03Satake.qxd 369 ANNALS OF GEOPHYSICS, VOL. 47, N. 2/3, April/June 2004 Key words historical earthquakes – seismic inten- sity – seismicity – Kuril subduction zone 1. Introduction Japan is one of the most seismically-active regions in the world, surrounded by four tec- tonic plates: the Pacific, Philippine Sea, Eurasia and North America plates (fig. 1). The last two, on which most Japanese islands are situated, are also known as the Amurian and Okhotsk plates, respectively (Seno et al., 1996; Heki et al., 1999). Most of great (M~8) earthquakes around Japan occur in subduction zones, in- cluding the Kuril trench where the Pacific plate subducts beneath Hokkaido, and the Nankai trough where the Philippine Sea plate subducts beneath southwest Japan. Japanese historical records document earthquakes as far back as the 7th century (Usami, 2002). Until around A.D. 1600, most historical records were kept in western Japan, particularly around Kyoto where the emper- or’s capital was located between the 8th and 19th centuries. Hence many earthquakes were reported in Kyoto and its vicinity. On the ba- sis of such historical documents, recurrence of great earthquakes along the Nankai trough with approximately 100 year intervals has been inferred from historical documents (Imamura, 1928; Ando, 1975). In A.D. 1603, the Shugun established a centralized govern- ment in Edo (present Tokyo; fig. 1), and each local government started keeping official records. Both the quantity and quality of his- Detection of Kuril subduction-zone earthquakes from remote historic records in Honshu, Japan, between 1656 and 1867 Kenji Satake Active Fault Research Center, Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology, Tsukuba, Japan Abstract Earthquakes before A.D. 1800 along the Southern Kuril trench, although before the start of written history on nearby islands, probably account for some of the earthquakes noted by local records in Honshu, hundreds of kilometers to the southwest. Earthquake historians have identified about 4800 felt earthquakes in Edo (present Tokyo) and about 3000 felt reports in selected local government records in Tohoku, northern Honshu, for the years A.D. 1656-1867. On the average, 19 earthquakes per year were felt in Edo. Of the Tohoku records, 361 (an average nearly 2 per year) were felt at multiple Tohoku locations; 95 of these (0.4 per year) were also felt in Edo. Since 1926, Tokyo has had a yearly average of 15 felt earthquakes with seismic intensity 2 or more on the Japan Meteorological Agency scale (corresponding to III or more on Modified Mercalli scale). For Tohoku the aver- age annual frequency is about 4. Among them, an average of 0.6 events per year also reached intensity 2 in Tokyo. About one quarter of these events occurred in the southern Kuril trench. If the seismicity is temporally constant, about 80 of the earthquakes recorded in 1656-1867 probably had a Kuril origin. Mailing address: Dr. Kenji Satake, Active Fault Re- search Center, GSJ/AIST, National Institute of Advanced Industrial Science and Technology, Site C7 1-1-1 Higashi, Tsukuba 305-8567, Japan; e-mail: kenji.satake@aist.go.jp 370 Kenji Satake torical records dramatically increased in the Edo period (A.D. 1603-1867), and earth- quakes were documented throughout Japan except for Hokkaido (fig. 2). The oldest documented earthquakes along the Southern Kuril trench occurred in the 19th century. Great earthquakes along the southern Kuril trench cause damage from ground shak- ing and tsunamis on nearby Hokkaido. Most of Hokkaido had long been occupied by native people (Ainu) and very few written records ex- isted until the 19th century. The oldest record in the Eastern Hokkaido is «Nikkan-ki», the official record of a temple in Akkeshi. It start- ed in A.D. 1804, and reports about 70 earth- quakes between 1816 and 1861. The earth- quake record, however, is not uniform: 32 Fig. 1. Plate tectonic configuration around the Japanese Islands. The Pacific plate subducts beneath northern Hon- shu, the largest island, and Hokkaido, the northernmost island, at the Japan and Kuril trenches, respectively. The Philippine Sea plate subducts beneath southwestern Japan at the Nankai trough. events were recorded between 1816 and 1821, followed by ten years with no reported earth- quakes. In this paper I attempt to detect the Kuril earthquakes from remote historical data. Be- cause of the short written history of Hokkaido, it seems difficult to infer the pre-19th century seismicity along the Kuril trench. However, great earthquakes along the southern Kuril trench are felt in Tohoku (northern Honshu) and Tokyo, and relatively uniform historical records exist in these regions. The seismic in- tensity distribution is examined from the Edo- period and modern data. Then an attempt is made to detect the Kuril earthquakes hidden in the historical earthquake records in Tohoku and Edo. 371 Detection of Kuril subduction-zone earthquakes from remote historic records in Honshu, Japan, between 1656 and 1867 2. Earthquakes along Kuril subduction zone Along the Kuril trench, where the Pacific plate subducts at a rate of about 8 cm/yr beneath Hokkai- do (Seno et al., 1996), great (M ~ 8) earthquakes repeatedly occurred in the 19th and 20th centuries (Kanamori, 1977; Fukao and Furumoro, 1979) (fig. 3). In the 20th century, five great interplate earthquakes occurred between 1952 and 1973. It should be noted that great earthquakes also occur within the subducted slab (e.g., in 1993 and 1994) Fig. 2. Yearly number of earthquake reports since the 7th century. The top figure shows total records for all Japan, the center figure is for Kyoto (includ- ing Nara), and the bottom figure is for Tokyo (in- cluding Kamakura). Historical earthquake data exists since around 7th century in the southwestern Japan, mostly around ancient capital of Kyoto. Since A.D. 1603 (in Edo period), historical data are available throughout Japan with exception for Hokkaido. His- torical records exist only after A.D. 1800 in Hokkai- do. Data are from Ueda and Usami (1990). and cause ground shaking and tsunami similar to those from interplate earthquakes (Satake and Tan- ioka, 1999). Great earthquakes also occurred in the 19th century (fig. 3), but their exact location, size and mechanisms are not well known. The Kuril earthquakes are associated with unusual intensity distributions. The isoseismals are not concentric around the epicentre, but are elongated along the Japanese arc because of the lower attenuation of seismic waves in the sub- ducting Pacific plate (Utsu, 1971). Figure 4 shows an example for the 1952 Tokachi-oki earthquake (Mw 8.1). Seismic in- tensity 5 (on the Japan Meteorological Agency scale; see fig. 4 for conversion to the Modified Mercalli scale) was recorded on the Pacific coast of Hokkaido near the source, and regions of intensity 4 and 3 extend along the western Hokkaido and northern Honshu. This earth- quake was felt as far as at Tokyo, where inten- sity 2 on the JMA scale was registered. Large Kuril earthquakes also cause tsunami damage on the Pacific coasts of Hokkaido and northern Honshu. The tsunami heights from the 1952 earthquake were mostly 2-4 m on the Hokkaido coast and 1-2 m on the northern Hon- shu coast (Watanabe, 1998). The tsunami caused extensive damage on Hokkaido’s Pacif- ic coast and lesser yet noticeable damage on To- hoku’s Sanriku coast. 3. Historical records Japanese historical documents describing earthquake damage have been systematically col- lected and published in three different phases: in 1899-1904 by Tayama, in 1941-1951 by Musha, and in 1980-1994 by Usami and his colleagues (Usami, 2002). The latest compilation, led by Us- ami, «Collection of Materials for the History of Japanese Earthquakes» (21 volumes) was pub- lished from Earthquake Research Institute, Uni- versity of Tokyo. The number of earthquakes (damage or felt reports) has dramatically in- creased in these three stages: approximately 1900, 8400 and 45 000. On the basis of these documents, seismic intensity at each location is estimated from descriptions of ground shaking and damage. The size and epicentre of earth- 372 Kenji Satake Fig. 3. Source regions of great interplate earthquakes along the southern Kuril trench. The 1968 and 1856 earth- quakes occurred around the corner of Kuril and Japan trenches. Magnitudes are Mw (moment magnitude) for the 20th century events and Mt (tsunami magnitude; Abe, 1999) for the 18-19th century events except for the 1893 event for which M was estimated by Utsu (1999). Mw Mw Mw Mw Mw M Fig. 4. Seismic intensity distribution (on the Japan Meteorological Agency scale) of the 1952 Tokachi-oki earth- quake. Conversion table for the JMA and Modified Mercalli scales is also shown. Mw 373 Detection of Kuril subduction-zone earthquakes from remote historic records in Honshu, Japan, between 1656 and 1867 quakes are further estimated from the seismic in- tensity distribution, by comparing modern earth- quake data and using empirical relations. As a re- sult, several tables of damaging earthquakes in Japan have been published (Usami, 2002). The seismic intensity distribution of some his- torical earthquakes in Tohoku is similar to that of modern Kuril earthquakes (fig. 5). An earthquake on 15 April 1674 caused slight damage to the cas- tle at Hachinohe, corresponding to seismic inten- sity 5 on JMA scale. The earthquake was strong- ly felt (intensity 3-4) in Tohoku and slightly felt (intensity 2) in Edo. On the basis of the intensity distribution, Usami (2002) estimated the epicentre off Hachinohe with M ~ 6. Another earthquake on 10 November 1692 was also felt strongly in Hachinohe without causing any damage (intensi- ty 4); it was slightly felt (intensity 2) in Edo. With- out any documented damage, this earthquake has not been included in the tables of damaging earth- quakes, and neither the epicentre nor magnitude has been estimated. Although the spatial extent is limited, the intensity distributions suggest a possi- bility of Kuril origin of both of these earthquakes. I examined selected historical documents that continuously recorded earthquakes in the Edo period. At Hirosaki, the official record of lo- cal government, «Tsugaru-han On’nikki» (about 3300 volumes) is a daily record of various events for 204 years (A.D. 1661-1864). Earthquake and tsunami damage in the territory was described in detail in this record. Even if no damage was caused, earthquakes were noted in daily records along with weather of the day. Such earthquake reports are found in about 1300 days during the 204 years, an average of 6.4 days per year (fig. 6). The daily records sometimes describe multi- ple earthquakes without specifying the exact number (e.g., «a few» or «several»), hence I de- cided to count the number of days («earthquake days») rather than number of earthquakes. Simi- lar documents report earthquakes on 730 days, or average of 3.6 days per year, at Hachinohe, and 950 days in total or average of 6.2 days per year at Morioka. In Edo, various records report about 4800 earthquakes in the Edo period, an average of 19 events per year (Ueda and Usami, 1990; fig. 6). Continuous records were also kept at Fig. 5. Seismic intensities estimated from historical documents for an earthquake on 15 April 1674 (left) and 10 November 1692 (right). For the 1674 event, epicentre (star) and M estimated by Usami (2002) are also shown. 374 Kenji Satake these, 95 events were also recorded in Edo or Nikko. Among the 361 events recorded at multiple locations, epicentres for 52 events have been pre- viously estimated on the basis of damage and in- tensity distributions (Usami, 2002). Additionally, about 20 events have been located off Hachinohe (fig. 5), but these could be of Kuril origin, as dis- cussed earlier. The earthquakes with known epi- centres include great (M ~ 8) earthquakes along the Japan trench. Along the northern Japan Fig. 6. Annual frequency of days in which earth- quakes were recorded in historical documents in three Tohoku locations (Hirosaki, Hachinohe, and Morio- ka) and Edo (Tokyo). See fig. 5 for the locations. Fig. 7. Annual frequency of earthquakes recorded at multiple locations. The bottom panel shows those recorded at least two locations in Tohoku. The cen- tral panel is for those recorded at multiple locations in Tohoku and Edo (Tokyo). The top panel indicates tsunami damage on the Sanriku coast. The white bars are trans-Pacific tsunamis, black bars are those from northern Japan trench and gray bars are from central Japan trench. Nikko, where guards of the first Shogun’s grave- yard kept daily records. From the list of earthquakes individually recorded in Tohoku and Edo, I selected those reported at multiple locations. If earthquake descriptions from different locations or docu- ments differ by up to one unit of time, a «Koku», or about two hours, I assume that they represent the same earthquake. Of the about 3000 descriptions from three localities in Tohoku, more than 2000 were reported at only one location and seem to be of local ori- gin. During the 212 years between 1656 and 1867, there are 361 events recorded at multi- ple locations in Tohoku region (fig. 7). Of 375 Detection of Kuril subduction-zone earthquakes from remote historic records in Honshu, Japan, between 1656 and 1867 trench, at the source area of the 1968 event (fig. 3), great (M ~ 8) earthquakes and their after- shocks occurred in 1677, 1763, and 1856. To the south along the central Japan trench, the Miyagi- oki earthquakes occurred in 1717, 1793, 1835 and 1861 in the Edo period, followed by 1897, 1936 and 1978 events (Utsu, 1999). During the Edo period 16 tsunamis were recorded on Sanriku, the Pacific coast of north- ern Honshu, but the origin of these tsunami are all known (Watanabe, 1998). Among these, five tsunamis (1687, 1700, 1730, 1751 and 1837) were trans-Pacific tsunamis from North or South America, supported by historical or pale- oseismological evidence. Four were from great earthquakes in the northern Japan trench (in- cluding one from the 1763 aftershock) and oth- er four were from the central Japan trench. The 1611 tsunami has been considered to be from a tsunami earthquake (Watanabe, 1998). A tsuna- mi in 1616, though included in Watanabe (1998), has been questioned of its existence (Yoshinobu Tsuji, manuscript in revision). The last one, in 1843, is considered to be from the penultimate event of the 1952 Tokachi-oki earth- quake along the southern Kuril trench (fig. 2). Unknown Kuril earthquakes in the Edo period, if any, apparently did not generate damaging tsunamis along the Sanriku coast, because such tsunami damage would have been documented. 4. Modern seismic intensity observations The Japan Meteorological Agency started systematic seismic intensity observations in 1926 at weather stations distributed throughout Japan. I used the JMA seismic intensity database (Ishiga- ki and Takagi, 2000) for the 76 years 1926-2001, and compared the modern and historical felt re- ports. Because there was no weather station in Hi- rosaki, I used nearby Aomori data. At Aomori, Morioka and Tokyo, the seismic intensity data have been available since 1926, but the Hachino- he data have been available only since 1936. The JMA seismic intensity observations de- tect more earthquakes than the historical reports. The average annual frequency of earthquakes with intensity 1 or more on JMA scale is 18 at Aomori, 48 at Hachinohe, 37 at Morioka, and 40 at Tokyo (table I; fig. 8). If the threshold is raised to seismic intensity 2 (corresponding to III on Modified Mercalli scale), then the average annu- al frequency becomes 7 at Aomori, 18 at Hachi- nohe, 13 at Morioka, and 15 at Tokyo. These numbers are comparable to annual earthquake days in the historical documents: 6.4, 3.6, 6.2 and 19, respectively, at these locations. It indi- cates that historical data are complete for modern seismic intensity 2 or higher, if the seismicity is assumed to be temporally constant. At Hachino- he and Morioka, the modern records show high- Table I. Felt earthquakes of historical and modern JMA data. Historical data JMA seismic intensity observation Annual frequency Period 1 or higher 2 or higher 3 or higher Period Hirosaki 6.4 1661-1864 17.5 7.2 1.9 Aomori: 1926-2001 Hachinohe 3.6 1665-1869 48.2 17.6 4.1 1936-2001 Morioka 6.2 1664-1796 37 13.1 3.4 1926-2001 Tokyo (Edo) 19 1601-1872 39.9 14.9 4.1 1926-2002 Tohoku 1.7 Two or more 9.8 3.6 0.9 Aomori, Hachinohe, Morioka Tohoku and Tokyo 0.4 1.4 0.6 0.1 Above three and Tokyo 376 Kenji Satake Fig. 8. Annual frequency of felt earthquakes reported by Japan Meteorological Agency (1926-2001). Shading indicates different intensity values on the JMA seismic intensity scale. Fig. 9. Epicentre distribution of earthquakes reported at three stations in Tohoku (open circles) and those reported at Tohoku and Tokyo stations (solid circles). The symbol size is proportional to earthquake magnitude. Triangles indicate the station locations. Nearly a quarter of these events occurred along the Kuril trench, in the region sur- round by dashed lines. 377 Detection of Kuril subduction-zone earthquakes from remote historic records in Honshu, Japan, between 1656 and 1867 er numbers; hence the historical data may be complete for intensity 3 or higher, for which the average annual frequency is 4 at both locations. However, considering the slight difference in counting methods (events in modern data where- as earthquake days that possibly contain multiple events in historical data) and the incompleteness of historical data, I conclude that historical data detect earthquakes with seismic intensity of 2 or higher on the JMA scale. Earthquakes recorded at multiple JMA sta- tions yield similar conclusions. During the 66 years between 1936 and 2001, the number of felt earthquakes at the three Tohoku stations (Ao- mori, Hachinohe and Morioka) is 276 if the threshold is intensity 2, and 68 for the threshold intensity of 3. The average annual frequencies are 4 and 1, respectively, comparable to that from historical data: 1.7. The number of events record- ed at Tohoku stations and Tokyo is 46 and 11 for intensity thresholds of 2 and 3, respectively. The annual frequencies are 0.6 and 0.1 events, again comparable to that of historical data, 0.4. Epicentres of earthquakes felt in Tohoku and Tokyo are distributed around northeast Japan (fig. 9). Many earthquakes are located on the Pacific coast of Hokkaido and Honshu, but inland earth- quakes, earthquakes along the eastern margin of Japan Sea, and deep events beneath the Russian coast are also included. Among them, 59 events (21%) of those felt (with intensity scale 2 or larg- er) only in Tohoku, and 11 events (24%) of those felt in both Tohoku and Tokyo occurred along the southern Kuril trench. 5. Edo-period Kuril seismicity A comparison of historical and modern earthquakes suggests historical data detect earth- quakes with seismic intensity of 2 or more on the JMA scale. Under an assumption that the histor- ical data are complete for this threshold, and fur- ther that the seismicity rate has not changed since the 17th century, the number of Kuril earth- quakes can be estimated for historical data set. Of the 361 earthquakes recorded at multiple To- hoku locations between A.D. 1656 and 1867, about 76 events (21%) are estimated to be of Kuril origin. Of the 95 events recorded at multi- ple Tohoku locations and Edo, about 23 events (24%) are also estimated to be of Kuril origin. Are the above assumptions, data complete- ness and temporally constant seismicity, valid? Regarding the second assumption, earthquakes often occur as clusters and the seismicity rate may change with time. In this study no declustering, including aftershocks, was made for either historical on modern records. Intraplate seismicity rate in Tohoku is reported to change with association of larger interplate earthquakes (Shimazaki, 1978). Be- cause the time period is very long (> 200 years for the Edo records and 75 years for modern data), effects of clustering and temporal change may not be significant. The present study cannot identify individual Kuril earthquakes, but the candidates are listed. Further investigation of historical documents could focus on these dates and might reveal more detailed information on individual earth- quakes. Historical documents are still being found from local archives. While it is unlikely that continuous documents as used here are newly found, historical records for shorter peri- od of time are likely to be found. The candidate tables will be therefore useful to examine such records. Acknowledgements The Japanese version of this paper with a table of the 361 events was published in Histor- ical Earthquakes (Rekishi Jishin). The full text can be downloaded at http://staff.aist.go.jp/ kenji.satake/Rekishijisin/05-Satake.pdf. Kazue Ueda provided me with her database for the in- dex to the collections and a summary of felt earthquakes (fig. 2). Akimichi Takagi provided me with his software to analyze the JMA seis- mic intensity database. Yuichi Nishimura care- fully reviewed the Japanese version of this pa- per. I thank two reviewers whose comments im- proved the presentations of the paper. I also thank Virginia García Acosta, Roger Musson, and Max Stucchi for their efforts to organize the workshop «Investigating the records of past earthquakes» and offering me an opportunity to participate. 378 Kenji Satake REFERENCES ABE, K. (1999): Quantification of historical tsunamis by the Mt scale, Zisin, J. Seismol. Soc. Jpn, 52, 369-377 (in Japanese with English abstract). 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