Multiscale Seismic Tomography Research Articles

Crustal structure beneath the Qilian Orogen Zone from multiscale seismic tomography

The Qilian Orogen Zone (QOZ), located in the north margin of the Tibetan Plateau, is the key area for understanding the deformation and dynamics process of Tibet. Numerous geological and geophysical studies have been carried out on the mechanics of the Tibetan Plateau deformation and uplift; however, the detailed structure and deformation style of the Qilian Orogen Zone have remained uncertain due to poor geophysical data coverage and limited resolution power of inversion algorithms. In this study, we analyze the P-wave velocity structure beneath the Qilian Orogen Zone, obtained by applying multi-scale seismic tomography technique to P-wave arrival time data recorded by regional seismic networks. The seismic tomography algorithm used in this study employs sparsity constraints on the wavelet representation of the velocity model via L1-norm regularization. This algorithm can deal efficiently with uneven-sampled volumes, and can obtain multi-scale images of the velocity model. Our results can be summarized as follows: (1) The crustal velocity structure is strongly inhomogeneous and consistent with the surface geological setting. Significant low-velocity anomalies exist in the crust of northeastern Tibet, and slight high-velocity anomalies exist beneath the Qaidam Basin and Alxa terrane. (2) The Qilian Orogen Zone can be divided into two main parts by the Laji Shan Faults: the northwestern part with a low-velocity feature, and the southeastern part with a high-velocity feature at the upper and middle crust. (3) Our tomographic images suggest that northwestern and southeastern Qilian Orogen Zones have undergone different tectonic processes. In the northwest Qilian Orogen Zone, the deformation and growth of the Northern Tibetan Plateau has extended to the Heli Shan and Beida Shan region by northward over-thrusting at the upper crust and thickening in the lower crust. We speculate that in the southeast Qilian Orogen Zone the deformation and growth of the Northern Tibet Plateau were of strike-slip style at the upper crust; in the lower crust, the evidence suggests ductile shear extrusion style and active frontage extension to the Alxa terrane. (4) The multi-scale seismic tomography technique provides multi-scale analysis and sparse constraints, which has allowed to us obtain stable, high-resolution results.

Multiscale Seismic Tomography and Earthquake Relocation Incorporating Differential Time Data: Application to the Maule Subduction Zone, Chile

Abstract We present a multiscale seismic tomography method incorporating differential time data. This is an extension of the double‐difference tomography method to teleseismic distances using a nested regional–global model parameterization. It allows inclusion of absolute and differential time data observed at any distance, potentially improving resolution of structure in areas where local or regional data are sparse. The algorithm, named teletomoDD , is quite flexible and is applicable to a variety of relocation and tomography problems, including simultaneously solving for global velocity structure. To illustrate the technique, we apply it to the Maule region of Chile, where an M w 8.8 earthquake occurred in 2010.

Multiscale seismic tomography and mantle dynamics

In this article we first introduce the methodology of multiscale seismic tomography and the way to interpret the obtained tomographic images, and then review the significant recent results of multiscale seismic tomography with emphasis on mantle plumes and subducting slabs. Global and regional tomography shows that most of the slab materials under Western Pacific and East Asia are stagnant in the mantle transition zone before finally collapsing down to the core–mantle boundary as a result of large gravitational instability from phase transitions. Local and teleseismic tomography studies have imaged clearly the subducting slabs and arc magma chambers in the upper-mantle wedge, indicating that geodynamic systems associated with arc magmatism and back-arc spreading are related to deep processes, such as convective circulation in the mantle wedge and dehydration reactions of the subducting slab. Because most hotspots are located in poorly instrumented continental and oceanic regions, 3-D crust and upper-mantle structure is determined for only a few hotspots such as Iceland, Yellowstone and Eifel which are covered by seismic networks, and plume-like slow anomalies are revealed under those hotspots. Global tomography has revealed deep mantle plumes under the major hotspots such as Hawaii, Iceland, Kerguelen, South Pacific and Africa. Strong lateral heterogeneities are revealed at the bottom of the mantle, which are associated with the deeply subducted slabs and the birth of mantle plumes. A thorough understanding of the deep Earth structure will only be achieved by a combination of more effective seismic imaging techniques and dense coverage of global seismic networks, particularly in the oceans.

Combining seismic reflection with multifold wide‐aperture profiling: An effective strategy for high‐resolution shallow imaging of active faults

Shallow imaging of fault zones is a challenging task. We investigate if this issue can be successfully addressed by combining reflection seismics with non‐standard multifold wide‐aperture profiling, a strategy pertaining to crustal scale exploration so far. Near‐offset reflection and multifold wide‐aperture data are simultaneously recorded along two profiles in a small basin crossed by an active fault in Southern Italy. Imaging consists of multiscale seismic tomography complemented by CDP processing of near‐offset reflections and of data spanning a large offset range. By combining smooth Vp and reflectivity images we pinpoint the fault location and yield a valuable picture of the basin. Performance of each exploration technique strongly varies in the two surveys depending on the local geology. Thus, the combined exploration strategy not only yields complementary images for enhanced interpretation, but also allows extending the range of applicability of shallow seismics for fault detection, reducing the risk of unsuccessful investigations.

Study on Method of Wavelet Multi‐Scale Crosshole Seismic Tomography in Complex Media

AbstractCrosshole seismic tomography in complex media is a very complex and nonlinear inverse problem. We generally cannot obtain a satisfactory clear image by using linearized tomographic method. This paper proposes a wavelet multiscale seismic tomography method. The forwarding traveltime is calculated by using the graph‐based ray tracing approach. The method effectively solves the difficulty of nonlinear tomography, overcomes the drawbacks of the linearized inverse methods and enhances the tomographic resolution. The results of physical modeling show that the method is suitable for complex media, has good practicability and effectiveness.