Mars Polar Science 2000 4072.pdf THE KATLA SUBGLACIAL VOLCANO: EARTHQUAKES AND EVIDENCE FOR CRUSTAL MAGMA CHAMBERS. Páll Einarsson and Bryndís Brandsdóttir, Science Institute, University of Iceland, Hofsvallagata 53, 107 Reykjavík, Iceland ([email protected], [email protected]) Seismological methods can be used effectively to sidence cauldrons were formed in the ice within the study the internal structure and dynamics of volca- caldera on both occasions. noes. Hypocenters of earthquakes provide information The most dramatic evidence for prehistoric activity on the location of physical events, focal mechanism of Katla is a layer from about 12 000 ybp found in the studies give information on their nature. Seismic GRIP ice core, thought to correspond to Ash Zone 1 in waves that have propagated through the roots of vol- Atlantic sediment cores and a deposit formed by la- canoes carry information on the physical state of the hars, ash fall and surges on the south flank of the vol- material there, e. g. location of magma chambers. cano [9], [10]. Ash layers from Katla have also been Eruptive activity is usually accompanied by seismic identified in the GRIP core at 75 400 and 77 500 ice activity which can give invaluable information on core years [9]. processes in the crust s.a. magma migration. Several Katla and the surrounding volcanic area has of the Icelandic central volcanoes, including the sub- shown persistent high seismic activity for more than glacial Katla volcano in South Iceland, are the source four decades. No volcanic activity has been detected of persistent seismic activity in spite of little other during this time, however, with the exception of apparent signs of volcanic unrest [1], [2]. Several probable small subglacial eruptions in 1955 and 1999. physical processes can cause these earthquakes, s. a. Seismic data of the period 1978-1985 are anlysed slow inflation or deflation of a shallow-level magma here. Epicenters of this period fall into two distinct chamber, intrusion of magma into the shallow crust, clusters under the Mýrdalsjökull glacier. One cluster or thermal cracking due to heating or cooling of brittle is located within the caldera of the Katla volcano, the crustal material. other is centered in the Goðabunga area, west of the The Katla volcano is an off-rift volcano, covered caldera and beneath the western caldera rim. All by the glacier Mýrdalsjökull, and located in the vol- available data are consistent with hypocentral depths canic flank zone in South Iceland. It has produced of 0-4 km. Earthquakes of both clusters have distinct mainly transitional alkalic basalt lavas and tephra [3], characteristics of volcanic earthquakes, i. e. emergent [4], but more silicic rocks are known from nunataks in P-waves and poor S-waves. These characteristics are the glacier [5]. A caldera, 10 km x 13 km in diameter, more pronounced for the Goðabunga cluster. The co- mapped beneath the glacier by radio echo sounding incidence of the Katla cluster with the caldera and an (Björnsson et al., this conference), is underlain by a area of S-wave shadowing suggests that it is related to zone of low P-velocity and high S-wave attenuation an active magma chamber of the Katla volcano. interpreted as a magma chamber at a depth of less Similarly we take the Goðabunga cluster as an indi- than 3 km [6]. The Eldgjá fissure to the north is cator of the existance of a second magma chamber. It structurally connected to the Katla volcano, and the is a matter of definition whether this chamber repre- similarity of the chemistry of Katla and Eldgjá lavas sents a separate central volcano or a second magma has led to the suggestion that Eldgjá eruptions were chamber of the Katla volcano. The two clusters to- fed by lateral flow from the Katla magma chamber [7]. gether with the Eyjafjallajökull volcano to the west The Eyjafjallajökull volcano to the west of Katla has a and its fissure swarm form a pronounced E-W struc- distinct, E-W striking fissure swarm that merges with ture and appear to be mechanically coupled. Two his- the radial fissure system of Katla. The volcanoes thus torical eruptions of Eyjafjallajökull are temporally appear to be tectonically connected. related to Katla eruptions. Earthquakes under Katla eruptions during the last 1100 years are Goðabunga show a clear seasonal correlation. Earth- relatively well known from historical records [8]. At quakes are several times more likely to happen in the least 17 eruptions occurred in this time, the latest latter half of the year than in the first [11]. Two fac- large ones in 1823, 1860 and 1918. These eruptions tors are likely to be effective here, the load of the gla- were accompanied by substantial jökulhlaups from the cier ice and the pore pressure in the crust beneath the glacier. Small subglacial eruptions appear to have glacier. Both factors would tend to reduce the friction occurred in 1955 and 1999 as suggested by seismicity, on fault planes in the autumn when melt water seeps both earthquakes and volcanic tremor recorded prior into the crust and increases the pore pressure. We ar- to small jökulhlaups. No eruption was visible but sub- gue that the pore pressure produces a larger effect than the change in the ice load. Mars Polar Science 2000 4072.pdf KATLA SEISMICITY: P. Einarsson and B. Brandsdóttir References: [1] Tryggvason, E. (1973) Bull. Seismol. Soc. Am., 63, 1327-1348. [2] Einarsson, P. (1991) Tectonophys., 189, 261-279. [3] Jakobsson, S. P. (1979) Acta Nat. Isl., 26, 103 pp. [4] Steinþórsson, S. et al. (1985) JGR, 90, 10,027-10,042. [5] Jóhannes- son, H. et al. (1982) Museum of Nat. Hist. and Iceland Geod. Surv. [6] Guðmundsson, Ó. et al. (1994) Geo- phys. J. Int., 119, 277-296. [7] Sigurdsson, H., and S. Sparks (1978) Nature, 274, 126-130. [8] Þórarinsson, S. (1975) Ferðafélag Íslands, Árbók, 125-149. [9] Grönvold, K. et al. (1995) Earth Planet. Sci. Lett., 135, 149-155. [10] Lacasse, C. et al. (1995) Bull. Volcanol., 57, 18-32. [11] Brandsdóttir, B., and Ei- narsson, P. (1992) In: Volcanic Seismology (eds. P. Gasparini and K. Aki), Springer Verlag, 212-222..