Earthquakes - Extreme Events: A Physical Reconstruction and Risk Assessment - page 174

microatolls grow through deposition of annual layers of calcium carbonate. The
shape of these layers reflects the nature of any sea-level changes occurring during
thelife of the coral (which can be several hundred years). During submergence
of the coral the uppermost layer, which is normally restricted by the lowest tide
level, experiences unrestricted growth and will form a 'cup shaped' morphology.
This can contrast to the earlier deposited layers which may be flatter in shape
reflecting a period of more stable sea level. During emergence a coral head devel-
ops a conical shape because of the progressive fall in the level of the lowest tide.
Episodes of sudden emergence and submergence are typically caused by vertical
displacements of the Earth's crust during major earthquakes. During episodes
of sudden submergence the microatoll layering shows a steep upward step from
a flatter surface. During sudden emergence the microatoll develops a 'hat-shape'
morphology where the hat 'brim' represents the continued growth for decades
after the event. The step from top of the bowl to the brim is a measure of the
vertical displacement during the event.
Natawidjaja (2002)analysed a 250 year long record of vertical deformation
from four coral microatolls above the Sumatran subduction zone. The most
prominent signal recorded by the corals is an 80 cm emergence event associated
with the M w 7. 7earthquake in AD 1935 which resulted in a 2.3 m slip on the
subjacent subduction interface. A 17 cm emergence occurred in AD 1797 and a
greater than 10 cm emergence occurred in AD 1743. Natawidjaja (2002)estimates
that the average slip on the subduction zone would have been about 0.5 m
(

M w 7.0) in 1743.
Earthquakes in AD 1833 and 1861 also appear to be registered by the 250
year long coral record. These events are the smallest earthquakes recorded here
resulting in a 10 cm die-down of the coral head growth in 1833 implying about
30 cm of slip on the subduction interface, and a 5 cm die-down in 1861 implying
about 15 cm of slip. Since the palaeoearthquake studies of Natawidjaja (2002)
and Zachariasen et al .(1999, 2000)thisregionexperienced the M w 9.3 earthquake
of December 2004 which resulted in at least 2 m of slip on the subduction
interface clearly representing the largest earthquake here for the last 250 years.
Further slip just to the south of the 2004 earthquake site then occurred in March
2005 with an 8.7 magnitude earthquake. Indeed this latter earthquake resulted
in substantial uplift/emergence of coral reefs and increased the land area of
islands considerably.

∼ M w 7.0) in 1797 and at least 0.3 m ( ∼

Conclusion

The severe shaking of the lithosphere during earthquakes results in sig-
nificant deformation of rock, sediment and soil layers. Fault terminations, sand