Katla is a threatening volcano in Iceland, partly covered by the Mýrdalsjökull ice cap. The volcano has a large caldera with several active geothermal areas. A peculiar cluster of long-period seismic events started on Katla's south flank in July 2011, during an unrest episode in the caldera that culminated in a glacier outburst. The seismic events were tightly clustered at shallow depth in the Gvendarfell area, 4 km south of the caldera, under a small glacier stream on the southern margin of Mýrdalsjökull. No seismic events were known to have occurred in this area before. The most striking feature of this seismic cluster is its temporal pattern, characterized by regular intervals between repeating seismic events, modulated by a seasonal variation. Remarkable is also the stability of both the time and waveform features over a long time period, around 3.5 years. No comparable examples have been found in the literature. Both volcanic and glacial processes can produce similar waveforms and therefore have to be considered as potential seismic sources. Discerning between these two causes is critical for monitoring glacier-clad volcanoes and has been controversial at Katla. For this new seismic cluster on the south flank we regard volcano-related processes as more likely than glacial ones for the following reasons: 1) the seismic activity started during an unrest episode involving sudden melting of the glacier and a jökulhlaup; 2) the glacier stream is small and stagnant; 3) the seismicity remains regular and stable for years; 4) there is no apparent correlation with short-term weather changes, such as rain storms. We suggest that a small, shallow hydrothermal system was activated on Katla's south flank in 2011, either by a minor magmatic injection or by changes of permeability in a local crack system.

Katla is one of the most active subglacial volcanoes in Iceland. A temporary seismic network was operated on and around Katla for 2.5 years. A subset of ~800 analyzed local earthquakes clustered geographically in four regions: (1) the caldera, (2) the western region, (3) the southern rim, and (4) the eastern rim of the glacier. Based on the frequency content of recorded seismograms, each event was labeled as volcano tectonic (VT), long period (LP), or ‘Mixed’. The southern cluster consists of LP events only, and the eastern cluster consists of VT events, while the western cluster is ‘Mixed’ although primarily LP. The caldera seismicity is confined to a subregion centered in the northeastern part of the caldera above 1 km below sea level (bsl) and gradually deepens away from its center to about 4 km depth. Deeper events are almost all VT, whereas LP events in the center of caldera locate at shallow depths. This is also where the velocities are lowest in the top 3 km of the crust of our 3‐D tomographic model. A high‐velocity core (~6.5 km/s) is found at 4 km bsl beneath this low‐velocity zone. We propose that a “subcaldera” may be developing within the present caldera and suggest a conceptual model for Katla volcano with a thin volume (~1 km thick) that may host hot rhyolitic material in the shallow crust below the relocated seismic activity and above the high‐velocity core. We interpret this core to consist of mafic cumulates resulting from fractionation of mafic intrusions and partial melting of subsiding hydrothermally altered rocks.

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Ocean-bottom seismographs (OBSs) are used to obtain seismic recordings offshore and are an increasingly important tool for investigating the globe. However, because OBS data cannot be time stamped using Global Positioning System (GPS) during deployment, correction for drift of the internal clock is required. This time drift is typically derived by synchronizing the clock before and after deployment. Linear correction is then applied using the timing deviation between GPS and the instrument’s internal clock at recovery, that is, the skew measurement. If synchronization measurements are missing, ambient noise cross-correlation functions (CCFs) are commonly used for time correction. When investigating recordings from a small-scale OBS network located on the Mohn’s mid-ocean ridge, we observed a remaining drift on the skew-corrected data. After recalculating the drift of the raw data using CCFs, we found that the skew-based time correction was incorrect. This was also verified with the observation of teleseismic P-wave arrivals. We describe a method to obtain properly time-corrected data and discuss the OBS timing issues in detail. The results shown were obtained using a software package that we developed for this specific purpose and made available as open-source software. Although we cannot explain the technical reason for the failure of skew correction, this study shows that skew corrections should not be trusted alone, and OBS timing should always be verified by either ambient noise correlations or P-wave arrival times.