Geoscience Reference
In-Depth Information
the waves' period and amplitude could be measured. Such traces became
all the more interesting in 1889, when a German astronomer discovered
by accident that his instruments in Potsdam had inadvertently recorded a
strong earthquake near Tokyo: they had detected waves from a spot 5,500
miles away on the earth's surface. The realization that earthquakes could be
mechanically recorded at such a distance inspired many new seismographic
inventions at the end of the nineteenth century.
But were these ingenious mechanisms any more reliable than human
witnesses? The first generation of seismographs was subject to small “self-
oscillations” due to lack of damping, making it hard to record long-period
motions. Few were able to record vertical motion. With further improve-
ments, seismographs became more trustworthy; but humans did not
necessarily become less so. At the turn of the twentieth century, scien-
tists studying seismographic curves discovered two kinds of seismic wave
traveling through the earth's core: a shorter period “primary” (compression)
wave and a longer period “secondary” (transverse) wave. Later research also
found several varieties of surface wave, which pass only through the crust.
It soon became clear that many apparent contradictions in the felt reports
of years past—conflicting accounts of jolts and rolling motion, “horizon-
tal” and “vertical” shocks—were resolved if one admitted that people, too,
could feel the differences between these types of wave.
23
Moreover, recent
evidence has suggested that humans may be more acute observers than even
the latest computerized seismographs. A 2008 study concludes that “events
with a magnitude smaller than 1, and even negative magnitudes [meaning,
on the logarithmic scale, fractions of a unit shock], can be felt, thus mak-
ing the human being an instrument eventually much more sensitive than
monitoring networks.”
24
nonetheless, by the early twentieth century, the availability of seismo-
graphs allowed many earth scientists to dream of turning their discipline
into a quantitative, objective science, modeled on physics. They trans-
formed what counted as evidence of the earth's history. out went data
filtered by human bodies; in came the “hard” evidence of seismographs
and accelerometers. This was the moment when scientists began to distin-
guish the “new seismology” from the “old.” Among the achievements of
the new seismology was the ability to use instrumental traces of the passage
of seismic waves as clues to the internal structure of the earth. The seismo-
graph became a telescope trained on the earth's hidden depths, where it
revealed a core of iron buried under a mantle of rock.
25
The next “revolu-
tion” came with the acceptance of plate tectonic theory, the “new geology,”
in the 1960s.
26
Plate tectonics overthrew the belief that the positions of
