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Triangulation scale error caused by the 1894 Shonai earthquake: a possible cause of erroneous interpretation of seismic potential along the Japan Trench
© The Author(s) 2018
- Received: 3 April 2018
- Accepted: 5 July 2018
- Published: 13 July 2018
- Crustal strain
- The 2011 Tohoku-oki earthquake
- The 1894 Shonai earthquake
- Shionohara baseline
- Baseline survey
The 2011 Mw9.0 Tohoku-oki earthquake was an unexpected giant earthquake for most seismologists. Matsuzawa (2011) summarized five reasons why seismologists believed that there would be no potential for a giant M9 earthquake along the Japan Trench: (1) The subducting Pacific plate is old and cold. Based on comparative subductology (Uyeda and Kanamori 1979), interplate coupling was considered small; (2) triangulation data for the last 100 years showed no E–W contraction; (3) there existed a high activity of small to middle sized earthquakes that were supposed to release tectonic strain at weakly coupled plate interface; (4) major earthquakes along the Japan trench were followed by large afterslip (e.g. Kawasaki et al. 2001), which suggested interplate coupling was not strong; (5) many small repeating earthquakes have been occurring along the Japan Trench (Igarashi et al. 2003), which indicates fault creep on the plate interface. Among these reasons, the lack of E–W contraction during the last 100 years was the most important since an E–W contraction is direct evidence of tectonic stress build-up associated with the subduction of the Pacific plate and interplate coupling. In this paper, we argue this understanding was probably inaccurate and try to explain why such misunderstanding occurred.
The Japanese triangulation network was established in the late nineteenth century to provide a reference for precise surveying and mapping of the whole country. Since Japan is located in a tectonically active region, conspicuous crustal deformation occurs associated with large earthquakes and volcanic activities. In addition, significant crustal deformation also occurs during interseismic periods, reflecting tectonic stress build-up due to plate motions. Resurveying of the triangulation network has provided us with important knowledge about crustal movements, and many studies have been conducted by utilizing those data (e.g. Muto 1932; Ando 1971; Harada and Kassai 1971; Nakane 1973a, b; Sato 1973; Fujii et al. 1986; Tada 1986; Hashimoto 1990; Hashimoto and Jackson 1993; Ishikawa and Hashimoto 1999).
The occurrence of the 2011 Tohoku-oki earthquake revealed that our evaluation about the seismic moment budget was wrong, implying a possible defect in the interpretation of the triangulation records. However, there has been no explanation why such misunderstanding occurred. In the following, we demonstrate that an unintentional error in the original triangulation in the l890′s has brought a significant bias in the reference system over a wide area in northeast Japan and caused misinterpretation of the crustal deformation which led to the underestimation of accumulated slip deficit along the Japan Trench.
Figure 1 compares the strain rate distribution in northeast Japan based on GPS data during 1996–2000 and that based on triangulation for 100 years (Ishikawa et al. 1998). E–W contraction is evident all over the Tohoku area in the GPS result. On the other hand, E–W contraction is not evident in the triangulation result over the twentieth century and significant N–S extension is identified. Similar pattern was reported by Hashimoto (1990) and Ishikawa and Hashimoto (1999) who corrected coseismic offsets due to major earthquakes to estimate interseismic crustal strain rate.
Discrepancies between the two results need some explanation. One possible explanation is that E–W contraction was a short-term feature that would be released by aseismic fault slips either as afterslips following large earthquakes or as unknown slow slip events during interseismic periods. Kawasaki et al. (2001) demonstrated that aseismic fault slips following M7 class earthquakes along the Japan trench contribute equally to or even larger than the coseismic slips. Plate boundary slips can release interseismic E–W contraction. But this interpretation cannot explain how to reproduce N–S extension identified in the triangulation.
Such a significant discrepancy between crustal strain patterns is found only in the Tohoku area. This indicates, if the baseline survey caused this discrepancy, the baseline in the Tohoku area should be responsible for the error. As is shown in Fig. 2, there are only two baselines in the Tohoku area, one is the Shionohara baseline in the Yamagata Prefecture in middle Tohoku, and the other is the Tsurunokotai baseline in the Aomori Prefecture. Since the discrepancy between the GPS and triangulation strain rate pattern is significant in middle-southern Tohoku, we considered the measurement of the Shionohara baseline might have a problem.
The Shionohara baseline (5129.5872 m) is composed of the two first-order triangulation control points at its eastern and western ends. This is the only original baseline in Japan whose both control points still exist at their original locations today, connected by a straight road for surveying. The baseline was constructed and measured from May to July in 1894. Then, surveys of a dense triangulation network around the Shionohara baseline was conducted from August to October in 1894. To the south of the Shionohara baseline, the Pacific coastal side was surveyed before 1894, while the Japan Sea coastal side was surveyed later. The whole network to the north was surveyed afterward (Fig. 2). The original survey as well as calculation records of the Shionohara baseline is preserved in the archives of the Geospatial Information Authority of Japan (former Geographical Survey Institute, the successor of the Military Land Survey that conducted the original triangulation survey). We checked the original calculation log for the Shionohara baseline. Detailed survey record is shown in Table 1. The overview of the baseline survey is summarized as follows.
Detailed survey record of the Shionohara baseline
Date and time
Date and time
Date and time
Date and time
Fault parameters of the 1894 Shonai earthquake and its tested ranges and assumed sampling in the Monte Carlo simulations
6.8, 6.9, 7.0, 7.1, 7.2
Fault depth (D)
Fault length (L)
Scaling from Mw (Takemura 2005)
Fault width (W)
W = 0.5 L
Determined from Mw, L and W
There can be a significant deformation due to postseismic deformation of the Shonai earthquake. Such a postseismic deformation was observed, for example, following the 2008 Iwate-Miyagi earthquake (Ohzono et al. 2012). According to Ohzono et al. (2012), the postseismic deformation may have added extension to the coseismic one on the hanging-wall side at the distance from 20 to 30 km, and the extension rate was around 1 ppm/year. Thus, it is reasonable to assume that, in the case of the Shonai earthquake, the coseismic change had a larger effect on the Shionohara baseline by an order of magnitude than the postseismic deformation.
In summary, we conclude that it is highly possible that the 1894 Shonai earthquake elongated the Shionohara baseline by as large as 5 cm or 10 ppm.
We also show distribution of the maximum shear strain change in Fig. 6b. The effect of the baseline bias is negligible (less than 1 ppm) for the maximum shear strain. The result is consistent with the fact that shear strain can be obtained only from angle measurements (e.g. Frank 1966). Fukushima et al. (2012) compared the shear strain distribution based on both triangulation data and GPS results before 2011 and concluded that E–W contraction has been continued over the twentieth century although the shortening might be enhanced in the southern Tohoku area. Our calculation result supports their conclusion.
We also tested correction of the possible 1894 Shonai earthquake in the original triangulation. For this purpose, we hypothesize two fault models of the 1894 Shonai earthquake (source parameters are shown in Table 4), corresponding to 5 and 10 cm of the Shionohara baseline elongation. Then we calculate coseismic displacement at benchmarks using the elastic dislocation code by Okada (1985) and corrected the triangulation angles measured before the 1894 earthquake. We conducted network adjustments using these corrected angles and baseline length for two cases and evaluate the expected crustal strain distribution (Additional file 1: Figure S1). These corrected crustal strain distributions show some notable changes from those in Fig. 7, for example, in the degree of the N–S extension due to the scale bias. However, the overall strain patterns do not change from those in Fig. 7 and the effect of coseismic angle changes is not serious. Since the hypothetical fault model contains large uncertainties, we prefer to present Fig. 7 to demonstrate the scale bias effects on the triangulation result.
As is already mentioned, the control points at both ends of the Shionohara baseline still exist at their original positions. The current length of the baseline may provide an additional constraint on the coseismic disturbance of the 1894 Shonai earthquake. Therefore, we conducted the baseline length survey in August 2012.
Measured coordinates and length of the Shionohara baseline
In order to compare the baseline length in 2012 with that in 1894, we need to consider effects of various earthquakes in the surrounding areas and interseismic deformation during last 100 years. Among them, the 2011 Tohoku-oki earthquake contributed to the largest baseline change. Two GEONET baselines, Mogami (MOGA)-Shinjo (SHIN) (15.6 km) and Shinjo (SHIN)-Tachikawa (TACH) (31.4 km) aligned in the E–W direction nearly parallel to the Shionohara baseline (see Fig. 4 for locations), had length changes of ~ 0.25 m (+ 16.0 ppm) and ~ 0.50 m (+ 15.9 ppm), respectively, including coseismic as well as postseismic changes until August 2012. Thus, the total effects of the 2011 Tohoku-oki earthquake on the Shionohara baseline are evaluated as an elongation of around + 16 ppm (8 cm). Based on the GPS data before 2011, baseline length change rate for Mogami-Shinjo was nearly 0 and that of the Shinjo-Tachikawa was − 3 mm/year (− 0.1 ppm/year). It is reasonable to assume the interseismic shortening ratio of the Shionohara baseline as 0–0.1 ppm/year.
Major earthquakes around the Shionohara baseline and calculated baseline changes
1894/10/22 Shonai *
Tanioka and Satake (1996)
Thatcher et al. (1980)
1900/5/12 Northern Miyagi
Yamanaka and Kikuchi (2004)
Yamanaka and Kikuchi (2004)
7.0, 7.5, 7.3,7.4, 6.9
1962/4/30 Northern Miyagi
1970/10/16 SE Akita
Seno et al. (1980)
1983/5/26 Japan Sea
2003/7/26 Northern Miyagi
Nishimura et al. (2003)
Takada et al. (2009)
We first summarize our hypothesis about the Shionohara baseline and related consequences. The Shionohara baseline was measured during May–July 1894, and the value of 5129.5872 m was obtained with an expected precision of ~ 1 ppm. On October 22 of the same year, the Shonai earthquake happened and the baseline length was increased by about 10 ppm. However, the baseline was never re-surveyed after the earthquake and the original value was used for network adjustment. As a result, the triangulation network in northeast Japan was defined with a negative isotropic scale bias of 5–10 ppm, or the network was defined 5–10 ppm smaller than its actual size. After the first survey, tectonic plate motion and interplate coupling at the Japan trench accumulated E–W contraction roughly at 0.1 ppm/year as a regional average. After 100 years from the first survey, the cumulative contraction reaches about 10 ppm in E–W direction. In the comparison of triangulation data, the E–W contraction signal was not identified since the original network was defined smaller by almost the equal amount to the cumulative contraction during 100 years. Instead, the comparison of triangulation surveys revealed extensive N–S extension because the original network size was underestimated in N–S direction, too. This hypothesis explains why we did not identify geodetic signals showing tectonic strain accumulation in the Tohoku area.
The N–S extension in the Tohoku area was pointed out as early as in 1971 by Harada and Kassai (1971) who compared the re-survey of the triangulation network in the Showa era (1948–1968) with the original solution in the nineteenth century. By using Frank’s (1966) method, Sato (1973) and Nakane (1973a, b) evaluate shear strain rate of Japan Islands. They also estimated the maximum contraction in the Tohoku area was in the E–W direction, but this estimate was consistent with the N–S extension since the applied method could not resolve the dilatation. Later, Hashimoto (1990) and Ishikawa and Hashimoto (1999) estimated horizontal crustal strain rates using multiple survey results and reached a similar conclusion to Harada and Kassai (1971). Hashimoto and Jackson (1993) discussed interseismic deformation using angle change rate and concluded interseismic coupling at the Japan Trench was weak. Though there have been so many studies using triangulation data, no reasonable interpretation has been given about the observed N–S extension signal before. The observational error of the original triangulation in the nineteenth century was considered as large as 10 ppm based on the internal consistency of the network adjustment (Komaki 1985). But all the previous authors considered that the observation error was random and they did not consider a possibility of the scale bias as pointed out in this study. Hashimoto (1990) and Ishikawa and Hashimoto (1999) calculated strain rates from baseline length change rates using network adjustment results of all the available surveys. In the calculation of baseline length change rate, even with a large formal error, data from the first survey are highly influential since they are the only data in the first half of the analysis period.
The measurement of the Shionohara baseline finished in early July of 1894. However, according to the record, angle measurements of the first-order benchmarks continued until October of the same year. We speculate that the survey team should know that the Shonai earthquake occurred just after their survey. Only 3 years before the Shonai earthquake, there occurred the 1891 Nobi earthquake, which heavily affected the nearby triangulation network and the first recovery survey took place. Thus, there remains a question why a similar recovery survey was not conducted around the Shionohara baseline. While the town of Sakata to the west of the source fault was heavily damaged, almost no damage was reported in Shinjo close to the survey area (Omori 1895). Probably the survey team did not expect such a significant deformation to occur at the baseline.
Scale bias derived from the baseline length error is very extensive as is shown in Fig. 6, but it does not cause any inconsistency in the adjustment result. So, it is very difficult to recognize from the network adjustment. On the other hand, erroneous angle measurements can be easily identified through the network adjustment with large residuals. It is a fatal fault that we have overlooked the possibility of such a scale error. Since everybody knows that triangulation survey has a weakness in its scale, we should have investigated the unbiased observables such as angles or shear strains. Also, those who discuss seismic potential using geodetic data should understand how those data were obtained and what kind of errors could be contained.
In spite of such annoying data errors, conventional geodetic data such as triangulation and leveling still keep their scientific value. Our observation history with precise instruments such as GPS is still only 25–30 years long. Many geological phenomena such as earthquakes and volcanic eruptions are all unique, and we still lack of experience to make forecast or prediction on what happens in the future. In order to fully utilize the legacy of old observations, we should pay the most careful attention to data quality and various errors contained in the observation data.
On the other hand, it was nothing but an unfortunate coincidence that such a bias sneaked in the triangulation data. If the baseline had been constructed in a different place, if the baseline had been designed in the N–S direction, if the earthquake had occurred several months earlier, if the earthquake had been a little smaller or even larger, if one of us had been careful enough to investigate such a possibility, we could have avoided such an erroneous interpretation. This example provides a very important lesson that we can never be too careful in preparing for future natural hazard.
We revisited the original record of the Shionohara baseline survey conducted in 1894 and confirmed that the survey was conducted normally with a good precision. But we also found a possibility that the baseline length was significantly affected by the Shonai earthquake that occurred only 30 km to the west of the baseline. We numerically investigated a possible effect of the Shonai earthquake to the Shionohara baseline length and found that an earthquake with magnitude 6.9–7.0 could effectively elongate the baseline length as much as 5 cm or 10 ppm. Network adjustment calculations demonstrate that an error of 10 ppm at the Shionohara baseline causes significant as well as extensive scale bias in the strain calculation. A bias of 10 ppm was large enough to conceal tectonic strain accumulation over 100 years and to create apparent N–S extension signal over the entire Tohoku area. The re-survey of the Shionohara baseline was done in August 2012, after the 2011 Tohoku-oki earthquake. In spite of a large uncertainty in the interseismic deformation rate, the result is consistent with the hypothesis of the baseline scale bias. Thus, our understanding of no significant elastic strain accumulation in the Tohoku area before the 2011 Tohoku earthquake was caused by the isotropic scale bias of ~ 10 ppm in the original triangulation network adjustment result, for which coseismic deformation of the 1894 Shonai earthquake was responsible.
TS planned the whole research, conducted data analysis and field survey, and wrote the manuscript. NM and YO cooperated in the field survey. All members read the manuscript and agreed on the content.
Critical comments by Paul Segall, an anonymous reviewer, and an associate editor Tomokazu Kobayashi were helpful to improve the manuscript. Angela Meneses-Gutierrez, Shinichi Nomura and Syota Suzuki are acknowledged for their support in the field survey. Takuya Nishimura and Hiroshi Yarai are acknowledged for their support in gathering triangulation data and reading original field/calculation logs in the archives. We also thank the Geospatial Information Authority of Japan for the use of GEONET data.
The authors have no competing interest.
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All the data used in this manuscript can be provided on request.
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This study was supported by JSPS KAKENHI Grant Numbers JP25282111 and JP26109003, and “Intensified Observation and Research on Strain Concentration Zone” project by the Ministry of Education, Culture, Sport, Science and Technology.
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