Preliminary Results from the Topliff Hill Paleoseismic Site: Evidence for 5-7 Basin and Range Normal Faulting Events in Tooele County, Utah Since the Late Pleistocene
Abstract
The Topliff Hills fault (THF) is a 25 km-long, west-dipping, normal fault in the eastern Basin and Range along the western slopes of the Topliff Hills and East Tintic Mountains. The THF, extending southward from the South Oquirrh Mountains fault (SOMF), may be seismically linked with the Oquirrh and Great Salt Lake faults to the north and the East Tintic fault (ETF) to the south, forming Utah's second longest fault system (> 250 km length) and a significant hazard to the major population centers in Salt Lake, Utah and Tooele counties. Previous work on the SOMF (Olig and others) revealed 5-7 surface-faulting earthquakes since the late Pleistocene with a recurrence interval of 12-25 ka and a 0.12 mm/a slip rate.
We established the Topliff Hill Paleoseismic Site (40.13, -112.203) topographically above and 1 km south of where the THF crosses the highstand shoreline of Pleistocene Lake Bonneville. We used structure-from-motion photogrammetry (SFM) and an eBee Plus unoccupied aerial system to construct a 3.8 km2, 6 cm digital surface model of the area. From analyses of etched shorelines on the hanging and foot walls of the THF, we infer 0.7-1.9 m of displacement since shoreline abandonment (18.5-15.2 ka). In May 2019 we investigated 2 trenches: Trench 1 (T1: 50 m long, up to 4 m deep) was cut across an 8 m scarp developed in a late Pleistocene fan and Trench 2 (T2: 25 m long, up to 1.5 m deep) was located 300 m north across a younger fan deposit with a poorly defined 1-2 m scarp. In T1 we measured 10 m of cumulative offset associated with 4 colluvial wedges that we infer are due to separate surface-rupturing earthquakes. Two additional hanging wall colluvial wedges on separate faults suggest 1-2 earlier events. In T2, a 0.3-0.4 m fault displacement provides evidence for 1 relatively young earthquake (possibly post-Bonneville). We are processing 14C and OSL samples to constrain ages for the 5-7 events documented at this site. The cumulative displacement and number of events show that slip-per-event may be variable, ranging from 0.5-3 m per event. This raises the possibility that the THF may alternately rupture independently or with the neighboring SOMF and ETF in larger magnitude earthquakes.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.T13H0314W
- Keywords:
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- 1242 Seismic cycle related deformations;
- GEODESY AND GRAVITY;
- 7209 Earthquake dynamics;
- SEISMOLOGY;
- 7221 Paleoseismology;
- SEISMOLOGY;
- 7290 Computational seismology;
- SEISMOLOGY