Reevaluation of Faulting and Related Seismic Hazards in the Lake Tahoe Basin
Reevaluation of Faulting and Related Seismic Hazards in the Lake Tahoe Basin
Dissertation Proposal
Gretchen C. Schmauder
December 13, 2011
Introduction
Geologists have been mapping the Lake Tahoe basin since the early 1860’s, when Josiah Whitney, the head of the California Geologic Survey, was charged with exploring the Sierra Nevada ranges. In the intervening years, generations of geologists, through field mapping and ever increasing technological advances, have provided a greater understanding of the region’s geologic past. The latest generation of exploration, including airborne Light Detection and Ranging (LiDAR), seismic multi-frequency CHIRP exploration, multi-beam sonar, and multispectral data acquisition, has allowed for improved accuracy in mapping the Sierra fault system. The improved maps will provide a framework on which a more detailed seismic hazard analysis may be presented.
Project Summary
The objective of this research is to provide accurate mapping of the active fault system in the Lake Tahoe basin using the latest generation of mapping tools, identify seismic activity related to the fault system, and provide an analysis of the seismic hazards, including liquefaction potential, ground shaking, and landslides presented by the active faulting.
Regional Geology
The Lake Tahoe basin is located on the eastern edge of the Sierra Nevada Microplate, in region known as the Sierra Nevada-Great Basin boundary zone (SNGBBZ). The SNGBBZ is characterized by a complex system of normal and strike-slip faults resulting in a series of en echelon, northwest trending half grabens (Schweickert et al, 2004), with Lake Tahoe filling the largest of these grabens. The Sierra Nevada microplate is a rigid segment of the crust and includes the Sierra Nevada range to the east and the northern portion of California’s Central Valley to the west. Evidence, including the development of large volcanic centers on releasing trans-tensional stepover faults, effusive volcanic eruptions, and abrupt derangement of E-W drainage systems (Busby et al, in press), suggest that the microplate formed at approximately 12 Ma. Results from recent geodetic data describe the Sierra Nevada microplate as moving in a counterclockwise direction around an Euler pole located west of California’s southern coast (Argus and Gordon, 1991). The Walker Lane belt, a trans-tensional zone accommodating the counterclockwise motion of the Sierra Nevada microplate to the stable North American plate,separates the microplate and the Basin and Range extensional province to the east.
Three main north trending, normal faults, the West Tahoe – Dollar Point Fault (WTDPF), North Tahoe – Stateline Fault, and the Incline Village Fault, cut the Lake Tahoe basin. The slip rates on these faults increases progressively to the west, with the highest slip rate occurring on the WTDPF. Slip on this fault is estimated to be approximately 0.4 to 0.8 mm per year (Brothers et al, 2009), while slip on the North Tahoe – Stateline fault is estimated at 0.4 to 0.46 mm per year and slip on the Incline Village fault is estimated to be approximately 0.1 mm per year (Kent et al, 2005).
Methods
Fault Mapping
The primary objective of this portion of the study is to accurately map the locations of the active faults and landslides in the Tahoe basin. The methods for this portion of the study are discussed below.
In 2010, the Tahoe Regional Planning Agency (TRPA) commissioned Watershed Sciences, Inc. (WSI) to collect LiDAR and Multispectral data over the region. WSI collected data over approximately 972 square kilometers. For the LiDAR collection, WSI used two Leica ALS50 Phase II laser system mounted in a Cessna Caravan 208B (Watershed Sciences, 2011). WSI processed the LiDAR data and provided the NSL with bare earth models of the Tahoe basin.
The multispectral data were collected with a XXXX.
After data collection and processing, I imported the LiDAR data into the Fledermaus environment for visual interpretation. Potential faults, landslides, and other features are identified on the data and their coordinates are recorded. [JL1]I input these coordinates into a hand held GPS and ground checked these features in the field. If, after ground checking, the features are determined to be faults and landslides, the coordinates are added to the new fault map.
In addition to the LiDAR data, The Nevada Seismological Laboratory (NSL), in collaboration with the Scripps Institute of Oceanography (SIO), and the U.S. Geological Survey (USGS), performed seismic multi-frequency CHIRP exploration in Lake Tahoe, Cascade Lake, and Fallen Leaf Lake. The NSLcollected the CHIRP data with an Edgetech SubScan system.
After collection, I converted the CHIRP data are into standard SEG-Y format and then processed the SEG-Y files using the SIOSEIS program. I imported the processed data into the Fledermaus environment and added it to the LiDAR data. The addition of the subsurface profiles to the LiDAR data allow for continuous mapping of the fault systems from the land into the lake. This method provides a more complete visualization of the fault system.
The team performed multi-beam bathymetry in Fallen Leaf Lake to provide information on the shape of the lake and to locate features of a possible underwater fault. The data were collected using a XXX and processed with XXX.
Seismicity
The primary objective of this portion of the study is to identify seismicity associated with the active fault system in the Lake Tahoe basin. The methods for this portion of the study are discussed below.
The NSL has been monitoring the seismicity in the region with a regional array of seismometers since the 1970’s. They operate a series of digital three-component strong motion, short period and broadband seismometers, as well as several analogue one-component seismometers around the perimeter of the lake and in the nearby mountains. As a result, the seismicity catalogue for the Lake Tahoe region is large.
For this portion of the study, I will obtain the data from the NSL seismometer network for the Lake Tahoe basin and process them using the USGS’s open file program HypoDD. HypoDD is a Fortran based program used to relocate an earthquake’s hypocenter. The program uses the double difference algorithm developed by Waldhauser and Ellsworth. According to Waldhauser and Ellsworth (2000),“The 'clouds' of hypocenters that are familiar to all seismologists typically sharpen into networks of discrete fault planes after relocation with hypoDD. Improvement in location precision of up to two orders of magnitudes over routine locations are achieved.” The purpose of relocating seismicity in the Tahoe basin is to identify planes of seismicity that may correlate to our mapped faults.
Hazard Analysis
The primary objective of this portion of the study is to identify hazards associated with the seismicity of the active fault system in the Lake Tahoe basin. The methods for this portion of the study are described in the following paragraphs.
Numerous hazards, including strong ground motion and liquefaction, tsunami, and landsliding, may be associated with seismicity along the active faults in the basin. An area of particular concern is the portion of the basin known as the Tahoe Keys. This area is a mixed use resort community built on imported fill immediately adjacent to the south shore of Lake Tahoe. Strong ground motion and liquefaction of the fill could be a major concern if a large earthquake were to occur in the region.
For this portion of the study,we will collect Refraction Microtremor (ReMi) data in the Tahoe Keys area. ReMi is a method used to determine shallow shear wave velocities for estimating earthquake site response (Louie, 2001). I will develop an exploration plan, placing two hundred meter long lines with 24 evenly spaced 4.5 hertz geophones in numerous locations in and around the Tahoe Keys. The data is collected with a DaqLink II 24 bit acquisition system and recorded with Vibrascope software. After collection, I will process the data the SeisOpt Vspect and SeisOpt Disper software modules from Optim™. After collection and processing, I will interpret and model the data using probable earthquake scenarios as determined from the previous portions of this study. [JL2]
Timeline
Spring 2011
Begin LiDAR analysis
Submit abstract to SSA – Memphis, TN
Literature review
Present poster at SSA
Summer 2011
Lake Tahoe Field Work, including LiDAR ground trothing truthing and Lake CHIRP
Submit Abstract to AGU – San Francisco, CA
Fall 2011
Continued literature review
Analyze CHIRP data
Combine LiDAR data, field data, and CHIRP data
Present at AGU
Write dissertation proposal
Begin writing Chapter 1 dissertation[JL3]
Winter 2011
Continue writing Chapter 1 of dissertation
Spring 2012
Edit and complete first dissertation chapter
Continued Literature review
Committee meeting and qualifying exam
Qualifying Exam
Submit first chapter to Geospheres
Begin Lake Tahoe ReMi data acquisition
Begin seismic relocation and modeling
Summer 2012
Complete Lake Tahoe ReMi data acquisition
Continue seismic modeling
Process ReMi data and complete ReMi modeling
Begin Chapter 2 of dissertation
Fall 2012
Continue seismic modeling
Edit and complete Chapter 2 of dissertation
Winter 2012
Final Thesis Proposal and Comprehensive Exam
Begin Chapter 3 of dissertation
Spring 2013
Edit and complete Chapter 3 of dissertation[JL4]
Write introduction and conclusion sections of dissertation
Summer 2013
Prepare defense
Defend
Schmauder ProposalPage 1
[JL1]Don’t sell yourself short! This was a lot of work. Use a full paragraph to tell us what you did yourself, and how. Make sure you show us in your presentation the results that you obtained. And explain what other people did, and where you collaborated with others. This comment also applies to the next two paragraphs.
[JL2]We will talk a lot about this- various methods, objectives, and results. How far will the hazard analysis go? All the way to 3d scenario shaking models? A probabilistic hazard map? Studies of submarine landslide potential?
[JL3]Not clear what the topic of each dissertation chapter is?
[JL4]I would also think in terms of paper submittal to journals- project title of paper and journal to submit to.