Lewis E. Hunter

LiDAR-Assisted Identification of an Active Fault near Truckee, California

Abstract We use high-resolution (1.5–2.4 points/m 2 ) bare-earth airborne Light Detection and Ranging (LiDAR) imagery to identify, map, constrain, and visualize fault-related geomorphology in densely vegetated terrain surrounding Martis Creek Dam near Truckee, California. Bare-earth LiDAR imagery reveals a previously unrecognized and apparently youthful right-lateral strike-slip fault that exhibits laterally continuous tectonic geomorphic features over a 35-km-long zone. If these interpretations are correct, the fault, herein named the Polaris fault, may represent a significant seismic hazard to the greater Truckee–Lake Tahoe and Reno–Carson City regions. Three-dimensional modeling of an offset late Quaternary terrace riser indicates a minimum tectonic slip rate of 0.4±0.1 mm/yr. Mapped fault patterns are fairly typical of regional patterns elsewhere in the northern Walker Lane and are in strong coherence with moderate magnitude historical seismicity of the immediate area, as well as the current regional stress regime. Based on a range of surface-rupture lengths and depths to the base of the seismogenic zone, we estimate a maximum earthquake magnitude ( M ) for the Polaris fault to be between 6.4 and 6.9.

Airborne LiDAR analysis and geochronology of faulted glacial moraines in the Tahoe-Sierra frontal fault zone reveal substantial seismic hazards in the Lake Tahoe region, California-Nevada, USA

We integrated high-resolution bare-earth airborne light detection and ranging (LiDAR) imagery with field observations and modern geochronology to characterize the Tahoe-Sierra frontal fault zone, which forms the neotectonic boundary between the Sierra Nevada and the Basin and Range Province west of Lake Tahoe. The LiDAR imagery clearly delineates active normal faults that have displaced late Pleistocene glacial moraines and Holocene alluvium along 30 km of linear, right-stepping range front of the Tahoe-Sierra frontal fault zone. Herein, we illustrate and describe the tectonic geomorphology of faulted lateral moraines. We have developed new, three-dimensional modeling techniques that utilize the high-resolution LiDAR data to determine tectonic displacements of moraine crests and alluvium. The statistically robust displacement models combined with new ages of the displaced Tioga (20.8 ± 1.4 ka) and Tahoe (69.2 ± 4.8 ka; 73.2 ± 8.7 ka) moraines are used to estimate the minimum vertical separation rate at 17 sites along the Tahoe-Sierra frontal fault zone. Near the northern end of the study area, the minimum vertical separation rate is 1.5 ± 0.4 mm/yr, which represents a two- to threefold increase in estimates of seismic moment for the Lake Tahoe basin. From this study, we conclude that potential earthquake moment magnitudes (M w ) range from 6.3 ± 0.25 to 6.9 ± 0.25. A close spatial association of landslides and active faults suggests that landslides have been seismically triggered. Our study underscores that the Tahoe-Sierra frontal fault zone poses substantial seismic and landslide hazards.

USE OF HI-RESOLUTION LIDAR IN DISCOVERING THE POLARIS FAULT, MARTIS CREEK DAM, TRUCKEE, CALIFORNIA

Martis Creek Dam, located in the Truckee Basin north of Lake Tahoe, was initially rated as one of the U.S. Army Corps of Engineers9 highest risk dams in the United States. While the dam has performed its flood control purpose, a history of excessive seepage during even moderate reservoir levels has prevented it from also fulfilling its potential water storage function. During seepage and seismic studies to assess and mitigate deficiencies, high resolution LiDAR data was obtained. This imagery provides an unprecedented representation of the ground surface that allows evaluation of geomorphology even in areas with a dense vegetation canopy. At Martis Creek Dam, this geomorphic analysis resulted in the recognition of a previously unknown and through-going lineament between the spillway and dam embankment. This feature extends to the southeast where several lineament splays are exposed on the East Martis Creek Fan. These lineaments were subsequently explored by paleoseismic trenching at two locations and confirmed as faults with Late Quaternary to Holocene displacement. Faulting was confirmed in both trenches as unique splays of a fault zone with several feet of apparent normal (vertical) slip and an unknown magnitude, but a potentially significant, strike-slip component. Faulting was observed near the ground surface in both cases and multiple fault events (a minimum of two) which are at least latest Pleistocene, and probably Holocene-active.

IDENTIFICATION OF THE POLARIS FAULT USING LIDAR AND SHALLOW GEOPHYSICAL METHODS

As part of the U.S. Army Corps of Engineers’ (USACE) Dam Safety Assurance Program, Martis Creek Dam near Truckee, CA, is under evaluation for earthquake and seepage hazards. The investigations to date have included LiDAR (Light Detection and Ranging) and a wide range of geophysical surveys. The LiDAR data led to the discovery of an important and previously unknown fault tracing very near and possibly under Martis Creek Dam. The geophysical surveys of the dam foundation area confirm evidence of the fault in the area.

A Case Study Illustrating Some Limitations of Airborne Geophysical Investigations for Munitions of Explosive Concern

In early 2005, a magnetic and electromagnetic airborne geophysical survey was conducted by Battelle/Oak Ridge National Laboratory over the former multi-range impact area of Fort Ord. Given that the multi-range area has tall vegetation and variable topography, the primary objective of the survey was to use a tool with wide area assessment capabilities to delineate special case areas with high densities of anomalies related to ‘Munitions of Explosive Concern’ (MEC) that might require special treatment (sifting?) or extraordinary amounts of time for excavating individual items. Approximately 25.62 km (6332 acres) of the 32.37 km (8000 acre) multi-range impact area was surveyed with the airborne magnetics system. The results of the survey included total field and pseudo-vertical-gradient (i.e. computationally-derived) magnetic field maps, sensor altitude maps, analytic signal maps, and an interpretation map delineating areas with higher and medium amplitude magnetic signals. The primary issue of concern is that it is not known how close the survey actually came to achieving the survey objectives, let alone detection limitations, since it is not known exactly what is located in the survey site. The resulting ordnance density interpretations are not reliable because they may not be complete. As part of the system calibration process, multi-altitude surveys (2 m, 4 m, 5.5 m) were performed over the well-documented geophysical prove-out grids developed for the Ordnance Detection and Discrimination Study located in the main survey area. From the calibration results two conclusions can be reached: 1) The flight altitude was too high to detect MEC the data cannot be used to infer estimates of concentrations that are present or not present; 2) The altitude was low enough to detect something but what was detected may or may not be indicative of a cluster. Large, single anomalies may be truly single hits or may be part of a much larger cluster. The main conclusion from this survey is that the airborne geophysical data can be used to indicate something is present but cannot be used to indicate whether an area is a special case area or not. Any airborne geophysical survey, and in particular an unexploded ordnance (UXO)-related investigation, must be analyzed with due consideration given to sensor altitude and compared to an area with known parameters.