We use group velocities from earthquake tomography together with group and phase velocities from ambient noise tomography (ANT) of Rayleigh waves to invert for the 3-D shear-wave velocity structure (5–70 km) of the Caribbean (CAR) and southern North American (NAM) plates. The lithospheric model proposed offers a complete image of the crust and uppermost-mantle with imprints of the tectonic evolution. One of the most striking features inferred is the main role of the Ouachita–Marathon–Sonora orogeny front on the crustal seismic structure of the NAM plate. A new imaged feature is the low crustal velocities along the USA-Mexico border. The model also shows a break of the east–west mantle velocity dichotomy of the NAM and CAR plates beneath the Isthmus of the Tehuantepec and the Yucatan Block. High upper-mantle velocities along the Mesoamerican Subduction Zone coincide with inactive volcanic areas while the lowest velocities correspond to active volcanic arcs and thin lithospheric mantle regions.
Crustal seismic models are important for several reasons. The first is the significant impact that crustal corrections have in mantle tomography (Bozdağ and Trampert, 2008; Lekić et al., 2010; Panning et al., 2010). Another is the strong dependency of earthquake location accuracy on the crustal velocity model.
Surface-wave earthquake-based global and regional tomography usually uses
long period velocity measurements (
Simplified tectonic map of the study area: physiographic provinces shown as gray lines (Sedlock, 1993; M. Moschetti, personal communication, 2011; Marshall, 2007); stations as red squares; and plate boundaries as black lines (Bird, 2003). Ap denotes Appalachian Plateau Province; B&R Basin and Range; CAVA Central America Volcanic Arc; CB Colombian Basin; ChB Chortis Block; CP Colorado Plateau; CR Colorado River; CT Cayman Trough; EPS East Pacific Rise; GB Grenada Basin; GCP Gulf Coastal Plain; GEP Gulf Extensional Province; GP Great Plains; IT Isthmus of Tehuantepec; ME Mississippi Embayment; MC Mesa Central; MP Motagua–Polochic fault system; Ou Ouachita Province; RG Rio Grande; RV Rivera Plate; SMOc Sierra Madre Occidental; SMOr Sierra Madre Oriental; SMS Sierra Madre del Sur; TMVB Trans-Mexican Volcanic Belt; VB Venezuela Basin; and YB Yucatan Block. Blue lines indicate main rivers. Highlighted yellow dashed black line indicates the Ouachita–Marathon–Sonora orogenic belt (OMS). Its extension into Mexico is taken from Handschy et al. (1987). The GEP location is taken from Zhang et al. (2007).
Recent global shear wave velocity models from surface waves image the crust
and uppermost mantle with 2
The data set used in this study consists of continuous recordings from nearly
100 broadband seismic stations of the Mexican and US national networks,
other global and regional networks, and temporary deployments. One of the
most important contributions of this study comes from the increased station
coverage in the region since the beginning of the 21st century. The
Mexican broadband National Seismic Network (IG) has expanded its coverage
towards the north and the south of the country; the regional Caltech network
(CI) has increased the coverage in California; and the deployment of the U.S. Geological Survey (USGS) Caribbean Network (McNamara et al., 2006) has
significantly improved the station coverage in the Caribbean. The
availability of data from several high-density temporal broadband networks,
such as the NARS array in Baja California (Trampert et al., 2003) and the
USArray Transportable Array in the continental US, has also increased the
station density in the western and northern boundaries of the region. Figure 1
shows the distribution of the 103 broadband stations used in this study
superimposed on a map showing the main tectonic features and physiographic
provinces of the area. We analyze 117 earthquakes of
We determine fundamental mode Rayleigh-wave group velocity dispersion curves
from the earthquake records applying FTAN (Frequency Time ANalysis) with the
PGSWMFA program from Ammon (1998). We invert these group velocity
measurements to obtain 2-D group velocity models by the method of Barmin
et al. (2001). This inversion procedure attempts to minimize a penalty function
(Eq. (15) of Barmin et al., 2001) that depends on three damping parameters.
These parameters are:
Path distribution of Rayleigh-wave group velocities at
From this second step we obtain group velocity maps for periods from 20 to
100 s on a 1
We use Rayleigh waves' group and phase velocity dispersion curves from 8 to
50 s obtained from ambient noise tomography on a 1
Rayleigh-wave group velocity perturbation maps at
Estimated resolution in km for group velocity maps at
We combine group velocity measurements from ambient noise and earthquake
tomography on each node of a 1
We simultaneously invert group and phase velocity measurements for a 1-D shear
wave velocity structure at each grid point by using a simple
parameterization of the medium consisting of 3 constant velocity layers over
a half-space. The model parameters (4 velocities and 3 thicknesses) can vary
across a wide range to obtain an optimized solution for the whole variety of
tectonic domains in the study area. We consider the media as a Poisson
solid, i.e.:
We use a modified code from Iglesias et al. (2001) to jointly invert phase and group velocities. This code solves the forward model with the subroutine SURFACE85 (Herrmann, 1987) and inverts with the simulated annealing algorithm (Goffe et al., 1994; Goffe, 1996). Simulated annealing is a global optimization method. The algorithm scans the possible solutions space to find the optimum model by reducing the searching vector length when it is close to a minimum and allowing misfit increases to avoid local minimums. The algorithm determines as the optimum model that which minimizes the misfit during a certain number of searching iterations. To assure the inversion of high quality dispersion curves, we only invert dispersion curves with velocity measurements at more than 3 discrete frequencies. By doing this we avoid inverting nodes with high resolution at narrow frequency ranges. We select as optimum models only those with velocity increasing with depth.
Examples of joining group velocity obtained from ANT (blue squares) and from earthquake tomography (empty circles) at four nodes of the inversion region representing different tectonic settings. The error bars denote resolution normalized by 2500 km at each period. The gray area limits the velocity overlapping and joining period range. Filled circles and continuous red lines indicate the combined dispersion curve.
The misfit of the dispersion measurements is computed as:
As the final step, we combine the 1-D shear models from each node to produce a 3-D shear wave velocity model.
The 3-D shear-wave velocity model obtained from inverting Rayleigh-wave group
velocities (10 to 100 s) and phase velocities (10 to 50 s) is sensitive to
velocity changes from 5 down to 70 km depth. The inversion fits periods
Rayleigh-wave group velocity maps from: earthquake tomography with
resolution
The model identifies different velocities between the Yucatan and Chortis continental blocks at 30 km depth (Fig. 9d). This seismological lower crustal difference agrees with the different origin and tectonic evolution proposed by several studies from geologic evidence and paleotectonic reconstructions (e.g., Burke, 1988; Rogers et al., 2007; Pindell and Kennan, 2009). It also reveals crustal heterogeneity on the Caribbean plate oceanic basins (Colombia, Venezuela, and Grenada) (Fig. 9c), despite the lower resolution of the model over this plate. The model also exhibits a high contrast between the upper and lower crustal velocities of the inland North American plate (Fig. 10).
Example of 1-D inversion of phase and group velocity at one node
of the grid situated on the TMVB.
Low upper-crust velocities (Fig. 9a) correspond to sedimentary basins along the Gulf Coastal Plain, the Gulf of California, the USA-Mexico border and the Motagua–Polochic fault system, while high velocities correlate with mountain ranges (e.g., the Sierra Madre Oriental, Sierra Madre Occidental, and Sierra Madre del Sur). These low velocities are observed down to approximately 5 km beneath the Gulf Coastal Plain, the Rio Grande drainage basin and the Colorado river mouth, but they reach down even further to 12 km beneath the Mississippi embayment (Figs. 9a, b, 10a). This low velocity anomaly beneath the Mississippi embayment agrees well with the sediment thickness model of Laske et al. (2013) and the velocity model of Bensen et al. (2009). Our model also shows low velocities along the USA-Mexico border with the lowest values coincident with the Rio Grande drainage basin, the major Holocene coastal depocenter west of the Mississippi delta.
Shear wave velocity maps at different depths (
The Ouachita–Marathon–Sonora orogen is a 3000 km long belt of deformed
Paleozoic rocks bordering the southern margin of the Laurentian (North
American) craton (Moreno et al., 2000; Poole et al., 2005). The eastern part
of this belt encloses low velocity areas beneath the Mississippi and Rio
Grande embayment (Fig. 9a). The location of the southern Laurentia margin
has been much debated (e.g., Moreno et al., 2000). Poole et al. (2005)
localized it along Chihuahua, Sonora, and Baja California, but Dickinson (2009) considers it still a genuine frontier of geoscience. Our results at
12 km depth (Fig. 9b) show the highest inland velocities (
Shear wave velocity along the cross-sections delineated in Fig. 9;
The extension in western North America during the late Oligocene to early
Pliocene has evolved from the continental-scale Basin and Range Province, to
a more limited region known as Gulf Extensional Province (GEP), and finally,
the deformation has been limited to the west of the GEP forming the Gulf of
California rift (Aragón-Arreola et al., 2005; and references therein).
The marine incursion over the rift formed the Gulf of California (GofC). At
present, the GofC hosts a zone of oblique extension that records the
transition from oceanic spreading centers and transform faulting in the
south (Londslade, 1989; Lizarralde et al., 2007) to the diffuse continental
deformation in the north (Oskin and Stock, 2003; González-Fernández
et al., 2005). We obtain a heterogeneous shear-wave velocity distribution
along the GofC in accordance with its different tectonic stages and with
results from several local studies (Aragón-Arreola and
Martín-Barajas, 2007; Persaud et al., 2007; Wang et al., 2009; Zhang
and Paulssen, 2012). Seismological data show a significant difference in
crustal thickness between the Sierra Madre Occidental core and its margins.
Several studies estimated the crustal thickness at the center of the Sierra
Madre Occidental around 36–40 km (Gomberg et al., 1989; Couch et al.,
1991). It thins towards the south and west to 25 km at the coast (Persaud et
al., 2007) where the crust has been thinned by extension that led to the
formation of the Gulf of California. Our model shows thinner crust beneath
the GofC (< 20 km) than in contiguous areas (Baja California
Peninsula and SMOc). We obtain
One of the novelties of this velocity model is that it clearly draws the
limits of the GEP province as high lower-crust velocities in contrast with
low velocities in the surrounding areas. For example, at 25 km depth the
contour between high (> 4.0 km s
Widely accepted Gulf of Mexico reconstruction models fit its opening from
158 to 130 Ma (e.g., Pindell and Kennan, 2009). During the extension of the
GOM, fragments detached from NAM, migrating to the south, and forming the
Yucatan Block and the northern portion of SAM plate. The GOM tectonic
evolution comprises seafloor spreading, and Yucatan Block rifting and
rotation (30–40
Some local seismic experiments of receiver functions infer thin crust
beneath the Veracruz Basin (e.g., Melgar and Pérez-Campos, 2011;
Zamora-Camacho et al., 2010). Our results confirm these observations,
revealing high velocities (
Several tomographic continental-scale studies (e.g., Alsina et al., 1996;
Van der Lee and Nolet, 1997; Vdovin et al., 1999; Godey et al., 2003; Bedle
and van der Lee, 2009) image the dichotomy between the low mantle seismic
velocities of the western North American and Caribbean plates and the high
velocities of their eastern parts. Our model shows this velocity contrast
from 50 km depth (Fig. 11) with great detail due to the large number of
stations used in Mexico and the Caribbean. We find low shear-wave velocities
in the western US, along Mexico and below the Chortis Block, and high
velocities in the central-east US, the Gulf of Mexico, the Isthmus of Tehuantepec,
the Yucatan Block, the central and eastern parts of the Caribbean plate, and
on the northern South American plate. At 50 km depth, the 4.30 km s
Along the Mesoamerican Subduction Zone high velocities at 50 km depth
coincide with a lack of active volcanism in certain areas (e.g., south of
Sierra Madre del Sur, part of the Isthmus of Tehuantepec), while low
velocities correspond to active volcanic arcs (e.g., TMVB and CAVA).
Regional and global seismic tomographic studies (Grand, 1994; Alsina et al.,
1996; Van der Lee and Nolet, 1997; Bijwaard and Spakman, 2000; Ritzwoller et
al., 2002; Ritsema et al., 2004) suggest that the lithospheric mantle has
been mostly removed and replaced by asthenospheric mantle in the region
between the Gulf of California and the Mesa Central, and from the US Basin
and Range Province to latitude 20
We invert group and phase velocities of fundamental mode Rayleigh waves to obtain a vertically polarized 3-D shear-wave velocity model (3DVSAM) of the crust and uppermost mantle of Mexico, the Gulf of Mexico and the Caribbean plate. We combine surface wave velocities from ANT and earthquake tomography. The model offers a picture of the seismic structure from 5 to 70 km depth of the region as a whole. Our model agrees with present and past tectonic processes in the region, coincides with crustal features showed in local studies, images with high detail the uppermost mantle, and exhibits some new seismological features. This model may be useful to constrain tectonic evolution models, localize regional earthquakes, simulate ground motions, and correct crustal effects in mantle tomography studies, among other possible applications.
The 3-D crustal and uppermost mantle shear-wave velocity model 3DVSAM is
available to download at:
B. Gaite designed and carried out the data processing. A. Villaseñor designed the research and collected the earthquake records. A. Iglesias developed the inversion code. I. Jiménez-Munt computed the gravity anomaly. All the authors interpreted the results. B. Gaite prepared the manuscript with contributions from the co-authors.
Seismic data come from the Mexican National Seismological Service (NSSM), and from the networks CI, CU, G, GE, II, IU, LI, NR, OV, TA and US through the IRIS Consortium. We acknowledge C. Valdés, A. Cárdenas, C. Cárdenas, I. Rodríguez, J. Pérez, J. Estrada, and S. I. Franco for providing NSSM data.
The maps and graphs were drawn with the Generic Mapping Tools (Wessel and Smith, 1998).
Funds provided by the REPSOL CO-DOS project supported B. Gaite. This is a contribution of the Team Consolider-Ingenio 2010 TOPO-IBERIA (CSD2006-00041). We thank D. García and A. Ugalde for their useful comments for improving this manuscript. We also are grateful to Taka'aki Taira and two anonymous reviewers for their constructive and helpful suggestions for enhancing the paper. Edited by: T. Taira