Michael Arnoff’s scientific contributions

Recent acquisitions of light detection and ranging (lidar) data along the Wasatch Front, and the proliferation of Structure from Motion (SfM) software present exceptional opportunities for detailed analysis of the Wasatch fault zone (WFZ). Between the Salt Lake City (SLCS) and Provo segments (PS), the WFZ takes a 7-km left bend along the Fort Canyon fault over Traverse Ridge. The Fort Canyon fault at the segment boundary is important because it bridges the most active and populated portions of the Wasatch fault system; the SLCS and PS rupturing together could produce an earthquake approaching Mw 7.5. As part of an effort to identify paleoseismic sites to constrain the timing of ground ruptures and importance of fault segmentation in rupture propagation through the Fort Canyon fault, we used lidar-based digital elevation models and derived products in concert with field and remote investigations to construct a 21-km long fault strip map documenting more than 159 topographic surface breaks that are inferred to be Holocene fault scarps. The segment boundary is more complex than neighboring parts of the SLCS and PS. Within the segment boundary the fault zone is wider, fault scarp lengths are shorter, the fault is more discontinuous, fault orientation is more variable, and scarp density is greater. Additionally, right-steps in the fault trace are associated with negative topography and left-steps are associated with positive topography, which can be explained by a component of right-lateral slip within the segment boundary. Mapping and fault outcrops in natural exposures and paleoseismic trenches demonstrate that Mw 6.6 and greater earthquakes have ruptured with meter-scale displacements through Traverse Ridge multiple times during the Holocene. This study also demonstrates that SfM derived high resolution digital surface models can be utilized to accurately locate even minor fault traces through sparsely-vegetated, semi-arid Basin and Range piedmonts, but also highlights the importance of lidar for mapping faults where they cut through higher, wetter, and more vegetated terrain.

Between the Provo and Salt Lake City segments the Wasatch fault is expressed as an 8-12 Km long network of discontinuous surface breaks. At both ends of the segment boundary the trend of the fault rotates by ~90 degrees, over a distance of ~2 km, from roughly N-S to E-W. The most prominent surface breaks (Holocene surface break apparent) range from 15 to 470 m in length with a mean length of ~120 m. The one sigma distribution of surface breaks includes lengths of 40 to 200 m. The width of faulting throughout the mapping area is consistently greater than 100 m with much of area presenting good evidence for Holocene ruptures extending across a fault-perpendicular zone of 500-750 m in width. Scarps within the segment boundary display relatively modest relief as compared to other portions of the Wasatch Fault. There is some evidence for a component of right lateral slip along the portions of the fault zone which trend east to west. Evidence includes 1) several apparent right-lateral offsets of stream channels, 2) negative topography between large right steps in the main surface trace of the fault, and 3) positive topography between large left steps in the main surface trace of the fault. In addition to mapping recent surface breaks and geomorphic surfaces (not shown here), the UVU geology field camps from 2013-2014 have identified and documented several opportunistic paleoseismic exposures across the segment boundary. Here we present preliminary logging from six of these exposures.