Possible Dual Earthquake–Landslide Source of the 13 November 2016 Kaikoura, New Zealand Tsunami
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A complicated earthquake (M w 7.8) in terms of rupture mechanism occurred in the NE coast of South Island, New Zealand, on 13 November 2016 (UTC) in a complex tectonic setting comprising a transition strike-slip zone between two subduction zones. The earthquake generated a moderate tsunami with zero-to-crest amplitude of 257 cm at the near-field tide gauge station of Kaikoura. Spectral analysis of the tsunami observations showed dual peaks at 3.6–5.7 and 5.7–56 min, which we attribute to the potential landslide and earthquake sources of the tsunami, respectively. Tsunami simulations showed that a source model with slip on an offshore plate-interface fault reproduces the near-field tsunami observation in terms of amplitude, but fails in terms of tsunami period. On the other hand, a source model without offshore slip fails to reproduce the first peak, but the later phases are reproduced well in terms of both amplitude and period. It can be inferred that an offshore source is necessary to be involved, but it needs to be smaller in size than the plate interface slip, which most likely points to a confined submarine landslide source, consistent with the dual-peak tsunami spectrum. We estimated the dimension of the potential submarine landslide at 8–10 km.
KeywordsNew Zealand 2016 Kaikoura earthquake tsunami numerical simulations landslide submarine mass failures spectral analysis
The USGS W-phase moment-tensor solution resulted in an oblique thrust fault mechanism including a right-lateral strike-slip component for the 2016 event (strike 219°, dip 38°, rake 128°) (mechanism shown in Fig. 1), which is close to the mechanism reported by Global CMT (strike 226°, dip 33°, rake 141°). Hollinsworth et al. (2017) reported a focal mechanism similar to Global CMT. Detailed mechanism solution by Duputel and Rivera (2017) revealed that the initial part of the rupture was of strike-slip type followed by large ruptures both on strike-slip and thrust faults. The earthquake was associated with major surface deformation in the form of uplift and subsidence on several inland and onshore faults (at least 12 major faults, Hamling et al. 2017) and an extensive coastal area was exposed to air due to co-seismic uplift (e.g., Hamling et al. 2017). According to various media and expert reports, tens of thousands of landslides of various sizes were observed following the earthquake (e.g., Massey et al. 2017). In terms of surface deformation, obviously the Kaikoura earthquake was among the most complex earthquakes worldwide. Aftershocks were distributed toward the NE of the epicenter (Fig. 1). The earthquake was followed by a tsunami reaching a maximum runup height of around 7 m in the near field (Power et al. 2017; Bradley et al. 2017), although the maximum zero-to-crest tide gauge height observed at the near-field station of Kaikoura was ~2.6 m (Fig. 3). Other tide gauges recorded zero-to-crest heights of: 0.67 m (in Sumner), 0.4 m (in Wellington), 0.2 m (in Castlepoint), and 0.16 m (in Chatham Island) (see Fig. 1 for the locations and Fig. 3 for the wave records). GeoNet reported tsunami runup heights of 4.1 and 4.4 m in the near field which caused some small damage to property with no death (Lane et al. 2017).
In terms of regional tectonic setting, the 2016 Kaikoura earthquake occurred in one of the world’s complex tectonic settings comprising a transition zone between two subduction zones connecting the Australian and Pacific plates: Puysegur Trench in the south and Hikurangi and Kermadec Trenches in the north (Fig. 1). The area is home to many faults with dominating strike-slip mechanisms among which is the Hope fault (Fig. 1), located close to the 2016 epicenter. However, the Hope fault was not the only one responsible for the 2016 rupture; field observations revealed evidence for rupture on various faults (Hamling et al. 2017).
Here, we characterize the 2016 Kaikoura tsunami by analyzing the physical properties of the tsunami, namely amplitude and period, using available tsunami observations. We then perform numerical simulations to shed light on the type of the tsunami source and to study tsunami propagation in the region.
2 Data and Methods
Tsunami data used in this study include nine tide gauge records with sampling interval of 1 min (see Fig. 1 for locations). The data were provided by the Intergovernmental Oceanographic Commission of UNESO sea-level monitoring facility (http://www.ioc-sealevelmonitoring.org/). To obtain tsunami waveforms, tidal signals were calculated by applying the TidalFit package of Grinsted (2008) and then removed from the tsunami records. Spectral analysis of tsunami waveforms was performed using the Welch’s (1967) averaged modified-periodogram method. Wavelet, time–frequency, analysis was conducted applying the program provided by Torrence and Compo (1998) using the Morlet mother function having a wavenumber of 6 and a scale width of 0.10. We applied the numerical model of Satake (1995) for tsunami propagation with the initial seafloor deformation calculated by the analytical formula of Okada (1985). Time step for tsunami simulation was 1 s for a total simulation time of 8 h. Bathymetry data used for tsunami simulations is from General Bathymetric Charts of the Oceans (GEBCO, Weatherall et al. 2015) having a resolution of 30 arcsec.
3 Tsunami Waveforms and Physical Properties
3.1 Tsunami Waveforms
3.2 Tsunami Spectra
Using the water depth of 100–500 m for the offshore area and applying the tsunami phase velocity equation (Eq. 5 in Heidarzadeh and Satake 2015b), the dominant period of 19 min implies a source dimension of 18–40 km for the tectonic source of the tsunami. The landslide source dimension is estimated at 8–10 km by using the dominant period of 4.2 min and the water depth at offshore slopes (i.e., 400–600 m).
3.3 Wavelet Analysis
4 Tsunami Simulations and Discussion
4.1 Tsunami Source
Source characterization and comparison of the observed and simulated waves from various earthquake models
Comparison of simulations with observation
Crustal and plate interface; the plate-interface component extends offshore
Onshore and offshore
Similar amplitude, longer period
Similar amplitude, similar period, except for the first peak
Crustal and plate interface; the plate-interface component is limited inland
Similar amplitude, similar period, except for the first peak
The contribution of plate-interface slip to the 2016 Kaikoura earthquake is not clear yet (as of May 2017). Since the occurrence of this event, there have been contradicting ideas about whether the earthquake ruptured the plate interface or not. Geodetic and coastal-uplift inversion by Hamling et al. (2017) showed that inclusion or exclusion of plate-interface slip does not change the results of inversion (i.e., the misfit between observation and simulations remains similar in both cases). Tsunami simulations conducted here indicates that an offshore plate-interface slip (as seen in EM1) is unlikely to be involved because it produces longer-period waves than tsunami observations. However, tsunami simulation is not capable of providing insights about the involvement of an inland plate-interface slip (as seen in EM3).
4.2 Regional Tsunami Propagation
Waveform analysis revealed that zero-to-crest tsunami amplitude was 257 cm at the Kaikoura tide gauge station, located within the co-seismic uplift zone; it was 67, 40, and 20 cm at the Sumner, Wellington and Castlepoint stations, respectively, located close to the rupture zone. Chatham Island tide gauge station, ~800 km to the east of epicenter, received tsunami amplitude of 13 cm. Most of the tsunami was confined within the shallow area between the east coast of New Zealand and Chatham Island which can be attributed to the presence of Chatham Rise in the region.
Fourier analysis revealed a dual-peak tsunami spectrum with two major peak periods of 4.2 and 19 min with a cutoff period of 5.7 min. The two major tsunami energy period bands are 3.6–5.7 and 5.7–56 min, which we attribute to potential landslide and earthquake sources of the tsunami, respectively. The timing of the potential landslide-generated waves was revealed by wavelet analysis. We estimate the dimension of the potential submarine landslide at 8–10 km.
Tsunami simulations reveal that a tsunami source with offshore plate-interface slip reproduces the near-field tsunami observation in terms of amplitude, but fails in terms of tsunami period by producing longer-period waves. On the other hand, a tsunami source without offshore plate-interface slip fails to reproduce the first (and the largest) peak of the tsunami observed in the near field, but matches fairly well both in terms of amplitude and period for the later waves. Tsunami simulation may indicate that an offshore forcing is necessary to be involved, but it needs to be smaller in size which most likely points to a confined submarine landslide source. This is consistent with the dual-peak tsunami spectrum of the observed tsunami waveforms. Furthermore, Fourier analysis for the simulated waves revealed spectral energy deficits for the simulated waves in the period band of 3.6–5.7 min, indicating that simulations lack a confined source with dominant period band of 3.6–5.7 min.
Tide gauge records are from the network operated by GNS Science and Land Information New Zealand, accessed from Intergovernmental Oceanographic Commission’s Sea Level Station Monitoring Facility (http://www.ioc-sealevelmonitoring.org/). The earthquake source model of USGS was used in this study (https://earthquake.usgs.gov/earthquakes/eventpage/us1000778i#executive). We thank Ian Hamling (GNS Science, New Zealand) for sharing his source model with us. Early results were presented at the annual tsunami symposium of the Japan tsunami community on 8, 9 December 2016 at the Kansai University, Osaka, Japan. We are grateful for the constructive comments from colleagues participating in this symposium. The manuscript benefited from constructive review comments by Alexander Rabinovich (Editor-in-Chief), Emily M Lane (NIWA, New Zealand), and an anonymous reviewer. We thank Yefei Bai for providing the Sumner tide gauge record. This research was funded by Brunel Research Initiative and Enterprise Fund 2017/18 (BUL BRIEF) at the Brunel University London to the lead author (MH).
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