PALEOEARTHQUAKE MAGNITUDES DERIVED FROM LIDAR-BASED MAPPING OF FAULT SCARPS IN THE NORTHWESTERN BASIN AND RANGE

Surface rupture length and mean displacement have been shown to be proportional to earthquake magnitude. Ideally, these two distinct measurements should produce similar estimations. In regions with long recurrence intervals, these methods provide one of the few means of assessing seismic hazards. High-resolution topographic data facilitates mapping and measuring surface ruptures in great detail in regions where scarps are well-preserved.

One such area is the northwestern Basin and Range. High-resolution lidar (1 point per 0.5 m²) data were acquired along ~80% of the Surprise Valley fault (SVF), a 95-km long, large-offset normal fault in NE California. Digital elevation models (DEMs), hillshades, and slopeshades were derived from the lidar data and provided a base for detailed mapping of scarps and Quaternary deposits along the SVF. A total of 78 topographic profiles were drawn from the DEM across fault scarps at localities where (1) no apparent sedimentation had occurred on the hanging wall post-rupture, (2) far-field slopes were the same (or nearly so) on the hanging- and footwalls, and (3) little to no modification from construction was present. Surface offsets and vertical displacements were calculated on these profiles using standard techniques.

Results indicate that scarps occur along ~64 km of the SVF in all Quaternary units, including older and younger fan deposits, landslides, alluvium, colluvium, and Pleistocene fan deltas; surface offset ranges from 0.8–22.2 m. At least 38 scarps preserve geomorphic evidence for multiple events, however, and apparent surface offset for the most recent event is limited to 0.6–4.9 m. The largest offsets are preserved in Pleistocene fan deltas; these most likely record multiple events even though they exhibit simple profiles that probably reflect rapid erosion to the angle of repose of coarse gravel. The smallest offsets (0.6–2.5 m) are preserved in active alluvial fans and are present along a length of ~42 km of the SVF; these scarps are interpreted to record only the most recent event. Estimates of the magnitude of the most recent event from surface rupture length (7.0) and measurements of displacement assuming a 68° fault (7.0-7.3) agree within error, and correlate well with estimates of 6.8–7.3 from paleoseismic trenching.