Quake Assesment - Japan Govt.

Source: Japan Times

Quake assessment projects nearly 10,000 dead in Tokyo

Latest projection takes into account lessons from the March 11 disaster

By MIZUHO AOKI

Staff writer

A massive quake beneath northern Tokyo Bay would kill about 9,700 people, the Tokyo Metropolitan Government said Wednesday.

In its latest damage projection report, the metropolitan government said approximately 70 percent of the area covered by Tokyo's 23 wards would suffer a destructive temblor of upper 6 or stronger on the Japanese seismic intensity scale.

This is worse than the previous estimate of six years ago, which envisaged an earthquake of magnitude 7.3 striking the greater Tokyo area, and reflects new findings from the March 11 quake and tsunami. It also takes into account a recent study by the science ministry that a massive inland quake threatening the metropolitan areas will top the 7-level Japanese seismic intensity scale.

"Based on our estimate of possible damage, we would like to work on preventive measures," said a Tokyo official, adding that the metropolitan government plans to draft a revised disaster prevention plan for Tokyo by September.

In the latest estimates, the metropolitan government's disaster-prevention council listed four possible types of earthquake that could strike Tokyo: a magnitude 7.3 quake originating below either the northern Tokyo Bay or the Tama region in western Tokyo; a magnitude 8.2 plate-boundary quake in the northwest of Kanagawa Prefecture and a magnitude 7.4 quake on an active fault in the Tachikawa area of western Tokyo.

Six different projections based on varying wind speed and time of a day were also considered.

Of the four types, the northern Tokyo Bay quake scenario would result in the most casualties and building damage, while in the worst of the six projections, the quake would hit at 6 p.m. in winter with the wind blowing at 8 meters per second, the highest possible speed based on past data.

In the worst-case scenario, upper 6 quake would strike about 444 sq. km of eastern Tokyo, while some areas facing Tokyo Bay, including Ota and Koto wards, may experience a level 7 quake.

The 9,641 death toll is up from 6,413 in the previous estimate. About 5,600 would likely be killed by collapsing buildings and some 4,100 by fire, it said.

The number of casualties is expected to be high in areas packed with wooden houses in east and southeast Tokyo, the report said.

The metropolitan government also projected there would be some 3.39 million evacuees and some 147,600 people would be injured.

Meanwhile, the number of destroyed buildings declined to 304,300 from 471,000 in the previous estimate, owing to a decline in the number of old wooden structures and reinforcement work done on many buildings, it said.

For the first time the metropolitan government estimated the height of tsunami for a magnitude 8.2 "Genroku"-type earthquake that hit Tokyo in 1703, a so-called plate-boundary earthquake like the temblor that struck on March 11, 2011.

In the report, tsunami of up to 2.61 meters would flood an area of 4.6 sq. km to a depth of 50 cm, but no one would likely be killed.

The metropolitan government estimated intensity scales and damage for every 50 sq. meters within the 23 wards and municipalities and every 250 sq. meters for the remaining areas, which is more detailed than the previous report, the official said.

The latest study by the metropolitan government was conducted in light of the March 11 quake and tsunami that took the lives of more than 15,000 people.

Research team releases new quake intensity maps for Tokyo

Tokyo could experience more intense shaking in the event of a magnitude-7 earthquake than has been previously forecast, according to a new intensity distribution map released by the science ministry on March 30.

According to a project team at the ministry that has been modeling the potential consequences of an earthquake centered directly under the capital, the intensity of shaking in Tokyo and Kanagawa Prefecture could reach the maximum 7 on the Japanese scale. While magnitude is a measurement of the energy released by the earthquake regardless of its depth, the "shindo" scale reflects the degree of shaking on the ground.

Previous maps made by the government's Central Disaster Management Council have never forecast that such quakes could register a 7 on the shindo scale, but the most recent map suggests that such intense shaking could hit Edogawa, Koto and Ota wards in Tokyo, as well as Kawasaki and Yokohama in Kanagawa Prefecture. Moreover, almost all of Tokyo's 23 wards could experience shaking of at least an upper 6 on the shindo scale.

The disaster council had previously projected that an earthquake on the Tachikawa fault could cause shaking that would reach an intensity of 7, but it had not said the same for a quake with an epicenter in the bay.

The ministry's team studied the subterranean structure of the Tokyo metropolitan area and found that a plate boundary capable of generating earthquakes was around 10 kilometers closer to the surface in some places than previously assumed by the disaster council. A shallower epicenter usually increases the intensity of shaking on the ground.

However, because of the complexity of Tokyo's subterranean structure, it is difficult to predict exactly where a quake will strike.

According to Kazuki Koketsu, a professor at the Earthquake Research Institute of the University of Tokyo and a member of the project team, "The entire southern Kanto region should prepare for the possibility of strong shaking from a quake directly under Tokyo."

The disaster council has produced maps for quakes with 18 different epicenters. Although there is no definite evidence that a quake has struck directly under Tokyo bay before, it was included in the modeling experiments because of the potentially catastrophic consequences on the capital.

Provisional calculations based on previous assumptions showed that around 25 million people would feel shaking of an intensity of at least lower 6 on the shindo scale.

The disaster council has predicted that in a worst-case scenario, a quake centered in the northern part of Tokyo Bay would kill 11,000 people and cause 112 trillion yen ($1.4 trillion) of damage.

In a different simulation conducted by researchers at Chiba University and other institutions, the arrival of rescue workers from areas west of Tokyo would be greatly delayed because of liquefaction under roads. Adachi, Itabashi, Kita, Shinagawa and Ota wards in Tokyo would be worst hit by such delays.

The latest study also calculated much greater damage to homes and infrastructure from a quake hitting the northern part of Tokyo Bay.

A total of 390,000 wooden homes would be totally destroyed, with 34,000 water pipes damaged. The worst-hit areas are likely to be Naka and Tsurumi wards in Yokohama, Sumida and Koto wards in Tokyo, and Chiba city's Chuo Ward in Chiba Prefecture, where the cities of Urayasu, Ichikawa, Funabashi and Ichihara will also be badly affected.

While the disaster council previously estimated that running water would be almost completely restored within four days of a major quake, the project team estimated that it would take two weeks to restore 70 to 80 percent of the water supply and sewage services.

The government has estimated that the southern Kanto region, which Tokyo is in, has a 70-percent chance of being hit by a magnitude-7 quake within 30 years.

The science ministry has compiled four different intensity distribution maps based on three different epicenters in northern Tokyo Bay, as well as for another quake centered between Chiba and Ibaraki prefectures, where quakes have struck in the past.

The intensity distribution maps have been posted to the science ministry's website at: (http://www.mext.go.jp/b_menu/houdou/).

http://www.japantimes.co.jp/text/nn20120403a9.html

http://www.japantimes.co.jp/text/nn20120401a1.html

Quakes around 1855 and 1894

One thing that I've noticed is that there has been other quakes that precede and follow both the 1855 Ansei Edo quake and the 1894 Meiji Tokyo quake.

Is this significant? Well, I have no idea, to be honest, but I thought that it was worthy of noting, especially as northeast Japan had a massive one on 11th March.
One of the quakes, 15th June 1896 Meiji-Sanriku earthquake, was in a similar region, along the Japan Trench, to that of the March 2011 quake.
It must also be mentioned that the Japan Trench has regularly produced some huge quakes which can occur in locations extending from offshore Aomori Prefecture to offshore Ibaraki Prefecture,
Three of the top six quakes ever recorded worldwide, including the 2011 Tōhoku quake, appear to be clustered in a 6.2-year span between 2004 and 2011; however, experts construe this as a statistical anomaly or random chance.

Anyway, here's the quakes:

December 23, 1854 Ansei-Tōkai earthquake 8.4M
December 24, 1854 Ansei-Nankai earthquake 8.4M
November 11, 1855 Ansei Edo earthquake
April 9 1858, Hietsu Earthquake 7.0-7.1M

April 7, 1889 Kumamoto earthquake 6.3M
October 28, 1891 Mino-Owari Earthquake 8.0M
June 20, 1894 Meiji Tokyo Earthquake
June 15, 1896 1896 Meiji-Sanriku earthquake 8.5M

Study on Seismic Intensity 1855 Ansei Edo quake

"Study on the distribution of seismic intensity of the 1855 Ansei Edo earthquake in the Kanto District"

By Takahisa Enomoto.

PDF:
http://library.jsce.or.jp/Image_DB/eq1994/proc/00037/380-0201.pdf

Depth of 1855 Ansei Edo quake

Location and Magnitude of the 1855 Ansei Edo Earthquake

Bakun, W. H.

American Geophysical Union, Fall Meeting 2005, abstract #S21A-0197

Japan Meteorological Agency intensity assignments from a set of recent calibration earthquakes were used to derive two intensity attenuation models suitable for estimating the location and magnitude M for historical earthquakes in Japan (Bakun, JGR, 2005). The "Honshu model" is appropriate for shallow crustal earthquakes. The "subducting-plate model" is appropriate for Pacific plate and Philippine Sea plate interplate and intraslab earthquakes. Intensities decrease more rapidly with distance from the source in the Honshu model than in the subducting-plate model, consistent with many studies showing that the colder, high-velocity, high-Q subducting Pacific and Philippine plates act as efficient waveguides for propagating strong shaking from earthquakes. M estimated from a set of intensity assignments is significantly larger for a Honshu-model source than for a subducting-plate model source. It is clear from the pattern of damage that the destructive 11 November 1855 Ansei Edo earthquake occurred near Tokyo, but the focal depth and causative seismogenic structure are controversial. M estimated using intensity assignments range from 7.2 and 7.4 for a subducting-plate-model source at 30 km and 70 km depth, respectively, to 7.7 for a shallow (5 km depth) Honshu-model source. The focal depth and causative seismogenic structure cannot be resolved from the pattern of intensity assignments. Historical accounts suggest that the 1855 Ansei Edo earthquake was preceded by alterations in groundwater and springs. In addition, 78 aftershocks were felt in Tokyo in the month immediately following the earthquake. These observations argue against a deep subducting-plate source. Surface faulting was not reported in 1855 and an order of magnitude more aftershocks were felt after the shallow crustal 1994 M7.3 Kobe earthquake -observations that suggest a somewhat deeper source for the 1855 Ansei Edo event. More felt aftershocks were reported in 1855 than after the 1987 off-Chiba M6.7 Philippine Sea plate intraslab earthquake (47 km focal depth). The 1855 aftershock activity and other historical accounts are apparently consistent with an M7.2 subducting-plate model source at about 30 km depth.

http://adsabs.harvard.edu/abs/2005AGUFM.S21A0197B

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Insights on the 1855 Ansei-Edo earthquake depth derived from attenuation tomography and comparison with the 23 July 2005 M=6 shock

Nakamura, R.

American Geophysical Union, Fall Meeting 2005, abstract #S21A-0198

The 3-D attenuation structure, Q, beneath the Kanto plain (Nakamura et al, 2003), Japan, has been obtained using short-period strong ground motion records from Kanto and Tohoku. Low Q is consistent with the Vp/Vs structure, and is associated with the active volcanic front (Nakajima et al, 2001). In addition, there is an unexpected low-Q zone oriented E-W beneath the Kanto plain. This low-Q band is coincident with high Poisson ratios, which have been interpreted as a serpentinized zone (Kamiya and Kobayashi, 2000). The heterogeneous attenuation structure must affect seismic ground motions for large events such as the damaging 1855 Ansei-Edo earthquake. First, we compare the distribution of seismic intensity between recent earthquakes and the Ansei-Edo shock. Despite the large and high quality intensity dataset for the 1855 shock, there are many difficulties in estimating its hypocenter. It is possible, for example, that different hypocenters produce the same seismic intensity distribution because of the complex attenuation structure. Considering these difficulties, we examine the ground motion of the Ansei-Edo and M=6 23 July 2005 earthquakes, and find out that the two intensity distributions are very similar. Second, we calculate the predicted ground motions using our 3-D Q structure for several trial 1855 hypocenters, principally in an attempt to infer whether the 1855 source was deep or shallow. Because of the observed high intensities characteristic of Kisarazu and the eastern Kanagawa prefecture shared by the Ansei-Edo and 23 July 2005 shocks, and the good match of these observations to the prediction using the 3-D Q structure, we infer a similar characteristic intensity distribution. We thus conclude that the 1855 Ansei-Edo earthquake may have occurred at the north end of Tokyo bay at a depth of about 70-80 km.

http://adsabs.harvard.edu/abs/2005AGUFM.S21A0198N

1855 Ansei Edo Quake quick notes.

Ansei Edo Quake 11th November 1855.

Magnitude: 7.0

Epicentre: Tokyo Bay. Now a built-up area. Closest station is Toyosu.
http://mapper.acme.com/?ll=35.65,139.8&z=11&t=M&marker0=35.65,139.8,1855%20Ansei%20Edo%20earthquake

Casualties: 7,000

Damage: Approx 50,000 houses detroyed by quake and subsequent fires.

1855 Ansei Edo Quake

Regarding the 1855 Ansei Edo Earthquake (M 6.9), there are two ideas: first, it has been caused by active fault movement close to the surface; second, it has occurred on the upper surface of the Philippine Sea Plate.

The Ansei Edo Earthquake is thought to have occurred in a shallow or moderately deep area near the mouth of the Arakawa River (Ara Kawa). It generated strong ground motion over a wide area in the Kanto region, and its ground motion is estimated to have been the equivalent of seismic intensity 6 in JMA scale in eastern Tokyo, northwestern Chiba Prefecture, and eastern Saitama Prefecture (Fig.5-20 and Fig.5-21). This earthquake caused extensive fire damage over a wide area. Liquefaction of the ground is also known to have occurred in several locations. There were more than 7,000 fatalities in all. While there was no report of the tsunami generated by this earthquake, a large number of aftershocks are thought to haUve occurred (Fig.5-22). Historical data reveal that several phenomena occurred before the earthquake, including the spring of groundwater and ground rumbling.