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Project Description
Collaborative Research: UNR & UNLV:
Lithospheric Architecture at a Magma Intrusion Event
Observed 30 km Below Lake Tahoe, California and Nevada
J. Louie, G. Biasi, R. Anooshehpoor, K. Smith U. of Nevada, Reno
C. M. Snelson, U. of Nevada, Las Vegas
C. Wilson, Lamont-Doherty Earth Observatory
Motivation for This EarthScope FlexArray Project
The recent detection of magma movement deep below Lake Tahoe by K. D. Smith et al. (2004) presents
one of the only modern observations of active injection of magma into continental crust. Further study of
this eature would help to answer fundamental questions relating to magma migration, melt extraction, and
crustal assimilation of mantle magmas. Additionlly, Study of this unusual event has raised many
questions about the structure and nature of the lower crust, most not well answered. We do not possess
even baseline constraints on the configuration of the lithosphere in the northern Sierra Nevada. Nor do we
have many constraints on the nature of the Sierra Nevada–Great Basin boundary zone (SNGBBZ). We
must be able to describe the configuration of this boundary in at least a fundamental way before we can
address questions such as whether the 2003 magmatic event was rare and isolated, or represents more
pervasive lithospheric processes.
Over the last 20 million years, Basin and Range extension has gradually encroached into the eastern edge
of the stable Sierra block. This progression has been marked by the creation of large volcanic centers
along the eastern edge of the Sierra (e.g. Coso and Long Valley). Clearly, the transition from the weak
crust of the Basin and Range to the relatively cold, strong Sierra block provides an ideal conduit to
channel mantle melts into the crust as has been repeatedly demonstrated over the last 5 million years
culminating in the massive Long Valley eruption nearly 800,000 years ago. Extensional encroachment
occurred more rapidly in the southern Sierra than in the northern Sierra as evidenced by the dramatic
topography of the southern Sierra. The 2003 magmatic injections may be an indication that extensional
strain has begun to modify the lower crustal structure of the Northern Sierra and has opened new
pathways for melt migration in the crust. Unfortunately, we have only sparse information about the state f
the crust and upper mantle in the northern Sierra Nevada, making it impossible to place the magmatic
event of 2003 in a broader, tectonic context. This limits what we can infer about magmatic processes that
produced the swarm.
The need for focused work on the northern Sierra Nevada is clear when the region it is compared to the
state of knowledge of the southern Sierra Nevada. Wernicke et al. (1996) summarized the results of the
multi-year, multidisciplinary southern Sierra Nevada seismic experiments. Careful work such as by Jones
et al. (1994) revealed the surprising lack of a deep crustal root below the topographically highest part of
the range. New tectonic models for the southern Sierra Nevada resulted quickly, such as the delamination
hypothesis of Ducea and Saleeby (1998). In contrast to the well-studied structure of the southern Sierra
Nevada (Jones et al., 1994; Fleidner et al., 1996; Wernicke et al., 1996; Jones and Phinney, 1998; Boyd et
al., 2004; etc.), information on the northern Sierra Nevada is relatively sparse *. Extrapolation of
information from the southern to northern Sierra Nevada may not be supportable.
Compilations such as by Braile et al. (1989) show the dearth of seismic constraints on the northern Sierra
Nevada region, with Eaton (1963) and Pakiser and Brune (1980) being the only experiments there until
*
Note: A search of GeoRef for 1980-2004 papers with keywords = (southern Sierra Nevada) or (northern
Sierra Nevada) and (crustal structure) yielded 7 citations for southern but 0 for northern.
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2002. The 2002 refraction work of Louie et al. (2004) was a reconnaissance experiment lacking any blasts
or on-line refraction sources in the Sierra Nevada. Although suggestive that a nonintuitively thicker (>50
km) crustal root may underlie the topographically muted northern Sierra Nevada, the 2002 survey could
not constrain the upper crust without local shots. This project is partly intended to remedy the
shortcomings of the reconnaissance results of Louie et al. (2004). With magma injection events observed
below the northern Sierra, it is now imperative to collect baseline geophysical data there.
In characterizing the transition between the California Peninsular Ranges and the Gulf extensional
province, Lewis et al. (2001) found tremendous dips on the Moho discontinuity, in places exceeding 20°.
With the suggestion of a >50 km root for the northern Sierra Nevada by Louie et al. (2004), such radical
lithospheric discontinuities may be a possibility for the SNGBBZ there. Yet the lack of baseline seismic
characterization is even more severe in considering this fundamental tectonic boundary. The refraction
results of Spieth et al. (1981) are too far west to address the transition, and the results of Catchings and
Mooney (1991) and Gilbert and Sheehan (2004) are too far east. A COCORP survey crossed the northern
Sierra Nevada, as reported in, for example, Nelson et al. (1986), Klemperer et al. (1986), Knuepfer et al.
(1987), and Klemperer (1987). However, this COCORP line was 50 km north of Lake Tahoe where the
SNGBBZ is complicated by interaction with right-lateral faulting of the Walker Lane. Thus we cannot
determine whether the magma injection below Lake Tahoe was interior to a geotectonic Sierra
Nevada block or a feature of the transition boundary zone itself.
The seismic refraction, tomography, and teleseismic imaging experiments proposed here for the Lake
Tahoe area are targeted to understand the structures and context of the 2003 deep magmatic event. Is the
shallow mantle unusual near north Lake Tahoe, or are similar conditions prevalent throughout the eastern
Sierra Nevada? What are the dimensions of the melt body and how does it facilitate melt extraction form
the mantle? Is this body a single reservoir or several small reservoirs? Are they a feature of the Sierra
Nevada block, or of the SNGBBZ transition that might give insight into the development of other young
volcanic centers along the Easter\n Sierra? Is there a deeper mantle source for the magma, and if so, what
is its distribution? If lithospheric anomalies are limited to just the area of the 2003 injection, then it may
have been an isolated and rare event or potentially associated with focused extensional strain within the
north-south structural transition expressed in the
surface faults at Lake Tahoe. The Lake Tahoe
Figure 1. Map showing the epicentral locations
area is a region is of relatively high extensional
of the deep (~30 km) earthquake swarm in 2003.
strain (Smith et al., 2004). If these anomalies
A perspective view on the locations of the deep
are part of a broader pattern, though, magma
earthquakes and an inferred “plane” to model the
injection of the lower crust could be a typical
intrusion is overlain in lower right.
feature of the boundary between the Sierras and
the extensional Great Basin, and perhaps of the
boundaries of extensional provinces in general.
Background and Rationale
Geodetic data indicate that the Sierra Nevada
block is moving about 13 mm/yr N40-45W
relative to stable North America. This motion
accounts for about 20-25% of the western North
American plate motion budget and is generally
oblique to north-striking down-to-the-east active
normal faults along the SNGBBZ in northern
Nevada and eastern California near the latitude
of Lake Tahoe.
This motion drives the
earthquake hazard of the region. In view of this
hazard, the Nevada Seismological Laboratory
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(NSL) has operated seismic stations in the
Lake Tahoe area for over 30 years.
As the SNGBBZ boundary has
evolved westward since 15 Ma (Dilles
and Gans, 1995) in a process that
incorporates Sierra granites into the
Basin Range Province (Surpless et al.,
2002), the normal fault systems at this
boundary have accommodated the
eastward collapse of the Sierra block
and development of Basin and Range
physiography.
Along the Sierra
Nevada front, north-striking normal
faults have developed in a leftstepping geometry from about the
latitude of Long Valley, extending
northward through the northern end of
Lake Tahoe. The Lake Tahoe basin
represents
the
westward,
and
youngest, expression of the SBGBBZ
having developed over the past 3 Ma.
To the east the Walker Lane Belt
(Stewart,
1988)
primarily
accommodates dextral plate-boundary
shear including extensional faulting in
overall transtensional tectonics.
In
other words, whereas the eastern front
of the Sierra Nevada is the focus of
down-to-the-east normal faulting
reflecting the westward evolution of
the Basin and Range into the Sierra
Nevada block, strike-slip faulting,
more representative of plate boundary
shear, characterizes the Walker Lane
Belt.
Figure 2: The geodetic displacements at Slide Mountain,
Nevada, before, during, and after the earthquake swarm
in the deep crust of late 2003.
The normal and strike-slip regimes in the upper crust along the Sierra Nevada front operate under
regionally consistent E-W directed extension (T-axis from focal mechanisms; Ichinose et al., 2001), with
the P-axis rotating locally to reflect either normal or strike-slip faulting. Along the SNGBBZ at the
latitude to of Lake Tahoe, Quaternary moment release is dominated by the normal fault systems, with
strike-slip faulting apparently playing a role in slip transfer between these primary normal-fault systems
(Schweickert et al., 2004).
Large volume Tertiary ash-flow tuff sequences that initiated around 30 Ma extend throughout northern
Nevada and eastern California near Lake Tahoe. Volcanism progressed throughout the Miocene and by
about 3 Ma had evolved to arc-related andesitic magmas and then waned. The most recent mapped
volcanics comprise a number of 1-2 Ma basaltic volcanic sources in the north Lake Tahoe area, and
additional source areas including the rhyolitic domes of the Steamboat Hill geothermal area in
metropolitan Reno east of the lake (Henry and Faulds, 2004). There is an ongoing debate concerning the
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most recent age of volcanism in the north Lake Tahoe area (Sylvestor 2004, personal communications);
and age dating is underway for some suspected young basalts at Tahoe. Only the Long Valley caldera
and Lassen Peak volcanic areas show volcanics younger than 1 Ma along the eastern front of the Sierra.
Evidence for magma injection in the SNGBBZ at lower crustal depth beneath north Lake Tahoe in late
2003 has recently been published in Science (Smith et al., 2004). A sequence of 1600 earthquakes
(Figure 1) from August 2003 through February 2004 define a planar structure at a depth of roughly 30 km
below the lake that was coeval with a 10 mm permanent displacement observed on a nearby GPS
instrument at Slide Mountain, Nevada (Figure 2). The observations were modeled as a 40 km2, 1-m thick,
lower-crustal dike injection event; and therefore the process accommodated about 1 m of extension in the
lower crust. The structure is oriented approximately N30ºW and dips 50º to the northeast. Observed EW extension in the Lake Tahoe region would support tensile failure for a structure with this geometry. In
addition, the volcanic dike in map view is situated within 4 mapped locations of 1-2 Ma basalts in the
north Lake Tahoe area (Henry and Faulds, 2004). Remarkably, the dike injection event apparently
triggered an increase in shallow seismicity (< 15 km) throughout the northern Lake Tahoe area that
included an M 4.2 earthquake in June 2004 at about 10 km epicentral distance from the deep crustal
sequence.
This inferred magmatic event represents a unique opportunity, possibly not to be repeated anytime soon,
to study a coherent magmatic intrusion in the northern Sierra Nevada. Thus it is reasonable to combine
the detailed analysis of the deep crustal context of the 2003 event with a broader look at the structure of
the northern Sierra Nevada at the latitude of Lake Tahoe, as proposed here. We also propose to apply
state-of-the-art imaging methods to the zone of the deep earthquake cluster (and presumed magmatic
intrusion) and to the problem of defining deep fault zones of the Tahoe Basin. The observation of magma
injection with the SNGBBZ also brings up fundamental questions regarding the evolution and westward
encroachment of the Basin and Range province that could contribute to our understanding of volcanic
hazards and public safety.
Broadly, this proposal will address two scales, one identified with the SNGBBZ faulting and magmatism
in the Tahoe Basin, and one more regional in scope that will provide the larger context. The proposed
work will address:
1. Is the dike related to the SNGBBZ frontal fault system or another of the Tahoe Basin faults?
Association with the frontal fault zone would require a fairly high angle in the mid-crust and a
more listric geometry eastward to join the surface expression of the frontal fault system.
2. Can we image lower crustal magma similar to those interpreted from the COCORP Sierra Valley
line (Klemperer et al., 1986; Knuepfer et al., 1987)? Are lower crustal magmas pervasive?
3. Can we specifically image a deeper magma source for the 2003 magma injection event under
Lake Tahoe? What is the depth of this source?
4. What is the nature of the boundary between the Sierra Nevada and the Great Basin? The
presence of volcanism in the physiographic Sierra Nevada suggests that the boundary is offset
between the crust and mantle as it is in the southern Sierra, but this remains to be demonstrated.
5. What is the structure of the Moho and crustal thickness through the SNGBBZ?
6. Are there other regions of the lower crust and upper mantle that suggest potential future magma
movement at depth?
Passive Tomography Experiments and Analysis
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Objectives: 1) Image the crustal and upper mantle structure in the Lake Tahoe area, including the region
of the inferred deep crustal magmatic injection in 2003; 2) Develop a velocity model for the structure
under the northern Sierra Nevada and Lake Tahoe that can be used to guide the teleseismic wave-equation
imaging at depth and to improve hypocentral locations for deep seismicity in this area; 3) Identify
shallow mantle regions with similar physical properties and presumably similar conditions for magmatic
activity; and 4) Link the study area to the broader context of subduction and evolution of the Sierra
Nevada.
Motivation: The crustal structure across the Sierra Nevada at the latitude of Lake Tahoe is poorly known.
Figure 2.5a provides the larger context of the northern Sierra Nevada and the study area. P-wave velocity
variations are shown for California and western Nevada at a depth of 50-70 km using the combined
California and Nevada permanent seismic arrays. Known effects of crustal thickening (Oppenheimer and
Eaton, 1984) were removed before inversion to improve resolution of the mantle contribution. The
western half of the Sierra Nevada range and eastern half of the Great Valley are high velocity relative to
model average. At these depths a silicic root would be lower velocity, so it appears rather to be cold and
perhaps eclogitic in composition. Its extent along strike indicates that it formed by subduction-related
processes, either with the Mesozoic construction of the arc or by modification during Cenozoic
subduction. Figure 2.5b shows volcanism since 13 Ma on an enlarged portion of the regional image.
Volcanism clearly concentrates east of the Sierra Nevada, but extensive volcanism north and west of Lake
Tahoe, which lies well within the physiographic Sierra, suggest that the mantle definition of the SNGBBZ
is 15-30 km to the west.
The detailed context for volcanism in Tahoe Basin and SNGBBZ is essentially unknown. The recent
magmatic event makes clear that at some level the system is active, but work proposed here will be
required before it can be known whether it was simply a local anomaly, whether it is part of a system with
similar source conditions, or whether it is the general state of the upper mantle beneath the eastern Sierra
Nevada. The active source portion of the proposed experiment discussed below is designed to evaluate
whether master faults of the Tahoe region or perhaps other fine-scale features work to localize magmatic
events.
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Caption, Figure 2.5. (a) P-wave tomographic image of California and Nevada at a depth of 50-70 km.
Teleseismic delays to permanent network stations of California and Nevada plus a local temporary array
in the southern Sierra Nevada (Biasi and Humphreys, 1992; Jones et al., 1994) were inverted in a model
with a total depth of 650 km. Block size is 30x35 km (EWxNS), extrapolated after inversion to 10x10 km
for plotting purposes. Outer red line encloses the region at this depth with crossing-ray constraint, which
can be interpreted as the resolved map region. Holes in the coverage exist in both the southern and
northern Sierra Nevada, generally corresponding to gaps between regional networks. LVC: Long Valley
Caldera; MTJ: Mendocino Triple Junction; SGVA: Southern Great Valley Anomaly; TRA: Transverse
Ranges Anomaly. Stratovolcanos labeled with white triangles. (b) Enlargement of (a) around the study
area. The prominent upper mantle velocity high velocity region is offset to the west some 15 to 30 km
from the eastern bound of the physiographic Sierra Nevada. A similar offset in the southern Sierras,
evident in (a), has been extensively studied (Jones et al., 1998; Ducea and Saleeby, 1998). Volcanism
since 13 Ma (C. Henry, pers comm.; circles: dated samples; stars: interpreted locations of volcanic
centers) indicate a fundamental difference in mantle conditions is present in shallow mantle depths.
Inversion block sizes and native station density are too coarse to resolve questions raised in this proposal.
Proposed Work:
Briefly, we propose an 18 month deployment of 40 broadband seismic stations (Figure 3). Data acquired
during this experiment will be supplemented by network station coverage of the California and Nevada
regional seismic networks. Where needed to fill in azimuth or ray-parameter coverage, recordings during
the experiment will be supplemented by archive data of the permanent seismic networks. The proposed
temporary array will entail relatively high density coverage in the central area to ensure adequate
resolution at upper mantle depths..
The usefulness of high-density teleseismic recording is shown in the Vp images shown in Figure 3.1.
Magma was erupted from five small centers (green triangles), four at about 1 Ma, and the fifth (LC)
approximately 120 Ka. Melt extraction increases seismic velocities, of itself and as it removes water from
the solid phases (esp. olivine) in the mantle (Karato and Jung, 1998; Hammond and Humphreys, 2000).
The only depth beneath the volcanic centers showing the predicted velocity increase is about 50 km.
Petrologic evidence shows the magma originated at about 53 km (the center of the layer shown in Fig.
3.1a, black oval in b). Of note as well, the lowest velocity regions in the 45-60 km depth slice (south and
east of LSM) are and have been substantially amagmatic. For reasons developed elsewhere (Biasi, in
prep.) this lowest velocity region is likely to be hydrated and subsolidus. Finally, the scale of the upper
mantle structures involved to produce the Quaternary volcanism here imaged are so small that imaging
with teleseismic shear waves could be difficult. At, say, five second periods, the wavelength is of order
20 km, and structures large enough to likely be imaged are probably just wet (Karato and Jung, 1998).
The southern Nevada example in Figure 3.1 shows that low velocities are not certain beneath regions of
recent volcanism, and that a nuanced interpretation that considers all available data may be necessary to
unravel the northern Sierra Nevada teleseismic images.
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Caption. Figure 3.1. (a) Direct image of a melt extraction region using P-wave tomography. Melt
extraction to Quaternary volcanic centers of Crater Flat (green triangles) should cause a 1-2% increase in
Vp in part by melt extraction and in part by dehydration of olivine. The one depth at which velocities
increase is imaged in the 45-60 km depth layer. One year of portable and network station teleseismic data
from southern Nevada was used to produce the image. Net station density is comparable to the proposed
northern Sierra Nevada experiment – 5-8 km in the center of the imaging area. This model shows that
4.5x4.5 km blocks can be resolved to a model depth of 80 to 100 km. Note that lowest mantle Vp regions
have not been volcanically active; these regions are likely to be sub-solidus and hydrated. LSM: Little
Skull Mountain; ESF: Exploratory Studies Facility, Yucca Mountain. (b) North-south cross-section
beneath the Crater Flat volcanic cones. The magmatic source is centered on ~50 km depth, consistent
with petrologic estimates.
The teleseismic array will consist of 40 Guralp 40-T broadband seismometers and RefTek 130
recorders to be taken from the pool of EarthScope flexible array instruments. They will cover an
approximate 2-D 60 km by 60 km grid centered on north Lake Tahoe, with portable stations at
about 10 km spacing (Figure 3). Figure 3 reflects realistic positions based on roads, federal land
units, and wilderness boundaries. The arrangement of the array will permit us to build a 3-D
tomographic model of the crust and upper mantle under the study region. NSL has 10 stations
within the proposed footprint of the teleseismic array. Arrivals obtained from these stations
during the experiment will provide the link between the data of the proposed experiment and the
earthquakes previously recorded on the network. Resolution will depend on a variety of factors
including the number of useable teleseisms that occur during the experiment; but we estimate
that we will be able to resolve features on the order of about 5 km in size to shallow mantle
depths beneath most of the array.
To increase the areal coverage of our broadband array, we plan to also analyze teleseismic arrivals at
stations of the surrounding CISN (California Integrated Seismic Network) and NSL networks. It is
important to analyze a box considerably larger and deeper than the focused study area so that unmodeled
structure from regions outside the analysis box can be mitigated (Evans and Achauer, 1993). In the
proposed study area this will be especially important since the subducting Gorda slab extends to a depth
of at least 400 km not far north of the project area. To reduce the teleseismic data to relative traveltime
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delays we propose to use a method similar to that used by VanDecar (1991) and by VanDecar et al.
(1995). In their methodology, teleseismic P and S waveforms are cross-correlated to give highly accurate
and robust estimates of P and S delay times and errors. The teleseismic inversion solves for velocity
perturbations to a background model, source-related delay (such as source mislocation and mistiming as
well as unmodeled structure outside the box) and station corrections (due to small-scale unmodelable
structure near the stations). The teleseismic tomographic inversion code itself is described in
Dueker et al. (1993).
.
Controlled-Source Experiments and Analysis
Objectives: 1) to test whether the magma injection has a detectable seismic signature. 2) if so, to
determine the extent of the magma body compared to its size inferred from the earthquake swarm beneath
north Lake Tahoe; 3) to locate
and measure the transition from
the Sierra Nevada to the Basin
and Range at the latitude of
39N (Lake Tahoe).
Motivation: Active source
experimentation provides better
spatial resolution at high
frequencies than can be
achieved
with
passive
teleseismic sources alone. We
expect that if the magma
intrusion is to be imaged,
higher frequencies of the active
source experiment will have the
greatest chance of success.
High frequencies are also more
likely to image faults and
determine whether the 2003
intrusion was assisted by a deep
fault structure of some sort.
Regardless of the detailed
success of the active source
component with the 2003
magmatic event, it is extremely
likely to succeed in recovering
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 Sierra Nevada and Great
Basin and its potential influence
on magma emplacement and
migration through the crust.
Figure 4. Proposed layout of the controlled-source seismic
experiment. Yellow dots represent the shot point locations and the
black lines represent station locations. The densest shot point
spacing is located over the anomaly at Lake Tahoe.
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Crustal tomographic inversion of the active 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.
Figure 5. Ray paths of forward and
back-scattered phases generated by an
impinging teleseismic P wave front
after Clouser and Langston (1995).
Solid lines mark P ray paths with S
rays marked by dashed lines.
Uppercase letters denote downgoing
phases with lowercase letters
symbolizing upgoing phases. The free
surface multiples returning to the
surface as S (e.g. PpSs) will be
contained in the receiver function. The
idealized vertical and radial
seismograms indicate the approximate
arrival and amplitude relationships for
some of the arrivals in the ray diagram
above.
of the controlled-source portion is similar
Snelson.
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. We anticipate that it will take a
week to deploy the receivers and approximately two nights
to shoot all of the shots. Thus, the controlled-source
experiment will last about 10 days. It will take
approximately one year to obtain the permits for the shots
and receivers, and a few months to have the shot holes
drilled. 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. The scope
to that accomplished in the Las Vegas Basin in 2003 by C.
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 further constraining the location of magma body (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.
Receiver Functions and Imaging
101
Objectives: 1) determine crustal thickness across a major tectonic
boundary from the Sierra Nevada to the Basin and Range; 2) constrain
the location of significant reservoirs of partial melt in the crust
beneath Lake Tahoe.
Motivation: The support mechanism for the high elevation of the
northern Basin and Range and the adjacent Sierra Nevada remains
unknown primarily because little is known about the crustal thickness
across this region. Most likely the difference in elevation between
the Sierra Nevada and the Basin and Range is also related to the
location of the magmatic intrusion beneath Lake Tahoe. Exploring
these connections through crustal-scale, converted-wave images of the
transition from the Sierra Nevada to the Basin and Range will
elucidate important features of tectonic-magmatic interactions.
Proposed Work:
We propose to use teleseismic converted phases recorded by the three
component passive array to map the crustal thickness and the location
of partial melt within the crust beneath the array using receiver
function analysis (Burdick and Langston, 1977; Langston, 1977; 1979;
Phinney, 1964) as applied by Wilson et al., (2003). Receiver
functions analysis exploits teleseismic P-to-S conversions to
identify velocity discontinuities in the lithosphere by using the Pwave arrivals recorded on the vertical component to enhance converted
arrivals recorded on the radial component (deconvolution). The
amplitude of the converted phases varies as a function of the
incidence angle of the impinging wave and the impedance contrast
across the interface. The delay time between the direct P arrival and
the converted phase are related to the depth to the conversion point
and the Vp/Vs ratio above the interface. We intend to use
teleseismic converted waves to determine the crustal thickness from
the High Sierra across the Lake Tahoe area and into the Basin and
Range to the east . Additionally, sufficiently large regions of
partial melt and/or areas of sufficiently low velocity should also
generate observable converted phases as demonstrated along the
southern Sierra Nevada front at Long Valley Caldera (Wilson et al.,
2003; Zandt et al., 2003; Steck and Prothero, 1988; 1994) and in the
Coso geothermal area (Wilson et al., 2003). In some cases, converted
phases from partial melt bodies have been effectively used to
determine seismic velocities within the partial melt region through
forward modeling of synthetic seismograms (Wilson et al., 2003; Zandt
et al., 2003)
In addition to basic receiver function analysis, we have designed the
passive seismic network to enable the application of a new
teleseismic depth migration scheme based on the shot-profile
formulation of wave equation migration (Wilson et al., 2004; Shragge
et al., 2005). The wave equation depth migration algorithm is well
suited to areas with complex velocity variations (e.g. sub-salt or
sub-melt imaging) and for multi mode imaging using forward-scattered
converted phases and back-scattered, reflected and converted phases.
111
We will use a combination of the wavespeed models derived from the
explosive and earthquake source travel times for three-dimensional
depth imaging for all imaging modes available in the teleseismic
wavefield. The images produced through depth migration will provide a
broad scale view of crustal structure across the transition from the
Sierra Nevada to the Basin and Range and will also help to locate
crustal melt reservoirs beneath the study area.
w
Education and Outreach
Nevada K-12 Network: The Nevada K-12 Seismic Network is a nationally unique program that
seamlessly integrates data collected from low-cost seismographs located in a small number of Nevada
schools (http://www.seismo.unr.edu/k12network) with existing Nevada regional seismic network
operations and the data at the IRIS DMC. Basic hardware for the network has been funded through the
Nevada Public Agency Insurance Pool (rural Nevada) and the Department of Energy (Las Vegas); it has
been underutilized for education and outreach due to limited funding. This network is a component of the
U.S. Educational Seismic Network (http://www.indiana.edu/~usesn). The Nevada K-12 network provides
an established communications network (network in terms of real-time technologies as well as personal
relationships) for public participation in EarthScope outreach activities on a number of levels. The
technologies of the Nevada K-12 network integrate live data from diverse sources, and this can include
USArray data and any data available through EarthScope. The incoming live feed creates a real-time
national and global perspective of active tectonics for Nevada students. Students can view the waveform
data, locate earthquakes, map locations, filter time-series data, import standard seismic data formats, and
control displays, in a robust and professional, yet user-friendly, interactive data-management
environment.
From the scientific perspective, K-12 students on the network have already supplied important data. For
example, the Nevada K-12 network stations at Virginia City High School, Douglas County High School,
and Storey County High School have contributed waveform data for locating the deep crustal swarm of
earthquakes beneath Lake Tahoe, an unprecedented sequence described in Science (Smith et al., 2004).
We propose to: 1) conduct a workshop for rural Nevada K-12 network schools (one workshop per year to
be conducted in Reno), 2) make site visits to rural schools for presentations on EarthScope to students,
and 3) perform basic site maintenance and upgrades on existing K-12 network stations. The workshop
will be based on the successful March 2004 workshop that was conducted in Las Vegas for 24 teachers
and high-school students from that area. Training in use of the PC networking software and basic
information on Nevada earthquakes, seismic hazard and the EarthScope program were provided. Basic
interpretation of seismograms and concepts of real-time systems and data management were also
presented. Each participant operated real-time seismic software during the one-day workshop. An
evaluation form was distributed and compiled to assess the workshop results. IRIS supplied EarthScope
materials that were distributed.
NSL Hallway Displays: The NSL has recently undertaken, with the aid of non-NSF funding, to upgrade
its hallway displays for the public. The NSL hosts nearly 2000 K-12 students per year on tours of our
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seismological lab, raising their awareness of earthquake hazard and of seismology in general. The new
displays will greatly enhance the experience with audio-visual clips of various aspects of earthquake
safety and of seismological investigations. Prominent display screens will be able to present seismic data
not only from the lab’s network but from the entire world. We propose to add data feeds from several
more stations in the Lake Tahoe area to these displays. We will also emphasize our work on this
EarthScope project in these tours by showing maps of the layout of the seismic experiments and showing
data collected in these experiments.
Educational Component: This project will help to achieve EarthScope educational goals in several
ways. Firstly, a post-doc and two graduate students will be supported by this proposed project. In
addition, many undergraduates will be used in the instrument deployment for the active seismic
experiment. These students will be trained on seismological equipment, field deployment methodology,
and rudiments of seismology. Their participation in an actual seismic experiment will expose them to
careers in geoscience and to one important type of experimental work done by geoscience professionals.
Time-Table
A time table incorporating the basic elements of this proposal is shown here. In the first year, we will
start the permitting process for the two field experiments. We estimate that this will take 3-6 months for
the passive and a year for the controlled-source part. We will deploy the passive experiment in the first
half of the first year, and run it for over a year (i.e., into the second half of the second year). The
controlled-source experiment will be executed in the second year and will be concurrent with the passive
recording time period. The final half-year of the project will be for data analysis and synthesis.
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, and the second year will be for the execution of the survey and 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. We have been successful in
previous years in getting a large number of volunteers to participate.
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The planning and execution of the passive tomography experiment and associated analysis will be the
responsibility of John Louie and Leiph Preston. Analysis of the data will be concentrated in the second
year of the project. Rasool Anooshehpoor will be supervising the deployment and maintenance of the
passive seismic array and data collection. He has extensive experience in deploying portable seismic
stations, having done so for every moderate to strong earthquake in Nevada and adjacent eastern
California since 1990. He has also been involved in the installation of the ANSS (Advanced National
Seismic System) stations in the urban centers of northern and southern Nevada. This component will
consist of permitting, obtaining instruments, deployment, data-gathering, and pick-up. Permits will be
sought in the first few months of the project so that a spring-summer 2005 deployment can be achieved.
We have already approached the USFS, in whose domain most of the stations will lay, concerning a
“blanket” permit for the sites.
The relocation of the deep-crustal earthquake swarm, and related shallow earthquakes, will be the
responsibility of David von Seggern. This aspect of the work is naturally performed near the beginning of
the project, as all necessary data is already available. Near the end of the project, the relocation effort will
be revisited with improved velocity models for the Lake Tahoe area.
The development of receiver functions will be the responsibility of Charles Wilson, who will also use the
teleseismic event recordings from the experiment to create scattered wave images of the crust beneath the
passive array.
The E&O component of the proposal will be carried out mainly by Ken Smith. He has already
established a leading-edge K-12 seismograph network in Nevada and is well versed in outreach activities
to schools, and he has established many contacts that will enable the E&O component to be successfully
implemented.
Intra-project communication will be handled by email and 2-3 meetings per year, generally associated
with annual meetings that we already attend (AGU, SSA). In addition, the PI’s Louie and Snelson are
often at each other’s respective campuses, affording additional contact points. The data will be modeled
by both UNR and UNLV and shared among the various investigators for integration purposes.
Externally, we will organize a special session at AGU (and possibly GSA) to show the results to the
larger community, present results at national meetings and at EarthScope meetings, and publish in peerreviewed journals.
Data Handling and Submission
Controlled-source and passive array data will be delivered immediately, as it becomes available, to the
IRIS DMC; a data report will be delivered within 2 years of acquisition of the data. Since PASSCAL will
handle submission of the data to the IRIS DMC, we do not have a significant requirement for graduate
student time in this respect. Investigators at UNR are familiar with CSS3.0/Antelope databases through
nearly five years of network operations based on Antelope, and they will be using these throughout the
experiments.
Results From Prior NSF Support
Results From Related Previous NSF Support of John Louie:
NSF EAR-00-01130: Evolution of the Sierra Nevada - Basin and Range boundary — tephrochronologic
and gravity constraints on the record in Neogene basin deposits,
6/2000-5/2002 for $55,182 between 3 PIs, Louie 2 weeks.
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Geophysical and tephrochronological investigations in the Gardnerville Basin, western Nevada show that
the deepest part of the basin is more symmetric than previously thought, perhaps due to a west-dipping
normal fault on the east side of the basin, active in Tertiary time. Small basins separated by
accommodation zones developed above the Genoa normal fault during Miocene-Pliocene motion.
Selected presentations:
P. Cashman, J. Trexler, T. Muntean, J. Faulds, J. Louie, G. Oppliger, R. E. Abbott, and M. Clark, 2003,
Neogene tectonic history of the Sierra Nevada - Basin and Range transition zone at the latitude of
Carson City, Nevada: geological and geophysical evidence: presented at Geol. Soc. Amer. Ann.
Mtg., Seattle, Nov. 2-5.
W. Thelen, J. Louie, M. Clark, and J. Scott, 2003, Geophysics and paleoseismology: the signature of
active faults in the Great Basin: poster presented at the XVI INQUA Congress, July 23-30, Reno,
Nevada.
Internet Dissemination: Links to presentations and data are at:
www.seismo.unr.edu/ftp/pub/louie/gardnerville/
Contributions to Education and Human Resources: This project provided practical and research
experience to undergraduate and graduate geology and geophysics majors, and funded the B.S., M.S., and
Ph.D. thesis research of five students.
NSF EAR-97-06255: Geophysical test of low-angle dip on the seismogenic Dixie Valley fault, Nevada,
9/1997-8/1999 for $91,313 between 3 PIs, Louie 2 weeks.
We conducted a 3-km-long seismic-reflection survey in Dixie Valley, central Nevada, with 125 shot
points. The reflection survey, along with shallow seismic refraction and reflection, gravity, and
electromagnetic measurements, tested whether the 1954 Dixie Valley Earthquake (Ms=6.8) produced slip
on a low-angle normal fault. Results show that earthquake slip took place along a fault plane of unusually
low dip (25-30 degrees). This is the first large historical earthquake for which slip on a low-angle normal
fault has been documented.
Publication: Abbott et al., 2001.
Internet Dissemination: Links to presentations, data, and publications are at:
http://www.seismo.unr.edu/ftp/pub/louie/dixie/
Contributions to Education and Human Resources: This project provided practical and research
experience to undergraduate and graduate students, helping several to gain employment, and funding
Ph.D. thesis research.
Results from Related Previous NSF Support of K. D. Smith:
NSF EAR-99-03200--Processes of active deformation and slip transfer along the Sierra Nevada-Great
Basin boundary zone in the Lake Tahoe basin, with R. A. Schweickert, M. M. Lahren, and R. Karlin.
Awarded $221,000 for three years, 8/1999- 10/2002.
The study integrated geological and earthquake observations of the faults, seismicity, and seismotectonics
of the Lake Tahoe Basin. Fault studies were conducted supplementing new bathymetry data from the
lake bottom to access the slip rates, fault extent and seismic hazard at Lake Tahoe. Microearthquake
deployments were conducted to constrain the sense of motion in different tectonics regimes around the
basin and to constrain the dip and activity rates on the Lake’s major normal faults. Several presentations
were conducted by the investigators at GSA meetings as well as presentations to numerous local groups at
Lake Tahoe outlining the structures, particularly on the lake bottom, and the seismicity of the lake area.
A fault map was generated that is currently available from the Nevada Bureau of Mines and Geology. A
paper describing the results of the study and the tectonic model (Schweickert et al., 2004) has been
accepted for publication in Tectonophysics.
Refs added
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Oppenheimer, D.H., and J.P. Eaton (1984). Moho orientation beneath Central California from
regional earthquake travel times, J. Geophys. Res. 89, 10267-10282.
Biasi, G.P. and E.H. Humphreys, P-wave image of the upper mantle structure of central
California and southern Nevada, Geophys. Res. Lett., 19, 1161-1164.
Dueker, K., E. Humphreys, and G. Biasi (1993). Teleseismic imaging of the western Unites
States upper mantle structure using the simultaneous iterative reconstruction technique, in
Seismic Tomography: Theory and practice, H.M. Iyer and K. Hirahara, eds., Chapman and Hall,
London.
Hammond, W. and E. Humphreys (2000). Upper mantle seismic wave velocity: effects of
realistic partial melt geometries, Journal of Geophysical Research, 105, 10975-10986.
Karato, S. and H. Jung (1998). Water, partial melting, and the origin of the seismic low velocity
and high attenuation zone in the upper mantle. Earth and Planetary Science Letters 157, 193207.