Journal of Seismology最新文献

Data assessment tools designed to improve data quality and real-time delivery of seismic and infrasound data produced by six seismoacoustic research arrays in South Korea are documented and illustrated. Three distinct types of tools are used including the following: (1) data quality monitoring; (2) real-time station state of health (SOH) monitoring; and (3) data telemetry and archive monitoring. The data quality tools quantify data gaps, seismometer orientation, infrasound polarity, digitizer timing errors, absolute noise levels, and coherence between co-located sensors and instrument-generated signals. Some of the tools take advantage of co-located or closely spaced instruments in the arrays. Digitizer timing errors are identified by continuous estimates of the relative orientation of closely spaced horizontal seismic components based on the root-mean-square error between a reference seismometer and each seismometer in the array. Noise level estimates for seismic and infrasound data are used to assess local environmental effects, seasonal noise variations, and instrumentation changes for maintenance purposes. The SOH monitoring system includes the status of individual ancillary equipment (battery, solar power, or components associated with communication) and provides the operator the capability to compare the current status to the historical data and possibly make remote changes to the system. Finally, monitoring data telemetry and overall data archival provide an assessment of network performance. This collection of tools enables array operators to assess operational issues in near real-time associated with individual instruments or components of the system in order to improve data quality of each seismoacoustic array.

We report solid-earth tidal modulation of early aftershocks of the July 2019 Ridgecrest earthquake sequence, which occurred close to the southeastern edge of the Coso geothermal field. We found that the frequency of early aftershocks in the northern part, close to the Coso geothermal field, was modulated by the solid earth tides as they exhibit a strong correlation with the peak shear stress and Coulomb stress imparted by the solid earth tides. However, aftershocks that occurred farther south of the Coso geothermal field in the same sequence exhibit a weak correlation with the solid earth tidal stress. Our analysis implies that the tidal modulation of the earthquake sequence in the northern part is due to its vicinity to the Coso geothermal fields in southern California, which contain high-pressure fluids and are well known for their susceptibility towards tidal triggering.

In this study, non-principal waves propagating in an isotropic elastic half-space covered by an orthotropic layer are examined. The main objective is to establish a formula for the SH transfer function induced by an vertically incident SH wave and a formula for the H/V ratio of surface waves. The peak frequencies of both the SH transfer function and the H/V ratio curve are examined for models with low to high impedance contrasts to verify the applicability of the quarter wave-length rule for both SH body waves and surface waves. It is numerically shown that the quarter wave-length rule applies well for non-principal SH body wave. Non principal surface waves are shown to be a composition of Love and Rayleigh waves, and their peaks follow the quarter wave-length rule only in the case of high impedance contrast. For medium or low impedance contrasts, the peak frequencies of surface waves could differ from the peak frequencies of SH body wave with relative differences up to (50%).

Underground human activities, such as mining, shale gas, and oil exploitation, waste-water disposal, or geothermal plants, can cause earthquakes; therefore, they are monitored by local seismic networks. An ideal seismic network has a triangulated grid, with spacing equal twice the minimal depth and no associated industry noise. In real cases, the network sensitivity is biased by stations placed near noisy roads, factories, or in a private garden, none located at optimal nodes. The sensitivity is also a function of the detection algorithm type and setting. The goal of this case study is to suggest a work-flow for network sensitivity calculation in case of no seismic activity. In other words: how small are the earthquakes that such seismic networks would detect if they were present? Such network sensitivity is a function of stations noise level, station-source geometry, and setting of the detection algorithm. A brief theory and work-flow description is followed by two real-case demonstrations from Czech Republic, Europe: first, a proof-test on a well-studied seismically active area of West Bohemia/Vogtland and second, an application to an uprising geothermal project in Litoměřice, where no seismic activity was detected in years of monitoring.

Force history inverted from long-period seismic records for a landslide has been widely used to extract its physical parameters. Most previous studies have adopted the point-source constant-mass landslide model during the inversion. In this study, we quantitatively investigate the effects of mass entrainment at different locations along the sliding path during the interpretation of inversion results. To demonstrate our analysis, we carried out the long-period seismic waveform inversion for the 2003 Qianjiangping landslide to obtain its force history and subsequently estimated its movement parameters. We propose a mass entrainment model based on the conservation of kinetic energy. The predictions of our mass entrainment model are verified using the inversion results of the Qianjiangping landslide and other events. Results from our mass entrainment model suggest that when half of the sliding mass is entrained at different locations along the sliding path, the estimated masses and maximum velocities vary between 50–100% and 83.38–122.26%, respectively. In particular, when the entrainment occurred at the slow-down phase, the estimated mass is less than 60% of the total mass, which is a good approximation of the initial sliding mass. The model can provide useful constraints for quantitative interpretations of landslide force history inversions of landslides.

In this work, we perform an evaluation of the coverage of the earthquake monitoring network of Taiwan. The capability of a general network is a function of an adequate number of optimally distributed nodes. For this case study, the evaluation is performed with a statistical approach which includes descriptive spatial statistics in combination with point pattern techniques. The spatial distribution of the nodes of the earthquake monitoring network is analyzed in comparison with the distribution of seismicity, completeness magnitude, active seismogenic sources, seismic hazard, and population distribution. All these data can be put in relationship with the objectives of an earthquake monitoring network; therefore, they can be used, in turn, to retrieve information about the consistency of the network itself. In particular, we investigate the “Real-time Seismic Monitoring Network” and the “Strong-Motion Earthquake Observation Network,” each one characterized by its own objectives, and therefore respectively compared with external information related to their purposes such as seismicity, seismogenic sources, seismic hazard, and population distribution. This simple and reliable approach reveals the high quality of the networks established in Taiwan. In general, it is able to provide quantitative information on the coverage of any type of network, identifying possible critical areas and addressing their future development.

Estimating the macroseismic parameters of earthquakes such as location, magnitude, and fault orientation can be helpful to complete the earthquake catalogs, especially on historical ones or the ones whose parameters were estimated with low accuracy. This study aims to estimate the macroseismic parameters of Tabas earthquake (1978.09.16) using its reported descriptions, Macroseismic Data Points (MDPs). MDPs include any descriptions of earthquake effects on building damages and environmental effects of each location affected by the earthquake. Macroseismic dataset of Tabas earthquake is collected from different documents. Thus, this dataset is used to estimate the macroseismic parameters of this earthquake: macroseismic location, magnitude, intensity, and source parameters. Results estimate the macroseismic epicenter of this earthquake at 33.49°N–56.94°E with the moment magnitude, Mw, 7.1 and the maximum macroseismic intensity value XI in both EMS and ESI intensity scales. Its estimated source orientation is consistent to Tabas fault system in the direction NNW-SSE with the strike, length, and width value of the source in 166.8°, 55.8 ± 7.5 km, and 18.2 ± 1.4 km, respectively. Moreover, for future studies, the locations near Khosro Abad, close to the macroseismic epicenter of this earthquake, are suggested for the next field studies, sampling the sediments related to the past evidences and dating them.

As a widespread geological hazard, the disaster development process of earth fissures is irreversible and difficult to control, which seriously affects the construction and safe operation of engineering facilities. However, few clear conclusions and special regulations have been given regarding the influence of earth fissures on the dynamic response characteristics of a site and earthquake prevention and disaster reduction measures. Therefore, the microtremor was used instead of earthquake motions to reveal the dynamic response of a site with fissures. The earth fissures in the Taiyuan Basin, which exhibit a large amount of activity, were used as representative examples. In order to reveal the dynamic response from several aspects, four methods, including the Fourier spectrum, the horizontal-to-vertical spectral ratio (HVSR), the response acceleration, and the Arias intensity, were employed. The results show that the spectrum peaks increase sharply at an earth fissure and return to a stable value approximately 20–25 m away from the fissure, indicating that the earth fissures have an amplification effect on the dynamic response of the site. Additionally, a greater amplification occurs on the hanging wall of the earth fissure. The influence range of the dynamic response of site can be divided into four areas. Suggestions on the seismic fortification intensity and setback distances were also proposed. After ground motion finite element simulation, the amplification effect of seismic response at the earth fissure site has been further confirmed. The amplification mechanism was summarized as the coupling of the changes in the soil properties caused by earth fissure activity, the catadioptric effect of the earth fissure interface, and the multiple amplifications caused by secondary fissures.

To preserve the analog records of historical earthquakes which occurred in and around Iran in digital form and to make them available for modern analysis techniques that require waveforms, we decided to digitize seismograms of destructive earthquakes that occurred between 1960 and 1990. The seismograms have been recorded at five seismic stations deployed in Tehran, Mashhad, Kermanshah, Tabriz, and Shiraz by the Institute of Geophysics of the University of Tehran (IGUT). The restoration project of the old analog seismograms archived in the IGUT started in 2018. Seismograms were sorted by station and date to create a hardcopy databank, and then those from large earthquakes and their larger aftershocks were chosen for the scanning process. Seismograms of 52 significant earthquakes and their aftershocks with magnitudes greater than 5.0 that occurred in and around Iran were scanned. Besides that, seismograms of several significant and huge teleseismic earthquakes that occurred between 1960 and 1990 were scanned. At present, almost 600 scanned seismograms are available. The current digitization process in IGUT is done manually and is based on waveform vectorization. The provided databank is one of the most important sources of information for research in seismology and active tectonics in and around Iran.

One of the objectives of seismic monitoring in mines is to detect strong and unexpected changes in the spatial and/or temporal behaviour of seismic parameters that could lead to rock mass instability and affect working places immediatelyx or in the short term. We present an influence based polygon-less method of detecting such conditions, taking into account the influence of ground motion generated by all available seismic events, regardless of their location, on a particular working place. The measures of influence are the peak ground velocity, PGV, and the cumulative absolute displacement, CAD, since their influences are moderated by the distance from the seismic source to the place of potential exposure. Both CAD and PGV are calculated for each point of interest from the ground motion prediction equation, GMPE, developed for a given mine. The CAD becomes CAID, i.e. cumulative absolute inelastic deformation, if the co-seismic strain, PGV(/v_{S}), exceeds the elastic threshold, e.g. (10^{-6}) for hard rock. Alerts and Alarms are triggered if the rates of CAID and/or its activity, ACAID, exceed predefined reference levels. Since they have different units, we normalise each by its reference level and at each time step select the one which is greater and call it the GMAP rating. All points that at a given time trigger an Alert define the exclusion zone and are monitored as time progresses.