(b. Nagoya, Japan, 8 September 1902;
d. Tokyo, Japan, 5 January 1995), seismology, Wadati-Benioff zone, ground subsidence.
Wadati is known as a seismologist who presented convincing evidence of the occurrence of earthquakes deeper than 300 kilometers (186 miles). He identified the intermediate and deep earthquake zone dipping away from oceanic trenches, now known as the Wadati-Benioff zone, thus establishing the basis of the plate tectonics hypothesis.
Early Life. Wadati’s grandfather was the fifth mayor of Sendai City, and president of local banks in the Tohoku district. His father died when Wadati was twenty years old and his brother died at the age of twenty-six. His wife pre-deceased him by several decades. In 1925 Wadati graduated from the Institute of Physics, Imperial University of Tokyo. After graduation, he was engaged in the observation of earthquakes at the Central Meteorological Observatory, where he discovered deep earthquakes.
Presentation of Convincing Evidence of Deep Earthquakes. In seismology, P waves are the primary, compressional waves, which travel fastest through the earth; S waves are the secondary, transverse waves. Shock waves are measured in at least three different locations to accurately determine the hypocenter of an earthquake. Arrival time refers to the arrival of the waves at the measuring stations. In 1928, in central Honshu (the largest of Japan’s four main islands), Wadati compared the consecutive curves of supposedly equal “S minus P” waves (showing the difference between the arrival time of S and P waves) and equal arrival time of P waves for the North Tazima earthquake, which occurred on 23 May 1925, and for the earthquake of 15 January 1927. The centers of the curves for both earthquakes were close to one another, so their epicenters must have been close. However, for the former earthquake the distances to the consecutive curves were less than those for the latter earthquake, while “S minus P” at the epicenter was about 10 seconds for the former and nearly 35 seconds for the latter. Based on the travel-time curves of a hypothetical (S-P) curve, he showed that the former earthquake took place at a depth of 30 kilometers (19 miles) and the latter was at a depth greater than 300 kilometers (186 miles).
The distribution of seismic intensity was different for the 1925 and 1927 earthquakes. For the former it was greatest near the epicenter and decreased gradually outward. For the latter, intensity II in the scale of the Central Meteorological Observatory (corresponding to IV on the Medvedev-Sponheuer-Karnik, or MSK scale) was noted not only near the epicenter, but also in the areas of northeast Honshu facing the Pacific Ocean.
A seismogram of the latter showed distinct P and S wave arrivals, and very short durations of pulsation, compared with that of the former. The period of the latter seismic waves was comparatively short, showing very rapid oscillations even when the distance from the epicenter was great. The usual dominant coda of surface waves following the P and S arrivals was not observed,
Wadati determined the hypocenters of twelve earthquakes deeper than 300 kilometers (186 miles) between 1924 and 1927, and showed that they occurred in a deep earthquake zone traversing central Honshu. In addition to that, he insisted that the destructive earthquakes in 1891, 1909, 1923, 1925, and 1927 took place in the same zone.
Discovery of the Wadati-Benioff Zone. Wadati compiled the data for these deep earthquakes and showed their geographical distribution in 1929 and 1931, and summarized them in 1935, demonstrating that most of the deep earthquakes occurred along the Traversing and Soya deep-focus earthquake zones. The former zone extended more than 2,000 kilometers (1,243 miles) from Vladivostok to the Bonin Islands, traversing central Honshu. The latter one ran northeastward along the Kuril Islands (Chishima Retto), extending from the northern part of the Sea of Japan through the Strait of Soya. In addition to those zones, the Kyushu deep-focus earthquake zone was revealed near the Ryukyu Islands, extending to northern Kyushu (the southernmost of the four main islands of Japan).
Based on the depth of the hypocenters of those earthquakes, Wadati drew the lines of equal focal depth for intermediate and deep earthquakes, and showed the inclined intermediate and deep earthquake zone dipping away from the oceanic trench toward the Asiatic continent northwestward and the Philippine Sea westward. A similar zone was suggested under Kyushu and its southern extension. Similar intermediate-deep earthquake zones were found in South America by S. W. Visser in 1936, and in the Philippines and Indonesian archipelagoes by Hendrik Petrus Berlage (1937). (Victor) Hugo Benioff (1954) showed the inclined intermediate-deep earthquake zone from the trench toward the surrounding continent in each area in the circum-Pacific region, based on the data presented by Beno Gutenberg and Charles Francis Richter (1953). He supposed the plane to be reverse faults, following Eduard Suess’s idea. Thus the intermediate-deep earthquake zone has been called the Wadati-Benioff zone after its discoverers.
Prehistory of the Discovery of Deep Earthquakes. Prior to the 1920s, earthquakes were supposed to occur at a depth shallower than 30 to 40 kilometers (19 to 25 miles), and the earth was supposed to be statically stable below a depth of 120 kilometers (75 miles) or so, based on the isostasy hypothesis. In 1922, while editing the International Seismological Summary, Herbert Hall Turner recognized an earthquake with a wave that arrived at the distant stations one minute or more earlier than the expected time calculated from the arrival time at nearby stations. He insisted that it must be due to a deep earthquake, but the occurrence of deep earthquakes was not accepted, and he had no proof from actual seismograms.
Turner published a paper regarding Wadati’s previously mentioned results, checking his hypocenters by the records of distant stations in 1929. Meanwhile, F. J. Scrase (1931) and V. C. Stechshulte (1932) examined seismo-grams of Turner’s supposed deep earthquake, and showed that his result was correct. Thus the discovery of deep earthquakes was attributed to Turner.
In Japan, Toshi Shida studied a strong earthquake that occurred on 21 January 1906, which shook seismo-grams over the whole globe. He concluded that the earthquake must have taken place deep in the earth, as the difference of arrival times in the Japanese islands was very small, and the arrival time was 10 seconds or more earlier, compared with those of ordinary earthquakes for observatories in foreign countries, according to the data in the International Seismological Summary. He traced the epicenter to central Honshu, but he did not publish the result, worrying about the isostasy hypothesis. A strong earthquake shook the Kansai area, in central Honshu, on 27 July 1926. Shida fixed the hypocenter near Lake Biwa, at a depth of 260 kilometers (162 miles). He read a paper on the discovery of deep earthquakes at the opening ceremony of the Beppu Geophysical Laboratory, the Imperial University of Kyoto on 28 October 1926, and the abstract was distributed to the guests and reprinted posthumously in Geophysics, the Japanese-language journal of geophysics of the university in 1937.
Estimation of the Magnitude of Earthquakes. In the paper describing deep earthquakes published in 1931, Wadati showed the relation between maximum amplitude and epicentral distance for thirty-one shallow earthquakes. He plotted the maximum ground motion at each station as ordinate (using a logarithmic scale) with the corresponding epicentral distances as abscissa. He pointed out that all the curves were concave upward, and their curvatures were more acute according to the size of the earthquakes. Seismograms of the shallow earthquakes showed the predominance of surface waves, and the maximum amplitudes were interpreted as being due to those waves, except at the areas near epicenters. He sought to apply this diagram to the estimation of the scale of destructive earthquakes. Richter adopted this diagram to define his magnitude scale in 1935.
Study of Ground Subsidence. Wadati was promoted to director of Osaka District Meteorological Observatory in and studied ground subsidence in Osaka. Two wells were drilled to observe ground subsidence. He pointed out the intimate relation between the subsidence and the level of ground water, and insisted that the subsidence was due to the shrinkage of mudstone in 1939 and 1940. Wadati and Takuzo Hirono (1942) developed the theory of subsidence due to consolidation of mudstone. Those studies were highly appreciated after World War II, when ground subsidence was a serious problem caused by the recovery of industries in Japan.
Education, Major Career, Honors, and Awards. Though he was seriously ill with tuberculosis while at the Central Meteorological Observatory and was placed on the retired list from 1929 to 1931, Wadati published many papers and several books on seismology from 1925 to 1935. He was promoted to director of the Central Meteorological Observatory in 1947, and to director-general of the Japan Meteorological Agency in 1956. He retired from the position because of the age limit in 1963.
Wadati was president of Saitama University from 1966 to 1972, of the Japan Academy from 1974 to 1980, and of the Japan Science Council from 1960 to 1962. He received an academy prize with an imperial gift for the discovery of deep earthquakes in 1932. He received an Order of Cultural Merits in 1985. He was an honorary member of the Seismological Society of America and the Royal Astronomical Society.
He died at age ninety-two of old age.
WORKS BY WADATI
“On the Mohorovicic Wave Observed in Japan.” Geophysical Magazine 1 (1927): 87–96.
“Shallow and Deep Earthquakes.” Geophysical Magazine 1 (1928): 162–202.
With Masuda Kunimo. “On the Travel Time of Earthquake Waves (Part VI).” Geophysical Magazine 8 (1934): 187–194.
“On the Activity of Deep-Focus Earthquakes in the Japan Islands and Neighbourhoods.” Geophysical Magazine 8 (1935): 305–325.
With Hirono Takuzo. “On Ground Subsidence in West Osaka (Third Report)” [in Japanese]. Report of the Institute of Disaster Science 6 (1942): 1–33.
Benioff, Hugo. “Orogenesis and Deep Crustal Structure: Additional Evidence from Seismology.” Bulletin of the Geological Society of America 65, no. 5 (1954): 385–400.
Gutenberg, Beno, and Charles F. Richter. Seismicity of the Earth and Associated Phenomena. Princeton, NJ: Princeton University Press, 1949.
Hirono Takuzo. “A Tribute to the Memory of Wadati Kiyoo.” Newsletter of the Seismological Society of Japan 6, no. 6 (1995): 35–36.