International Journal of Geosciences, 2012, 3, 303-313
http://dx.doi.org/10.4236/ijg.2012.32032 Published Online May 2012 (http://www.SciRP.org/journal/ijg)
The Role of Fluids in Promoting Seismic Activity in Active
Spreading Centers of the Salton Trough, California, USA
Musa Hussein, Aaron A. Velasco, Laura Serpa, Diane Doser
Department of Geological Sciences, University of Texas at El Paso, El Paso, USA
Received January 26, 2012; revised February 21, 2012; accepted March 14, 2012
ABSTRACT
We interpret seismic activity in the active spreading centers of the Salton Trough to indicate 1) a magmatic intrusion in
the lower crust beneath the active Brawly, Cerro Prieto, Imperial, Elsinore, and San Jacinto fault systems; and 2) fluids
in the upper crust that have been released from that magmatic body. The absence of a magmatic body and fluids at the
location of fossil spreading centers along the Sand Hill and Algodones faults ndicated by little or no seismic activity in
those areas. We show several lines of evidence to point out that both melt and fluids related to the seismic activity. In
particular, receiver function analysis, Vp/Vs ratios, and tomographic data reveal low velocity zones coincide with the
location of the active spreading centers. High Vp/Vs ratios and low velocity zones in the lower crust and upper mantle
attributed to melt inclusion, while low Vp/Vs ratios in the upper crust are attributed water inclusions. Frequency-mag-
nitude distributions characterized by high b-values in southern California; high b-valu es have also been associated with
crustal fluids. A crustal scale model developed from the receiver functions, gravity, and magnetic data supports the ex-
istence of a magmatic intrusion within about 20 km of the surface southwest of the Salton Sea, that intrusion extends for
70 km in a SW-NE direction.
Keywords: Crustal Model; Magmatic Intrusion; Salton Trough; Seismic Activity; Spreading Centers
1. Introduction
Mantle fluids have been interpreted to be associated with
the San Andreas fault system in central and southern Ca-
lifornia [1]. The most promising source of mantle derived
fluids comes from mantle wedge material, which was po-
tentially hydrated and serpentinized during subduction
prior to the formation of the San Andreas fault and which
would subsequently dehydrate after the plate boundary
along California transitioned from subduction to strike
slip tectonics [2]. Fluids appear to enter the brittle fault
zone at or near lithostatic pressure and thus may contri-
bute to fault weakening because of high fluid pressure at
seismogenic depths [3,4]. Seismicity and tomography su-
ggest that for seismic events to occur in the lower crust,
fluids as well as a rheologically strong lower crustal layer
must be present in order to restrict the rise of fluids.
There has been little consideration of whether ponded
magma and exsolved fluids at the base of the crust could
similarly trigger lower crustal events [5]. One of the few
studies to point out that high pore pressure would be ne-
cessary to trigger such lower crustal events has been that
of [6], who studied lower crustal earthquakes in the East
African Rift and found no difference in rupture characte-
ristics between upper and lower crustal events (based on
mb-Mw relation). This suggests that similar fluid trigger-
ing processes may occur throughout the crust as pro-
posed by [5].
In this study, we examine the relationship between
fluids and seismicity in active spreading centers (Brawly,
Cerro Prieto, Imperial, Elsinore, and San Jacinto) of the
Salton Trough and compare those areas with areas of
little or no earthquake activity in nearby fossil spreading
centers (Sand Hill and Algodones). We used data from
the southern California broadband seismic network and
data collected by the EarthScope US Array seismic de-
ployment for our analysis. We also obtained data from
existing gravity databases at the University of Texas, El
Paso (UTEP) (http://www.research.utep.edu/paces)-Pan
American Center of Earth and Environmental Studies-
(PACES) that is currently hosted at the CYBER-ShARE
Center of Excellence at UTEP. Aeromagnetic data were
obtained from the US Geological Survey. The results of
this study indicate a magmatic intrusion at a depth of
about 20 km to the southwest of Salton Sea. That intru-
sion extends for 70 km in SW-NE direction.
2. Tectonic Setting
The Salton Trough (Figures 1 and 2) is a highly ex-
tended terrane at the northern extent of sea-floor spread-
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opyright © 2012 SciRes. IJG

M. HUSSEIN ET AL. 304
Figure 1. Location map showing the study area and adja-
cent areas. Dashed line A-A’ showing the location of our
model. Square box showing the area of Figure 2.
Figure 2. Topographic and structural map of the Salton
Trough showing the active spreading centers (gray polygon),
and fossil spreading centers (After Lonsdale, 1989).
ing in the Gulf of California [7]. The area has high heat
flow and inferred to be in the early stages of transitioning
from continental crust to oceanic crust [8].
The Salton Trough results from northward progression
of California [8-10]. The initial opening of the Gulf of
California occurred about 12 - 10 Ma, shortly after sub-
duction ceased along the continental margin of Mexico
[9,11]. The Salton Trough apparently represents a present
day onshore analog to theses Gulf of California evolving
spreading centers [10].
The Salton Trough region is seismically active [12]
with most of the seismicity in the region occurring along
the Imperial and Brawley fault zones (Figure 2, [8]). The
Cerro Prieto, Imperial, Brawley, and San Andreas faults
form an echelon series of east-stepping, right lateral faults
linked by pull apart basins referred to as the Cerro Prieto,
Brawley, and Salton Buttes (Figure 2) spreading centers
by [10]. These spreading centers have probably changed
position more than once. Thus, for example, the parallel
Sand Hills and Algodones faults inferred to represent
fossil transforms or spreading centers parallel to, but ap-
proximately 45 km east of the Imperial fault zone [9,13].
The Imperial Valley—Salton Trough is highly active
seismically [8]. Large earthquakes have occurred on Im-
perial fault in 1940 (Ms 7.1) and in 1979 (Ms 6.9). Ref-
erence [8] suggested based on geodetic, seismic, and heat
flow data that the imperial and Brawley fault systems are
young (3000 to 100,000 years old) and have migrated
northwest into Imperial Valley. The southern most part
of the active San Andreas fault extends northwest from
the southeast corner of the Saton Trough [12]. Further
northeast, smaller Late Cenozoic strike slip faults also
known from surface exposure and these likely represent
parts of the evolving transform system [7]. We suppose it
is possible to classify The Cerro Prieto, Imperial, Braw-
ley, and San Andreas as seisomogenic fault based on the
seismic activity along these faults.
3. Data and Data Analysis
3.1. Receiver Functions
A receiver function is the seismic response of the earth
beneath a seismic station to an incoming P-wave. In par-
ticular, a receiver function maps P-to-S converted energy
due to impedance contrasts (i.e., changes in velocity and
density) in the earth. First-order information about the
crustal structure can be derived from the radial receiver
function, which is dominated by P-to-S converted energy
from velocity discontinuities in the crust and upper man-
tle [14]. Receiver functions can be used to determine
crustal thickness, Vp /Vs ratios, and the lateral variation of
the crustal thickness [15]. In receiver function estimation,
the foundation of the iterative deconvolution approach is
least squares minimization of the difference between the
observed horizontal component seismogram and pre-
dicted signal generated by convolution of an iteratively
updated spike train with the vertical component seismo-
gram [16]. The iterative time-domain approach has seve-
ral advantages, such as the ability to estimate the percent
fit and the long period stability by a priori constructing
the deconvolution as a sum of Gaussian pulses [16]. We
compute receiver functions using the iterative time de-
convolution with Gaussian width (Ga) factors of 2.5,

M. HUSSEIN ET AL. 305

1.75, and 1 which is equivalent to applying low pass fil-
ters with cutoff frequencies of 1.2, 0.9, and 0.5 Hz, re-
spectively.
components to radial and transverse directions. From the
waveform data, we computed the radial and transverse
receiver functions using iterative deconvolution, keeping
data with an 80% or greater fit. We also manually in-
spected each radial receiver function to ensure quality.
We then stacked the radial receiver functions using the
approach of [15]. For stacking we ran the H-K stacking
codes, we set the weight to be (0.3/0.5/0.2) (0.4/0.3/0.3)
and (0.5/0.4/0.1).
We collected data from 27 broadband seismograph sta-
tions (listed in Table 1 and shown in Figures 3 and 4) in
the vicinity of the Salton Trough, which recorded data
from 2000 to 2009.
Specifically, we collected broadband seismic wave-
form data for teleseismic earthquakes with M > 5.5. Our
approach of analysis consist of several steps, the first
step is copying receiver function for stations into diffe-
rent directories, stepping through each year and 100 day
Julian day increment. Loop over Julian days 000, 100,
200, 300 for each K (0, 1, 2, 3).
The time separation between Ps and P can be used to
estimate crustal thickness H, given the average crustal
velocities of Vs and Vp:
Data available directly from the Incorporated Research
Institutes for Seismology (IRIS), Data Management Cen-
ter (DMC), using the Standing Order of Data (SOD),
which allowed for automated rotation of the horizontal

22 22
11
ps
t HVs pVp p






where p is the ray parameter of the incident wave [15].
Of concern is the trade-off between the crustal thickness
and crustal velocities, since tPs represents the differential
Table 1. Stations codes, coordinates, estimated Vp/Vs, crustal thickness and n umbe r of re ceiver functions used in this study.
No. Station Code Longitudes Latitudes Est. Vp/Vs Est. Crustal Thickness No. of RF
1 Cl-ADO –117.43 34.55 1.74 ± 0.01 36.49 ± 0.32 168
2 Cl-BBR –116.92 34.26 1.95 ± 0.10 29.00 ± 0.34 205
3 Cl-BC3 –115.45 33.66 1.80 ± 0.11 25.50 ± 0.32 216
4 Cl-BEL –116.00 34.00 1.76 ± 0.12 28.59 ± 0.12 245
5 Cl-BFS –117.66 34.24 1.81 ± 0.10 31.00 ± 0.33 202
6 Cl-DAN –115.38 34.63 1.69 ± 0.08 28.03 ± 0.28 411
7 Cl-DVT –116.10 32.66 2.07 ± 0.11 20.50 ± 0.58 18
8 Cl-GLA –114.83 33.05 1.67 ± 0.12 27.00 ± 0.38 493
9 Cl-HEC –116.34 34.83 1.79 ± 0.09 28.44 ± 0.27 260
10 Cl-IRM –115.15 34.16 1.80 ± 0.10 2.00 ± 0.33 239
11 Cl-NEE –114.60 34.82 1.87 ± 0.11 25.96 ± 0.38 27
12 Cl-PDM –114.14 34.30 1.73 ± 0.10 27.96 ± 0.32 230
13 Cl-RRX –117.00 34.86 1.88 ± 0.10 30.49 ± 0.33 253
14 Cl-SWS –115.80 32.94 1.61 ± 0.09 27.96 ± 0.27 166
15 Cl-GMR –115.66 34.78 1.77 ± 0.11 26.57 ± 0.42 147
16 Cl-MUR –117.20 33.60 1.77 ± 0.09 31.15 ± 0.30 171
17 TA-109C –117.11 32.89 1.64 ± 0.11 38.56 ± 0.34 30
18 TA-112A –114.58 32.54 1.79 ± 0.08 25.06 ± 0.46 29
19 TA-Y12C –114.52 33.75 1.63 ± 0.09 33.49 ± 0.25 83
20 TA-W13A –113.90 35.10 1.77 ± 0.09 28.91 ± 0.30 54
21 TA-X13A –113.80 34.60 1.76 ± 0.09 26.98 ± 0.35 65
22 TA-Y13A –113.83 33.81 1.72 ± 0.12 30.4± 0.35 73
23 NR-NE70 –115.26 32.42 2.35 ± 0.06 30.48 ± 0.21 8
24 NR-NE71 –115.91 31.69 1.83 ± 0.10 32.50 ± 0.38 136
25 NR-NE72 –116.06 30.85 1.85 ± 0.10 32.10 ± 0.33 34
26 AZ-MONP –116.42 32.89 1.78 ± 0.09 30.49 ± 0.26 249
27 AZ-PFO –116.46 33.61 1.81 ± 0.11 27.05 ± 0.38 419

M. HUSSEIN ET AL. 306
Figure 3. Contour map of the Moho depth (crustal thick-
ness) in km based on receiver func tion data. Data were col-
lected from 27 stations (shown as red triangles). Stations
codes are shown inside small squares beside each station.
Figure 4. Vp/Vs contour map, high Vp/Vs located to the
south and southwest of the Salton Se a. Gray circle show s an
approximate location of the magmatic intrusion.
travel time of S with respect to P in the crust.
In our analysis, we ran script to calculate the ray pa-
rameters for all events and stations in the study area.
The dependence of H on Vp is not as strong as the de-
pendence on Vs or more precisely on the Vp /Vs ratio K.
For example using a Vp of 6.3 km/s and Vp/Vs ratio of
1.732 for 30 km thick crust, one gets:
∆H = (H/Vp) ∆Vp = 4.3 ∆Vp (km)
which means that the uncertainty of H is <0.5 km for a
0.1 km/s in Vp. However, the thickness is highly de-
pendent on the Vp/Vs as shown by:
∆H = (H/Vp) ∆k = −40.2 ∆k (km)
i.e., a 0.1 change in k can lead to about 4 km change in
the crustal thickness. This ambiguity reduced by using
later phase, which provide additional constraints:


22
21
PpSs PsPs
t
HVs p



so that both K and H can be estimated [17-19].
We contour the crustal thickness and Vp/Vs ratios
(Figures 3 and 4 respectively) using a minimum-curva-
ture algorithm to interpolate values to a rectangular grid.
Moho dome shape beneath the area of active upper crust
extension suggested being primarily the product of mag-
matic activity in the lower crust and upper mantle fol-
lowing the interpretation of [20] for a similar feature in
Death Valley, California.
Reference [20] suggests that the doming of the lower
crust of Death Valley accommodated by the addition of
magma to the lower crust.
The Moho is about 20 km deep southwest of the Sal-
ton Sea and increases to 32 km east of the Salton Sea.
The average Vp/Vs ratio is 1.80 in the study area in gen-
eral; this ratio increases to the south and southeast of the
Salton Sea becoming 2.1 and 2.3, and decreases to the
east to 1.6. Reference [21] concluded that high concen-
tration of high Vp/Vs ratio might be caused by the pla-
gioclase rich mafic composition of the lower crust; be-
cause high crustal S-wave and Rayleigh wave phase ve-
locity in the same area exclude the possibility of crustal
fluids or partial melting as a possible reason for high
Vp/Vs. Reference [22] concluded that the low velocity
with low Vp/Vs zones in the upper crust are caused by the
inclusion of H2O and that low velocity with high Vp/Vs
zones in the lower crust and uppermost mantle are caused
by melt inclusions. Magnetotelluric survey in similar
areas revealed conductive zones located at a depth of
below 15 - 20 km; this is consistent with the low velocity
with high Vp /Vs zone [23]. Reference [23] inferred that
the low velocity anomaly could be explained by the par-
tially molten rock.
3.2. Frequency-Magnitude Distribution
(b-Values)
The b-value is a critical parameter for seismic hazard
assessment and an indicator of the presence of fluids and/
or partial melt. The earthquake distributions (number of
events vs magnitude) typically follow a power law; and
the negative slope of that plot is the b-value [24]. Normal

M. HUSSEIN ET AL. 307
fault events have the highest (b) values (b ≈ 1.1), thrust
events the lowest (b ≈ 0.7) and strike slip events inter-
mediate values (b ≈ 0.9) [24].
Southern California Earthquake Data Center
(http://www.data.scec.org/index.html) seismic activity re-
cord shows numerous seismic events for September,
2009, that are focused along the San Jacinto, Elsinore,
Cerro Prieto and Imperial faults, while seismic activity is
weak or absent along the fossil spreading centers of the
Sand Hill and Algodones faults (Figure 5). Reference
[25] analyzed earthquakes from the local catalog of events
in the Southern California region from 07/01/1944 to
03/01/1990 using magnitudes between M = 4.1 and M =
7.7. The cumulative and the binned density are shown in
Figure 6. Slopes in Figure 6 are negative. This observa-
tion may imply the number of normal fault events falls
off more rapidly with increasing magnitude than is the
case for strike-slip or thrust events. b-values for the com-
plete catalog are shown in Table 2. The cumulative dis-
tribution seems to have the property of self-similarity
across the entire range of magnitude to about M = 6.6
(Figure 6; from [25]). Reference [26] shows the relation
between the depth and the b-values in southern Califor-
nia (Table 3).
They separate the data into five depth ranges from 0 -
3, 3 - 6, 6 - 9, 9 - 12, and 12 - 15 km for calculating
b-values, they restricted the magnitude to larger than M =
2.0 for detection and completeness and smaller than M =
5.5. Table 3 shows a decrease in b-va lues with increas-
ing earthquake depth, with exception for the intervals 9 -
12 km and 12 - 15 where the b-value is increasing by a
very small portion.
Reference [27] showed that areas of high b-values
from 1.3 ± 0.05 up to 1.5 ± 0.05 could be attributed to the
Table 2. “b-Values” for complete catalog (After Knopoff,
2000).
Magnitude
Range
“b-Value” Total Catalogue
(Main and after shocks)
“b-Value”
Main Shocks
4.1 - 7.7 0.97 ± 0.03 0.87 ± 0.05
4.1 - 6.6 0.96 ± 0.03 0.88 ± 0.05
4.1 - 4.8 0.98 ± 0.07 0.99 ± 0.12
Table 3. Relatio n between de pth and “b-Value s” (After Mor i
and Abercrombie, 1997).
Magnitude Range Depth (km) “b-Value”
2.0 - 5.0 0 - 3 1.026 ± 0.008
2.0 - 5.0 3 - 6 1.063 ± 0.010
2.0 - 5.0 6 - 9 0.967 ± 0.016
2.0 - 5.0 9 - 12 0.923 ± 0.016
2.0 - 5.0 12 - 15 0.928 ± 0.021
Figure 5. The seismic map of September 2009. Seismic ac-
tivity focused along the active spreading centers and absent
along the fossil spreading centers (Map from Southern Ca-
lifornia Earthquake Data Center) .
Figure 6. Cumulative and differentiated (shaded) distribu-
tion of the magnitudes of all earthquakes in Southern Cali-
fornia region from 7-1-1944 to 3-1-1990 (After Knopoff).
presence of high thermal gradients due to the emplace-
ment of magmatic fluids. As magmatic fluids injected
into the system and temperatures expected to rise around
the intrusion and the crust would weaken due to its inabi-
lity to accumulate high amounts of stress. The high tem-
perature, weaken crust and expanding sill could cause the
formation of numerous small fractures [27].
A relatively large number of smaller earthquakes is ex-
pected to accompany the formation of small fractures and
would alter the frequency-magnitude distribution of earth-
quake toward higher b-valu es because of developing de-
viatoric tension [27].
Reference [28] plotted b-values in Koyna-Warna re-
gion, India, and concluded that a systematic increase in
b-values from 8 km depth to 12 km for Koyna earth-

M. HUSSEIN ET AL.
IJG
308
quakes indicates the presence of pore fluids at a depth
greater than 8 km, and stated that low b-value areas at 5
km depth show localized high stresses that are favorable
for future rupture.
magnetic anomaly map (Figure 8). We used Montage
Oasis soft ware for contouring and Adobe illustrate for
final production. Both maps were projected using UTM
projection.
3.3. Gravity and Aeromagnetic Data 4. Model Development
We obtained gravity data from UTEP-PACES We created a crustal scale model (Figure 9) to explore
the deep structures of the active and fossil spreading
centers using the unfiltered Bouguer gravity and mag-
netic anomaly maps with the receiver function analysis.
We divided our targeted features into “active” and “fos-
sil” spreading centers based on the amount (magnitude
and frequency) of seismic activity corresponding with
known faults and the style of the potential field anomaly.
That is, we expect high amplitude Bouguer gravity ano-
malies to correspond to the rifts because of the thin crust
and shallow Moho which is typical for rift settings.
(http://www.research.utep.edu/paces) website that is cur-
rently hosted at the Cyber-ShARE Center of Excellence
at UTEP. We used 40,784 Bouguer gravity points to cre-
ate the Bouguer gravity anomaly map (Figure 7). The
gravity data were merged from a variety of surveys and
cover the US and the border region. Average error for
this data set ranges from 0.05 to 2 mGal (Al-Douri, per-
sonal communication, 2009). Reference [29] of the US
Geological Survey using a digital elevation model calcu-
lated terrain corrections and a technique based on the
approach of [30]. A Bouguer gravity correction was
made using 2670 kg/m3 as the reduction density. Aero-
magnetic data were obtained from the US Geological
Survey with a grid spacing of 1 km [31]. We used
112,436 aeromagnetic measurement points to create the
Crustal Model
The starting point for our model was to incorporate the
depth to the Moho that was determined from the receiver
Figure 7. Bouguer gravity anomaly map of the Salton Trough area. Fossil spreading centers and active spreading centers are
gravity high related to basaltic volcanism of Salton Buttes, and due to the presence of shallow Moho and magmatic intrusion.
Dashed line A-A’ shows the location of the crustal profile. Circle labeled as 1 shows the approximate location of the active
spreading centers; circle 2 shows the approximate location of the fossil spreading centers. SD: San Diego; T: Tijuana; SF:
San Felipe.

M. HUSSEIN ET AL. 309
Figure 8. Magnetic anomaly map of the Salton Trough. Fossil spreading centers are magnetic medium to high related to ba-
saltic rocks; active spreading centers are magnetic low associated with the fluids in the upper crust. Dashed line A-A’ shows
the location of the crustal profile. Circle labeled as 1 shows the approximate location of the active spreading centers; circle 2
shows the approximate location of the fossil spreading centers. SD: San Diego; T: Tijuana; SF: San Felipe.
function analysis and the densities for the upper, middle,
lower crust, and upper mantle from previous studies
[7,12,32]. Magnetic susceptibilities estimated from [33].
The density (D) for the upper crust (sediments and me-
tasedimenatry composition) ranges from 2500 to 2600
kg/m3, while magnetic susceptibility (S) ranges from
0.002 to 0.007, density of the middle crust is 2750 kg/m3,
and (S) is 0.065 and mostly quartzofeldspathic composi-
tion, density of the lower crust is 2950 kg/m3 (gabbroic
composition), and (S) is 0.004. Density of the magmatic
body is 3100 kg/m3, its magnetic susceptibility is 0.0,
and the mantle density is 3300 kg/m3 with a magnetic
susceptibility of 0.0.
The profile (A-A’) is about 450 km long, and crosses
the central part of the study area (see Figure 7 for profile
location). The depth to the Moho, determined from the
receiver function, varies from 38 km at the starting point
(A), decreases to 26 km southwest of the Salton Sea, and
increases to 32 km at the end point (A’) of the model.
Magmatic intrusion is modeled at a depth of about 20 km
to the southwest of Salton Sea as shown by the crustal
model (Figure 9). That intrusion extends for 70 km in
SW-NE direction as interpreted from the model. Mag-
matic intrusion released fluids that promote seismic ac-
tivity in the active spreading centers.
5. Discussion
We used several data sets to interpret seismic activity in
the active spreading centers of Salton Trough. Table 4
summarizes the differences in observations and models
between the active and fossil (inactive) spreading centers.
We incorporate receiver function analysis with gravity
and magnetic data to create a subsurface crustal scale
model that reveals magmatic intrusion underlies the ac-
tive spreading centers. Vp/Vs ratios are high in the area
where magmatic intrusion presents and indicates partial
melt in the lower crust; b-va lues are high in these same
areas and indicate fluids in the upper crust.
We interpret upper crustal seismicity within the active
spreading centers to promote by fluids ex-solved from

M. HUSSEIN ET AL. 310
Figure 9. Interpretative model for gravity and aeromagnetic data profile A-A’ (Central region of Salton Trough). This model
is about ~450 km long, covers the central part of the study area. The depth to the Moho, according to the receiver functions,
varies from 25 km at the central point of the model south of Salton Sea [D = Density (kg/m3), S = Susceptibility (dimen-
sionless), M = Magnetization (A/m), MI = Magnetic inclination (degree), MD = Magnetic declination (degree)].
Table 4. Summary of the major differences between active and fossil spreading centers of the Salton Trough as extracted
from incorporating different data sets.
Compared characteristi cs Active spreading centers Fossil spreading centers
Depth to Moho Shallow Moho (thin crust) Deeper Moho (thick crust)
Vp/Vs High in lower rust Low in the lower crust
b-Values High b-values Low b-values
Gravity anomalies High gravity amplitudes Low gravity amplitudes
Magnetic anomaly Low magnetic amplitude High magnetic amplitude
Seismic activity Seismically active No seismic activity
Magmatic underplating Magmatic body at depth of 20 km No evidence for any magmatic intrusions
the magmatic intrusion that underlies the spreading cen-
ters at a depth of about 20 km because cyclic fluid pore
pressure increase weakens faults [34]. Earthquake acti-
vity in the lower crust has been identified at several rifts
(e.g. Taupo Volcanic Zone, New Zealand, [5]) suggest-
ing the possibility of fluid involvement. Both seismicity
and tomography suggest that in order for these lower
crustal events to occur, fluids needed, as well as a rheo-
logically strong lower crust that restricts these fluids to
its base. The strength of the crust is governed by strain
rate, temperature and rock composition as well as pore
fluid pressure [5].
The locations of the active spreading centers are char-
acterized by high amplitude gravity anomalies because
the crust is thin and the Moho is shallow beneath active
spreading centers according to our receiver function
analysis (Figures 3 and 4). Low gravity anomaly ob-
served to the west and south of the Salton Sea, we relate
this low anomaly to the existence of fluids or partial melt.
We assume the crust is gabbroic in composition, consis-
tent with our model density of 2950 kg/m3. This compo-
sition could provide the strong lower crust that restricts

M. HUSSEIN ET AL. 311
fluids to its base. The active spreading centers are char-
acterized by a low magnetic anomaly inferred to be the
response from molten material and/or fluids. In contrast,
high amplitude magnetic anomalies prevail in the fossil
spreading centers due to the magnetization of cooled
gabbro and less fluid in the crust.
Several lines of evidence indicate the role of fluid in-
volvement in seismic activity within the active spreading
centers of the Salton Trough. High Vp /Vs to the south
and southwest of the Salton Sea provide evidence for
such activity (Figure 4).
The crustal scale model suggests a partially molten,
mafic intrusion underlies the location of the active
spreading centers (Figure 9). We interpret the fluids to
be derived from that magma intrusion. Our model also
shows a doming of the Moho beneath the spreading cen-
ters (between San Jacinto and San Andreas faults, Figure
9) which is suggested to results from magmatic activity
and intrusion in the lower crust and upper mantle. Refe-
rence [35] concluded that the regional upper mantle is
very heterogeneous with the greatest amplitude of veloc-
ity and Vp/Vs perturbations in the upper 200 km and
mantle structure that correlate spatially with major physi-
ographic provinces of southern California.
The largest low velocity, high Vp/Vs anomaly is found
beneath the Salton Trough and is attributed to relatively
high partial melt in the asthenosphere resulting in litho-
spheric thinning. Reference [4] concluded from the mag-
nitudes of Vp, Vs, and Vp/Vs perturbations, and know-
ledge of regional 1-D velocity and attenuation, that the
asthenosphere contains up to 1% partial melt extending
to depths of 150 - 200 km.
b-values from frequency-magnitude plots for south-
west California are high. According to [25,26], areas of
high b-values are attributed to the presence of a high
thermal gradient due to the emplacement of magmatic
fluids [27,28]. Earthquakes occur on strike slip and/or
normal faults in these areas. The lack of coincidence be-
tween the epicenteral area of seismic activity and the
surface outlet of sub-crustal fluids suggest that the es-
caping fluids do not follow vertical pathway on moving
from their deep source to the surface [36]. Pressurized
fluids can escape from their source along high perme-
ability pathways in the upper crust [37].
6. Conclusion
Seismic activity is restricted to the active spreading cen-
ters of the Brawly, Cerro Prieto, Imperial, Elsinore, and
San Jacinto fault zones, these are strike-slip faults that
extend from 15 km (Brawley fault) to 210 km (San Ja-
cinto fault zone) in the active spreading centers of the
Salton Trough rift (Figure 2). Spreading is limited or
absent in the fossil spreading centers at the Sand Hill and
Algodones faults (Figure 2). We associate seismic activ-
ity with the presence of a magmatic intrusion beneath the
active spreading centers and with fluids derived from this
intrusion. Several lines of evidence support the existence
of magma intrusion and fluids: 1) low velocity and high
Vp/Vs ratios in the lower crust as shown by tomography
and receiver function analysis; 2) low amplitude mag-
netic anomaly in the location of active spreading centers
attributed to the fluids and/or melts; and 3) a high ampli-
tude gravity anomaly that underlies the active spreading
characteristics of the inferred magma intrusion can be
recognized over a distance of 70 km in a NE-SW direc-
tion along the fault zones of the active spreading centers
(Figure 9). Frequency-magnitude distribution b-values
are high in southwest California including the study area;
high b- values are attributed to high thermal gradient due
to the emplacement of magmatic fluids.
7. Acknowledgements
We would like to thank, Dr. Terry Pavlis, Dr. William
Cornell and Dr. Vladik Kreinovich for helpful discus-
sions. We would like also to thank Carlos Montana for
the technical support. The work partially supported by
NSF HRD 0734825 Cyber-ShARE Center of Excellence.
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