Tae-Kyung Hong

Pn Travel-Time Tomography of the Paleo-Continental-Collision and Rifting Zone around Korea and Japan

Abstract A recent dense deployment of seismic stations in South Korea and Japan allows regional seismic imaging of the far-east Asian region that experienced continental collisions and riftings. We perform seismic imaging based on a mantle-lid P -wave ( Pn ) travel-time tomography to exploit the tectonic imprints in the lithosphere. The average Pn velocity in the region is estimated to be 7.95±0.03 km/sec. The inverted Pn velocities illuminate the tectonic structures. High velocities of ∼8.15 km/sec are observed in the Precambrian massif regions, while low velocities of ∼7.8 km/sec are associated with the fold belt and sedimentary basin regions in the southern Korean Peninsula. The Pn velocity is estimated to be low in the backarc basins, including the Ulleung and the Yamato basins, while it is estimated to be higher on the continental fragments, including the South Korea plateau and the Oki bank. The high-velocity structures along the Japanese Islands support the separation of the Japanese province from the Eurasia plate. The high-velocity anomaly along the east coast of the Korean Peninsula around the Hupo bank suggests intrusion and solidification of high-density material in the lower crust and mantle lid.

Probabilistic Analysis of the Korean Historical Earthquake Records

Seismic‐hazard assessment for earthquakes with long recurrence intervals requires long earthquake records. Historical records for seismic damage can complement instrumental earthquake records, which is particularly useful for low‐seismicity regions. Uncertainty in epicenters and magnitudes, however, hinders the full use of historical earthquake records. A probabilistic method to determine the epicenters of historical events is presented. The epicenters of historical events are determined probabilistically considering the nature of seismicity such that the spatial distribution of seismicity is nearly stationary with time in a constant tectonic environment. The probabilities for historical events to have occurred at given locations can be calculated using an instrumental seismicity density function and a distance‐dependent weighting function. A location is selected randomly from a set of candidate source locations, of which probabilities are greater than a given prescribed value. The epicentral seismic intensity for the determined source location is calculated using an intensity–distance relationship. The magnitude is determined using an intensity–magnitude relationship. The method is verified with analysis of seismic intensity data for earthquakes in California. The method is applied to an ∼1900 year long Korean historical seismic record, and the magnitudes of historical earthquakes are estimated. The Gutenberg–Richter b values for the historical events are determined to be 0.73. It is observed that the northwestern and southern peninsula, and the southeastern offshore region have high seismic‐hazard potentials.

Scattering Attenuation of 2D Elastic Waves: Theory and Numerical Modeling Using a Wavelet-Based Method

The passage of seismic waves through highly heterogeneous media leads to significant scattering of seismic energy and an apparent attenuation of seismic signals emerging from the heterogeneous zone. The size of this scattering attenuation depends on the correlation properties of the medium, the rates of P- and S-wave velocities, and frequency content of the incident waves. An estimate of the effect can be obtained using single scattering theory (first-order Born approximation) for path deviations beyond a minimum scattering angle; smaller deviations require consideration of multiple scattering or a representation in terms of travel-time perturbations. Although an acoustic treatment provides a quantitative reference, full elastic effects need to be taken into consideration to get an accurate attenuation rates. The use of a wavelet-based modeling technique, which is accurate and stable even in highly perturbed media, allows an assessment of the properties of different classes of stochastic media (Gaussian, exponential, von Karman). The minimum scattering angle for these stochastic media is in the range of 60° to 90°. The wavelet-based method provides a good representation of the scattered coda, and it appears that methods such as finite differences may overestimate scattering attenuation when the level of the heterogeneity is high.

Lg wave propagation in the area around Japan: observations and simulations

Regional wavefields are strongly influenced by crustal structure heterogeneity variations along their propagation paths. Observations of such effects between the Asian continent and the Japanese subduction zone across the Sea of Japan (East Sea) have been strongly assisted recently by the development of high-density seismic networks in Japan and Korea, as well as the supercomputer-based three-dimensional finite difference method seismic wave propagation simulations using detailed heterogeneous crustal models. In this study, an Lg propagation map derived from 289,000 ray paths connecting sources to observation stations reveals efficient Lg wave propagation from continental Asia to Kyushu through the Korean Peninsula, and to Hokkaido, thus indicating a laterally consistent crustal structure extending from continental Asia to Japan. However, the Lg wave propagation to the Japanese main island of Honshu is totally blocked as it crosses the continental-oceanic boundary surrounding the Sea of Japan. Three-dimensional (3-D) finite-difference method seismic wave propagation simulations performed using a detailed crustal structural model allow us to clearly visualize the way in which an Lg wave develops from a shallow source in the crust and its propagation in the crustal waveguide by means of multiple post-critical S wave reflections in the continental structure. The sudden thinning of the continental crust at the edge of the Asian continent adjacent to the oceanic crust in the Sea of Japan, which involves a thickness change from 30 to 10 km within a 100-km distance (with the thinner crust extending over 600 km), decreases the Lg wave energy by 10%. It has been confirmed that 50% of this Lg wave energy loss occurs during wave passage along the thinner crust and that the other 50% results from conversion into P energy in the overlying seawater.

Seismic discrimination of the 2009 North Korean nuclear explosion based on regional source spectra

Seismic discrimination of an underground nuclear explosion (UNE) based on regional waveforms in continental margins is challenging due to large variations among waveforms. The 2009 North Korean UNE test was conducted in the far eastern Eurasian plate. The UNE was recorded by densely-located regional seismic stations, and regional waveforms exhibit highly path-dependent amplitude and arrival time features due to complex crustal structures. Regional source spectra are calculated by correcting for the path effects on the waveforms. A two-step approach is proposed for stable inversion of source-spectral parameters and path parameters. Characteristic overshoot features are observed in the source spectra, particularly strong in Pn. The path parameter, Q, is determined uniquely regardless of the source-spectral model implemented, which suggests stable separation of path effects from waveform records. The estimated source spectra fit well to a theoretical UNE source-spectral model. The fitness between the estimated and theoretical source-spectral models allows us to discriminate UNEs from natural earthquakes. Also, the P/S source-spectral ratio is observed to be an effective discriminant of UNE.

Seismotectonic Properties and Zonation of the Far-Eastern Eurasian Plate Around the Korean Peninsula

Regional seismotectonics provides crucial information for seismic hazard analysis, which is difficult to address with short-term earthquake records. The far-eastern Eurasian plate around the Korean Peninsula presents a stable intraplate environment with diffuse seismicity, of which responsible tectonics and active faults are difficult to identify. Combined analysis of instrumental and historical earthquake records is required for assessment of long-term seismicity properties. Seismotectonic provinces are identified from the spatial distribution of seismicity properties controlled by the medium properties and stress field. The boundaries of the seismotectonic provinces are defined considering the medium properties that can be inferred from geological, geophysical and tectonic features. The Gutenberg–Richter frequency–magnitude relationships and maximum magnitudes for the seismotectonic provinces are determined using instrumental and historical earthquake records. The validity of maximum magnitude estimation is tested with synthetic data. A parametric method, the Tate–Pisarenko method, produces more accurate estimates than non-parametric methods. A modified Tate–Pisarenko method is proposed for estimation of maximum magnitudes for incomplete short-term earthquake catalogs. The maximum magnitude of events for the whole region is approximately the same as the average of the maximum magnitudes of events for subdivided provinces, causing apparent variation in maximum magnitudes depending on the number of seismotectonic provinces. Consideration of a reasonable number of seismotectonic provinces may be needed for proper assessment of seismic hazard potentials is recommended. The combined analysis of historical and instrumental earthquake records suggests maximum magnitudes greater than 7 around the peninsula.

The 12 September 2016 ML5.8 midcrustal earthquake in the Korean Peninsula and its seismic implications

The seismicity in the Korean Peninsula has increased since the 2011 Mw9.0 Tohoku-Oki megathrust earthquake. Two strike-slip earthquakes with magnitudes of ML5.1 and 5.8 occurred in the southeastern Korean Peninsula on September 12, 2016. The two events occurred within 48 minutes. The ML5.8 earthquake was the largest event in the Korean Peninsula since 1978 when national seismic monitoring began. Both events produced strong high-frequency ground motions. More than 500 aftershocks with local magnitudes greater than or equal to 1.5 followed the events for two months. An unreported subsurface strike-slip fault with a dip of 65∘ to the east and a strike of N27∘ E was responsible for the earthquakes. The fault ruptured at depths of 11-16 km, resulting in a rupture plane of ∼26 km2. The aftershock distribution displayed horizontal streaks at a depth of ∼14 km, which was consistent with the focal mechanism solutions from long-period waveform inversion. The number of aftershocks decreased exponentially with time. The two ML5.1 and 5.8 earthquakes produced regional Coulomb stress changes of -4.9 to 2.5 bar. The spatial distribution of the aftershocks correlated with the Coulomb stress changes. The peak dynamic stress induced by strong ground motions reached 14.2 bar. The groundwater levels changed coseismically in some regions of decreased static stresses. The earthquakes on previously unidentified faults raised attention for the potential seismic hazards by earthquakes with long recurrence intervals.

Seismic Investigation of the 26 March 2010 Sinking of the South Korean Naval Vessel Cheonanham

A South Korean naval vessel sank on 26 March 2010. A seismic event with magnitude (ML) of 1.5 was observed at the time of vessel-sinking. Seismic waveforms were collected from three local stations. The event origin time is refined based on the P and S arrival times at the stations. The calculated event location and time are close to the reported vessel-sinking location and time, suggesting that the observed seismic event is associated with the vessel-sinking. The amplitudes of S waves are comparable to those of P waves. Seismic waves coupled from acoustic waves are observed, providing additional constraints of epicentral distance and source type. The coupled acoustic waves have a dominant frequency of ∼32 Hz. The acoustic waves and high P=S amplitude ratios suggest an underwater explosion. The body- wave magnitude based on Pn is determined to be 1.46, which is consistent with the reported ML. P energy is dominant at around 8.5 Hz, with multiple frequencies of 17.7 and 34.6 Hz. The primary frequency suggests the water-column thickness in the source region to be 44 m, which agrees with the reported value.

Prediction of ground motion and dynamic stress change in Baekdusan (Changbaishan) volcano caused by a North Korean nuclear explosion

Strong ground motions induce large dynamic stress changes that may disturb the magma chamber of a volcano, thus accelerating the volcanic activity. An underground nuclear explosion test near an active volcano constitutes a direct treat to the volcano. This study examined the dynamic stress changes of the magma chamber of Baekdusan (Changbaishan) that can be induced by hypothetical North Korean nuclear explosions. Seismic waveforms for hypothetical underground nuclear explosions at North Korean test site were calculated by using an empirical Green’s function approach based on a source-spectral model of a nuclear explosion; such a technique is efficient for regions containing poorly constrained velocity structures. The peak ground motions around the volcano were estimated from empirical strong-motion attenuation curves. A hypothetical M7.0 North Korean underground nuclear explosion may produce peak ground accelerations of 0.1684 m/s2 in the horizontal direction and 0.0917 m/s2 in the vertical direction around the volcano, inducing peak dynamic stress change of 67 kPa on the volcano surface and ~120 kPa in the spherical magma chamber. North Korean underground nuclear explosions with magnitudes of 5.0–7.6 may induce overpressure in the magma chamber of several tens to hundreds of kilopascals.

Joint Determination of Event Epicenter and Magnitude from Seismic Intensities

Characteristic features of seismicity with long recurrence intervals can be deduced from analysis of historical earthquake records that inherently suffer from uncertainty in the event locations and magnitudes. A novel method to determine the event epicenters and magnitudes jointly from seismic intensities is proposed. The probability for a set of event epicenter and magnitude is assessed by accounting the fitness between the observed and reference seismic intensities, spatial‐occurrence probability based on seismicity density distribution, and temporal‐occurrence probability from the Gutenberg–Richter magnitude–frequency relationship. A set of event epicenter and magnitude yielding the peak probability is chosen. The validity of the method is tested for both synthetic and instrumental seismic‐intensity data, confirming high accuracy. The method is applied effectively to historical events with written seismic damage records. It is found that the errors generally decrease with increasing number and azimuthal coverage of seismic‐intensity data, and increase with epicentral distances. The method appears to be promising for historical earthquakes of which source properties are poorly known. The method is applicable for assessment of the properties of long‐period seismicity, which is crucial for assessment of potential seismic hazards.