TESTING VISIBILITY OF LANDSLIDE DEPOSITS BY DEEP REFRACTION-MICROTREMOR AND H/V ANALYSIS, WASHOE VALLEY, NV

Landslide hazard and recurrence is based on presence and extent of known landslide deposits. However, in tectonically subsiding basins landslide deposits can become buried, resulting in an incomplete record. To complicate matters, once buried landslide deposits are difficult to identify, generally requiring expensive borehole access. Therefore the goal of this study is to test whether deep refraction microtremor (a.k.a. ReMi) and horizontal-over-vertical seismic spectral methods will show systematic variability in shear wave velocity over a known buried landslide deposit relative to an area without landslide deposits. The larger maximum clast size of landslide compared to alluvial deposits should lead to differences in shear-wave velocities.

The Carson Range bounds the west side of Washoe Valley, NV with the Sierra Nevada Frontal Fault system separating the two. The Slide Mountain landslide complex sits in the northwestern part of Washoe Valley. The complex contains debris from at least ten Quaternary landslides that were sourced on Slide Mountain and exited onto the valley floor at Ophir Creek. No other landslide complex is apparent along the range front.

To test whether the landslide complex will have a distinct shear-wave velocity profile, we collected passive-source seismic data on two linear arrays on the western side of Washoe Valley. One array was placed across the toe of the Slide Mountain landslide complex near Ophir Creek. The second array was ~2 km south in an area with no apparent landslide deposition. Each line consisted of 100 Fairfield 3‑component seismic nodes with ~22 m spacing for a total line lengths of 2.2 km. The nodes collected data for ~4 hours for each array. The data are being processed using VsSurf^™ from Terēan^™ and initial 1D shear-wave velocity profile results show good constraints to at least 700 m depth. Ultimately, if successful, this technique will be a way to help understand long-term frequency and variability of landsliding events, and can potentially guide drilling programs to determine more information on buried landslide deposits.