Scholarly article on topic 'Wide-area land subsidence caused by “the 2011 Off the Pacific Coast of Tohoku Earthquake”'

Wide-area land subsidence caused by “the 2011 Off the Pacific Coast of Tohoku Earthquake” Academic research paper on "Earth and related environmental sciences"

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Abstract of research paper on Earth and related environmental sciences, author of scientific article — Tetsuro Imakiire, Mamoru Koarai

Abstract This paper gives an overview of the crustal deformation caused by “the 2011 off the Pacific Coast of Tohoku Earthquake” (hereinafter: “the Tohoku Earthquake”) as detected by the GPS Earth Observation Network (GEONET), a GPS continuous observation system operated by the Geospatial Information Authority of Japan (GSI), and of ground deformation resulting from liquefaction triggered by the earthquake motion as determined by a leveling survey. A very large part of the Japanese archipelago was affected by the crustal deformation caused by the mainshock. The subsidence in the Pacific coastal area of the Tohoku region is especially remarkable. Based on a resurvey after the earthquake, the GSI revised the coordinates and heights of survey marks, including the Origin of the Japanese Horizontal Control Network and the Origin of the Japanese Vertical Control Network. We estimated the geometry of the seismogenic fault of the Tohoku Earthquake, as well as a slip model for the boundary between the Pacific Plate and the North American Plate, based on crustal deformation data. Postseismic crustal deformation was also observed by GEONET. Even though the area that coseismicaly subsided is undergoing partial uplift from postseismic deformation, such uplift is not rapid enough to make up for the coseismic subsidence within the next several years. The earthquake caused liquefaction in large parts of the Kanto Region, especially on the Tokyo Bay side and at the lower reaches of the Tone River. Liquefaction was concentrated in areas of land reclamation and former river channels, where it is easy to recognize using time-series geospatial information such as olden topographic maps and olden aerial photographs. To determine the amount of land subsidence (cm) due to liquefaction in Urayasu City, the GSI carried out a leveling survey and compared the differences between pre- and post-earthquake LIDAR survey data with the relative subsidence obtained by leveling.

Academic research paper on topic "Wide-area land subsidence caused by “the 2011 Off the Pacific Coast of Tohoku Earthquake”"

The Japanese Geotechnical Society

Soils and Foundations

www.sciencedirect.com journal homepage: www.elsevier.com/locate/sandf

SOILS AND

FOUNDATIONS

Wide-area land subsidence caused by ''the 2011 Off the Pacific Coast of

Tohoku Earthquake''

Tetsuro Imakiiren, Mamoru Koarai

Geospatial Information Authority of Japan, Japan

Received 27 March 2012; received in revised form 16 August 2012; accepted 16 October 2012 Available online 11 December 2012

Abstract

This paper gives an overview of the crustal deformation caused by ''the 2011 off the Pacific Coast of Tohoku Earthquake" (hereinafter: ''the Tohoku Earthquake'') as detected by the GPS Earth Observation Network (GEONET), a GPS continuous observation system operated by the Geospatial Information Authority of Japan (GSI), and of ground deformation resulting from liquefaction triggered by the earthquake motion as determined by a leveling survey. A very large part of the Japanese archipelago was affected by the crustal deformation caused by the mainshock. The subsidence in the Pacific coastal area of the Tohoku region is especially remarkable. Based on a resurvey after the earthquake, the GSI revised the coordinates and heights of survey marks, including the Origin of the Japanese Horizontal Control Network and the Origin of the Japanese Vertical Control Network. We estimated the geometry of the seismogenic fault of the Tohoku Earthquake, as well as a slip model for the boundary between the Pacific Plate and the North American Plate, based on crustal deformation data. Postseismic crustal deformation was also observed by GEONET. Even though the area that coseismicaly subsided is undergoing partial uplift from postseismic deformation, such uplift is not rapid enough to make up for the coseismic subsidence within the next several years.

The earthquake caused liquefaction in large parts of the Kanto Region, especially on the Tokyo Bay side and at the lower reaches of the Tone River. Liquefaction was concentrated in areas of land reclamation and former river channels, where it is easy to recognize using time-series geospatial information such as olden topographic maps and olden aerial photographs. To determine the amount of land subsidence (cm) due to liquefaction in Urayasu City, the GSI carried out a leveling survey and compared the differences between pre- and post-earthquake LIDAR survey data with the relative subsidence obtained by leveling. © 2012 The Japanese Geotechnical Society. Production and hosting by Elsevier B.V. All rights reserved.

Keywords: The 2011 off the Pacific Coast of Tohoku Earthquake; Crustal deformation; Fault model; GEONET; Ground subsidence; Leveling survey; LIDAR survey; Liquefaction

1. Overview of the coseismic crustal deformation

The 2011 off the Pacific Coast of Tohoku Earthquake (hereinafter: ''the Tohoku Earthquake") occurred at 14:46

"Corresponding author. E-mail address: imq@gsi.go.jp (T. Imakiire). Peer review under responsibility of The Japanese Geotechnical Society.

on March 11, 2011. It was the most powerful earthquake ever recorded in Japan, with a magnitude of 9.0 (as measured by the Japan Meteorological Agency (JMA)). It caused severe damage on the Pacific coast from the Tohoku Region to the Kanto Region, especially from the huge tsunami.

Remarkable coseismic crustal deformation was detected by GEONET throughout Northeastern Japan. Fig. 1 shows a contour map of horizontal displacement and Fig. 2 shows such a map of vertical displacement (GSI, 2011a). The maximum displacement was recorded at the Oshika site in

0038-0806 © 2012 The Japanese Geotechnical Society. Production and hosting by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.sandf.2012.11.007

From : 2011/03/01 21:00 - 2011/03/09 21:00 To : 2011/03/11 18:00 - 2011/03/11 21:00

* Fixed Station : Misumi (950388)

Fig. 1. Crustal deformation (horizontal) associated with the 2011 off the Pacific Coast of Tohoku Earthquake that occurred on March 11, 2011. Displacement vector is estimated by the relative position referred to Misumi station in Yamaguchi prefecture. Coordinates used for the calculation are R3 solution for the position before the earthquake, and Q3 solution for the coordinates after the earthquake. $ is the epicenter of mainshock of the Tohoku earthquake.

Ishinomaki City, Miyagi Prefecture. Oshika moved about 5.3 m east-by-southeast and subsided about 1.2 m.

It is notable that large parts of the Tohoku Region and the Kanto Region experienced crustal deformation from the Tohoku Earthquake. The GEONET sites in Iwate Prefecture to the north, Yamagata Prefecture to the west and Ibaraki Prefecture to the south were displaced more than 1 m horizontally. Even in the Tokyo metropolitan area, about 0.2 m of horizontal movement was observed. Subsidence in the Pacific coastal area was also remarkable. Subsidence of several tens of centimeters was recorded at many sites from Miyagi to the Ibaraki Prefecture coast, which were affected by high tides after the earthquake.

Because the GSI recognized that the coordinates of control points and the height of benchmarks had greatly changed, we announced on March 14 that those control points and benchmarks would no longer be usable for surveys. Those control points and benchmarks are indispensable for reconstruction, as infrastructure and buildings were seriously damaged and should be repaired quickly. Therefore, the GSI decided to revise the basic survey results. For this purpose, the GSI resurveyed and recalculated these monuments in the Tohoku, Kanto and Chubu regions. Corrected coordinates of GPS-based control points were published on May 31, 2011, and those of other control points and leveling benchmarks were published on October 31, 2011. The Origin

From : 2011/03/01 21:00 - 2011/03/09 21:00 To : 2011/03/11 18:00 - 2011/03/11 21:00

er Fixed Station : Misumi (950388)

Fig. 2. Crustal deformation (vertical) associated with the 2011 off the Pacific Coast of Tohoku Earthquake that occurred on March 11, 2011. Displacement vector is estimated by the relative position referred to Misumi station in Yamaguchi prefecture. Coordinates used for the calculation are R3 solution for the position before the earthquake, and Q3 solution for the coordinates after the earthquake. $ is the epicenter of mainshock of the Tohoku earthquake.

of the Japanese Horizontal Control Network and the Origin of the Japanese Vertical Control Network were also affected by the crustal deformation caused by the Tohoku Earthquake, so the datum values for the origins were also corrected, and new values were announced on October 21, 2011.

Table 1 shows the old and new datum values for the Origin of the Japanese Vertical Control Network. That origin decreased (subsided) by 24.0 mm. Fig. 3 shows how

Table 1

Datum value of Origin of Japanese Vertical Control Network

Height from the average sea level of Tokyo Bay

Old value 24.4140 m (Before the Tohoku Earthquake)

New value 24.3900 m (After the Tohoku Earthquake)

Fig. 3. Height change along the leveling survey route after the 2011 off the Pacific Coast of Tohoku Earthquake (unit: cm).

the height of the benchmarks along the leveling routes where resurvey was carried out have changed (GSI, 2011b). Basically, areas where the height change was estimated to exceed 5 cm based on GEONET data were selected for resurvey. One leveling route, remote from any other leveling route, at the boundary of Niigata and Nagano prefectures is the area affected by the March 12 earthquake (M6.7), which is considered to be one of the earthquakes triggered by the Tohoku Earthquake.

The subsidence determined by the leveling survey is consistent with the results of GEONET observations. Even in inland parts of the Tohoku and Kanto regions, the height change exceeds 10 cm. It should be noted that long-term crustal deformation that accumulated after the old coordinates and height values were published in 2002 is included in the height change. Even though the changes of values for data, control points and benchmarks are mainly due to the coseismic and post-seismic crustal deformation caused by the Tohoku Earthquake, we cannot separate them from long-term pre-earthquake deformation.

Though both horizontal and vertical data were revised, this report addresses only vertical data changes. More detailed information about the survey results revision is available at the website of the GSI (GSI, 2011b).

As nearly 30% of the control points in Japan were given new coordinate values in this revision process, the GSI updated the system of survey results as "Geodetic Coordinates 2011'' from the former ''Geodetic Coordinates 2000.'' The GSI is providing conversion tools for the coordinates of public survey results, so that local governments will be able to easily correct their survey results that were based on the old system into new ones without much extra work. When the evacuation order is rescinded, the GSI will complete a resurvey and recalculation for remaining areas where new survey results are not available because of the nuclear power plant accident in Fukushima.

The deformation described above is mainly explained by the crustal deformation, which is related to the fault motion of the mainshock of the Tohoku Earthquake. In addition to this crustal deformation, there were local ground deformation phenomena caused by the strong motion of the earthquake. Such local ground deformation, e.g. liquefaction, is discussed in a later section.

2. Fault models

We assume most of the coseismic displacement at the GEONET sites to be crustal deformation elastically caused by the fault motion of the Tohoku Earthquake. The GSI constructed three fault models for the Tohoku Earthquake. The first is a preliminary fault model that consists of two rectangular faults with a uniform slip in an elastic half-space (GSI, 2011c). The parameters of those faults were estimated based on the formula introduced by Okada (1985). This model shows that the major rupture has a total length of about 380 km and a fault width of about 90-130 km. A reverse fault motion is inferred. Slip amounts for a northern segment and a southern one are estimated to be about 25 m and about 6 m, respectively. The total moment magnitude (Mw) is 8.9.

The second model is a distributed slip model using coseismic surface displacement data observed by GEONET (GSI, 2011c). We assumed that the coseismic slip occurred at the interface between the Pacific Plate, which is subducting from the east, and the North American plate, where the Tohoku region is located. The slip is estimated by geodetic inversion based on the method of Yabuki and Matsu'ura (1992). The slip area, i.e., the area where the slip is estimated to be larger than 4 m, extends more than 400 km north-south, or nearly 450 km in the major axis along the Japan trench, and the width is about 200 km east-west. The largest slip, estimated to be near the epicenter, is about 27 m (Ozawa et al., 2011). The total amount of energy coseismically released, or the seismic moment based on this slip model, is 3.90 x 1022 Nm, equivalent to a moment magnitude (Mw) of 9.0, assuming a rigidity of 40 GPa. This moment magnitude

Slip distribution model based on GEONET and seafloor crustal deformation data

From 10 March 2011 to 12 March 2011 (F3 solution)

Reference site: Fukue(950462) +SGO by JCG5

Fig. 4. The slip distribution model of the 2011 off the Pacific Coast of Tohoku Earthquake, based on GEONET observations on land and seafloor crustal deformation from GPS/acoustic observations by Japan Coast Guard (a slip distribution model of the plate interface). North-south dashed lines are contours for the plate boundary depth, in 20 km interval.

value is consistent with other estimation results based on a seismic wave inversion and tsunami inversion analysis.

The third model (Fig. 4) is the advanced slip distribution model using GEONET data and seafloor crustal deformation data observed by the Japan Coast Guard (GSI, 2011c; Sato et al., 2011b). The horizontal movement observed at the Miyagi-1 seafloor site is as large as 24 m, and the vertical movement there is about 3 m of uplift. It is notable that the center of the slip area is estimated to be nearer to the Japan Trench, or more eastward, than with the above model. Furthermore, the estimated maximum slip is more than 56 m, much greater than the above estimation. This extremely large slip means that the plate boundary around this area was stuck very firmly before the earthquake, and had been accumulating strain energy for a long time.

3. Postseismic crustal deformation

Remarkable postseismic crustal deformation has been observed by GEONET. This deformation is attributed to afterslip at the plate boundary around the seismogenic fault of the mainshock. Fig. 5 shows postseismic horizontal and vertical movement vectors for Northeastern Japan (Nishimura, 2011). The horizontal movement east to southeast is notable. It means that the crustal block of northeastern Japan is moving eastward from the afterslip at the plate boundary between the North American Plate and the Pacific Plate.

However, there is an important difference between the coseismic movement and the postseismic one. Although all GEONET sites were found to have subsided coseismically,

Fig. 5. Crustal deformation after the 2011 off the Pacific Coast of Tohoku Earthquake in the case of the Misumi site, showing (left) horizontal deformation as of September 18, 2011, and (right) vertical deformation as of September 18, 2011. $ is the epicenter of mainshock of the Tohoku earthquake.

some sites in Miyagi Prefecture and others around Choshi were found to have uplifted after the earthquake. It is considered that this change from subsidence to uplift was caused by the shift of the area where the slip was occurring.

Fig. 6 shows a slip distribution model for postseismic crustal deformation (GSI, 2011c). The most significant feature of this slip model is that the center of the slip area is slightly west of the center of the coseismic slip area. This means that postseismic slip at the plate boundary occurs mainly in a zone deeper than that of the coseismic rupture zone. It is known that the afterslip of major interplate earthquakes occurs in the area neighboring the seismogenic zone of the mainshock. Such a phenomenon was seen in the case of the ''2003 Off Tokachi Earthquake" (Murakami et al., 2006). The afterslip at the deeper plate boundary is a naturally expected process after the Tohoku earthquake.

The geomorphological evidence, such as that of coastal terraces, shows that the coastal areas of the Tohoku district have been undergoing long-term uplifting. However, geodetic observations, including leveling surveys from 100 years ago, tidal observations for several decades and GPS observations, show that the Pacific coast of the Tohoku region had been subsiding before the Tohoku Earthquake. There was a hypothesis that coseismic uplift caused by a major interplate earthquake would exceed the inter-seismic

subsidence. However, the relevant area has undergone subsidence as a result of the Tohoku Earthquake.

One event that may cause inter-seismic uplift is slow long-term slippage at the deeper part of the plate boundary. A geological study on the sediment along the Pacific coast of Eastern Hokkaido suggests that postseismic uplift continued for several decades after a major earthquake in the 17th century (Sawai et al., 2004).

Another possibility is that a different type of earthquake caused the uplift. For example, a simulation study on an interplate earthquake along the Nankai Trough suggests that a special type of earthquake, which ruptures deep parts of the plate boundary, occurs once per several occurrences of the normal reoccurring earthquakes (Hori et al., 2009). Such an earthquake might have been able to uplift the coastal area of Tohoku region.

We do not have exact evidence to determine which kind of event will occur after the Tohoku Earthquake. However, the postseismic uplift seen in the coastal region slowed once several months passed after the mainshock. The amount of postseismic deformation can be modeled as a logarithmic function of the time after the mainshock (GSI, 2012). The parameters estimated for this model based on postseismic deformation data show that most GEONET sites in the Tohoku region will not recover from the coseismic

Coseismic and postseismic slip distribution model From 10 March 2011 to 16 July 2011 (F3 solution and R3 solution) Contour interval:8.0m (Coseismic)

Fig. 6. Slip distribution model for postseismic crustal deformation of the 2011 off the Pacific Coast of Tohoku Earthquake, based on GEONET observations (blue lines), overlaying the coseismic slip distribution based on GEONET observations on land and seafloor crustal deformation from GPS/ acoustic observations (red lines). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

subsidence within several hundreds of years, which is considered the recurrence time for a major M9 class earthquake in this region. This analysis suggests that a mechanism different from ongoing postseismic slip is the reason for the long-term uplift of the Pacific coast of the Tohoku Region. We should carefully monitor the crustal deformation over a period of time to study the mechanism of inter-seismic uplift.

4. Liquefaction damage from the 2011 off the Pacific Coast of Tohoku Earthquake

The Tohoku Earthquake caused heavy liquefaction damage to buildings, public facilities and lifelines in large

parts of the Kanto District, especially on the Tokyo Bay side and on the lower plains along the Tone River.

Conditions under which liquefaction occurs are as follows: strong earthquake motion, high groundwater level and loosely deposited sand. Places with a high possibility of liquefaction are reclaimed land, former river channels, and lowlands between sand dunes and sandbars. To know the potential for liquefaction, it is important to determine the history of the land. The GSI has archived time-series geospatial information, such as from olden topographical maps and olden aerial photos, and it provides these archived data to the general public. It is possible to understand the history of the land and liquefaction-prone locations from time-series geospatial information.

We surveyed the damage at the liquefaction sites and researched geographical conditions of areas remarkably damaged by liquefaction using time-series geospatial information. We mainly used time-series geospatial information, such as olden French-style maps (Japanese: jinsoku-sokuzu), olden topographical maps, previous aerial photos, land condition maps and landform classification maps for river improvement.

4.1. Liquefaction damage in the Hinode area of Itako City, Ibaraki Prefecture

Although liquefaction occurred at many places in Itako City, Ibaraki Prefecture, which is at the lower reaches of the Tone River, liquefaction damage was especially serious in the Hinode area (Fig. 7). Typical liquefaction damage of the Hinode area is shown in Fig. 8. Although the main

Fig. 7. Index map for the sites referred in the figures to describe the liquefaction (Base map using the Cyber Japan Web System by GSI of Japan).

Fig. 8. Liquefaction situation of Hinode area, Itako City, Ibaraki Prefecture. (Base map using the Cyber Japan Web System by GSI of Japan).

roads were restored to some extent by June 2011, leaning utility poles were left "as is'' (Fig. 8A). The school buildings showed an apparent uplift of about 25 cm at Hinode Junior High school (Fig. 8B). The damage tended to be more severe in the southern area. Street gutters were deformed by lateral spreading; cave-in and other damage to the lid of a gutter are seen in Fig. 7C or Fig. 7D. The jinsoku-sokuzu map (Fig. 9) published by the Japanese army about 130 years ago shows that the Hinode area is land that was reclaimed by the drainage of a small bay, which means the area is susceptible to liquefaction.

4.2. Liquefaction damage in the Kinu area of Shimotsuma City, Ibaraki Prefecture

There is a low-lying former river channel in a band across the Kinu area of Shimotsuma City (Fig. 7). This landform is shown on the "Mitsukaido" Land Condition Map published by the GSI.

This former river channel sank, and land subsidence and relative groundwater level rise are presumed. Service on neighboring roads was suspended as a result of intense cracking and undulation of the road (A in Fig. 10).

Fig. 9. Overlaying "Jinsoku-sokuzu" and Base Map Information of Hinode area, Itako City, Ibaraki Prefecture (using the Historical Agricultural Environmental Inspection System by National Institute for Agro-Environmental Sciences).

Fig. 11. (a) Jinsoku-sokuzu of Kinu area, Shimotsuma City, Ibaraki Prefecture, (b) Topographical map "Mitsukaido" with 1/50,000 published in 1934, (c) Aerial photo taken by US army in1947 (USA-R388-61).

Liquefaction damage, such as cracking, differential settlement and the tilting of buildings occurred even in the new residential area in the former river channel. The houses were destroyed by the severe subsidence of reclaimed land and the collapse of foundations (B in Fig. 10).

The main stream of the Kinu River was shown as meandering in a jinsoku-sokuzu map from about 130 years ago (Fig. 11a). A new river channel was under construction in the olden topographic map (1:50,000 scale) published about 75 years ago (Fig. 11b). In the U.S. army aerial photo, it is clear that the former river channel is now a paddy field (Fig. 11c).

5. Liquefaction in Urayasu City

Urayasu City is one of the areas that were most severely affected by liquefaction caused by the strong motion of the Tohoku Earthquake (Fig. 7). We carried out a leveling survey in Urayasu City and compared the results with the differences in LIDAR survey data measured pre-and post-earthquake, and upward displacement values of piled buildings to determine surface subsidence by liquefaction.

5.1. Results of the leveling survey near Shin-Urayasu Station

Since the liquefaction damage of Urayasu City in the Tokyo Bay area was severe, the amount of land subsidence caused by liquefaction was surveyed by leveling, and the author analyzed the severity of liquefaction.

Leveling was carried out using the block reference point near Shin-Urayasu Station on the JR Keiyo Line in August 2011. The leveling route was set perpendicular to the coastline. The results were compared with the observation results of the block reference point in November and December 2006. Assuming that the subsidence at point A is zero where liquefaction was not observed relative subsidence in the area after 2006 was computed.

The leveling survey results are shown in Fig. 12. Subsidence in the liquefaction area ranged from several centimeters to several tens of centimeters. A tendency was seen for the subsidence of recently reclaimed land near the coastline to be large and for the subsidence of old reclaimed land to be small. Near point H, land subsidence of about 4 cm was observed for the five years from 2004 to 2009 by leveling survey. The subsidence in Fig. 12 contains not only that from liquefaction but also that from pre-earthquake consolidation.

Fig. 12. Results of leveling survey in Urayasu City (modified from Koarai et al., 2011). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 13. Overlaying the amount of change by the difference of two LIDAR data and results of leveling (modified from Koarai et al., 2011). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

5.2. Comparison of the difference between pre- and post-earthquake LIDAR data, and leveling survey data

To understand ground deformation produced by earthquake-generated liquefaction, Urayasu City (2011) released an elevation difference map on its website. This map was created by comparing two seasons of airborne laser survey data (LIDAR data). Pre-earthquake LIDAR

data were surveyed in December 2006; post-earthquake LIDAR data were surveyed in April 2011.

An overlay of the change of elevation produced by determining the difference between pre- and post-earthquake LIDAR data and the data from the leveling survey is shown in Fig. 13. It is thought that the results also include fundamental subsidence for the previous five years. Land subsidence is observed in whole area; deeper

subsidence was observed in the area on the south side of the Tokyo Bay Highway. Crustal deformation from the Tohoku Earthquake is shown to have caused about 4 cm of subsidence in the Tokyo Bay area, and about 4 cm of fundamental land subsidence in five years. The vertical accuracy of the LIDAR survey is about 15 cm. Because depression of 30 cm or more is considered significant with

Fig. 14. Relationship between results of leveling and difference of LIDAR data (after Koarai et al., 2011).

respect to LIDAR data, only depressions of 30 cm or more are judged to be significant for the following discussion.

A large amount of sedimentation is particularly apparent on the west side of the Sakai River (Imagawa, Takasu), at Meikai and at Hinode. Notable land subsidence due to liquefaction is found along the line a-b-c-d and b-e-f in Fig. 13. When field survey was performed, severe sand volcanoes and ground deformation were found to have occurred.

Near point J, where the leveling survey shows about 45 cm of subsidence, an area with large subsidence has the shape of a circle. About 40 cm of uplift for buildings was measured at several apartments near point J by field survey. Therefore, it can be considered that the subsidence of a block reference point properly reflects the surrounding land subsidence.

The relationship between subsidence at a block reference point as determined by leveling survey and by the average value of the difference of LIDAR data in nine meshes near the block reference point are shown in Fig. 14. Since the amount of lowering as determined by the difference in the two LIDAR data is large in places where large amounts of subsidence as determined by leveling occurred, it can be judged that the correspondence between the data is generally good.

However, absolute value correspondence is not necessarily good. In Fig. 14, the amount of lowering as determined by the difference of the two LIDAR data tends to be larger than that determined by leveling. Although a problem remains in the absolute value of the difference of LIDAR

Fig. 15. Overlaying difference of each period LIDAR data and values of uplift of large buildings (modified from Koarai et al., 2011). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

data, LIDAR data are sufficiently accurate for judging relative amounts of lowering, such as the degree of subsidence, and for seeing the tendency of overall subsidence distribution. The result is considered effective toward finding out the amount of land subsidence accompanying liquefaction in Urayasu City.

5.3. Comparison between the difference between pre- and post-earthquake LIDAR data and the uplift value for large buildings

Research Institute of Environmental Geology, Chiba (2011) measured the amount of uplift of structures with a stake foundation (from which the amount of land subsidence that can be found) and made the results public on their website. The data excluded the factors of uplift of buildings caused by consolidation settlement as much as possible, with the aim of measuring only the apparent uplift of buildings caused by the earthquake. The data were measured with high density except in the single-family house areas and areas without buildings. The amount of surface subsidence by liquefaction is expected to be measured quite accurately, since neither the amount of subsidence by crustal deformation or consolidation settlement is included. This data is considered very useful for calculating the regional subsidence by liquefaction. An overlay of the values of uplift of buildings and the difference in DEM by two period LIDAR data is shown in Fig. 15. The uplift of buildings is greater in the area on the south side of the Tokyo Bay Highway. The Tomioka (A in Fig. 15) and Mihama (B in Fig. 15) districts on the north side of the Keiyo Line and along the Sakai River, the Takasu district (C in Fig. 15), the Meikai district (D in Fig. 15) and the Hinode district (E of Fig. 15) saw notable uplift, while the seaside (i.e., the southeast side) of the Hinode and Meikai districts relatively little uplift is seen. This distribution qualitatively agrees with the area where the amount of lowering as determined by the difference in LIDAR data tends to be large.

6. Summary

Significant subsidence along the Pacific coast of the Tohoku Region resulted from crustal deformation caused by the Tohoku Earthquake. This wide-area subsidence can be explained by the elastic deformation of the crust derived from movement at the seismogenic fault, which occurred as the mainshock on March 11, 2011.

Another kind of remarkable subsidence caused by the Tohoku Earthquake is liquefaction, which occurred in large parts of the Kanto District. The areas most severely affected by liquefaction were reclaimed land such as

former river channels, which can be recognized from olden maps and olden aerial photos.

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