar (SAR) interferometry depends on the ability to maintain phase coherence over appropriate time intervals. Using ERS 1 C band (λ=5.66 cm) SAR imagery, we studied the seasonal and temporal changes of the interferometric SAR coherence for fresh lava, weathered lava, tephra with weak water reworking, tephra with strong water reworking, and fluvial deposits representing the range of typical volcanic surface materials in the Katmai volcano group, Alaska. For interferograms based on two passes with 35 days separation taken during the same summer season, we found that coherence increases after early June, reaches a peak between the middle of July and the middle of September, and finally decreases until the middle of November when coherence is completely lost for all five sites. Fresh lava has the highest coherence, followed by either weathered lava or fluvial deposits. These surfaces maintain relatively high levels of coherence for periods up to the length of the summer season. Coherence degrades more rapidly with time for surfaces covered with tephra. For images taken in different summers, only the lavas maintained coherence well enough to provide useful interferometric images, but we found only a small reduction in coherence after the first year for surfaces with lava. Measurement of volcanic deformation is possible using summer images spaced a few years apart, as long as the surface is dominated by lavas. Our studies suggest that in order to make volcanic monitoring feasible along the Aleutian arc or other regions with similar climatic conditions, observation intervals of the satellite with C band SAR should be at least every month from July through September, every week during the late spring/early summer or late fall, and every 2–3 days during the winter.

Journal of Geophysical Research B: Solid Earth↗