There are records from the past that show pretty major events in that area, Toba comes to mind.
Krakatoa comes to mind. In 1883 all three volcanoes in that system exploded quite spectacularly, much more violently than even Mt. St. Helens. I was watching the documentary on the 1883 eruption just last night. If I remember correctly, they had pretty strong evidence of an even bigger eruption from the same system just a century or two before the 1883 eruption.
I would say that there is much to be concerned about regarding the Sumatran volcanoes.
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