Program For Array Seismic Studies Research Articles

Insights into mantle structure and flow beneath Alaska based on a decade of observations of shear wave splitting

AbstractSKS shear wave splitting measurements from three Program for Array Seismic Studies of the Continental Lithosphere experiments (Broadband Experiment Across the Alaska Range, Alaska Receiving Cross Transect of the Inner Core, and Multidisciplinary Observations Of Subduction), which form a north/south transect across Alaska, show a remarkably simple pattern of two large anisotropy domains. In the northern domain, extending from the 70 km contour of the subducting Pacific plate north to the Arctic Ocean, fast directions are consistently in the NE‐SW direction. These directions are essentially parallel to the absolute plate motion direction in northern Alaska and parallel to the strike of the subducting plate above the mantle wedge, suggesting that they represent some combination of plate‐scale asthenospheric flow in the upper mantle and flow along the subducting plate in the mantle wedge. A strong wedge component beneath the Alaska Range is required to explain systematics of splitting delay times. In the southern domain, which extends south from the 70 km depth contour to the subducting plate, fast directions are in the NW‐SE direction, a 90° rotation from the northern domain. These fast directions are parallel to the dip of the subducting plate in the direction of convergence and represent entrained flow beneath the subducting slab; the Pacific Plate absolute motion approximately parallels local convergence. Two major factors seem to control flow in these regions, absolute plate motion in the north and the subduction of the Pacific plate in the south, although both subduction‐driven wedge flow and absolute plate motion contribute to the southern part of the northern regime.

Crustal radial anisotropy across Eastern Tibet and the Western Yangtze Craton

AbstractPhase velocities across eastern Tibet and surrounding regions are mapped using Rayleigh (8–65 s) and Love (8–44 s) wave ambient noise tomography based on data from more than 400 Program for Array Seismic Studies of the Continental Lithosphere and Chinese Earthquake Array stations. A Bayesian Monte Carlo inversion method is applied to generate 3‐D distributions of Vsh and Vsv in the crust and uppermost mantle from which radial anisotropy and isotropic Vs are estimated. Each distribution is summarized with a mean and standard deviation, but is also used to identify “highly probable” structural attributes, which include (1) positive midcrustal radial anisotropy (Vsh > Vsv) across eastern Tibet (spatial average = 4.8% ± 1.4%) that terminates abruptly near the border of the high plateau, (2) weaker (−1.0% ± 1.4%) negative radial anisotropy (Vsh < Vsv) in the shallow crust mostly in the Songpan‐Ganzi terrane, (3) negative midcrustal anisotropy (−2.8% ± 0.9%) in the Longmenshan region, (4) positive midcrustal radial anisotropy (5.4% ± 1.4%) beneath the Sichuan Basin, and (5) low Vs in the middle crust (3.427 ± 0.050 km/s) of eastern Tibet. Midcrustal Vs < 3.4 km/s (perhaps consistent with partial melt) is highly probable only for three distinct regions: the northern Songpan‐Ganzi, the northern Chuandian, and part of the Qiangtang terranes. Midcrustal anisotropy provides evidence for sheet silicates (micas) aligned by deformation with a shallowly dipping foliation plane beneath Tibet and the Sichuan Basin and a steeply dipping or subvertical foliation plane in the Longmenshan region. Near vertical cracks or faults are believed to cause the negative anisotropy in the shallow crust underlying Tibet.

Data Products at the IRIS DMC: Stepping Stones for Research and Other Applications

In 2010, the Incorporated Research Institutions for Seismology (IRIS) Data Management Center (DMC) expanded the preexisting effort to generate, archive, and distribute data products derived from the extensive data archives of the center, which include the IRIS Program for Array Seismic Studies of the Continental Lithosphere (PASSCAL), Global Seismographic Network (GSN), EarthScope, and many other data sources. This expansion included dedicating two new full‐time staff to data products and to the development of a new system to manage and distribute them. With community guidance provided by a data products working group (DPWG), DMC has developed a number of freely available unique and increasingly popular data products for the seismological community. In addition, DMC is also archiving and distributing other data products that are produced by community members. A complete list of data products managed by DMC is maintained here: http://www.iris.edu/dms/products. Many of these are available from the Searchable Product Depository (SPUD, http://www.iris.edu/spud), the DMC data product management system. Traditionally, data products result from specialized processing applied during research by the principal investigators and their teams. Products span the range of simple plots or animations derived from waveform data to highly refined models. Some waveform‐processing techniques (e.g., receiver functions) have gained wide community use and can be derived by widely accepted algorithms. Such first‐order data products make ideal candidates for routine generation to provide a standard for the user community. Data products are typically derived from raw data to serve as insightful summaries of the raw time series data, or as quality control metrics, or as foundations or stepping stones to more specialized products and research. The overall goal of the DMC data product effort is to produce and manage a variety of community‐approved and utilized data products that complement the raw data managed at the DMC and ultimately serve …

남극 지진 및 빙권 신호 관측을 위한 초광대역 지진계 설치

극지연구소는 2010 ~ 2011년 제 24차 남극하계탐사기간 동안 남극 킹조지섬에 3성분 초광대역 지진계를 성공적으로 설치하였다. 미국 IRIS PASSCAL에서 개발한 지진관측시스템을 활용하였으며, 이는 POLENET 지진관측시스템과 완벽히 호환된다. 지진계설치 및 운용의 목적은 크게 두가지로 나누어 지는데, 첫번째로 킹조지섬 주변의 지진 및 빙권관련 신호를 관측하는데 있으며, 두번째로는 극한환경에서의 지진관측장비 성능검사이다. 관측이 성공적으로 수행된다면, 다음 목표로 관측시스템의 추가확보를 통하여 남극반도 라르센 빙붕시스템과 같은 빙붕 주변으로 관측장소를 이동하여 지권과 빙권간 상호작용을 이해하고, 빙권의 움직임등 동역학적인 해석을 위한 연구를 수행하고자 한다. 이 보고서에서 우리는 지진파 배경잡음분석을 통하여 지진관측시스템 성능 및 특성 파악에 관한 내용과 관측시스템 운용관련 정보를 제공하고자 한다. Korea Polar Research Institute (KOPRI) has successfully installed two autonomous very broadband three-component seismic stations at the King George Island (KGI), Antarctica, during the 24th KOPRI Antarctic Summer Expedition (2010 ~ 2011). The seismic observation system is originally designed by the Incorporated Research Institutions for Seismology Program for Array Seismic Studies of the Continental Lithosphere Instrument Center, which is fully compatible with the Polar Earth Observing Network seismic system. The installation is to achieve the following major goals: 1. Monitoring local earthquakes and icequakes in and around the KGI, 2. Validating the robustness of seismic system operation under harsh environment. For further intensive studies, we plan to move and install them adding a couple more stations at ice shelf system, e.g., Larsen Ice Shelf System, Antarctica, in 2013 to figure out ice dynamics and physical interaction between lithosphere and cryosphere. In this article, we evaluate seismic station performance and characteristics by examining ambient noise, and provide operational system information such as frequency response and State-Of-Health information.

An Improved Method to Extract Very-Broadband Empirical Green's Functions from Ambient Seismic Noise

The extraction of Green’s functions by cross correlation of continuous seismic records at station pairs is best achieved in a diffuse wave field, where energies radiated by random sources have equal power. To partially satisfy the diffuse wave‐field condition, original seismic records must be normalized as they are highly nonstationary and may have amplitude variations of orders of magnitude within the cross‐correlation time window. We adapt a frequency–time normalization method to obtain seismograms with an even spectrum at all times within the data‐processing unit. Compared with the commonly used one‐bit normalization, the new method improves the signal‐to‐noise ratio of empirical Green’s functions by a factor of ∼2 and increases the effective data‐recording duration by a factor of 4, assuming random local and instrument noise. It yields useful Rayleigh waves at periods up to 300 s for Program for Array Seismic Studies of the Continental Lithosphere (PASSCAL)–type deployments and 600 s for permanent stations with very‐broadband sensors. Thus, the new method makes it possible to extend surface‐wave signals to frequencies beyond those in traditional earthquake‐based surface‐wave tomography at both the high‐ and low‐frequency ends.

Rayleigh wave phase velocity maps of Tibet and the surrounding regions from ambient seismic noise tomography

Ambient noise tomography is applied to the significant data resources now available across Tibet and surrounding regions to produce Rayleigh wave phase speed maps at periods between 6 and 50 s. Data resources include the permanent Federation of Digital Seismographic Networks, five temporary U.S. Program for Array Seismic Studies of the Continental Lithosphere (PASSCAL) experiments in and around Tibet, and Chinese provincial networks surrounding Tibet from 2003 to 2009, totaling ∼600 stations and ∼150,000 interstation paths. With such a heterogeneous data set, data quality control is of utmost importance. We apply conservative data quality control criteria to accept between ∼5000 and ∼45,000 measurements as a function of period, which produce a lateral resolution between 100 and 200 km across most of the Tibetan Plateau and adjacent regions to the east. Misfits to the accepted measurements among PASSCAL stations and among Chinese stations are similar, with a standard deviation of ∼1.7 s, which indicates that the final dispersion measurements from Chinese and PASSCAL stations are of similar quality. Phase velocities across the Tibetan Plateau are lower, on average, than those in the surrounding nonbasin regions. Phase velocities in northern Tibet are lower than those in southern Tibet, perhaps implying different spatial and temporal variations in the way the high elevations of the plateau are created and maintained. At short periods (<20 s), very low phase velocities are imaged in the major basins, including the Tarim, Qaidam, Junggar, and Sichuan basins, and in the Ordos Block. At intermediate and long periods (>20 s), very high velocities are imaged in the Tarim Basin, the Ordos Block, and the Sichuan Basin. These phase velocity dispersion maps provide information needed to construct a 3‐D shear velocity model of the crust across the Tibetan Plateau and surrounding regions.

Characterizing the 410 km discontinuity low‐velocity layer beneath the LA RISTRA array in the North American Southwest

Receiver functions recorded by the 54‐station 920 km long Program for Array Seismic Studies of the Continental Lithosphere–Incorporated Research Institutions for Seismology Colorado Plateau/Rio Grande Rift Seismic Transect Experiment (LA RISTRA) line array display a pervasive negative polarity P to S conversion (Pds) arrival preceding the positive polarity 410 km discontinuity arrival. These arrivals are modeled as a low‐velocity layer atop the 410 km discontinuity (410‐LVL) and are inverted for a velocity profile via a grid search using a five‐parameter linear gradient velocity model. Model parameter likelihood and correlations are assessed via calculation of one‐ and two‐dimensional marginal posterior probability distributions. The maximum likelihood model parameter values found are top velocity gradient thickness of 0.0 km with a 4.6% (−0.22 km/s) shear velocity reduction, a 19.8 km constant velocity layer, and bottom gradient thickness of 25.0 km with a 3.5% (+0.17 km/s) shear velocity increase. The estimated mean thickness of the 410‐LVL is 32.3 km. The top gradient of the 410‐LVL is sharp within vertical resolution limits of P to S conversion (<10 km), and the diffuse 410 km velocity gradient is consistent with hydration of the olivine‐wadsleyite phase transformation. The 410‐LVL is interpreted as a melt layer created by the Transition Zone Water Filter model. Two secondary observations are found: (1) the 410‐LVL is absent from the SE end of the array and (2) an intermittent negative polarity P525s arrival is observed. We speculate that upper mantle shear velocity anomalies above the 410 km discontinuity may manifest Rayleigh‐Taylor instabilities nucleated from the 410‐LVL melt layer that are being shed upward on time scales of tens of millions of years.

Field Geophysics Class at Volcán Tungurahua, Ecuador

Ecuador's erupting Volcán Tungurahua was the recent site of a 3‐week graduate‐level geophysical course on volcanoes, hosted by Ecuador's Instituto Geofisico of the Escuela Politecnica Nacional (IG‐EPN) and the Department of Earth Science at the New Mexico Institute of Mining and Technology (NMT). Sixteen students from 12 universities and four countries participated in the intensive course, which entailed broadband seismometer and infrasound sensor deployment followed by subsequent data processing, analysis, interpretation, and result synthesis. Hardware for the course was provided by the Incorporated Research Institutes for Seismology (IRIS) through the Program for Array Seismic Studies of the Continental Lithosphere (PASSCAL) as well as the IG‐EPN and NMT geophysics programs.

Teleseismic Receiver Function and Surface-Wave Study of Velocity Structure beneath the Yanqing-Huailai Basin Northwest of Beijing

Abstract Shear-wave velocities beneath the Yanqing-Huailai Basin, 90–140 km northwest of Beijing, are estimated from the joint inversion of surface-wave phase velocities and teleseismic receiver functions. The data set is from a temporary broadband seismic network supported by the Program for Array Seismic Studies of the Continental Lithosphere (PASSCAL) in the basin and includes 34 teleseismic events from 2003 to 2005. Receiver functions from the teleseismic events are similar for the stations around the Yanqing-Huailai Basin and exhibit little variation with azimuth. The velocity models constrained by receiver functions and surface-wave dispersion curves are also similar. The resulting models reflect the low-velocity basin sediments to 2 km followed by a positive velocity gradient to 15 km with shear-wave velocity increasing from 2.0 to 3.55 km/sec. Evidence of a midcrust low-velocity layer starts at 15 km with a shear velocity decrease to 3.3 km/sec that extends to approximately 25 km. The total crustal thickness is 38–42 km with a smooth Moho transition to an upper-mantle shear velocity of 4.3 km/sec. The low-velocity zone is consistent with recent extension, geothermal activity, and earthquake locations above this depth. The average shear velocity model for the basin has similarities to other regional and global models but provides more detailed structure in the uppermost and lower portions of the crust. The new model includes the effect of the sediments in the basin, the low-velocity layer, and the gradual Moho transition. Predicted P - and S -travel times are 1–3.5 sec slower than the previous models at regional distances.

Seismic Recording on Drifting Icebergs: Catching Seismic Waves, Tsunamis and Storms from Sumatra and Elsewhere

Under the Incorporated Research Institutions for Seismology (IRIS) Program for Array Seismic Studies of the Continental Lithosphere (PASSCAL) project SOUTHBERG, we have operated for the past three years a number of seismic stations on giant Antarctic icebergs parked or drifting slowly along the margins of the Ross Sea. The purpose of this deployment was to investigate in situ the characteristics and origin of high-frequency tremors emanating from inside the icebergs, tremors that previously had been detected in the far field as hydroacoustic T phases recorded in French Polynesia (Talandier et al. 2002, 2006). Similar observations were made later in the Indian Ocean by Chapp et al. (2005). In this general context, the purpose of this paper is to report on the ancillary functioning of iceberg-sited seismometers as teleseismic observatories, and in particular their recording of the two Sumatra mega-earthquakes of 26 December 2004 and 28 March 2005. These data are available upon request from the authors. We are in the third and final year of the SOUTHBERG deployment. In November–December 2003, we installed four seismic stations on iceberg C16 (including an STS-2 broadband instrument at the central station C16A) and left one of them to winter over during 2004. Because all stations work on solar power, the station shut down on 28 May 2004 after the arrival of permanent winter darkness at that latitude, but it woke up and started recording uneventfully on 25 September 2004 around the austral spring equinox, thus providing a total of 141 days of seismic recording, which went beyond our expectations. Bolstered by this performance, we left four stations to winter over in 2005: one at C16A (hereafter referred to as C16 for short), one on iceberg B15A, one on iceberg B15K (a fragment of B15A that detached in 2004), and one on …

Variations in Short-period Geophone Responses in Temporary Seismic Arrays

Geophones with resonant frequencies of 1–4 Hz are commonly used as short-period sensors for site-response studies ( e.g. , Bonilla et al. , 1997; Hartzell et al. , 1997; Frankel et al. , 1999; Hartzell et al. , 2000) and crustal-reflection, refraction, and tomography experiments ( e.g. , Brocher et al. , 2001; Fuis et al. , 2003). In many cases these studies require only arrival-time picks, but site-response and attenuation measurements require accurate ground-velocity data from these sensors. Moreover, these geophones are often used to record seismic phases at or below their nominal resonant frequencies ( e.g. , Bonilla et al. , 1997; Hartzell et al. , 2000; Dutta et al. , 2001; Pratt et al. , 2003b; Pratt et al. , 2003a). For studies that require accurate ground-velocity measurements or that use a variety of sensor types, the instrument response must be removed from the data using either the manufacturer's specifications or results from geophone calibration tests. Geophone placement, particularly any tilt to the sensor, can alter the instrument response significantly. The geophone response ideally is measured while the sensors are in the ground, but such in situ calibrations generally are not carried out because of time constraints. More commonly, it simply is assumed the geophones have the response characteristics specified by the manufacturer. In a recent site-response study in the Seattle area, named the Seattle Seismic Hazard Investigation of Puget Sound (Seattle SHIPS 2002; Pratt et al. , 2003b), we used 72 three-component geophones from the Program for Array Seismic Studies of the Continental Lithosphere (PASSCAL) instrument pool to determine the amplitudes of both horizontal and vertical ground shaking at sites over the Seattle sedimentary basin (Pratt et al. , 2003b). During the experiment, we used the signal-coil calibration method (Rogers …

IRIS Seismology Program marks 20 years of discovery

The year 2004 marked the twentieth anniversary of the Incorporated Research Institutions for Seismology (IRIS) Consortium and its four core programs: the Program for Array Seismic Studies of the Continental Lithosphere (PASSCAL), the Global Seismic Network (GSN), Education and Outreach (E&O),and the Data Management System (DMS).IRIS activities (www.iris.edu) are supported principally through the U.S. National Science Foundation's (NSF) Earth Sciences Instrumentation and Facilities Program, and through the community efforts of IRIS's more than 150 member institutions in the United States and abroad. The consortium provides fundamental capabilities for the exploration of the Earth's interior through the collection and distribution of seismographic data. PASSCAL is the IRIS facility program dedicated to seismographic data collection associated with temporary deployments using highly portable instrumentation.

Lithospheric assembly and modification of the SE Canadian Shield: Abitibi‐Grenville teleseismic experiment

This paper presents the results of a joint Lithoprobe‐Incorporated Research Institutions for Seismology (IRIS)/Program for Array Seismic Studies of the Continental Lithosphere (PASSCAL) teleseismic experiment that investigates portions of the Grenville and Superior Provinces of the Canadian Shield along the Québec‐Ontario border. Data from a 600‐km‐long, N‐S array of 28 broadband seismographs deployed between May and October 1996 have been supplemented with additional recordings from an earlier 1994 deployment and from stations of the Canadian National Seismograph Network and the Southern Ontario Seismic Network. Relative delay times of P and S waves from 123 and 40 teleseismic events, respectively, have been inverted for velocity perturbations in the upper mantle and reveal a low‐velocity, NW‐SE striking corridor that crosses the southern portion of the line at latitude 46°N and lies between 50 and 300 km depth. Multievent S K S‐splitting results yield an average delay time of 0.57±0.22 s and a direction of fast polarization of N93°E±18°, which is consistent with an earlier interpretation as being due to fossil strain fields related to the last major regional tectonic event. Subtle variations in splitting parameters over the low‐velocity corridor may suggest an associated disruption in mantle fabric. Profiling of radial receiver functions reveals large and abrupt variations in Moho topography, specifically, a gradual thickening in crust from 40 to 45 km between latitudes 45°N and 46°N, which is followed by an abrupt thinning to 35 km at 46.6°N, some 65 km southeast of the Grenville Front. This structure is interpreted as a subduction suture extending the full length of the Front and punctuating a major pre‐Grenvillian (Archean‐Proterozoic) episode of lithospheric assembly in the southeast Canadian Shield. The low‐velocity mantle corridor, by contrast, is better explained as the extension of the Monteregian‐White Mountain‐New England seamount hotspot track below the craton and is here postulated to represent interaction of the Great Meteor plume with zones of weakness within the craton developed during earlier rifting episodes.

Seismic anisotropy and mantle flow from the Great Basin to the Great Plains, western United States

Shear wave splitting and P, SKS, and S travel time residuals are calculated for teleseismic arrivals recorded on the Colorado Plateau‐Great Basin Program for Array Seismic Studies of the Continental Lithosphere (PASSCAL) portable broadband seismic deployment and for permanent stations in the western United States. Little shear wave splitting is observed for broadband recordings in the northern Colorado Plateau, the Rocky Mountains, or the central Great Basin. The transition between the Colorado Plateau and the Great Basin is marked by moderate shear wave splitting (1.0 s) and unusually late teleseismic phase arrivals. This suggests material with a higher content of mantle melt or volatiles than regions to either side. Splitting in the transition between the Colorado Plateau and Great Basin is part of a pattern of fast polarizations that align in a semicircle, surrounding a central Great Basin region of null (no splitting) measurements. Away from the California plate boundary, splitting to the north and south of our study region aligns roughly parallel to the absolute plate motion of the North American plate. No simple spatial relation of splitting with geological and geophysical features such as mountain ranges, velocity anomalies, gravity, magnetics, or heat flow is evident in most of the western United States. However, splitting in the Great Basin is compatible with asthenospheric flow. The smallest shear wave splitting delay times coincide with the Eureka Low in heat flow, also having low S velocity at 300 km depth. We suggest that the circumferential pattern of fast polarization directions ringing a central region of nulls in the Great Basin is caused by mantle flow, by the interaction of mantle up welling and the absolute motion of the North American plate.

Mantle discontinuity structure beneath the Colorado Rocky Mountains and High Plains

Analysis of mantle discontinuity structure using converted P to S (PaS) phases beneath Colorado from the Program for Array Seismic Studies of the Continental Lithosphere (PASSCAL) Rocky Mountain Front (RMF) experiment reveals significant topography at the 410 and 660 km depth discontinuities and corresponding transition zone thickness variations. A stack of all radial receiver functions resolves the 410 and 660 km discontinuities at average depths of 419 and 677 km, respectively. Imaging of lateral variations in mantle discontinuity structure is accomplished by geographically binning the Pds conversion points and then stacking the receiver functions in each bin to form spatial images, analogous to common depth point stacking. Corrections for lateral velocity heterogeneity are calculated using the local S wave tomographic model of Lee and Grand [1996] and a constant ∂lnVs/∂lnVP scaling of 1.3. This scaling value is determined from the relative scaling between teleseismic P and S wave travel time residuals measured from the Rocky Mountain Front deployment. Mantle discontinuity images using 150 km square bins show 20 km of 410 km discontinuity topography, 30 km of 660 km discontinuity topography, and up to 40 km of transition zone thickness variation. Features of the discontinuity structure include a 20 km depression of the 660 km discontinuity beneath western Colorado and a gradual 10 km dip of the 410 km discontinuity beneath the High Plains. The thickening of the transition zone beneath southwest Colorado is consistent with the presence of the subducted Farallon slab in this region as imaged by Van der Lee and Nolet [1997]. In general, our results show that the transition zone discontinuity structure is more complex than that predicted by the simple model of olivine phase boundaries modulated by vertically coherent thermal anomalies.

Estimation of crustal stochastic parameters from seismic exploration data

Stochastic models for the crystalline crust produce synthetic seismograms that compare well with recorded data for a variety of crustal ages and tectonic environments. In this paper, we explore the parameter space describing such stochastic models as a basis for formulating the inverse problem; that is, we wish to estimate the parameters which define a stochastic model from the recorded backscattered wave field. We base the estimation on approximate relations between the primary reflectivity series, which is the ideal wave field response of a medium, and various seismic gathers. A two‐dimensional lateral correlation method is used to investigate the sensitivity of synthetic wave fields to horizontal characteristic length. A derived empirical relationship relates the scale length to the half width of the correlation coefficient. A horizontal wavenumber‐time domain spectral analysis successfully estimates the horizontal characteristic length (ax) and the fractal dimension (D) of the stochastic medium from which the wave field was backscattered. We employ a Monte Carlo based optimization scheme to fit the observed spectral values to a one‐dimensional von Kármán function and simultaneously estimate values for ax and the Hurst number ν. The method yields consistent results when tested on synthetic data. Application of this technique to data sorted into different domains shows that zero‐offset sections yield the best estimate. We also present results of our estimates using this method on a Program for Array Seismic Studies of the Continental Lithosphere (PASSCAL) dynamite source shot gather and a vibroseis‐source Consortium for Continental Reflection Profiling (COCORP) data set recorded in the Basin and Range Province.

Upper mantle shear structure beneath the Colorado Rocky Mountains

A tomographic inversion of teleseismic shear wave delays has been performed using data collected in the 1992 Program for Array Seismic Studies of the Continental Lithosphere Rocky Mountain Front experiment. The shear wave residuals used as data were corrected for known crustal, sediment and topographic differences and thus were sensitive to mantle shear velocity variations beneath the Colorado array. Large differences in shear wave travel time are observed across Colorado and into western Kansas with a maximum difference of near 5 sec between the central Rockies in Colorado and a station in western Kansas. The shear wave residuals were inverted for shear wave velocity variations in the upper 350 km of the mantle beneath Colorado and Kansas. Large differences in mantle shear velocity are found between the Rockies and the Kansas Great Plains. The average shear velocity in the upper 200 km is about 9% greater beneath Kansas than beneath the Rockies. At greater depths, lateral variations are far smaller and not as well correlated with surface tectonics. In a qualitative sense, there appear to be three mantle structures present beneath the study area. Beneath the Rockies the upper mantle is extremely slow. Both to the west and east of the Rockies we find intermediate mantle shear velocity. There is then an abrupt increase in mantle shear velocity near the Colorado‐Kansas border. We interpret the abrupt increase in mantle shear velocity near the Colorado‐Kansas border to mark the boundary of cratonic lithosphere. The slow velocity zone beneath the Rockies appears to be upwelled asthenosphere as is seen beneath the Rio Grande Rift to the south. To match the seismic contrast between the mantle beneath Kansas and the Colorado Rockies requires a temperature contrast of at least 350° C as well as partial melt beneath the Rockies. The hot mantle beneath the Rockies implies that the mantle density is low and the high topography of the Rockies is at least partially due to buoyant mantle.

Crustal thickness variations across the Colorado Rocky Mountains from teleseismic receiver functions

Variations in crustal thickness from the Great Plains of Kansas, across the Colorado Rocky Mountains, and into the eastern Colorado Plateau are determined by receiver function analysis of broadband teleseismic P waveforms recorded during the 1992 Rocky Mountain Front Program for Array Seismic Studies of the Continental Lithosphere (PASSCAL) experiment. The receiver functions are calculated using a time domain deconvolution approach and are interpreted in terms of a single crustal layer, with thickness determined by a grid‐search comparison of observed receiver functions with synthetics. The average crustal thicknesses determined by these methods are Kansas Great Plains, 43.8±0.4 km; Colorado Great Plains, 49.9±1.2 km; Colorado Rocky Mountains, 50.1±1.3 km; and northeast Colorado Plateau, 43.1±0.9 at latitudes of 38°–40°N. The main variations in crustal thickness that we observe are between the Kansas Great Plains and the Colorado Great Plains and between the Rocky Mountains and the Colorado Plateau. There is not a significant crustal thickness difference between the Colorado Great Plains and the Colorado Rocky Mountains. Together with gravity data and mass balance calculations, these results are incompatible with the hypothesis that the compensation of the Rocky Mountains relative to the Great Plains is accommodated purely by an Airy‐type crustal root or any other mechanism that restricts compensation solely to the crust and requires significant support for the excess topography of the Rocky Mountains to come from the mantle. Models with a rigid elastic plate may match receiver function estimates of crustal thickness but underpredict the amplitude of the gravity low over the Rockies. Our favored model includes lateral variations in crustal velocities obtained from refraction studies and crustal thickness variations constrained by the receiver functions. These models indicate that there is a profound transition in mantle density structure near the eastern range front.

Active thrust front of the Greater Caucasus: The April 29, 1991, Racha earthquake sequence and its tectonic implications

Although fault‐bounded thrust sheets are common in the geological record, seismic evidence for their motion is sparse. The April 29, 1991, Racha earthquake (Ms = 7.0), the largest instrumentally recorded earthquake in the Greater Caucasus, is one of the largest recent earthquakes in continental thrust belts and provides evidence on mechanisms of thrust sheet motion. Using data from a deployment of Program for Array Seismic Studies of the Continental Lithosphere (PASSCAL) digital seismographs and various other instruments, we locate 1952 aftershocks occurring between May 7 and June 30, 1991. The aftershocks form a zone ∼70 km long and 10–25 km wide striking E‐W, following the Racha ridge at the southern boundary of the Greater Caucasus thrust system. Teleseismic body waves are inverted for source parameters of the mainshock and the two largest aftershocks. The solutions show thrust faulting with centroid depths of 3–10 km, comparable to depths of locally recorded aftershocks (∼2–12 km). The shallow‐dipping nodal plane, the aftershock distribution, and surface geology demonstrate that the main event was caused by faulting on a thrust system dipping NNE at 20°–31° bounding the southern slope of the Greater Caucasus. This fault system thrusts the Greater Caucasus structures south over the Dzhirula basement massif. The inferred fault geometry suggests that the active fault is either a detachment between sediments and Dzhirula basement or cuts through the basement at shallow depths. The 1500‐m‐high Racha ridge overlies the aftershock zone and is a likely consequence of repeated similar earthquakes. Hence the 1991 earthquake sequence shows that the western Greater Caucasus is accommodating plate convergence at a rate possibly comparable to the eastern Greater Caucasus (a few millimeters per year). Along‐strike geological discontinuities above and below the thrust surface correspond to the eastern end of the mainshock rupture area. No strong evidence for transfer structures could be found along strike, suggesting that differences in collisional style between the western and eastern Greater Caucasus may reflect differences in mechanical properties rather than differences in convergence rate. A June 15, 1991, event and its aftershocks, southeast of the primary aftershock zone along strike, show fault planes and slip vectors rotated ∼41° clockwise from the mainshock. This rotation is consistent with an along‐strike change in direction of the thrust front, near 44°E longitude, and demonstrates strong local structural or topographic control on slip direction. The rotation requires along‐strike shortening within the Greater Caucasus thrust system at a rate comparable to the rate of thrusting.

Mantle anisotropy beneath the Tibetan Plateau: evidence from long-period surface waves

We investigate long-period surface waves ( T > 100 s) propagating across the Tibetan Plateau, based on the seismic records from a PASSCAL portable array and nearby Chinese Digital Seismic Network (CDSN) stations. Significant quasi-Love waveform anomalies, associated with fundamental Rayleigh-Love coupling, are consistently observed, suggesting that strong lateral gradients in azimuthal anisotropy exist beneath the plateau. More intense surface-wave scattering is observed at intermediate periods ( T ≈ 50 s), which suggests that the cause of the long-period quasi-Love waves lies in the upper mantle, not the thickened crust of the plateau. A detailed analysis of the data indicates that the long-period waveform anomalies are generated beneath the central Tibetan Plateau, where the structural trend implies deep deformations induced by continental collision. The absence of quasi-Love anomalies at the westernmost station of the Program for Array Seismic Studies of the Continental Lithosphere (PASSCAL) array suggests that the east-west extent of this mantle deformation is limited. Some, but not all, of the quasi-Love observations are consistent with SKS splitting observations. Both sets of observations predict a strong gradient in anisotropic properties in central Tibet. However, the quasi-Love waveforms are absent from records collected in the northern plateau for northerly propagation paths, which is not consistent with a gradient at the northern edge of the plateau, as suggested by SKS studies. This discrepancy indicates that the simple models used to interpret both body- and surface-wave data may be inadequate. Either significant P-wave anisotropy exists under the plateau, or the S-wave anisotropy does not possess a uniformly horizontal symmetry axis, or strong east-west gradients in anisotropy bias our data-synthetic comparison.