6 February 2013 Mw8.0 Santa Cruz Islands Tsunami
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Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Nat. Hazards Earth Syst. Sci. Discuss., 3, 1949–1970, 2015 www.nat-hazards-earth-syst-sci-discuss.net/3/1949/2015/ doi:10.5194/nhessd-3-1949-2015 NHESSD © Author(s) 2015. CC Attribution 3.0 License. 3, 1949–1970, 2015 This discussion paper is/has been under review for the journal Natural Hazards and Earth 6 February 2013 System Sciences (NHESS). Please refer to the corresponding final paper in NHESS if available. Mw 8.0 Santa Cruz Islands Tsunami Source of the 6 February 2013 Mw 8.0 F. Romano et al. Santa Cruz Islands Tsunami Title Page F. Romano, I. Molinari, S. Lorito, and A. Piatanesi Abstract Introduction Istituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata 605, 00143, Rome, Italy Conclusions References Received: 27 February 2015 – Accepted: 3 March 2015 – Published: 16 March 2015 Tables Figures Correspondence to: F. Romano ([email protected]) Published by Copernicus Publications on behalf of the European Geosciences Union. J I J I Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion 1949 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Abstract NHESSD On 6 February 2013 an Mw 8.0 subduction earthquake occurred close to Santa Cruz Islands at the transition between the Solomon and the New Hebrides Trench. The en- 3, 1949–1970, 2015 suing tsunami caused significant inundation on the closest Nendo Island. The seis- 5 mic source was studied with teleseismic broadband P waves inversion optimized with 6 February 2013 tsunami forward modeling at DART buoys (Lay et al., 2013), and with inversion of tele- Mw 8.0 Santa Cruz seismic body and surface waves (Hayes et al., 2014). The two studies also use different Islands Tsunami hypocenters and different planar fault models, and found quite different slip models. In particular, Hayes et al. (2014) argued for an aseismic slip patch SE from the hypocen- F. Romano et al. 10 ter. We here develop a 3-D model of the fault surface from seismicity analysis and re- trieve the tsunami source by inverting DART and tide-gauge data. Our tsunami source model features a main slip patch (peak value of ∼ 11 m) SE of the hypocentre, and Title Page reaching to the trench. The rake direction is consistent with the progressively more Abstract Introduction oblique plate convergence towards the Solomon trench. The tsunami source partially Conclusions References 15 overlaps the hypothesized aseismic slip area, which then might have slipped coseismi- cally. Tables Figures 1 Introduction J I J I On 6 February 2013 an Mw 8.0 earthquake occurred in the Pacific Ocean nearby the archipelago of Santa Cruz Islands. The hypocenter (165.138◦ E 10.738◦ S, Back Close 20 depth ∼ 29 km, USGS, http://comcat.cr.usgs.gov/earthquakes/eventpage/usc000f1s0# Full Screen / Esc summary) is located at the subduction interface between the Australia and the Pacific plates, 76 km West from Lata, the main city of Nendo Island (Figs. 1 and 2). Printer-friendly Version This earthquake, the largest in 2013, occurred on a complex section of the Australia- Pacific plate boundary at the northern end of the New Hebrides trench (Hayes et al., Interactive Discussion 25 2012), nearby a short segment of dominantly strike-slip plate motion that marks the transition between Vanuatu and the Solomon Islands subduction zones. This seg- 1950 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | ment is characterized by a complex tectonic regime that becomes progressively more oblique westward as revealed by the focal mechanisms of the local seismicity (Fig. 1). NHESSD In this region the relative convergence velocity between Australia and Pacific plates is 3, 1949–1970, 2015 ∼ 9.4 cmyr−1 (DeMets et al., 2010). 5 The Santa Cruz Islands earthquake generated a tsunami that struck the Nendo Island, in particular the city of Lata with waves higher than 1 m. Several runup and 6 February 2013 flowdepth measurements have been collected during a field survey conducted on some Mw 8.0 Santa Cruz islands of the archipelago few days after the earthquake (Fritz et al., 2014), reporting Islands Tsunami maximum tsunami wave heights of about 11 m in the western part of the Nendo Island. F. Romano et al. 10 In addition, the tsunami propagated in the Pacific Ocean, also reaching the coasts of Hawaii (Lay et al., 2013). Seismic and tsunami source of this earthquake have been previously studied with Title Page different methodologies (Lay et al., 2013; Hayes et al., 2014a), highlighting some dif- ferences between the resulting models in terms of both slip patches position and slip Abstract Introduction 15 amplitude. Hayes et al. (2014a) studied the Santa Cruz Islands earthquake by inverting Conclusions References teleseismic body and surface waves; Lay et al. (2013) performed a teleseismic broad- band P waves inversion optimized with tsunami forward modelling at DART buoys. Tables Figures These studies used different hypocenters and different planar fault models; in partic- ular, Lay et al. (2013) adopted both hypocenter and fault plane shallower than those J I 20 used by Hayes et al. (2014a). The best-fitting source model in Hayes et al. (2014a, hereinafter HA14) has a main patch of slip centred around the hypocenter with a max- J I imum slip of about 4 m and a second smaller patch located SE of the Nendo Island Back Close and characterized by relatively low slip (∼ 0.5 m). On the other hand, the source model Full Screen / Esc in Lay et al. (2013, hereinafter LA13) features two patches with slip larger than 10 m; 25 the first patch is located around the hypocenter, whereas the second one is shallower Printer-friendly Version and located SE of the hypocenter. The surface projection of the slip in LA13 is roughly consistent with the HA14 patches even if they are at different depths (and featuring Interactive Discussion quite different slip values), because of the different fault planes used. In addition, the 1951 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | LA13 source model is more efficient in terms of tsunami waves excitation than that of HA14 and quite well predicts the tsunami observations recorded at the DART buoys. NHESSD The usual pattern of the aftershocks distribution following a great subduction earth- 3, 1949–1970, 2015 quake should show a large number of events occurring at the subduction interface not 5 dislocated, eventually also bordering the broken asperities (Aki, 1979). On the other hand, as already extensively discussed (Hayes et al., 2014a; Lay et al., 2013), after 6 February 2013 the 2013 6 February event, very few events were located along the subduction inter- Mw 8.0 Santa Cruz face. Furthermore, most of early aftershocks in the epicentral area (∼ 200 events within Islands Tsunami 48 h from the mainshock, USGS catalogue, http://earthquake.usgs.gov/earthquakes/) F. Romano et al. 10 showed strike-slip and normal mechanism, including two earthquakes with Mw > 7 oc- curred in the upper crust portion of the Pacific plate and in the outer-rise trench region. Hayes et al. (2014a) proposed a block-like motion behaviour of the Pacific upper plate Title Page to explain these observations. In particular, they argued that a large number of anoma- lous right-lateral strike-slip events located southeast of Nendo Island were triggered by Abstract Introduction 15 significant aseismic slip along a portion of the megathrust south-eastward from the epi- Conclusions References central area. However, LA13 model features significant coseismic slip on this portion of the fault; these differences may be due to the different data used and/or to the different Tables Figures fault models adopted in the inversions. Here we study the coseismic tsunami source of the Santa Cruz Islands earthquake J I 20 by inverting the available tsunami waveforms. We compute the Green’s functions at the DART buoys and tide gauges using a 3-D fault model that honours the complex J I geometry of the subduction interface. After retrieving the tsunami source model, we Back Close discuss it in comparison with LA13 and HA14 source models. Full Screen / Esc 2 Tsunami data and fault model Printer-friendly Version Interactive Discussion 25 The tsunami generated by the Santa Cruz Islands earthquake propagated both in the North and South Pacific Ocean and it has been observed in the open sea at sev- eral DART buoys and at some tide gauges located along the coasts of Solomon and 1952 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Fiji Islands. We select 5 DART buoys (52 403, 52 406, 51 425, 55 012, and 55 023, http://www.ndbc.noaa.gov/dart.shtml) and 3 tide gauges (Lata Wharf, Honiara, and NHESSD Lautoka, http://www.ioc-sealevelmonitoring.org) that distinctly recorded a tsunami sig- 3, 1949–1970, 2015 nal and that allows a good azimuthal coverage (Fig. 2, further details in Supplement). 5 Before using the tsunami data in the inversion, we remove the tide from the original signals by using a robust LOWESS procedure (Barbosa et al., 2004). 6 February 2013 The fault model geometry can greatly influence the results of source inversion. Mw 8.0 Santa Cruz Adopting a fault geometry that honours the complexities of the subduction interface Islands Tsunami then may help to reduce the epistemic uncertainties associated to forward modelling F. Romano et al. 10 (Romano et al., 2014). This is particularly true for earthquakes of this size occurring in subduction zones characterized by strong variations of strike and/or dip (e.g. Hayes et al., 2014b), even more so in complex tectonic environments like the Santa Cruz Title Page Islands region. Thus, analysing the aftershocks distribution occurred after the 6 February main- Abstract Introduction 15 shock, the local seismicity, and considering the rupture area expected for a M8 event, Conclusions References we built a 3-D non-planar fault model with variable strike and dip angles in order to account for such geometrical complexities of the subduction interface on both the Tables Figures New Hebrides and Solomon trenches (Bird, 2003).