Aftershocks after the M 6.9 Mindanao, Philippines earthquake

How the rupture zone is measured

An aftershock is a smaller earthquake triggered as the crust settles around the fault that ruptured. They cluster along that rupture, so the area worth searching scales with the size of the mainshock rather than being a fixed radius.

The 81 km zone above comes from the Wells & Coppersmith (1994) rupture-length relation, doubled because aftershocks routinely extend about one rupture length beyond each end of the fault, then floored at 50 km and capped at 400 km. Anything outside it is counted as separate regional seismicity, not part of this sequence.

Aftershocks are most frequent in the hours right after the mainshock and taper off over days to months. A quiet day does not mean the sequence has ended.