Seismological and Thermochronological Constraints on the Thermal State and Present-Day Seismogenic Depths of the Central Alpine Fault, New Zealand

Session: Exploring Rupture Dynamics and Seismic Wave Propagation Along Complex Fault Systems
Type: Oral
Date: 4/29/2020
Time: 05:30 PM
Room: 110 + 140
Description: 

The Alpine Fault is a major transpressive plate-bounding fault that forms the western boundary of the Southern Alps orogen in the South Island of New Zealand. Paleoseismological studies reveal the fault to regularly rupture in large (Mw 7–8) earthquakes and to have last ruptured in 1717 CE; it is understood to now be in the late stages of its typical <300-yr interseismic period. Recent borehole measurements and previous thermal modelling results reveal high temperatures in the crust near the central Alpine Fault. We revisit those observations in conjunction with a newly developed microseismicity catalog to investigate the effects of exhumation on the depths of earthquakes and the brittle–ductile transition throughout the orogen. The new catalog contains 7,719 relocated earthquakes, which we analyse jointly with published thermochronological data using a 1-D thermal model to determine orogenic uplift rates and the corresponding temperature structure of the crust. The calculated exhumation rates vary from 1 to 8 mm/yr, with the maximum values of 8±3 mm/yr obtained in the area of highest topography near Aoraki/Mount Cook, a finding consistent with previous geologic and geodetic observations. Our analysis provides estimates of the temperature of the brittle-ductile transition beneath the Southern Alps and the temperature at which each earthquake in the catalog occurred. We obtain a mean brittle–ductile transition temperature of ~450–470°C, which is consistent with a feldspathic crustal rheology and high strain rates. The corresponding cut-off depths for seismicity near the central Alpine Fault vary over an along-strike distance of ~100 km by a factor of two, from ~20 km at either end of the central section to ~10 km near Aoraki/Mount Cook. This pronounced along-strike variation is likely to affect down-dip and along-strike patterns of coseismic slip in future Alpine Fault earthquakes.

Presenting Author: John Townend

Authors