ALBERT

Description: 〈span〉〈div〉ABSTRACT〈/div〉The FMNEAR method (for determination of Focal Mechanism using NEAR‐source records) has already proved its efficiency for continental earthquakes recorded by a large number of well‐distributed stations. Installed in January 2015, it has been running in real time in Chile, in the specific tectonic conditions of the active convergent margin of the Nazca plate, where most of the earthquakes occur offshore. About 3 yr of data were collected, containing thousands of earthquakes, for which 80% have low‐to‐moderate moment magnitudes (Mw〈4.6). Here, we show results of the FMNEAR inversion, processed in real time at the National Seismological Center of Chile. We compare our solutions with the solutions provided by the U.S. Geological Survey and the Global Centroid Moment Tensor. Our results tested on all the inverted earthquakes common to the three methods show that the main characteristics of the source are well retrieved by the FMNEAR inversion. With the present‐day seismic network installed in Chile, FMNEAR was able to provide more than 6200 automatic event solutions, from which about 1000 were considered as reliable. All the inversions were conducted a few minutes after the earthquake origin time, with a calculation time varying from a few seconds for the smallest events to half an hour for the biggest and most complicated earthquakes. The FMNEAR approach showed its efficiency in inverting even small earthquakes in Chile (Mw〈4.5), if the azimuthal station coverage and the number of near stations are sufficient. By implementing FMNEAR in other tectonic settings and continuing to probe its results in the active seismic context of Chile, we expect to improve its capacity to provide viable and meaningful information in real time on the local seismic activity.〈/span〉

Description: 〈span〉〈div〉ABSTRACT〈/div〉The FMNEAR method (for determination of Focal Mechanism using NEAR‐source records) has already proved its efficiency for continental earthquakes recorded by a large number of well‐distributed stations. Installed in January 2015, it has been running in real time in Chile, in the specific tectonic conditions of the active convergent margin of the Nazca plate, where most of the earthquakes occur offshore. About 3 yr of data were collected, containing thousands of earthquakes, for which 80% have low‐to‐moderate moment magnitudes (Mw〈4.6). Here, we show results of the FMNEAR inversion, processed in real time at the National Seismological Center of Chile. We compare our solutions with the solutions provided by the U.S. Geological Survey and the Global Centroid Moment Tensor. Our results tested on all the inverted earthquakes common to the three methods show that the main characteristics of the source are well retrieved by the FMNEAR inversion. With the present‐day seismic network installed in Chile, FMNEAR was able to provide more than 6200 automatic event solutions, from which about 1000 were considered as reliable. All the inversions were conducted a few minutes after the earthquake origin time, with a calculation time varying from a few seconds for the smallest events to half an hour for the biggest and most complicated earthquakes. The FMNEAR approach showed its efficiency in inverting even small earthquakes in Chile (Mw〈4.5), if the azimuthal station coverage and the number of near stations are sufficient. By implementing FMNEAR in other tectonic settings and continuing to probe its results in the active seismic context of Chile, we expect to improve its capacity to provide viable and meaningful information in real time on the local seismic activity.〈/span〉

Description: We calculate well-resolved corner frequencies and radiated energies for a set of 96 earthquakes divided into two clusters located in the same tectonic setting but with different depths in northern Chile. Fifty-three shallow events with a mean depth of 20 km are analyzed, along with 43 intermediate-depth earthquakes with a mean depth of 110 km. We deduce and compare their static (stress drop Δσ) and dynamic (rupture energy EG and radiation efficiency ηR) source parameters and test the implications of different common assumptions on their depth dependence, such as stress drop invariance, strain drop invariance, or constant rupture velocities. Our data show that, in this zone of Chile, most of these models imply higher rupture velocities at depth than for shallow earthquakes. In these cases, high stress drop, high fracture energy release rate, and higher radiation efficiency are observed for intermediate-depth earthquakes, suggesting short and impulsive ruptures. ©2019. American Geophysical Union. All Rights Reserved.

Description: The Lacq area in southwest France has been associated with continuous moderate induced seismic activity since 1969. However, the mechanisms driving this induced seismicity are not fully understood: reservoir depletion has been proposed as the main factor, and more recently wastewater injection has been suggested to play a more important role (Grasso et al., 2021). The interpretation of these mechanisms relies heavily on the quality of earthquake locations, which we prove to be weak due to a lack of local instrumentation for several years. In order to provide the most complete and reliable induced event catalog for the studies of the Lacq induced seismicity mechanisms & seismic hazard, we made an exhaustive compilation, analysis and improvement of all available catalogs. We also provided new earthquake detections & relocations in a 3D velocity model from past and present temporary deployments never used for studying the Lacq area. Important remaining location uncertainties lead us to also carefully sort the events according to their location confidence, defining 3 classes of events (unconstrained location, location constrained within 2-3 km and 1-2 km respectively). This new harmonized catalog and the identification of well-constrained events, covering 50 years of induced seismicity, allow us to propose that wastewater injection is almost certainly the main mechanism driving the seismicity, with (i) most of the constrained events located within the reservoir boundaries and (ii) the released seismic energy variations following variations in injection operations at different scales. In particular, we have also highlighted a change in the injection-seismicity relationship around 2010–2013. From 2013, despite lower injection volumes, seismicity remained persistent and some clusters of earthquakes were detected predominantly in spring, summer, and early autumn, except in winter periods. From 2016, we observed a strong temporal relationship between days with higher rate/volume injections (approximately above 400m3/day) and both clustered events and higher magnitude earthquakes (greater than 2.4).