Erosion Rates on Newly Uplift Marine Terraces Following the 2016
Kaikōura Magnitude 7.8 (Mw) Earthquake
Abstract
Since 1973 micro-erosion meters (MEM) have been used at Kaikōura
Peninsula to determine lowering rates on inter-tidal shore platforms.
Rates measured over two, two year periods (1973-1975 and 1994-1996) and
at decadal scales (20-30 years) demonstrate that platform surface
lowering is on average 1.1 mm/yr. The 14 November 2016 Kaikōura
magnitude 7.8 (Mw) earthquake caused an instantaneous uplift of 0.8-1.0
m of the peninsula. The uplift offers the rare opportunity to examine
how such an event alters processes and rates of erosion on these shore
platforms, since these are now partially marine terraces as the inner
margins of some platforms are now above high tidal levels (but perhaps
not storm surge). Since the earthquake, 42 MEM sites have been measured
seven times at 3 monthly intervals. Most recently in September 2018. MEM
sites show widely varying responses to the uplift. Erosion rates are at
some MEM sites three times the previous annual rate while other sites
show significant amounts of rock swelling (3-4 mm in 6 months), or
aggradation as weathered rock fragments are no longer removed by wave
action. The coseismic uplift has fundamentally changed the process
regime operating on these platforms. Zones of maximum wetting and drying
have migrated seaward causing previously slow eroding (< 1
mm/yr) MEM sites to accelerate to twice the pre-earthquake rates.
Erosion rates demonstrate rapid adjustment of the platform surface to
this disturbance and illustrate how uplifted marine terraces can be
rapidly eroded despite being above sea level. The preservation of the
new marine terrace is probably dependent on further uplift within the
next 300-400 years, otherwise erosion by lowering and backwear will
likely remove the new surface. This scenario has significant
implications for marine terrace preservation and the recording of
coseismic events in the landscape.