ALBERT

Description: Hekla is one of the most active and dangerous volcanoes in Iceland presenting a high hazard to air travel and a growing tourist population. It is hence important to monitor its seismic activity in real-time. However, until now the pre-eruption warning time is only around one hour. A temporary seismic network deployed by us around Hekla summit in 2012 recorded unexpected background micro-seismicity (Eibl et al., 2014). Seismic monitoring directly on the edifice could provide a possible means to early-warning if micro-seismicity on Hekla increases prior to an eruption. In addition, the monitoring of a fissure eruption close up is expected to better understand how it initiates in detail. This prompted the installation of the Hekla Real-Time Seismic Network (HERSK) in 2018 (Möllhoff et al., 2018a/b). We experienced logistical difficulties especially in winter months, mainly in relation to power provision. In this project we build on the first phase of HERSK to (1) test novel ways of powering stations that transmit real-time data in very harsh environments and (2) to work towards a real-time event detection and location system dedicated to seismic activity at Hekla volcano. The development of the real-time system necessitates the derivation of a velocity model which we derive by inverting observed microseismicty data. This opens the way to image the internal structure of Hekla volcano. This research is part of project EUROVOLC that has received funding from the European Union’s Horizon 2020 research and innovation actions under grant agreement No 731070. Waveform data are available from the GEOFON data centre, under network code XE and embargoed until Jan 2025. The seismic waveforms were recorded during meter-scale hydraulic fracturing experiments in the Äspö Hard Rock Laboratory (HRL) in Sweden (Zang et al., 2017). This dataset only contains a subset of the data recorded during the experiments, monitored by a complementary monitoring system. The two seismometers contained in this dataset (A89 and A8B) were located in galleries adjacent/close to the injection borehole (see Fig. 2 in Niemz et al., 2021). The experiments were conducted at the 410m-depth level of the Äspö HRL. Each of the six experiments (HF1 to HF6) consisted of multiple stages with an initial fracturing and three to five refracturing stages (see injection parameters contained in this dataset). The six injection intervals were located along a 28m-long injection borehole. The borehole was drilled sub-parallel to the minimum horizontal compressive stress direction. The distance of the two seismometers to the injection intervals in the injection borehole is between 17 m and 29 m for sensor A89 and 52 m to 72 m for sensor A8B. A89 and A8B correspond to BB1 and BB2 in Niemz et al., 2021. For more details regarding the experimental setup, see Zang et al., 2017; Niemz et al., 2020; and Niemz et al., 2021. The records of the two seismometers show long-period transients that correlate with the injection parameters. These transients are the response of the seismometers to a tilting of the gallery floor. The extracted tilt time series provide independent insight into the fracturing process during the hydraulic stimulations (Niemz et al., 2021).