During the 20th century, improvements in and greater use of seismic
instruments such as seismographs enabled scientists to learn that earthquakes
tend to be concentrated in certain areas, most notably along the oceanic
trenches and spreading ridges. By the late 1920s, seismologists were beginning
to identify several prominent earthquake zones parallel to the trenches that
typically were inclined 40–60° from the horizontal and extended several hundred
kilometers into the Earth. These zones later became known as Wadati-Benioff
zones, or simply Benioff zones, in honor of the seismologists who first
recognized them, Kiyoo Wadati of Japan
and Hugo Benioff of the United
States. The study of global seism city
greatly advanced in the 1960s with the establishment of the Worldwide Standardized
Seismograph Network (WWSSN) to monitor the compliance of the 1963 treaty
banning above-ground testing of nuclear weapons. The much-improved data from
the WWSSN instruments allowed seismologists to map precisely the zones of
earthquake concentration world wide.