Repetitive fracturing during spine extrusion at Unzen volcano, Japan

O D Lamb,J E Kendrick,Y Lavallée,K Umakoshi,A J Hornby,S De Angelis

doi:10.5194/se-6-1277-2015

Abstract

Abstract. Rhythmic seismicity associated with spine extrusion is a well-documented phenomenon at a number of dome-forming volcanic systems. At Unzen volcano, Japan, a 4-year dome-forming eruption concluded with the emplacement of a spine from October 1994 to February 1995, offering a valuable opportunity to further investigate seismogenic processes at dome-forming volcanoes. Using continuous data recorded at a seismic station located close to the dome, this study explores trends in the seismic activity during the extrusion of the spine. We identify a total of 12 208 volcano-seismic events in the period between October 1994 and February 1995. Hourly event counts indicate cyclic activity with periods of ∼ 40 to ∼ 100 h, attributed to pulsatory ascent defined by strain localisation and faulting at the conduit margins. Waveform correlation revealed two strong clusters (a.k.a. multiplets, families) which are attributed to fracturing along the margins of the shallow, ascending spine. Further analysis indicates variable seismic velocities during the spine extrusion as well as migration of the cluster sources along the spine margins. Our interpretation of the results from seismic data analyses is supported by previously published field and experimental observations, suggesting that the spine was extruded along an inclined conduit with brittle and ductile deformation occurring along the margins. We infer that changes in stress conditions acting on the upper and lower spine margins led to deepening and shallowing of the faulting sources, respectively. We demonstrate that the combination of geophysical, field and experimental evidence can help improve physical models of shallow conduit processes.

Highlights

Lava dome growth and collapse represent a major volcanic hazard globally (Sparks, 1997)
We present a detailed characterisation of the seismicity associated with the extrusion of a spine during the late stage of the 1991–1995 dome-building eruption at Unzen volcano, Japan
Single-station detection with a short-term average/long-term average (STA/LTA) algorithm detected a total of 12 208 volcano-seismic events between 1 October 1994 and 28 February 1995 at Unzen volcano (Fig. 1a)

Summary

Introduction

Lava dome growth and collapse represent a major volcanic hazard globally (Sparks, 1997). The transition from effusive to explosive activity of a dome may be rapid and can generate destructive pyroclastic flows (Calder et al, 2002). This range of behaviour presents a significant challenge for forecasting and hazard mitigation, for example, the 1991–1995 eruption at Unzen, Japan Continuous monitoring of growing and unstable lava domes is central to risk mitigation. The movement of magma during domebuilding eruptions is a rich source of geophysical signals, earthquakes. Analysis of shallow (< 3 km) volcanic earthquakes has highlighted the importance of discerning their characteristics for the purpose of hazard assessment (Neuberg, 2000; Chouet and Matoza, 2013)

Results

Discussion

Conclusion