Tomographic images and 3D earthquake locations of the seismic swarm preceding the 2001 Mt. Etna eruption: Evidence for a dyke intrusion

Abstract

On July, 12, 2001, Mt. Etna experienced a sudden increase of seismic activity heralding one of the most intense eruptions of the past 30 years. Between July 12 and July 18, when the eruption started, thousands of small magnitude earthquakes occurred and were recorded by a dense seismic network run by the Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Catania (INGV‐CT). Hypocentral depths of earthquakes were very shallow, mostly located above 3 km b.s.l. and clustered near the summit area. The high quality seismic dataset gives us the unique opportunity to study the process of magma migration before the eruption. In this study we present the three‐dimensional earthquake locations and the velocity structure obtained by a tomographic inversion. The shallowness of seismicity allowed us to enhance the details of the structure beneath the summit craters, in a volume poorly defined by previous tomographic studies. The presence of a high Vp‐body previously observed at Mt. Etna is confirmed at shallow depth beneath the southeastern part of the summit area. The earthquakes preceding the eruption onset concentrated at its western border. A low Vp/Vs anomaly is found at 0–1 km depth, just at the top of the volume where the magma intruded before the eruption. This anomalous zone can be considered as molten material wealthy in gas. The relocated seismicity occurs in a cylinder below the vents activated along the fracture system and exhibited an upward migration until the eruption. All these results show evidence for the emplacement of a near‐vertical dyke striking about N‐S and a few kilometres south of the summit craters.

Relating seismic velocities, thermal cracking and permeability in Mt. Etna and Iceland basalts

“…These results clearly demonstrate that lava-flow basalt from Etna contains a much higher level of crack damage than the columnar basalt from Iceland that is formed in an intrusive environment. Our results can explain the low seismic velocities (approximately 3-4 km/s) inferred for basaltic volcanic edifices, which are essentially formed from piles of lava flows, and the higher values (approximately 5-6 km/s) observed for intrusive cooled magma bodies, such as dykes emplaced in the volcanic edifice [9,10,20,21].…”

“…Changes in P-and S-wave velocity have been interpreted as being due to cooling intrusive bodies or molten rocks (e.g. [9,10]). However, despite the improvement in local seismic tomography techniques, no corresponding direct measurements are available, and there is a paucity of quantitative experimental data.…”

Seismic tomography of Central São Miguel, Azores

“…Coso geothermal system [Wu and Lees, 1999;Sanders et al, 1995], and low-density caldera infilling materials (e.g. Taupo volcanic zone [Sherburn et al, 2003] [Nicholson and Simpson, 1985;Bonner et al, 1998;Patané et al, 2002]. In spite of that, in volcanic environments high Vp/Vs values at shallow depths can be generally associated with saturated highly fractured materials (as in Vesuvio [Scarpa et al, 2002] and Kilauea [Hansen et al, 2004]).…”

Source Separation on Seismic Data: Application in a Geophysical Setting

“…The lower regions of the model were constructed from the tomographic model of [12]. As tomography models are known to be poorly constrained near the Earth's surface, a low P-wave velocity near surface gradient ranging from 1600m/s at the surface to 3000m/s at 500m depth was added to the model.…”