© Ye, et al./GRL Vol. 39A diagram of the Philippine Trench showing historic large earthquakes of magnitude 7.0 or great, as well as the motions of tectonic plates.
"Typically, if you locate aftershocks they sort of outline the fault that ruptures. This time they didn't," said Thorne Lay, a geophysicist at the University of California, Santa Cruz who was part of the team. "Instead they were shallower and had very different fault geometries."
The 7.6 quake happened at a fault within the Philippine Sea Plate, which is subducting beneath the Philippine Microplate. Some of the unusual aftershocks were so called "intraplate" ruptures, like the original quake, but happened at a shallower depth.
And others of the unusual aftershocks were located west of the epicenter, within the Philippine Trench itself, where the plate is subducting. These are called interplate aftershocks because they happen at the boundary between two plates.
That segment of the trench hasn't seen a major earthquake in at least 400 years, but the shocks may be a sign that the plate boundary is linked to the intraplate rupture and that it is building up strain in preparation for a big one.
No one will know until GPS equipment is installed and scientists collect more data, Lay said.
