Fault model of the Te Araroa earthquake, New Zealand,
using ocean bottom pressure records#Tatsuya Kubota, Tatsuhiko Saito (NIED), Yoshihiro Ito (DPRI),
Yoshihiro Kaneko (GNS Science), Laura Wallace (UTIG),
Syuichi Suzuki, Ryota Hino (Tohoku Univ.), Stuart Henrys (GNS Science)
On September 1, 2016 (UTC), the Te Araroa earthquake (Mw 7.0), with anormal-faulting mechanism, occurred in the subducting plate in the Hikurangi
subduction zone. By comparing the regional onshore seismograms of this event andthe smaller precursor events, Warren-Smith et al. (2018) suggested that this event ispossible to have a larger source dimension (i.e., larger fault length and width) thanthat expected from typical fault scaling relationships. However, they could not
discuss it in detail since this earthquake occurred far from the coast and the azimuthal
coverage of the seismic stations was poor. When this event occurred, ocean bottompressure gauges (OBPs) were installed ~170 km south of the source area and theyclearly observed tsunamis from the source to OBPs (direct waves) and from coastalreflections (reflected waves) (Figure 1). Since tsunami data contain uniqueinformation about the area of seafloor deformation (the extent of the tsunami source)
and the tradeoff between the fault dimension and the rupture velocity is much lesssignificant than the seismic waves, tsunami data have an advantage in constraining
earthquake source dimensions. In this study, we investigated the tsunami dataassocialted with the Te Araroa earthquake to estimate the finite fault model, in order
to discuss the fault dimensions. We note that we used the reflected waves, whichhave not been explicitly used by the previous researches, for earthquake source
modelling, in addition to the direct waves.
We first constrained the centroid horizontal location and source dimension,
based on the grid-search approach. As a result, we obtained a centroid near theGCMT centroid and found that the models using coastal reflections require a source
dimension larger than ~ 30 km long. Using these results as a prior information, wethen estimated the slip distribution by inverting tsunami waveforms (Figure 1a). We
obtained the maximum slip of 0.9 m and Mo = 4.3 × 1019 Nm (Mw 7.03; µ = 40MPa). Using this model, we calculated the shear stress change distribution (Figure1b) and obtained the energy-based stress drop ΔσE (e.g., Noda et al., 2013) as 1.0
MPa. This value was consistent with the typical stress drop values (~ 1 – 10 MPa)(e.g., Ye et al., 2016) although is on the low end of these, and was consistent withthe large fault dimension suggested by Warren-Smith et al. (2018).
Using the tsunami waves from both the direct and the coastal reflected waves
observed by offshore OBPs, we could constrain the stress drop value of the Te Araroa
earthquake in detail. This study shows that the information from the direct and thecoastal-reflected tsunami data observed by the offshore OBPs provides us tighter
constraints on the source parameter estimation of offshore moderate earthquakes,
which is usually difficult to obtain from the onshore seismic data alone.

Figure 1. (a) Slip distribution obtained from the finite fault inversion. The areasurrounded by green lines is the subfaults with slip larger than 20% of the maximum
slip. The iso-depth contour lines are drawn at 1000 m interval. (b) Distribution of
the shear stress change on the fault. Positive value denotes the stress reduction. (c)Comparison of the observed (black) and calculated (red) waveforms.