Upheaval Down Under

Abstract

GEOLOGY New Zealand sits astride a transition from a west-dipping subduction zone toward the north (responsible for the volcanism of the North Island) to an east-dipping subduction zone toward the south. The transition forms a system of right-lateral strike-slip faults that have produced the dramatic topography of the South Island, as well as several large earthquakes. One of these was the 1855 magnitude 8.2 temblor on the Wairarapa Fault just east of the city of Wellington. Rodgers and Little remeasured offsets produced by this earthquake and conclude that the ground slipped by as much as 18 m, an enormous amount for a strike-slip fault. For comparison, the devastating 1906 San Francisco earthquake produced a maximum of about 6 m of slip at the surface. Furthermore, the earthquake extended laterally only about 150 km (versus 480 km for the 1906 quake). An earlier quake may have produced surface slip of 14 m. The authors explain the paradox of a huge slip and short surface rupture by suggesting that the Wairarapa Fault extends deep into the crust, connecting with the northern-dipping subduction zone at depth. — BH J. Geophys. Res. 111 , B12408 (2006).