SCEC INSTANeT News
	Invitation to Field Review of Hebgen Lake Paleoseismic Sites

Last July, researchers from the US Geological Survey (Denver, Bozeman, and Menlo Park), Lawrence Livermore National Laboratory, and Montana State University (the Hebgen Lake Paleoseismology Working Group) jointly conducted paleoseismic investigations at three sites on the Hebgen fault and Red Canyon fault in southwest Montana. These subparallel en echelon faults were the primary structures that ruptured during the 1959 Hebgen Lake earthquake. Even though the Hebgen Lake earthquake was one of the largest earthquakes in the intermountain west, the earlier history of these faults is poorly understood. These investigations, which use classical paleseismology and cosmogenic isotope dating of alluvial surfaces and a bedrock exposure, were designed to evaluate the timing of prehistoric events and determine whether the Hebgen and Red Canyon faults ruptured synchronously prior to 1959. The northernmost trench site is at Cabin Creek at the north end of the Hebgen fault. This trench has now been backfilled but the scarp, terraces, trench logs, and dating results with will provide the focus for discussion. At the Section 31 site near the middle of the Hebgen fault two open trenches across the main fault and graben that expose multiple events will be on view. Immediately south of these trenches is a faulted limestone exposure that has been the location of sampling and Cl 36 analysis used for interpretation of earthquake recurrence. The final stop is southeast of Grayling Creek, near the southeast end of the Red Canyon fault. Here, one trench across a major graben exposes the 1959 event and prior deformation. An informal field review is planned for Thursday, October 26 for all interested parties. This review was originally planned for August, but the fires in the region forced this revised schedule. The trenches will be filled immediately following the field review, so this is the last chance to see the exposures and talk about our interpretations. The trip will convene at the Cabin Creek site at 9:00 AM and spend approximately two hours.  Following this anticipate approximately three hours at Section 31, including the discussion of cosmogenic dating work performed on the bedrock scarp (and lunch!), and another two hours at Grayling Creek. Plans are to finish between 4:00 and 5:00pm. Please bring a bag lunch and water/beverage. The most convienient lodging is located in West Yellowstone, ~20 miles from Cabin Creek. Directions to Cabin Creek from West Yellowstone, MT: Take Highway 191/287 north from W. Yellowstone (~8 mi) and turn left onto Highway 287 (a sign points to Hebgen Lake). About 12 miles travel west on 287 should allow you to see a sign for Cabin Creek recreational area, on your right. Directions to Cabin Creek from Big Sky, MT: Take Highway 191 south until a junction with Highway 287. Turn right towards Hebgen Lake on 287 for ~12 miles until you see a sign (on the right) for Cabin Creek. Directions to Cabin Creek from Ennis, MT: Take Highway 287 south from Ennis. Pass the Earthquake Visitor's Center. Cabin Creek will be ~4 miles on your left. Please notify Kathy Haller, haller@usgs.gov, and Heidi Stenner, hstenner@usgs.gov, if you plan to attend. Looking forward to seeing all of you on October 26 at Hebgen Lake! HEBGEN LAKE PALEOSEISMOLOGY WORKING GROUP: Lucilla Benedetti, Lawrence Livermore National Laboratory; Marc Caffee, Lawrence Livermore National Laboratory; James Essex, U.S. Geological Survey; Kathy Haller, U.S. Geological Survey; John Hamilton, U.S. Geological Survey; Dean Hancock, U.S. Geological Survey; Suzanne Hecker, U.S. Geological Survey; Rain Hintz, Montana State University; Dave Lageson, Montana State University; Michael Machette, U.S. Geological Survey; Kenneth L. Pierce, U.S. Geological Survey; Cal Ruleman, Montana State University; David Schwartz, U.S. Geological Survey; Heidi Stenner, U.S. Geological Survey; Hiroyuki Tsutsumi, Kyoto University (Japan); Jerome Van der Woerd, Lawrence Livermore National Laboratory; Toshikazu Yoshioka, Geological Survey of Japan

Phone 213/740-5843 Fax 213/740-0011 e-mail: SCECinfo@usc.edu