PALEOSEISMIC INVESTIGATIONS OF THE SOUTHERN EAST CACHE FAULT, NORTHERN UTAH

The East Cache fault (ECF), northern Utah, is 80 km long and consists of northern, central, and southern segments. The southern segment, southern Cache Valley, Utah, has been interpreted to consist of three fault strands that cut a block of Tertiary Salt Lake Formation (Tsl) west of Paleozoic bedrock. Four trenches were excavated across these three mapped or inferred strands of the southern segment to determine the faults' Quaternary rupture history, and to better evaluate the seismic hazards of this rapidly urbanizing area. The western trench was located across an inferred fault trace in Quaternary Bonneville beach deposits, the central trench across the strand that cuts Tsl, and the eastern trench was sited near the contact between Paleozoic and Quaternary/Tertiary units. The excavation of the western strand shows that no fault exists where previously mapped. This “strand” was likely mistaken for the Lake Bonneville shoreline highstand deposits and landforms. Two trenches were excavated to evaluate the central strand, however no fault was found in this area. Reasons for this are likely due to thicker sediments and a deeply buried fault, suggesting there is no Holocene surface rupture. The excavation of the eastern strand reveals the main strand of the ECF, where Paleozoic dolostone is faulted against Quaternary sediments (OSL dates pending). No Holocene rupture is interpreted for the main strand of the ECF at this location. These results may indicate that the southern segment of the ECF is more inactive relative to the central segment, earthquake activity has not ruptured the surface recently, and/or the segment is long overdue for a surface-rupturing event.