Journal of Seismology最新文献

The focal mechanisms of small-magnitude earthquakes can be challenging due to sparse networks, limited availability of velocity models, and a lack of suitable approaches. In this study, three techniques were used to determine the focal mechanism of the Burewala earthquake ((M_w) (sim ) 4) in Pakistan, which occurred on March 18, 2022. Full waveform inversion yielded different results for the local and regional velocity models. Although the regional velocity models showed a better waveform fit, the thrust mechanism solutions obtained did not match first-motion polarities. Therefore, cyclic scanning of polarity solutions was employed to combine polarities and waveforms, but the solutions were unreliable due to poor station coverage. Therefore, the inversion of waveform envelope technique was used, which is less sensitive to velocity models; hence, it can be used for sparse networks. The mechanism (i.e., normal faulting with a minor strike-slip component) was obtained for local and regional velocity models. The obtained solutions were consistent with the waveform inversion results based on the local velocity model. Thus, envelope inversion is deemed a more suitable technique for analyzing small-magnitude earthquakes.

The study of the strong ground motion is of utmost relevance because the amplitude of seismic waves and their frequency content could severely damage structures. As both the amplitude and the frequency content directly depend on the seismic source, a proper description and simulations of the earthquakes’ rupture process are required. This means that realistic source models should incorporate a heterogeneous distribution of rupture parameters that generates self-arrested ruptures. One of these models is a heterogeneous energy-based (HE-B) method, which can describe the kinematic rupture process based on the distribution of residual energy (({E}^{res})). This parameter defines zones in faults where the accumulated energy is larger than the dissipated energy. In this context, this study presents the spatial variations of radiated energy, corner frequency, and stress drop at far-field distances as a consequence of the heterogeneous distribution of positive residual energy. It is found that the rupture of asperities, determined by large values of ({E}^{res}), strongly shifts the frequency content and generates a Doppler effect of the frequency content. That is, the location in the far-field in direction where the asperity is being ruptured generates traveling waves characterized by an increase of the observed corner frequency, which corresponds to the corner frequency measured by the observer. This implies that different station measures different frequency content implying different estimations of the source parameters. Besides, the variability of the observed corner frequency could break the scaling between the corner frequency and the magnitude. Nevertheless, it is also found that the average observed corner frequency, when considering all the points or stations, is almost the same as that obtained for the seismic source. A similar property is found for radiated energy and stress drop. These results show that the ground motion at a given location varies depending on the heterogeneities of the section of the fault being ruptured.

The central part of the Pannonian Basin is characterised by low to medium seismicity. North central Hungary is one of the most dangerous areas of the country in terms of earthquakes, which also includes the area of the Mór Graben where some of the largest earthquakes occurred in Hungary’s history. Recent activity has been observed in the Mór Graben. It has been established that earthquake swarms occur quite frequently in the graben. To further study these events, we deployed a temporary seismic network that operated for 20 months. Using the temporary network stations as well as permanent stations from the Kövesligethy Radó Seismological Observatory and the GeoRisk Ltd. networks we registered 102 events of small magnitudes. In this paper, we demonstrate and compare three different event detection methods based on the registered waveforms by the permanent and temporary stations to find the optimal one to collect a complete swarm list in the Mór Graben. After the hierarchical cluster analysis, we relocated the hypocentres using a multiple-event algorithm. Our results demonstrate that the most successful detector in this case is the “Subspace detector.” We managed to create a complete list of the events. Our results indicate that the Mór Graben is still seismically active.

Two closely timed moderate-sized earthquakes (e.g., the M_L 6.6 Guanshan and M_L 6.8 Chihshang earthquakes) occurred in eastern Taiwan on 17–18 September 2002. To understand the rupture relationship between the two earthquakes, we used a teleseismic P-wave inversion and rupture directivity analysis to investigate the source parameters of the M_L 6.6 Guanshan earthquake. The teleseismic P-wave inversion method assumed earthquake rupture to be a single source to determine the focal mechanism, seismic moment, and azimuth-dependent source duration for the Guanshan earthquake. The rupture directivity analysis using the azimuth-dependent source duration showed that the Guanshan earthquake unilaterally ruptured along a west-dipping fault plane (Central Range fault) with a rupture velocity (Vr) of approximately 2.8 km/s from the hypocenter toward the deeper part of the fault. Furthermore, through time-domain deconvolution, the source time function of the Guanshan earthquake approximated an isosceles triangle, which also indicated that the earthquake rupture was relatively simple. Here, we propose a fault-based relationship between the M_L 6.6 Guanshan and M_L 6.8 Chihshang earthquakes. After the M_L 6.6 Guanshan earthquake, the source area was relocked, preventing the Chihshang earthquake from rupturing southward. Instead, it ruptured the fault from south to north.

Seismicity parameters can simplify the understanding of the intrinsic complications that arise in the state of stress across the hypocentral areas of interest. We studied variations of the spatial and temporal changes of these parameters by three different methods: maximum curvature, entire magnitude range, and hierarchical space time point process model across the July 2019 Ridgecrest earthquake region. In order to verify the estimations, the Utsu’s test has also been applied. According to the results, seismicity parameters show heterogeneous distribution in this area. The implemented methods provide comparable b-values; however, the b-value displays relatively lower values in northwest and higher values in southeast. Seismicity rate comparison for two periods before and after the M7.1 shock favors change in the b-value. Based on the employed catalog, seismic activity accelerated about half an hour before the M6.4 event. Whereas 2 days before the M7.1 earthquake, seismic activity was low and accelerated approximately 1 day prior to the same event. So there is a clear difference in pre M6.4 and pre M7.1 seismic activity patterns. Moreover, the b-value and magnitude of completeness show decrease before the M7.1 shock, and spatial changes of the b-value expose obvious differences with depth.

A dataset of simulated ground motions is created for seven recorded and previously validated, along with three hypothetical earthquakes in Turkey. This dataset has potential uses in engineering practice and research by both seismological and engineering communities. The simulated ground motion dataset with extensive information on the simulations and ground motion intensity parameters for each simulated motion is presented in an open-access online repository. A two-level randomization scheme is proposed to account for the uncertainties in input parameters and source-to-site geometries. An investigation of the magnitude-distance ranges in the simulated dataset, as well as the distribution of ground motion intensity measures, showed that the created dataset fills the gaps observed in recorded ground motion datasets. Pulse-like motions in the dataset are identified, and the relationship between pulse periods and earthquake magnitude is shown to agree with other relationships in the literature which are derived from recorded ground motions. The effects of source-to-site geometry and uncertainties in the following four input parameters are investigated: magnitude ((Mw)), stress drop, ((Delta tau)), time-averaged shear-wave velocity in the upper 30 m (({V}_{S30})), and high-frequency attenuation parameter (({kappa }_{0})). The dataset is validated by investigating the variability and inter-period correlation of normalized residual spectral acceleration values ((epsilon )), calculated using a ground motion model (GMM). The variability of (epsilon) is found to be consistent with the variability of GMMs. However, inter-period correlations of (epsilon) are shown to be larger than predictions of empirical models based on recorded earthquakes.

Probabilistic seismic hazard analysis (PSHA) is the primary method for determining the earthquake forces as input to structural seismic evaluation and design. Epistemic uncertainty has been incorporated into the PSHA process using a logic tree. One of the main challenges in using logic trees is determining ground motion prediction equations (GMPEs) and their branches’ weights. In this paper, regarding the different definitions of probability, the philosophy of GMPE selection and logic tree branches’ weight allocation in the PSHA is investigated. The results show that the classical and frequency definitions of probability are not applicable in the selection and weight allocation process. We suggest that the best way to allocate weight can be obtained by combining the inter-subjective and propensity probability definitions. To evaluate the effect of weight allocation on the PSHA results, PSHA was performed for a site in Tehran using different selection and weighting approaches. The results of the numerical example show up to a 50% variation in the spectral acceleration in the range of common building periods. We show that the issue of GMPE selection and weight allocation has not been adequately addressed in the current procedures of PSHA. So, it is necessary to develop specific agendas in this field.

Due to the complex nonstationarity of ground motion in time–frequency domain, the traditional methods of comparing and evaluating earthquake waveforms have not enough ability and accuracy to distinguish the details and changing features of the similar waves, which makes the similarity evaluation of waveform is difficult to be quantified accurately. The similarity degree of different signals can be calculated precisely according to dynamic time warping (DTW) algorithm, so it can be used for waveform comparison and similarity evaluation. In order to improve the traditional method, a method based on DTW distance is proposed to identify the earthquake waveform and analyze the ground motion characteristics. Based on the statistical analysis of a great quantity of earthquake waves, the changes law of DTW distance considering amplitude, time lag, noise signal ratio, site type, and the comprehensive effect is obtained. DTW distance is proved to be used as a compatible evaluation standard for waveform refinement. It is verified that DTW distance and vector norm are essentially equivalent. In the analysis of ground motion, DTW distance is implicated in the equivalent amplitude and energy of earthquake waves. The physical connotation of DTW distance is demonstrated by analyzing the data of the station array, and the results show that the distribution of DTW distance can accurately imply the time–space variation effect of the earthquake in the region. The reasonable reference range of DTW distance is defined by statistical method, and the corresponding evaluation standard of synthetic multi-point ground motion with real characteristics is proposed. In the synthetic accuracy evaluation of artificial ground motion with spatial variation effect, the combination of ground motions with more real characteristics can be obtained by evaluating and optimizing the waveforms according to the variation rule and range of DTW distance.

The characterization and prediction of wind turbine (WT) emissions are important steps in reducing their impact on humans or sensitive technologies such as seismic stations or physics experiments. Here, WT ground motion emissions are studied along two measurement lines set up at two wind farms on the Eastern Swabian Alb, southwest Germany. The main purpose of the data analysis is to estimate amplitude decay rates from vertical component data and surface wave phase velocities excited by the permanent motion of the WT towers. Phase velocities as well as geological information serve as input to build realistic subsurface models for numerical wave field simulations. Amplitude A decay rates are characterized by b-values through (Asim 1/r^b) depending on distance r and are derived from peaks in power spectral density (PSD). We find an increase of (b_text {PSD}) with frequency from 0.5 to 3.2 for field data. For low frequencies (1.2 Hz and 3.6 Hz), (b_text {PSD}) ranges from 0.5 to 1.1, hence close to the geometrical spreading factor of surface waves ((b_text {PSD}=1)). Anelastic damping and scattering seem not to be significant at these frequencies which also shows in numerical simulations for quality factors (Q=50-200). We also find that the emitted wavefields from several WTs interfere, especially in the near-field, and produce strong local ground motion amplitudes. The inclusion of a steep topography present in low mountain ranges adds more wave field distortions which can further increase the amplitudes. This needs to be considered when predicting WT induced ground motions.

A new, improved approach in the design of broadband seismometers is presented. The design results in the implementation of a high performance, low cost, and simple-to-operate instrument. The proposed seismometer is based on a modified accelerometer followed by a continuous time integrator for providing velocity voltage output. It has a broadband response, flat in velocity from 120 s to 75 Hz, high sensitivity 1200 V/(m/s), and 40 V_pp differential output range. The acceleration integration method provides high performance at low frequencies, with self-noise well below the New Low Noise Model at the range 80 s–16 Hz. The mechanical system provides a perfectly linear response of its displacement sensing system. Evaluation, classification, and noise determination of the presented instrument are performed in terms of direct experimental measurements, simulations, and calculations based on raw data from the proposed sensor and from a commercial product with approximately equivalent performance. Its technical features and performance specifications guarantee accurate sensing of local events, with maximum power at the frequency range of 5 to 10 Hz, but also make it ideal for the recording of distant tectonic activity, where extremely weak motions at long periods are expected. The whole design is robust, lightweight, and weatherproof, comprising in this way a useful tool to geoscientists.