Dong-Hoon Sheen

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.

Magnitude scaling relationships using P waves for earthquake early warning in South Korea

The scaling relationships of the peak displacement, Pd, and the maximum predominant period, τpmax, of P waves were investigated to estimate magnitudes for earthquake early warning in South Korea. Pd and τpmax were measured for 504 vertical records from 70 earthquakes at distances of 20 to 100 km. The earthquakes occurred between 2001 and 2011 and ranged from ML = 3.0 to 5.2. Since the events were generally low to moderate in magnitude, the parameter for a real-time high-pass filter was adjusted and the first 3 seconds of the P-waves were processed. The scaling relationships of Pd and τpmax obtained from iterative regressions were M = 1.17 log(Pd) + 0.87 log(R) + 6.57 and M = 3.30 log(τpmax) + 5.75, respectively, where R is the epicentral distance in kilometers, Pd is in centimeters, and τpmax is in seconds. The average errors of the magnitude estimates obtained from the mean of the Pd magnitude and τpmax were 0.06 magnitude units for the calibration data but 0.37 for a recent magnitude 3.9 event, which implies that the scaling relationships can be used in these forms but the relationships still need to be improved with more data to be useful for mitigating damage from future earthquakes around the Korean Peninsula.

Microseisms from huge Indian Ocean storms in May 2007

Ocean waves are acknowledged to be the cause of microseisms ubiquitous in seismograms. The excitation and source locations of microseisms, however, remain enigmatic. In this study, the characteristics of microseisms generated by extraordinary storms in the Indian Ocean in the middle of May, 2007 were investigated. Spectral analysis showed that two huge ocean-swell systems, one that began around May 9 and the other around May 14, generated strong microseisms: single-frequency (SF) microseisms ranging from 0.04–0.06 Hz and double-frequency (DF) microseisms ranging from 0.08–0.12 Hz. The dispersion of ocean waves generated a progressive frequency shift of microseismic peaks. A comparison among microseisms at deep-ocean islands and coastal seismic stations indicated that most of the dispersed DF microseisms followed SF microseisms generated in coastal areas. However, a dispersed DF microseism can also occur with no preceding excitation of an SF microseism; this has been observed at deep-ocean islands and coastal stations simultaneously, and implies that some of the DF microseisms recorded at inland stations may have been generated in the deep ocean. Frequency-wavenumber analysis of the DF band indicated the presence of non-dispersed body-wave microseisms when the swells were in the middle of the ocean. However, microseisms showing a progressive frequency shift of spectral intensity propagated dominantly as surface-waves and were observed when the swells were close to the coast and when a favorable condition for wave-wave interaction reached in the deep ocean.

Efficient Finite Difference Calculation of Partial Derivative Seismic Wavefield Using Reciprocity and Convolution

Summary Seismic waveform inversion is commonly studied by the least-squares method which transforms the inverse problem to an iterative minization problem of residuals between the synthetic model response and observed data. One can solve this problem using the gradient method, the Gauss-Newton method or the full Newton method. The performance of the inverse method depends on the algorithm for calculating the partial derivative seismic wavefield due to perturbations in the subsurface. In this work, 2-D elastic staggered-grid finite difference method is used to explicitly calculate the partial derivative wavefields by utilizing the sourcereceiver reciprocity and the convolution.

Application of the Maximum‐Likelihood Location Method to the Earthquake Early Warning System in South Korea

Abstract The purpose of most earthquake early warning systems (EEWSs) is promptly to identify damaging earthquakes occurring at local distances from a seismic station or network. However, mislocation of a large regional or teleseismic event that is relatively innocuous to the local society can cause an EEWS to issue a serious false alarm. To solve this problem, we applied the recently developed, robust maximum‐likelihood earthquake location method (MAXEL) to the EEWS in South Korea, which is adjacent to several major plate boundaries capable of producing very large earthquakes at regional distances. MAXEL, which is based on the maximum‐likelihood method using the concepts of equal differential times of P arrivals and Student’s t distribution, was optimized for the application and event identification criteria were introduced. Although the maximum number of arrivals for locating an earthquake was controlled to allow for fast computation and successful operation of the EEWS, it was possible to maintain location accuracy. Synthetic tests based on current Korean seismic networks showed that MAXEL could locate local events very well and also handle regional earthquakes effectively. In addition, offline tests using observed data also clearly confirmed the robustness of MAXEL to outliers and its superior capability for discriminating local events from regional or teleseismic events. Therefore, it is expected that MAXEL could significantly improve the stability and accuracy of the phase association and event location of EEWSs and that MAXEL has great potential for earthquake early warning.

Estimation of Wave Parameters for Probabilistic Tsunami Hazard Analysis Considering the Fault Sources in the Western Part of Japan

Probabilistic tsunami hazard analysis (PTHA) is based on the approach of probabilistic seismic hazard analysis (PSHA) which is performed using various seismotectonic models and ground-motion prediction equations. The major difference between PTHA and PSHA is that PTHA requires the wave parameters of tsunami. The wave parameters can be estimated from tsunami propagation analysis. Therefore, a tsunami simulation analysis was conducted for the purpose of evaluating the wave parameters required for the PTHA of Uljin nuclear power plant (NPP) site. The tsunamigenic fault sources in the western part of Japan were chosen for the analysis. The wave heights for 80 rupture scenarios were numerically simulated. The synthetic tsunami waveforms were obtained around the Uljin NPP site. The results show that the wave heights are closely related with the location of the fault sources and the associated potential earthquake magnitudes. These wave parameters can be used as input data for the future PTHA study of the Uljin NPP site.

Analysis of Uniform Hazard Spectra for Metropolises in the Korean Peninsula

The uniform hazard spectra for seven major cities in Korea, Seoul, Daejeon, Daegu, Busan, Gwangju, Ulsan, and Inchon are suggested. Probabilistic seismic hazard analyses were performed using the attenuation equations derived from seismology research in Korea since 2000 and the seismotectonic models selected by expert assessment. For the estimation of the uniform hazard spectra, the seismic hazard curves for several frequencies and PGAs were calculated by using the spectral attenuation equations. The seismic hazards (annual exceedance probability) calculated for the 7 metropolises ranged from about to and averaged out at about with a log standard deviation of about 0.085 at 0.2 g. The uniform hazard spectra with recurrence intervals of 500, 1000, and 2500 years estimated by using the calculated mean seismic hazard on the frequencies presented peak values at 10.0 Hz, and the log standard deviations of the difference between metropolises ranged from about 0.013 to 0.209. In view of the insignificant difference between the estimated uniform hazard spectra obtained for the considered metropolises, the mean uniform hazard spectrum was estimated. This mean uniform hazard spectrum is expected to be used as input seismic response spectrum for rock sites in Korea.

Microseismic Monitoring Using Seismic Mini-Array

It was introduced a seismic mini-array that could monitor microseismicity efficiently and analyzed seismic data obtained from the mini-array that was operated from December 19, 2012 to January 9, 2013. The mini-array consisted of a six channel data logger, a central 3 components seismometer, and a tripartite array of vertical sensors centered around the 3 components seismometer as an equilateral triangle with about 100 m aperture. All seismometers that had the same instrument response were connected a 6 channel data logger, which was set to record seismograms at a sampling rate of 200 sps. During the three weeks of campaign, a total of 16 microearthquakes were detected. Using time differences of P wave arrivals from the vertical components, S-P time from 3 components seismometers, and back azimuth from the seismic array analysis, it was possible to locate the hypocenter of the microearthquake even with one seismic miniarray. The epicenters of two nearest microearthquakes were a quarry site located 1.3 km from the mini-array. The records of quarry blasting confirmed the our analysis.

Basin model inversion using information theory and seismic data

We present a new approach to basin-model inversion in which uncertain parameters in a basin model are estimated using information theory and seismic data. We derive a probability function from information theory to quantify uncertainties in the estimated parameters in basin modeling. The derivation requires two constraints: a normalization and a misfit constraint. The misfit constraint uses seismic information by minimizing the difference between calculated seismograms from a basin simulator and observed seismograms. The information-theory approach emphasizes the relative difference between the so-called expected and calculated minima of the misfit function. The synthetic-model application shows that the greater the difference between the expected and calculated minima of the misfit function, the larger the uncertainty in parameter estimation. Uncertainty analysis provides secondary information on how accurately the inversion process is performed in basin modeling.

Wave propagation in poroelastic media: A velocity‐stress staggered‐grid finite‐difference method with perfectly matched layers

We present a velocity-stress staggered-grid finitedifference method to simulate wave propagation in heterogeneous poroelastic media. Biot’s theory expressed in terms of velocity and stress is to obtain a set of first order hyperbolic equations. This formulation is discretized into a staggered grid both in space and time domain with the harmonic average of the material properties to account for heterogeneous media. To simulate the wave propagation in an unbounded media, the perfectly matched layer method is used as an absorbing boundary condition. Numerical simulations of Biot’s theory show the existence of a slow P wave in porous media and are also qualitatively consistent with previous analytic predictions of the wave propagation in fluid saturated poroelastic media.