Musa Hussein

Receiver Functions and Gravity Data

Abstract

This is an integrated and comparative study of two California rifts. The available data make a comparative and integrated analysis worthwhile. For this work we have used receiver functions, controlled source seismic, gravity and magnetic data to constrain crustal structure. Analysis of gravity data shows that the anomalies in the Salton Trough are deeper than anomalies of Death Valley. Our modeling suggests the Moho is 21 km deep south of the Salton Sea and deepens to 33 km in the region west of the Salton Trough, while in Death Valley the Moho is 26 km deep in the central part of the basin and deepens to 32 km on either side. Another significant difference between the two basins is the density of the lower crust, which is 2950 kg/m3 for the Salton Trough and 2750 kg/m3 for Death Valley. Density of the upper crust varies from 2750 kg/m3 to 2450 kg/m3 in the Salton Trough and from 2650 kg/m3 to 2450 kg/m3 in Death Valley. Sedimentary rocks and meta-sedimentary rocks in Death Valley are thick and reach a depth of 15 km, while in the Salton Trough the depth of sedimentary rocks and meta-sedimentary rocks is 8-9 km. The Salton Trough is formed from magmatism in the lower crust and sedimentation in the upper crust. Rising of upper mantle material causes uplifting, thinning, and crustal extension (rifting) in the central part of the Salton Trough south of the Salton Sea, and in the southern part of Death Valley. Magnetic anomalies are shallow in both regions. The anomalies in Death Valley show higher relief (~ 420 nT, compared to 250 nT) than in Salton Trough. Salton Trough magnetic anomalies are almost flat with some exceptions in the marginal areas.

Receiver Functions

A receiver function is the seismic response of the earth beneath a seismic station to an incoming P-wave. The physical earth property that controls what is observed in a receiver function is the P-to-S impedance structure. Different studies use seismic receiver functions to determine crustal thickness and Vp/Vs ratios, and to determine the lateral variation of the Moho depth (Zhu and Kanamori, 2000). Receiver functions can provide a very good point measurement of crustal thickness under a broadband station and are not sensitive to crustal P velocity. The impedance structure in the crust and upper mantle reflects many important tectonic and geodynamic processes. For example in regions of lithospheric extension, one would expect to find a thin crust and therefore a shallow Moho. Data for this part was downloaded from http//www.earthscope.org and http//www.seis.sc.edu.

Musa Hussein

Locations of seismograph stations within the study area (from Earthscope.org).

Musa Hussein

Information on seismograph stations where receiver functions have been determined.

No. / Station Code / Type of Deployment / Geographic Location / State / Longitude / Latitude / Ellipsoidal Elevation WGS-84 (m) / Start Data Flow / Most Recent Data / Data Type / Sample rate
2 / CI-ADO / Transportable Array (removed) / Victorville / CA / 117.4 W / 34.5 N / 908 / 4/1/2004 / 6/23/2004 / Seismic Broadband, 40 sps
3 / CI-BBR / Transportable Array / Big Bear Solar Observatory / 116.9 W / 34.3 N / 2069 / 4/1/2004 / 6/13/2006 / Seismic Broadband, 40 sps
4 / CI-BC3 / Transportable Array / Desert Center / CA / 115.5 W / 33.7 N / 1137 / 4/1/2004 / 6/12/2006 / Seismic Broadband, 40 sps
5 / CI-BEL / Transportable Array / Joshua Tree National Park / CA / 116.0 W / 34.0 N / 1388 / 4/1/2004 / 6/13/2006 / Seismic Broadband, 40 sps
6 / CI-BFS / Transportable Array / Mt Baldy Forest Station / 117.7 W / 34.2 N / 1312 / 4/1/2004 / 6/13/2006 / Seismic Broadband, 40 sps
9 / CI-DAN / Transportable Array (removed) / Needles / CA / 115.4 W / 34.6 N / 428 / 4/1/2004 / 4/4/2006 / Seismic Broadband, 40 sps
11 / CI-DNR / Transportable Array (removed) / Anza / CA / 116.6 W / 33.6 N / 1274 / 4/1/2004 / 4/22/2004 / Seismic Broadband, 40 sps
12 / CI-DVT / Transportable Array / Ocotillo / CA / 116.1 W / 32.7 N / 881 / 4/1/2004 / 6/13/2006 / Seismic Broadband, 40 sps
16 / CI-GLA / Transportable Array / Glamis / 114.8 W / 33.1 N / 610 / 4/1/2004 / 6/13/2006 / Seismic Broadband, 40 sps
17 / CI-GOR / Transportable Array (removed) / Vista / CA / 117.2 W / 33.2 N / 138 / 4/1/2004 / 4/22/2004 / Seismic Broadband, 40 sps
20 / CI-HEC / Transportable Array / Baker / CA / 116.3 W / 34.8 N / 920 / 4/1/2004 / 6/13/2006 / Seismic Broadband, 40 sps
21 / CI-IRM / Transportable Array / Eagle Mtn. / CA / 115.1 W / 34.2 N / 567 / 4/1/2004 / 6/13/2006 / Seismic Broadband, 40 sps
28 / CI-NEE / Transportable Array / Needles / CA / 114.6 W / 34.8 N / 170 / 4/1/2004 / 6/13/2006 / Seismic Broadband, 40 sps
30 / CI-PDM / Transportable Array / Lake Havasu City / AZ / 114.1 W / 34.3 N / 144 / 4/1/2004 / 6/13/2006 / Seismic Broadband, 40 sps
32 / CI-RRX / Transportable Array / Barstow / CA / 117.0 W / 34.9 N / 439 / 4/1/2004 / 6/13/2006 / Seismic Broadband, 40 sps
36 / CI-SDR / Transportable Array (removed) / El Cajon / CA / 116.9 W / 32.7 N / 113 / 4/1/2004 / 4/22/2004 / Seismic Broadband, 20 sps
40 / CI-SWS / Transportable Array / Westmorland / CA / 115.8 W / 32.9 N / 140 / 4/1/2004 / 6/13/2006 / Seismic Broadband, 40 sps
45 / AZ-MONP / Transportable Array / Monument Peak / CA / 116.4 W / 32.9 N / 1920 / 4/15/2004 / 6/13/2006 / Seismic Broadband, 40 sps
46 / AZ-PFO / Transportable Array / Pinon Flats Observatory / CA / 116.5 W / 33.6 N / 1259 / 4/15/2004 / 6/13/2006 / Seismic Broadband, 40 sps
64 / TA-109C / Transportable Array / Miramar / CA / 117.1 W / 32.9 N / 150 / 5/10/2004 / 6/13/2006 / Seismic Broadband, 40 sps
67 / CI-BCC / Transportable Array (removed) / Bear Creek Country Club / 117.3 W / 33.6 N / 391 / 6/23/2004 / 8/2/2005 / Seismic Broadband, 40 sps
71 / TA-Y12C / Transportable Array / Blythe / CA / 114.5 W / 33.8 N / 196 / 11/17/2004 / 6/13/2006 / Seismic Broadband, 40 sps
109 / CI-MUR / Transportable Array / Murrieta / 117.2 W / 33.6 N / 562 / 10/12/2005 / 6/13/2006 / Seismic Broadband, 40 sps
120 / TA-X13A / Transportable Array / Yucca / AZ / 113.8 W / 34.6 N / 889 / 3/15/2006 / 6/13/2006 / Seismic Broadband, 40 sps
121 / TA-Y13A / Transportable Array / Salome / AZ / 113.8 W / 33.8 N / 356 / 3/16/2006 / 6/13/2006 / Seismic Broadband, 40 sps
133 / TA-W13A / Transportable Array / Kingman / AZ / 113.9 W / 35.1 N / 1988 / 3/28/2006 / 6/10/2006 / Seismic Broadband, 40 sps
142 / CI-GMR / Transportable Array / Granite Mountains Research Center / 115.7 W / 34.8 N / 1326 / 5/3/2006 / 5/26/2006 / Seismic Broadband, 20 sps

Musa Hussein

Data extracted from EARS results.

Station / Longitudes / Latitudes / Est. Vp/Vs / Est. Crustal Thickness
AZ-LVA2 / -116.53 / 33.35 / 1.67 / 30.5
XF94-LGNA / -116.41 / 32.84 / 1.84 / 30.25
Cl-DGR / -117.01 / 33.65 / 1.78 / 33.25
Cl-DVT / -116.1 / 32.66 / 1.62 / 34.5
Cl-BAR / -116.67 / 32.68 / 1.76 / 40
AZ-ASBS / -116.47 / 33.62 / 1.77 / 29.25
TA-Y12C / -114.52 / 33.57 / 1.87 / 25
Cl-PLM / -116.86 / 33.35 / 1.99 / 29.25
AZ-BZN / -116.67 / 33.49 / 1.72 / 31
Cl-GLA / -114.83 / 33.05 / 1.66 / 27
Cl-BC3 / -115.45 / 33.66 / 1.83 / 24
Cl-BBR / -116.92 / 34.26 / 1.96 / 28.75
TS-BAR / -116.67 / 32.68 / 1.91 / 31.75
Cl-SDD / -117.66 / 33.55 / 2.09 / 26.5
AZ-CRY / -116.74 / 33.57 / 1.77 / 32
XF94-HONY / -116.64 / 32.9 / 1.74 / 34.25
TS-SVD / -117.1 / 34.1 / 1.88 / 33.25
AZ-YAQ / -116.35 / 33.17 / 1.84 / 23
TS-DGR / -117.01 / 33.65 / 1.79 / 32.75
II-PFO / -116.46 / 33.61 / 1.71 / 29.5
Cl-BCC / -117.26 / 33.58 / 1.80 / 31.75
AZ-GLA / -114.83 / 33.05 / 1.71 / 26
AZ-FRD / -116.6 / 33.49 / 1.70 / 32.23
AZ-SHUM / -116.44 / 33.63 / 1.71 / 29.5
XF94-FEAR / -115.93 / 32.77 / 1.68 / 32.23
Cl-BEL / -116 / 34 / 1.75 / 28.25
TS-PFO / -116.46 / 33.61 / 1.71 / 29.25
AZ-SOL / -117.25 / 32.84 / 1.95 / 22.5
AZ-KNW / -116.71 / 33.71 / 1.77 / 33.25
AZ-SMTC / -115.8 / 32.94 / 1.76 / 38
TS-SMCT / -115.72 / 32.95 / 1.60 / 22.5
AZ-HWB / -116.96 / 33.03 / 1.88 / 34.75
AZ-TRO / -116.43 / 33.52 / 1.84 / 26.75
AZ-SND / -116.61 / 33.55 / 1.80 / 31
AZ-MONP / -116.42 / 32.89 / 1.81 / 30.25
TS-GLA / -114.83 / 33.05 / 1.64 / 27.5
Cl-DNA / -115.38 / 34.64 / 1.68 / 28.25
AZ-ELKS / -116.45 / 33.58 / 1.73 / 28.5
TA-109C / -117.11 / 32.89 / 2.04 / 23
NR-NE70 / -115.26 / 32.42 / 1.61 / 22.5
AZ-BVDA2 / -116.37 / 33.33 / 1.87 / 22.5
XF94-BLSY / -116.88 / 32.91 / 1.89 / 31
Cl-HEC / -116.33 / 34.83 / 1.82 / 28.25
Cl-SVD / -117.1 / 34.11 / 1.91 / 32.23
XF94-PINE / -116.53 / 32.83 / 1.81 / 29.25
Cl-MUR / -117.2 / 33.6 / 1.71 / 27.5
AZ-PFO / -116.46 / 33.61 / 1.72 / 29.5
XF94-MICA / -116.12 / 32.65 / 1.63 / 28.75
AZ-SMER / -117.17 / 33.46 / 1.76 / 32.23
AZ-RDM / -116.85 / 33.63 / 1.81 / 32.23
NR-NE71 / -115.91 / 31.69 / 1.80 / 33
Cl-JCS / -116.6 / 33.09 / 1.78 / 35
Cl-SWS / -115.8 / 32.94 / 1.67 / 26.5
AZ-SMER2 / -117.17 / 33.46 / 1.78 / 33.25
XF94-ALPN / -116.75 / 32.87 / 1.80 / 35
Cl-IRM / -115.15 / 34.16 / 1.77 / 27.25
AZ-WMC / -116.67 / 33.57 / 1.78 / 31
Cl-PDM / -114.14 / 34.3 / 1.74 / 27.75

Musa Hussein

Contour map showing depth to the Moho (km) based on receiver function information.

Musa Hussein

Gravity Data

Gravity data was modeled to determine the density variations of subsurface rocks. Modeling is a powerful tool to map in detail the anatomy of the plate boundaries, especially when combined with analysis of seismic and well data. Integration of gravity models with previous seismic studies (e.g. Parsons and McCarthy, 1996; Parsons et al. 2001; Fuis et al. 1984) provided additional constraints on the composition and structure of the crust and upper mantle. For instance, seismic studies have difficulty distinguishing eclogite and dunite due to their similar velocity and Vp/Vs ratios. However, these two rock types have different densities. Therefore, long wavelength gravity anomalies may provide crucial information needed to distinguish deeper crustal lithologies that have similar seismic properties. Gravity should also help detect lateral changes in lithology and perhaps important geometrical features.

Gravity Data Filtering

The term filtering can be applied to various techniques that attempt to separate anomalies on the basis of wavelength and or trend (Blakely, 1996). The term separate is a good intuitive one because the idea is construct an image (anomaly map) and then use filtering to separate anomalies of interest to the interpreter from other interfering anomalies. In high pass, low pass or band pass filtering we remove certain wavelengths from the anomaly map.

We can apply several filters to our data, in particular strike filtering, wavelength filtering, and regional-residual filtering. Strike (directional filter) is very good for removing directional features from a grid. The cosine function makes the filter smooth, so directional ringing problems are not a problem. The rejection (or pass) can be narrowed or widened by setting the degree of the cosine function.

A low pass filter smoothes the input data by the application of a convolution filter to the data. The filter is called “low –pass” because it allow slow wave numbers (low frequencies) to pass the out put channel. Features in the data that are shorter than the short wavelength cutoff will be removed. High-pass filter is seldom use because of Gibb’s Phenomena (ringing). A regional-residual filter, which is another smooth filter.

The anomalies of the study area are deep, with two trends, an E-W trend in the eastern portion of the study area intersected by a NW-SE trend representing the present trough.

The effects of the certain filters are sometimes minimum; they don’t show a significant change to the data when applied, which consider to be an acceptable thing, because these filters remove or enhance the anomalies (depend on your filter), if no significant change appears that means the anomalies are not changing with certain depth or direction, while if there is a change within the anomaly map that means the anomalies them self are changing.

Musa Hussein

Bouguer anomaly map of the study area.

Musa Hussein

Location of gravity stations.

Locations of gravity models.

Musa Hussein

Interpretative model for gravity and aeromagnetic data model A-A` (Northern region of Salton Trough).

Interpretative model for gravity and aeromagnetic data model B-B` (Central region of Salton Trough).

Interpretative model for gravity and aeromagnetic data model C-C` (Southern region of Salton Trough).

Interpretative model for gravity and aeromagnetic data model D-D`.