ALBERT

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: 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 (~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. Online Material: Figures illustrating seismograms, process procedure, imaged reflectors, and resolved velocity structure for six broadband stations and one short-period station.

Description: 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°. 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.

Description: 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.

Description: Located between the very active Japan and Ryukyu subduction zones and the northern China plate, the Korea Peninsula has been considered a part of the stable Eurasia continent and is very quiet in seismic and tectonic activity. Although there were many significant damaging earthquakes reported in historical times, seismic hazard in Korea has long been overlooked. Modern earthquake activity in the Korean Peninsula is very low and is not well recorded, at least until 1998 when the modernization of the Korean National Seismic Network was implemented. Thus, modern earthquake data are not adequate for evaluating seismic hazard in the Korean Peninsula. On the other hand, the historical earthquake catalog, which includes documented earthquake information from around the Korean Peninsula and can be dated back to as early as A.D. 2, provides the only available long-term database for the investigation of temporal and spatial patterns of earthquake activity. The importance of seismic hazard assessment has significantly increased in modern times because of the recent construction of many critical facilities, such as nuclear power plants, super-computer centers, large hospitals, and high-technology centers, throughout the entire Korean Peninsula. Although uncertainties on the historical earthquake locations and their magnitudes are expected to be large, information obtained from this historical earthquake catalog can at least provide a long-term scientific basis for an estimation of seismic hazard in Korea. For the entire Korean Peninsula, seismic hazard is evaluated in terms of the spatial distribution of seismicity and relative seismic energy release over the 2000 years of the historical record. Results from our preliminary analysis clearly demonstrate that seismic activity in the Korean Peninsula can be categorized into four prominent seismic zones, inside which seismic hazard is much higher than that in the surrounding regions. These four seismic zones include: (1) the western Korean seismic zone extending from Seoul to Pyongyang, which is characterized by a few concentrated regions of high seismicity and a high relative seismic energy release; (2) the eastern Korean seismic zone, which is characterized by a low seismic rate but a high relative seismic energy release from a few large historical events; (3) the northeastern Korean seismic zone, which is probably related to the deep Japan subduction-zone earthquakes underneath northeast China and has a very low seismicity but a very high relative energy release; and (4) the southern Korean seismic zone, which is characterized by many scattered patches of high seismicity and a few zones of high seismicity and high relative seismic energy release from a few large historical events. Among the three most seismically active regions near Pyongyang, Seoul, and Pusan, the probability of occurrence for an earthquake of magnitude greater than 5.0 is estimated to be about 1%, 2%, and 3% per year, respectively. Since significant damaging earthquakes (M〉 or =7.0) have occurred in these three regions in historical times, an effective assessment of seismic hazard potential in the Pyongyang, Seoul, and Pusan regions cannot be overlooked.