【地球科学】ニュージーランドのアルパイン断層の極端な状態

Extremely high temperatures and fluid pressures, much greater than those expected, have been measured in a borehole in the Alpine Fault in New Zealand, a Nature paper reports. The fault is expected to rupture in a magnitude-8 earthquake in the next few decades and the study could have broad implications for understanding earthquakes and fault zone geology.

The Alpine Fault is a major plate boundary fault that runs most of the length of New Zealand’s South Island. The fault produces large earthquakes approximately every 300 years, and it last ruptured in 1717. Rupert Sutherland and colleagues drilled a scientific borehole into the fault to examine the hydrothermal conditions therein.

They collected comprehensive rock, mud and seismological observations and found that temperatures and fluid pressures in the borehole were much higher than expected, particularly in the ‘hanging wall’ above the fault, where the average temperature gradient reached around 125 degrees Celsius per kilometre of depth. Values above 80 degrees Celsius per kilometre tend to be associated with volcanic regions, but there is no evidence for volcanism near this site. The authors conclude that these extreme hydrothermal conditions result from rapid fault movement, which transports rock and heat from greater depths, and fluid movement through fractured rocks, which concentrates heat into valleys.