Archaeoseismology and Palaeoseismology in the Alpine ... - Tierra

Prior to the present study all evidence of non‐seismic
activity, which could affect results, had to be discounted.
Intensive study of the paleoclimate, the geology of the
cave and its hyrdrogeological setting, allowed us to
exclude evidence for damage from numerous sources.
They include anthropogenic activity, ice creep,
underground glaciers, frost action, erosion, soil creep,
effects of water circulation, flooding, debris flow,
incations caused by non‐earthquake related activity and
slope movements (Becker et al., 2006).
The cave is ca. 50m2 in area and divided into two
chambers. In an upper chamber speleothems are
abundant. For this chamber a plan was made in order to
document sample location (Fig. 3A). Throughout this
chamber evidence of collapses is visible in broken
speleothems (Fig. 3B), fallen stones and a fallen segment
of its wall. In the lower chamber chalk and cherts are
exposed, while only few speleothems are found, and
those are predominantly below cracks (Fig. 3C).
DATING OF BROKEN SPELEOTHEMS
Broken speleothem samples were collected from all parts
of the upper chamber of the cave (yellow markers in Fig.
3). The samples consist of broken stalagmites and
stalactites and flowstone cores drilled from the cave floor
exposing fallen and embedded speleothems. For each
seismite sample a seismic contact (i.e. the precise location
at which the laminae of the speleothems are broken. See
Fig. 4) was established and the closest laminae to each
contact were sampled with a small drill. The last lamina
which grew before the break event was termed a pre‐
seismic sample, while the first one to grow after the break
event was termed a post‐seismic sample (Fig. 4).
In this study, dating with the U‐Th method was conducted
on the Multiple Collector Inductively Coupled Plasma
Mass Spectrometer (MC‐ICP‐MS) at the Geological Survey
of Israel. The MC‐ICP‐MS produces high analytical
precision results and enables work on small samples (ca.
0.3g) that, due to an ability to sample individual laminae,
give ages at high resolutions. The ages found for Denya
Cave were calculated as single samples using the age
equation according to Broecker and Kaufman (1965).
Ages determined for seismite samples were seen to form
clusters at very specific time intervals. Since some
samples had high detrital content, their ages had to be
corrected (Kaufman et al., 1998). In order to correct for
detrital Th, the samples were divided according to the age
groupings and plotted on a three dimensional isochron
where x=230Th/238U, y=234U/238U and z=232Th/238U
(Ludwig and Titterington, 1994), using the Isoplot3.7
program. Ongoing research indicates that those clusters,
when falling on a reasonably accurate three dimensional
isochron plot, are considered to be groupings of a single
age indicating one seismic event.
RESULTS
A total of 68 speleothem samples was taken and
inspected from Denya Cave, 32 of which were broken and
thus, identified as likely to indicate seismic activity. Some
of those samples had more than one seismite (i.e. a
record of a single seismic event). The ages obtained from
1 st INQUA‐IGCP‐567 International Workshop on Earthquake Archaeology and Palaeoseismology
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those seismites vary greatly. They indicate seismic activity
during the last 350 ka; older samples are in secular
equilibrium and cannot be further dated.
A seismic event is associated with the age clusters, but
only when seismites appear to be sound indicators of a
physical break in the speleothems examined and when
there are pre‐ and post‐seismic event samples in the
same cluster. Results further show that during the last 80
ka at least five seismic events occurred, causing damage
to speleothems in Denya cave (~11, 22, 28, 37 and 55 ka).
Indicators for an event at ~5 ka were also found, but they
are not strongly substantiated by the types of seismites
representing them. Indicators for older events that
affected the cave were found, but due to the relatively
large error on these ages, we could not identify clusters.
CONCLUSIONS
Fig. 4: A seismite from Denya Cave, sample DN‐7
The dating of speleothems using the U‐Th method,
combined with their subsequent correction for detrital
Th, employing the three dimensional isochron plots,
represents a new approach to identifying seismic events
in the Quaternary. This research in Denya Cave, Mt
Carmel, Haifa, indicates that speleothems there recorded
five seismic events that caused damage to the cave during
the last 80 ka. The latest such event occurred between 11
and 10 ka. Yet another, younger event, at ~5 ka may also
have taken place, but additional evidence is necessary to
validate that observation.
Acknowledgements: This research was funded by the Israel
Science Foundation, Grant no. 1006/2004 to A. Agnon. The
authors wish to thank M. Stein, G. Hartman, A. Vaks, and T.
Zilberman for useful discussions. All the staff of the Geological
Survey of Israel who helped with all technical matters: S.
Ashkenazi, Y. Mizrachi, C. Hemo, N. Tapliakov, I. Segal and O.
Yoffe. L. Magagritz, M. Laskow, E. Braun, M. Kanari, U. Ryb for