Sumatran-Andaman Islands Megaquake Revised To M9.3

[David]

The National Earthquake Information Center (NEIC), part of the U.S.G.S., provided some amazing updates this afternoon.

The thesis is that the northern two-thirds of the aftershock zone broke slowly, not generating regular seismic waves but lower frequency normal modes.

There was also concern that the conventional way of determining moment magnitude may saturate for these superquakes (or megaquakes). The consequence is that the Sumatran quake may be an M9.3 instead of an M9.0, which would make it the second-largest quake in recorded history (the Chile 1960 event is still top spot).

Other points from the talk:

• The break looks like three quakes in a row when the source function is determined: 9.0, 8.1 & 8.1. It took eight minutes for the fault to break through.

• Seismologists expect measurable normal mode energy through March of this year -- in other words, the Earth is still "ringing."

• The tsunami in northern Indonesia was 34 meters (eleven stories) & arrived ten minutes after the quake shaking began.

• The tsunami was measured on nearly every tidal gauge on the planet.

A UK Royal Navy ship got three-dimensional images of the seabed near the epicenter. In the photos, the flat Indian plate shows up in purple & blue, & turns green & yellow at the Burma microplate -- you can clearly see where one plate is subducting under another, causing the latter to rumple up "like a carpet," one of the sailors described it.

Buckling Plate

Large areas of the subduction zone fault to the south were not involved in the Dec. 26 quake, which isn't good news. If tension wasn't released on those portions, the plate that is subducting underneath the other is still dragging it down -- it has yet to rebound, & there's no telling when it might. When it does, it could prove equally catastrophic.