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We explore the ambient noise wave field recorded on different optical fibers interrogated during distributed acoustic sensing (DAS) campaigns in Iceland and Germany. DAS is a relatively new technology that allows for measurements of acoustic strain signals at densely spaced observation points along buried optical fibers. Ambient noise interferometry, in particular single-channel auto-correlation, is potentially a powerful technique for resolving the structure of the subsurface medium and characterizing its properties below a seismic sensor. Combining these two techniques to extract the ambient noise auto-correlation functions from horizontal strain rates all along the densely spaced locations on the fiber we obtain coherent signals that correlate over multiple observation points. We are challenged by the separation of the super-positioned non-stationary ambient noise field and the seismic response of the subsurface. We test our processing workflow and results against independent methods. (1) A controlled vibro-seis experiment and (2) seismic ambient noise analysis of a large-N three-component seismic nodal array which was co-located with the optical fiber in Iceland., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

We present the detection and localization of local seismic events from a seismic network consisting of more than 350 short-period and broadband stations deployed in the Eifel volcanic region, centered around the Laacher See. The network spans an area of 180 km by 120 km. Using a combination of stacking and migration method and characteristic function generated by a re-trained neural network which are imaging P and S wave phase arrivals, we are able to detect and automatically locate seismicity in the network from continuous waveform data, including those events associated with dormant volcanic processes. The Eifel region has been geologically active for millions of years, with the most recent volcanic explosive eruption 13,000 years ago at the Laacher See. While the area is currently considered volcanic dormant, the ongoing seismic activity provides insight into the magmatic activity at depth. Our results can help to improve our understanding of the transcrustal magmatic system beneath intraplate volcanic fields and potential for future volcanic activity in the Eifel. The method and example presented here can be applied to volcanic provinces elsewhere support volcanic monitoring and fast, automatic processing of seismicity., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

On February 6, 2023, southeastern Turkey was hit by two of the most devastating earthquakes in the instrumental period of the country, with Mw 7.8 and Mw 7.5, respectively. Both earthquakes caused massive damage in Turkey and Syria. In this study, we analyze the rupture processes of main- and aftershocks by combining different seismic source characterization techniques using teleseismic, regional, and local data. We perform finite source inversion and back projection-based analyses for the two main shocks and invert for probabilistic centroid moment tensor solutions of both main and aftershocks (M≥4). The first earthquake was bilateral and ruptured a seismic gap along the East Anatolian Fault, with rupture first propagating to the north-east for ~200 km, and in a latter phase propagating to the SSW, probably coming to a halt only on a branch extending into the Mediterranean Sea. The total length of the rupture likely exceeds 500 km. The second event ruptured the EW-oriented Sürgü-Misis Fault to the NW of the first event. It shows a highly concentrated rupture near the epicenter, Rupture directivity analyses for M≥5.3 earthquakes provide additional insights into dynamic source aspects. Preliminary moment tensor solutions of numerous aftershocks indicate a remarkable variability of rupturing mechanisms, suggesting stress changes and the activation of multiple faults in the vicinity of the main ruptures. With our work, we aim to shed light onto multiple aspects of the complex rupture evolution and hope to provide new insights towards a better understanding of the devastating 2023 Türkiye earthquake sequence., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

The GFZ organizes annually International Training Courses on “Seismology, Seismic Data Analysis, Hazard Assessment and Risk Mitigation”. They provide theoretical fundamentals and practical training for geo-scientists and engineers from countries in Asia, Africa, and Latin America. The courses are being supported by the German Federal Foreign Office and are a contribution to the global UN Sendai Framework for Disaster Risk Reduction (UNDRR).In line with the growing demand by participants in former courses for mainly practice-oriented training and workshop discussions related to case studies, the course program comprises, besides introductory and state-of-the-art review lectures on the various subjects of earthquake seismology and risk assessment, extensive practical exercises, demonstrations, workshop discussions and scientific excursions. Generally, the course program aims at developing interdisciplinary problem understanding, acquaintance with the theoretical fundamentals and basic features of modern instrumentation, commonly used models and algorithms as well as developing practical skills in data evaluation and analysis. The courses are being announced through the GFZ website, the EMSC, and IRIS as well as printed information. Additional ad-hoc sponsors are mainly from the hosting institutions and governments in case of regional courses in Asia, Africa, or Latin America.Since 1992 these 4-week courses are held alternately as world-wide open courses in Potsdam or as regional courses in earthquake-prone countries in Asia, Africa, or Latin America (2006 Kyrgyzstan, 2008 Costa Rica, 2010 Turkey, 2012 Morocco, 2014 Colombia, 2016 Myanmar, and 2018 Ghana). Many of the former course participants became project partner in ongoing research projects of German and European institutions., 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)

The ICDP project "Drilling the Eger Rift" focuses on geodynamic processes in W-Bohemia and Vogtland, such as earthquakes and subsurface fluid flows. Therefore, three boreholes have been drilled and will be instrumented with 3D seismic arrays. The pilot 3D array has 10 borehole geophones with a 10Hz corner frequency and is surrounded by a surface array with 12 4.5Hz geophones. The data is recorded by Earth Data Loggers with a sampling rate of 1000Hz.The goal is to locate seismic noise sources up to a distance of 15km that may be linked to fluid migration in W-Bohemia. We analyze continuous seismic noise records of more than 20 stations from regional networks and our seismic array. Differential PSDs were calculated in different frequency ranges to find continuous tremor-like seismic sources before the earthquake swarm in April 2021. Our next step is to locate these sources using regional networks and the 3D seismic array.The analysis revealed an increase in seismic noise between 4-8Hz for a 2-hour period on April 5th, 2021, one day before the earthquake swarm started. Seven stations in the region show a median differential PSD level at least three times higher than the differential PSD level for the entire day. These stations are located up to a distance of 12km to the Nový Kostel focal zone, where most of the earthquakes occurred. The increase is also compared to Radon concentrations measured at Hartoušov (W-Bohemia) to find possible coincidence., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)