ALBERT

Description: 〈span〉〈div〉ABSTRACT〈/div〉In November 2014, a temporary land and marine seismic network was deployed to monitor the drilling of an exploratory well in the Canary Channel (eastern Canary Islands). This region is characterized by low‐seismic activity; however, because of the increased awareness of the potential seismic hazard caused by hydrocarbon exploitation activities, the drilling operations were monitored with an unprecedented level of detail for an activity of this kind. According to the reported earthquakes, there was not a measurable increase in seismicity in the vicinity of the well. Overall seismic activity was low, which is consistent with the historical seismicity records. Harmonic tremor, explained here as resonances of the instrument‐seafloor system generated by bottom water currents in the area, was commonly detected on the ocean‐bottom seismometer (OBS) recordings. The marine network data also revealed dozens of nonseismic short‐duration signals per day that appear similar to other events on OBS recordings throughout the world. We suggest that they may be caused by direct perturbations on the OBS, mostly induced by ocean currents in the Canary Channel.〈/span〉

Description: 〈span〉〈div〉ABSTRACT〈/div〉The April 2016 Pedernales earthquake ruptured a 100 km by 40 km segment of the subduction zone along the coast of Ecuador in an Mw 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 Geofísico at the Escuela Politécnica 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 catalog 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 ∼100  km to the northeast of the mainshock in the epicentral region of an Mw 7.8 earthquake in 1958. In December, an increase in seismicity occurred ∼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.〈/span〉

Description: 〈span〉〈div〉ABSTRACT〈/div〉The April 2016 Pedernales earthquake ruptured a 100 km by 40 km segment of the subduction zone along the coast of Ecuador in an Mw 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 Geofísico at the Escuela Politécnica 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 catalog 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 ∼100  km to the northeast of the mainshock in the epicentral region of an Mw 7.8 earthquake in 1958. In December, an increase in seismicity occurred ∼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.〈/span〉

Description: A megathrust subduction earthquake ( M w  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 ( M w  6.7) and 16:46 ( M w  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 M w  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 V S 30 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.

Description: 〈span〉〈div〉ABSTRACT〈/div〉In November 2014, a temporary land and marine seismic network was deployed to monitor the drilling of an exploratory well in the Canary Channel (eastern Canary Islands). This region is characterized by low‐seismic activity; however, because of the increased awareness of the potential seismic hazard caused by hydrocarbon exploitation activities, the drilling operations were monitored with an unprecedented level of detail for an activity of this kind. According to the reported earthquakes, there was not a measurable increase in seismicity in the vicinity of the well. Overall seismic activity was low, which is consistent with the historical seismicity records. Harmonic tremor, explained here as resonances of the instrument‐seafloor system generated by bottom water currents in the area, was commonly detected on the ocean‐bottom seismometer (OBS) recordings. The marine network data also revealed dozens of nonseismic short‐duration signals per day that appear similar to other events on OBS recordings throughout the world. We suggest that they may be caused by direct perturbations on the OBS, mostly induced by ocean currents in the Canary Channel.〈/span〉