Earthquake Contributions to Coastal Cliff Retreat

Abstract

Abstract. Modeling suggests that steep coastal regions will experience increasingly rapid erosion related to climate change induced sea level rise. Earthquakes can also cause intense episodes of coastal cliff retreat, but coseismic failures are rarely captured in the historical record used to calibrate most cliff retreat forecast models. Here, we disaggregate cliff-top retreat related to strong ground motion and non-seismic sources, providing a unique window into earthquake contributions to long-term coastal cliff retreat. Widespread landsliding and up to c. 19 m of coastal cliff-top retreat occurred in the area of Conway Flat during the 2016 Kaikōura (New Zealand) earthquake despite relatively low (c. 0.2 g) peak ground accelerations. While coastal cliff-top retreat has been spatially and temporally variable over the past 72 years, historical aerial imagery suggests that large earthquake induced landslide triggering events disproportionately contribute to an average 0.25 m/year retreat at Conway Flat. The 2016 Kaikōura earthquake represents c. 24 % of the total cliff-top retreat over the past 72 years and c. 39 % of cliff-top retreat over the past 56 years. Additionally, significant retreat between 1950 and 1966 is likely the result of local seismicity. Together these two events account for c. 57 % of cliff-top retreat over the past 72 years. Earthquake-related debris piles at the base of the cliffs have been rapidly eroded in the 5 years since the 2016 Kaikōura earthquake (more than 25 % loss of debris volume) and there will likely be little evidence of the earthquake within the next decade. In regions with similar lithologic and coastal conditions, evidence of past widespread single-event cliff-top retreat may be limited or non-existent. The coastal cliffs at Conway Flat demonstrate the potential to significantly underestimate future cliff-top retreat using historical records.

Uplifted Pleistocene Marine Terraces at Active Margins: Modeling Reveals the Effects of Sea Reoccupation and Coseismic Uplift on Uplift Rate Calculation

“…Although we analyze β z and β x separately, downwear and backwear operate simultaneously in nature: their individual contribution to terrace erosion and shaping strongly depends on lithology, climate, and surface elevation relative to the local tidal range (Dickson et al., 2022; Payo et al., 2015), and can be amplified by phenomena such as intense storm surges or earthquake‐induced landsliding (e.g., Bloom et al., 2023; Brooks et al., 2012). Furthermore, in nature the erosional parameters ( β z , β x , h wb ) can vary depending on site‐specific response to local factors.…”

Coastal earthquake induced landslide susceptibility during the 2016 Mw 7.8 Kaikōura earthquake, New Zealand

“…The steep decrease in modelled PGA/PGV observed near the coast (Figure 8) could be a result of increasing distance from the seismic source south of Kaikōura. Here, coastal landslides concentrate within weaker actively eroded lithologies that fail at lower ground motions (Bloom et al, 2022b).…”

“…These units are, in places, overlain by less consolidated Pleistocene alluvial, fluvial, and beach deposits. Portions of the region's steep coastline, primarily south of the Haumuri Bluffs at Conway Flat (Figure 1), form steep coastal cliffs c. 50 to 150 m in height, many of which are subject to wave action at high tide (Bloom et al, 2022b). North of the Haumuri Bluffs, however, most coastal hillslopes are uplifted and buffered from direct wave action by shore platforms that have, in places, been anthropogenically modified to facilitate road and rail corridors (Figure 2; Mason et al, 2017;Stringer et al, 2021).…”

“…As a result of low coastal population density, little work has been done to estimate long term coastal retreat rates for the South Island of New Zealand. The few studies that have estimated retreat (Bloom et al, 2022b;Kirk 1975Kirk , 1977 suggest highly variable rates modulated by lithology and topography. Average rainfall measured in the township of Kaikōura is c. 721 mm yr -1 (Macara, 2014) but is highly spatially variable along the Kaikōura coast ranging from c. 675 to c. 1500 mm yr -1 (NIWA, 2022).…”

“…In many regions, oversteepening results from a combination of uplift and wave action that actively undercuts coastal cliffs (Emery and Kuhn, 1982). In the Kaikōura region, however, most steep coastal slopes, with the exception of those at Conway Flat (Bloom et al, 2022b), are currently isolated from direct wave action by recent uplift which forms shore platforms. Ages of these uplifted platforms (Howell and Clark, 2022) suggest that this isolation has lasted for several hundred years at the least.…”