HR: 16:00h
AN: S12E-01 INVITED     [PDF]
TI: Quaternary Extensional Tectonics of the Basin and Range Province
AU: * Machette, M N
EM: machette@usgs.gov
AF: U.S. Geological Survey, MS 966, Box 25046, Denver, Co 80225-0046 United States
AB: The Basin and Range province is experiencing rapid growth, increasing water demand, and an average of about 11 mm/yr of NW-directed extension. None of these trends are compatible. Most Quaternary faults trend N, have a normal-dip movement, and border strongly uplifted or tilted ranges. Although these faults are spectacular, the slip rates and recurrence intervals for M 6.5+ ground ruptures are ca. 0.1 mm/yr and 10$^{4}$ yr, respectively. Obviously, some faults are considerably more active, such as the Genoa fault (2-3 mm/yr) and the Wasatch fault (1-1.5 mm/yr). However, the many hundreds of less remarkable Basin and Range (B\&R) faults are poorly documented. Paleoseismic studies of some of these faults reveal average slip rates of 0.05-0.15 mm/yr and recurrence intervals measured in tens of thousands of years. Offsetting this relatively benign hazard are the sheer bulk of faults that riddle the province. A new USGS compilation of faults found ca. 600 structures in Nevada and 150 in New Mexico, 15% of which have been active in the past 15 k.y. Except for historic ruptures in the province, there is little spatial association of faulting and recorded seismicity. For example, the Wasatch fault zone is poorly expressed on Utah seismicity maps, and the Thousand Springs fault was aseismic well before 1983 Borah Peak, Idaho earthquake. Similar examples are common in the B\&R, especially its southern half. For the most part, the normal faults are aseismic and locked, but can be loaded to near the point of failure. Conversely, the CNSB has been the preferred site for historic earthquakes larger than M 6.5. From 1872 to 1954, seven large earthquakes ripped through this NNE-trending belt: an average of one rupture every 14 years. Paleoseismic investigations of the CNSB have shown that this rate and spatial pattern of activity is anomalous. There is no evidence for similar precursory activity in the past 50 k.y. and there has been almost 50 years of quiescence since the last large earthquake. So one of the most perplexing questions is "why here and where next". With the advent of GPS monitoring we are starting to better identify the active faults of the B\&R. Now the challenge is to compare geologically determined rates and styles of deformation to contemporary strain fields determined by GPS and see if the regions of accelerated extension are relicts of the recent past or precursors of the future.
DE: 7221 Paleoseismology
DE: 7223 Seismic hazard assessment and prediction
DE: 7230 Seismicity and seismotectonics
SC: Seismology [S]
MN: 2003 Fall Meeting