Rapid response to the earthquake emergency of May 2012 in the Po Plain, northern Italy

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Profiling the Quito basin (Ecuador) using seismic ambient noise

International audienceQuito, the capital of Ecuador, with more than 2.5 M inhabitants, is exposed to a high seismic hazard due to its proximity to the Pacific subduction zone and active crustal faults, both capable of generating significant earthquakes. Furthermore, the city is located in an intermontane piggy-back basin prone to seismic wave amplification. To understand the basin’s seismic re sponse and characterize its geological structure, 20 broad and medium frequency band seismic stations were deployed in Quito’s urban area between May 2016 and July 2018 that continuously recorded ambient seismic noise. We first compute horizontal-to-vertical spectral ratios to determine the resonant frequency distribution in the entire basin. Secondly, we cross-correlate seismic stations operating simultaneously to retrieve inter-stations surface-wave Green’s func tions in the frequency range of 0.1 - 2 Hz. We find that Love waves traveling in the basin’slongitudinal direction (NNE-SSW) show much clearer correlograms than those from Rayleigh waves. We then compute Love wave phase-velocity dispersion curves and invert them to obtain shear-wave velocity profiles throughout the city. The inversions highlight a clear difference in the basin’s structure between its northern and southern parts. In the center and northern areas, the estimated basin depth and mean shear-wave velocity are about 300 m and 1700 m.s−1, respectively, showing resonance frequency values between 0.6 and 0.7 Hz. On the contrary, the basement’s depth and shear-wave velocity in the southern part are about 1000 m and 2600m.s−1, having a low resonance frequency value of around 0.3 Hz. This difference in structure between the center-north and the south of the basin explains the spatial distribution of low frequency seismic amplifications observed during the Mw 7.8 Pedernales earthquake in April 2016 in Quito

Comparison of Observed Ground‐Motion Attenuation for the 16 April 2016 M w 7.8 Ecuador Megathrust Earthquake and Its Two Largest Aftershocks with Existing Ground‐Motion Prediction Equations

International audienceA megathrust subduction earthquake (Mw 7.8) struck the coast of Ecuador on 16 April 2016 at 23:58 UTC. This earthquake is one of the best‐recorded megathrust events to date. Besides the mainshock, two large aftershocks have been recorded on 18 May 2016 at 7:57 (Mw 6.7) and 16:46 (Mw 6.9). These data make a significant contribution for understanding the attenuation of ground motions in Ecuador. Peak ground accelerations and spectral accelerations are compared with four ground‐motion prediction equations (GMPEs) developed for interface earthquakes, the global Abrahamson et al. (2016) model, the Japanese equations by Zhao, Zhang, et al. (2006) and Ghofrani and Atkinson (2014), and one Chilean equation (Montalva et al., 2017). The four tested GMPEs are providing rather close predictions for the mainshock at distances up to 200 km. However, our results show that high‐frequency attenuation is greater for back‐arc sites, thus Zhao, Zhang, et al. (2006) and Montalva et al. (2017), who are not taking into account this difference, are not considered further. Residual analyses show that Ghofrani and Atkinson (2014) and Abrahamson et al. (2016) are well predicting the attenuation of ground motions for the mainshock. Comparisons of aftershock observations with the predictions from Abrahamson et al. (2016) indicate that the GMPE provide reasonable fit to the attenuation rates observed. The event terms of the Mw 6.7 and 6.9 events are positive but within the expected scatter from worldwide similar earthquakes. The intraevent standard deviations are higher than the intraevent variability of the model, which is partly related to the poorly constrained VS30 proxies. The Pedernales earthquake produced a large sequence of aftershocks, with at least nine events with magnitude higher or equal to 6.0. Important cities are located at short distances (20–30 km), and magnitudes down to 6.0 must be included in seismic‐hazard studies. The next step will be to constitute a strong‐motion interface database and test the GMPEs with more quantitative methods

Resolving source mechanisms of microseismic swarms induced by solution mining

International audienceIn order to improve our understanding of hazardous underground cavities, the development and collapse of a ∼200 m wide salt solution mining cavity was seismically monitored in the Lorraine basin in northeastern France. The microseismic events show a swarm-like behaviour, with clustering sequences lasting from seconds to days, and distinct spatiotemporal migration. Observed microseismic signals are interpreted as the result of detachment and block breakage processes occurring at the cavity roof. Body wave amplitude patterns indicated the presence of relatively stable source mechanisms, either associated with dip-slip and/or tensile faulting. Signal overlaps during swarm activity due to short interevent times, the high-frequency geophone recordings and the limited network station coverage often limit the application of classical source analysis techniques. To overcome these shortcomings, we investigated the source mechanisms through different procedures including modelling of observed and synthetic waveforms and amplitude spectra of some well-located events, as well as modelling of peak-to-peak amplitude ratios for the majority of the detected events. We extended the latter approach to infer the average source mechanism of many swarming events at once, using multiple events recorded at a single three component station. This methodology is applied here for the first time and represents a useful tool for source studies of seismic swarms and seismicity clusters. The results obtained with different methods are consistent and indicate that the source mechanisms for at least 50 per cent of the microseismic events are remarkably stable, with a predominant thrust faulting regime with faults similarly oriented, striking NW–SE and dipping around 35°–55°. This dominance of consistent source mechanisms might be related to the presence of a preferential direction of pre-existing crack or fault structures. As an interesting byproduct, we demonstrate, for the first time directly on seismic data, that the source radiation pattern significantly controls the detection capability of a seismic station and network

The 2016 M-w 7.8 Pedernales, Ecuador, earthquake : rapid response deployment

The April 2016 Pedernales earthquake ruptured a 100 km by 40 km segment of the subduction zone along the coast of Ecuador in an M-w 7.8 megathrust event east of the intersection of the Carnegie ridge with the trench. This portion of the subduction zone has ruptured on decadal time scales in similar size and larger earthquakes, and exhibits a range of slip behaviors, variations in segmentation, and degree of plate coupling along strike. Immediately after the earthquake, an international rapid response effort coordinated by the Instituto Geofisico at the Escuela Politecnica Nacional in Quito deployed 55 seismometers and 10 ocean-bottom seismometers above the rupture zone and adjacent areas to record aftershocks. In this article, we describe the details of the U.S. portion of the rapid response and present an earthquake cata-log from May 2016 to May 2017 produced using data recorded by these stations. Aftershocks focus in distinct clusters within and around the rupture area and match spatial patterns observed in long-term seismicity. For the first two and a half months, aftershocks exhibit a relatively sharp cutoff to the north of the mainshock rupture. In early July, an earthquake swarm occurred similar to 100 km to the northeast of the mainshock in the epicentral region of an M-w 7.8 earthquake in 1958. In December, an increase in seismicity occurred similar to 70 km to the northeast of the mainshock in the epicentral region of the 1906 earthquake. Data from the Pedernales earthquake and aftershock sequence recorded by permanent seismic and geodetic networks in Ecuador and the dense aftershock deployment provide an opportunity to examine the persistence of asperities for large to great earthquakes over multiple seismic cycles, the role of asperities and slow slip in subduction-zone megathrust rupture, and the relationship between locked and creeping parts of the subduction interface

Repeating Aftershocks of the 16th April 2016 Mw 7.8 Pedernales (Ecuador) Earthquake Underline the Interplay Between Afterslip and Seismicity

International audienceRepeating earthquakes are earthquakes that repeatedly break a single, time-invariant fault patch. They are generally associated with aseismic slip, which is thought to load asperities, leading to repeated rupture. Repeating earthquakes are therefore useful tools to study aseismic slip and fault mechanics, with possible applications to earthquake triggering, loading rates and predictability. In this study, we analyze one year of aftershocks following the 16th April 2016 Mw 7.8 Pedernales earthquake in Ecuador to find repeating families, using data recorded by permanent and temporary seismological networks. We focus on a small area north of the mainshock containing about 900 catalogued events, where seismicity during both the inter-seismic and post-seismic periods has been previously linked to aseismic slip. We calculate waveform cross-correlation coefficients (CC) on all available catalogue events, which we use to sort 195 events into 84 preliminary families of 2 to 18 events, using a minimum CC of 0.9. These events were then stacked and used to perform template-matching on the continuous data. In total, 387 earthquakes were classified into families, including 12 from the one-year period before the mainshock. We later relocated these earthquakes using a double-difference method, which confirmed that most of them did have overlapping sources. Repeating earthquakes seem to concentrate largely around the area of largest afterslip release. We also find an increase in the recurrence time of repeating events with time after the mainshock over the year of the postseismic period. Overall, our results suggest that most aftershocks are driven by afterslip release. The presence of new repeating families beyond the first month after the mainshock could be a sign of afterslip migration downdip from the early postseismic areas near the trench. Meanwhile, the increase in repeating earthquakes' recurrence times with time highlights a possible timeframe for the afterslip's deceleration