What do the Ansei-Nankai and Ansei-Tokai earthquakes of 1854 have in common with the 1944-1946 Tononkai and Nankai earthquakes in Japan?   They each suffered massive aftershocks shortly thereafter.    The Ansei-Nankai and Ansei-Tokai earthquakes were 8.4 magnitude and only 31 hours apart.  Worse, the aftershocks were nearly as bad.

And the same scenario could apply this time, says UC Davis seismologist John Rundle, and Tokyo is at the most risk.  Friday's magnitude 9.0 temblor has been followed by hundreds of
powerful aftershocks that have migrated southwards.   "Initially, the major aftershocks were confined to the region near Sendai, but the steady southward march of the aftershocks is cause
for alarm for Tokyo and surrounding regions." 

Typically, an earthquake of magnitude 9 would be followed, in no particular order, by an aftershock of magnitude 8, ten aftershocks of about magnitude 7 and many smaller aftershocks, he says.  But the magnitude 8 has not happened yet.  If it happens in Tokyo Bay, it could set off a tsunami that would devastate the densely populated region, similar to the events of September 1, 1923 during the great Kanto earthquake (magnitude 7.9).

Rundle's research uses computer modeling to understand systems that can go through abrupt and catastrophic changes, such as earthquake faults. He has collaborated with researchers at NASA's Jet Propulsion Laboratory and other universities on Quakesim, which studies earthquake fault systems and has produced earthquake forecasts for California and other parts of the world.

Last year, Rundle forecast that of four Japanese cities - Tokyo, Osaka, Niigata and Sendai - Sendai was the second-most likely to be hit by a major earthquake within 150 miles over the next year. Tokyo was the most at risk, he calculated. He has updated this forecast as of 3 p.m. on March 13.