A Brief History of Seismology Part 3: The Classical Period of Seismology during World War II

If the 19^th century and its major representatives presented Earth science with new instrumental improvements, observations, and studies of earthquakes as well as their wave generation, then the beginning of the 20^th century, especially between the period between 1880 and 1920, transformed seismology "from being primarily a descriptive natural history of earthquakes to having a large (and mathematical) component concerned with the wave propagation within the Earth¹."

From the Expansion of Seismic Stations to the International Seismological Association

This naturally meant that some of the ideas that were thought of before 1920 were now being improved. This period lasted for over forty years, from 1920 to 1960, and is today regarded as the so-called "Classical Period" in the history of seismology. Although this time may have been important in science and led to a huge change in physics, it didn't apply to seismology as it hasn't provided any substantial changes in its techniques and aims. This was partially due to a major setback from both World Wars but still, some improvements took place, such as an increase in the number of seismic stations that were being set up by individual organizations. Along with the increase in the number of seismic instruments some "global", as well as international organizations engaged in seismology, the most important being The International Seismological Association (founded in 1904 and revived in 1992). It was founded with the aim of promoting international cooperation and later created perhaps one of the most important entities for global seismic research: The International Seismological Summary (ISS). Along with good grounds for a "virtual" global network, the ISS also made the information about earthquake research collected worldwide available to individual researchers and stimulated further studies of seismic waves as well.

Picture 1: One of the modern seismic stations.

The Beginning of Instrumental Seismology

For some scientists, wave-propagation studies were a great research opportunity while for others, they were considered as an abandonment of the most useful part of the subject of earthquakes as seismology departed from its focus on earthquakes and their effects. Instead, such efforts resulted in new, this time more sensitive instruments, for example, sensitive electromagnetic seismometers and the so-called strain seismometer, which enabled better (local) seismic networks. This also led to a better quality of data and eventually to a shift from Germany to America where seismology focused on the application to the so-called subsurface exploration, usually in the oil industry. For instance, a portable seismograph invented by L. Mintrop (1880–1956), a German mine surveyor and geophysicist, was developed and formed the beginning of the exploration of seismology dominated by some US companies. What started as an experiment soon became the beginning of the so-called instrumental seismology.

Instrumental seismology definitely contributed to a period of rapid progress in global seismology. The studies and observations provided by the instrumental seismologists were fruitful as they began to focus on the global and local Earth structure. This way they not only figured out the seismic-wave velocity structure of the mantle and the core but also constructed epicenters and travel times of the earthquakes using the global data. Moreover, they showed that the "Earth is, in fact, a nearly spherically symmetric body, with only a few major internal discontinuities¹" such as in the Earth's core, which even obtained an inner core, and therefore "said to have completed the discovery phase of the inside of the Earth, though not its detailed mapping¹." Such earthquake studies, specifically the studies of crustal velocity structure, have also led to some other findings, for example, the finding of the crust-mantle discontinuity. For this purpose, several programs "that used large-scale seismic refraction to determine crustal structure¹" expanded and this changed significantly the branch of seismology after World War II.

Picture 2: Earth's structure as we know it nowadays.

From Larger to Smaller Earthquakes and the Birth of a Magnitude Scale

During the Classical Period, seismology not only provided the science with improved seismic instruments and a better understanding of the Earth's structure but also greatly improved the instrumental recording of earthquakes that were, at first, rather blurred. Nevertheless, it was the improvement of instrumental measurements that eventually led to a rather impressive and more complete compilation of earthquake catalogs. "The first major advance from instrumental measurements came from the dense local network in Japan, which enabled K. Wadati to distinguish (in 1927) between deep and shallow earthquakes¹." The results of K. Wadati (1902–1995), a seismologist at the Central Meteorological Observatory of Japan who researched deep earthquakes, have finally brought some clearance in understanding deep earthquakes and showed that these were relatively restricted in their distribution.

Along with a better understanding of deep earthquakes and their distribution seismic geography also benefited from other research: in comparison to Japan and Wadati's research of deep earthquakes, the seismologists working on the local network in Southern California detected rather smaller earthquakes and started to distinguish them from the larger and damaging ones. What may seem like an unimportant contribution, led C.F. Richter (1900–1985), an American seismologist and geophysicist, to transform Wadati's idea into a »magnitude« scale later used to describe the size of the earthquakes. Together with B. Gutenberg (1889–1960), a German-American seismologist, C.F. Richter extended the concept of a magnitude scale "from southern California shocks to earthquakes throughout the world. The magnitude scale, combined with the distinction between earthquakes of various depths, and the more reliable locations possible with improved travel times, came together in Gutenberg and Richter's study of the seismicity of the Earth¹." This also marked the birth of the later famous Richter scale used not only to describe the magnitude, e.g. the distribution of earthquakes by size, and the depth of earthquakes but also their effects.

Picture 3: Earthquake magnitude scale and its effect.

Earthquake distribution was definitely one of the greatest contributions of the so-called Classical Period of Seismology but the same could not be said of the understanding of the source of the seismic waves, which remained of the biggest mysteries during 1960 and thereafter.

Sources:
1) Agnew, Duncan Carr. History of Seismology. Accessed on 16-Mar-2022. Available at: https://igppweb.ucsd.edu/~agnew/Pubs/agnew.a66.pdf