A Synchronised Deep Source and Repetitive triggering of Long-Period Tremors in Aso volcano, Japan: implications/assessments on magmatic processes and upcoming eruptions

In the southwest Japan, long-period tremors (LPTs) in Aso volcano have been observed since the pioneering work by Sassa (1935) and more recently by Kawakatsu et al (2000). LPTs typically have a resonance period of ~15 seconds and they are repetitive and time-invariant in their location and mechanism. While surface degassing/eruption and magmatic heating (or hot gas) are often invoked to trigger LPTs, seismic observation indicative of direct triggering is lacking. Recently, Niu and Song (2018, AGU) analysed 6 year continuous data from broadband seismometers and borehole tiltmeters of V-net in Aso volcano. Focusing in the ultra long-period band (> 100 sec), they identified a repetitive, synchronised deep source triggering LPTs. Moment tensor inversion locates the deep, predominantly isotropic source at about 3.2 km depth, just between the LPT source at sea level and the inferred magma chamber at about 6 km depth. Zhou, Niu and Song (V056, this meeting) also demonstrates excitation of long-period (~100 sec) Rayleigh waves by F-net that corroborates this finding.

Repetitive triggering of LPT by the deep source suggests small (~1000-4000 m³ per event), but frequent magma or/and gas ascent, with the deep source region potentially serving as a buffer between the LPT source and the magma chamber. The duration of the deep source event is typically ~100 sec and the volume change rate associated with each deep source is typically on the order of 10-40 m³/s. However, two events minutes prior to the 2016 phreatomagmatic eruption result in volume change of 2.25x10⁵ - 3.37x10⁵ m³ per event, and the volume change rate increases up to about 7500-11250 m³/s. These estimates are 2-4 order of magnitude larger than average volume change rate over longer time scale, but apparently the same order of magnitude as the magma discharge rate inferred from the plume height. Concerning a conduit radius of 10 m, the inferred magma ascent velocity prior to the 2014 Strombolian eruption and 2016 phreatomagmatic eruption is about 0.03-0.12 m/s and 20-35 m/s, respectively, contemplating different flow regimes and eruption styles. Finally, in the deep source region, accumulative volume changes prior to the 2014 eruption and during the inter-eruption periods of the 2015 and 2016 eruptions are apparently in general agreement with the estimates from tephra deposits.