AN EARTHQUAKE CLUSTER FOLLOWED THE DRYING OF PLEISTOCENE LAKE CHEWAUCAN, CENTRAL OREGON BASIN AND RANGE

New paleoseismic data on the Ana River fault in Summer Lake Basin documents a cluster of three M~7 normal slip earthquakes in the past 7-13Ka, at least three times the number expected from the average recurrence interval over the previous ~70Ka when the fault was under the lake. Building on earlier work of Pezzopane & Weldon (1992) and Langridge (1998), the University of Oregon Neotectonics and Field Camp classes in 2009 logged new trenches across the Ana River fault, which is the northern splay of the ~50 km long Winter Rim fault zone. This abstract presents and interprets these class projects.

The new data confirm three earthquakes since 7-13Ka (previous work suggested at least 2 and possibly 3 events) with displacements of ~1 meter, at least as great as the average displacements of the longer-interval Pleistocene earthquakes that occurred under deep water conditions. C-14 samples and analysis of likely Mazama pumice collected from colluvial wedges of the three earthquakes since the Pleistocene lake dried will provide constraints on the timing of the individual events. The most recent two earthquakes offset a lake terrace that appears to contain Mazama pumice (~7 Ka), suggesting a brief mid Holocene (4-7 Ka) 1300 m high lake stand, 20 m higher than the widely recognized late Holocene (<4 Ka) “Neopluvial” lake level.

These results support the hypotheses (previously proposed by others) that 1) drying of Pleistocene lakes can significantly affect the occurrence of earthquakes, at least for normal faults within the Basin and Range, 2) slip rates and recurrence intervals on faults used for seismic hazard purposes that are based on intervals that include a cluster of earthquakes may significantly over-estimate the actual hazard, and 3) because short intervals within a cluster are associated with offsets similar in size to those associated with much longer intervals outside the cluster, time- or slip-predictable models of earthquake recurrence cannot be applied to these types of earthquakes (if any).