By Heidi Soosalu
Hekla is a volcano with special seismic characteristics. During non-eruptive periods it is virtually aseismic, and does not give any obvious long-term or intermediate-term precursory warning before eruptions.
The eruption-related seismicity starts only 30-80 minutes before its onset. Hundreds of small volcano-tectonic earthquakes (magnitude < 3), related to the intrusion of magma, occur during the first hours, when the eruption is violent and explosive. This seismicity soon calms down, together with the eruptive activity. Later the activity consists mainly of peaceful streaming of lava and occasional gas bursts. Only very few earthquakes occur after the onset hours.
Volcanic tremor – continuous low-frequency vibration of the ground – starts simultaneously with the eruption, and continues throughout it. It is by far most vigorous during the explosive onset, and decreases together with the settling eruptive activity.
The few earthquakes at Hekla and its immediate neighbourhood during the non-eruptive times are typically small (magnitude < 2), and not related to the Hekla volcano itself. Rather they follow a same kind of distribution as the events of the Mid-Atlantic transform, South Iceland seismic zone, west of it. They are grouped loosely to two north-south lineaments similar to the seismic zone faults, and occur mainly at the depths of 8 – 12 km, like the earthquakes in the eastern end of the seismic zone.
Thus Hekla has a dual nature in the seismic sense: on one hand the seismicity is ruled by the tectonics of the South Iceland seismic zone, and on the other hand by the volcano's internal processes. The seismic zone is the dominant factor during the non-eruptive periods.
The volcano-related seismicity of Hekla occurs primarily in the beginning of the eruptions. Numerous small earthquakes are produced when the general stress field adjusts to intruding magma and to opening of the eruptive fissures. However, the most persistent volcano-related seismic signal at Hekla is the volcanic tremor that starts when the magma reaches the surface and continues until the end of the eruption. The earthquakes during the later phases of the eruption are few and, again, tend to follow the seismicity pattern of the South Iceland seismic zone.
Einarsson P, Björnsson S (1976) Seismic activity associated with the 1970 eruption of volcano Hekla in Iceland. Jökull 26:8-19
Soosalu H, Einarsson P (1997) Seismicity around the Hekla and Torfajökull volcanoes, Iceland, during a volcanically quiet period, 1991-1995. Bull Volcanol 59:36-48
Soosalu H, Einarsson P (2002) Earthquake activity related to the 1991 eruption of the Hekla volcano, Iceland. Bull Volcanol 63:536-544
Soosalu H, Einarsson P, Jakobsdóttir S (2003) Volcanic tremor related to the 1991 eruption of Hekla volcano, Iceland. Bull Volcanol 65:562-577