Seismic P Wave Velocity Model From 3‐D Surface and Borehole Seismic Data at the Alpine Fault DFDP‐2 Drill Site (Whataroa, New Zealand)

V Lay,R S Kofman,S Buske,J D Eccles,A Constantinou,S B Bodenburg,D Lawton,M Bertram,A R Gorman,M K Savage,K Hall,R Kellett,J Townend,D R Schmitt

doi:10.1029/2019jb018519

Abstract

AbstractThe New Zealand Alpine Fault is a major plate boundary that is expected to be close to rupture, allowing a unique study of fault properties prior to a future earthquake. Here we present 3‐D seismic data from the DFDP‐2 drill site in Whataroa to constrain valley structures that were obscured in previous 2‐D seismic data. The new data consist of a 3‐D extended vertical seismic profiling (VSP) survey using three‐component and fiber optic receivers in the DFDP‐2B borehole and a variety of receivers deployed at the surface. The data set enables us to derive a detailed 3‐D P wave velocity model by first‐arrival traveltime tomography. We identify a 100–460 m thick sediment layer (mean velocity 2,200 ± 400 m/s) above the basement (mean velocity 4,200 ± 500 m/s). Particularly on the western valley side, a region of high velocities rises steeply to the surface and mimics the topography. We interpret this to be the infilled flank of the glacial valley that has been eroded into the basement. In general, the 3‐D structures revealed by the velocity model on the hanging wall of the Alpine Fault correlate well with the surface topography and borehole findings. As a reliable velocity model is not only valuable in itself but also crucial for static corrections and migration algorithms, the Whataroa Valley P wave velocity model we have derived will be of great importance for ongoing seismic imaging. Our results highlight the importance of 3‐D seismic data for investigating glacial valley structures in general and the Alpine Fault and adjacent structures in particular.

Highlights

IntroductionRecent paleoseismological research reveals that the central and southern portions of the Alpine Fault in proximity to the Whataroa Valley rupture in MW 7–8 earthquakes remarkably consistently with average recurrence intervals of 263 ± 68 and 291 ± 23 years, respectively (Cochran et al, 2017; Howarth et al, 2018)
The new data consist of a 3-D extended vertical seismic profiling (VSP) survey using three-component and fiber optic receivers in the Deep Fault Drilling Project (DFDP)-2B borehole and a variety of receivers deployed at the surface
Our results highlight the importance of 3-D seismic data for investigating glacial valley structures in general and the Alpine Fault and adjacent structures in particular

Summary

Introduction

Recent paleoseismological research reveals that the central and southern portions of the Alpine Fault in proximity to the Whataroa Valley rupture in MW 7–8 earthquakes remarkably consistently with average recurrence intervals of 263 ± 68 and 291 ± 23 years, respectively (Cochran et al, 2017; Howarth et al, 2018). The last major earthquake on the Alpine Fault occurred in 1717 A.D., and the conditional probability of a MW 8 earthquake in the 50 years exceeds 27–29% (Biasi et al, 2015; Cochran et al, 2017; Howarth et al, 2018). Viewed on a regional scale, the central Alpine Fault appears as a straight boundary (e.g., Norris & Cooper, 2001; Sutherland et al, 2006).

Results

Discussion

Conclusion