Birth of a subduction zone.

Birth of a subduction zone 

A massive earthquake that struck the seafloor south of New Zealand in May appears to have signaled the very early stages of subduction there -- the same process that long ago created deep ocean trenches around the Pacific. Subduction occurs when two crustal plates collide and one dives below the other.

The magnitude 8.2 shock, the world's largest in 12 years (SN: 6/3/89, p.340), occurred at the Macquarie ridge, a chain of mountains and troughs that runs south from New Zealand and forms the boundary between the Pacific and Australian plates. This and other local ruptures over the past century illuminate the complex dance between the plates in the area. Like passing trains headed in opposite directions, the Australian plate is moving northwest in relation to the Pacific plate. The large quakes that struck in 1981 and 1989 reflect this motion because the ruptures occurred along vertical faults that allow the plates to slip past each other.

Yet the Macquarie ridge also generates smaller quakes along dipping fault planes. Quakes of this type indicate the two plates are pressing together as they slide past each other, as if the trains moved closer together as they passed.

These smaller, compressional earthquakes suggest subduction is just beginning along the Macquarie ridge, says Susan L. Beck of the Lawrence Livermore (Calif.) National Laboratory, who investigated the ruptures with Larry J. Ruff and Bart W. Tichelaar of the University of Michigan at Ann Arbor. She notes that the underwater ridge has not spawned extremely large compressional earthquakes during the last century. This suggests that many separate dipping faults lace the area and have not yet connected to form one large fault plane -- a necessary step in subduction.

Beck cautions, however, that historical records of quakes in this remote region span only a relatively recent period. If huge compressional quakes have ruptured the area in centuries past, subduction could be much farther along, she says. Earthquakes in coming decades will help scientists pin down the progression.