Subtle changes in strain prior to sub-Plinian eruptions recorded by vault-housed extensometers during the 2011 activity at Shinmoe-dake, Kirishima volcano, Japan

Earth, Planets and Space

Table 1. Observed changes in strain (unit: 10⁻⁹) during sub-Plinian eruptions (SP1–3) and estimated isotropic point source parameters, together with estimation errors due to possible uncertainties in the observed values^*1.

Event (duration)	Observed strain change			Estimated source parameters and errors
	E1 (×10⁻⁹)	E2(×10⁻⁹)	E3(×10⁻⁹)	Azimuth^*2 (deg)	Depth^*3 (km)	Volume change (× 10⁶ m³)
SP1 (15:30–18:50, Jan 26)	111±5	−68±5	25 ±5	+2.9±2.4	4.2±1.3	0.89±0.07
SP2 (01:50–05:00, Jan 27)	135±5	−84±5	30±5	+2.7±1.9	4.4±1.0	1.10±0.07
SP3 (16:30–17:50, Jan 27)	71±5	−44±5	16±5	+2.7±3.7	3.9±2.2	0.57±0.07
Magma effusion (Jan 28–30)	333±20	−239±20	39±20	+2.7±3.0	5.7±1.2	3.07±0.26

^*1Errors in observed values during magma effusion are assumed to be larger than those for SP1–3, as the duration of the effusive event was considerably longer than the duration of the sub-Plinian eruptions; this leads to changes in non-volcanic strain.
^*2Azimuth values are measured anticlockwise from the E1 direction.
^*3Depth and volume change values are estimated by assuming the horizontal distance to the source is 11.6 km, as inferred from GPS data analysis (GSI, 2011).