Geology topics

Walter D. Mooney

Publications and source records attributed to Walter D. Mooney.

At least 55 records · Page 3Linked to original sources

USGS2012

Deep crustal structure of the Cascade Range and surrounding regions from seismic refraction and magnetotelluric data

Several regional seismic refraction and magnetotelluric (MT) profiles have been completed across the Cascade Range and surrounding geologic provinces in California, Oregon, and Washington. Analysis of three MT and two seismic refraction profiles in Oregon and a coincident MT and refraction profile in northern California show a high degree of correlation between resistivity and velocity models. The main feature that is evident in both data sets is a highly conductive (2-20 ohm m) zone that occurs at depths of 6-20 km and largely within a midcrustal velocity layer of 6.4-6.6 km/s, overlying a lower crust with velocities of 7.0-7.4 km/s. Accretionary structures in the southern Washington Cascades have been shown to be related to stress release in the area of Mount St. Helens. In order to explain the similar structures in the MT and refraction models for Oregon and California, a model is proposed involving the effects of metamorphic zonation to produce the velocity structure, combined with metamorphically produced fluids and partial melt to produce the deep conductor.

California, Oregon, Washington↗

USGS2012

Crustal structure of the northwestern Basin and Range Province from the 1986 Program for Array Seismic Studies of the Continental Lithosphere Seismic Experiment

A portion of northwestern Nevada was imaged to determine the crustal structure and to assess reported differences between refraction versus reflection determinations of Moho depth and how the crustal composition and structure has been influenced by volcanic and extension mechanisms. Interpretation of the refraction/wide-angle reflection data suggests that the crust is fairly uniform in thickness and varies by less than 5 km over the 280 km east-west profile and 3 km over its 220 km north-south length. The velocity structure is characterized by five layers: 1) an uppermost crust, composed of sedimentary rocks and basement that has an average velocity of 5.7 km s-1; 2) a middle crust that extends to a depth of 18-22 with an average velocity of 6.1 km s-1; 3) a 10-12 km thick lower crust with an average velocity of 6.6 km s-1; 4) a 2-5 km thick transitional crust-mantle boundary defined by a 7.6 km s-1 velocity; and 5) an upper mantle with an average Pn velocity of 7.9-8.0 km s-1. A uniform upper mantle composition across the Basin and Range is suggested and the homogeneity of the velocity structure beneath the western Basin and Range argues for a youthful Moho and crust that has been reworked by province-wide late Cenozoic extension, episodic magmatism, and underplating. -from Authors

Journal of Geophysical Research↗

USGS2012

Crustal seismicity and the earthquake catalog maximum moment magnitudes (Mcmax) in stable continental regions (SCRs): Correlation with the seismic velocity of the lithosphere

A joint analysis of global seismicity and seismic tomography indicates that the seismic potential of continental intraplate regions is correlated with the seismic properties of the lithosphere. Archean and Early Proterozoic cratons with cold, stable continental lithospheric roots have fewer crustal earthquakes and a lower maximum earthquake catalog moment magnitude ( M cmax ). The geographic distribution of thick lithospheric roots is inferred from the global seismic model S40RTS that displays shear-velocity perturbations (δ V S ) relative to the Preliminary Reference Earth Model (PREM). We compare δ V S at a depth of 175 km with the locations and moment magnitudes ( M w ) of intraplate earthquakes in the crust ( Schulte and Mooney, 2005 ). Many intraplate earthquakes concentrate around the pronounced lateral gradients in lithospheric thickness that surround the cratons and few earthquakes occur within cratonic interiors. Globally, 27% of stable continental lithosphere is underlain by δ V S ≥3.0%, yet only 6.5% of crustal earthquakes with M w >4.5 occur above these regions with thick lithosphere. No earthquakes in our catalog with M w >6 have occurred above mantle lithosphere with δ V S >3.5%, although such lithosphere comprises 19% of stable continental regions. Thus, for cratonic interiors with seismically determined thick lithosphere (1) there is a significant decrease in the number of crustal earthquakes, and (2) the maximum moment magnitude found in the earthquake catalog is M cmax =6.0. We attribute these observations to higher lithospheric strength beneath cratonic interiors due to lower temperatures and dehydration in both the lower crust and the highly depleted lithospheric root.

Earth and Planetary Science Letters↗

USGS2012

Exploring the Earth's crust: History and results of controlled-source seismology

This volume contains a comprehensive, worldwide history of seismological studies of the Earth’s crust using controlled sources from 1850 to 2005. Essentially all major seismic projects on land and the most important oceanic projects are covered. The time period 1850 to 1939 is presented as a general synthesis, and from 1940 onward the history and results are presented in separate chapters for each decade, with the material organized by geographical region. Each chapter highlights the major advances achieved during that decade in terms of data acquisition, processing technology, and interpretation methods. For all major seismic projects, the authors provide specific details on field observations, interpreted crustal cross sections, and key references. They conclude with global and continental-scale maps of all field measurements and interpreted Moho contours. An accompanying DVD contains important out-of-print publications and an extensive collection of controlled-source data, location maps, and crustal cross sections.

GSA Memoir↗

USGS2011

Overview of the 2010 Haiti earthquake

The 12 January 2010 M w 7.0 earthquake in the Republic of Haiti caused an estimated 300,000 deaths, displaced more than a million people, and damaged nearly half of all structures in the epicentral area. We provide an overview of the historical, seismological, geotechnical, structural, lifeline-related, and socioeconomic factors that contributed to the catastrophe. We also describe some of the many challenges that must be overcome to enable Haiti to recover from this event. Detailed analyses of these issues are presented in other papers in this volume.

Earthquake Spectra↗

USGS2011

Three‐dimensional model for the crust and upper mantle in the Barents Sea region

The Barents Sea and its surroundings is an epicontinental region which previously has been difficult to access, partly because of its remote Arctic location (Figure 1) and partly because the region has been politically sensitive. Now, however, this region, and in particular its western parts, has been very well surveyed with a variety of geophysical studies, motivated in part by exploration for hydrocarbon resources. Since this region is interesting geophysically as well as for seismic verification, a major study [ Bungum et al ., 2004] was initiated in 2003 to develop a three‐dimensional (3‐D) seismic velocity model for the crust and upper mantle, using a grid density of 50 km. This study, in cooperation between NORSAR, the University of Oslo (UiO),and the U.S.Geological Survey (USGS), has led to the construction of a higher‐resolution, regional lithospheric model based on a comprehensive compilation of available seismological and geophysical data. Following the methodology employed in making the global crustal model CRUST5.1 [ Mooney et al ., 1998], the new model consists of five crustal layers: soft and hard sediments, and crystalline upper, middle, and lower crust. Both P ‐ and S ‐wave velocities and densities are specified in each layer. In addition, the density and seismic velocity structure of the uppermost mantle, essential for Pn and Sn travel time modeling, are included.

Eos, Earth and Space Science News↗

USGS2011

IASPEI workshop: Seismic modeling of laterally varying structures

During the past 10 years, significant progress has been made in the methods of collection and analysis of seismic reflection and refraction data. This progress has led to the development of new models for the structure and composition of the earth's crust, based on sophisticated analysis of numerous profiles in many areas of geologic importance. The third triannual meeting of the IASPEI (International Association of Seismology and Physics of the Earth's Interior) Commission of Controlled Source Seismology was convened in Park City, Utah, on August 11–17, 1980, to bring together seismologists and geologists to explore and assess the progress of controlled source techniques (controlled sources include explosions, air guns, and Vibroseis‐type sources), and to evaluate its significance in terms of current models of the seismic velocity structure and composition of the crust and upper mantle. Particular attention was paid to the progress and problems in the modeling of two‐ and three‐dimensional structures.

Eos, Transactions, American Geophysical Union↗

USGS2011

Pore-fluid migration and the timing of the 2005 M8.7 Nias earthquake

Two great earthquakes have occurred recently along the Sunda Trench, the 2004 M9.2 Sumatra-Andaman earthquake and the 2005 M8.7 Nias earthquake. These earthquakes ruptured over 1600 km of adjacent crust within 3 mo of each other. We quantitatively present poroelastic deformation analyses suggesting that postseismic fluid flow and recovery induced by the Sumatra-Andaman earthquake advanced the timing of the Nias earthquake. Simple back-slip simulations indicate that the megapascal (MPa)–scale pore-pressure recovery is equivalent to 7 yr of interseismic Coulomb stress accumulation near the Nias earthquake hypocenter, implying that pore-pressure recovery of the Sumatra-Andaman earthquake advanced the timing of the Nias earthquake by ∼7 yr. That is, in the absence of postseismic pore-pressure recovery, we predict that the Nias earthquake would have occurred in 2011 instead of 2005.

Lithosphere↗

USGS2011

Don't forget about the Christchurch earthquake: Lessons learned from this disaster

In the aftermath of the devastating magnitude-9.0 earthquake and tsunami that struck the Tohoku region of Japan on March 11, attention quickly turned away from a much smaller, but also highly destructive earthquake that struck the city of Christchurch, New Zealand, just a few weeks earlier, on Feb. 22. Both events are stark reminders of human vulnerability to natural disasters and provide a harsh reality check: Even technologically advanced countries with modern building codes are not immune from earthquake disasters. The Christchurch earthquake carried an additional message: Urban devastation can be triggered even by moderate-sized earthquakes.

Christchurch↗

USGS2010

The North American upper mantle: Density, composition, and evolution

The upper mantle of North America has been well studied using various seismic methods. Here we investigate the density structure of the North American (NA) upper mantle based on the integrative use of the gravity field and seismic data. The basis of our study is the removal of the gravitational effect of the crust to determine the mantle gravity anomalies. The effect of the crust is removed in three steps by subtracting the gravitational contributions of (1) topography and bathymetry, (2) low-density sedimentary accumulations, and (3) the three-dimensional density structure of the crystalline crust as determined by seismic observations. Information regarding sedimentary accumulations, including thickness and density, are taken from published maps and summaries of borehole measurements of densities; the seismic structure of the crust is based on a recent compilation, with layer densities estimated from P-wave velocities. The resultant mantle gravity anomaly map shows a pronounced negative anomaly (−50 to −400 mGal) beneath western North America and the adjacent oceanic region and positive anomalies (+50 to +350 mGal) east of the NA Cordillera. This pattern reflects the well-known division of North America into the stable eastern region and the tectonically active western region. The close correlation of large-scale features of the mantle anomaly map with those of the topographic map indicates that a significant amount of the topographic uplift in western NA is due to buoyancy in the hot upper mantle, a conclusion supported by previous investigations. To separate the contributions of mantle temperature anomalies from mantle compositional anomalies, we apply an additional correction to the mantle anomaly map for the thermal structure of the uppermost mantle. The thermal model is based on the conversion of seismic shear-wave velocities to temperature and is consistent with mantle temperatures that are independently estimated from heat flow and heat production data. The thermally corrected mantle density map reveals density anomalies that are chiefly due to compositional variations. These compositional density anomalies cause gravitational anomalies that reach ~250 mGal. A pronounced negative anomaly (−50 to −200 mGal) is found over the Canadian shield, which is consistent with chemical depletion and a corresponding low density of the lithospheric mantle, also referred to as the mantle tectosphere. The strongest positive anomaly is coincident with the Gulf of Mexico and indicates a positive density anomaly in the upper mantle, possibly an eclogite layer that has caused subsidence in the Gulf. Two linear positive anomalies are also seen south of 40°N: one with a NE-SW trend in the eastern United States, roughly coincident with the Grenville-Appalachians, and a second with a NW-SE trend beneath the states of Texas, New Mexico, and Colorado. These anomalies are interpreted as being due to (1) the presence of remnants of an oceanic slab in the upper mantle beneath the Grenville-Appalachian suture and (2) mantle thickening caused by a period of shallow, flat subduction during the Laramie orogeny, respectively. Based on these geophysical results, the evolution of the NA upper mantle is depicted in a series of maps and cartoons that display the primary processes that have formed and modified the NA crust and lithospheric upper mantle.

Journal of Geophysical Research B: Solid Earth↗

USGS2010

Global significance of a sub-Moho boundary layer (SMBL) deduced from high-resolution seismic observations

We infer the fine structure of a sub-Moho boundary layer (SMBL) at the top of the lithospheric mantle from high-resolution seismic observations of Peaceful Nuclear Explosions (PNE) on superlong-range profiles in Russia. Densely recorded seismograms permit recognition of previously unknown features of teleseismic propagation of the well known Pn and Sn phases, such as a band of incoherent, scattered, high-frequency seismic energy, developing consistently from station to station, apparent velocities of sub-Moho material, and high-frequency energy to distances of more than 3000 km with a coda band, incoherent at 10 km spacing and yet consistently observed to the end of the profiles. Estimates of the other key elements of the SMBL were obtained by finite difference calculations of wave propagation in elastic 2D models from a systematic grid search through parameter space. The SMBL consists of randomly distributed, mild velocity fluctuations of 2% or schlieren of high aspect ratios (???40) with long horizontal extent (???20 km) and therefore as thin as 0.5 km only; SMBL thickness is 60-100 km. It is suggested that the SMBL is of global significance as the physical base of the platewide observed high-frequency phases Pn and Sn. It is shown that wave propagation in the SMBL waveguide is insensitive to the background velocity distribution on which its schlieren are superimposed. This explains why the Pn and Sn phases traverse geological provinces of various age, heat flow, crustal thickness, and tectonic regimes. Their propagation appears to be independent of age. temperature, pressure, and stress. Dynamic stretching of mantle material during subduction or flow, possibly combined with chemical differentiation have to be considered as scale-forming processes in the upper mantle. However, it is difficult to distinguish with the present sets of Pn/Sn array data whether (and also where) the boundary layer is a frozen-in feature of paleo-processes or whether it is a response to an on-going processes; nevertheless, the derived quantitative estimates of the SMBL properties provide important constraints for any hypothesis on scale-forming processes. Models to be tested by future numerical and field experiments are, for example, repeated subduction-convection stretching of oceanic lithosphere (marble-cake model) and schlieren formation at mid-ocean ridges. It is also proposed that the modeling of the observed blocking of Sn and Pn propagation at active plate margins offers a new tool to study the depth range of tectonics below the crust-mantle boundary. Finally, the deduced schlieren structure of the SMBL closes an important scale gap of three to four orders of magnitude between structural dimensions studied in petrological analysis of mantle samples (xenoliths or outcrop of oceanic lithosphere) and those imaged in classical seismological studies of the lithosphere.

International Geology Review↗

USGS2010

Lithospheric buoyancy and continental intraplate stresses

Lithospheric buoyancy, the product of lithospheric density and thickness, is an important physical property that influences both the long-term stability of continents and their state of stress. We have determined lithospheric buoyancy by applying the simple isostatic model of Lachenbruch and Morgan (1990). We determine the crustal portion of lithospheric buoyancy using the USGS global database of more than 1700 crustal structure determinations (Mooney et al., 2002), which demonstrates that a simple relationship between crustal thickness and surface elevation does not exist. In fact, major regions of the crust at or near sea level (0-200 m elevation) have crustal thicknesses that vary between 25 and 55 km. Predicted elevations due to the crustal component of buoyancy in the model exceed observed elevations in nearly all cases (97% of the data), consistent with the existence of a cool lithospheric mantle lid that is denser than the asthenosphere on which it floats. The difference between the observed and predicted crustal elevation is assumed to be equal to the decrease in elevation produced by the negative buoyancy of the mantle lid. Mantle lid thickness was first estimated from the mantle buoyancy and a mean lid density computed using a basal crust temperature determined from extrapolation of surface heat flow, assuming a linear thermal gradient in the mantle lid. The resulting values of total lithosphere thickness are in good agreement with thicknesses estimated from seismic data, except beneath cratonic regions where they are only 40-60% of the typical estimates (200-350 km) derived from seismic data. This inconsistency is compatible with petrologic data and tomography and geoid analyses that have suggested that cratonic mantle lids are ??? 1% less dense than mantle lids elsewhere. By lowering the thermally determined mean mantle lid density in cratons by 1%, our model reproduces the observed 200-350+ km cratonic lithospheric thickness. We then computed gravitational potential energy by taking a vertical integral over the computed lithosphere density. Our computed values suggest that the thick roots beneath cratons lead to strong negative potential energy differences relative to surrounding regions, and hence exert compressive stresses superimposed on the intraplate stresses derived from plate boundary forces. Forces related to this lithosphere structure thus may explain the dominance of reverse-faulting earthquakes in cratons. Areas of high elevation and a thin mantle lid (e.g., western U.S. Basin and Range, East African rift, and Baikal rift) are predicted to be in extension, consistent with the observed stress regime in these areas.

International Geology Review↗

USGS2010

Cold cratonic roots and thermal blankets: How continents affect mantle convection

Two-dimensional convection models with moving continents show that continents profoundly affect the pattern of mantle convection . If the continents are wider than the wavelength of the convection cells (-3000 km, the thickness of the mantle ), they cause neighboring deep mantle thermal upwellings to coalesce into a single focused upwelling. This focused upwelling zone will have a potential temperature anomaly of about 200°C, much higher than the 100°C temperature anomaly of upwelling zones generated beneath typical oceanic lithosphere. Extensive high-temperature melts (including flood basalts and late potassic granites) will be produced, and the excess temperature anomaly will induce continental uplift (as revealed in sea level changes) and the eventual breakup of the supercontinent. The mantle thermal anomaly will persist for several hundred million years after such a breakup. In contrast, small continental blocks (<1000 km diameter) do not induce focused mantle upwelling zones. Instead, small continental blocks are dragged to mantle down-welling zones, where they spend most of their time, and will migrate laterally with the downwelling. As a result of sitting over relatively cold mantle (downwellings), small continental blocks are favored to keep their cratonic roots . This may explain the long-term survival of small cratonic blocks (e.g., the Yilgarn and Pilbara cratons of western Australia, and the West African craton). The optimum size for long-term stability of a continental block is <3000 km. These results show that continents profoundly affect the pattern of mantle convection . These effects are illustrated in terms of the timing and history of supercontinent breakup, the production of high-temperature melts, and sea level changes. Such two-dimensional calculations can be further refined and tested by three-dimensional numerical simulations of mantle convection with moving continental and oceanic plates.

International Geology Review↗

USGS2010

Seismology of the continental crust and upper mantle

More seismological studies of the continental crust and sub‐crustal lithosphere of the United States have been completed in the past four years than at any other similar period, and a continued growth in activity is likely to continue for years to come. Several trends account for this phenomenon. First, the interest in seismic reflection studies generated initially by COCORP results in this country [ Brown and others , 1986], and later by the British BIRPS results [ Matthews and Gheadie , 1986], has led to the development of several other seismic reflection programs. Among the most active of these research programs are those of the University of Wyoming [ Smithson and others , 1986], Virginia Polytechnic Institute, CALCRUST [ Henyey , 1986], and the U.S. Geological Survey (USGS) [ Hamilton , 1986]. In Canada, the Lithoprobe program has achieved remarkable results in a variety of geographic locations [ Green et al. , 1986]. A second trend is the resurgence of interest in seismic‐refraction/wide‐angle reflection profiling. The year 1978 marked the beginning of this increased activity when several large projects were conducted in the western U.S., such as the Yellowstone‐Snake River Plain experiment organized by the University of Utah. Since 1979, a large amount of refraction/wide‐angle reflection data has been collected by the USGS and by university groups utilizing large numbers of state‐of‐the‐art industry seismographs in cooperative experiments. A third trend is the increased sophistication of other seismic methods such as teleseismic delay‐time methods, tomography, and receiver‐transfer functions. In these studies, permanent or temporary recording arrays have been used to determine local and regional crustal and upper‐mantle structure with impressive resolution.

Reviews of Geophysics↗

USGS2010

Poroelastic stress-triggering of the 2005 M8.7 Nias earthquake by the 2004 M9.2 Sumatra-Andaman earthquake

The M9.2 Sumatra-Andaman earthquake (SAE) occurred three months prior to the M8.7 Nias earthquake (NE). We propose that the NE was mechanically triggered by the SAE, and that poroelastic effects were a major component of this triggering. This study uses 3D finite element models (FEMs) of the Sumatra-Andaman subduction zone (SASZ) to predict the deformation, stress, and pore pressure fields of the SAE. The coseismic slip distribution for the SAE is calibrated to near-field GPS data using FEM-generated Green's Functions and linear inverse methods. The calibrated FEM is then used to predict the postseismic poroelastic contribution to stress-triggering along the rupture surface of the NE, which is adjacent to the southern margin of the SAE. The coseismic deformation of the SAE, combined with the rheologic configuration of the SASZ produces two transient fluid flow regimes having separate time constants. SAE coseismic pore pressures in the relatively shallow forearc and volcanic arc regions (within a few km depth) dissipate within one month after the SAE. However, pore pressures in the oceanic crust of the down-going slab persist several months after the SAE. Predictions suggest that the SAE initially induced MPa-scale negative pore pressure near the hypocenter of the NE. This pore pressure slowly recovered (increased) during the three-month interval separating the SAE and NE due to lateral migration of pore fluids, driven by coseismic pressure gradients, within the subducting oceanic crust. Because pore pressure is a fundamental component of Coulomb stress, the MPa-scale increase in pore pressure significantly decreased stability of the NE fault during the three-month interval after the SAE and prior to rupture of the NE. A complete analysis of stress-triggering due to the SAE must include a poroelastic component. Failure to include poroelastic mechanics will lead to an incomplete model that cannot account for the time interval between the SAE and NE. Our transient poroelastic model explains both the spatial and temporal characteristics of triggering of the NE by the SAE.

Earth and Planetary Science Letters↗

USGS2010

Proceedings of the 25th Himalaya-Karakoram-Tibet Workshop

For a quarter of a century the Himalayan-Karakoram-Tibet (HKT) Workshop has provided scientists studying the India-Asia collision system a wonderful opportunity for workshop-style discussion with colleagues working in this region. In 2010, HKT returns to North America for the first time since 1996. The 25th international workshop is held from June 7 to10 at San Francisco State University, California. The international community was invited to contribute scientific papers to the workshop, on all aspects of geoscience research in the geographic area of the Tibetan Plateau and its bounding ranges and basins, from basic mapping to geochemical and isotopic analyses to large-scale geophysical imaging experiments. In recognition of the involvement of U.S. Geological Survey (USGS) scientists in a wide range of these activities, the USGS agreed to publish the extended abstracts of the numerous components of HKT-25 as an online Open-File Report, thereby ensuring the wide availability and distribution of these abstracts, particularly in the HKT countries from which many active workers are precluded by cost from attending international meetings. In addition to the workshop characterized by contributed presentations, participants were invited to attend a pre-meeting field trip from the Coast Ranges to the Sierra Nevada, to allow the international group to consider how the tectonic elements of the Pacific margin compare to those of the Himalayan belt. Following the workshop, the National Science Foundation (NSF) sponsored a workshop on the 'Future directions for NSF-sponsored geoscience research in the Himalaya/Tibet' intended to provide NSF Program Directors with a clear statement and vision of community goals for the future, including the scientific progress we can expect if NSF continues its support of projects in this geographic region, and to identify which key geoscience problems and processes are best addressed in the Himalaya and Tibet, what key datasets are needed, and how NSF can best support the evolving need for interdisciplinary investigations. This workshop also has clear societal relevance. Recent earthquakes have brought international attention to active tectonics and earthquake hazards in the HKT region. Prominent examples include the Mw 7.8 Kokoxili (Qinghai, China) earthquake of 2001, the Mw 7.6 Kashmir (Pakistan) earthquake of 2005, the Mw 7.9 Wenchuan (Sichuan, China) earthquake of 2008, and this year the Mw 6.9 Yushu (Qinghai, China) earthquake. Geological and geophysical field work conducted both before these earthquakes, as well as in response to them, has helped to define the active faults and regional tectonics in the HKT region. The research presented at this workshop provides the framework necessary for improved seismic hazard assessments in this region. The organizers gratefully acknowledge the support of NSF's Continental Dynamics Program and its Office of International Science and Engineering, through award EAR-0965796. We thank San Francisco State University's Sheldon Axler, Dean of the College of Science and Engineering, and Toby Garfield, Director of the Romberg Tiburon Center, for use of their conference facilities; and the Department of Geosciences, particularly Deb Shulman and Miriam Knof, for administrative support. The California Academy of Sciences generously hosted a reception for visiting delegates, and Brad Ritts (Chevron Exploration Technology Company), Todd Greene (California State University, Chico) and John Shervais (Utah State University) together co-led the pre-conference field trip. Technical editing of this volume was led by Roxanne Renedo (U.S. Geological Survey) with assistance from Margaret Milia (Stanford University). We are grateful to the U.S. Geological Survey (USGS) Earthquake Hazards Program and the USGS Menlo Park (California) Publishing Service Center for making this online report possible.

Open-File Report↗

USGS2009

Crustal structure across the Three Gorges area of the Yangtze platform, central China, from seismic refraction/wide-angle reflection data

We present active-source seismic data recorded along a 300 km-long profile across the Three Gorges area of the western Yangtze platform, central China. From west to east, the profile crosses the Zigui basin, Huangling dome and Jianghan basin. The derived crustal P -wave velocity structure changes significantly across the Tongchenghe fault that lies at the transition from the Huangling dome to the Jianghan basin. West of the Tongchenghe fault, beneath the Zigui basin and the Huangling dome, we observe a ~ 42 km thick crust of relatively low average velocity (6.3–6.4 km/s). In contrast, east of the Tongchenghe fault, beneath the Jianghan basin, the crust is only 30 km thick and has a high average velocity (6.6–6.7 km/s). A west–east variation in crustal composition along the Tongchenghe fault is also inferred. West of the fault, P -wave velocities suggest a felsic composition with an intermediate layer at the base of the crust, whilst, east of the fault, felsic, intermediate, and mafic crustal layers are apparent. Our results suggest that the crust beneath the Jianghan basin has been thinned by rifting, accompanied by intrusion of the lower crust by mafic dikes and sills. The west-to-east division of the crust in the Three Gorges area coincides with first-order geophysical contrasts in gravity, topography, crustal and lithospheric thickness.

Tectonophysics↗

USGS2009

Validation of the rupture properties of the 2001 Kunlun, China (Ms = 8.1), earthquake from seismological and geological observations

We determine the finite-fault slip distribution of the 2001 Kunlun earthquake ( M s = 8.1) by inverting teleseismic waveforms, as constrained by geological and remote sensing field observations. The spatial slip distribution along the 400-km-long fault was divided into five segments in accordance with geological observations. Forward modelling of regional surface waves was performed to estimate the variation of the speed of rupture propagation during faulting. For our modelling, the regional 1-D velocity structure was carefully constructed for each of six regional seismic stations using three events with magnitudes of 5.1–5.4 distributed along the ruptured portion of the Kunlun fault. Our result shows that the average rupture velocity is about 3.6 km s −1 , consistent with teleseismic long period wave modelling. The initial rupture was almost purely strike-slip with a rupture velocity of 1.9 km s −1 , increasing to 3.5 km s −1 in the second fault segment, and reaching a rupture velocity of about 6 km s −1 in the third segment and the fourth segment, where the maximum surface offset, with a broad fault zone, was observed. The rupture velocity decelerated to a value of 3.3 km s −1 in the fifth and final segment. Coseismic slip on the fault was concentrated between the surface and a depth of about 10 km. We infer that significant variations in rupture velocity and the observed fault segmentation are indicative of variations in strength along the interface of the Kunlun fault, as well as variations in fault geometry.

Qinghai Province↗