Seismicity in far western Nepal reveals flats and ramps along the Main Himalayan Thrust

Summary Unravelling relations between lateral variations of mid-crustal seismicity and the geometry of the Main Himalayan Thrust system at depth is a key issue in seismotectonic studies of the Himalayan range. These relations can reveal along strike changes in the behavior of the fault at depth related to fluids or the local ramp-flat geometry and more generally of the stress build-up along the fault. Some of these variations may control the rupture extension of intermediate, large or great earthquakes, the last of which dates back from 1505 CE in far western Nepal. The region is also associated to lateral spatio-temporal variations of the mid-crustal seismicity monitored by the Regional Seismic Network of Surkhet-Birendranagar. This network was supplemented between 2014 and 2016 by 15 temporary stations deployed above the main seismic clusters giving new potential to regional studies. Both absolute and relative locations together with focal mechanisms are determined to gain insight on the fault behavior at depth. We find more than 4000 earthquakes within 5 and 20 km-depth clustered in three belts parallel to the front of the Himalayan range. Finest locations reveal close relationships between seismic clusters and fault segments at depth among which mid-crustal ramps and reactivated tectonic slivers. Our results support a geometry of the Main Himalayan Thrust involving several fault patches at depth separated by ramps and tear faults. This geometry most probably affects the pattern of the coseismic ruptures breaking partially or totally the locked fault zone as well as eventual along strike variations of seismic coupling during interseismic period.

Strong following earthquake forecasting by a pattern recognition approach in California

<p>During seismic clusters, strong earthquakes (e.g. the mainshocks) are sometimes followed by another strong following earthquake, very dangerous because it strikes already damaged structures. To forecast the occurrence of such subsequent large earthquakes (SLE), we proposed a pattern recognition approach based on seismological features. The method, called NESTORE, has been successfully applied to northeastern Italy and western Slovenia (Gentili and Di Giovambattista, 2020) and to all of Italy (Gentili and Di Giovambattista, 2017). In this study, we will present the results of the application of NESTORE to California seismicity. NESTORE method is adaptive and depends on the region analyzed. During the supervised training phase, some features are selected as the best-performing ones in the analyzed area, which will be used for classification. Tests of this method demonstrate good performance for California seismicity.</p><p> </p>

Lateral migration patterns toward or away from injection wells for earthquake clusters in Oklahoma

<p>Exploring the connections between injection wells and seismic migration patterns is key to understanding processes controlling growth of fluid-injection induced seismicity. Numerous seismic clusters in Oklahoma have been associated with wastewater disposal operations, providing a unique opportunity to investigate migration directions of each cluster with respect to the injection-well locations. We introduce new directivity migration parameters to identify and quantify lateral migration toward or away from the injection wells. We take into account cumulative volume and injection rate from multiple injection wells. Our results suggest a weak relationship between migration direction and the cluster-well distances. Migration away from injection wells is found for distances shorter than 5-13 km, while an opposite migration towards the wells is observed for larger distances, suggesting an increasing influence of poroelastic stress changes. This finding is more stable when considering cumulative injected volume instead of injection rate. We do not observe any relationship between migration direction and injected volume or equivalent magnitudes.</p>

On seismic clusters, swarms and repeating earthquakes in Central Chile.

<p> Along the Chilean subduction segment, the seismicity tends to display characteristics of mainshock-aftershocks sequences. However, besides large and destructive earthquakes, central Chile has been also characterized by the occurrence of localized seismicity clusters with weak to moderate magnitudes, appearing either in form of repeated short-duration swarms or in form of sustained long-lasting activity. Seismic swarms were observed prior to large earthquakes and were hypothesized as possible precursors, although they did not always develop into major earthquakes. The origin and driving processes of this localized seismic activity have not yet been identified. Here, we characterize the seismicity at two seismic clusters in Central Chile, by analyzing hypocentral locations, spatio-temporal migration, magnitude, and inter-event time distributions and moment tensors. Both clusters are characterized by weak to moderate seismicity and manifest as clear seismicity rate and Benioff strain anomalies. We discuss these seismic clusters over a period of 18 years (2000-2017) and investigate their interactions with the Maule earthquake. We find repeating thrust earthquakes on the slab interface at one cluster beneath Vichuquén slipping at a rate comparable to the tectonically accumulated one. At the offshore Navidad cluster, the seismicity occurs in forms of swarms, with the largest episodes in 2001, 2002, 2004, 2012, 2014, 2016 and 2017 showing some rough temporal recurrence. Moment tensor indicates the occurrence of similar thrust mechanisms along a west-dipping structure across the subducting plate. Clusters persist before and after the Maule earthquake. However, at the Vichuquén cluster, the increased seismicity rate following the Maule earthquake remains to date higher than the background rate and the system is still far from recovery.</p>