Fig. 1: Aso volcano system and observations of synchronous VLP and tilt offset.

a The topographic relief near the Aso caldera is overlaid with collocated seismometer and tiltmeter (open triangles), VLP source in the shallow crack-like conduit²⁹ (thick bar), the geodetically inferred magma chamber^45,46 (open circle), and the low-velocity zone⁴⁷ (dashed circle). BCU and BYA chambers are inferred from levelling data in 1993–1994 and 1993–2004, respectively⁴⁵. New Mogi chamber is inferred from GNSS (2004–2008) and levelling data (1998–2008)⁴⁶. The star marks the active Naka-dake first crater. The upper inset displays Aso volcano located in Kyushu Island, southwest Japan. The source of the tilt offset is marked as a red cross, corresponding to a shallow magma storage zone (SMSZ) connected to the top of the chamber roof. The beach ball displays the deviatoric component of the source moment tensor. The depths of BCU, BYA, new-Mogi, low-velocity zone, and crack-like conduit can be referred to Fig. 3c. b Tilt waveforms at four V-net stations near the October 8, 2016 phreatomagmatic eruption. The waveforms are low-pass filtered at 0.05 Hz with a 4^th-order casual Butterworth filter. The linear trend determined in the first 10 min is used to remove the background trend. “LE” and “LN” denote eastside and northside down tilts, respectively. “Event 1” and “Event 2” denote the tilt offset that occurred concurrently with two VLPs. The red line marks the timing of the 2016 eruption.