Search

Thunder-induced seismic waves recorded at dense seismic stations in Seoul, South Korea are analysed for inversion of thunder source spectra. Thunder-induced seismic waves from four local thunder events are analysed. A theory is introduced for the inversion of acoustic source spectra from thunder-induced seismic waves. In the course of source-spectral inversion, the propagation and acoustic-to-seismic coupling effects are counted. The thunder-induced seismic signals were well identified at distances of <∼20 km. Direct acoustic-to-seismic coupled seismic waves present apparent phase velocities of sound speed in atmosphere (340 m s−1). Thunder-induced seismic waves are dominant at high frequencies (>20 Hz). Vertical peak ground accelerations of thunder-induced seismic waves in local regions (0.024–0.110 m s−2 at distances of 2.4–3.7 km) are equivalent to the ground motion levels induced by a moderate-size (∼ M 5) earthquake at regional distances. The thunder-induced acoustic waves in the atmosphere are obtained by removing the acoustic-to-seismic coupling effect and site-response effect from the observed thunder-induced seismic waves. The quality factors for acoustic wave attenuation in the atmosphere are determined. Urban landscapes and atmospheric effects cause strong acoustic attenuation over atmospheric absorption. Acoustic thunder source spectra are determined by stacking the inverted acoustic spectra at all stations. The peak frequencies of acoustic thunder source spectra are around 34–36 Hz, suggesting the acoustic energy per unit length in lightning strikes to be ∼4 × 106 J m−1. Local seismic records are applicable for the investigation of thunder and lightning properties. [ABSTRACT FROM AUTHOR]

Major earthquakes in continental regions may cause significant damage. Preexisting fault system across megacity receives high attention for possible seismic damages. Earthquake occurrence mechanism is important to assess the geohazard potentials. Continental‐scale Quaternary fault system is developed across the Seoul metropolitan area where the population is the largest in the Korean Peninsula. Historical seismic‐damage records suggest potential seismic hazards in the Seoul metropolitan area. We investigate the fault motions and spatial distribution of earthquakes in the Seoul metropolitan area using a matched‐filter technique that is based on stacked waveform crosscorrelation functions among densely‐deployed seismic stations. The analysis detects 1103 earthquakes that include 360 events with magnitudes (ML) of −0.6 to 2.0 around the Chugaryeong fault and 34 events with magnitudes of −0.5 to 2.7 around Wangsukcheon, Pocheon, and Yeseonggang faults. The seismicity suggests a set of near‐vertical subparallel (or orthogonal) faults that develop from the major faults. A major fault system behaves as a backbone structure that makes branch faults develop, producing seismicity including major earthquakes. The backbone structure may control the fault development that conforms to the ambient stress field. The backbone faults may play a role to increase geohazard potentials. Plain Language Summary: The Seoul metropolitan area is the most populated region in the Korean Peninsula. Historical seismic damage records and major Quaternary faults suggest high seismic hazard potentials in the region. The identification of earthquake‐spawning faults is crucial for seismic hazard mitigation. We illuminate the earthquake‐spawning faults from micro to small earthquakes in the Seoul metropolitan. We perform a matched‐filter analysis to find earthquakes in adjacent locations of reported events. Recently‐deployed temporary dense seismic stations allow us to detect microearthquakes in high ambient‐noise environments of the megacity. We detect 1103 earthquakes that include 360 events with magnitudes (ML) of −0.6 to 2.0 around the Chugaryeong fault and 34 events with magnitudes (ML) of −0.5 to 2.7 around Wangsukcheon, Pocheon, and Yeseonggang faults. The earthquake distribution suggests the presence of active near‐vertical subparallel or conjugate faults along the major Quaternary faults that may behave as backbones of earthquake‐spawning faults. Key Points: Micro to small earthquakes occur around Quaternary faults in the Seoul metropolitan areaSubvertical strike‐slip faults develop along the Quaternary faults, responding to the ambient stress fieldClustered earthquakes occur along conjugate or subparallel faults [ABSTRACT FROM AUTHOR]

An approximately 40‐km long high‐resolution reflection seismic profile (P3) was acquired in the metropolitan area of Seoul in South Korea for the purpose of fault system imaging in a highly noisy and challenging urban environment. Two 12t seismic vibrators (mini‐vibs) were used as the seismic source. Data were recorded using a dual element seismic spread; 20 m spaced 421 wireless seismic recorders connected to 10 Hz geophones and 20 micro‐electro‐mechanical‐based landstreamer sensors (2 m sensor spacing) attached to one of the vibrators. The purpose of the dual spread employed was to delineate both near‐surface and deep structures. The processing results show good quality and the processing work was complemented by different analysis to further constraints the geological interpretation. The survey results provide evidence for the 3D geometry of three fault systems, including Chugaryeong, Pocheon, and Wangsukcheon faults. A gently westerly dipping set of reflectivity underlying a dome‐shaped package of reflectivity is interpreted as a fault, and could project to the known surface position of the Pocheon fault. The dome‐shaped reflectivity is interpreted as folded and faulted dyke or sill systems. Downward continuation of the interpreted fault intersects the sub‐vertical Chugaryeong fault in a zone where the current seismicity is observed, suggesting that these two major fault systems may have jointly evolved in the form of splay faults. Reflections from the Wangsukcheon fault are also present in the data and interpreted to dip approximately 60° to the east, in an opposite direction to the two other faults. Plain Language Summary: An approximately 40‐km long high‐resolution reflection seismic profile was acquired in the metropolitan area of Seoul in South Korea for the purpose of in‐depth fault system imaging in a highly noisy and challenging urban environment. Two different types of receivers were used simultaneously to achieve good results from both shallow and deep structures. Processing results have good quality and are complemented with different analysis to further constraints the geological interpretation. The survey results provide evidence for the 3D geometry of three fault systems, including Chugaryeong, Pocheon, and Wangsukcheon faults. A gently westerly dipping structure underlying a dome‐shaped structure is interpreted as a fault, and could project to the known surface position of the Pocheon fault. A dome‐shaped reflectivity is interpreted as folded and faulted dyke or sill systems. Downward continuation of the interpreted fault intersects the sub‐vertical Chugaryeong fault in a zone where the current seismicity is observed, suggesting that these two major fault systems may be connected and jointly evolved. Results from the Wangsukcheon fault are also present in the data and it is interpreted to approximately dip 60° to the east, in an opposite direction to the two other faults. Key Points: Reflection seismic imaging of fault systems in Seoul metropolitan areaSuggested dependency of the regional seismicity from Chugaryeong fault systemDeep geometry reconstruction of three regional fault systems [ABSTRACT FROM AUTHOR]

We investigate the upper‐crustal seismic velocity structure in the Seoul metropolitan area, where about 20 million people live. The Chugaryeong fault zone (CFZ) is placed in this area, but the seismic hazard potential remains unclear. We conducted ambient noise tomography to illuminate the high‐resolution upper‐crustal structure in the Seoul metropolitan area. We analyzed continuous vertical seismic records for ∼5 months from a dense seismic array with 77 broadband stations. Group velocity dispersion curves and tomographic maps were extracted between 0.5 and 10 s periods. We inverted 3‐D group velocity tomography models up to a depth of ∼10 km from the group velocity maps. The shear‐wave velocity model is consistent with the geological features. High‐velocity anomalies at shallow depths are correlated with the surface topography and geology. The CFZ is located at a low velocity below the 5 km depth and presented as the simplified model. The large VS contrast regions are located beneath NS‐trending faults. The cross‐sections coincide with the near‐vertical strike‐slip faults in this area. In the southern region of the Seoul metropolitan area, low‐velocity anomalies correlate with high heat flow regions. Our results effectively suggest high resolution upper‐crustal structures and subsurface hidden faults in the urban area. Plain Language Summary: The Chugaryeong fault zone (CFZ) runs across the Seoul metropolitan area, where more than 20 million people live. The Chugaryeong fault system was repeatedly reactivated in the Quaternary. This study investigates the upper crustal structures in the Seoul metropolitan area using ambient seismic tomography based on densely deployed seismic stations. The seismic velocity structures are correlated with surface topography and geology. Low velocities are present in the southern Seoul metropolitan area of high heat flows. Vertically consistent low shear‐wave velocity anomalies are observed at depths of 5–10 km in the CFZ. Lateral variations in seismic velocities reveal the fault structures, which is consistent with seismic activities. The lateral gradients of shear‐wave velocities illuminate the near‐vertical fault geometry. The upper‐crustal shear‐wave velocity model may provide information to assess seismic hazard potentials in the Seoul metropolitan area. Key Points: High‐resolution upper‐crustal structures in urban region are derived from ambient noise tomography based on dense seismic stationsAmbient noise tomography reveals near‐vertical strike‐slip fault zones with low velocity anomalies in the Seoul metropolitan areaThe fault zones run across the Seoul metropolitan area, producing earthquakes [ABSTRACT FROM AUTHOR]

Quaternary faults run across the Seoul metropolitan area that is the highest population region in the Korean Peninsula. Active fault identification and seismic hazard potential assessment are crucial for public safety. Densely deployed permanent and temporal seismic stations enabled us to detect micro to small earthquakes, allowing us to identify earthquake‐spawning faults in Seoul metropolitan area. The source parameters of 455 earthquakes in 2004–2020 are refined. The Gutenberg‐Richter b value is 0.94. Dominant focal depths are 4–15 km. The focal mechanism solutions of 64 earthquakes are determined using seismic‐wave polarities and amplitude ratios. Strike‐slip earthquakes are dominant in the region. Earthquakes are clustered around the Chugaryeong fault system. The dominant strikes of fault planes range from N20°E to N45°E in the northern and southern Seoul metropolitan areas, suggesting branch fault development locally. The earthquakes in middle‐northern Seoul present N‐S directional strike‐slip motions at depths ∼7.5 km along the Chugaryeong fault, suggesting seismically active near‐vertical faults subparallel with Chugaryeong fault. Plain Language Summary: The Seoul metropolitan area is the largest population region in the Korean Peninsula. More than 20 million people live in the area where Quaternary faults run across. Historical literatures presented high seismic damages in the region. Active fault identification and seismic hazard potential assessment are crucial for public safety. Dense seismic stations were deployed to investigate the seismicity and earthquake‐spawning faults in the Seoul metropolitan area. We refined the source locations of 455 earthquakes in 2004–2020. The focal mechanisms solutions of 64 earthquakes were determined using the seismic‐wave polarities and amplitude ratios. Strike‐slip earthquakes are dominant in the region. The strikes of fault planes range from N20°E to N45°E in the northern and southern Seoul metropolitan areas. Branch faults appear to develop locally according to the ambient stress field. Microseismicity are clustered locally. The earthquakes in middle‐northern Seoul present N‐S directional strike‐slip motions at depths ∼7.5 km along the Chugaryeong fault. The Chugaryeong fault system appears to be seismically active. Key Points: Earthquake‐spawning faults run across the Seoul metropolitan areaChugaryeong fault behaves as a single continuous fault, producing earthquakesBranch faults develop locally by the ambient stress field [ABSTRACT FROM AUTHOR]