A 32-year long earthquake!
Yes, you read that right - an earthquake that lasted for 32 years has actually happened. Scientists have determined that from 1829 to 1861 a slow slip event progressed around the Indonesian island of Sumeulue, just off the island of Sumatra (incidentally the same place where the massive 2005 earthquake-tsunami occurred). As such slow events do not produce the rapid jolts that more 'normal' earthquakes do, they are not associated with huge amounts of damage, however, their slow progress can destabilise surrounding tectonic arrangements and in turn, stimulate the more 'normal' and damaging events.
Damage to Sumatran town following 2005 tsunami. Source: U.S. Navy photo by Photographer's Mate 2nd Class Philip A. McDaniel File# 050102-N-9593M-031 (public domain).
So why is this region so susceptible to earthquakes (and in turn, tsunamis)? Well, the Indo-Australian Plate is subducting beneath the Eurasian Plate right along the south/west coast of Sumatra - in fact, you can see this happening in the map above, with the deep ocean to the west of Sumatra (the Indo-Australian plate) suddenly becoming shallower as you approach Sumatra (the Eurasian Plate). Sometimes this subduction process comes to a halt and tensions build up; once tensions reach a certain threshold, the rock fractures and an earthquake happens. This new research however, shows that such fracturing need not be instantaneous but can happen slowly, over many years.