Chin-wu Chen | National Taiwan University (original) (raw)

Papers by Chin-wu Chen

Geophysical Research Letters, 2014

We present a three-dimensional (3D) hybrid method that interfaces the spectral-element method (SE... more We present a three-dimensional (3D) hybrid method that interfaces the spectral-element method (SEM) with the frequency-wavenumber (FK) technique to model the propagation of teleseismic plane-waves beneath seismic arrays. The accuracy of the resulting 3D SEM-FK hybrid method is benchmarked against semi-analytical FK solutions for 1D models. The accuracy of 2.5D modelling based on 2D SEM-FK hybrid method is also investigated through comparisons to this 3D hybrid method. Synthetic examples for structural models of the Alaska subduction zone and the central Tibet crust show that this method is capable of accurately capturing interactions between incident plane waves and local heterogeneities. This hybrid method presents an essential tool for the receiver-function and scattering-imaging community to verify and further improve their techniques. These numerical examples also show the promising future of the 3D SEM-FK hybrid method in high-resolution regional seismic imaging based on waveform inversions of converted/scattered waves recorded by seismic array. effective seismic input for non-linear soilstructure interaction systems, Earthq. Eng. Struct. Dyn., 12, 107-119. Bostock, M. G., S. Rondenay, and J. Shragge (2001), Multiparameter two-dimensional inversion of scattered teleseismicbody waves 1. Theory for oblique incidence, J. Geophys. Res., 106 (12), 30,771-30,782. Burdick, S., M. V. de Hoop, S. Wang, and R. D. V. D. Hilst (2014), Reverse-time migration-based reflection tomography using teleseismic free surface multiples, Geophys. J. Int., 196, 996-1017, doi:10.1093/gji/ggt428. Cao, Q., P. Wang, R. van der Hilst, M. de Hoop, and S.-H. Shim (2010), Imaging the upper mantle transition zone with a generalized Radon transform of SS precursors, (2003a), Coupling the spectral element method with a modal solution for elastic wave propagation in global Earth models, Geophys. J. Int., 152, 34-67.

A variety of methodologies have been developed over the years to analyze converted and scattered ... more A variety of methodologies have been developed over the years to analyze converted and scattered seismic waves, ranging from single station applications to high-resolution imaging using dense arrays of broadband seismometers. A key step in the data preprocessing chain for these imaging techniques is the 'source-normalization', which requires the construction and application of a deconvolution operator to remove the extended

Geophysical Journal International, 2010

We investigate the applicability of an array-conditioned deconvolution technique, developed 6 for... more We investigate the applicability of an array-conditioned deconvolution technique, developed 6 for analyzing borehole seismic exploration data, to teleseismic receiver functions and data 7 preprocessing steps for scattered wavefield imaging. This multichannel deconvolution tech-8 nique constructs an approximate inverse filter to the estimated source signature by solving an 9 overdetermined set of deconvolution equations, using an array of receivers detecting a com-10 mon source. We find that this technique improves the efficiency and automation of receiver 11 function calculation and data preprocessing workflow. We apply this technique to synthetic 12 experiments and to teleseismic data recorded in a dense array in northern Canada. Our results

Geophysical Research Letters, 2014

OCEANS 2014 - TAIPEI, 2014

ABSTRACT T waves excited by earthquakes propagate along the SOFAR channel with low transmission l... more ABSTRACT T waves excited by earthquakes propagate along the SOFAR channel with low transmission loss, and therefore can be recorded on land-based seismic stations and hydrophones located thousands of kilometers away from earthquake epicenters. Early T-wave observations are mostly based on recordings by land-based stations due to the mechanics of the energy conversion of acoustic waves into seismic phases. Recently, T-wave signals have also been detected by ocean-bottom seismometers (OBS) at deep ocean basin offshore eastern Taiwan, raising the question of how deep ocean environment affects the generation and propagation of T waves. In this study, we examined the seismic waveform data recorded at 31 OBSs deployed in Okinawa Trough and Huatung Basin from 2006 to 2012. During this time period, there are 440 regional earthquakes with magnitude larger than 5 in the Western Pacific Ocean. A total of 68 T-wave events are identified using the criteria that significant energy in the dominant frequency of ~2-10 Hz and time duration longer than 100 seconds. Most of these events were generated by shallow-depth (less than 50 km) earthquakes, with only one exception by deep source of 225 km. Among these 68 events, 19 events were recorded on 3 OBSs located at 4500-m depth of Huatung Basin, where the depth of minimum sound speed is around 1100 m. To understand how acoustic energy scatters from the SOFAR channel into the ocean bottom, we apply the acoustic parabolic equation (PE) theory to simulate acoustic propagation in the presence of ocean sound speed perturbations. The simulations indicate that sound speed perturbations indeed affect the acoustic propagation pattern, part of which may then reach deep ocean regions. We further take into account possible effects of seafloor topography on generating scattered and surface waves along the ocean-crust interface.

Both amplitude and travel time of diffracted waves are commonly used to constrain the velocity st... more Both amplitude and travel time of diffracted waves are commonly used to constrain the velocity structure of the lowermost mantle. Amplitudes are especially sensitive to the variation of velocity gradients, while travel times indicate the average velocity in the region under inspection. As an alternative to direct travel time measurement, the ray parameter of the diffracted waves is useful when

It has been shown in recent years that lithospheric discontinuities may hold the key to better un... more It has been shown in recent years that lithospheric discontinuities may hold the key to better understanding the processes responsible for the formation and evolution of continents. Archean cratons have remained stable for billions of years, and therefore provide a unique window into the tectonic processes that took place during the time when they were formed. In this study, we present a new receiver function (RF) image of the lithospheric mantle beneath the Slave craton. We analyze P-to-S (Ps) converted waves from a dataset consisting of 135 events recorded at a linear array of 30 broadband seismic stations spanning ~400 km across the Slave craton. Our RF image shows a consistent positive velocity gradient across the array at depths from 34-41 km, indicating the location of Moho. Furthermore, we observe a pronounced southward- dipping negative gradient at ~103-134 km depths beneath central Slave. This low velocity layer is spatially coincident with an electrical conductive anomaly derived from previous magnetotelluric experiments, and with the upper boundary of the petrologically-constrained ultra-depleted region. One possible explanation for this geophysical/petrological boundary is that it represents a compositional interface marked by alteration minerals that cause the low seismic velocities (e.g., phlogopite) and inter-granular graphite films that cause the conductive anomaly. We speculate that this front may have originated through metasomatism associated with a subduction event that played an important role in the assembly of the Slave craton.

ABSTRACT Archean cratons have remained stable for billions of years, and therefore provide a uniq... more ABSTRACT Archean cratons have remained stable for billions of years, and therefore provide a unique window into the tectonic processes that took place during the time when they were formed. In this study, we present a new receiver function (RF) image of the lithospheric mantle beneath the Slave craton. We analyze P-to-S (Ps) converted waves from a dataset consisting of 62 events recorded at a linear array of 23 broadband seismic stations of POLARIS and MIT, spanning ~400 km across the Slave craton. Our RF image shows a consistent positive velocity gradient across the array at depths from 34-41 km, indicating the location of Moho. Furthermore, we observe a pronounced southward- dipping negative gradient at ~103-134 km depths beneath central Slave. This low velocity layer is spatially coincident with an electrical conductive anomaly derived from previous magnetotelluric experiments, and with the upper boundary of the petrologically-constrained ultra- depleted region. One possible explanation for this geophysical/petrological boundary is that it represents a compositional interface marked by alteration minerals that cause the low seismic velocities (e.g., phlogopite) and inter-granular graphite films that cause the conductive anomaly. We speculate that this front may have originated through metasomatism associated with a subduction event that played an important role in the assembly of the Slave craton.

ABSTRACT The origin of Archean cratonic lithosphere is the subject of much debate, but many see s... more ABSTRACT The origin of Archean cratonic lithosphere is the subject of much debate, but many see subduction as a key process of assembly. Therefore, remnants of subduction structure and associated volatiles should be preserved in the cratonic lithosphere. In this study, we synthesize coincident seismic, magnetotelluric, and petrological results that suggest the existence of such structure and volatile alteration in the lithosphere of the central Slave craton, in the northwestern Canadian Shield. We construct a receiver function image of the Slave lithosphere by analyzing P-to-S converted waves from a dataset consisting of 62 events, recorded at a linear array of 23 broadband seismic stations of the POLARIS-MIT network. Our image shows a southward-dipping, negative seismic velocity gradient at ~96-124 km depth beneath the central Slave craton. This feature marks the top of a low velocity layer that is spatially coincident with an electrical conductive anomaly derived from previous magnetotelluric experiments, and with the upper boundary of a petrologically-constrained ultra-depleted region. Furthermore, kimberlite pipes from the central Slave craton contain volatile-rich mantle xenoliths that originated in the same depth range as the seismic and electrical anomalies. We interpret this geophysical/petrological boundary as a compositional interface marked by alteration minerals (e.g. phlogopite) that cause the seismic anomaly, and inter-granular graphite films that cause the conductive anomaly. We suggest that this mineralization represents relict metasomatism associated with a subduction event that played an important role in the assembly of the Slave craton.

The Mw 7.9Wenchuan earthquake of 12 May 2008 was the most devastating earthquake in China in the ... more The Mw 7.9Wenchuan earthquake of 12 May 2008 was the most devastating earthquake in China in the past 30 years in terms of human losses and property damage. The main shock ruptured with about 9 m of slip along the Longmen Shan fault zone located the boundary of Tibetan plateau and Sichuan basin. About 5-6 m maximum vertical offset was identified in the field survey after the earthquake. Nine borehole strainmeters installed at western Foothills in Taiwan orogenic belt captured significant step-like variation of areal strain. The areal strain increasing was observed from 0.01 to 0.2 microstrain at five boreholes located in Chiayi area southwestern Taiwan and Hisnchu area northwest Taiwan. The areal strain decreasing was also observed from 0.05 to 4 mricostrain at three boreholes located at Hisnchu and Taipei area of northwest Taiwan. This big earthquake also case water level to sscillate and undergo sustained changes occurred concurrenly with S waves and Love waves. We attribute these step-like transient variations of areal strain to the dynamic triggering of the Wenchuan earthquake.

ABSTRACT The regional spatial variations of teleseismic waveforms reflect lateral velocity pertur... more ABSTRACT The regional spatial variations of teleseismic waveforms reflect lateral velocity perturbations in the crust and upper mantle beneath the receiver array. The waveform effects of a cold, high velocity slab are early arrivals, reduced amplitudes, and broadening waveforms. Taiwan, sitting atop the convergent boundary between the Eurasia and the Philippine Sea Plates with flipping of subduction polarities offshore to the south and to the northeast, is an ideal natural laboratory to observe these effects. The Taiwan Integrated Geodynamic Research (TAIGER) project deployed six dense linear arrays across Taiwan to collect data of marine air gun and natural earthquake sources between Feb. 2009 and June 2009. In this study, we examine the variations of teleseismic P and S across the NS array, using data of earthquakes from the Kermadec, Tonga, Vanuatu, New Britain, and New Guinea regions. All, with epicentral distances ranging from 35° to 80°, have similar back azimuths that are perpendicular to the azimuth of the NS array. Having calibrated with theoretical arrival times from 1-D model, we measure the peak-to-trough amplitudes, widths, and relative arrival times of P and S (0.1-1.2Hz bandpass) from displacements of vertical and transverse components, respectively. Results show that covariances of measurements of the three parameters generally comply with the effects of a subducting slab. Pattern of the effects can be grouped into three segments from south to north. Combined with results of ray tracing and depth contours of slab top surfaces, observations of the south and north segments are explained as imprints of the Manila slab and the Ryukyu slab, respectively. On the other hand, the middle segment in central Taiwan exhibits the most prominent features of slab effects suggesting the presence of an aseismic slab deep in the upper mantle. The closely spaced array allows not only the detailed spatial variations to be traced but also the northern extension of the aseismic slab to be determined.

ABSTRACT Seismic coda waves and converted phases have been used extensively to image detailed sub... more ABSTRACT Seismic coda waves and converted phases have been used extensively to image detailed subsurface structures underneath seismic arrays, based on methods such as receiver functions, Kirchhoff migration and generalized Radon transform (GRT). Utilizing the same coda and converted waves, we propose to image both discontinuity interfaces and 3D velocity anomalies by combining full numerical simulations of wave propagation with adjoint methods recently adopted in global and regional tomography inversions. The `sensitivities' of these coda/converted waves to density, P and S velocities are calculated based on the interaction of the forward wave field that produces the main P phase, and the adjoint wave field generated by injecting the coda/converted phases at array stations as virtual sources, similar to the computation of isochrons in previous techniques. The density kernels generally highlight discontinuity interfaces and sharp velocity contrasts, while P and S velocity kernels provide hints to the update of volumetric velocity structures. The application of numerical solvers also allows the incorporation of 3D regional tomography models as background velocity models, providing better focusing of velocity anomalies. We show the feasibility of this technique on a synthetic case built based on the imaging geometry for Slave craton in the northwestern Canadian Shield by the POLARIS broadband seismic network. The main challenge of this technique lies in reproducing the forward wave field generated by tele-seismic sources in a limited simulation domain encompassing only local heterogeneous structures underneath array receivers. For simple homogeneous and layer-over-half-space background models, this can be solved by setting the incoming plane waves as initial conditions based on analytical formulae. For more sophisticated background models, a hybrid spectral-element solver is implemented by defining a fictitious boundary encompassing all local heterogeneities within the simulation domain. Displacements on the boundary elements in the absence of 3D heterogeneities are computed by other semi-analytical methods (such as normal-mode or reflectivity), and injected to the numerical solver to simulate the full and scattered wavefield inside and outside the fictitious boundary.

Geophysical Research Letters, 2014

Geophysical Journal International, 2010

Geophysical Research Letters, 2014

OCEANS 2014 - TAIPEI, 2014