Constraining maximum event magnitude during injection-triggered seismicity

Understanding mechanisms controlling fluid injection-triggered seismicity is key in defining strategies to ameliorate it. Recent triggered events (e.g. Pohang, Mw 5.5) have exceeded predictions of average energy release by a factor of >1000x, necessitating robust methodologies to both define critical antecedent conditions and to thereby constrain anticipated event size. We define maximum event magnitudes resulting from triggering as a function of pre-existing critical stresses and fluid injection volume. Fluid injection experiments on prestressed laboratory faults confirm these estimates of triggered moment magnitudes for varied boundary conditions and injection rates. In addition, observed ratios of shear slip to dilation rates on individual faults signal triggering and may serve as a measurable proxy for impending rupture. This new framework provides a robust method of constraining maximum event size for preloaded faults and unifies prior laboratory and field observations that span sixteen decades in injection volume and four decades in length scale.

Relating spatial-temporal b-value patterns of induced earthquakes of the Groningen gas field to differential stress changes due to reservoir extraction

<p>The Gutenberg-Richter law describes the frequency-magnitude distribution of seismic events where its slope, the 'b-value', is commonly used to describe the relative occurrence of large and small events. Statistically significant b-value variations have been measured in laboratory experiments, mines, and various tectonic regimes (Wiemer & Wyss, 2002). An inversely proportional dependency of the b-value on the differential stress has been observed across different scales (Amitrano, 2003; Schorlemmer et al., 2005). Layland-Bachmann et al. (2012) have shown that this could explain the observed pattern of induced seismicity spatial-temporal b-value variations in Enhanced Geothermal Systems. In our study, we look for a similar relation applied to the Groningen gas field in the Netherlands.</p><p>It is well known that the poroelastic changes in differential stress during gas extraction are influenced by the offset of the reservoir layer across the fault. Recently, Jansen et al. (2019) and Lehner (2019) proposed an analytical solution for stress changes on offset faults due to reservoir depletion. In a parallel study, we extended this solution to include the development of aseismic slip under slip weakening and the derivation of the onset of seismic slip.<br>We utilize this formulation to derive the onset of seismic slip on theoretical faults of variable fault offset, dip, and reservoir thickness. Subsequently, we map our theoretical faults onto the pre-existing faults in the Groningen gas field, deriving fault segment-specific depletion levels at which the segment would become seismically active. We then simulate reservoir depletion conditions over time and assign an event magnitude to fault segments that move past their seismic activation depletion. To assign a magnitude, we use the observation that b-values are inversely proportional to differential stress, which is governed by the pore pressure depletion. Hence, we assume a simple inverse linear relation with pore pressure depletion. Each event magnitude is then randomly drawn from the probability density function of the Gutenberg-Richter distribution with the b-value assigned.<br>We aim to compare the obtained catalogue and its b-value distribution both in time and space to the observed event-size distribution of the Groningen gas field as derived by Muntendam-Bos and Güdük (EGU abstract 2021).</p>

Typology of Hazard Event Severity Metrics for Multi-Hazard Research

<p>In the scholarly field of hazards, adverse impacts of a hazard event are interpreted as the result of interactions among hazard elements, exposure of entities of value, and vulnerability of the exposed entities. The severity of hazard elements is usually communicated as a magnitude or intensity. Such hazard event magnitude or intensity metrics correspond to the expected damages due to a hazard event given an average exposure and vulnerability. These severity metrics can be used to facilitate hazard communication and enhance emergency management. However, hazard event severity metrics for singular hazard types such as the earthquake Richter magnitude and the Saffir-Simpson hurricane wind scale cannot be readily adapted for multi-hazard comparative analyses. The first and foremost challenge to such comparative analyses is a lack of conceptual framework to systemically classify different hazard event severity metrics. In this presentation, we introduce a four-dimensional typology of hazard event severity metrics for hazard research within a multi-hazard context. The four dimensions include the spatial, temporal, applicational, and indicial dimensions. Based on a literature review on 67 existing hazard event magnitude or intensity scales for 21 singular hazard types, we demonstrate that the proposed typology can be applied to classify hazard event severity metrics. We further implement the proposed typology to two newly developed equivalent hazard event severity metrics called the Gardoni Scale and the Murphy Scale to showcase the utility of the proposed typology in facilitating quantification of hazard severity across different hazard event types.</p>

Solar Flare Build-Up and Release

Flares and coronal mass ejections should follow a pattern of build-up and release, with the build-up phase understood as the gradual addition of stress to the coronal magnetic field. Recently Hudson (Mon. Not. Roy. Astron. Soc.491, 4435, 2020) presented observational evidence for this pattern in two isolated active regions from 1997 and 2006, finding a correlation between the waiting time after the event, and the event magnitude. In this article we systematically search for related evidence in the largest 14 active regions of Solar Cycle 24, chosen as those with peak sunspot area exceeding 1000 millionths of the solar hemisphere (MSH). The smallest of these regions, NOAA 12673, produced the exceptional flares SOL2017-09-06 and SOL2017-09-10. None of these regions showed significant correlations of waiting times and flare magnitudes, although two hinted at such an interval-size relationship. Correlations thus appear to be non-existent or intermittent, depending on presently unknown conditions.

Early rupture signals predict the final earthquake size

SUMMARY When a seismic rupture starts, the process may evolve into multiple ways, generating different size earthquakes. Contrasting models have been proposed to describe the evolution of the rupture process while limited observations at the scale of real earthquake data are available, so that a unifying theory is still missing. Here we show that small and large earthquake ruptures are different before the arrest and they do not exhibit a common, size-independent, universal behaviour. For earthquakes with magnitude 4 < M < 9 occurred in Japan, we measure the initial rate of the P-wave peak amplitude and show that this quantity is correlated to the final event magnitude and not affected by distance attenuation, thus being a proxy for the initiation time of the rupture process. While opening new views on the rupture preparation process, our findings can have significant implications on the effective development of fast and reliable methods for source characterization and ground shaking prediction.