Crustal and upper-mantle structure in the Eastern Mediterranean from the analysis of surface wave dispersion curves

The dispersive properties of surface waves are used to infer earth structure in the Eastern Mediterranean region. Using group velocity maps for Rayleigh and Love waves from 7100 s, we invert for the best 1D crust and uppermantle structure at a regular series of points. Assembling the results produces a 3D lithospheric model, along with corresponding maps of sediment and crustal thickness. A comparison of our results to other studies finds the uncertainties of the Moho estimates to be about 5 km. We find thick sediments beneath most of the Eastern Mediterranean basin, in the Hellenic subduction zone and the Cyprus arc. The Ionian Sea is more characteristic of oceanic crust than the rest of the Eastern Mediterranean region as demonstrated in particular by the crustal thickness. We also find significant crustal thinning in the Aegean Sea portion of the backarc, particularly towards the south. Notably slower Swave velocities are found in the uppermantle, especially in the northern Red Sea and Dead Sea Rift, central Turkey, and along the subduction zone. The low velocities in the uppermantle that span from North Africa to Crete, in the Libyan Sea, might be an indication of serpentinized mantle from the subducting African lithosphere. We also find evidence of a strong reverse correlation between sediment and crustal thickness which, while previously demonstrated for extensional regions, also seems applicable for this convergence zone

A Top to Bottom Lithospheric Study of Africa and Arabia

We study the lithospheric structure of Africa, Arabia and adjacent oceanic regions with fundamental-mode surface waves over a wide period range. Including short period group velocities allows us to examine shallower features than previous studies of the whole continent. In the process, we have developed a crustal thickness map of Africa. Main features include crustal thickness increases under the West African, Congo, and Kalahari cratons. We find crustal thinning under Mesozoic and Cenozoic rifts, including the Benue Trough, Red Sea, and East, Central, and West African rift systems. Crustal shear wave velocities are generally faster in oceanic regions and cratons, and slower in more recent crust and in active and formerly active orogenic regions. Deeper structure, related to the thickness of cratons and modern rifting, is generally consistent with previous work. Under cratons we find thick lithosphere and fast upper mantle velocities, while under rifts we find thinned lithosphere and slower upper mantle velocities. There are no consistent effects in areas classified as hotspots, indicating that there seem to be numerous origins for these features. Finally, it appears that the African Superswell has had a significantly different impact in the north and the south, indicating specifics of the feature (temperature, time of influence, etc.) to be dissimilar between the two regions. Factoring in other information, it is likely that the southern portion has been active in the past, but that shallow activity is currently limited to the northern portion of the superswell

The Surface Wave Magnitude for the 9 October 2006 North Korean Nuclear Explosion

Surface waves were generated by the North Korean nuclear explosion of 9 October 2006 and were recorded at epicentral distances up to 34°, from which we estimated a surface wave magnitude (M_s) of 2.94 with an interstation standard deviation of 0.17 magnitude units. The International Data Center estimated a body-wave magnitude (m_b) of 4.1. This is the only explosion we have analyzed that was not easily screened as an explosion based on the differences between the M_s and m_b estimates. Additionally, this M_s predicts a yield, based on empirical M_s/yield relationships, that is almost an order of magnitude larger than the 0.5–1 kt reported for this explosion. We investigate how emplacement medium effects on surface wave moment and magnitude may have contributed to the yield discrepancy

An Analysis of the Mt. Meron Seismic Array

We have performed a quick analysis of the Mt. Meron seismic array to monitor regional seismic events in the Middle East. The Meron array is the only current array in the Levant and Arabian Peninsula and, as such, might be useful in contributing to event location, identification, and other analysis. Here, we provide a brief description of the array and a review of the travel time and array analysis done to assess its performance

Final report for "Development of generalized mapping tools to improve implementation of data driven computer simulations" (LDRD 04-ERD-083)

Probabilistic inverse techniques, like the Markov Chain Monte Carlo (MCMC) algorithm, have had recent success in combining disparate data types into a consistent model. The Stochastic Engine (SE) initiative was a technique that developed this method and applied it to a number of earth science and national security applications. For instance, while the method was originally developed to solve ground flow problems (Aines et al.), it has also been applied to atmospheric modeling and engineering problems. The investigators of this proposal have applied the SE to regional-scale lithospheric earth models, which have applications to hazard analysis and nuclear explosion monitoring. While this broad applicability is appealing, tailoring the method for each application is inefficient and time-consuming. Stochastic methods invert data by probabilistically sampling the model space and comparing observations predicted by the proposed model to observed data and preferentially accepting models that produce a good fit, generating a posterior distribution. In other words, the method ''inverts'' for a model or, more precisely, a distribution of models, by a series of forward calculations. While powerful, the technique is often challenging to implement, as the mapping from model space to data needs to be ''customized'' for each data type. For example, all proposed models might need to be transformed through sensitivity kernels from 3-D models to 2-D models in one step in order to compute path integrals, and transformed in a completely different manner in the next step. We seek technical enhancements that widen the applicability of the Stochastic Engine by generalizing some aspects of the method (i.e. model-to-data transformation types, configuration, model representation). Initially, we wish to generalize the transformations that are necessary to match the observations to proposed models. These transformations are sufficiently general not to pertain to any single application. This is a new and innovative approach to the problem, providing a framework to increase the efficiency of its implementation. The overall goal is to reduce response time and make the approach as ''plug-and-play'' as possible, and will result in the rapid accumulation of new data types for a host of both earth science and non-earth science problems

A Model-Based Signal Processing Approach to Nuclear Explosion Monitoring

This report describes research performed under Laboratory Research and Development Project 05-ERD-019, entitled ''A New Capability for Regional High-Frequency Seismic Wave Simulation in Realistic Three-Dimensional Earth Models to Improve Nuclear Explosion Monitoring''. A more appropriate title for this project is ''A Model-Based Signal Processing Approach to Nuclear Explosion Monitoring''. This project supported research for a radically new approach to nuclear explosion monitoring as well as allowed the development new capabilities in computational seismology that can contribute to NNSA/NA-22 Programs

Seismic Source and Path Calibration in the Korean Peninsula, Yellow Sea

Two significant seismic events were analyzed using the crustal velocity model developed under this contract. The M{sub W} = 4.55 Korea earthquake of January 20, 2007 occurred in the Republic of Korea on land and within the dense digital seismic network. Using P-wave arrivals from 60 broadband, short-period and acceleration stations, the event occurred at 37.68N, 128.58E at a depth of 7.5 km at 20070120115653.8. Source inversion was performed using the accelerometer recordings in the 0.05-0.20 Hz band the broadband data in the 0.02-0.10 Hz band, with identical focal mechanisms and source depths of 9 and 11 km, respectively. This is the largest event on land in South Korea since the M{sub W} 4.7 event on December 13, 1996. Forward modeling of the waveforms at INCN and MDJ indicates the ability of the current model to match observations on the Korean Peninsula and the effect of significant pulse shape modification for paths that partially cross the Sea of Japan. The results of using the local network data provide a ground truth point for other studies analyzing seismic events on the peninsula. The isotropic seismic moment of the October 9, 2006 North Korea explosion was estimated from the Rayleigh-wave spectral amplitudes observed at MDJ and INCN. Very little Love wave signal was observed, indicating weak tectonic release. The explosion yield was investigated using the Denny and Johnson (1991) model relating yield to the observed isotropic moment as a function of depth of burial and material properties. Sensitivity analysis highlights the strong effect of the assumed velocity and density structure in the upper kilometer of the Earth and the assumed depth of burial on the estimated yield. The crustal velocity model developed under this contract provides strong constraints on the expected shear-wave velocities in the shallow parts of the crust. Issues to be investigated include the effect of wave propagation through the Eastern Sea (Sea of Japan) to stations in South Korea, and the effect of attenuation on isotropic moment estimates over longer paths, e.g., to the station BJT in Beijing

Crustal thinning between the Ethiopian and East African Plateaus from modeling Rayleigh wave dispersion

The East African and Ethiopian Plateaus have long been recognized to be part of a much larger topographic anomaly on the African Plate called the African Superswell. One of the few places within the African Superswell that exhibit elevations of less than 1 km is southeastern Sudan and northern Kenya, an area containing both Mesozoic and Cenozoic rift basins. Crustal structure and uppermost mantle velocities are investigated in this area by modeling Rayleigh wave dispersion. Modeling results indicate an average crustal thickness of 25 {+-} 5 km, some 10-15 km thinner than the crust beneath the adjacent East African and Ethiopian Plateaus. The low elevations can therefore be readily attributed to an isostatic response from crustal thinning. Low Sn velocities of 4.1-4.3 km/s also characterize this region

Model-Based Signal Processing: Correlation Detection With Synthetic Seismograms

Recent applications of correlation methods to seismological problems illustrate the power of coherent signal processing applied to seismic waveforms. Examples of these applications include detection of low amplitude signals buried in ambient noise and cross-correlation of sets of waveforms to form event clusters and accurately measure delay times for event relocation and/or earth structure. These methods rely on the exploitation of the similarity of individual waveforms and have been successfully applied to large sets of empirical observations. However, in cases with little or no empirical event data, such as aseismic regions or exotic event types, correlation methods with observed seismograms will not be possible due to the lack of previously observed similar waveforms. This study uses model-based signals computed for three-dimensional (3D) Earth models to form the basis for correlation detection. Synthetic seismograms are computed for fully 3D models estimated from the Markov Chain Monte-Carlo (MCMC) method. MCMC uses stochastic sampling to fit multiple seismological data sets. Rather than estimate a single ''optimal'' model, MCMC results in a suite of models that sample the model space and incorporates uncertainty through variability of the models. The variability reflects our ignorance of Earth structure, due to limited resolution, data and modeling errors, and produces variability in the seismic waveform response. Model-based signals are combined using a subspace method where the synthetic signals are decomposed into an orthogonal basis by singular-value decomposition (SVD) and the observed waveforms are represented with a linear combination of a sub-set of eigenvectors (signals) associated with the most significant eigenvalues. We have demonstrated the method by modeling long-period (80-10 seconds) regional seismograms for a moderate (M{approx}5) earthquake near the China-North Korea border. Synthetic seismograms are computed with the Spectral Element Method for a suite of long-wavelength (2 degree) seismic velocity models based on the MCMC method. We are working on higher resolution (1 degree) models for the same region and methods to increase the frequency content of the synthetic seismograms