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in the epicenter area in the Visp valley during the first shock of the ground - often
reported as detonations or cracking shocks - was evidence enough for the geologist
to consider the hypothesis that the subterranean hollow stratum would lead to a sub-
sequent undulating movement of the surface in the neighboring countries, moving
towards the north much farther than towards the south. His basic assumption, still
accepted today, is based on the idea that earthquakes depend on definite conditions
that are present in the ground. In terms of these ideas, the cause of the shock is
explained by the fall of a heavy body from the vault of a cave, or the collapse of a
mountain rock to the base of a hollow layer. The direct effect of such a subterranean
shock is similar to a shock to water covered with ice. Since the ground can be more
easily compressed than water, the shock will propagate deep downward. But since
the static vault of the ground prevents the shock to reach the Earth's surface, the
undulation will, in contrast to an uncovered fluid, propagate also horizontally in all
directions. The direct shock can thus be felt only under the location of the shock,
at any other place it is covered by the undulation that already has started. It might
have been this hypothesis that led Volger to scale the damage and plot a map. This
very precise map evidently shows that the propagation of the main shock on July
25, 1855 was at least three and a half times stronger towards the north than the
south. Volger assumed that this effect was caused by the washout of strata in the
structure of the northern wing of the basin below the Gorner-Visp valley between St.
Niklaus and Stalden. On the other hand, Volger strongly denied the explanation that
earthquakes are cut off by mountain ridges, in this case by the Jura and the Alps.
In the framework of his collapse theory, Volger provided the following plausible
explanation of different behaviors of mountains and buildings during earthquakes:
while mountains are comparable to large ships, below which several seismic waves
propagate simultaneously so that the mountains as such cannot start to vibrate them-
selves, in contrast to high buildings and in particular towers, which are subject to
different processes due to their small ground area in comparison to their height. With
this hypothesis Volger reassessed Werner's (Abraham Gottlob Werner, 1749-1817)
original idea of the collapse of whole strata rather than providing new ideas to the
causes of earthquakes (Oeser 2003).
5 Early Macroseismic Maps
On the other hand, the depiction of macroseismic maps is the most fascinating part
of Volger's work. Equivalent with the collection of seismic data, earthquake-map
plotting has its history, too, going back to the above mentioned map design by
Mogiol for the Nice earthquake of 1564. In the first half of the 19th century, P.N.C.
Egen and J.J. Noggerath already drew earthquake maps showing seismic inten-
sity. After these pioneer cartographic earthquake portrayals, the two maps of the
Visp 1855 event, both designed by A. Petermann, the first based on preliminary,
the second on detailed macroseismic information provided by Volger, led to an ad-
vanced step of macroseismic study both concerning the collection of data as well as
map plotting.
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