[Ed. – It sounds as if what we’ve realized is that activity we had evidence of was basically all one “slow-slip” earthquake. This one culminated in a magnitude 8.5 quake in 1861. Very interesting read.]
When a magnitude 8.5 mega-earthquake struck off the Indonesian island of Sumatra in February 1861, it caused the land to convulse, stirring up a wall of water that crashed on nearby shores and killed thousands of people.
Now, it seems that tragic event was no isolated incident: It actually marked the end of the longest earthquake ever recorded, which crept through the subsurface for a whopping 32 years. Known as a slow-slip event, these kinds of quakes have been known to unfurl over days, months, or years. But the newly described event lasted more than twice as long as the past record-holder, scientists report in Nature Geoscience. …
[S]low-motion earthquakes release energy built up from the shifts of tectonic plates. But instead of unleashing it in a ground-rattling burst, slow quakes sluggishly release strain over time, and so are not hazards on their own. Still, the subtle shifts of the subsurface potentially load strain on adjacent zones along a fault, which could increase the risk of a bigger temblor nearby.