Earthquakes - Extreme Events: A Physical Reconstruction and Risk Assessment - page 173

seismic event can be identified by a growth response occurring synchronously
in at least two separate trees. The ring-width analysis from undisturbed trees is
combined into a site and species specific control series. Ring-width plots from
disturbed trees are then compared to the control chronology in order to identify
episodes of severe growth change resulting from past seismic events (Sheppard
and Jacoby, 1989).
Large earthquakes can also induce significant forest disturbance where trees
are literally shaken to the ground resulting in high mortality rates. Where trees
are long lived, the age structure of these vegetation communities can reveal a
story about the history of earthquakes when other possible disturbance mecha-
nisms are eliminated. Wells et al .(2001)were able to identify four major earth-
quake events on the South Island of New Zealand, based upon the age structure
of forests in the Westland region. These earthquake events occurred around
AD 1820--1830, 1710--1720, 1610--1620 and 1460. They found that over 80% of the
1412 ha forested area studied currently comprises simple, first generation cohorts
of trees established after the last catastrophic earthquake disturbance. This left
only about 14% of the area inhabited by complex mature all-aged forest. This
means that there appears to be a region wide over-abundance of mature trees
and a lack of small to intermediate size trees. This feature of the demographics
of the forest population had long intrigued scientists studying the area. This
'regeneration gap' had previously been thought to be possibly due to climatic
cooling, an increased frequency and severity of droughts and decreased pre-
cipitation. However, Wells et al .(2001)suggest that major earthquakes play a
significant role in structuring the forest ecosystems and that the reasons for the
regeneration gap is likely to be due to the long period of time since the last
major earthquake on the Alpine Fault which was about 280 years ago. It is likely
that three of the major disturbances to the forest were due to earthquakes on the
Alpine Fault and the last disturbance was due to an earthquake to the south of
theregion in AD 1826. Interestingly, it appears that this last earthquake caused
asubstantial tsunami around the shores of New Zealand's South Island which
has been identified in sediment cores from Okarito Lagoon, Westland (Goff et al .,
2004).
Coral records of earthquakes
Coral microatolls can record earthquakes where they are subject to
episodes of submergence or emergence due to vertical movements of the Earth's
crust during major earthquakes. Coral microatolls are a reliable recorder of
sea levels. The upward limit of coral growth is limited by the lowest tides
above which the coral becomes exposed to the atmosphere and can die. Coral