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Mofettes are gas emission sites where high concentrations of CO2 ascend through conduits from as deep as the mantle to the Earth's surface and as such provide direct windows to processes at depth. The Hartoušov mofette, located at the western margin of the Eger Graben, is a key site to study interactions between fluids and swarm earthquakes. The mofette field (10 mofettes within an area of 100 m × 500 m and three wells of 28, 108, and 239 m depth) is characterized by high CO2 emission rates (up to 100 t/d) and helium signatures with (3He/4He)c up to 5.8 Ra, indicating mantle origin. We compiled geological, geophysical, geochemical, and isotopic data to describe the mofette system. Fluids in the Cheb basin are mixtures between shallow groundwater and brine (>40 g/L at a depth of 235 m) located at the deepest parts of the basin fillings. Overpressured CO2-rich mineral waters are trapped below the mudstones and clays of the sealing Cypris formation. Drilling through this sealing layer led to blow-outs in different compartments of the basin. Pressure transients were observed related to natural disturbances as well as human activities. External (rain) and internal (earthquakes) events can cause pressure transients in the fluid system within hours or several days, lasting from days to years and leading to changes in gas flux rates. The 2014 earthquake swarm triggered an estimated excess release of 175,000 tons of CO2 during the following four years. Pressure oscillations were observed at a wellhead lasting 24 h with increasing amplitudes (from 10 to 40 kPa) and increasing frequencies reaching five cycles per hour. These oscillations are described for the first time as a potential natural analog to a two-phase pipe–relief valve system known from industrial applications. [ABSTRACT FROM AUTHOR]

Based on an analysis of continuous monitoring of farm animal behavior in the region of the 2016 M6.6 Norcia earthquake in Italy, Wikelski et al., 2020; (Ethology, 126(9), 2020, 931) conclude that animal activity can be anticipated with subsequent seismic activity and that this finding might help to design a "short‐term earthquake forecasting method." We show that this result is based on an incomplete analysis and misleading interpretations. Applying state‐of‐the‐art methods of statistics, we demonstrate that the proposed anticipatory patterns cannot be distinguished from random patterns, and consequently, the observed anomalies in animal activity do not have any forecasting power. [ABSTRACT FROM AUTHOR]

Fluid driven seismicity has been observed worldwide. The occurrence of intraplate seismicity triggered by pore pressure perturbations is a widely accepted process. Past analysis of earthquake swarms in the NW-Bohemia/Vogtland region provided evidence for the diffusion of pore pressure fronts during the migration of earthquakes within each swarm. Here, we test the hypothesis whether the diffusion of hydraulically induced pore pressure perturbations from the surface to the hypocentral depth could be a valid trigger mechanism.We test this hypothesis for the earthquake swarms in the Nový Kostel focal zone based on the analysis of 121 earthquake swarms and microswarms which occurred between 1992 and 2016. During the past 30 yr, 90 per cent of the earthquake swarms clustered beneath the village Nový Kostel within a depth range between 6 and 12 km. The spatial distribution of the swarms suggests a main diffusion pathway to the hypocentres, namely conduits along a NW-SE trending fracture zone which can be correlated at the surface with a part of the Libocký creek. A secondary branch of this fault zone exits in the southern part below the Horka reservoir. The filling of this reservoir began in 1969 and might have triggered the first significant earthquake swarm during the last century within the Nový Kostel area in the year 1985/1986. Since then, both the number of potentially triggered swarms as well as the number of microswarms increased with time. According to a robust statistical evaluation, hydraulically induced pore pressure perturbations could have triggered at least 19 per cent of the analysed earthquake swarms assuming effective diffusivities of 0.5 m2 s-1. Thus, hydroseismicity seems to be a valid option to explain at least a part of the earthquake swarms. The alternate hypothesis that ascending/intruding magmatic fluids trigger the remaining part of the earthquake swarms is another potential but weak option. [ABSTRACT FROM AUTHOR]

In recent years, there has been increasing interest in the study of seismic noise interferometry as it can provide a complementary approach to active source or earthquake-based methods for imaging and continuous monitoring the shallow structure of the Earth. This meaningful information is extracted from wavefields propagating between those receiver positions at which seismic noise was recorded. Until recently, noise-based imaging relied mostly on Rayleigh waves. However, considering similar wavelengths, a combined use of Rayleigh and Love wave tomography can succeed in retrieving velocity heterogeneities at depth due to their different sensitivity kernels. Here, we present a novel one-step algorithm for simultaneously inverting Rayleigh and Love wave dispersion data aiming at identifying and describing complex 3-D velocity structures. The algorithm may help to accurately and efficiently map the shear wave velocities and the Poisson ratio of the surficial soil layers. In the high-frequency range, the scattered part of the correlation functions stabilizes sufficiently fast to provide a reliable estimate of the velocity structure not only for imaging purposes but also allows for changes in the medium properties to be monitored. Such monitoring can be achieved with a high spatial resolution in 3-D and with a time resolution as small as a few hours. In this paper, we describe a recent array experiment in a volcanic environment in Solfatara (Italy) and we show that this novel approach has identified strong velocity variations at the interface between liquids and gas-dominated reservoirs, allowing localizing a region which is highly dynamic due to the interaction between the deep convection and its surroundings. [ABSTRACT FROM AUTHOR]

Aftershocks of the magnitude Mw 6.4 Bingöl earthquake of 1 May 2003 (eastern Turkey) were monitored by a local temporary seismic network. The spatial distribution of the aftershocks shows that the earthquake did not occur on the left-lateral East Anatolian fault, but perpendicular to it along a north-northwest-striking right-lateral fault. The event ruptured an area of 20 km by 8 km, but did not reach the Earth's surface. The aftershocks of the 2003 Bingöl earthquake and those of the 1992 Erzincan earthquake seem to indicate the existence of a new seismically active right-lateral shear zone. Indeed, we show that historical earthquakes along both the North and East Anatolian fault led to positive changes in Coulomb failure stress for north-northwest-oriented right-lateral strike-slip faults in the Bingöl epicentral area. Moreover, the existence of such faults is supported by remote-sensing data. [ABSTRACT FROM AUTHOR]

The existence of a causal link between large earthquakes and volcanic unrest is widely accepted. Recent observations have also revealed counterintuitive negative responses of volcanoes to large earthquakes, including decreased gas emissions and subsidence in volcanic areas. In order to explore the mechanisms that could simultaneously explain both the positive and negative responses of volcanic activity to earthquakes, we here focus on the role played by topography. In the laboratory, we shook a volcanic edifice analogue, made of gel, previously injected with a buoyant fluid. We find that shaking triggers rapid migration of the buoyant fluid upward, downward, or laterally, depending on the fluid's buoyancy and storage depth; bubbly fluids stored at shallow depth ascend, while low-buoyancy fluids descend or migrate laterally. The migration of fluids induced by shaking is two orders of magnitude faster than without shaking. Downward or lateral fluid migration may decrease volcanic gas emissions and cause subsidence as a negative response, while upward migration is consistent both with an increase in volcanic activity and immediate unrest (deformation and seismicity) after large earthquakes. The fluid migration is more efficient when the oscillation frequency is close to the resonance frequency of the edifice. The resonance frequency for a 30-km-wide volcanic mountain range, such as those where subsidence was observed, is ~0.07 Hz. Only large earthquakes are able to cause oscillation at such low frequencies. [ABSTRACT FROM AUTHOR]