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Lee-Ann Bradley

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The torrential rains that accompanied Hurricane Mitch in October and November of 1998 triggered thousands of landslides in the moderate to steep terrain bordering the Motagua and Polochic Rivers in eastern Guatemala. Using aerial photographs taken between January and March 2000 we mapped all visible landslides larger than about 15 m in minimum dimension in a study area of 10,000 km2 encompassing twenty 1:50,000-scale topographic map quadrangles. Rainfall from Hurricane Mitch was exceptional because it was geographically widespread, prolonged over a period of about a week, moderate to heavy in intensity, and occurred at the end of the rainy season when the ground already had a high moisture content. As documented...
Categories: Publication; Types: Citation; Tags: Open-File Report
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Despite the role of the Alaska-Aleutian megathrust as the source of some of the largest earthquakes and tsunamis, the history of its pre–twentieth century tsunamis is largely unknown west of the rupture zone of the great (magnitude, M 9.2) 1964 earthquake. Stratigraphy in core transects at two boggy lowland sites on Chirikof Island’s southwest coast preserves tsunami deposits dating from the postglacial to the twentieth century. In a 500-m-long basin 13–15 m above sea level and 400 m from the sea, 4 of 10 sandy to silty beds in a 3–5-m-thick sequence of freshwater peat were probably deposited by tsunamis. The freshwater peat sequence beneath a gently sloping alluvial fan 2 km to the east, 5–15 m above sea level...
Categories: Publication; Types: Citation; Tags: Geosphere
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Fault scarps recently discovered on Airborne Laser Swath Mapping (ALSM; also known as LiDAR) imagery show Holocene movement on the Lake Creek-Boundary Creek fault on the north flank of the Olympic Mountains of northwestern Washington State. Such recent movement suggests the fault is a potential source of large earthquakes. As part of the effort to assess seismic hazard in the Puget Sound region, we map scarps on ALSM imagery and show primary field and laboratory data from backhoe trenches across scarps that are being used to develop a latest Pleistocene and Holocene history of large earthquakes on the fault. Although some scarp segments 0.5-2 km long along the fault are remarkably straight and distinct on shaded...
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Sources of seismic hazard in the Puget Sound region of northwestern Washington include deep earthquakes associated with the Cascadia subduction zone, and shallow earthquakes associated with some of the numerous crustal (upper-plate) faults that crisscross the region. Our paleoseismic investigations on one of the more prominent crustal faults, the Darrington–Devils Mountain fault zone, included trenching of fault scarps developed on latest Pleistocene glacial sediments and analysis of cores from an adjacent wetland near Lake Creek, 14 km southeast of Mount Vernon, Washington. Trench excavations revealed evidence of a single earthquake, radiocarbon dated to ca. 2 ka, but extensive burrowing and root mixing of sediments...
Categories: Publication; Types: Citation; Tags: Geosphere
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