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Spine growth and seismogenic faulting at Mt. Unzen, Japan.

Hornby, Adrian J. and Kendrick, Jackie E. and Lamb, Oliver D. and Hirose, Takehiro and De Angelis, Silvio and von Aulock, Felix W. and Umakoshi, Kodo and Miwa, Takahiro and Henton De Angelis, Sarah and Wadsworth, Fabian B. and Hess, Kai-Uwe and Dingwell, Donald B. and Lavallée, Yan (2015) 'Spine growth and seismogenic faulting at Mt. Unzen, Japan.', Journal of geophysical research : solid earth., 120 (6). pp. 4034-4054.


The concluding episode of activity during the recent eruption of Mt. Unzen (October 1994 to February 1995) was characterized by incremental spine extrusion, accompanied by seismicity. Analysis of the seismic record reveals the occurrence of two dominant long‐period event families associated with a repeating, nondestructive source mechanism, which we attribute to magma failure and fault‐controlled ascent. We obtain constraints on the slip rate and distance of faulting events within these families. That analysis is complemented by an experimental thermomechanical investigation of fault friction in Mt. Unzen dacitic dome rock using a rotary‐shear apparatus at variable slip rates and normal stresses. A power density threshold is found at 0.3 MW m−2, above which frictional melt forms and controls the shear resistance to slip, inducing a deviation from Byerlee's frictional law. Homogenized experimentally generated pseudotachylytes have a similar final chemistry, thickness, and crystal content, facilitating the construction of a rheological model for particle suspensions. This is compared to the viscosity constrained from the experimental data, to assess the viscous control on fault dynamics. The onset of frictional melt formation during spine growth is constrained to depths below 300 m for an average slip event. This combination of experimental data, viscosity modeling, and seismic analysis offers a new description of material response during conduit plug flow and spine growth, showing that volcanic pseudotachylyte may commonly form and modify fault friction during faulting of dome rock. This model furthers our understanding of faulting and seismicity during lava dome formation and is applicable to other eruption modes.

Item Type:Article
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Publisher statement:Hornby, A. J., J. E. Kendrick, O. D. Lamb, T. Hirose, S. De Angelis, F. W.von Aulock, K. Umakoshi, T. Miwa, S. Henton De Angelis, F. B. Wadsworth, K.‐U. Hess, D. B. Dingwell, and Y. Lavallée (2015), Spine growth and seismogenic faulting at Mt. Unzen, Japan. Journal of Geophysical Research: Solid Earth, 120, 4034-4054. doi: 10.1002/2014JB011660. To view the published open abstract, go to and enter the DOI.
Date accepted:27 April 2015
Date deposited:31 May 2018
Date of first online publication:01 June 2015
Date first made open access:No date available

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