well, all day i have had it in my mind that the mag was M 7.4 and that is what i wrote for the ground failure and intensity interpreti e posters.
they are updated online and here is a tweet showing them.
here is the #EarthquakeReport:
@earthjay impressive overview (as always, great work). 7.6 in the upper plate... I missed that one! 'Good' news wrt tsunamis, very bad news for local people!
@earthjay Focal mechanism seems to point right away to thrust slip on low angle plane, you think it is on the steep eastward dipping? In the block diagram profiles there is a steep fault, but dipping clearly westward? Your main point is that the tsunami would be much larger if it would have been on the subduction interface?
@berndandeweg i agree that the earthquake likely occurred on west dipping fault plane. could be on fault in upper plate (like a splay fault) or the slab is shallower than the USGS slab model.
@judithgeology suggests the latter.
possibly larger, if event not on megathrust. if on megathrust, seems more likely it would extend further updip with larger slip area, especially in shallow region of megathrust. (?)
In general, more compact, shallow slip patches would yield larger tsunamis - the energy of the tsunami is proportional to the square of the height of the wave. So it's hard to argue that the earthquake being on the megathrust or on a parallel upper-plate fault is responsible for the moderate tsunami.
Predicting coastal wave heights in tsunamis is notoriously difficult, so I'm not sure what wave should be "expected" in any case.
@judithgeology @earthjay in the Japanese subduction zone, there is an intraplate deformation zone in the downgoing plate, parallel to the subduction-megathrust, with quite some seismicity. Might this be something like that? @earthjay
@berndandeweg @earthjay Interesting! I don’t think that really matches what we’re seeing in the Philippines, though - the earthquake was shallow than slab2.0, not deeper.
agreed, not sure there is a double zone in the philippines (unclear, but may not be able to tell given paucity of seismometers and uncertain depths? compared to Japan, which has an abundance of seismometers).
while the megathrust in the Philippines could be shallower than slab 2.0 (seems reasonable! if not probable) alternate hypotheses include: there could be two splays of the megathrust (as appears to exist in southern cascadia: Smith et al., 1993 JGR v. 98 no. B5, p. 8153-8171)) or a splay fault responsible for the Philippine event (?).
most of the events after the mainshock were deeper. so, maybe the mainshock was on this shallower fault, which triggered slip on the deeper fault. (?)
thank you!
this paper does not have any information that helps us understand if there is a single megathrust (possibly shallower than slab 2.0), multiple slabs (like possibly in southern Cascadia), or splay faults.
are there any more recent papers with more detailed information about the fault geometry here?
@earthjay @judithgeology complex zone.. Here a couple of thrusts head north towards the hypocnter of the last quakes