Geoscience Reference
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(flood/rainfall) events. The results and comparisons were only moderately
successful in connecting rainfall events and seismic events to landslide activity.
Fantucci and Sorriso-Valvo (1999)suggested that the relationship between rainfall
and landslides is considerably more complex than the one between seismically
triggered mass movements. They found only two concurrences where rainfall
and seismic activity occurred at the same time as a landslide (an earthquake
between AD 1908 and 1910 and a 1910 rainfall event). While the identification
of landslide events and their causative events was moderately successful, there
wasaclear distinction between the dendrochronological series from the unsta-
ble and undisturbed areas (Fig. 7. 7 ). Hence, they were able to derive a record of
active mass movements.
Ring-width patterns are also analysed for cross dating and to possibly say
something about why the tree died or was subjected to impacts such as tilting.
Ring-width patterns are usually studied from each tree in the data set. Cross
dating is achieved by measuring all of the individual annual ring widths from
multiple radii (two to four) for each tree sample and by making statistical anal-
yses between samples. Essentially, cross dating involves the identification of a
ring or set of rings with a common characteristic separate from the other rings.
If characteristic X occurs in the outermost series of rings of a tree (Tree 1) and
thesame characteristic occurs in an inner ring of another tree (Tree 2) then the
former tree (Tree 1) is older or started growing before the latter tree (Tree 2).
The relative ages of trees in an area can then be determined by counting the
rings either before or after the rings displaying characteristic X .Ifthe absolute
ages of any of the rings in a tree that contains a ring with characteristic X can
be obtained, then accurate ages for any of the other rings can be determined.
This of course is subject to the rings being annual growth features. The tree-ring
patterns also allow the identification of the last growth ring produced before
thetree died. The anatomy of this ring can provide clues to the cause of death
of the tree (Jacoby et al ., 1992).
Jacoby et al .(1992)undertook a study of tree rings from trees submerged in
Lake Washington, USA following three identified prehistoric landslide events.
The presence of fire scars (the characteristic X in this case) in the sampled trees
(Douglas firs) aided the cross dating of samples. These trees can survive repeated
fires because of their unusually thick bark, but such fires can cause distinct
trauma or scarring in subsequent rings. These scars provide excellent markers
forthe cross dating. The Jacoby et al .(1992)tree-ring analysis found that the
trees had all died in the same year and season. It was suggested that the trees
died between fall and early spring as the outer ring of all the trees was fully
developed but without any initiation of the next year's growth. Radiocarbon
dates for all three landslides overlap, as did the age of one of the trees. This
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