The direct occasion for the theoretical considerations by v. SEEBACH about earthquakes was the so-called
Central-German earthquake on 6th March, 1872, which he described in a monograph. V.SEEBACH carried out
a comparison to other earthquakes in Europe and it turned out that it had a considerable geographical coverage:
with the exeption of the Rhineland earthquake on 29th July, 1846, covering according to NÖGGERATH
probably an area of 3.848 square miles, and the Wallis earthquake that covered, according to VOLGER, an area
of 3.700 square miles, the Central-German earthquake had, after having excluded "suspicious" reports from
locations such as Passau, Munich, Heidelberg, etc., an estimated area of tremor of at least 3.100 square miles and
thus a large area than most other European earthquakes of this time. Also the Neapolitan earthquake on 16th
December, 1857, that covered according to MALLET, 1.843,7 German geographic square miles, was smaller in
its geographic extent, but much more violent in its effects. |
For v. SEEBACH the large spatial extent of the Central German earthquake was already pointing at the enormous depth of the earthquake centre and he hoped to determine the exact position of this centre by using his new measurement method. In addition, underground observations in coal mines showed that individual locations in a seismic area are in a "local earthquake shadow", although v. SEEBACH could not explain this fact. But these data together with the data of previous earthquakes showed that certain places on the surface of the earth remained undestroyed amidst an area of destruction, a phenomenon that HUMBOLDT, who is explicitly cited by SEEBACH in this connection, had called "earthquake bridges".
The different reports showed "satisfactory consensus" about the way the earthquakes were experienced. Generally they were described as an "undulatory" gradually shifting movement of the ground". Most of the reports that v. SEEBACH knew of mention two shocks or "swells", and some people even stated that the second shock was stronger than the first one. In some places only one shock occurred, while in other places 3, 4 or even 6 shocks or waves were experienced. A fact that v. SEEBACH tried to explain as the effect of "local refractions and reflections". Reports about possible secondary shocks said to have preceded or followed the main shock on 6th March, at 4 p.m. are rejected by SEEBACH as a sensory illusion or as a "trembling of the ground caused by other factors" that was erroneously ascribed to the earthquake.
So only two shocks remained for him that were noticeable on the surface of the earth as undulatory movements of the ground. But when we apply the theory of longitudinal and transversal waves, the two shocks can be related to a single impulse of the origin, due to the different propagation speeds of the two kinds of waves. V. SEEBACH has some basic reservations due to the different time intervals between the two shocks at different locations:
"Although we are tempted to consider the two shocks to be only the result of a single shock that was propagated by longitudinal as well as by transversal vibrations, the observations of the time interval between the two shocks at different locations are by far not precise enough to prove it; according to the theory we would have to expect that the first shock would have to be the stronger one."
The huge differences in the temporal indications of the duration of earthquakes, between one second and 5-6 minutes, can be attributed according to SEEBACH to the lack of experience of the observer in estimating short periods of time. He believes that it is impossible to determine the limit of these errors of estimates and where real differences begin. Although v. SEEBACH considers the cause of the Central German earthquake to be unknown, as an expert of volcanism he focusses his attention on the sound phenomena that accompanied the earthquake. Often these sound phenomena were compared to the rolling sound of distant thunder or to the noise of car passing by. But in some places a strange roaring thunder during complete absence of wind, which was compared to the noise by many birds flying by. When reading such reports v. SEEBACH remembers his own experiences when he climbed the volcanoe of Telica in Nicaragua and one occasion during his stay in Santorini:
"The air hardly moved and we were scared three times by a strange loud noise. It started like a strong wind, became stronger and like a roaring thunder of a blowing engine in a furnace and rattling and clattering, like a train passing by in a certain distance, until the noise receded and suddenly stopped. It was a strange and loud thundering and roaring, and each time we were not sure about the direction from where the noise came from, from above or below. First, I thought it would be a sudden thunderstorm, while my colleagues thought it would be like the roar of a rapids or the rumbling down of a block of rock. Only one year later on Santorini, I was sure that this noise was of subterranean and volcaneous origin and doubtlessly caused by strongly compressed gases that escaped."
The reports deviated from each other concerning the question whether these sound phenomena were heard before or after the actual earthquake. Many reports did not give any information in this respect. In addition it was very difficult to determine whether these noises did not emerge in the buildings themselves as an effect of the earthquake.
V. SEEBACH also criticized the sources concerning the indications of the directions of the earthquake shocks. The directed arrows that he drew in his map according to these reports did not show any rules. For SEEBACH the main reason for these differences is that the direction of the wave movement was confused with the so-called "azimuth" in astronomical terminology that indicates only the direction (cardinal point) but not the origin of the wave movement. Often the direction of the impulse of the inertia moment of a foreign or one's own body that is diametrically opposed to the real direction of the shock, was confused with the direction of the propagation of the wave movement, although it had been known for a long time (see the predecessors of MALLET) that the upright bodies hit by a shock will fall into the opposite direction.
The fact that v. SEEBACH is also influenced by MALLET's theory of sea quakes is shown by the special attention that he pays to the rare reports about wave movements in rivers and lakes located in the area hit by an earthquake. Although there are reports about westward movements on ponds, marshy pools or ditches, but these observations do not prove the direction of the shock, only the general direction (cardinal point), since it remains undecided whether the observed waves are those that MALLET called "forced" or just those that follow them.
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