ALBERT

Description: The 9 February 2010 M w  3.1 Siheung earthquake occurred close to the metropolitan Seoul City in the central Korean peninsula and was widely felt by residents in Seoul. The shock occurred at a depth of 12 km in Precambrian crystalline bedrock with predominantly strike-slip faulting on a near vertical fault plane striking east-southeast–west-northwest. A rupture radius of ~0.12±0.01 km is estimated for the M w  3.1 shock, with a stress drop of 15.2 MPa. Sixteen small repeating earthquakes, spread in time up to about five and a half years, are detected based on their waveform similarities in a stable continental region setting. These shocks cluster along a trend striking ~110°, which suggests a nodal plane of the mainshock focal mechanism striking N109° is the likely fault plane. The repeating events around Siheung, Korea, show a recurrence interval of 0.77±0.37 year/event for shocks of magnitude range M w  0.6–3.1. The waveform cross-correlation detector provided identification of small repeating earthquakes with similar waveforms on an expanded time window, which may suggest the existence of such repeating earthquakes in stable regions worldwide with relatively low seismicity. Such data may lead to improvements in delineating seismogenic features, understanding the earthquake source properties, and evaluating the earthquake hazards associated with infrequent occurrence of such earthquakes in stable regions. Online Material: Figures showing locations from HYPOINVERSE single event location and double-difference methods, and source time functions determined using empirical Green’s function analysis.

Description: Accurate earthquake locations and their focal mechanisms can illuminate the distribution and mode of deformation at rifted continental margins. The Pacific–North America (Pa–NA) plate boundary within the Gulf of California (GoC) provides an excellent opportunity to explore the evolution and kinematics of rifting, as continental extension in the north transitions to seafloor spreading in the south. From October 2005 to October 2006, an array of eight four-component ocean-bottom seismographs deployed in the GoC recorded seismicity as part of the Sea of Cortez Ocean-Bottom Array (SCOOBA) experiment. By combining the data with those from the onshore Network of Autonomously Recording Seismographs (NARS)-Baja array, we detect and locate ~700 earthquakes ( M w  2.2–6.6) mainly along the northwest–southeast-striking transform faults that delineate the plate boundary. For 36 events ( M w  3.5–6.6) with high signal-to-noise ratio in a long-period (10–20 s) band, we determine deviatoric moment tensors and associated double-couple focal mechanisms by regional waveform inversion. Many focal mechanisms are consistent with right-lateral strike-slip faulting along the Pa–NA transform fault system, which suggests that the transform faults primarily accommodate seismic deformation within the gulf. In addition, we capture a swarm of events on Baja California along the right-lateral northwest–southeast-striking Las Viboras–El Azufre fault, which may be related to ongoing geothermal activity and volcanic deformation within the peninsula. The combination of high-resolution earthquake locations and focal mechanisms improves our understanding of the distribution of seismic deformation within the greater extensional zone in the southern GoC. Online Material: Earthquake catalog of the 695 events detected and located during our combined on-/offshore deployment of seismometers.

Description: 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 ~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. Online Material: Interactive visualizations of hypocentral distributions.

Description: We assess seismological evidence bearing on claims that North Korea conducted a small nuclear test on 12 May 2010 in the vicinity of known underground nuclear tests (UNTs) in 2006, 2009, 2013, and 2016. First, we use Lg -wave cross correlation and more traditional methods to locate the 2010 event between about 4 and 10 km southwest of the 2009 test. Second, we compare the relative sizes of regional P and S waves, using stations within 400 km of the known North Korean nuclear tests, to assess the nature of the event. We measured P / S ratios at different frequencies, at first using data from the open station MDJ in northeast China, for training sets of earthquakes and of explosions. We developed a linear discriminant function (LDF) that, in application to P / S measured at MDJ, is most effective in separating the earthquake and explosion populations. MDJ lacks usable data for the event of interest, but we obtained regional data from stations of the nearby Dongbei Broadband Seismographic Network (DBSN) for the 12 May 2010 event and for nearby UNTs conducted in 2006 and 2009. When our LDF is applied to DBSN data, and to data from stations SMT and NE3C in China, the LDF values measured from P / S ratios from known explosions are explosion-like; but for the 12 May 2010 event, the LDF values are earthquake-like for frequencies between 6 and 12 Hz. Our method for characterizing earthquakes and explosions on the basis of their regional signals can be widely applied. Measurements of P / S based on the three-component waveform data provide better discrimination power than do those based on vertical-component data alone. Electronic Supplement: Tutorial material on the Mahalanobis distance-squared measure, three-component linear discriminant function (LDF) analysis, tables of measurements of the log 10 P / S spectral ratios obtained from waveforms recorded at station MDJ for the two training sets and three-component discrimination analysis, and figures of log ( P / S ) values measured at 8 Hz from vertical-component waveforms at station MDJ for two training sets and probability distributions for D .