Robert van der Hilst

Massachusetts Institute of Technology | MIT · Department of Earth Atmospheric and Planetary Sciences

Vp/Vs models provide important complementary information to Vp and Vs models, relevant to lithology, rock damage, partial melting, water saturation, etc. However, seismic tomography using body wave traveltime data from local or regional earthquakes does not constrain Vp/Vs well due to the different resolution of Vp and Vs models, with the Vp models...

Resolving both crustal and shallow-mantle heterogeneity, which is needed to study processes in and fluxes between crust and mantle, is still a challenge for seismic tomography. Body wave data can constrain deep features but often produce vertical smearing in the crust and upper mantle; in contrast, surface wave data can provide good vertical resolu...

P-wave travel-time residuals from USArray helped improve the scale and consistency with which the mantle beneath North America is resolved. Beginning in 2008, we published a series of P-wave velocity models based on a global ray theoretical inversion of USArray and global catalog data.Here,we present the final model update, MITP-USA-2016MAY, which...

We use virtual deep seismic sounding (VDSS) and data from ~1000 broadband seismic stations to provide high-resolution estimates of crustal structure in the western Cordillera of the United States (U.S.). The most robust result is the geographic distribution of residual topography (that is, the difference between observed elevation and that expected...

We introduce a new algorithm for joint inversion of body wave and surface wave data to get better 3-D P wave (Vp) and S wave (Vs) velocity models by taking advantage of the complementary strengths of each data set. Our joint inversion algorithm uses a one-step inversion of surface wave traveltime measurements at different periods for 3-D Vs and Vp...

S U M M A R Y We propose a method to invert surface wave dispersion data directly for 3-D variations of shear wave speed, that is, without the intermediate step of phase or group velocity maps, using frequency-dependent ray tracing and a wavelet-based sparsity-constrained tomographic inversion. A fast marching method is used to compute, at each per...

The lateral expansion of the southeastern Tibetan Plateau causes devastating earthquakes, but is poorly understood. In particular, the links between regional variations in surface motion1–3 and the deeper structure of the plateau are unclear. The plateau may deform either by movement of rigid crustal blocks along large strike-slip faults4,5, by con...

Continental China consists of a complex amalgamation of geotectonic units and has experienced strong and widespread tectonic deformation since the Mesozoic. To understand its geological structure better, we conducted a systematic receiver function analysis using a total of 83 509 teleseismic traces in the time period of 2009-2010 recorded by 798 br...

Surface-wave tomography is usually applied at regional or planetary scale but represent an interesting approach also for near surface applications. We present a surface wave dispersion tomography that does not require the construction of phase or group velocity maps and which inverts path-specific dispersion data directly for 3-D Swave velocity var...

Surface-wave tomography often involves the construction of phase (or group) velocity maps through linearized inversion of measured phase (group) arrival times. Such inversions require a priori information about the medium (that is, a reference model) in order to calculate source-receiver paths, which is inaccurate for complex media, and requires re...

The reverse-time migration of global seismic data generated by free-surface multiples is regularly used to constrain the crustal structure, but its accuracy is to a large extent determined by the accuracy of the 3-D background velocity model used for wave propagation. To this improve the velocity model and hence the accuracy of the migrated image,...

A large part of global plate motion on mid-ocean ridge transform faults (RTFs) is not accommodated as major earthquakes. When large earthquakes do occur, they often repeat quasiperiodically. We focus here on the high slip rate (~14 cm/yr) Gofar transform fault on the equatorial East Pacific Rise. This fault is subdivided into patches that slip duri...

Very accurate timing of seismic recordings is critical for modern processing techniques. Clock synchronization among the instruments constituting an array is, however, difficult without direct communication between them. Synchronization to Global Positioning System (GPS) time is one option for on-land deployments, but not for underwater surveys as...

The lateral expansion of the southeastern Tibetan Plateau causes devastating earthquakes, but is poorly understood. In particular, the links between regional variations in surface motion and the deeper structure of the plateau are unclear. The plateau may deform either by movement of rigid crustal blocks along large strike-slip faults, by continuou...

The lateral expansion of the southeastern Tibetan Plateau causes devastating earthquakes, but is poorly understood. In particular, the links between regional variations in surface motion 1–3 and the deeper structure of the plateau are unclear. The plateau may deform either by movement of rigid crustal blocks along large strike-slip faults 4,5 , by...

The lateral expansion of the southeastern Tibetan Plateau causes devastating earthquakes, but is poorly understood. In particular, the links between regional variations in surface motion and the deeper structure of the plateau are unclear. The plateau may deform either by movement of rigid crustal blocks along large strike-slip faults, by continuou...

Significance Deep in the Earth’s interior, the region just above the core–mantle boundary exerts control on mantle convection and heat loss from the core. It has long been thought that the so-called D″ region is separated from a more uniform mantle above by a single interface, often attributed to a phase transition in Mg perovskite. Systematic deep...

Converted and multiply reflected phases from teleseismic events are routinely used to create structural images of the crust-mantle boundary (Moho) and the elasticity contrasts within the crust and upper mantle. The accuracy of these images is to a large extent determined by the background velocity model used to propagate these phases to depth. In o...

[1] We present a refined 3D crustal model beneath SE Tibet from ambient noise adjoint tomography. Different from ray-theory based tomography, adjoint tomography in this study incorporates a spectral-element method (SEM) and takes empirical Green's functions (EGFs) of Rayleigh waves from ambient noise interferometry as the direct observation. The fr...

We present a method that extends both the applicability and the quality of virtual deep seismic sounding (VDSS)—a technique for estimating crustal thickness that is robust even if the crust–mantle transition is complex or the crustal thickness is large. The results are important for studies of crustal contributions to isostasy and for understan...

[1] With P-to-S converted waves recorded at seismic stations of the U.S. Transportable Array, we image the fine structure of upper mantle and transition zone (TZ) beneath the western U.S. We map the topographies of seismic discontinuities by stacking data by common conversion points along profiles. Systematic depth and amplitude measurements are pe...

Surrounded by seismicity and other manifestations of active deformation, the Ordos plateau, or the western portion of the North China craton (NCC), is a uniquely stable terrane in Asia. Results from virtual deep-seismic sounding and crustal receiver functions suggests that the crust under the eastern Ordos is thicker (at least 60 km) than expected...

SUMMARY Surface wave tomography often involves the construction of phase (or group) velocity maps through linearized inversion of measured phase (group) arrival times. Such inversions require a priori information about the medium (that is, a reference model) to calculate source–receiver paths as well as regularization. Surface wave eikonal tomograp...

We present a wave equation prestack depth migration to image crust and mantle structures using multi-component earthquake data recorded at dense seismograph arrays. Transmitted P and S waves recorded on the surface are back propagated using an elastic wave equation solver. The wave modes are separated after the reverse-time continuation of the wave...

On a global scale, seismicity on oceanic transform faults that link mid-ocean ridge segments is thermally controlled. However, temperature cannot be the only control because the largest earthquakes on oceanic transform faults rupture only a small fraction of the area that thermal models predict to be capable of rupture. Instead, most slip occurs wi...

Over the past three decades, tremendous progress has been made with the mapping of mantle heterogeneity and with the understanding of these structures in terms of, for instance, the evolution of Earth's crust, continental lithosphere, and thermo-chemical mantle convection. Converted wave imaging (e.g., receiver functions) and reflection seismology...

The solution of large linear tomographic inverse problems is fundamentally non-unique. We suggest to explore the non-uniqueness explicitly by examining the null-space of the forward operator. We show that with the null-space shuttle it is possible to assess robustness in tomographic models, and we illustrate the concept for the global P-wave model...

Due to present computational limitations, migration by the one-way wave equation remains an integral tool in seismic exploration. For the realistic interpretation of common image point gathers, it is necessary that migration be free from artifacts from caustics and turning waves. In order to permit situations where turning waves occur, we perform o...

Online material : P -wave tomography model for the United States; scripts for plotting cross-sections. In 2011, the Transportable Array of USArray (http://www.iris.edu/USArray/), the seismological component of EarthScope (http://www.earthscope.org/), continued to extend east of the Rocky Mountains and the Great Plains and into the Midwest and Gulf...

Over the past three decades, tremendous progress has been made with the mapping of mantle heterogeneity and with the understanding of these structures in terms of, for instance, the evolution of Earth's crust, continental lithosphere, and thermo-chemical mantle convection. Converted wave imaging (e.g., receiver functions) and reflection seismology...

We have developed a method for converted wave imaging, which we refer to as Array Receiver Functions and which is based on Reverse Time Migration (of teleseismic wavefields), to image contrasts in Earth's structure in the crust and upper mantle. The traditional receiver function method is based on ray theory and assumes that the depth to and proper...

The mechanism of Tibetan formation and evolution is debated, with rigid block extrusion, distributed thickening, injection of Indian crust into Tibetan lower crust, and channel flow. These competing models have been proposed for the uplift and deformation of Tibet, each with different implications for the eastern part of Tibet plateau. For investig...

Global seismology and exploration seismics have long lived in parallel universes, with little cross-fertilization of methodologies and with interaction between the associated communities often limited to company recruitment of students. Fortunately, this traditional separation of technology and people has begun to disappear. This is driven not only...

Wave-equation migration of teleseismic converted and free-surface reflected phases has been successful in helping to constrain the structure of the crust and upper mantle, but its accuracy is to a large extent determined by the accuracy of the 3-D background velocity model used in the wave propagation. To this end, we wish to apply the technique of...

In the first part of our study, we use Rayleigh and Love wave Green's functions estimated from ambient seismic noise to study crustal structure and radial anisotropy in the tectonically complex and seismically active region west of Sichuan Basin and around the Eastern Himalaya Syntaxis. Seismic noise records are from 372 stations of 3 seismic array...

The southeast margin of the Tibetan Plateau lies between the heartland of the plateau to the west and the stable south China block to the east, spanning from western Sichuan to central Yunnan in southwest China. A channel flow model in which a weak zone exists in the mid-to-lower crust has been proposed to explain the low-gradient topographic slope...

The Hawaiian hotspot is often attributed to hot material rising from depth in the mantle, but efforts to detect a thermal plume seismically have been inconclusive. Most tomographic models reveal anomalously low wavespeeds beneath Hawaii, but the depth extent of this structure is not well known. S or P data used in traveltime inversions are associat...

Using data from the US Transportable Array, we combine observations of P-to-S single and multiple scattered waves to constrain the transition zone (TZ) structure beneath the Western US. From stacking the data by common conversion points along profiles, we produce depth images of seismic discontinuities. Systematic depth and amplitude measurements a...

The complex tectonic history that has shaped the western United States can be expected to have left an imprint on the crust and mantle below. As a corollary, the current configuration of crustal and upper mantle structures provides vital clues to understanding the tectonic history and associated dynamic mantle processes. Body wave conversions at a...

During the last decade, sharp decreases of wave speed have been reported regionally near or within the transition zone and associated with the presence of low-velocity layers in the deep mantle. These low velocity layers are thought to represent mantle rock dehydration-induced partial melting. Here, from global and regional observations of scattere...

Inter-station Green's functions estimated from ambient noise studies have been widely used to investigate crustal structure. However, most studies are restricted to continental areas and use fundamental-mode surface waves only. In this study, we recover inter-station surface (Scholte-Rayleigh) wave empirical Green's function (EGFs) of both the fund...

The Hawaiian hotspot is often attributed to hot material rising from depth in the mantle, but efforts to detect a thermal plume seismically have been inconclusive. To investigate pertinent thermal anomalies, we imaged with inverse scattering of SS waves the depths to seismic discontinuities below the Central Pacific, which we explain with olivine a...

To address the tectonic and magmatic modifications of the Pacific Northwest lithosphere, including transformation of the Farallon oceanic terrane “Siletzia” into continent, we study the crust and uppermost mantle of the Pacific Northwest with fundamental-mode Rayleigh-wave ambient noise tomography using periods 6–40 s, resolving isotropic shear-wav...

We use seismic prospecting data on a 40 × 40 regular grid of sources and receivers deployed on a 1 km × 1 km area to assess the feasibility and advantages of velocity analysis of the shallow subsurface by means of surface-wave tomography with Green's functions estimated from crosscorrelation. In a first application we measure Rayleigh-wave dispersi...

1] Understanding the geotectonic evolution of the southeastern Tibetan plateau requires knowledge about the structure of the lithosphere. Using data from 77 broadband stations in SW China, we invert Rayleigh wave phase velocity dispersion curves from ambient noise interferometry (T = 10–40 s) and teleseismic surface waves (T = 20–150 s) for 3‐D het...

The D" region at the base of the mantle has been characterized by seismologically inferred 3D heterogeneity, including multiple interfaces, localized low velocity zones, and anisotropy. The occurrence of the post-perovskite (PPV) phase transition with a steep Clapeyron slope of 11 to 13 MPa/K, close to the core-mantle boundary, has been a prime can...

Noise cross-correlation has been used to recover surface wave Green's functions between receivers. However, most noise cross-correlation studies are restricted to land seismic stations and few studies have observed higher-mode surface waves. We apply noise cross-correlation on three-component broadband data recorded by 30 ocean bottom seismometers...

Passive techniques are now widely used for seismic imaging, for example ambient noise tomography. A recent development of passive techniques, namely the Passive Image Interferometry, allows the monitoring of temporal variations of seismic velocities from the continuous recording of ambient noise. The sensitivity of the method is as high as 0.01% in...

Both P and S wave tomography studies reveal long wavelength fast anomaly at the mantle base of East Asia, which is probably related to ancient episodes of subduction of oceanic lithospheres. We use Generalized Radon transform (GRT) to image the lowermost mantle beneath East Asia, which is well sampled both by ScS and SKKS data. We selected data ass...

We combine results from seismic tomography and plate motion history to investigate structure and evolution of slabs of subducted lithosphere in the lower mantle beneath western Pacific. We selected earthquakes and stations along a wide corridor from the Aleutians to Australia. We measured P and S differential travel times by waveform multi-channel...

Due to the underdetermined nature of large tomographic inverse problems, a sizable nullspace exists. It is therefore important to investigate the uncertainty of tomographic models produced by inverse problems with multiple solutions. The nullspace shuttle (Deal and Nolet, 1996) has been designed to exploit components of the nullspace, along with a...

During Cenozoic time two dynamic systems, the India/ Eurasia intracontinental convergent and the West Pacific/Indonesia subduction systems, have been the major controls for the tectonic development of Southeastern Asia and its adjacent offshore basin and arcs. The boundary between these two systems is broad and fluctuated temporally and spatially b...

Finite-frequency wave-equation migration of teleseismic converted phases and free-surface multiples has proven to be an effective method for imaging structures in the crust and upper mantle using better wave dynamics. The accuracy of these images, however, depends on the accuracy of the smooth wavespeed model through which the data are migrated. Mi...

We summarize results of our seismological studies of the lithosphere beneath Tibet and SW China. Joint analysis of geological, geodetic, and seismological data suggests that the Tibetan plateau formed through interplay between continental collision between India and Asia in the west and ocean floor subduction along the western Pacific island arcs a...

In recent years, progress has been made in seismology to constrain the depth variations of the transition zone discontinuities, e.g. 410 km and 660 km discontinuities, which can be used to constrain the local temperature and chemistry profiles, and hence to infer the existences and morphology of mantle plumes. Taking advantage of the abundance of n...

Since the advent of global tomography more than 30 years ago much progress has been made with the mapping of mantle heterogeneity and anisotropy associated with mantle convection. To distinguish between end-member flow models represented by canonical layered versus whole mantle convection and thermal versus thermo-chemical convection, much emphasis...

The US Transportable Array of three-component broadband seismic stations gives us an unprecedented opportunity to study the mantle structure at high resolution and at the scale of a whole continent. From 428 earthquakes, recorded between 2004 and 2009 and processed into receiver functions, we analyze P-to-S converted-waves, including multiple rever...

A variety of different plume models have been proposed to explain the pattern of uplift, rifting, and volcanism observed throughout East Africa and western Arabia. Some studies advocate for a single, lower-mantle-originating plume impinging on the lithosphere beneath either southern Ethiopia or Afar. Other studies support multiple upwellings beneat...

We model empirical Green's Functions (EGFs) from ambient noise interferometry using spectral-element method (SEM) by putting point force at the station locations of a temporary array (Oct. 2003 - Sept. 2004) in the southeastern Tibet and the Sichuan Basin. The synthetic EGFs calculated from a 3-D model by Yao et al. (2008) well capture the characte...

Accretion of the Siletzia oceanic terrane to the Pacific Northwest of the United States ~50 Ma led to a series of tectonic and magmatic adjustments including the establishment and heterogeneous development of the Cascade volcanic arc across Siletzia, and large-scale back-arc crustal extension. To address volcanic arc development and the tectonic an...

We use Rayleigh and Love wave Green’s functions estimated from ambient seismic noise to study crustal structure and radial anisotropy in the tectonically complex and seismically active region west of Sichuan Basin and around the Eastern Himalaya Syntaxis. Seismic noise records are from 372 stations of 3 seismic arrays: MIT and Lehigh arrays (Nov 20...

1] We use Rayleigh and Love wave Green's functions estimated from ambient seismic noise to study crustal structure and radial anisotropy in the tectonically complex and seismically active region west of the Sichuan Basin and around the Eastern Himalaya Syntaxis. In agreement with previous studies, low velocity zones are ubiquitous in the mid‐lower...

In this study, we present a method for the joint inversion of receiver function and ambient noise based on Bayesian inverse theory (Tarantola, 1987, 2005). In our method, the nonlinear inversion method of the complex spectrum ratio of receiver functions (Liu et al., 1996) has been extended to perform the joint inversion of the receiver function and...

In this study, we present a method for the joint inversion of receiver function and ambient noise based on Bayesian inverse theory (Tarantola, 1987, 2005). The nonlinear inversion method of the complex spectrum ratio of receiver functions (Liu et al., 1996) is extended to perform the joint inversion of the receiver function and ambient noise with a...

Improving seismic hazard mitigation of the densely populated metropol-itan area of and around the capital of Taiwan requires detailed knowledge of the 3D crustal structure of Taipei basin. The high levels of ambient noise and the low levels of regional seismicity of this region complicate investigations of crustal structure with traditional seismic...

Online material: Updated 3-D tomography model and plotting script with examples.

Tomographic images of the mantle beneath East Asia were obtained from the inversion of traveltime data from global and regional seismograph networks and from temporary arrays on and around the Tibetan plateau. Our results are consistent with previous studies but the unprecedented resolution of mantle heterogeneity provides new insight into the larg...

The D″ region at the base of the mantle is characterized by seismologically inferred 3D heterogeneity, including multiple interfaces, localized low velocity zones, and anisotropy. The occurrence of the post-perovskite (PPV) phase transition with a steep Clapeyron slope of 11.5–13 MPa/K, close to the core–mantle boundary, is a prime candidate for ex...

We developed P- and S- wave velocity structures in the crust and upper mantle in the Arabian-Eurasian collision zone and surrounding areas, including Iran, Arabia, Eastern Turkey, and the Caucasus. We first obtain Pn and Sn velocities using local and regional arrival time data. Second, we obtain the 3-D crustal P and S velocity models. Third, we ex...

We demonstrate the feasibility of using inverse scattering for high-resolution imaging of discontinuities in the upper mantle beneath oceanic regions (far from sources and receivers) using broadband wavefield observations consisting of SS and its precursors. The generalized Radon transform (GRT) that we developed for this purpose detects (in the br...

There exist much 3D seismic heterogeneities at the base of the mantle, which may be attributed to post-perovskite ( PPV) phase transition or chemical heterogeneities. To study the irregular seismic structure that may be caused by the undulations of the PPV phase boundary, we have carried out finite-element simulations with high local resolution in...

The tectonic history of continental China is complicated and the associated lithospheric deformation is large; the use of the anisotropic propagation of surface waves is an important way to investigate the history of lithospheric deformation pattern. We measured inter-station Rayleigh-wave phase velocity dispersion from cross-correlation of narrow...

Surface waves are of increasing interest in seismic prospecting. Traveltime tomography based on dispersion measurements is often used to process surface-wave data, but it has limitations due to the a priori information it requires. The surface-wave eikonal tomography proposed here does not require such a priori information. In complex scattering en...

In this study, we present a method for the joint inversion of receiver function and ambient noise based on Bayesian inverse theory (Tarantola, 1987, 2005). The nonlinear inversion method of the complex spectrum ratio of receiver functions (Liu et al., 1996) is extended to perform the joint inversion of the receiver function and ambient noise with a...

We apply a three-dimensional (3D) generalized Radon transform (GRT) to scattered P-waves from 575 local earthquakes recorded at 68 temporary network stations for passive-source imaging of (near-vertical) structures close to the San Andreas Fault Observatory at Depth (SAFOD) site. The GRT image profiles through or close by the SAFOD site reveal near...

Taking advantage of the abundance of natural sources (earthquakes) in western Pacific subduction zones and the many seismograph stations in the Americas, we use inverse scattering - a generalized Radon transform - of SS precursors to image the transition zone discontinuities underneath Hawaii and the Hawaii-Emperor seamount chain. The GRT makes use...

Seismic structure of the crust and underlying upper mantle of Tibet reflects the physical state of the rock at depth and can offer essential constraints on the dynamics and evolution of the plateau. Data from a number of broad-band seismic experiments conducted in recent years, together with data from permanent stations in the region, produce dense...

Knowledge of the structure at or near mantle discontinuities - and of the length scales over which radial and lateral variations occur - is important for our understanding of Earth's thermal and chemical state and its evolution and dynamics. We aim to improve our knowledge of the mantle transition zone through a concerted effort of seismic imaging...

We investigate the potential of Earth's free oscillations coupled modes as a tool to constrain large-scale seismic anisotropy in the transition zone and in the bulk of the lower mantle. While the presence of seismic anisotropy is widely documented in the uppermost and the lowermost mantle, its observation at intermediate depths remains a formidable...

In 2006 a temporary seismic array of 297 broadband seismograph stations was deployed in west Sichuan region ( 26-32N; 100-105E), SW China, by State Key Laboratory of Earthquake Dynamics,Institute of Geology (IG) of Chinese Earthquake Administration (CEA). The goal of this project is to understand better the structure of the crust and upper-mantle a...

While global and regional tomography continues to provide constraints on mantle heterogeneities, discrepancies on small-scale (less than ~ 1000 km) structure limit our understanding of mantle dynamics. Tomography models which employ regularly sampled grids tend to over-parameterize areas with poor data sampling or average out small-scale structures...

Surface wave tomography is aimed at imaging near-surface heterogeneities of a medium. In order to get high-resolution images, one needs numerous seismic sources, to sample the area with a great number of source-receiver paths. On the other hand, the passive cross-correlation method (also called passive seismic interferometry) is a way to 'create' d...

Studying the crust and upper mantle beneath SE Tibet and Sichuan Province, SW China, is important for (at least) two reasons. First, this region has been central in discussions about the mechanisms of the formation and (eastward) expansion of the Tibetan plateau and, in particular, about the role of lower crustal flow. Second, as the southern part...

Estimates of (radial and azimuthal) seismic anisotropy help understand the style, direction, and - with some assumptions - the history of deformation of material deep in the Earth's interior. Surface waves can constrain azimuthal anisotropy in the lithosphere with much better depth resolution than shear wave splitting measurements. Since surface wa...

Since 2006, the State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquake Administration, has deployed a dense seismograph array in SW China in order to understand the earthquake dynamics in this region. Using about one year continuous recordings at 156 stations over western Sichuan(100°~105°E, 29°~32°N), we obtained the s...

Receiver functions are commonly used in regional seismology to give obtain images of strong subsurface discontinuities beneath regional arrays. These images, however, are subject to error due to poorly constrained uncertainties in mantle wavespeed heterogeneity and the assumption of locally horizontal interfaces. In order to simultaneously better c...

In recent years, we used the generalized Radon transform (GRT) of ScS and SKKS wavefields (separately) to image the lowermost mantle beneath Central America (Wang et al., JGR, 2006; Van der Hilst et al., Science, 2007; Wang et al., GJI, 2008) and East Asia (Shang et al., AGU, 2008). Since the quality of the GRT imaging operator depends on the apert...

Empirical Green's functions (EGFs) between receivers can be obtained from seismic interferometry through cross-correlation of pairs of ground motion records. Full reconstruction of the Green's function requires diffuse wavefields or a uniform distribution of (noise) sources. In practice, EGFs differ from actual Green's functions because wavefields...

Green's functions (GFs) of surface wave propagation between two receivers can be estimated from the cross-correlation of ambient noise under the assumption of diffuse wavefields or energy equipartitioning. Interferometric GF reconstruction is generally incomplete, however, because the distribution of noise sources is neither isotropic nor stationar...

Online material: MIT P -wave tomography model for the United States created using travel-time residuals from the USArray Transportable Array from April 2004 to December 2008. As the Transportable Array of USArray (http://www.iris.edu/USArray/), the seismology component of EarthScope (http://www.earthscope.org/), progresses eastward across the Unit...