Validation of the 3-D phase-weighted relative back projection technique and its application to the 2016 Mw 7.8 Kaikōura earthquake

Abstract

Back projection methods are increasingly used for mapping large earthquake ruptures in space and time. Unlike other seismological source imagingmethods, back projections require no prior knowledge of earthquake source parameters or fault geometries except for the epicentre and average depth, making them a convenient tool for studying complex, multisegmented ruptures. We developed a new 3-D Phase-Weighted Relative Back Projection method (3-D PWBP) that yields improved spatial resolutions by enacting two advances. First, we exploit both the phase and amplitude of the seismogram signal to enhance the distinction of correlated signals. Second, we implement a 3-D velocity model to provide more accurate traveltimes. We vindicate these refinements with several synthetic tests and an analysis of the 1997 Mw 7.2 Zirkuh (Iran) earthquake, which we show ruptured mostly unilaterally southwards at ∼3.0 km s-1 along its ∼125 km-long, mostly single-stranded surface rupture. Then, we apply the new method to the more complex case of the 2016 Mw 7.8 Kaikoura (New Zealand) earthquake, which we demonstrate is divided into two major stages separated by a gap of ∼8 s and ∼30-40 km. The overall rupture speed is ∼1.7 km s-1 and the overall duration is ∼84 s, considerably shorter than some earlier estimates. We see no clear evidence for a continuous failure of the subduction interface that underlies the known, surface-rupturing crustal faults, though we cannot rule out its involvement in the second major stage in the northern part of the rupture area. The late (∼80 s) peak in relative energy is likely a high-frequency stopping phase, and the rupture appears to terminate southwest of the offshore Needles fault.