Magma intrusion and effusion at Sinabung volcano, Indonesia, from 2013 to 2016, as revealed by continuous GPS observation

We analyzed continuous Global Positioning System (GPS) data from Sinabung to capture and model the migration of magma from the pre-eruptive and syn-eruptive time period between June 2013 and January 2016. We divided this time into four periods of significant deformation: two extensional stages followed by two contractional stages. Using a grid search method, we determined the location and volume change of a Mogi source for each deformation stage. Cumulative volume changes during the contraction periods were approximated by an exponentially decaying function with time. Period 1 began in June 2013 with slight extension, for which an inflation source was modeled at a depth of 3–8 km below sea level (bsl) and a volume change of 0.3–1.8 Mm³. Seismicity in period 1 was marked by a notable increase in deep high frequency volcano tectonic earthquakes (VTs) beginning in July 2013 and shallow VTs in September 2013. Period 2 began in late October 2013 with accelerated extension, with at least 1 cm extension in the baseline length. During period 2 the modeled inflation source ascended to a shallower depth of 0.9 (0.4–2.1) km below sea level (bsl) beneath Sinabung with a change in volume of + 0.39 (+ 0.18–+0.60) Mm³, and with accelerated rates of volume increase during the time period when the magma migrated to the surface. Seismicity during period 2 was marked first by an increase in the incidence of shallow volcano-tectonic (VT) earthquakes and later by repetitive self-similar hybrid events as the magma migrated to the surface. Period 3 began in January 2014, after the appearance of the lava dome, and was marked by rapid steady contraction of ~ 3 cm through March 2014. The modeled source located at 8.4 (7.4–9.9) km bsl beneath the eastern flank of Sinabung with a volume change of − 20.51 (− 26.89 to − 14.12) Mm³. Period 4 began in April 2014 with decelerating contraction, and the modeled deformation center shifted to the northeast, reaching a depth of 12.2 (10.1–14.8) km bsl between Sinabung and Sibayak volcanoes and a change in volume of − 88.26 (− 123.87 to − 52.66) Mm³. Approximately 2/3 of the total volume change related to contraction occurred between January 2014 and May 2016, and the current activity of Sinabung is expected to decrease gradually and almost terminate in the early 2020s, assuming no new intrusion or deformation rate changes. Both of the eruptions at Sinabung in 2010 and Unzen in 1991–1995 show characteristics of ground inflation and subsequent deflation, indicating magma migration and effusion processes similar to the current Sinabung activity. The inflation before the 2010 Sinabung eruptions likely started before 2007 and is an indication of magma intrusion before the 2010 and 2013 eruptions.