Earthquakes - Extreme Events: A Physical Reconstruction and Risk Assessment - page 164

Samples of these sedimentary sequences are often taken in the form of cores
obtained from aboard a vessel. The cores can be several metres long and are
often sliced into smaller sections before being stored in refrigerated conditions.
The cores are analysed via detailed visual description, X-radiography, magnetic
susceptibility, major element analysis, grain size distribution and microfossil
presence and abundance. Sedimentation rates can be determined after obtaining
aseries of radiocarbon ages at selected intervals, or from sections of the core
that appear to show distinct changes in sedimentation style.
Karlin and Abella (1996)undertook a study of sediments in Lake Washing-
ton, USA, in order to investigate the history of large earthquakes in the north-
west Pacific region. They used magnetic susceptibility profiles to characterise
thesediment lithology, which they found to be more reliable than grain-size
measurements alone. The magnetic susceptibility analysis was able to identify
concentrations of magnetic minerals, which better identified individual sedi-
mentation events. In most of the samples, a peak in the magnetic susceptibility
was associated with a 1100 year old turbidite layer near the top of the cores.
Karlin and Abella (1996) attributed this layer to a palaeoearthquake, which had
also been identified in studies using tree-ring analysis (Jacoby et al . 1992). Subsi-
dence of the land during this earthquake caused trees, formerly growing on the
lake shore, to become submerged into the lake water. The magnetic susceptibil-
ity peaks also varied directly with concentrations of an aluminosilicate (Al 2 O 3 )
and inversely with concentrations of silica (SiO 2 ) and organic matter content
as measured by loss-on-ignition (LOI). These results suggest that the sediments
deposited into the lake over the past 3000 years are a mixture of a terrigenous
fraction of detrital aluminosilicates and a lake-derived biogenic fraction of sili-
cates (diatoms) and organic matter. The silica and organic carbon reflect more
normal lake sedimentation. The combination and alternating relative concentra-
tions of these sediments and the aluminosilicates suggest that there had been
periodic changes in the terrestrial detrital sediment influx which is probably a
function of earthquake-generated sediment fluxes to the lake.
The presence and absence of shallow water planktonic and non-planktonic
species within sediment cores from lakes in these settings can also help identify
major changes in sedimentation or slumping of the basin walls in prehistoric
times. Organisms that normally live in shallow water, and the narrow littoral
edge of a lake, can be transported rapidly into deeper water sediments through
sudden slumping of the basin walls. Karlin and Abella (1996) also observed such
changes within their cores from the deeper water sediments in Lake Washington.
Anormallyriver-dwelling species was found in cores from the centre of the lake,
at the same core interval where the aluminosilicate peak occurred, suggesting
there had been an abrupt, short-lived change in the character of the water. There