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Potential geomorphic consequences of a future Great (Mw 8.0+) Alpine Fault earthquake, South Island, New Zealand

Copernicus Publications on behalf of European Geosciences Union
Natural Hazards and Earth System Sciences
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The Alpine fault in New Zealand's South Island has not sustained a large magnitude earthquake since c. AD 1717. The time since this rupture (295 years) is close to the average inferred recurrence interval of the fault (~300 years) and the Alpine fault is therefore expected to generate a large magnitude earthquake in the near future. Previous ruptures of this fault are inferred to have generated Mw 8.0 or greater earthquakes and to have generated, amongst other geomorphic hazards, large-scale landsliding and landslide dams throughout the Southern Alps. There is currently 85% probability that the Alpine fault will cause a Mw 8.0+ earthquake within the next 100 years. While the seismic hazard is fairly well understood, that of the consequential geomorphic activity is less well-studied, and these consequences are explored herein. They are expected to include landsliding, landslide damming, dambreak flooding, debris flows, river aggradation, liquefaction, and landslide-generated lake/fiord tsunami. Using evidence from previous events within New Zealand as well as analogous international examples we develop first-order estimates of the likely magnitude and possible locations of the geomorphic effects associated with earthquakes. Landsliding is expected to affect an area >30,000 square kilometres and involve >1 billion cubic metres of material. Landslide dams are expected to occur in many narrow, steep-sided gorges in the affected region. Debris flows will be generated in the first long-duration rainfall after the earthquake and will continue to occur for several years as rainfall (re)mobilises landslide material. In total more than 1000 debris flows are likely to be generated at some time after the earthquake. Aggradation of up to 3 m will cover an area >125 square kilometres and is likely to occur on many West Coast alluvial fans. The impact of these effects will be felt across the entire South Island and are likely to continue for several decades.
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... Area of Figure 12 is shown as a red rectangle. Source: This work includes Toit u Te Whenua Land Information New Zealand data, which are licensed by Toit u Te Whenua Land Information New Zealand for re-use under the Creative Commons Attribution 4.0 International licence [Color figure can be viewed at wileyonlinelibrary.com] in alpine sections of the river (Robinson & Davies, 2013;Wells & Goff, 2006) and (2) flooding and sedimentation-induced damage to farms and property, and infrastructural lifelines (Orchiston et al., 2018). Most of the understanding of related sediment fluxes is based on (1) decadal landslide inventories in the Southern Alps (Hovius et al., 1997); (2) experience from non-earthquake-related landslides in individual catchments (e.g., Korup, 2005); and (3) volumes of landslidegenerated sediment from elsewhere in New Zealand or globally, calibrated against modern sediment loads and an assumption of steady state processes in the Southern Alps where denudation balances mountain building. ...
... (2013) of 2-4 m and assume no sediment bypass in the calculation. It is likely that the catchments of the range front fans generate disproportionately high yields compared with the region-wide average sediment yields proposed by Robinson and Davies (2013) because of their proximity to the fault, steeper than average slopes and weak cataclastic rocks. Our 95% CI estimates of the duration of the relaxation phase (duration of aggradation) range from effectively instantaneous to nearly two centuries based on OxCal models (Table 2) These average rates obscure the likely punctuated and locally thick sedimentation that would make maintaining pasture and farm infrastructure impossible, as demonstrated by the Poerua River aggradation following the 1999 Mt Adams landslide (Davies & Korup, 2007). ...
... Blagen et al. (2022) suggest that this effect may lead to less aggradation at fan heads and more towards fan toes, with a more rapid progression of aggradation down-fan. Robinson and Davies (2013) noted that few alluvial fans are currently aggrading and that most fan heads are incised, as we see at the Te Taho fans. Therefore, most West Coast alluvial fans responding to Alpine Fault earthquakes are likely to have relatively short response times as Davies and Korup (2007) (Wang et al., 2017). ...
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We examined the stratigraphy of alluvial fans formed at the steep range front of the Southern Alps at Te Taho, on the north bank of the Whataroa River in central West Coast, South Island, New Zealand. The range front coincides with the Alpine Fault, an Australian-Pacific plate boundary fault, which produces regular earthquakes. Our study of range front fans revealed aggradation at 100- to 300-year intervals. Radio- carbon ages and soil residence times (SRTs) estimated by a quantitative profile devel- opment index allowed us to elucidate the characteristics of four episodes of aggradation since 1000 CE. We postulate a repeating mode of fan behaviour (fan response cycle [FRC]) linked to earthquake cycles via earthquake-triggered land- slides. FRCs are characterised by short response time (aggradation followed by inci- sion) and a long phase when channels are entrenched and fan surfaces are stable (persistence time). Currently, the Te Taho and Whataroa River fans are in the latter phase. The four episodes of fan building we determined from an OxCal sequence model correlate to Alpine Fault earthquakes (or other subsidiary events) and support prior landscape evolution studies indicating ≥M7.5 earthquakes as the main driver of episodic sedimentation. Our findings are consistent with other historic non- earthquake events on the West Coast but indicate faster responses than other earth- quake sites in New Zealand and elsewhere where rainfall and stream gradients (the basis for stream power) are lower. Judging from the thickness of fan deposits and the short response times, we conclude that pastoral farming (current land-use) on the fans and probably across much of the Whataroa River fan would be impossible for several decades after a major earthquake. The sustainability of regional tourism and agriculture is at risk, more so because of the vulnerability of the single through road in the region (State Highway 6).
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... In addition, earthquakes can cause a large amount of landslide deposits to accumulate in channels or on slopes. Once these loose materials are transported from the slope to the channel, they are often transported out of the basin by debris flow, resulting in a series of geological hazards and geomorphic consequences (Fan et al., 2019;Robinson and Davies, 2013). Furthermore, catastrophic earthquakes can cause surface deformation, cracks and severe vegetation destruction, destabilizing the geological environment and leading to higher landslide activity in subsequent years or even decades Fan et al., 2019;Huang and Li, 2014;Koi et al., 2008;Wasowski et al., 2011;Xiong et al., 2020;Zhang and Zhang, 2017). ...
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... Various geomorphological indexes have been proposed to quantify fault activity (Troiani and Seta 2008;Arrowsmith and Zielke 2009;Parul et al. 2013;Troiani et al. 2014;Gao et al. 2015;Liu et al. 2015;Liu and Du 2016;Kaushal et al. 2017;Wang et al. 2017;He et al. 2018;Pavano et al. 2018;Sharma et al. 2018;Wang et al. 2019). Terrain analysis of seismogenic faults has been well documented in studies areas around the world (Cello et al. 2000;Lin et al. 2003;Pucci et al. 2003;Maschio et al. 2005;Irikura 2012; Mahmood and Gloaguen 2012 ;Robinson and Davies 2013 ;Koukouvelas et al. 2018), and has become a major method for studying neotectonics, seismogeology, and paleoseismology (Papanikolaou et al. 2015). In addition, Noriega et al. (2006) and Chen et al. (2013) demonstrated that deflected streams can be used to compute fault slip rate in history by sediment dating on the offset stream channels. ...
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... As identified in previous studies, the Franz Josef township is threatened by a number of hazards [7]. A review of existing studies and consultations with experts have identified several components of the disaster system in the Franz Josef area [9,11,13,[51][52][53][54][55][56]. The hazard elements used in the present modelling are earthquake shaking (due to the activity of the Alpine fault and other sources), rainfall, flooding from the Waiho river, landsliding, and landslide dam in the Callery catchment; while the potentially vulnerable elements are houses, roads and the stopbanks of the Waiho river. ...
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