The huge 9.0 earthquake that hit Japan was actually an aftershock—and is likely to be followed by more large aftershocks in the coming weeks.
Nature reports that the huge 9.0 earthquake that hit Japan at 2.46 p.m. on Friday (local time) will be followed by "a number of major aftershocks in the next weeks"—and, surprisingly, that the March 11 quake was itself an aftershock of a 7.2 quake that also took place below the Pacific sea floor near the northern Miyagi Prefecture on March 9.
John McCloskey, a geophysicist at the University of Ulster in Coleraine, Northern Ireland, told Nature:
Although certainly very big, today's quake was not totally unexpected. Technically, it was in fact an aftershock of the weaker quake earlier in the week—even though it may sound odd that an aftershock can be stronger than the main shock.
According to Nature, "McCloskey's group had just completed computer calculations of the changes in seismic stress caused by the 9 March quake when the big one struck." They quickly realized that the smaller quake had significantly increased stress in the fault zone, leading directly to the 9.0 quake just east of Sendai.
What's more, the two quakes have "probably also affected the stress field further south along the fault zone, critically increasing the earthquake risk in the Tokyo region." McCloskey warned that some of these aftershocks "may be as large as, or even stronger than, the quake that last month devastated Christchurch in New Zealand."
Japan is probably the most seismically-prepared country on earth, but more serious tremors could undoubtedly hamper relief efforts.
Image: Seismicity 1990-Present, Near East Coast of Honshu, Japan, courtesy USGS. Plate boundaries in yellow.