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1

Imaging Extensional Versus Strike
-
Slip Tectonics

in the Northern Walker Lane


John Louie, Graham Kent, and Kenneth D. Smith

Nevada Seismological Laboratory, University of Nevada, Reno

Patricia Cashman

and James Trexler

Dept. of Geological Sciences and Engineering, University of Nevada, Reno


PROJECT DESCRIPTION


TABLE OF CONTENTS

Introduction and
Intellectual Merit

................................
................................
...........................
1

Proposed Work


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................................
................................
.............................
6


Seismic
-
reflection surveying

................................
................................
................................
......
6


Microearthquake (MEQ) recording

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................................
.........................
1
1

Expected Results and Broader Impacts


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................................
..................
1
1

Compliance with EAR Data Policy
................................
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...........................
1
4

Results of Previous NSF support in the last 5 years: G. M. Kent

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.........
1
4

Results of Previous NSF support in the la
st 5 years: J. H. Trexler and P. Cashman

.........
1
5


Introduction and
Intellectual Merit

The Walker Lane, a zone of complex faulting in the western Great Basin adjacent
to the Sierra Nevada, currently accommodates 20%
-
25% of the Pacific
-
North American
plate relati
ve motion (e.g., Dixon et al., 1995; Thatcher et al., 1999), and may be an
incipient intracontinental transform fault (Faulds et al., 2005a, b; Faulds and Henry,
2006). Accommodation of the plate boundary shear changed with the opening of the
Gulf of Cali
fornia about 6 Ma (Oskin and Stock, 2003). The Walker Lane has been
divided into structural domains characterized by (1) east
-
northeast
-
striking faults with
left
-
lateral displacement; and (2) northwest
-
striking faults with right
-
lateral
displacement (Stew
art, 1988) (Fig. 1).
T
he Walker Lane (as traditi
onally mapped)
intersects the Si
erran frontal system
of normal faults
in

the vicinity of Reno, Nevada.

The Neogene sedimentary basins of northwest Nevada and northeast California record
the development of intra
-
plate deformation along the eastern side of the Sierra Nevada

(e.g., with geometries as estimated geophysically by
Saltus

and Jachens
, 1995).

The e
arthquake hazards presented to the people and the economy of the Reno

Tahoe

Carson City urban area crucially depend on fault length, segmentation, and
event mechanism. Among the swarm of faults cutting the region (fig. 1), which present
the greatest hazard
?
Strike
-
slip faults spanning several basins may be expected to
rupture in longer segments and larger events than
geometrically complex
normal faults
that are contained to the boundaries of one basin

(Wesnousky, 2008)
.
Our lack of
knowledge of the region’s

tectonic framework, and how faults may or may not link up
to break in large events (up to Richter magnitude 7.5),
call into question any attempts to
assess the earthquake hazard or risk.


2

The Genoa fault
(
Pease, 1979
)
and the Mount Rose fault zone

(Sawyer,

1999)

form
the Sierra range
-
front fault between Carson
Valley and Reno (fig. 1).
These two north
-
striking faults are poorly characterized with respect to their components of right
-
lateral
versus normal displacement history and future
potential.
The Genoa
fault is a norma
l
fault that is thought to have
a minor component of dextral motion (Pease, 1979;
Surpless, 1999).
With
its

curved trace, tilt fanning in the Neogene section,
and
largest
displacement in the middle

section, the Genoa fault itself must have
almost exclusively
dip slip.

Total vertical offset,

based on gravity, is on the order of 3.2 to 3.7 km

and has
been on
-
going since at least Pliocene time, based on tilt fanning of the 2 Ma
-

7 Ma
Neogene section
(
fig. 2;
Cashman et al., 2009)
.
A 700
-
m
-
long seismic reflection survey
hinted at the depths and dip of major stratigraphic transitions in the center of the basin
(fig. 3), but was too limited in extent to fit the reflections into any structural framework.
Trenching stud
ies document two l
ate Holocene,
large
-
displacement events
on the
Genoa fault
(Ramelli et al., 1999).


Figure 1: (left
)
Regional location map

taken from Cashman et al. (2009)
, showing the Neogene
basins along the Sierra Nevada

Basin and Range transition zone. Map modified

fr
om Cashman
and Fontaine (2000).
Light gray is the Walker Lane

proper
; dark gray is outcrop areas of
Neogene sediment
ary rocks.
Buried Neogene sedimentary rocks mo
re extensive than these
surface
exposures. “Northern Walker Lane”, as used herein, contains

the
Pyramid Lake
domain

(characterized by nor
thwest
-
striking dextral faults) and

Carson domain

(cha
racterized
by
northeast
-
striking sini
stral faults
)
, and part of the

Walker Lake domain

(characterized by
northwest
-
striking

dextral faults)
, of Stewart

(198
8)
;
and is
extended here to include the basins

3

east of Lake Tahoe
.

(right) Google map of the northern Walker Lane showing

fault
s

from the
USGS Qfaults database (USGS et al., 2006
; fault age keyed to
line
color

with warmer colors
more recent
),

the locations of the TRK and MNZA reflection profiles of Frary et al. (2009; 2010)
as white lines, the paleoseismic trench and
high
-
resolution profile presented by Kell
-
Hills et al.
(2010b)

at the yellow pin by MRFL
, and the two
reflection

survey routes p
roposed here across
the South Reno basin and Carson Valley, in pink.


Ramelli et al. (1994) explored selected Mt. Rose fault traces and evaluated the
potential for past ruptures, also proposing some of the rupture to be dextral strike slip.
This fault syst
em, on the south end of the Reno
-
area basin, is enormously complex with
a large number of short segments of unknown age and activity (fig. 1).
A similar swarm
of short, straight fault traces can also be seen on the eastern side of Carson Valley in fig.
1.
Whether these faults might generate normal or strike
-
slip events is unknown. Since
these traces could bring rupture
from the Genoa


Mt. Rose system
into downtown
Reno, it is important to clarify the nature of their activity.



Figure 2:
Gravity

profile and geology model along the proposed Carson Valley reflection line (fig.
1). The
two
-
dimensional modeling assumed basement

rocks as Mesozoic plutonic and
metamorphic units averaging 2.75 g/cc, upper basin

fill as Quaternary and Neogene
sediments
a
veraging 2.3 g/cc, and deeper basin fill as sediments averaging 2.4 g/cc.

Note that the fanning
of dips in the Neogene section (angles shown above cross
-
section) record progressive tilting
along the basin
-
bounding fault through deposition (approx. 7

Ma
-

2
.5 Ma).

Taken from
Cashman et al. (2009).


4


Active seismicity fails to confirm the preference for normal
-
slip on the Genoa


Mt. Rose fault system. Most of the recorded seismicity

has been sho
wn to be strike slip.
Studies of Tahoe
-
area tectonics (Schweicker
t et al., 2004) and the
Double Springs Flat
earthquake (Ichinose et al., 1999)

conclud
ed that the major normal faults
showed little
se
ismicity, with strike
-
slip f
aulting in the step
-
overs between the normal
-
fault systems
dominating the
activity along the e
astern Sierra.

Recent observations from the Reno, Nevada area from a new paleoseismic
trench, and new seismic
-
reflection surveys show active tectonics that are hard to fit into
this accepted framework for the Walker Lane region. The n
ew trenching and shall
ow
seismic imaging of
the obvious range
-
front

fault in the Mount Rose zone shows
a low
-
angle
normal
fault (fig. 4)
.

This surprising result, presented by Kell
-
Hills et al. (2010b),
followed paleoseismic trenching of the fault (at the MRFL on fig. 1) in Sept
. 2009 by A.
Sarmiento and S. Wesnousky. This discovery, proved by trench observation coupled
with a Nov. 2009 high
-
resolution seismic study, demands new thinking about the
tectonics of the Genoa fault system as it approaches Reno from the south.


Figure
3: Short r
eflection profile recorded
near the middle of the proposed

Carson Valley

profile
(fig. 1), interpreted at right.
This unmigrated stacked section is plotte
d at approximately 1:1
vertical
exaggeration for the velocities found by NMO analy
sis. The i
nterpretation (right)
identifies three west
-
dipping reflections as deep as 1000 m.
Their dips increase with depth and
age, consistent with the fanning of dips seen in surface exposures (Fig. 2).
At the left are the
depth, stacking veloci
ty and dip interpre
ted for each
reflection
.
Loss of fold near the

ends of this
very short (720 m)
profile allows interpretation only within its center.

Taken from Cashman et
al. (2009).



5

The

observations

of Cashman et al. (2009) and Kell
-
Hills et al. (2010)

point to the
need

for seismic
-
reflection imaging of the two principal Tertiary
-
to
-
Quaternary
sedimentary basins associated with the Genoa fault and the Mount Rose fault zone, to
characterize the disp
lacement histories of these faults. We propose to conduct two full
-
scale s
eismic
-
reflection studies of the basins along two west
-
east routes about 7 km long
each (fig. 1).
The surveys will reveal the
internal
stratigraphy and deformation of these
two

basins, and allow separation of Tertiary from Quaternary displacement histories
.

This investigation of faulting history will test hypotheses of westward
propagation of Basin and Range extension (e.g., Dilles and Gans, 1995; Surpless et al.,
2002), northward propagation of dextral slip related to changes in the plate boundary
configuration (e.g., Faulds et al., 2005
a
), as well as specific questions about fault
geometry and displacement related to accommodation zones between major fault
systems.
Most of the region’s Tertiary basin fill is hidden below Quaternary deposits.
Seism
ic imaging of faulting and stratigraphy within the basins is the only practical way
to acquire additional tests of this hypothesis.
The basins likely record deformation
history since the mid
-
Tertiary, and should show the shift in style from
mid
-
Tertiary

ea
st
-
west extension to the current trans
-
tension
, as proposed by Faulds et al. (2005a)
.

To investigate

faulting in the northernmost of the basins along the Genoa


Mt.
Rose fault system, a
collaboration
between
UNR
, Boise State, the USGS, and
nees@UTexas per
formed minivibe
and hammer
surveys of the
basin below the Reno
urban area under USGS sponsorship (white lines at MNZA and TRK on fig. 1, right).
Frary et al. (2009; 2010) and Louie et al. (2009) showed that the
June 2009
minivibe
surveys
achieved

clear str
atigraphic and fault imaging
despite urban noise. The Reno
-
area basin had previously been characterized from geothermal
-
well and gravity data by
Abbott and Louie (2000) and Widmer et al. (2007), demonstrating that no more than 200
m of Quaternary sediments

overlie up to 1.5 km of Tertiary basin fill.


Figure
4
: velocity optimization and prestack depth migration of high
-
resolution reflection survey
along paleoseismic trench across the Mount Rose fault (at MRFL in fig. 1), presented by Kell
-

6

Hills et al.
(2010b). The low
-
angle normal fault observed in the trench

(shown in yellow)

can be
traced to 40 m depth where it cuts the slip surface of an older landslide

(shown in red)
.


Direct fault imaging
has been

essential in the Reno
-
area

basin, to see faults that
do not offset basement significantly. Fig. 5
from Kell
-
Hills et al. (2010a) shows one of
hundreds of

raw shot records from
the
MNZA

line
exhibiting negative
-
moveout
reflections, and a
section of our
imaging to date

that may be s
h
owing fault
-
plane
reflections. The negative
-
moveout reflections in the shot records appear to be sidewall
bounces of direct energy off the shallow fault zone (one of the Mt. Rose strands). Such
strong sidewall reflections have been observed rarely, such as

in COCORP shot records
across the San Andreas fault at Parkfield and across the Garlock fault, in California.
Louie et al. (1988) imaged the San Andreas reflections as steeply dipping structure
bounding the active fault zone, to 3 km depth.
Louie and Qin
(1991)

imaged the steeply
dipping walls of a transtensional pull
-
apart basin along the Garlock fault. On the
MNZA line, the sidewall reflections migrate into east
-
dipping structures at either 70° or
30° dip (Kell
-
Hills et al., 2010a). Additional analysis w
ill tell which dip is more likely for
this fault, and thus whether it is likely to have much of a strike
-
slip component of
motion. A strike
-
slip component is unlikely for a shallow
-
dipping normal fault. We
conducted further work with L. Liberty of Boise St
ate in March 2010, doing high
-
resolution hammer reflection surveys in the area of the fault sidewall reflections along
the MNZA line. Preliminary results (not shown) confirm the locations of possible faults
seen in June 2009, and the depth of the volcanic
flows at the bottom of the sediments.

Figure 6 shows the result of first
-
arrival picking, velocity optimization, and
prestack depth migration by Kell
-
Hills et al. (2010a) on a section of the TRK lines, in
downtown Reno (fig. 1)
.

Clearly imaged interruptions of east
-
dipping stratigraphy,
right up to the surface, suggest west
-
dipping normal faulting accompanied by eastward
tilting in the Quaternary and older section. This is consistent with surface exposures
and gravity models
west
of Reno (
Bell and Garside, 1987; Trexler et al., 2000; Abbott
and Louie, 2000;

Trexler and Cashman, 2007
), but is the opposite of fault
-
related tilting
in Carson Valley (Cashman et al., 2009).
However, such a significant and recent west
-
dipping normal faul
t does not fit well with traditional thinking that the Genoa


Mt.
Rose system is the range
-
bounding fault on the east side of the Sierra Nevada. Reversals
in the sense of subsidence from east to west along this fault system point to more
complex
tectonics
.


7


Fig.
5
: (left) Correlated shot record from the east end of the MNZA survey

(fig. 1)
, showing
possible

linear
sidewall reflections originating at mapped faults
, similar to those see by Louie et
al. (1988) from the San Andreas fault at Parkfield, Calif
.

(right) Preliminary velocity
optimization and prestack depth migration (PSDM) results from this part of MNZA. The
vertical scale is depth in meters in this 1:1 section. East is to the right. From Kell
-
Hills et al.
(2010a).

The basin floor of Tertiary volc
anics is cut and displaced by a fault in the northern
segment of the Genoa


Mt. Rose system. Advanced imaging has located two possible downdip
paths for the fault, at either 30° and 70° dip, leaving open the question of whether this important
fault has pr
edominantly dextral strike slip and is steeply dipping, or predominantly normal slip
at shallow dip. Presented by Kell
-
Hills et al. (2010a).


Proposed Work

Seismic
-
r
eflection
s
urveying


Optim Inc., a

seismic acquisition firm with an
established record of basin and fault imaging in Nevada will perform
14
.8 line
-
kilometers of innovative 2D

surveying.

The UNR group, together with Honjas and
Pullammanappallil at Optim, have been directly imaging faults in

the western Great
Basin for more than a dozen years. Honjas et al. (1997) performed 2d velocity
optimization from tens of thousands of first
-
arrival picks on about a thousand prestack
reflection records in a grid of seismic lines covering the northern Dix
ie Valley
geothermal field. The direct imaging showed the existence of basinward step
-
faults,
hidden from the surface by younger alluvial deposits. Chavez
-
Perez et al. (1998) applied
the same analysis to existing COCORP data across Death Valley, imaging th
e Black
Mountains range
-
front fault and quantifying its heave. Abbott et al. (2001) conducted a
reflection survey in southern Dixie Valley, directly imaging the shallow
-
dipping 1954
M7.2 rupture below up to 1 km of alluvium using the same combination of ve
locity
optimization and depth migration. Louie and Pullammanappallil (2007) and Louie et al.
(2008) reviewed that work and added the 2003 shallow imaging of a low
-
angle normal
fault in the west Ruby Mountains. Louie et al. (2007) elucidated these, plus muc
h
additional Optim work, for the Nevada geothermal industry.


8

Figure
7
, an example of commercial Optim analysis taken from Louie et al.
(2007), shows how powerful the combination of a
industry
-
scale reflection survey with
a

high
-
quality

1st
-
arrival pick set
, velocity optimization, and PSDM can be. At this
Nevada geothermal prospect, only the range
-
front fault is manifested at the surface. The
other normal faults are blind and hidden by younger alluvium. Optim achieved clear,
direct fault
-
plane reflection ima
ges of these faults despite their not significantly
offsetting the basin floor, nor being associated with significant lateral velocity changes.

The interior stratigraphy of the alluvial fans is also imaged.

The faults are located with
the most possible con
fidence for seismic data: within one lateral wavelength, rather
than within a much larger Fresnel zone that would have to be allowed if the image was
of lower quality. In most geothermal prospects in central Nevada, the geothermal
resource is controlled by

these hidden faults, and it is crucial to locate and drill them.



Figure 6: PSDM of part of the TRK1 reflection line in downtown Reno, with layered Quaternary
and Tertiary stratigraphy cut and tilted by possible west
-
dipping normal faults, from
Kell
-
Hills
et al. (2010a).

The west dip of the basin fill is consistent with the west
-
dipping Neogene section
exposed along the east flank of the Carson Range, west of Reno
; but it opposes the east dips found
throughout the Genoa


Mt. Rose fault systems,
aligned with these faults to the south
.


In the first project year Optim will survey a
7.2
-
km transect across the south
Reno basin to characterize the history of the Mount Rose fault zone, and an
7.6
-
km
transect across Carson Valley to investigate the Geno
a fault

(pink lines on fig. 1, right)
.
These routes cross the full width of both basins where Tertiary sediments and volcanics
are obscured below
Quaternary sediments. This coverage will allow the most active
basin
-
bounding faults to be imaged, as well as
the hidden Tertiary stratigraphy and
structure.

Both surveys will deploy
at least two
industry
-
sized heavy vibrators
. Each will
have a standard 60,000
-
lb (27
-
ton) hold
-
down weight with a 4
-
ton reaction mass, for
vertical forcing. As well, we will use a

new design of wireless, portable 3
-
component
(3C) geophone recorder
s. Geophones will extend along each profile at 110
-
ft (33.5
-
m)

9

intervals. All 220+ 3C geophones along a line will record every vibrator location. This
procedure assures the longest offsets

and deepest
-
penetrating
1
st
-
arrival information
and velocity constraint. This full
-
offset recording technique is used
by Optim for
geothermal exploration,
as in fig. 7, with excellent results.

A full suite of field tests will
be run at the beginning of each line before the walk
-
through
, to refine the parameters
beyond the typical values given here. If data quality begins to deteriorate along a line,
we will run further tests before proceeding.




With the dense,
660+ channels of
2D
-
3C data

on each line
, P
-
velocity
optimization analyses by the Seismo Lab group will extend to recover S
-
velocity and
Poisson’s ratio sections on both lines, and use the velocity sections to derive prestack
depth
-
migrated cross
-
section images of P
-
P, S
-
S, as well as P
-
S and
S
-
P converted
-
wave
reflections. Processing and imaging will be done in the first and second project years by
PIs Louie and Kent, and the graduate student at UNR, with consultations to
Pullammanappallil at Optim. The data
-
analysis procedures will include:


a.

Pi
cking of vertical and horizontal first arrivals

from the correlated and summed 3C
vibrator records. Following Pullammanappallil and Louie (1994), this will be the
essential data set allowing complete characterization of lateral velocity variations in
Fig. 7: 2d seismic
-
reflection

(wiggle traces)

and velocity
sections

(colors)

from a Nevada geothermal prospect,
showing direct detection and imaging of hidden faults
below the basin floor. The data were collected and
processed with velocity optimi
zation and prestack depth
migration (PSDM) by Optim. From Louie et al. (2007).


10

the u
pper portion of the basin, where they are strongest. It is anticipated that 300,000
traces will need to be picked for their first arrivals. The graduate student supported
by this project will do most of this work over a period of 2
-
4 months, using the most

convenient of the ProMAX, SPW, OpendTect, or JRG seismic software pl
atforms that
we have available.

b.

Development of 2d optimized velocity models

from the
P and S
1st
-
ar
r
ival picks of the
prestack data. This task will be completed
in collaboration with

subc
ontractor Optim
using their proprietary SeisOpt® technology, an extensively tested and industry
-
vetted implementation of the simulated
-
annealing travel time optimization of
Pullammanappallil and Louie (1994), making use of finite
-
difference travel times
(e
.g., Vidale, 1988). This nonlinear optimization assures the best possible
representation of lateral velocity discontinuities. No assumptions are made about
the form or smoothness of the discontinuities. If insufficient depth coverage of the
optimized veloc
ities from the 1st
-
arrival time picks results,
we

will conduct
additional optimizations, including for instance prestack reflection coherency in the
objective function (Pullammanappallil and Louie, 1997).

c.

Preprocessing and filtering of the prestack records

will be performed by
the PIs and the
graduate student
in collaboration with

contractor Optim
. Given the
ex
urban setting
of the surveys, and the use of a single geophone per recording channel rather than a
geophone group array, the prestack data records
will likely be

dominated by strong,
low
-
velocity surface waves. Data quality will thus profit from application of the
Hale and Claerbout (1983) Butterworth dip filter, as did the direct fault imaging of
Kanbur et al. (2000) at the Upheaval Dome, Utah impac
t structure.

d.

Prestack depth
migration (PSDM)

of all the prestack reflection records through the
optimized velocities will bring the steeply dipping fault
-
plane reflections into focus
at their true locations. The use of finite
-
difference travel times (e.g.
, Vidale, 1988)
and properly assessed lateral
-
velocity variations from Optim’s SeisOpt® results will
allow the proper placement of reflection depth points along even strongly curved
raypaths. Optim and the PI
s

and graduate student at UNR will collaborate o
n this
step as well. We will include migration operator antialiasing control (Lumley et al.,
1994), and possibly Bayesian signal/noise separation

(Harlan et al., 1984). These
enhancements were employed in PSDM work by Kanbur et al. (2000) and Louie et
al.
(2002), as well as by Optim for the numerous geothermal imaging examples
shown in Louie et al. (2007), such as in figure
7
.

e.

The PIs and graduate student will interpret the PSDM images for quality and accuracy
,
comparing for instance basin depths derived fr
om Abbott and Louie (2000) in south
Reno and from Cashman et al. (2009) in Carson Valley against the major impedance
contrasts shown by the PSDM. Additional processing will be done as necessary. We
will identify and interpret direct fault images within the

sections (as suggested in
fig. 5), making sure fault interpretations are internally consistent with models of
basin structure.

f.

As in

Louie et al. (2002), we will produce mu
lti
component images of P
-
P, S
-
S, P
-
S, and S
-
P
reflections from each of the three
components of recorded seismograms.

Three components

11

of data times four components of images will yield a total of twelve component
cross
-
sectional images for each survey line. Comparison of a reflector’s response on
each of the component images will allow

an assessment of physical properties at the
reflective interface. For example, both a wedge of porous sediment caught between
hard volcanic flows, and an isolated hard volcanic flow within sediment will
produce strong S
-
S reflections. The sediment wedge w
ill also produce strong P
-
P
reflections, while an isolated flow may not. We hope to correlate multicomponent
reflector response against geological information such as the location of the
boundary between Tertiary volcanics and hard Tertiary sediments, and
the
boundary between Tertiary and Quaternary sediments. As well, the
multicomponent images will help us distinguish between fractured fault zones and
dipping sedimentary boundaries. Such distinctions are hard to make in the single
-
component images recorded

by the USGS, as in fig. 5 (right).

g.

PIs Cashman and Trexler will interpret the seismic sections produced by Louie, Kent, and
the graduate student to produce a comprehensive geologic interpretation of each line,
accounting for all known geophysical and geol
ogical data sets
, including the MEQ study at
the south Reno line
.

Trexler will analyze the internal stratigraphy of the Neogene
and Quaternary basin fill, correlating units in the seismic records with known (and
locally dated) facies in surface exposures a
nd well logs. Cashman will interpret the
Neogene
-
present deformation history of each basin, focusing on fault sense and slip
history. Both of these can be constrained by known surface faults and by the tilt and
offset histories recorded in Neogene and Qu
aternary sediments.
This comprehensive
interpretation will address the following questions: the sense, amount and age of
offset on the basin
-
bounding and the intra
-
basin faults; the existence of major west
-
dipping fault systems along the east flanks of the

south Reno and Carson Valley
basins;
the reason for the dip reversal between the west
-
dipping Neogene section in
Carson Valley and the east
-
dipping Neogene section in the Reno basin;
the existence
of previously unrecognized faults in the subsurface; the p
otential continuity of a
major fault or faults between segments of the Genoa


Mt. Rose fault system; the
presence and nature of any accommodation zones
(Faulds and Varga, 1998)
linking
major fault segments; and the thicknesses
and deformation

of Tertiary,

Quaternary
,

and younger basin fill.

h.

The geologic interpretation will be iterated

in the sense that all models produced will be
checked for consistency with known geophysical and geological constraints. The
MEQ mechanisms

developed by Smith

and
geologic in
terpretations by Cashman
and Trexler will feed back into
Louie and Kent’s

interpretation of the
multicomponent
seismic data, and possibly into the processing. All of these
iterations will be conducted as an interdisciplinary collaboration.

i.

The PIs
and the
graduate student
, starting with the geologic interpretation,

will make an
assessment of the reflection response of Tertiary versus Quaternary faults

from the
multicomponent images
, after separating the two sets of faults in the interpretation.
The
PSDM and

direct fault imaging gives us the rare opportunity to examine the
physical properties of the buried faults. Some questions we hope to collect data on

12

include: Are any of the faults simple lateral velocity boundaries? Are they low
-
velocity zones? Do they s
how a reflection signal consistent with increased porosity?
Decreased porosity and work hardening?

Are there systematic differences in
physical properties between faults active in the Tertiary, but not later, and those
currently active?

j.

The PIs

and the gra
duate student
, starting with the geologic interpretation,

will also make an
assessment of fault reflection response versus vertical fault offset,
where total offset can be
assessed from stratal offsets across the fault. We will look for any correlations
be
tween fault age, offset, depth, and other factors against reflection attributes such
as amplitude, phase, and frequency.
Multicomponent
AVO (amplitude versus offset
or incidence angle) studies of the fault reflections may be possible.

Microearthquake (MEQ)

recording


To assess and locate microearthquakes along
the Sierra Nevada range
-
front fault between two urban geothermal areas, about a dozen
L4
-
3C seismometers will be deployed over a small region for 18 months

(fig. 8)
. This
would qualify as an EarthScop
e/USArray Flexible Array deployment
, with data
telemetered in
real time to the Nevada Seismo Lab, and then streamed to the IRIS
-
DMS
.
MEQ r
ecording will be in concert with
the

pair of seismic reflection lines across the
two
Sierra Nevada range
-
front
basins.
Station geometry will be optimized to most
effectively take advantage of the existing permanent network.
MEQ recording
will help
the reflection imaging identify and characterize faults and tectonic style affecting the
Neogene basins.

The N. Tahoe
area extending into the southern Truckee Meadows
produces regular small magnitude events (Ichinose et al., 1999).

PI Smith and his technical staff will be responsible for the MEQ recording and
interpretation.

UNR
will contribute

at least
two stations
,

and
8 to 12

RAMP

station
setups

will be requested from
PASSCAL
. UNR

instruments
may be deployed in
advance of project funding
.

Fig. 8 shows existing stations as yellow and red dots, and
seven blue dots, indicating the region across which the RAMP stations may
be
deployed.

The two broadband stations in the area of the south Reno reflection line (red dots
on fig. 8)
are not expected to contribute significantly in contraining local MEQs
. Station
WCN may be relocated

before the project begins

(the site Considering
BMHS on thick
sediments, Figure 8 illustrates a lack of adequate controls for event as small as M0.0.
Also, note the closes
t short period stations

(yellow dots on fig. 8)
are nearly 10 km away
from the focus of the
south Reno
experiment
,

and
are limited to
analog
data resolution

(e.g.,

MPK; effectively 10
-
bit dynamic range).

Therefore, we are limited in our ability to
locate small events (M
~
0.
0
) in the
south Reno area. Also, Smith

ran a small portable
instrument deployment for
a geothermal pr
oducer

in 2001 that showed a number of
small events (many shallow), some
right below the proposed south Reno line
.

That
short term experiment showed numerous small magnitude events in the south Reno
area (termed the “UBOAT” experiment from 2000
-
2001; data
have been submitted to
the IRIS data center).
There were also events located at depth just south of the range
front
,

most likely in the Carson Range footwall block
;
and most likely an

extens
ion of N.
Tahoe
-
area seismcity.
The MEQ

deployment will establish
a completeness threshold of

13

magnitude

0.0, or better, producing the most comprehensive view of the micro
-
seismicity, and earthquake focal mechanisms, in this important structural transition
zone along the eastern Sierra. MEQ locations and mechanisms will
a
lso
contribute to
better understanding of the structural controls on local geothermal resources.


Expected Results and Broader Impacts

The nature and activity of the faults along the Sierra Nevada range front are
crucial inputs to an earthquake
-
hazard ass
essment of the Reno

Carson City and Lake
Tahoe urban
areas of Nevada and California. The fault imaging and MEQ results will
allow probabilistic assessment of the hazard presented by each of the many fault
strands crossed by the 14.8 km of survey line. Our
results will show, as well, the
presence of any previously unmapped faults.


14


Figure 8: Northern Nevada seismic network coverage in the project area. The yellow dots are
short
-
period stations most sensitive to small earthquakes. Blue dots show the proposed

microearthquake (MEQ) deployment area.


The possible linking and simultaneous rupture of the Genoa and Mount Rose
fault system provides the longest and most dangerous fault rupture threatening these
urban areas, with event magnitudes of 7.5 a possibility.

Since dePolo et al. (1996) the

15

most hazardous event for the Reno urban basin has been thought to be such a long
rupture into the city from the south. Fig. 9 shows a deterministic prediction of ground
shaking for an M7.5 event on the Genoa


Mt. Rose Fault

system, making the
assumption that all the segments can link into one long rupture (Louie and Larsen,
2007). This scenario injects an unexpected amount of energy into the Reno
-
area basin,
resulting in shaking velocities exceeding 15 cm/s.

The proposed ref
lection surveys and MEQ recording will thus provide critical
hazard assessment, helping to protect the region’s urban population and economy.
Evaluation of the faulting and tectonic history of the basins as proposed here will
provide information on the mos
t important earthquake scenarios affecting the region.
Despite the importance of imaging these fault systems, survey efforts funded by the
USGS will be restricted for the foreseeable future to the denser urban areas. USGS
-
NEHRP funding has been directed to
ward locating the faults within densely populated
areas. Such restrictions will not allow investigation of fault history through the
stratigraphy and structure of the south Reno and Carson Valley basins, which are not
densely urban.



Figure 9:
Development of an M7.5 Genoa


Mt. Rose earthquake scenario using deterministic
physics
-
based methods, from Louie and Larsen (2007). (left) Basin
-
thickness map assembled from
multiple sources including Abbott and Louie (2000) for Reno
-
area details, and
Sal
tus

and
Jachens (
1995
) for the rest of western Nevada. Zero basin thickness is green; dark blue is about 1
km thickness; the Lake Tahoe basin

has an unknown greater depth. (center) Snapshot of
computed
wave propagation at 32 s after rupture initiation

at t
he south end of the Genoa fault
zone
, with strong directivity of 2
-
sec
-
period waves
into the Reno
-
area basin, top center. (right)
Maximum ground
-
motion map for the computed scenario, with a red circle at the epicenter and
red arrow along the straight, nort
hward
dextral
-
normal

oblique
rupture 85 km long

and 15 km
wide
. Computed ground motion of 5 cm/s or greater are yellow, to a maximum of 16 cm/s in the
west Reno sub
-
basin.
Note the
computation

predicts only 3 cm/s ground motion in the other
basins along th
e rupture.


16


In addition, the project will support one graduate student and provide part
-
time
research experience for two or three undergraduate students. The graduate student will
get experience with initiating a project, reflection data collection, PSMD
imaging, high
-
precision earthquake source
-
parameter estimation, supervision of undergraduate
workers, and presentation and publication of results. All of the students will benefit
from participating in the close collaboration between geologists and geophy
sicists in the
interpretation of the results, and from the experience of working with a diverse research
team. These benefits will help to strengthen Nevada’s scientific and technical workforce.


Compliance with EAR Data Policy

All seismic data resulting
from this project will be contributed to the IRIS Data
Management System in accordance with PASSCAL guidelines. A real
-
time feed from
the telemetered MEQ recording stations will be delivered to the DMS and made open
for immediate free access. Event sets
, m
echanisms,

and catalogues will be contribut
ed
within 2 years of recording.

Al
though not recorded with

PASSCAL facilities, all seismic
-
reflection records
developed by this project will be contributed to the IRIS
-
DMS as soon as practical after
data acquisiti
on, and not later than
the end of the first project year
.
Initial delivery will
be in the PIC KITCHEN format, or as requested by IRIS.
Processed products

such as
time picks, velocity models, and the multicomponent images will also be contributed
within 2 y
ears of recording.

Presentation and publication of imaging and MEQ results will be
comprehensive,
with all geophysical results posted in standard electronic seismic data
-
exchange formats on the Western Basin and Range Community Velocity Model website:
htt
p
://crack.seismo.unr.edu/wbrcvm
. At least two presentations on the results will be
made each project year at international professional conferences such as AGU, GSA,
SSA, SEG, or AAPG. Results will also be submitted to peer
-
reviewed journals for
publication.


Results of Previous NSF support in the last 5 years: G. M. Kent

G. M. Kent, A. J. Harding

and S. C. Singh (BIRPS co
-
funded), PIs: OCE
-
9633774,
$997,428 (U.S. amount), 3/1/97
-
2/28/00, The anatomy of a ridge
-
axis discontinuity
(ARAD) 3D seismic

experiment; OCE
-
9911802, $294,297 (U.S. amount), 4/01/2000
-
3/31/03, Amplitude Variation with Offset Studies and Pre
-
stack Imaging of the ARAD
3D Reflectivity Volume.

The ARAD 3D seismic experiment was conducted aboard the R/V Maurice
Ewing during Septembe
r
-
October of 1997 and centered on the 9°03’N overlapping
spreading center (OSC) along the East Pacific Rise.

Key elements of this survey
included: (1) the first 3D reflection survey of a mid
-
ocean spreading center, and (2) a
coincident 3D crustal tomograph
y experiment.

The 3D images of crustal reflectivity and
velocity have provided considerable insight into crustal structure and melt dynamics
beneath this prototypical feature. The observed distribution of crustal magma

17

accumulations beneath this overlapper

appears to be inconsistent with either a simple,
broadly symmetrical structure for the OSC, or with models which depict the limbs of
the OSC as attenuated ends of magmatic systems fed largely by horizontal flo
w of melt
from distant sources (
Kent et al.
, 2
000)
.

3D reflectivity images also reveal the presence of
Moho reflections beneath the melt lens, suggesting the formation of “zero
-
age”

Moho
(
Singh et al., 2006
)
.

Additionally, travel
-
time variations between the melt lens and Moho
reflections suggest the p
resence of considerable amounts of melt distributed within the
lower crust beneath the northern limb of the OSC.

The amplitude variation with offset
(AVO) pattern of magma chamber reflections shows a coincident region of higher melt
fraction overlying this

anomalous lower crustal region, supporting the conclusion of
additional melt at depth.

Of interest, this region of high melt fraction also corresponds
to the location of
a

recently discovered hydrothe
rmal field
.

Thus far, eight journal
articles have been
written describing results from the ARAD 3D dataset, including two
Nature, three Geology, one GRL and two JGR papers.

The latest manuscript was
recently published in Nature (2006) attesting to the richness of this first 3D reflection
dataset of a mid
-
ocean

ridge.

A subset of published articles include:



Singh, S. C., A. J. Harding, G. M. Kent, M. C. Sinha, V. Combier, S. Bazin, C. H. Tong, P.
J. Barton, R. W. Hobbs, R. S. White and J. A. Orcutt
, 2006
, Seismic reflection
images of the Moho underlying melt

s
ills at the East Pacific Rise:

Nature, 442,

287
-
290.

Kent, G. M., Singh, S. C., Harding, A. J., Sinha, M. C., Orcutt, J. A., Barton, P. J., White,
R. S., Bazin, S., Hobbs, R. W., Tong, C. H., and Pye, J. W.,
2000,
Evidence from
three
-
dimensional seismic reflectivity images for enhanced melt supply beneath
mid
-
ocean
-
ridge discontinuities:

Nature, 406,

614
-
618
.


Results of Previous NSF su
pport in the last 5 years: J.

H. Trexler

and P. Cashman

EAR0510915: “Distribution

and Kinematics of Late Paleozoic Deformation from
Southeastern California to Northeast Nevada” was awarded to a collaborative proposal
including Boise State University and University of Nevada Las Vegas, to study the
tectonic history of the upper Paleozoi
c rocks of eastern California, Nevada and Idaho.

This investigation has funded 6 graduate students from the University of Nevada, and
resulted in 3 publications in peer
-
reviewed journals, 9 abstracts at national and regional
meetings, and two MS degrees co
mpleted, one near completion.