Using lower crustal earthquakes below Askja volcano, central Iceland to map melt movements and plumbing geometry

Abstract

During summer 2006 approximately 100 lower crustal earthquakes (13 - 27 km depth) were recorded around Askja volcano on the plate spreading boundary in central Iceland, using a local array of 20 broadband seismometers. Continuous measurements from summer 2007 to summer 2009, using both temporary and permanent stations, have identified many more such earthquakes. In total over 600 events, with local magnitudes of 0 to 1.7, are located within this normally aseismic ductile region. The brittle-ductile boundary in the Askja region is well defined by the sharp lower cut-off of persistent upper crustal seismicity at depths of 5 - 7 km. Tension axes from fault plane solutions of the upper-crustal earthquakes show a close alignment with the local plate spreading direction of 106°, indicating that plate spreading dominates the stress field in the upper crust. The lower crustal earthquakes are distinctly different in appearance to the upper crustal earthquakes with a lower frequency content and generally emergent P- and S-phases. There is also a clear mid crustal gap between the upper and lower crustal seismicity. The lower crustal events generally occur in swarms lasting several minutes, with hypocentres from a single swarm located close to one another. These earthquakes can be divided into three main clusters, the largest of which is located at the northern boundary of the youngest caldera and adjacent to a geodetically proposed magma chamber, extending away 10 km to the north east, along the fissure swarm. The second cluster is located 20 km further to the north east, between a shield volcano and a subglacially erupted table mountain. The third cluster is 10 km east of Askja, north of another shield volcano. The largest cluster can be subdivided into two smaller sub-clusters, each of them semi-vertical, extending over a depth range of about 8 km with the slightly shallower of the sub-clusters closer to the caldera. We propose that these lower crustal earthquakes are related to the plumbing system of the volcano and that they are caused by magma movements. We suggest that high strain rates at the tips of propagating dikes allow these earthquakes to be generated within a normally aseismic lower crustal region.