Controlled-Source Experiments and Analysis
Objectives: 1) To develop a background velocity structure in the study area; 2) to locate and
image the transition from the Sierra Nevada to the Basin and Range (SNGBBZ) at the latitude of
39N (Lake Tahoe); 3) to define the structure of the Moho and lower crust in this transition zone.
Motivation: Controlled-source experimentation provides better spatial resolution at high
frequencies than cannot be achieved with passive teleseismic sources alone. These data will
provide the background velocity structure necessary for better location of the earthquake swarms.
In addition, these data will provide the first-order structure of the crust and the crust-mantle
boundary. Neither of these is well known from previous studies. Defining the mid-crust to upper
mantle of this region is essential in understanding the transitional boundary between the
SNGBBZ and its potential influence on magma emplacement and migration through the crust.
Crustal tomographic inversion and forward modeling of the controlled-source data will provide
the first realistic velocity structure at the latitude of Lake Tahoe.
Proposed Work: To accomplish these objectives, we plan to survey two lines (lines a and b in
Figure 4). The swarm of earthquakes reported by Smith et al. (2004) will be the center of our two
lines. The swarm has a strike of N30ºW with a 50ºNE dip (Figure 1). Line a will trend
perpendicular to the dike swarm whereas Line b will parallel the feature. This constitutes an
effort to produce a 3D tomographic image of the feature.
We plan to deploy RefTek 125 receivers ("Texans") every 500m along the lines and place shot
points every 10 km near the source of the swarm and every 100 km outboard. The shots will
range in size from 250 lbs to 3000 lbs along the lines. The density of receivers will provide a
generalized image of the deep crust and upper mantle that can be used in a joint inversion with
the earthquake hypocenters.
The receivers for both lines will be laid out initially, and then all shots will be recorded by all
receivers on both lines. We estimate that there will be 12 shot points and 1200-1500 receivers for
the total controlled-source experiment. This will allow a limited 3D coverage similar to other
experiments of this type.
The data from the controlled-source portion of the experiment will be processed initially as 2D
data along the two lines and then as 3D data when all of the shots and receivers are taken
together. The goal will be to produce a good image of the lateral extent of the swarm and the
crust-mantle in the SNGBBZ transition. We plan to integrate these data with the earthquake data
and wide-angle reflections to produce a joint inversion the crust-mantle structure (Zelt and Smith,
1992; Hole and Zelt, 1995; Zelt and Barton, 1998). Although the well understood trade-off
between reflector depth and the velocities above will still exist, the presence of direct P-wave
arrivals in the inversion constrain the upper crustal velocities, thus reducing this trade-off. The
explosion sources will also help constrain velocities within the mid-to-upper crust of our 3-D
study area, providing much better earthquake locations and velocities in the lower crust than
would be available from the earthquake data alone.
Time Table
Scouting for the profiles and shots will take about 2 weeks starting in January 2006. Permitting
for the project will begin in January 2006 using the permit office at UNLV to assist in paperwork
and obtain these permits. The bulk of the line is under the jurisdiction of the US Forest Service
and/or the BLM, which will simplify the permitting process; but there will to be a significant
effort devoted to the portion of the line in Lake Tahoe. It is estimated that permitting will take 6
months. Surveying for the receiver stations will take place from June to July 2006 as well as
drilling for the shot holes. The acquisition of the seismic data will take place in August 2006. It
will take about 10 days to deploy and retrieve the receivers and approximately two nights to shoot
all of the shots. The scope of the controlled-source portion is similar to that accomplished in the
Las Vegas Basin in 2003 by C. Snelson. The data will be reduced and processed at UNLV from
August to October 2006. Forward modeling and tomographic inversion of these data should
begin around October 2006 and continue into the Spring 2007. At the end of Spring 2006, these
data will be integrated with the passive source data and also be available to the earthquake
modeling group to jointly invert to further refine their models.
Project Management
The planning and execution of the controlled-source survey will be the responsibility of Catherine
Snelson and John Louie. This component will consist of permitting, surveying, drilling, drill
watching, loading of the shots, shooting, deploying, pick-up, and clean-up. The shooting will be
overseen by Catherine Snelson and executed by a local blaster. The first year will be devoted to
permitting the shots and receivers for this survey, for the execution of the survey, and the second
year will be for the processing and interpretation of the data. The receiver deployment will be
done by volunteers from UNLV and UNR, as well as from the local communities and institutions.
An IRIS undergraduate intern will be requested to help during the summer of 2006. We have
been successful in previous years in getting a large number of volunteers to participate.