Tae-Seob Kang

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.

The 12 September 2016 Gyeongju earthquakes: 1. Observation and remaining questions

Two earthquakes (ML 5.1 and 5.8) ruptured branches of the Yangsan Fault System in Gyeongju, S. Korea on September 12, 2016. After the ML 5.8 earthquake, aftershock earthquakes continued to occur, including two notable earthquakes (ML 4.3 and 4.5) on September 12 and 19, 2016. This paper details the early reports of the Yangsan Fault System in the Gyeongsang Basin from various geological and geophysical/seismological perspectives. Based on a review and an initial seismological analysis of the results of the three earthquakes (ML 5.1, 5.8, and 4.5), we present and discuss the following topics: (1) the tectonic setting and the geophysical/seismic structure of the Yangsan Fault System, (2) historical seismicity and inferred seismic hazard from historical (literature) data, and (3) source mechanisms of the three earthquakes. In the end, we highlight some of the outstanding issues with regard to earthquakes and future research topics.

The 12 September 2016 Gyeongju earthquakes: 2. Temporary seismic network for monitoring aftershocks

The ML 5.8 earthquake in Gyeongju, southeastern Korea, on September 12, 2016 11:32:54 (UTC) was the largest earthquake on the Korean Peninsula since instrumental monitoring began in 1978. It was preceded by an ML 5.1 foreshock and is being followed by numerous aftershocks. Within an hour of the mainshock, the first temporary seismic station to monitor aftershocks was installed at about 1.5 km east of the announced epicenter. The current temporary seismic network consists of 27 stations equipped with broadband sensors covering an area of ∼38 × 32 km in the mainshock region. This is the first high-density aftershock monitoring array in the Korean Peninsula. Initial results, using data from both the regional seismic networks and the aftershock monitoring array, indicate that earthquakes during the first 10 days following the mainshock are related to the Yangsan Fault System. Establishment of an official rapid-response team to monitor aftershocks of major earthquakes is advised.

An analysis of the infrasound signal from the Miyagi-Oki earthquake in Japan on 16 August 2005

Following the 16 August 2005 Miyagi-Oki earthquake in Japan, coherent atmospheric infrasonic waves were observed at regional distances (1200–1500 km) using three seismo-acoustic arrays on the Korean Peninsula. A source-location procedure was applied to the distinct long-duration infrasonic signals to construct earthquakegenerated infrasound source regions on the Japanese island arc. The results showed that the long-duration infrasonic signal was attributable to extensive seismic ground motions on land areas from the southwestern through to the northeastern part of the island arc as well as regions close to the earthquake epicenter. In many coherent infrasonic signals, an effect of seismic ground motions in sedimentary basins could be identified as a source of infrasound radiation from the large earthquake. These observations and interpretations were confirmed using predictions of possible infrasound arrival azimuth variation by converting real seismological data from the dense Japanese seismic network.

The offshore Uljin, Korea, earthquake sequence of April 2006: seismogenesis in the western margin of the Ulleung Basin

An unusual earthquake sequence comprising 11 events with magnitude range ofML 2.0 to 3.2 occurred off the eastern coast of the southern Korean Peninsula in April 2006. Since there is no obvious mainshock in this sequence unlike a typical mainshock-aftershock sequence, the seismicity pattern shows the characteristics of swarm behavior. Focal mechanism of the largest event (ML 3.2) on the 29 of April is normal to strike-slip faulting. Hypocenter relocations of nine events improve the epicenter locations that fall within an area with a radius of about 0.7 km while depths are less well constrained with ranges of 1.6 km to 13.0 km. We propose that a swarm behavior of the sequence is closely related to the marginal geometry of the Ulleung Basin and the regional stress regime. The fact that epicenters of the April 2006 sequence are at the same transitional zone of continental to rifted continental crust as that of the 29 May 2004,Mw 5.1, earthquake indicates that the Ulleung Fault is seismically active.

Relocation of earthquakes beneath the East Sea of Korea: uncertainty of hypocentral parameters caused by refracted waves

In addition to direct waves, refracted (head) waves such as Pn and Sn have been used to determine hypocentral locations, especially when the number of direct waves is inadequate to determine the hypocentral parameters of local earthquakes. However, refracted waves tend to increase errors and the non-uniqueness problems of hypocentral parameters compared with direct waves, if the true velocity structure is not known. Beneath the East Sea of Korea (Japan Sea), conditions for determining hypocentral parameters of earthquakes are unfavorable. To estimate these parameters, this study applied the computational algorithm VELHYPO, which uses the best-fitting velocity model obtained from the results of grid searching methods. The accuracy of hypocentral parameters obtained from direct and different combinations of refracted waves was examined using a synthetic data set. The accuracy tests indicated that hypocentral parameters determined by direct waves alone tend to be more accurate and reliable than those determined by direct and refracted waves combined. The accuracy of hypocentral parameters increases as the grade of the weighting factor for refracted waves decreases when compared with that for direct waves. We relocated 72 earthquakes that occurred beneath the East Sea by applying VELHYPO and giving a lower weighting factor grade to refracted waves than to direct waves. The relocated epicenter distribution shows a close relationship with the regional fault distribution. We compared the hypocentral parameters determined by this study and the Korea Institute of Geoscience and Mineral Resources.

Characteristics in hypocenters of microseismic events due to hydraulic fracturing and natural faults: a case study in the Horn River Basin, Canada

For two to three decades, microseismic monitoring has been popular in the development of unconventional resources, because the fracture network generated by hydraulic fracturing mainly controls the productivity, and microseismic monitoring enables direct measurements for imaging the fracture network. Nevertheless, some refinements are required to make this method more practical. One challenge is to quantify the effects of pre-existing natural fractures for generating microseismic events. We determine the hypocenters of microseismic events occurring in a shale gas play in the Horn River Basin, Canada, and report several interesting spatial and temporal features of the hypocenter distributions. Automatic phase-picking is applied to waveform data recorded at 98 shallow buried three-component geophones, and phases thought to be from the same event are associated. The initial hypocenters of events are determined by iterative linear inversion algorithm then relocated using a double-difference algorithm, where relative travel time measurements are obtained with the waveform cross-correlation. We group events into many clusters based on fracking stages and their hypocenters, and then define the best-fitting plane of hypocenters for each cluster. Most strikes of the best-fitting planes are consistent with the direction of local horizontal stress maximum, indicating that hydraulic fracturing induces most microseismic events. However, the best-fitting planes of several clusters have strikes similar to those of pre-existing faults or fractures, indicating that pre-existing natural faults or fractures can affect the generation of microseismic events. In addition, some observations suggest that natural fractures can affect the temporal evolution of the spatial occurrence pattern of microseismic events. We observed specific migration patterns of microseismic events around known faults in the study area. Although further work is required for complete understanding of this phenomenon, our observations help elucidate the nature of microseismic generation.

Automatic determination of first-motion polarity and its application to focal mechanism analysis of microseismic events

A method for automatically determining first-motion polarities is developed, with a view to its application to the focal mechanism analysis of massive microseismic events caused by hydraulic fracturing. The method is based on two assumptions: the existence of a point source, which has negligible effects of fault finiteness and rupture directivity; and a laterally homogeneous structure, which has azimuthally isotropic propagation characteristics. Under these assumptions, the event waveforms recorded at each station share a common source time function shape, with varying amplitudes, that is dependent on the radiation pattern from the source. With respect to the reference waveform with the highest signal-to-noise ratio (SNR) among all the waveforms at each station for an event, the relative polarities of waveforms at other stations are estimated using cross correlation analysis. The absolute polarities of each waveform are then defined using the reference waveform, whose SNR can be further enhanced using the stacked waveforms after corrections of relative polarities and time lags between the reference and target waveforms have been made. Then, a grid-search algorithm can be incorporated to invert focal mechanisms using the automatically measured polarities for a given event. Since the procedure requires only the evaluation of a cross-correlation coefficient, it can be incorporated into an automated algorithm. The method is applied to determine the focal mechanism solutions of microseismic events occurring in a shale gas play where commercial production is ongoing. Two types of focal mechanism solutions are found to be dominant: vertical dip-slip, and strike-slip. The strike-slip events occur due to re-activation of natural fractures connecting cracks opened by hydraulic fracturing. For the vertical dip-slip events, it was observed that their strikes are consistent with the direction of maximum horizontal stress, which supports the theory that slips occur on the horizontal bedding planes due to the opening of vertical cracks generated by hydraulic fracturing.

Deep Fault Plane Revealed by High‐Precision Locations of Early Aftershocks Following the 12 September 2016 ML 5.8 Gyeongju, Korea, EarthquakeShort Note

An M-L 5.8 earthquake, which is large for a stable continental region, occurred in southeastern Korea on 12 September 2016. Ten days of data from a temporary seismic network deployed immediately after the mainshock are combined with data from permanent seismic stations to determine high-precision locations of early aftershocks to reveal the geometry of the causative structure at depth. Well-constrained relative earthquake hypocenters and focal mechanisms are used to define the subsurface fault plane with a strike of similar to N28 degrees E and dip of similar to 78 degrees to the east-southeast. This fault plane extends from 12 to 15 km depth and may have been responsible for most of the early earthquakes in the Gyeongju earthquake sequence. A pre-existing weak zone in a strike-slip duplex that formed from subsidiary Riedel shears beneath the Yangsan fault system may have been reactivated to nucleate the mainshock and aftershocks.

Imaging of Lithospheric Structure Beneath Jeju Volcanic Island by Teleseismic Traveltime Tomography

Jeju Island (JI) is an intraplate volcanic field located at the continental margin of Northeast Asia. This volcanic island has been formed by multiple eruptions from the Pleistocene to the Holocene (~3.7 ka), which have yielded hundreds of monogenetic volcanic cones and a central basaltic shield. To understand the volcanic structures and mechanism beneath JI, we deployed 20 broadband temporary seismometers across the island for over two years (October 2013 to November 2015). We investigated the crustal and upper mantle structures in JI for the first time using the gathered data. Through teleseismic traveltime tomography, we obtained images of the lithospheric structure related to the volcanic system. A major finding was the identification of a prominent low-velocity anomaly (< 0.3 km/s in P wave velocity relative to the surrounding high-velocity region) beneath the summit of the central shield volcano at greater depths (50–60 km), which separates into low-velocity zones at shallower depths (10–45 km). Based on previous geological observations, the anomalies were interpreted as a magmatic system, potentially with partial melting. Moreover, relatively high velocity zones were consistently imaged to the north, east, and west of the island, indicating relatively thick lithospheric structures at the southern margin of the continental lithosphere beneath the Korean Peninsula. Based on the geometries of the imaged structures, we suggest that a focused decompressional melting at sublithospheric depths and complex magma interactions within the lithosphere resulted in the characteristics of JI volcanism as intraplate magmatic activities that are isolated in space and confined in time.