18 Aug 2022
 | 18 Aug 2022

Earthquake Contributions to Coastal Cliff Retreat

Colin K. Bloom, Corinne Singeisen, Timothy Stahl, Andrew Howell, and Chris Massey

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.

Colin K. Bloom et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2022-643', Mark Dickson, 06 Sep 2022
    • AC1: 'Reply on RC1', Colin Bloom, 24 Sep 2022
  • RC2: 'Comment on egusphere-2022-643', Colin V. Murray-Wallace, 28 Dec 2022
    • AC2: 'Reply on RC2', Colin Bloom, 02 Feb 2023

Colin K. Bloom et al.


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Short summary
Earthquakes can cause damaging coastal cliff retreat but we have a limited understanding of how these infrequent events influence long-term retreat. This makes planning for this hazard a challenge. In this study, we use historic aerial images to measure coastal cliff-top retreat at a site in New Zealand. We find that earthquakes account for close to half of long-term retreat at this site and our results have helped us to develop tools for estimating the influence of earthquakes at other sites.