Arthur J. Rodgers

Crustal structure of northern and southern Tibet from surface wave dispersion analysis

[1] Group and phase velocities of fundamental mode Rayleigh waves, in the period range of 10 to 70 s, are obtained for southern and northern Tibet. Significant variations in crustal velocity structure are found. The group velocity minimum for Tibet occurs at � 33 s and the minimum is � 0.12 km/s lower for southern Tibet than for northern Tibet. At periods greater than 50 s, however, group velocities are up to 0.2 km/s faster in southern Tibet. The group and phase velocities are inverted for layered S wave models. The dispersion observations in southern Tibet can only be fit with a low-velocity layer in the middle crust. In contrast, the velocity models for northern Tibet do not require any lowvelocity zone in the crust. The S wave velocity of the lower crust of southern Tibet is � 0.2 km/s faster than the lower crust of northern Tibet. In southern Tibet the sub-Moho velocity increases with a positive gradient that is similar to a shield, while there is no velocity gradient beneath northern Tibet. The high-velocity lower crust of southern Tibet is consistent with the underthrusting of Indian continental lithosphere. The most plausible explanation of the mid-crustal low velocity zone is the presence of crustal melt resulting from H2O-saturated melting of the interplate shear zone between the underthrusting Indian crust and overflowing Asian crust. The lack of a pronounced crustal low-velocity zone in northern Tibet is an indication of a relatively dry crust. The low S wave velocity in the lower crust of northern Tibet is interpreted to be due to a combination of compositional differences, high temperatures, presumably caused by a high mantle heat flux, and possibly small amounts of partial melt. Combined with all available observations in Tibet, the new surface wave results are consistent with a hot and weak upper mantle beneath northern Tibet. INDEX TERMS: 7205 Seismology: Continental crust (1242); 7218 Seismology: Lithosphere and upper mantle; 7255 Seismology: Surface waves and free oscillations; 8102 Tectonophysics: Continental contractional orogenic belts; KEYWORDS: Tibet, crustal velocity structure, surface wave, Rayleigh waves, continental collision

Lithospheric structure of the Qiangtang Terrane, northern Tibetan Plateau, from complete regional waveform modeling: Evidence for partial melt

We report models of P and S wave velocity and attenuation for the the crust and uppermost mantle of the Qiangtang Terrane, northern Tibetan Plateau, inferred by fitting reflectivity synthetic seismograms to observed complete regional waveforms. The data are three-component broadband seismograms recorded by the 1991–1992 IRIS-PASSCAL (Incorporated Research Institutions for Seismology-Program for Array Seismic Studies of the Continental Lithosphere) Tibetan Plateau Experiment and Global Seismic Network stations in the region. The Qiangtang Terrane has thick crust (65±5 km) with P and S wave velocities of 6.1–6.3 and 3.34–3.43 km/s, respectively, yielding an anomalously high crustal Poissons ratio of 0.29±0.02. Seismic velocities of the upper mantle of the Qiangtang Terrane are normal for P waves and slow for S waves (8.10 and 4.35–4.41 km/s, respectively) with a high mantle Poissons ratio of 0.29±0.01. Attenuation in the crust and upper mantle is high (QP = 100–200 and QS = 44–89). Modeling of the broadband P waveforms suggests that a decrease in mantle velocity occurs at about 160 km depth in the mantle; however, this is not unambiguously supported by the data and modeling. The crust and uppermost mantle of the Qiangtang Terrane probably contains partial melt based on the high Poissons ratio, low shear wave velocities, and low Q. The absence of high-frequency Sn and the presence of volcanism of mantle lithospheric origin support the presence of partial melt. Crustal and uppermost mantle structure in the Qiangtang Terrane is different from that for the Lhasa Terrane (immediately to the south) based on results of our previous studies. The average crustal P and S wave velocities are 4% faster and 2% slower, respectively, in the Qiangtang Terrane relative to the Lhasa Terrane. This yields a significant difference in the crustal Poissons ratio with values of 0.29 for the Qiangtang Terrane and 0.25 for the Lhasa Terrane. Differences in the uppermost mantle P and S wave velocities and Poissons ratios of these two adjacent terranes cannot be explained by temperature differences alone. Using the mantle temperature estimates of McNamara et al.. [1997] we suggest that partial melt of an ultramafic composition beneath the Qiangtang Terrane fits the velocity and Poissons ratio estimates.

Amplitude corrections for regional seismic discriminants

Abstract — A fundamental problem associated with event identification lies in deriving corrections that remove path and earthquake source effects on regional phase amplitudes used to construct discriminants. Our goal is to derive a set of physically based corrections that are independent of magnitude and distance, and amenable to multivariate discrimination by extending the technique described in Taylor and Hartse (1998). For a given station and source region, a number of well-recorded earthquakes is used to estimate source and path corrections. The source model assumes a simple Brune (1970) earthquake source that has been extended to handle non-constant stress drop. The discrimination power in using corrected amplitudes lies in the assumption that the earthquake model will provide a poor fit to the signals from an explosion. The propagation model consists of a frequency-independent geometrical spreading and frequency-dependent power law Q. A grid search is performed simultaneously at each station for all recorded regional phases over stress-drop, geometrical spreading, and frequency-dependent Q to find a suite of good-fitting models that remove the dependence on mb and distance. Seismic moments can either be set to pre-determined values or estimated through inversion and are tied to mb through two additional coefficients. We also solve for frequency-dependent site/phase excitation terms. Once a set of corrections is derived, effects of source scaling and distance as a function of frequency are applied to amplitudes from new events prior to forming discrimination ratios. Thus, all the corrections are tied to just mb (or M0) and distance and can be applied very rapidly in an operational setting. Moreover, phase amplitude residuals as a function of frequency can be spatially interpolated (e.g., using kriging) and used to construct a correction surface for each phase and frequency. The spatial corrections from the correction surfaces can then be applied to the corrected amplitudes based only on the event location. The correction parameters and correction surfaces can be developed offline and entered into an online database for pipeline processing providing multivariate-normal corrected amplitudes for event identification. Examples are shown using events from western China recorded at the station MAKZ.

S wave velocity structure of the Arabian Shield upper mantle from Rayleigh wave tomography

[1] The shear wave velocity structure of the shallow upper mantle beneath the Arabian Shield was modeled by inverting Rayleigh wave phase velocity measurements between 45 and 140 s together with previously published Rayleigh wave group velocity measurements between 10 and 45 s. For measuring phase velocities, we applied a modified array method to data from several regional networks that minimizes the distortion of raypaths caused by lateral heterogeneity. The new shear wave velocity model shows a broad low-velocity region to depths of � 150 km in the mantle across the Shield and a narrower low-velocity region at depths � 150 km localized along the Red Sea coast and Makkah-Madinah-Nafud (MMN) volcanic line. The velocity reduction in the upper mantle corresponds to a temperature anomaly of � 250– 330 K. These findings, in particular the region of continuous low velocities along the Red Sea and MMN volcanic line, do not support interpretations for the origin of the Cenozoic plateau uplift and volcanism on the Shield invoking two separate plumes. When combined with images of the 410 and 660 km discontinuities, body wave tomographic models, a S wave polarization analysis, and SKS splitting results for the Arabian Peninsula, the anomalous upper mantle structure in our new velocity model can be attributed to an upwelling of warm mantle rock originating in the lower mantle under Africa that crosses through the mantle transition zone beneath Ethiopia and moves to the north and northwest under the eastern margin of the Red Sea and the Arabian Shield. In this interpretation, the difference in mean elevation between the Arabian Platform and Shield can be attributed to isostatic uplift caused by heating of the lithospheric mantle under the Shield, with the significantly higher elevations along the Red Sea coast possibly resulting also from lithospheric thinning and dynamic uplift.

Joint inversion for three-dimensional S velocity mantle structure along the Tethyan margin

Abstract : For purposes of studying the lateral heterogeneity as well as for ultimately predicting seismograms for this region, we construct a new 3-D S-velocity model by jointly inverting a variety of different seismic data. We jointly invert regional waveforms, surface wave group velocity measurements, teleseismic S arrival times, and crustal thickness estimates from receiver functions, refraction lines, and gravity surveys. These data types have complementary resolving power for crust and mantle structures, vertical and lateral variations, shallow and deep mantle features, local and global structure. Therefore, a joint inversion of these data sets might help unravel the complexity of this tectonically diverse area. These measurements are made from a combination of mantle investigation of the deep suture between Europe and Africa (MIDSEA), Program for Array Seismic Studies of the Continental Lithosphere (PASSCAL), GeoScope, Geofon, Global Seismographic Network (GSN), International Deployment of Accelerometeres (IDA), MedNet, national networks, and local deployments throughout the study region which extends from the western Mediterranean region to the Hindu Kush and encompasses northeastern Africa, the Arabian peninsula, the Middle East, and part of the Atlantic Ocean for reference. We have fitted the waveforms of regional S and Rayleigh waves from over 3800 seismograms using Partitioned Waveform Inversion. We include over 3000 crustal thickness estimates from receiver functions, gravity measurements, and refraction profiles. We include Rayleigh wave group velocities for hundred thousands of paths transecting the region. We have over 3000 teleseismic S arrival times measured through cross correlation and over 170000 from picks originally reported to the International Seismological Centre (ISC).

Upper mantle structure beneath the Arabian Peninsula and northern Red Sea from teleseismic body wave tomography: Implications for the origin of Cenozoic uplift and volcanism in the Arabian Shield

Upper mantle structure between 150 and 400 km depth is imaged beneath the Arabian Shield and northern Red Sea by modeling P and S traveltime residuals from teleseismic events recorded on the Saudi Arabia National Digital Seismic Network, the 1995–1997 Saudi Arabian PASSCAL experiment, and three permanent stations (RAYN, EIL, and MRNI). Relative traveltime residuals were obtained using a multichannel cross-correlation method and inverted for upper mantle structure using VanDecars inversion method. The resulting images reveal a low-velocity region (∼1.5% for the P model and ∼3% for the S model) trending NW–SE along the western side of the Arabian Shield and broadening to the northeast beneath the Makkah-Madinah-Nafud volcanic line. We attribute the low velocities to a mantle thermal anomaly that could be as large as 330 K and that is associated with the Cenozoic uplift of and volcanic centers on the Shield. Our tomographic images are not consistent with models invoking separate mantle upwellings beneath the northern and southern regions of the Shield and instead favor single plume or superplume models. We also find little evidence for low velocities beneath the northern Red Sea, suggesting that there might not be a geodynamic link between rifting in the Red Sea and plateau uplift and volcanism in the Shield.

Low crustal velocities and mantle lithospheric variations in southern Tibet from regional Pnl waveforms

We report low average crustal P-wave velocities (5.9–6.1 km/s, Poissons ratio 0.23-0.27, thickness 68–76 km) in southern Tibet from modelling regional Pnl waveforms recorded by the 1991–1992 Tibetan Plateau Experiment. We also find that the mantle lithosphere beneath the Indus-Tsangpo Suture and the Lhasa Terrane is shield-like (Pn velocity 8.20–8.25 km/s, lid thickness 80–140 km, positive velocity gradient 0.0015–0.0025 s−1). Analysis of relative Pn travel time residuals requires a decrease in the mantle velocities beneath the northern Lhasa Terrane, the Banggong-Nujiang Suture and the southern Qiangtang Terrane. Tectonic and petrologic considerations suggest that low bulk crustal velocities could result from a thick (50–60 km) felsic upper crust with vertically limited and laterally pervasive partial melt. These results are consistent with underthrusting of Indian Shield lithosphere beneath the Tibetan Plateau to at least the central Lhasa Terrane.

Source Parameters for Moderate Earthquakes in the Zagros Mountains with Implications for the Depth Extent of Seismicity

Six earthquakes within the Zagros Mountains with magnitudes between 4.9 and 5.7 have been studied to determine their source parameters. These events were selected for study because they were reported in open catalogs to have lower crustal or upper mantle source depths and because they occurred within an area of the Zagros Mountains where crustal velocity structure has been constrained by previous studies. Moment tensor inversion of regional broadband waveforms has been combined with forward modeling of depth phases on short-period teleseismic waveforms to constrain source depths and moment tensors. Our results show that all six events nucleated within the upper crust (<11 km depth) and have thrust mechanisms. This finding supports other studies that call into question the existence of lower crustal or mantle events beneath the Zagros Mountains.

Partitioning of Seismoacoustic Energy and Estimation of Yield and Height‐of‐Burst/Depth‐of‐Burial for Near‐Surface Explosions

Explosions near the Earths surface excite both seismic ground motions and atmospheric overpressure. The energy transferred to the ground and atmosphere from a near-surface explosion depends on yield (W) as well as the height-of-burst/ depth-of-burial(HOB/DOB)forabove/belowgroundemplacements.Wereportanalyses of seismic and overpressure motions from the Humble Redwood series of low-yield, near-surface chemical explosions with the aim of developing quantitative models of energy partitioning and a methodology to estimate W and HOB/DOB. The effects of yield, HOB, and range on amplitudes can be cast into separable functions of range andHOBscaledbyyield.WefindthatdisplacementoftheinitialPwaveandtheintegral of the positive overpressure (impulse) are diagnostic of W and HOB with minimal scat- ter. An empirical model describing the dependence of seismic and air-blast measure- ments on W, HOB/DOB, and range is determined and model parameters are found by regression. We find seismic amplitudes for explosions of a given yield emplaced at or above the surface are reduced by a factor of 3 relative to fully contained explosions below ground. Air-blast overpressure is reduced more dramatically, with impulse reduced by a factor of 100 for deeply buried explosions relative to surface blasts. Oursignalmodelsare usedtoinvertseismicandoverpressure measurementsforW and HOB and we find good agreement (W errors <30%, HOB within meters) with ground- truth values for four noncircular validation tests. Although there is a trade-off between W and HOB for a single seismic or overpressure measurement, the use of both meas- urement types allows us tolargelybreak this trade-off and better constrainW and HOB. However, both models lack resolution of HOB for aboveground explosions.

Upper Mantle Shear and Compressional Velocity Structure of the Central US Craton: Shear Wave Low‐Velocity Zone and Anisotropy

One-dimensional upper mantle velocity structure is estimated by modeling P, SV and SH body-waveforms sampling the central United States. The resulting model features a thick mantle lid with a positive velocity gradient. A significant shear wave low-velocity zone (6% velocity reduction) is required below the lid however the compressional velocities are not reduced. The paths studied lie along the fast axes of teleseismic shear wave splitting measurements and absolute plate motion. We observe splitting of upper mantle refracted shear-waves, with the transverse component slow relative to the radial component. In order to fit the shear wave arrivals and satisfy teleseismic shear-wave splitting observations we infer azimuthal anisotropy (maximum 1.6%) distributed throughout the mantle lid and low-velocity zone, with increasing anisotropy with depth.