Journal of Seismology最新文献

We determine average attenuation functions and estimates of the quality factor Q for both P- and S-waves in the northern Gulf of California, Mexico. We use seismograms from the Broadband Seismological Network of the Gulf of California (RESBAN) operated by the Centro de Investigación Científica y de Educación Superior de Ensenada, Baja California (CICESE). The database consisted of 64 earthquakes with Mw between 4.5 and 6.6 and hypocentral distances between 30 and 350 km. Attenuation functions were determined from a nonparametric model obtained by inverting observed spectral amplitudes of 25 frequencies between 0.1 and 25.12 Hz. To estimate ({Q}_{p}) and ({Q}_{s}), three geometric dispersion functions were defined: one frequency dependent and two frequency independent. We find that the estimates of (Q) depend strongly on the geometric dispersion function adopted. Estimates of (Q) obtained for the hypocentral distance from 30 to 350 km indicate that P-wave attenuation is larger than S-wave attenuation regardless of the geometrical spreading function used. When using the frequency-dependent geometric dispersion, we estimate that ({Q}_{P}=224.6{f}^{1.10}) and ({Q}_{S}=244.7{f}^{1.17}). In general, the high values of ({Q}_{p}) and ({Q}_{s}) suggest that the northern Gulf of California consists of a continental crust, possibly containing rocks with low fluid content and that the likely high pressure present in that region could generate a decrease in attenuation due to the closure of pores in the rock. In addition, the values of the ({Q}_{s}/{Q}_{p}) ratio suggest that the rocks in that region must have a low fluid content.

Abstract  

The Leipzig-Regensburg fault zone is documented as a band of seismic activity extending northwards from the earthquake swarm region NW-Bohemia/Vogtland at the Czech-German border area and is intersected by several Hercynian fault zones. Along the fault zone, there are several earthquake swarm areas, the northernmost of which are Schöneck and Werdau. In this study, we investigate the presumably fluid-induced earthquake swarm activity of the Schöneck and Werdau area. For this purpose, we apply two methods: local earthquake tomography and receiver functions to identify the structural composition of the crust, the areas affected by fluids and the origin of the fluids. We detected potential fluid paths characterised by high Vp/Vs ratios and granite intrusions nearby the swarms characterised by low Vp/Vs anomalies. Receiver function analysis yields the Moho at 25 to 33 km depth and two seismic discontinuities at 55 km and 68 km depth.

The unmatched seismic theoretical rotations from the experimental data led the researchers to develop the reduced micropolar theory in seismology. The study here mainly focuses on the finite-fault simulations for rotations and strains in the near-fault region for a causative strike-slip fault of the (M_w) 6.5 event. Firstly, the parametric investigation is performed on additional material parameters, viz. micropolar couple modulus and microinertia density to the rotations and strains. Secondly, the seismic source parameters such as rupture velocity, slip velocity, burial fault depth, earthquake magnitude, hypocenter location and slip amplitude are varied to see the effect of these parameters on rotations and strains seismograms. The results in different scenarios are compared to the classical elastic medium and reduced micropolar medium. The rotations obtained using reduced micropolar theory are comparatively high to the rotations of classical elastic theory. Although, the obtained displacements in both theories are almost the same. The normal strains in both theories are equivalent, while the shear strains differ as the shear strains in reduced micropolar theory are asymmetric and rotation dependent. The increment in the value of microinertia density increases the rotations, however, the converse is true in the case of micropolar couple modulus. The parametric analysis results demonstrate that near-fault ground rotations and strains are highly sensitive to changes in the seismic source parameters. For instance, modelling the medium homogeneous decreases the amplitude and duration of seismograms sharply compared to layered media. Finally, peak ground contours of displacements, rotations and strains are presented for different hypocenter locations using grid point simulation, and it is found that a change of hypocenter location alters the spatial distribution of peak values of these quantities in the near-fault region of the surface plane. Nevertheless, the maximum limit of peak values over the entire ground surface is near equal for the different hypocentral locations.

The identification of unnatural earthquake events is one of the tasks of earthquake rapid report. The identification accuracy is of great significance for improving the quality of earthquake catalog and seismological research. In this study, a 7-layer convolutional neural network model was constructed to identify unnatural earthquakes. First, the three-component seismic waveform was input to obtain the waveform image classifier, and then, the time–frequency spectrum of blasting and collapse was input to obtain the time–frequency spectrum classifier. The two classifiers were used to identify natural earthquake, blasting, and collapse. The model was trained and tested using 3386 seismic events of Shandong seismic network from 2017 to 2022. The events identified as blasting by the waveform image classifier were reidentified by the time–frequency spectrum classifier. Finally, the identification accuracy of natural earthquake, blasting, and collapse is 97.50%, 95.87%, and 86.84%, respectively, with an average accuracy rate of 96.13%. The experimental results show that the two-step convolutional neural network can extract the characteristics of seismic signals from multiple angles, which get a good result in seismic event classification.

Abstract

Recent earthquake damage distributions have demonstrated that the influence of local geology on ground shaking is a significant factor in engineering seismology. So, calculating the site effect is a priority to get a trustworthy assessment of the seismic risk for a location, in addition to studying the local seismic sources. The signal amplitude can be amplified by this effect throughout a range of periods. The site effect has been calculated using a variety of computational and experimental techniques, such as seismic noise measurements. In this study, to calculate the site effect, the analysis of accelerograms recorded by Iran’s strong motion network of the Road, Housing, and Urban Development Research Center was used. Here, 294 records from 63 stations were used to calculate the H/V (horizontal to vertical spectral ratio) curve as well as the near-surface high-frequency attenuation parameter (κ₀). The classification method is based on determining the peak period at each station. To examine site effect consideration, we use the hybrid method composed of the finite difference method for low frequencies (< 1 Hz) and a stochastic finite fault method for high-frequency radiation (> 1 Hz) to simulate an earthquake scenario on the Niavaran fault, which is located north of Tehran, Iran. According to the findings, different site classes cause spectral amplitude variations ranging from 11 to 28% at different periods (T = 0.2, 0.5, 1.0, and 4.0 s).

There is a pressing need for a homogonous tsunami catalogue for the Indian Ocean as nearly 20% of tsunami events worldwide affect the region. Any study on tsunami hazard assessment necessitates a homogenous tsunamigenic earthquake catalogue. The existing records of strong tsunamigenic earthquakes have the magnitudes expressed in moment magnitude (M_W), body wave magnitude (m_b), local magnitude (M_L), and surface wave magnitude (M_S). This study deals with developing regional magnitude correlation equations for tsunamigenic earthquakes of the Indian Ocean. The present investigation estimates the threshold magnitude and focal depth for an earthquake to turn tsunamigenic. It is found that earthquakes above M_W ≥ 5.9 and focal depth ≤ 80 km have the potential to generate a tsunami in the region. The moment magnitude is the most proper scale to characterize the size of large tsunamigenic earthquakes as it is more directly related to the released energy and does not suffer saturation. Hence, equations have been developed to convert surface wave magnitude (M_S) to moment magnitude (M_W) using three types of regression models viz. standard regression (SR), inverse standard regression (ISR), and orthogonal standard regression (OSR). The efficacy of these models has been compared in terms of R-squared and residual analysis. This study indicates that OSR is the best-suited regression model for developing magnitude correlation equations for the three zones of the Indian Ocean region under study. Also, a single unified conversion equation for the whole of the Indian Ocean has been derived with rational accuracy.

This article presents updated seismic hazard curves, spectra, and maps of ground motion intensity measures for the northern region of the Dominican Republic (DR) obtained using a probabilistic seismic hazard analysis (PSHA). The analysis performed uses as input data an earthquake recurrence model based on fault slip rates derived from GPS measurements published in the aftermath of the 2010 Haiti earthquake. The seismicity rate data are used to calibrate a composite characteristic earthquake model, which is combined with a Poisson process to provide a temporal characterization of earthquake occurrence. The seismic hazard curves and maps presented include parameters such as (horizontal) peak ground acceleration and pseudo-spectral response accelerations at 0.2s and 1.0s periods for 5% damping at firm rock sites. The results show that the ground motion parameters with a 2% probability of exceedance (PE) in 50 years determined in this study are up to 46% larger than the corresponding parameters specified in the current DR building code seismic hazard maps for the northern DR. Moreover, the design response spectra for a site in the city of Santiago specified in the code is significantly lower than the 2% PE in 50 years uniform hazard spectra determined in this study for vibration periods smaller than 0.5s, a range that includes the majority of the structures that define the built environment of the DR.

Standard seismological practices use a 1-D velocity model to calculate and determine earthquake hypocenters. For Northern Thailand, a minimum 1-D velocity model with station delays by applying the VELEST code is first presented here, which can be applied for earthquake location determinations as well as an initial model for 3-D seismic tomography studies. Altogether 614 P- and 689 S-wave travel time data from 145 events were manually picked from earthquake waveforms recorded by 13 seismic stations operated under the Thai Meteorological Department (TMD) from October 2009 through March 2021. A set of five velocity models with 5-km-layer thicknesses down to 40 km depth were tested with earthquake locations to obtain the best-fit velocity models. Results provided minimum travel-time differences between observed and calculated P and S first arrival times. After 13–20 iterations, a reduction of RMS (root-mean-square) values of the travel time residuals approaching a final minimum was observed. The vertical distribution of the hypocenters indicates that seismicity is concentrated in the upper 20 km depth range below northern Thailand. Only few events are found at deeper levels. The 1-D velocity model has slightly lower velocity values than the global velocity model (ak135 and iasp91). Station delays of P- and S-waves are in the range of −0.8 s and +0.7 s, indicating laterally varying geology correlating with near-surface geology. Positive delay times are related to softer sedimentary rocks and sediments, and negative delay times to igneous rock outcrops.