Source Properties of Moderate-Magnitude Earthquakes to Quantify Variations in Seismogenic characteristics Along the Alpine Fault, New Zealand.

Abstract:

The Alpine Fault is a major active continental transform fault that is late in its typical cycle of large earthquakes: extensive paleoseismic research has revealed that the central section of the Alpine Fault ruptures in M7+ earthquakes every 291±23 years and last ruptured in 1717 AD. The paleoseismic results also reveal that some small-scale segments of the fault, which coincide with pronounced along-strike changes in fault characteristics, act as conditional barriers to rupture. The geometry, seismicity rates and geology of the Alpine Fault change along its three principal sections but it is unclear whether source properties (e.g. stress drop, directivity) of near-fault seismicity also vary between fault segments, and whether these properties can elucidate, or have some influence on the conditional segmentation of the Alpine Fault during large earthquake rupture.

To constrain whether preferred propagation direction and the rupture process influence the conditional segmentation of Alpine Fault earthquakes, we have computed Brune-style stress drops and directivity estimates of moderate-magnitude earthquakes occurring on and close to the Alpine Fault, using an empirical Green’s function (EGF) approach. We use data from dense, temporary seismometer networks, including DWARFS (Dense Westland Arrays Researching Fault Segmentation), a new two-part network designed to constrain seismogenic behaviour near key transitional boundaries. Our results investigate the spatial variability of these source properties along the length of the Alpine Fault, focussing on whether earthquakes at the rupture segment boundaries behave differently to those in the middle of previously identified rupture segments.