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Changes in seismic complexity could reveal the dynamic changes inside a volcanic system prior to an eruption, which can be utilized to improve eruption forecasting. Permutation Entropy (PE) is a robust method to quantify the complexity of a time series, by calculating it directly using the amplitude. Examples of successful PE applications are eruptions forecasting at Strokkur Geyser, Iceland, the 1996 eruption of Gjálp, Iceland, and the eruptions of Shinmoedake volcano, Japan, in 2011, 2017, and 2018. While PE could show temporal changes prior to an eruption, these features are not always prominent. To improve this method, we calculated not only PE by using seismic amplitudes, but also Phase Permutation Entropy (PPE) by using seismic instantaneous phases. We calculated both PE and PPE for seismic station FLUR from January 2014 to December 2015, which covers the eruption period of Holuhraun in Iceland. During the unrest period, both PE and PPE show changes in entropy, but their patterns are not always the same. While PE shows a strong increase which marks the onset time of dyke propagation two weeks before the main eruption, it does not show a clear transition toward the main eruption. In contrast, PPE only shows a gradual change when the dyke started migrating. Jowever, it displays strong changes at the possible subglacial eruptions on 23 August, the small eruption on 29 August, and the main eruption on 31 August 2014. This proves the capability of PE and PPE to contribute to the framework of eruption forecasting., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

A powerful seismic sequence struck the Turkey-Syria border on Feb. 6 with three earthquakes having M>6.5. At 01:17 UTC, a M7.8 earthquake occurred that was followed 11 minutes later by a M6.7, and around nine hours later by a M7.5. In the first 24 hours, 20 aftershocks with M>5 and 68 events with M>4 have been registered. Recently, we have improved and tested a Bayesian simulation-based workflow for spatio-temporal early seismicity forecasting based on ETAS model. It exploited the versatility of the Bayesian inference to adaptively update the forecasts based on the incoming information from the ongoing sequence. This workflow is demonstrated and verified through retrospective early seismicity forecasting of Central Italy 2016 and the 2017-2019 western Iran seismic sequences. We test this workflow to predict the spatial distribution of events and their uncertainties for various forecasting intervals within this seismic sequence. Bayesian updating is first employed to learn the ETAS model parameters conditioned on the registered events (that already took place). Then, plausible sequences of events during the forecasting interval are adaptively generated. To this end, we strive to simulate those plausible sequences by embedding a branching process formulation inside the proposed workflow as an alternative to the piece-wise stationary integration of the conditional rate. The latter could be a new feature to the forecasting workflow, while its efficiency in providing early forecasts is explored during this study. This work has been supported by PRIN-2017 MATISSE project No 20177EPPN2, funded by Italian Ministry of Education and Research., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

The emplacement of fresh magma at depth should be detectable with an appropriate fluid monitoring strategy. Eifel volcanic fields are accompanied by hundreds of CO2-rich springs and mofettes. Fluids can ascent at velocities of several 100 m/d through the crust, leading to estimated travel times of several months from the lower crust to the surface. Thus, online monitoring is mandatory to catch potential fluid transients related to magmatic processes at depth – in addition to repeated sampling campaigns for the isotopic composition of the gases. Monitoring targets are fluid emission sites with high CO2 concentrations and increased mantle helium signatures. In November 2020, real-time fluid monitoring started with focus on the East Eifel. As of today, ten sites are operative including abandoned CO2 wells, mofettes, CO2-rich springs, as well as a coldwater geyser in the West Eifel. Wellhead pressure or water level are measured at sampling rates up to 1 Hz, while water temperature, conductivity, and radon concentration are measured in 1 to 5 minute intervals. We report on short-term (several days) and long-term (several years) pressure transients in the fluid system. The latter are tentatively related to the recovery of a formerly exploited CO2 reservoir in the East Eifel as predicted from a physical model capable to explain observed local ground deformation above the reservoir. The presented examples demonstrate the need for a synoptic interpretation of meteorological, hydrogeological, geophysical, and geodetic data to provide the basis for an assessment of the activity state of the Eifel volcanic fields., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

We develop a simple fracture mechanics model for the stable growth of a penny-shaped shear crack under friction. We assume equilibrium between released strain energy and frictional energy and suggest a stability criterion that combines the tip stress intensity in fracture mechanics with the threshold criterion of frictional strength in a Coulomb failure approach. Fracture growth causes a slow but steady buildup of stress. On a larger fault, patches of increased shear stress may develop with the time to failure and the size of the patch depending on the background stress and the constitutive relations. The model predicts a self-regulating feedback loop that keeps stress intensity low until a strength threshold is exceeded at the point of high stress. At this point, instability occurs and a rupture nucleation phase may start culminating in a co-seismic rupture. The available fracture energy scales with the third power of the fracture length, while the slip on the fracture is a linear function of the fracture length.The new model may explain how a slow preparatory phase transitions into rupture nucleation. It suggests that large earthquakes tend to develop on faults with intermediate stress levels. Faults with very low stress may not produce stress asperities and remain stable, while faults with very high stress tend to produce smaller earthquakes after a short preparation period., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

In the past decades, the Epidemic Type Aftershock Sequence (ETAS) model has been widely used for describing the occurrence and clustering of earthquakes in space and time. Because of large depth uncertainties in the past, this model is two-dimensional and uses only epicentral information. Current earthquake catalogs provide high-precision depth values that contain valuable information. Thus, we extend the spatiotemporal ETAS model to 3D versions exploiting the full hypocentral information of earthquake catalogs by incorporating a depth-dependent kernel into the trigger function. To explore its most appropriate parametric form, we first examine different triggering functions for the depth difference of the aftershock-mainshock pairs identified by the Nearest-Neighbor (NN) method, showing that a magnitude-dependent power-law kernel fits best the earthquakes data in Southern California. Therefore, in order to incorporate the earthquakes' depth information in the ETAS model, we extend the established 2D spatial power-law kernel by using hypocentral instead of epicentral distances. Furthermore, in our extended 3D-ETAS model, we consider space-dependent background activity, where we additionally allow for depth-dependent aftershock productivity. A first application to data from Southern California shows a good performance of the 3D model. We also find that the aftershock productivity strongly depends on depth, similar to the seismic moment released by the mainshocks, which may be related to depth-dependent seismic coupling., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

We have recently developed a new physics-based seismicity model assuming pre-existing populations of faults that respond to changes in Coulomb stress. In contrast to the traditional Coulomb Failure (CF) model, which assumes instantaneous triggering when stress exceeds a threshold value, the new model replaces instantaneous triggering with an average time to failure that depends exponentially on the absolute stress value. The prediction for critical pre-stress conditions is identical to the widely used rate-state (RS) friction model. Thus, CF and RS predictions are both special cases of the new model. However, the new stress response model can also account for subcritical initial stress conditions, which are particularly relevant for induced seismicity in intraplate regions.Here we show the application of the new model to the Groningen gas field, which is one of the largest onshore gas fields in production in Europe and a frequently considered site for the study of induced seismicity. The long delay between the start of production and the onset of seismicity indicates subcritical pre-stresses. We estimate the pre-stress distribution based on the background stress field and the existing fault structure. Using this pre-stress distribution and the estimated temporal evolution of the 2D stress field in the reservoir, we show that the delayed onset, the peak in 2013-2015 and the recent decline in observed seismicity are well explained by the model., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)