Slow slip and frictional transition at low temperature at the Hikurangi subduction zone

Shallow portions of faults exhibit brittle, stick-slip behaviour that gives way to more stable sliding with increasing depth, limiting the depths to which earthquake-inducing slip can occur. The increase of temperature with depth is often assumed to transform friction in the fault from velocity-weakening (unstable) to velocity-strengthening (stable), and to change conditions from brittle to ductile1-5. A temperature of 350 degrees C has been suggested to mark the base of the frictionally unstable portion of the fault, which becomes free slipping at depth where temperatures rise above 450 ffi C (refs 2,4,6). Here, we show that both slow-slip events and the geodetically observed transition from fault locking to free slip at the Hikurangi subduction zone east of the North Island, New Zealand occur at temperatures as low as 100 degrees C, suggesting that temperature is not a primary control on either slow-slip or fault-locking processes at the Hikurangi margin. Although globally shallow slow-slip events are rare, five out of seven events observed so far at the Hikurangi margin are less than 15km deep.