The moment magnitude () 5.5 earthquake that struck South Korea in November 2017 was one of the largest and most damaging events in that country over the past century. Its proximity to an enhanced geothermal system site, where high-pressure hydraulic injection had been performed during the previous 2 years, raises the possibility that this earthquake was anthropogenic. We have combined seismological and geodetic analyses to characterize the mainshock and its largest aftershocks, constrain the geometry of this seismic sequence, and shed light on its causal factors. According to our analysis, it seems plausible that the occurrence of this earthquake was influenced by the aforementioned industrial activities. Finally, we found that the earthquake transferred static stress to larger nearby faults, potentially increasing the seismic hazard in the area.

.[1] On May 11th 2011, a rather small earthquake caused nine fatalities in the city of Lorca, SE-Spain. We analyze seismograms from a dense network to characterize the source of this earthquake. We estimate an oblique reverse faulting mechanism, moment magnitude of 5.2 and a shallow hypocenter (4.6 km), at only 5.5 km epicentral distance from the city center. Double difference relocations yield a $5 km long, NE-SW trending distribution of aftershocks SW of the mainshock, suggesting a SW propagating rupture along the Alhama de Murcia fault. We use the Mw 4.6 foreshock and an Mw 3.9 aftershock as empirical Greens functions to estimate apparent source time functions, observing a clear directivity effect. We model apparent durations with a unilateral and asymmetric bilateral rupture, in both cases obtaining rupture directivity of $N220°E, towards Lorca. In addition to the near epicenter and shallow depth, directivity may have contributed to the significant impact.

A crucial issue to characterize hydraulic fractures is the robust, accurate and automated detection and location of acoustic emissions (AE) associated with the fracture nucleation and growth process. Waveform stacking and coherence analysis techniques are here adapted using massive datasets with very high sampling (1 MHz) from a hydraulic fracturing experiment that took place 410 m below surface in the Äspö Hard Rock Laboratory (Sweden). We present the results obtained during the conventional, continuous water-injection experiment HF2 (Hydraulic Fracture 2). The resulting catalogue is composed of more than 4000 AEs. Frequency-magnitude distribution from AE magnitudes (M AE) reveals a high b-value of 2.4. The magnitude of completeness is also estimated approximately M AE 1.1 and we observe an interval range of M AE between 0.77 and 2.79. The hydraulic fractures growth is then characterized by mapping the spatiotemporal evolution of AE hypocentres. The AE activity is spatially clustered in a prolate ellipsoid, resembling the main activated fracture volume (~ 105 m 3), where the lengths of the principal axes (a = 10 m; b = 5 m; c = 4 m) define its size and its orientation can be estimated for a rupture plane (strike ~ 123°, dip ~ 60°). An asymmetric rupture process regarding to the fracturing borehole is clearly exhibited. AE events migrate upwards covering the depth interval between 404 and 414 m. After completing each injection and reinjection phase, the AE activity decreases and appears located in the same area of the initial fracture phase, suggesting a crackclosing effect.

The 2013 seismic sequence at the Castor injection platform offshore Spain, including three earthquakes of magnitude 4.1, occurred during the initial filling of a planned Underground Gas Storage facility. The Castor sequence is one of the most important cases of induced seismicity in Europe and a rare example of seismicity induced by gas injection into a depleted oil field. Here we use advanced seismological techniques applied to an enhanced waveform dataset, to resolve the geometry of the faults, develop a greatly enlarged seismicity catalog and record details of the rupture kinematics. The sequence occurred by progressive fault failure and unlocking, with seismicity initially migrating away from the injection points, triggered by pore pressure diffusion, and then back again, breaking larger asperities loaded to higher stress and producing the largest earthquakes. Seismicity occurred almost exclusively on a secondary fault, located below the reservoir, dipping opposite from the reservoir bounding fault.