On 8 August 2020, northwest North Carolina experienced an Mw 5.1 earthquake that caused damage to buildings and roads in the city of Sparta. A regional centroid moment tensor solution shows that the earthquake was the result of slip on a reverse fault with a minor strike-slip component. Interferometric Synthetic Aperture Radar (InSAR) data, from the Japan Aerospace Exploration Agency’s Advanced Land Observing Satellite #2 (ALOS2) satellite, reveal a deformation field that is more complex than expected from a single reverse fault earthquake. The data also reveal an apparent fault rupture at the Earth’s surface that caused damage to local roads. Modeling of the InSAR deformation field indicates the fault rupture is associated with a very shallow normal faulting event with an equivalent Mw of about 5.1 that overprinted the reverse fault deformation field and possibly occurred aseismically.

We apply the single-station microtremor horizontal-to-vertical spectral ratio (HVSR) method to image the geometry of Pohang Basin, South Korea, which experienced the greatest earthquake damage in Korea during a series of anthropogenic earthquakes between November 2017 and February 2018. We collected and analysed the ambient seismic noise at 124 temporary stations. The resonance frequencies, which vary significantly across the area (0.35–19.86 Hz), were inverted to constrain the depth of the major impedance contrast, which is interpreted to be the sediment–bedrock interface beneath each station. The sedimentary layer thickness is generally thin in the north and thickens to the southern and central parts of the basin, where the depth to bedrock is up to 340 m. We compare the HVSR results with six borehole observations in the area, whose depth to bedrock ranges from 189 to 359 m. The sediment thicknesses obtained via the direct borehole measurements and HVSR method are comparable with each other. The resultant three-dimensional shape of the sedimentary basin provides crucial information for the microzonation of the Pohang area for seismic risk mitigation. It also provides a realistic initial velocity model for three-dimensional tomographic inversions to elucidate the detailed subsurface structure of the region.

The 2017 Pohang earthquake (ML 5.4) is the second largest earthquake occurred in an intraplate in modern Korea and is considered the largest induced earthquake from an EGS system around the world. The mainshock was proceeded by a few foreshocks and followed by a few thousands of aftershocks. Numerous densely distributed seismic stations in local and regional distances were deployed to monitor this earthquake sequence. Original hypocenters in the Pohang region were located using HYPODD that is independent on crustal structures. A comprehensive crustal Vp and Vs model was recently available from an invited committee of foreign experts based on well logs and regional seismic data. This model is then revised, especially the uppermost few hundred meters, based on results from a study of S to P converted waves from shallow interfaces beneath various stations, from the traditional Wadati plots analysis, and from the interpretation of two short seismic reflection/refraction profiles. From continuous data, 5 to 10 folds of additional earthquakes than the original manually picked events can be identified and located. P and S arrival times from all earthquakes are re-picked from continuous data and are relocated using the revised model and Hypoellipse program. Temporal and spatial distribution of relocated seismicity at depths range from 3 to 7 km are more clustered and confined than that from the original catalog. A few thin vertical cross-sectional views of hypocenters parallel and perpendicular to the seismicity reveal that seismicity propagates along multiple NE-SW trending faults beneath the Pohang basin and extending NE offshore into East Sea. These fault system is sandwiched between the Yongshan fault and a few other secondary faults to the south. The main shock (5.4) and the two largest aftershocks (4.3 and 4.6) as well as their associated aftershocks show predominantly NE-SW strike-slip with reverse faulting propagating along three different adjacent faults. Geometry of active faults and their tectonic implications will be presented and discussed in the meeting.

Yellow Sea and East Sea regions near Korea are two of the most seismically active marginal seas in the Far East. While offshore earthquakes in the Yellow Sea may be attributed to potential micro-plate boundaries, East Sea earthquakes may be associated to the seaward extension of many active faults on land or the deformation boundary between oceanic and continental crust. However, offshore earthquake locations using local seismic network are always subjecting to large uncertainties due to poor spatial coverage of seismic stations, discrepancies on velocity models, and limitations on traditional location technologies. For instance, it is not uncommon that the same earthquake within Yellow Sea may be reported independently more than tens to hundreds of km apart in Chinese and Korean catalogs while there is no mechanism for earthquake data exchange between the two countries. Multiple seismic array method can be applied to improve epicenter location of offshore earthquakes. Seismic stations in Korea can be integrated into three arrays based on their latitude. Apparent azimuths and apparent velocities of the incoming seismic waves (mainly Pn) from a regional earthquake to each array can be reliably determined. Epicenter of a regional earthquake can thus be located by tracing seismic rays following the back azimuths derived from multiple arrays. Offshore earthquakes in the East Sea and Yellow Sea regions are located at shallow depth within crust that Pn waves are expected to be the first arrival phase at many Korean stations. Thus, offshore earthquakes can be reasonably located using Pn arrivals. In the Yellow Sea case, the apparent velocity ~8.0 km/sec is observed for all arrays suggesting a typical continental Pn waves propagating across the continent-continent transition region into Korea. In the East Sea case, the apparent velocity of ~6.8 km/sec or lower is observed for all arrays suggesting a typical oceanic Pn wave propagating across the oceanic-continental margin into Korea. A better relocated earthquake location in the offshore region is essential for our understanding of regional tectonics and earthquake hazard assessment.

The strong-motion downhole array (SMDA) in Taipei basin is examined, and the data quality, glitches, and systemic errors in its data are discussed. This seismic network is an array of arrays: the SMDA comprises a total of 32 triggered strong-motion acceleration seismometers spanning eight sites. Each site has one seismometer at the surface and an additional two to four seismometers each collocated at the individual boreholes. Polarity reversals, swapped components, clock desynchronization, and bad components (flat-line signal or aseismic noise) have all been observed and are shown to be occasional issues. Signal-to-noise ratios are generally excellent. The lack of known orientations at depth is the primary issue regarding data quality of the SMDA. Orientations of each borehole seismometer were recorded at the time of installation, but those initial values are not reliable for subsequent events. Furthermore, redetermined azimuthal orientations are inconsistent from event to event, suggesting that the borehole seismometer orientations change over time. Orientation wander is not associated with instrument maintenance. An iterative method to reliably determine borehole seismometer orientations is introduced. Data from each borehole seismometer are rotated and compared with that from a collocated reference station on the surface with known orientation until a maximum correlation is reached. This method is reliable for most events, but may become unreliable for local events in which the incidence of incoming seismic waves is near vertical and shorter wavelengths introduce complicated wave propagation within Taipei basin. Temporal analysis of borehole seismometer orientations shows not only drifting of azimuthal orientations over time, but also erratic changes in orientation in multiples of 90 degrees. This behavior is suggestive of frequent polarity reversals and/or swapped components on one or both of the two horizontal components of each borehole seismometer.

A 1D normal moveout (NMO)-corrected and stacked pseudoprofiling method was applied to analyze the characteristic features shown on primary P- and S-wave coda and on Sp waveforms from local microearthquakes in an attempt to image prominent reflectors and to resolve shallow crustal velocity structure (similar to 5 km) in the upper Mississippi embayment. Acoustic well log data were used to constrain the P-wave velocity in the upper 5 km. Events at close distances and with clear P and S arrivals were selected to ensure reliable NMO correction for reflections and transmissions. The observed reflections and transmissions are important controlling factors on modeling waveforms. We analyzed local earthquake data recorded at all broadband and one short-period station of the Cooperative New Madrid Seismic Network. Despite polarity differences among P, S, and Sp waveforms, consistent reflectors in the sedimentary section can be imaged across the three wave types. Correlation with a basement-penetrating well indicates that reflectors at the base of the Upper Cretaceous-Holocene Mississippi Embayment Supergroup, the base of the Cambrian-Ordovician Knox Group, and the high-velocity lower Upper Cambrian Bonneterre Formation are shown in pseudoprofiles among stations in the upper Mississippi embayment. Our study finds that a one-layer homogeneous velocity model of sediments in the ranges of 1.95-2.42 km/s for V-P and 0.60-0.73 km/s for V-S overlying a half-space of Paleozoic rocks with velocities in the ranges of 6.0-6.2 km/s for V-P and 3.26-3.6 km/s for V-S can represent shallow crustal structure in the upper Mississippi embayment. Differential times of P-PpPhp and S-SsShs appear linearly proportional to sediment thicknesses, which best fits a one-layer sediment structure with average V-P 2: 042 +/- 0: 041 km/s and V-S 0: 709 +/- 0: 051 km/s, in the least-squares sense predicted by the wave propagation effects.

Determination of reliable hypocenters of earthquakes is crucial to earthquake seismology and to evaluate hazards associated with earthquakes. There are many associated computer codes for this purpose; however, most of the location algorithms are designed to determine hypocentral parameters based on previously determined velocity models. In contrast, we employed a location method that is independent of the initial velocity model, using a genetic algorithm (GA) to determine an optimal 1D velocity model and the locations of earthquakes. Using this GA, we relocated earthquakes that occurred in the New Madrid Seismic Zone (NMSZ) in the central United States between October 1989 and August 1992. The goal of this work was to delineate the possible fault planes by reliable relocation of those earthquakes and to determine a 1D velocity structure for the NMSZ. A total of 502 earthquakes recorded by 37 Portable Array for Numerical Data Acquisition (PANDA) stations were used in the relocation study. In the relocation process, the root mean square travel-time residuals were reduced by similar to 35%, corresponding to an average of 2.3 km deeper in depth, 0.7 km shift in latitude, and 0.8 km shift in longitude compared with those in the initial catalog locations. The hypocenters of the earthquakes can be subdivided into four groups based on their spatial distributions. The group that corresponds to the Cottonwood Grove fault (CGF) in the southwestern NMSZ represents a very steep plane, whereas the other three groups fall into Reelfoot fault (RF). We inverted P- and S-wave travel times from the new hypocentral parameters to determine 1D velocity models. The resulting eight-layered velocity models consist of a 2 km thick surface layer followed by seven 2 km thick layers, with V-P ranges from 5.36 to 6.74 km/s and V-S ranges from 2.83 to 3.90 km/s for both CGF and RF regions.

Seismic hazard in the active collision zone of southeastern Taiwan has been poorly known. Although the area has experienced only a few magnitude 6 earthquakes during the modern seismic observation period, there are good geological and instrumental evidences that the area has repeatedly experienced magnitude 7 earthquakes, including a sequence in 1951. We estimated 3-D P-and S-wave velocity models by applying an earthquake tomography method to a large number of high-quality arrival-time data collected by a regional seismic network and a local seismic array. Earthquakes in the region that occurred between 1991 and 2011 were then relocated using the 3-D velocity models. The 3-D velocity models and seismicity depict the current deformation structures in the region. Numerous earthquakes have occurred along a narrow east-dipping seismic zone beneath the southern Coastal Range (CR) near Chengkung. The seismogenic structure extends from the surface eastward to depths greater than 20 km. It is characterized by low-V-p, low-V-s and low-V-p/V-s. Soft materials within the seismogenic structure may include fluids as well as cracks with high aspect ratios resulting from extended periods of tectonic loads. Soft sediments with high fluid contents between rigid materials are regarded as significant weak zones where stress concentration and nucleation of numerous earthquakes can easily occur. Beneath the western boundary of the Longitudinal Valley (LV), a vertical or steeply west-dipping strike-slip fault is identified. Apparently, significant slip partitioning is taking place to accommodate the oblique tectonic motions. In the eastern offshore, seismic velocities indicate that the materials are derived from deeper depth and that significant amounts of water have been introduced to form serpentines at shallow depth. The central LV and CR have been largely aseismic since a series of magnitude 7 earthquakes in 1951. A large amount of tectonic stress is stored in the region. Thus, the area has high potential to release seismic energy through future large earthquakes.

High-quality first-arrival data collected with a high-density temporary seismic array and regional seismic network were used to construct a P-wave velocity model and identify the precise location of earthquakes in the active collision zone of southeastern Taiwan. A crustal-scale weak zone, defined by high seismicity, is characterized by a steeply east-dipping lower-velocity anomaly and represents a boundary between an uplifted upper-mantle and the Luzon volcanic arc. The main features of the weak zone, with its associated low-velocity zone and seismicity pattern, vary significantly along the collisional boundary of southeastern Taiwan. Along the weak zone, unlithified sediments or highly fractured materials are compacted as the Philippine Sea plate moves toward the Eurasian plate. In areas of more advanced collision, soft material is more compacted and can therefore store significantly greater amounts of strain energy. This energy is released through episodic earthquakes.

High-resolution 3-D VP and VS velocity models around the source region of the Mw7.6 Chi-Chi, Taiwan, earthquake show significant lateral and vertical variations. The mainshock occurred within a narrow low velocity zone along the Chelungpu fault. A sudden increase of velocity and seismicity took place across the Shuilikeng fault to the east. Most aftershocks were located in areas of high VP and VS. A sharp east-dipping zone extending from the Shuilikeng fault on the surface to a depth of about 15 km separates the low velocity region in the west from the high velocity region in the east. An apparent high-angle west-dipping seismic zone lies at depths of 15–30 km beneath the western Central Range. Numerous aftershocks with normal faulting were located west of the middle segment of the Chelungpu fault.

Normal Pn waves are commonly observed in Taiwan from shallow regional earthquakes at epicentral distances larger than 120 km, similar to the observations in many other continental regions. However, the critical distances to observe Pn waves for shallow eastern Taiwan earthquakes vary with azimuth corresponding to a significant variation of crustal thickness. In particular, anomalous Pn waves are commonly observed for shallow eastern Taiwan earthquakes recorded on seismic stations at epicentral distances as small as 60 km along the collision zone suture, the Longitudinal Valley. For the same event, normal Pn waves are observed at other seismic stations elsewhere on the island. The apparent velocity of the anomalous and normal Pn waves from the same event is 7.8 +/- 0.15 km/sec, which is consistent with the average Pn velocity in the Taiwan area. Thus, the unusually short critical distance for Pn waves in eastern Taiwan suggests that the crust beneath the collision zone suture must be very thin and the upper mantle beneath the Longitudinal Valley must be relatively elevated compared with that beneath the other parts of Taiwan. Assuming a simple 1 D layered velocity model, the Moho depth beneath the suture zone can thus be estimated at similar to 23 +/- 2 km. This observation is consistent with the recent report from a high-resolution 3D tomographic inversion that a narrowly confined, anomalously elevated, and north-northeast-south-southwest elongated oceanic upper mantle was imaged beneath the Longitudinal Valley from Hualien in the north to Taitung in the south (Kim et al., 2005, 2006). Furthermore, the preceding observations may also support the interpretation that the conduction of excess heat supply from the elevated hot oceanic upper mantle into the adjacent mid-to-lower continental crust over a long period of geological time may play an important role in the crustal deformation beneath the continent, including metamorphism, thickening, and uplifting.

Traditional local-earthquake location using a horizontally layered homogeneous velocity model is limited in its resolution and reliability due to the existence of frequently overlooked 3D complexity of the real Earth. During traditional 3D seismic tomography, simultaneous earthquake relocation using the resultant 3D velocity model has produced reliable earthquake locations; however, only a small subset of events are typically used and thus relocated in the inversion. The rest of the events in a catalog must then be relocated using the 3D models. The repeated calculation of travel times across 3D V P and V S models is also not efficient and not practical for a routine network earthquake location when the very time-consuming exact 3D raytracing is used. Because high-resolution earthquake data are now available from many modern seismic networks, representative high-resolution 3D V P and V S models for a region can be better determined. By taking advantage of recently available high-speed computer technology and large disk space, we implemented a simple algorithm to efficiently locate every local earthquake using the best available regional 3D V P and V S models. Once the V P and V S information for all cubic cells in a 3D grid model are determined, P and S travel times from each grid point to all seismic stations can be calculated and stored on disk files for later usage. During the iteration process for earthquake location, travel times from a trial hypocenter to all recording stations can be determined simply by a linear interpolation from those of the adjacent eight grid points available in the previously stored disk files without the need for raytracing. The iterations continue until the hypocenter adjustments at the end of the last iteration are below the given criteria and the travel-time residual, or the difference between the observed and the calculated travel times, is a minimum. Therefore, any local earthquake can be efficiently and reliably located using the available 3D velocity models. This simple location program has been applied to relocate earthquakes in the New Madrid Seismic Zone (nmsz) of the central United States and in the central eastern Taiwan region. Preliminary results in both regions reveal that earthquake hypocenters can be efficiently relocated in spite of the very significant lateral structural variations. Tests with data from Taiwan further demonstrate that the resolution of seismic tomography and the relocated seismicity is sensitive to relative distribution of seismic-network stations and background seismicity. Thus, this single-event location program can be applied to relocate all earthquakes in a seismic-network catalog and, more importantly, to allow routine earthquake location for any seismic network using the available 3D velocity models.