Long-period seismic events with strikingly regular temporal patterns on Katla volcano's south flank (Iceland)

Abstract

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.

The Katla volcanic system imaged using local earthquakes recorded with a temporary seismic network

“…The nearly monochromatic low-frequency events at the southern glacier rim were located at a very shallow depth by Sgattoni et al [2016]. Their locations in our 3-D velocity model are almost identical.…”

“…As these events are clustered in a small shapeless distribution, they appear not associated with either a vertical or a horizontal structure, e.g., like a fault. These events are more likely associated with a volcanic or hydrothermal process than a glacial one, based on the regular and stable seismicity observed over several years and their location beneath thin and stagnant ice [ Sgattoni et al ., ].…”

“…Following a tremor burst on 8–9 July 2011, a new epicentral zone emerged on the southern rim of the glacier near Gvendarfell Rise. The Gvendarfell cluster also reveals clear seasonal behavior with a peak in seismicity in July/August [ Sgattoni et al ., ].…”

Tectonic and Anthropogenic Microseismic Activity While Drilling Toward Supercritical Conditions in the Larderello‐Travale Geothermal Field, Italy

“…Additionally, the relatively narrow frequency content, high similarity, unclear wave arrivals, and periodic occurrence of the type‐2 events recall long‐period (LP) signals that have often been associated with fluid motion at depth (e.g., magma motion in magmatic reservoirs). For example, such features are strikingly similar to LP events recorded at Katla volcano, Iceland, where the occurrence of well machinery is unlikely (Sgattoni et al, ). However, the frequency content of type‐2 events (6–8 Hz) is higher than the one of typical LPs, which is normally lower than 5 Hz (Figure S5).…”

Short-Term Seismic Precursors to Icelandic Eruptions 1973–2014

“…The 2011 event was studied in considerable detail. It was preceded by a peculiar sequence of small earthquakes that clustered on the southern flank of the volcano and continued for several years after the event (Sgattoni et al, 2016). Two kinds of contiuous tremor were identified, one associated with the water flood that issued from the glacier, the other originated near the two ice cauldrons where the flood originated (Sgattoni et al, 2017).…”