Earthquakes - Extreme Events: A Physical Reconstruction and Risk Assessment - page 159

after an earthquake may be as small or large as those during coseismic uplift.
Japanese and Alaskan experience suggests the rates of aseismic movement gen-
erally decrease with distance from the subduction zone and with time following
an earthquake. The long-term aseismic uplifts along coasts bordering subduction
zones, however, occur at much slower rates than the short-term uplift occurring
shortly after an earthquake.
Emergent shorelines are former shorelines that have been uplifted and no
longer lie at an elevation where normal coastal processes can operate. Many
such shorelines have arrived at their present elevation due to coseismic uplift
during the Holocene. Historically, shorelines have been observed to experience
rapid uplift during large earthquakes. As a consequence, it has been suggested
that higher, older shorelines were likewise raised coseismically during seismic
events. Where a number of raised shorelines are present, it is possible to estimate
therecurrence and magnitude of the earthquakes responsible for their relative
elevations. In this sense, the heights of these shorelines are taken to be indirect
measures of the extent of plate-slip motion in the subduction zone. However,
three major assumptions are required when these features are used to estimate
palaeoseismic frequency and magnitude. These assumptions are:

(1) that long-term uplift rates are constant;

(2) theage and height data should approximate either time-predictable or

displacement-predictable models of earthquake recurrence; and

(3) each great earthquake should be represented by a separate shoreline

that is separate from shorelines that have formed during aseismic pro-
cesses.

The third point is the most important of the three assumptions because it
is often difficult to distinguish coseismic and aseismically uplifted shorelines
along coasts bordering active tectonic margins. This is because storms can build
successively lower beach berms on gently sloping coastlines that are rebounding
from glacioisostatic depression. Also, the rates of sediment supply may exceed
therate of any sea-level rise -- hence the coast appears to be experiencing uplift
but is in fact accreting. It is also possible for large storms to reoccupy emerged
strandlines (shorelines) and rework previously deposited beach sediments.
Reconstructing palaeoearthquakes using this method first requires identifi-
cation of individual shorelines and then determinations of their elevation and
distribution. Uplifted shorelines are often marked by sandy beaches or marine
terraces cut in consolidated bedrock, including coralline limestones. The ele-
vation of the shorelines can be measured from aerial photographs and in the
field using standard topographic surveying techniques. Lithological and strati-
graphic analysis of identified beach deposits (or former coral reefs) must also