ALBERT

Description: 〈span〉〈div〉Abstract〈/div〉The reliability and accuracy of the ground‐motion prediction equations (GMPEs) are of prime interest while evaluating seismic hazard for any region. The regular updates and minimization of the uncertainties associated with the coefficients of the GMPEs are important for improving ground‐motion predictions and consequent performance of seismic hazard maps.Thus, in the present study, we propose an update of the GMPEs estimated by 〈a href="https://pubs.geoscienceworld.org/bssa#rf32"〉Sharma 〈span〉et al.〈/span〉 (2013)〈/a〉 in The Geysers geothermal area. The update is done using the huge dataset available and by extending the magnitude range as well as distance range. The previous dataset used by 〈a href="https://pubs.geoscienceworld.org/bssa#rf32"〉Sharma 〈span〉et al.〈/span〉 (2013)〈/a〉 was composed of 212 earthquakes recorded at 29 stations with the magnitude range between 1.3≤Mw≤3.3 and distance range between 0.6≤Rhypo≤20  km. The new dataset encloses 10,974 induced earthquakes recorded at 29 stations with the magnitude range between 0.7≤Mw≤3.3 and distance range between 0.1≤Rhypo≤73  km. We compute updated GMPEs for peak ground velocity (PGV), peak ground acceleration (PGA), and 5% damped spectral acceleration (SA) (T) at T 0.05, 0.1, 0.2, 0.5, and 1.0 s.The mean ground‐motion predictions of the updated model proposed in the present study and the associated uncertainties are compared with the previous model proposed by 〈a href="https://pubs.geoscienceworld.org/bssa#rf32"〉Sharma 〈span〉et al.〈/span〉 (2013)〈/a〉 and with other models specifically developed for small‐magnitude earthquakes. The GMPEs are derived using a nonlinear mixed‐effect regression technique that accounts for both interevent and intraevent dependencies in the data. We also demonstrate the dependency of aleatory (random) uncertainties and epistemic (informative) uncertainties on source, medium, and site properties. We also concluded that the medium is behaving homogeneously in terms of peak ground‐motion attenuation by analyzing uncertainties associated with different ground‐motion periods.〈/span〉

Description: The analysis of earthquake focal mechanisms provides information about the stress regime, fault geometry, and deformation processes acting in a given region. Generally, the techniques aimed at determining focal mechanism are designed to work in a specific magnitude range operating both in the time and frequency domain and using different data (e.g., P polarities, S -wave polarization, S / P -amplitude ratios, etc.). In this article, we present a new method, Bayesian inversion of spectral-level ratios and P -wave polarities (BISTROP), that can be applied to both small and moderate-to-large magnitude events. BISTROP uses a Bayesian approach to jointly invert the long-period spectral-level P / S ratios and the P polarities to infer the fault-plane solutions. We apply this method to analyze synthetic data as well as those generated by real earthquakes. We find that the obtained solutions for moderate earthquakes are comparable with those obtained using moment tensor inversion, and they are more constrained with respect to the solutions obtained using only P -polarity data for small earthquakes.

Description: One of the main challenges of modern engineering seismology is the mitigation of the adverse consequences of earthquakes. Although modern engineering techniques allow for the designing of earthquake-resistant structures, the ability to predict more reliable ground-motion estimates and associated uncertainties is needed. With this aim, this study investigated the possibility of using data recorded during an earthquake to improve the empirical ground-motion prediction equations (GMPEs). In particular, we propose a procedure that updates the coefficients of an initial GMPE to account for the specific features of a seismic source and propagation medium. We applied the technique in the immediate postevent time period of three well-recorded earthquakes that occurred recently in Italy and caused casualties and significant damage: the 6 April 2009 M w  6.3 L’Aquila earthquake, the 20 May 2012 M w  5.9 Emilia earthquake, and the 25 October 2012 M w  5.2 Pollino earthquake. For possible future development, using the same earthquakes and the networks with which they were recorded, we also explored the potential of the technique as a possible real-time application, as in the case of earthquake early warning systems.

Description: Ground-motion prediction equations (GMPEs) play a crucial role for estimating the seismic hazard in any region using either a deterministic or a probabilistic approach. Indeed, they represent a reliable and fast tool to predict strong ground motion, given source and propagation parameters. In this article, we estimated GMPEs for the South Korea peninsula. GMPEs were computed for peak ground displacement, peak ground velocity, peak ground acceleration, and spectral accelerations (damping at 5%) at 13 different periods from 0.055 to 5 s. We analyzed data from 222 earthquakes recorded at 132 three-component stations of the South Korea Seismic Network, from 2007 to 2012, with local magnitude ranging between 2.0 and 4.9 and epicentral distances varying from 1.4 to ~600 km. A nonlinear mixed effects technique is used to infer the GMPE coefficients. This technique includes both fixed and random effects and accounts for both inter- and intraevent dependencies in the data. Station-specific corrective coefficients were estimated by a statistical approach and were included in the final ground-motion prediction model. Finally, predictions for peak ground acceleration and spectral acceleration are compared with observations recorded for an M L  5.1 earthquake that occurred in 2014, the data for which were not included in the modeling. Online Material: Figures showing final ground-motion prediction equation models versus observations, and intra- and interevent residuals.

Description: 〈span〉〈div〉Abstract〈/div〉The reliability and accuracy of the ground‐motion prediction equations (GMPEs) are of prime interest while evaluating seismic hazard for any region. The regular updates and minimization of the uncertainties associated with the coefficients of the GMPEs are important for improving ground‐motion predictions and consequent performance of seismic hazard maps.Thus, in the present study, we propose an update of the GMPEs estimated by 〈a href="https://pubs.geoscienceworld.org/bssa#rf32"〉Sharma 〈span〉et al.〈/span〉 (2013)〈/a〉 in The Geysers geothermal area. The update is done using the huge dataset available and by extending the magnitude range as well as distance range. The previous dataset used by 〈a href="https://pubs.geoscienceworld.org/bssa#rf32"〉Sharma 〈span〉et al.〈/span〉 (2013)〈/a〉 was composed of 212 earthquakes recorded at 29 stations with the magnitude range between 1.3≤Mw≤3.3 and distance range between 0.6≤Rhypo≤20  km. The new dataset encloses 10,974 induced earthquakes recorded at 29 stations with the magnitude range between 0.7≤Mw≤3.3 and distance range between 0.1≤Rhypo≤73  km. We compute updated GMPEs for peak ground velocity (PGV), peak ground acceleration (PGA), and 5% damped spectral acceleration (SA) (T) at T 0.05, 0.1, 0.2, 0.5, and 1.0 s.The mean ground‐motion predictions of the updated model proposed in the present study and the associated uncertainties are compared with the previous model proposed by 〈a href="https://pubs.geoscienceworld.org/bssa#rf32"〉Sharma 〈span〉et al.〈/span〉 (2013)〈/a〉 and with other models specifically developed for small‐magnitude earthquakes. The GMPEs are derived using a nonlinear mixed‐effect regression technique that accounts for both interevent and intraevent dependencies in the data. We also demonstrate the dependency of aleatory (random) uncertainties and epistemic (informative) uncertainties on source, medium, and site properties. We also concluded that the medium is behaving homogeneously in terms of peak ground‐motion attenuation by analyzing uncertainties associated with different ground‐motion periods.〈/span〉

Description: The accurate modeling of ground motion for induced-seismicity hazard estimation is critically dependent on how amplitudes scale with distance near the hypocenter. A rich database of ground motions from small events recorded at close distances in the Geysers region of California has been used to constrain the near-distance saturation effects that control the maximum observed ground motions and intensities for shallow-induced events. The results of this study support the modeling of these effects using an equivalent point-source concept, in which the effective source depth increases from a value near 1 km at moment magnitude ( M ) of 2 to a value near 3 km at M  4. This near-distance saturation behavior can be applied to the development of ground-motion models for induced seismicity in any region.

Description: Preferential direction in rupture propagation of earthquakes is known to have strong consequences on the azimuthal distribution of the ground motion. While source directivity effects are well established for large seismic events, their observation for moderate and small earthquakes are still restricted to a few cases. This is mainly due to intrinsic difficulties in recognizing source directivity unambiguously for less energetic/shorter ruptures. Therefore, we propose the use of multiapproach analysis for revealing the possible directivity for small-to-moderate earthquakes, taking advantage of the different sensitivity of each approach to various source and propagation characteristics. Here, we demonstrate that the application of six diverse and independent methods converges in giving consistent information on the rupture kinematics of the 2013 December 29, M w  = 5.0 earthquake. The results indicate a distinct rupture propagation direction toward S-SW, which correlates with observed asymmetry of damage and felt area. Overall, we conclude that the use of a single technique cannot provide a univocal solution, whereas the application of distinct analyses helps to strongly constrain source kinematics and should be preferred, in particular when dealing with small-to-moderate earthquakes.

Description: Intrusions are a ubiquitous component of mountain chains and testify to the emplacement of magma at depth. Understanding the emplacement and growth mechanisms of intrusions, such as diapiric or dike-like ascent, is critical to constrain the evolution and structure of the crust. Petrological and geological data allow us to reconstruct magma pathways and long-term magma differentiation and assembly processes. However, our ability to detect and reconstruct the short-term dynamics related to active intrusive episodes in mountain chains is embryonic, lacking recognized geophysical signals. We analyze an anomalously deep seismic sequence (maximum magnitude 5) characterized by low-frequency bursts of earthquakes that occurred in 2013 in the Apennine chain in Italy. We provide seismic evidences of fluid involvement in the earthquake nucleation process and identify a thermal anomaly in aquifers where CO 2 of magmatic origin dissolves. We show that the intrusion of dike-like bodies in mountain chains may trigger earthquakes with magnitudes that may be relevant to seismic hazard assessment. These findings provide a new perspective on the emplacement mechanisms of intrusive bodies and the interpretation of the seismicity in mountain chains.

Description: The occurrence of the M w  6.0 South Napa California earthquake, on 24 August 2014 at 03:20 a.m. local time, triggered discussion in the seismological community about the level of damage associated with such a moderate-magnitude event and about the geometry and orientation of the causative fault. In addition, coulomb static stress change mapping does not seem to be able to fully explain near-source aftershock distribution. Here, we find clear evidence of a north-northwest source directivity from the analysis of the spatial distribution of peak ground motion. The area of the highest values of the estimated peak dynamic strain field, computed accounting for fault extent and source directivity, agrees with the near-source aftershock distribution. This might suggest that, in addition to coulomb static stress change, dynamic strain also contributed to the triggering of near-source Napa earthquake aftershocks. The approach used here might be useful to identify areas likely prone to aftershock occurrence.