Our ability to evaluate present and
future earthquake hazards is based
on understanding the past behaviour of seismogenic
(earthquake-producing) faults. Basic data for earthquake-hazard
evaluation include the location, length, and amount of fault
displacement; intensity of shaking; size or magnitude; and dates of
previous earthquakes. This information is generally available for
earthquakes in recent decades since local, regional, and global seismic
networks of recording instruments have been used. To study historic
earthquakes that predate the use of recording instruments, we rely on
written accounts, coupled with geologic and geomorphic evidence. The
historic record ranges from a few hundred years in the United States to
a few thousand years in China. For prehistoric earthquakes, the only
source of information is the geologic and geomorphic record.
Paleoseismicity is defined as the study of the occurrence, size,
timing, and frequency of prehistoric earthquakes. Paleoseismicity,
which most often utilizes the Pleistocene and Holocene (last 1.65 m.y.
and last 10 k.y., respectively) geologic and geomorphic record, extends
over a much longer time period than the limited historic record of
seismic activity. This is particularly important because the recurrence
intervals for large, damaging earthquakes on many fault segments in the
most tectonically active regions on Earth are often a few hundred to a
few thousand years. In regions with lesser tectonic activity,
recurrence intervals may be tens- to hundreds-of-thousands of
years. In particular, paleoseismological study followed several
main lines of investigation: 1) detailed mapping of fault scarps
produced by (and geomorphic features displaced by) contemporary and
recent large earthquakes and study of its geology and geomorphology; 2)
trenching along the fault scarp; 3) study of strandline features
uplifted or downdropped by earthquakes, and 4) study of deposits
related to seismic shaking such as turbidites, landslides, rockfalls,
plus study of tsunami deposits. Observations that can be obtained from
these investigations provide information to estimate some of the main
parameters that characterize a seismogenic fault: extent of the rupture
and distribution of slip during an individual earthquake, time elapsed
since the last large earthquake, recurrence intervals and variations in
recurrence intervals and earthquake size, and slip rates.