Journal of Seismology最新文献

Induced earthquakes tend to be shallow, while tectonic events often occur in deeper parts of the Earth. A well-estimated hypocenter with uncertainties may help to evaluate whether an event is of an induced or tectonic origin. In this study, we focus on the development of a hypocenter method that helps to better define the source location of an earthquake and reduce the spatial error of the measurement. The hypocenter and the uncertainty is obtained by using the P- and S-wave phase time difference for a station and the P-wave traveltime differences between pairs of stations simultaneously in the hypocenter analysis. The uncertainty inherent to an imperfect reference velocity model, modelling, instrumental inaccuracy and phase time picking is propagated into the spacial hypocenter solution. A refined hypocenter methodology is successfully tested in a synthetic experiment with shallow ((sim ) 5 km), intermediate ((sim ) 10 km) and deep source points ((sim ) 15 km). The synthetic experiment indeed shows that it is possible to separate earthquakes by their depth solution, hence offering an indication that the event is either induced or tectonic. Case studies are presented of estimations of hypocenters and error ellipses for (1) induced seismicity at sites for gas storage in salt domes, geothermal production and gas extraction as well as (2) tectonic events.

The clipping of near-field seismic waveforms caused by instrument limitations results in the waste of lots of waveforms and hinders the effective advancement of seismic research projects like source parameter inversion, near-fault ground motion characteristics, and earthquake early warning (EEW). In this paper, we propose to use the cubic spline interpolation method to restore the clipping waveform to solve the near-field data loss. We evaluate the recoverability of seismic waveforms with different clipped levels through artificial clipping experiments and compare the recovery waveforms with the projection onto convex sets (POCS) method. The results show that the restoration deviation of the cubic spline interpolation method is less than 1% at the clipped point, for the records whose clipped amplitude does not exceed 50% of the peak. In addition, the restored phase feature of the cubic spline interpolation is closer to the real waveform than the POCS method. To verify the performance of this method in the realistic earthquake, we apply it to the Barkam M_w 5.9 earthquake. The results show that the peak ratio of the restored clipping records to the measured non-clipping records is close to 1, and the relative error of the response spectrum is less than 0.1. Finally, we apply this method to successfully restore about 60% of near-field clipped records from the 2022 Lushan M_w 5.8 earthquake, which provided more data support for the study of the near-field ground motion.

The new permanent seismic station SVZ started working in Arctic (Severnaya Zemlya archipelago, Bolshevik Island), we discuss the first results of the local seismicity study. We have processed 46 local seismic events for 2017–2022. Data processing using a single seismic sensor is difficult, but using waveforms and spectral-temporal analysis of P, S, and surface waves, it is possible to separate shallow earthquakes from glacial events. We distinguish two types of icequakes—glacier-related events and pulses. An increase in seismic background noise is related to processes of glacial nature in summertime. Data processing allows to obtain the main peculiarities of local seismicity, i.e., mutual connection of glacial processes and crustal seismicity. We show the presence of icequakes induced by local crustal events; their epicenters confined to the ice domes edges. Spatio-temporal sequence and the rate of events migration show that the glacial events are a result of the defusing deformations process in the glaciers caused by the impact of shallow tectonic earthquakes. Thousands of pulsed and high-frequency micro-oscillations associate with the sea ice sheet or lake ice. The temporal variation of events’ number and energy per day correlates with air temperature temporal variations. Moreover, there is a sharp increase in the number of pulses with sharp temperature changes. The greatest activity of this pulsed events is in winter and especially in March, connected with ice surface destruction. In case a network deployment is not feasible, even a single seismic station allows to get a useful information about the glacial and seismic processes.

Although earthquake early warning (EEW) systems have advanced significantly, accurately determining earthquake parameters from the initial 3 s of P-wave motion remains challenging. Factors such as the complexity of the earthquake source and variability of ground motion due to site conditions contribute to this difficulty. This article aims to investigate how local site conditions impact the correlation between EEW parameters and earthquake magnitude, to better understand the influence of site conditions on the accuracy of EEW systems. Specifically, the study examines the effect of variation site conditions on commonly used EEW parameters, such as average characteristic period (τ_c) and peak displacement amplitude (P_d), for different site classes. A dataset of 432 strong-motion records with magnitude ranging from 5 to 7.3 was analyzed and site characterization information from the Kiban Kyoshin Network (KiK-net) in Japan was used. A linear relationship between EEW parameters (τ_c, P_d) and magnitude for the combined dataset (all data), as well as separate datasets based on site classes C (very dense soil and soft rock) and D (stiff soil site), was developed, and then the statistical parameters, correlation coefficient value (R), and standard deviation error (SD) in the linear regression analysis were compared. The study finds that τ_c and P_d have a significant correlation with magnitude when separate correlations are developed for site classes C and D. Absolute residual error and percentage error analyses were carried out. It was found that magnitude prediction errors were reduced particularly for class D sites. Overall, the study suggests need for use of site class based magnitude prediction equations in earthquake early warning, especially for softer soil sites.

Bhaktapur city, one of the historical cities of Nepal, is among the old settlements within Kathmandu Valley that were frequently devastated during major earthquakes. Historical evidence clearly states the high seismic vulnerability of Bhaktapur. So, this study characterizes the free field of the Bhaktapur district in terms of periods. For this, the horizontal to vertical (H/V) technique was used to determine the predominant period of the study area, measuring microtremor data at 200 m × 200 m spacing in the core city area, whereas 400 m × 400 m spacing of measurement was done at the outskirts of the city with rare settlement. The spatial distribution map of the predominant period is prepared with the period obtained from 609 measuring points. The map shows that the predominant period ranges from 0.1 to 4 s and then categorized into four different zones—A (0.10–0.6 s), B (0.6–1.0 s), C (1.0–1.7 s), and D (1.7–4.0 s). The central parts of the study areas are spotted with longer periods (1.7–4.0 s), where the density of the dweller is high. The structural period of 28 reinforced concrete buildings was also determined to understand the level of danger of the buildings due to soil-structure resonance. The predominant period of the buildings ranges from 0.12 to 0.42 s. The possibility of soil structure resonance condition is higher for six buildings, medium for two buildings, and low for the rest of the buildings. The results can be applied to urban planning, seismic hazard mitigation, conservation, and restoration of heritage monuments.

On 29 November 2022, an earthquake of M_L 5.0 (M_w 4.8) occurred onshore South Evia Island (central Greece) preceded by a M_L 4.7 (M_w 4.6) event. The pattern of relocated aftershocks indicates the activation of a single, near-vertical fault segment, oriented NW-SE at shallow crustal depths (6–11 km). We suggest that both events ruptured a blind, left-lateral strike-slip fault, about 5 km southeast of village Almyropotamos. We observed that a clear foreshock activity (N=55 events) existed before the two moderate events. The impact of the static stress loading on neighboring fault planes diminishes after a distance of 7 km from the November 2022 epicenters, where the static stress falls below +0.1 bar. We further explore triggering relationships between the 29 November events and the late December 2022 moderate events (M_L 4.9) that occurred about 60 km toward NW in the Psachna and Vlahia regions of central Evia. We present evidence of possible delayed dynamic triggering of the late December 2022 central Evia sequence, based on marked changes in seismicity rates and on measured peak ground velocities (PGVs) and peak dynamic strains, both exhibiting local maxima in their map distributions. The causes of the delayed triggering may be related to the well-known geothermal field in central/north Evia and the NW-SE strike of the seismic fault.

Seismic shaking of an area is strongly affected by the local geology. The so-called local site effects must be considered for the estimation of seismic effects on structures and urban planning. Thus, the seismic microzonation is the process aimed at identifying and mapping the subsoil local response in a given area, usually at urban/municipality scale and in terms of ground shaking parameters and susceptibility to ground instabilities. In Italy, for areas that can be schematised as a 1D subsoil model (e.g. alluvial plain), a simplified approach is proposed to quantify the seismic amplification (amplification factor, AF). This approach consists of tables of correspondences, called seismic abacuses, available for the whole national area as well as for some regional territories, and derived for simplified subsoil models. In this work, the results of the comparison between the AF values retrieved from national abacuses applied in the Friuli Venezia Giulia (Italy) plain municipalities and those from 1D numerical simulations are presented. In general, the abacuses underestimate the local seismic site effects a part for sites with a shallow bedrock. No correlations/trends were identified between the AF derived from abacuses and those from numerical simulations. Moreover, considering the elastic acceleration response spectra, it emerges that in the 49.5% of the FVG analysed sites the abacuses approach, even though it underestimates the real seismic response, is a more suitable approximation compared to the soil class simplified approach proposed by the Italian regulation. Finally, what emerges is that the limit of 30 m, as indicated in the Italian regulation, to consider a deep or shallow bedrock seams underestimated, and the AFs are not correlated with the seismic bedrock depth when it is higher than 100 m.

Numerous ground-motion models (GMMs) that predict the intensities of surface ground motions have been previously developed based on regression analysis (RA). This study develops GMMs to estimate 5% damped pseudo-spectral accelerations (PSAs) for 30 periods (0.01–7.0 s) for within-rock ground motions, based on machine learning (ML) methods (i.e., two ensemble methods (random forest (RF) and gradient boosting (GB)) and an artificial neural network (ANN)). GMMs are developed separately for four earthquake types (main and aftershocks of active crustal region events and those of subduction zone interface events), considering the differences in the characteristics of each earthquake type. We utilize 20,041 ground motions recorded at 575 borehole stations in Japan during 602 earthquakes with moment magnitudes greater than 5.0 and rupture distances shorter than 300 km. The prediction performances of GMMs based on RF, GB, ANN, and RA are evaluated by the standard deviations of the total, between-event, and within-event residuals. The GMMs based on the three ML methods (RF, GB, and ANN) perform better than the RA-based models. The RF-based GMMs resulted in the most accurate prediction of the PSAs of within-rock ground motions with a small bias and variance, which can enhance the seismic designs and seismic hazard assessments for underground structures.

A new cellular automaton model is developed to examine the nature of temporal sequences of earthquakes. The model takes the space dependence of fault strength into account and assumes that an earthquake produces a continuous area with the stress dropped to zero. In the model, viscous slip is also introduced on the fault and earthquakes are realizable only when the stress accumulates faster than the viscous relaxation. The analysis reveals that the sequences of earthquakes generally satisfy the power law relation between the intensity and frequency of earthquakes so that earthquakes may be in the state of self-organized criticality. On the other hand, periodicity appears in some sequences that consist of the groups of high seismic activity repeated between calm intervals with an almost constant period. Therefore, self-organized criticality and periodicity coexist in these sequences and the claim that earthquakes are unpredictable because of self-organized criticality may be inadequate.