A Brief History of Seismology Part 2: The Birth of the New Seismology

Considering earthquakes as a manifestation of divine wrath, attributing them to winds, considering the to be a part of the subtle essence of qi, as well as the invention of the first modern seismoscope and the first systematic catalogs of shocks – the 18th century and the beginning of the 19th century, led to some important discoveries in the field of seismology. Although these were fruitful to some extent, earthquake science at that time was still lacking an appropriate method of recording earthquake motion. Nevertheless, the world of Earth science was at the forefront of the »new seismology«.

Seismology as a profession

It was Italians who invented the so-called »tromometer«, but the Japanese government was the one who started bringing foreign experts to Japan to develop seismology as a profession. Among the most important ones was John Milne (1850–1913) who later led the foundation of the Seismological Society of Japan.¹ Such and other similar organizations, for example, the Meteorological Agency, soon led to seismology becoming a university department; Sekiya Seikei (1855–1896), a Japanese geologist and one of the first seismologists, for example, became the first professor of seismology in 1886.

As earthquake science was starting to hit off, it also tried to develop some kind of a system or routine of reporting earthquakes and to record the time history of felt shaking. J.A. Ewing (1855–1935), a Scottish physicist and engineer, and Milne were among the first scientists who developed the first good records of ground shaking around 1880. Although the records turned out to be just a »few simple pulses and not the purely longitudinal motion« as well as much smaller and more complicated than first assumed, they exposed another problem in trying to explain the observed ground motion. Science wasn't namely sure about how much of the ground motion actually comes from earthquakes and how much from the complexity of the Earth's wave propagation.

The holistic approach to Seismology and the invention of the first instruments

Earthquake scientists, including the leading seismologist Milne, have, nevertheless, started to study earthquakes as a whole, interpreting all of their aspects and elastic waves, while including instrumental measurements. Combining theoretical knowledge with instrumental improvements eventually led to the birth of »the new seismology«. Although these focused more or less on local shaking, they provided the basis for a different approach and measurements, some of which were sensible and some of which were purely experimental. Out of primarily astronomical interest, E. Von Rebeur-Paschwitz (1861–1895), a German astronomer, geophysicist, and seismologist, »built sensitive horizontal pendulums for measuring tidal tilts« that soon showed some transient disturbances and correlated with one of the Japanese earthquakes. In other words, this demonstrated that one could actually measure and record distant earthquakes; such discoveries soon led to new development in seismology as well as in seismic instruments, for example in Italy and Japan.

This way the so-called »global seismology« – instead of local – started to gain interest; Milne, for example, delivered a design for an inexpensive seismometer, which led to the development of a global network of seismic instruments. These were low-gain and undamped and able to detect larger shocks but unfortunately unfit to also show their waves and other details. Seismology at this point was already developed to some extent but a major breakthrough came a bit later with the arrival of advancement in instrumentation. In this field, two seismologists, E. Wiechert (1861–1928), a German physicist and geophysicist, and B.B. Golicyn (1862–1916), a Russian physicist, played an important part: Wiechert introduced his inverted-pendulum sensor, the first properly damped seismometer, in 1904, while Golicyn »applied electrodynamic sensors and photographically recording galvanometers to create instruments of unprecedently high sensitivity and accuracy.«²

Sorting out different waves of the Earth

Such instruments were later slowly installed at different observatories around the world, which highlighted a new problem: observing different »phases« they first had to sort them out and relate them to different kinds of Earth's waves. These were also closely connected to the studies on Earth's interior, which were an active subject in the 19^th century, and according to which the Earth’s depths were hot and dense, while it still wasn’t clear what parts were liquid (and gaseous) or solid. Due to Earth’s solid surface, the elastic waves were able to travel, which was an idea proposed by John William Strutt (1842–1919), an English mathematician, in 1885. Although his findings and observations contributed to the general understanding of the so-called wave (Rayleigh) scattering, it »was not known which of these wave types would occur, and how much they might become indistinguishably confused during propagation. Given that the Earth was an inhomogeneous body, and that rocks were anisotropic, seismologists had a wide range of options available to explain the observations.«³

Picture 1: Earth's interior.

A variety of observations and studies enabled the seismologists to make a clear and obvious distinction between »main phase« (surface waves) and the so-called »preliminary tremors« (body waves). The latter were later, in 1900, classified in two phases by R.D. Oldham (1858–1936), a British geologist; these caused some confusion as to the measurements and the particle motion were different from the ones from Rayleigh's theory. That's why Emil Wiechert (1861–1928), a German physicist and geophysicist together with some other scientists in 1904 proposed the idea of marking them with the letters P, S, and L, which only referred to the timing (or form) but not also to their type. Wiechert and his students at Göttingen also became leaders and experts of the subject; along with G. Herglotz (1881–1953), a German Bohemian Physicist, he came up with the »first solution to the geophysical inverse problem of deducting wave velocities from travel times.«⁴ It wasn't merely Wiechert that contributed to wave-propagation studies – there were also others, for example, A. Mohorovičić, »who used data from the 1909 Kulpa Valley earthquake in Croatia to study travel times at relatively short distances, and found additional phases.«⁵

Picture 2: The representation of the P-, S- and L-waves.

Source:

Agnew, Duncan Carr. History of Seismology. Retrieved 2022-03-16. Source: https://igppweb.ucsd.edu/~agnew/Pubs/agnew.a66.pdf