Invited Talks¶
Some invited talks in MIGG, with some PPTs in MIGG Google Drive (groupmeeting.NTU/talks.MIGG).
2021¶
Adjoint inversion of passive seismic datasets for sedimentary velocity structures¶
2021/08/05 by Guoliang Li
Abstract
Accurate seismic images of crust are essential for assessing seismic hazards and elucidating tectonic processes that shape surface landforms. Although California and Nevada have been studied extensively using various seismic datasets and tomographic methods, the region lacks a seismic model that can accurately define both shallow (<6 km) sedimentary basins and the deeper crust. In recent decades, Rayleigh wave ellipticity has been successfully extracted from both earthquake events and noise cross-correlation functions and used by many studies to constrain sedimentary velocity structures. In my study, I try to employ the adjoint tomography method (or finite frequency method) to invert the Rayleigh wave travel time and ellipticity information for both sedimentary and crystalline crustal structures.
To achieve that goal, I first build an initial 3D shear wave speed model for the adjoint tomography by jointly inverting Rayleigh wave ellipticity, phase velocity, and teleseismic P waveforms based on the traditional Ray-theory. And then, I developed a code package that enables the SPECFEM3D_Cartesian package to mesh the 3D geometry of the curved earth surface in order to fast and accurately simulate waveforms in an area of 1200 km × 1200 km. After that, I devised the misfit function for Rayleigh wave ellipticity and calculated the corresponding adjoint sources and Frechét kernels. In the final step, I will do synthetic tests to verify the contribution of Rayleigh wave ellipticity kernels in resolving sedimentary structures.
Seismic Observayion and Imaging Using Dense Seismic Arrays with Nodes and Distributed Acoustic Sensing (DAS)¶
2021/03/18 by Feng Cheng
Abstract
Recently, dense seismic deployments, with hundreds to thousands of short-period nodal instruments or distributed acoustic sensing (DAS) systems, have been widely used in seismological studies. These dense arrays have very close station spacings ranging from several meters to hundreds of meters to record well-sampled and unaliased wavefields in local or regional settings. Data acquired by such dense systems promote the development of new array-based seismic imaging methods and greatly improve our understanding of fine-scale subsurface properties, microseismic activities and earthquake rupture processes. During the presentation, Dr. Cheng will briefly present his works on dense seismic array observation and high-resolution seismic imaging, including geology structure delineation with 1D linear dense array, geothermal reservoir exploration using 2D dense array, hidden geothermal systems characterization using DAS array. Discussions related to dense array deployment, observation, imaging will be encouraged during and after the presentation.
Reference
Behm, M., Cheng, F., Patterson, A., & Soreghan, G. S. (2019). Passive processing of active nodal seismic data: estimation of Vp/Vs ratios to characterize structure and hydrology of an alpine valley infill. Solid Earth, 10(4), 1337–1354. https://doi.org/10.5194/se-10-1337-2019
Crustal and Upper Mantle Structure Beneath Central Myanmar Inferred from Receiver Functions¶
2021/02/02 by Yiming Bai
Abstract
The Eocene collision of India into Eurasia dominates the tectonics of Southeast Asia. To the east, the Indian plate is obliquely subducting along the Burma arc into the upper mantle. Affected by this, the country of Myanmar has undergone complex tectonic processes and is of high seismic risk. Yet, the subsurface seismic structure of this region has not been not fully investigated. In this talk, he will present some recent work on the crustal and upper mantle structure beneath central Myanmar and adjacent regions inferred from receiver functions and other techniques. The seismic data used were primarily collected from a China-Myanmar temporary experiment.
Reference
Bai Y., Yuan X., He Y., Hou G., M. Thant, K. Sein, Ai Y., et al. (2020). Mantle Transition Zone Structure Beneath Myanmar and Its Geodynamic Implications. AGU Journals: Geochemistry, Geophysics, Geosystems, 21(12): 1-18. https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2020GC009262
2020¶
What can we learn from the surface fractures nearby the Ridgecrest earthquakes?¶
2020/11/24 by Xiaohua Xu
Abstract
Contemporary earthquake hazard models hinge on an understanding of how strain is distributed in the crust and the ability to precisely detected mm-scale deformation over broad regions of active faulting. Satellite radar observations revealed hundreds of previously unmapped linear strain concentrations (or fractures) surrounding the 2019 Ridgecrest earthquake sequence. We documented and analyzed displacements and widths of 169 of these fractures. While most fractures are displaced in the direction of the prevailing tectonic stress (prograde), a large number of them are displaced in the opposite (retrograde) direction. We constructed fault slip model and computed static Coulomb stress change for a suite of receiver fault orientations consistent with those exhibiting surface fractures. Comparing static stress change models to the phase gradient maps, we were able to determine that these surface fractures can largely be explained with static stress change from the earthquake sequence, with most antithetic motions are associated with compliant fault deformation, where most prograding ones being shallow frictional slip. We further explore a conceptual model that can explain these observations. One important implication from the model is that much of the “off-fault” strain in the Mojave shear zone is due to permanent inelastic deformation on many small faults. This is in support of the hypothesis that compensation of shallow slip deficit occurs on distributed shallow structures.
Reference
Xu X, Sandwell D T, Ward L A, et al. Surface deformation associated with fractures near the 2019 Ridgecrest earthquake sequence[J]. Science, 2020, 370(6516): 605-608.
Three-dimensional thermochemical model and melt distribution beneath Northeast China¶
2020/11/12 by Anqi Zhang
Anqi is a Postdoc at Department of Ocean Science and Engineering, Southern University of Science and Technology. her research is mainly focused on using multiple geophysical data to infer the subsurface temperature/composition etc., via a multi-observable probabilistic inversion method.
Abstract
Intraplate volcanisms spanning from the Late Cretaceous to present are widespread in Northeast China (NEC). Numerous geophysical and geochemical studies have been carried out during the past few decades to explore the origin and mantle dynamic of these intraplate volcanisms. However, the hypotheses put forward for the geodynamic evolution of the region are diverse and often controversial, such as mantle plume, mantle upwelling originates from the transition zone, small-scale convection, and the influx of fluid released from the stagnant slab of the Pacific plate in the mantle transition zone. Here we use a probabilistic inverse method to jointly invert Rayleigh wave dispersion data, surface heat flow measurements, geoid height, and absolute elevation data. The output is a 3D model of temperature, bulk density, seismic velocity, and compositional structure of the lithosphere beneath NEC. Overall, our results reveal high temperature anomalies beneath all active volcanoes as expected. The resulting lithosphere-asthenosphere boundary (LAB) depth shows significant correlations with independently collected geochemical parameters of young basalts (< 5 Ma) in NEC. This observation manifests lithosphere thickness effect on compositional variations of continental basalts (the lid effect) and emphasizes the consistency between the geophysical and geochemical data. We also estimate distribution of melt in the uppermost mantle beneath the NEC from joint inversion, which is in good agreement with the surface locations of young basalts. Our predicted melt distribution indicates that Changbaishan (CBS) basalts are produced by partial melting of asthenospheric mantle underlies relatively thin lithosphere. The mantle upwelling beneath the CBS volcano may originate from deep mantle sources, which causes large degree of partial melting at 60 km depth. In contrast, basalts in the Xingmeng belt at the west of Songliao basin, such as the Halaha and Abaga volcano, may be derived from relatively low-degree partial melting caused by localized asthenospheric upwelling at shallow depths.
Crustal deformation in the southeastern (SE) Tibet revealed by the receiver function analysis¶
2020/10/15 by Mijian Xu
Abstract
We employ receiver function and common conversion point stacking analysis with the seismic waveforms recorded by the dense ChinArray and other local seismic stations to accurately define the Moho topography in SE Tibet.
We find that the Moho under the Tibetan Plateau is much deeper than that under the surrounding Yangtze Craton and Indochina block; abrupt Moho changes are found across the southeastern plateau margin, similar to that under the eastern plateau margin.
We interpret these sharp Moho variations across the plateau margin to have developed when the Tibetan Plateau was extruded southeastward in the late Miocene. Subsequent gravity collapse resulted in crustal extension and gentle topographic variation, while the sharp Moho slope was preserved.
references
Xu, M., Huang, Z., Wang, L., Xu, M., Zhang, Y., Mi, N., et al. (2020). Sharp lateral Moho variations across the SE Tibetan margin and their implications for plateau growth. Journal of Geophysical Research: Solid Earth, 125, e2019JB018117.
Xu, M., Huang, H., Huang, Z., Wang, P., Wang, L., Xu, M., … & Yuan, X. (2018). Insight into the subducted Indian slab and origin of the Tengchong volcano in SE Tibet from receiver function analysis. Earth and Planetary Science Letters, 482, 567-579.
Lift and Relax for PDE-constrained Inverse Problem in Seismic imaging¶
2020/08/26 by Fang Zhilong
Abstract
We present Lift and Relax for Waveform Inversion (LRWI), an approach that mitigates the local minima issue in seismic full waveform inversion (FWI) via a combination of two convexification techniques. The first technique (Lift) extends the set of unknown variables to their products, arranged as a moment matrix. This algebraic idea is a celebrated way to replace a hard polynomial optimization problem by a semidefinite programming approximation. Concretely, both the model and the wavefield are lifted from vectors to rank-2 matrices. The second technique (Relax) invites to consider the wave equation, not as a hard constraint, but as a soft constraint to be satisfied only approximately a technique known as wavefield reconstruction inversion (WRI). WRI weakens wave-equation constraints by introducing wave-equation misfits as a weighted penalty term in the objective function. The relaxed penalty formulation enables balancing the data and wave-equation misfits by tuning a penalty parameter. Together, ”Lift” and ”Relax” help reformulate the inverse problem as a set of constraints on a rank-2 moment matrix in a higher dimensional space. Such a lifting strategy permits a good data and wave-equation fit throughout the inversion process, while leaving the numerical rank of the rank2 moment matrix to be minimized down to one. Numerical examples indicate that compared to FWI and WRI, LRWI can conduct successful inversions using an initial model that would be considered too poor, and data with a starting frequency that would be considered too high, for either method in isolation.
Deformation of the Continental Lithosphere¶
2020/08/24 by Zhongxiong Cu
Deformation of the Continental Lithosphere: Insight from the Continental-Continental Collision in the Southern Tibetan Plateau and Intraplate Deformation in Central Mongolia
Temporal seismic velocity change using repeating earthquake¶
2020/08/19 by Yixiao Sheng
Yixiao Sheng’s PhD Researches
Seismic velocity and attenuation based on earthquake and ambient noise surface-wave¶
2020/08/12 by Fabrizio Magrini
Fabrizio Magrini’s PhD Researches
Ambient noise tomography data preprocessing, waveform modelling and full waveform inversion¶
2020/07/30 by Yang Yang
Yang Yang’s PhD Researches
Seismic regional tomography of the southern Puna plateau¶
2020/06/18 by Jing Chen
Jing Chen, Sofia-Katerina Kufner, Xiaohui Yuan,Benjamin Heit, Hao Wu, Dinghui Yang, Bernd Schurr, Suzanne Kay, Lithospheric delamination beneath the southern Puna plateau resolved by local earthquake tomography. preprint