Chang-Eob Baag

Joint Analysis of Teleseismic Receiver Functions and Surface Wave Dispersion using the Genetic Algorithm

Teleseismic P-wave receiver function data and Rayleigh-wave phase velocity measurements are combined using the genetic algorithm, a global optimi- zation technique, to model crustal structure in southern Korea. The two datasets complement each other because receiver functions are sensitive to shear-wave ve- locity contrasts in layered structures, while surface wave dispersion is sensitive to averages of shear-wave velocities. The genetic algorithm is more useful than line- arized inversion in regions where there is little a priori information about local velocity structure because it is not sensitive to the initial model. The stability and variability of resulting crustal model parameters are quantified by using a Monte Carlo technique in specifying a suite of initial models. Depths to the Moho discon- tinuity in southern Korea were estimated to be 29-30 km for stations near the western coast and 33-36 km for inland stations. A well-resolved crustal low-velocity zone was inferred for some stations.

Crustal Structure in Southern Korea from Joint Analysis of Teleseismic Receiver Functions and Surface-Wave Dispersion

We estimated crustal structures under 18 broadband stations in southern Korea by combining receiver functions and surface-wave dispersion with the genetic algorithm (ga). Estimated crustal structures were analyzed together with previously determined structures under four stations (GKP, INCN, SNU, and TJN) in Chang et al. (2004). The trend of Moho depths estimated from the ga inversion generally coincides with the surface topography, ranging from 26 km to 36 km in the inland. However, the Moho depth distribution does not agree with the topography in the region around the Chugaryeong fault, which extends approximately north-northeast–south-southwest in the central Korean Peninsula. The shallow Moho depth under this region may be related to consequential crustal thinning processes along the fault caused by extensional tectonic movements. Another discrepancy is found in the Gyeongsang basin formed in a retroarc setting by the subduction of the Izanagi Plate in the early Cretaceous. A thick crust observed in the basin may be caused by two factors—maturity of the basin and underplating of magma materials. Average crustal velocities vary from 6.02 km/sec to 6.51 km/sec in southern Korea. This variation indicates that crustal structures in southern Korea involve diverse velocity profiles that change rapidly with distance. Remarkably, a clear velocity discontinuity is observed at the depth range of 8–10 km under several stations.

Crustal structure of the continental margin of Korea in the East Sea (Japan Sea) from deep seismic sounding data : evidence for rifting affected by the hotter than normal mantle

Abstract Despite the various opening models of the southwestern part of the East Sea (Japan Sea) between the Korean Peninsula and the Japan Arc, the continental margin of the Korean Peninsula remains unknown in crustal structure. As a result, continental rifting and subsequent seafloor spreading processes to explain the opening of the East Sea have not been adequately addressed. We investigated crustal and sedimentary velocity structures across the Korean margin into the adjacent Ulleung Basin from multichannel seismic (MCS) reflection and ocean bottom seismometer (OBS) data. The Ulleung Basin shows crustal velocity structure typical of oceanic although its crustal thickness of about 10 km is greater than normal. The continental margin documents rapid transition from continental to oceanic crust, exhibiting a remarkable decrease in crustal thickness accompanied by shallowing of Moho over a distance of about 50 km. The crustal model of the margin is characterized by a high-velocity (up to 7.4 km/s) lower crustal (HVLC) layer that is thicker than 10 km under the slope base and pinches out seawards. The HVLC layer is interpreted as magmatic underplating emplaced during continental rifting in response to high upper mantle temperature. The acoustic basement of the slope base shows an igneous stratigraphy developed by massive volcanic eruption. These features suggest that the evolution of the Korean margin can be explained by the processes occurring at volcanic rifted margins. Global earthquake tomography supports our interpretation by defining the abnormally hot upper mantle across the Korean margin and in the Ulleung Basin.

Crustal Structure in Southern Korea from Joint Analysis of Regional Broadband Waveforms and Travel Times

Regional broadband waveforms and travel times of seismic phases are combined to model crustal structure in regions where the signal-to-noise ratio (snr) of waveforms is low. The utilization of travel-time data enables us to overcome the problem occurring in waveform inversion with low snr data: the distortion of deep velocity structure. The genetic algorithm (ga) adopted as a search algorithm is useful in regions where there is little a priori information about crustal structure. After verifying the robustness of the joint inversion technique by numerical tests, we applied the technique to southern Korea in order to get a simple one-dimensional average crustal model, using broadband waveforms bandpassed between 0.03 and 0.3 Hz and travel times of Pg , PmP , and Pn waves recorded at ten broadband and five short-period stations from the 2 June 1999 Gyeongju earthquake ( M L 3.4), Korea. The P -wave velocities in the resulting three-layered crustal model are 5.67 ± 0.06, 6.05 ± 0.05, 6.67 ± 0.02, and 7.88 ± 0.02 km/sec from the top layer to the half- space, respectively, and the depths of layers are 5.1 ± 0.8, 16.7 ± 1.0, and 31.9 ± 1.0 km. Synthetic waveforms and travel-time curves corresponding to the resulting velocity model fit observed seismograms generally well. The synthetics are preceded by the observed waveforms in the southwestern part of Korean Peninsula, inferring the presence of a high-velocity zone in that area.

Moho Depth and Crustal VP/VS Variation in Southern Korea from Teleseismic Receiver Functions: Implication for Tectonic Affinity between the Korean Peninsula and China

We estimated Moho depths and V P / V S ratios of the crust under 21 broadband stations in southern Korea by using a grid search in the crustal thickness– V P / V S ratio ( H-κ ) domain. The Moho depth varies from 25.9 km to 32.5 km, and the V P / V S ratio ranges from 1.71 to 1.82 inland. Moho depths in the southernmost area of the Korean Peninsula were estimated shallower than those of the previous results obtained assuming a Poisson solid in the joint analysis of receiver functions and surface-wave dispersion. This southernmost area is roughly in accord with the Yeongnam massif, where relatively high V P / V S ratios of 1.78–1.82 are estimated. On the contrary, comparatively low V P / V S ratio measurements (1.71–1.76) are generally distributed in the Gyeonggi massif, which is located in the central area of the Korean Peninsula. The major factor for the high V P / V S ratios in the Yeongnam massif is thought to be the plagioclase-rich mafic composition of the lower crust rather than partial melting or crustal fluids, because high crustal S -wave velocities are reported in the Yeongnam massif. The mafic composition might have been supplied by the magmatic underplating. From the clearly divided feature of V P / V S ratios in southern Korea and the V P / V S ratio similarities between southern Korea and China, it seems that the Yeongnam massif might be related to the Sino-Korea craton, whereas the Gyeonggi massif is related to the Yangtze craton.

Parallel implementation of a velocity-stress staggered-grid finite-difference method for 2-D poroelastic wave propagation ☆

Abstract Numerical simulation of wave propagation in poroelastic media demands significantly more computational capability compared to elastic media simulation. Use of serial codes in a single scientific workstation limits the size of problem. To overcome this difficulty, a parallel velocity-stress staggered-grid finite-difference method is developed for efficient simulation of wave propagation in 2-D poroelastic media. The finite difference formulation of Biots theory has the properties of fourth order accuracy in space and second order accuracy in time. The model is decomposed into small subdomains for each processor. After each processor updates wavefields within its domain, the processors exchange the wavefields via message passing interface (MPI). The parallel implementation reduces the computational time and also allows one to study larger problems. From our numerical experiment, consistent with other 1-D experiments, it is found that the presence of heterogeneity of porous medium can produce significant P-wave attenuation in the seismic frequency range.

The 29 May 2004,M w =5.1, offshore Uljin earthquake, Korea

We present results of a preliminary analysis for source parameters of the 29 May 2004 offshore Uljin earthquake. The analysis is based on a set of broadband and short period velocity seismograms, and accelerograms recorded by seismic networks in southern Korea. The estimated parameters of the mainshock are as follows: origin time 19∶14∶25.82 in Korean local time (10∶14∶25.82 UT), latitude 36.626°N, longitude 130.054°E, depth 18 km, magnitudeMw 5.1, stress drop 28 bars, one focal plane 337°, 56° and 78°, and the other plane 178°, 36° and 107° in strike, dip and rake, respectively. Three aftershocks ofMw 2.3, 2.0 and 2.4 occurred within source region of the mainshock, and two other events ofMw 2.5 and 3.5 at epicenters beyond the rupture area are presumed to be triggered dynamically by the 29 May 2004 earthquake. The largest ground acceleration, 2.62%g, was instrumentally recorded at Pohang, 90.4 km south-west of the mainshock epicenter. The maximum instrumental intensity is estimated as V and VI in Modified Mercalli Intensity scale excluding and including local effects, respectively. There is a possibility that the mainshock-aftershock sequence and one of the dynamically triggered events are directly related to the Ulleung Fault.

Rapid and Accurate Two-Point Ray Tracing Based on a Quadratic Equation of Takeoff Angle in Layered Media with Constant or Linearly Varying Velocity Functions

A rapid and accurate computational method is introduced for two-point ray tracing in horizontally layered media with constant or linearly varying vertical velocity distributions. The horizontal distance between the source and a receiver can be expressed in a nonlinear form in terms of the takeoff angle at the source, with the use of Snell9s law for layered media, in which the velocity of each layer is constant or linear with depth. This nonlinear equation is expanded in a Taylor series with terms up to the second order about the initial value of the takeoff angle of the ray for two-point ray tracing. This expansion yields a quadratic equation with respect to the correction angle, which is the angular difference between the true and calculated takeoff angles at the source or the starting point of the ray. The takeoff angle of the ray is updated in an iterative way by solving this quadratic equation for the correction angle. The initial value of the takeoff angle in the iteration is reasonably estimated by considering geometric aspects of ray paths and dynamic properties of rays. A primary estimation of the initial value is obtained by adjustment of ray parameter values, starting from the takeoff angle of the straight line extending from the source to the receiver and assuming that the media are homogeneous, with a velocity equal to an average velocity obtained by weighting on the ray segment length within each layer along the ray path. An improved initial takeoff angle is obtained from the dynamic properties of the rays in layered medium, such as constancy of the ratio of distances from the central ray to any nearby two rays measured at any points along the central ray so long as the takeoff angle differences among these rays are small. The convergence rate of computation in this method is much faster than that of Newton9s method and requires only a few iterations, even for large horizontal distances between the source and receivers. This method also works for layered media that embed low-velocity layers. The computational results show that the initial values of takeoff angle calculated in this article are not far from the true value and provide stable and rapid convergence in two-point ray tracing. Therefore, the accuracy and convergence rate of this method are sufficient enough to be applied to a wide range of seismic problems.

Finite-Difference Seismic Simulation Combining Discontinuous Grids with Locally Variable Timesteps

A locally variable timestep scheme that matches with discontinuous grids in the finite-difference method is developed for the efficient simulation of seismic-wave propagation. The first-order velocity-stress formulations are used to obtain the spatial derivatives using finite-difference operators on a staggered grid. In the case of a media interface with high velocity contrast, the computational domain consists of two regions with different grid spacings. Each region roughly covers the medium of the lower or higher wave propagation velocity. There is a small overlap of the two regions, called the transitional zone, within the higher velocity medium. A grid three times coarser in the high-velocity region compared with the grid in the low-velocity region is used to avoid spatial oversampling. Temporal steps corresponding to the spatial sampling ratio between both regions are determined based on local stability criteria. The wave field in the margin of the region with the smaller timestep is linearly interpolated in time using the values calculated in the region with the larger one within the transitional zone. Since the temporal interpolation is a 1D operation performed separately from the spatial interpolation strategy employed to connect two regions with different grid spacings, the proposed scheme is not restricted to 2D or 3D problems with a specific order of accuracy of the spatial finite-difference approximation. The use of the locally variable timestep scheme with discontinuous grids results in remarkable savings of computation time and reductions in memory requirements, with the efficiency depending on the simulation model.

An Efficient Finite-Difference Method for Simulating 3D Seismic Response of Localized Basin Structures

We present an efficient technique to simulate seismic wave propagation in structures including basins using localized discontinuous grids in combination with locally variable timesteps. The technique uses a 3D fourth-order, staggered-grid, finite-difference method based on the velocity-stress formulations of the elastodynamic equations. Discontinuous grids have 3D blocky form; thus, the boundary between the two adjacent regions of different grid spacings could be extended to the position of the Earth9s free surface. As an example of application, ground motions in a small-scale basin in the Hongseong area, Korea, is simulated considering a hypothetical source of the 7 October 1978 Hongseong, M L 5, earthquake. The results show that the method can achieve distinguished reductions of computational memory and thus CPU time for models of the localized 3D structures compared with methods on grids with constant spatial and temporal steps.