Your TL;DR for Tokyo’s worst-case earthquake scenario

The summarized earthquake report can be found on Tokyo’s disaster prevention website here (Japanese):
https://www.bousai.metro.tokyo.lg.jp/_res/projects/default_project/_page_/001/021/571/20220525/n/00n.pdf

And the full official report can be found here (Japanese):
https://www.bousai.metro.tokyo.lg.jp/taisaku/torikumi/1000902/1021571.html

A complete list of all recorded earthquakes in Japan by the Headquarters for Earthquake Research (earthquake return rates, 30-year probabilities, etc.):
https://www.jishin.go.jp/main/choukihyoka/ichiran.pdf

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#INTRO

Although recent news reports mostly discuss about the single worst case earthquake scenario, there are actually several other scenarios that can also occur. After the Great East Japan Earthquake in 2011, Japan’s Cabinet Office and the Headquarters for Earthquake Research selected several likely earthquake scenarios that can impact Tokyo. Each scenario was chosen based on its probability of happening soon or how destructive they could potentially be.

The goal of the report is to have a realistic simulation of human/city damage. This helps people efficiently make rules, plan emergency responses, and spend money without doing too little or going overboard. For example, it wouldn’t make sense to spend millions and waste time and manpower building a 12.5 meter high tsunami wall (the actual height of the new wall in Rikuzentakata, Iwate) around Tokyo if the simulations predict only a 2.6 meter tsunami (the current simulated prediction).

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#LIMITATIONS

Japan is experienced dealing with earthquakes. However, they note that our knowledge of the earthquake mechanisms is still limited. Not only that, there are only a limited number of past large earthquakes they can pull data from to help estimate future damage.

The simulation is also based on a number of assumptions, like damage to transportation (trains, cars, etc.), lifelines (electricity, water, communications, etc.) and the exact location of where the earthquake will happen (different parts of Tokyo will have different ground motions or ground liquification). So the damage numbers do not represent all of the damage that could be expected. It’s also possible that people will respond differently than in past disasters or how they responded to disaster questionnaires.

But even so, the report aims to paint an overall “picture” of a disaster scenario based on Japan’s history and the latest sciences and data available.

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#SIMULATED CONDITIONS

Conditions were simulated to be:

* Winter season
* 6:00 PM
* 8 m/s wind speed

The time of day, season, and weather conditions change throughout the year. However, fire hazards are highest during the evening winter when people are home and fire appliances are being used. Some homes are old and are at risk of collapse. There are also many people around offices, downtown areas, and stations at risk of falling objects. Railways and roads are packed at rush hour and can make it difficult for emergency services to respond. Wind speeds were assumed to be normal at 4 m/s and strong at 8 m/s.

Using data from recent earthquakes that killed over 300 people in Japan, the simulated deaths and injuries are calculated by various factors such as:

* Time of day
* Population density
* Population indoors/outdoors
* Population age
* Traffic flow/density
* Building age
* Building retention rates
* Building type (wood vs non-wood)
* Falling down
* Falling indoor/outdoor objects
* Falling block walls
* Falling vending machines
* Steep slope failure
* Land liquification
* People who have difficulty escaping on their own
* Fire damage (failure to escape, buried alive, or caught in fire while escaping)
* Tsunami (public’s evacuation awareness, evacuation speeds by terrain, population by floor)

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#FALLING VENDING MACHINES

The “falling vending machines” factor might seem strange, but there were over 4,045,800 vending machines reported in Japan in December 2020. This is an example of how they calculate one of the factors in the simulation.

In the simulation, vending machines are treated as concrete walls, but corrected for the average width of the machines. In 1978, an earthquake near Miyagi caused nearly 14,000 walls to collapse and fatally crushed 18 people. This earthquake led to a major revision of earthquake codes in Japan. But even with the revision, in 2018 an earthquake near Osaka caused concrete walls to kill 2 people, including a a 9-year old girl on her way to school.

The high number of vending machines installed in the country are placed in locations with high foot traffic, and are at risk of falling over during earthquakes, especially if they’re not properly fitted.

Injury/death calculations were done by:

* Vending machine installation ratio by district
* Daytime/nighttime population
* Rate of installed vending machine fall prevention measures
* Vending machine fall rate from the 1995 Kobe Earthquake (this earthquake killed over 6,400 people, 90% due to being crushed or suffocated)
* Number of deaths/injuries from fallen stone walls due to the ground acceleration of the 1978 Miyagi Earthquake

With this, they calculate 205 people being fatally crushed by concrete walls, vending machines, etc. if there’s a M7.3 earthquake in southern Tokyo (the worst case scenario), and 7,057 people being seriously injured.

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#THE 5 TOKYO SCENARIOS

Here are the 5 likely scenarios that can affect the Tokyo area:

(Note: I rounded all numbers down to nearest 100 for easier reading. Numbers may not add up. For exact numbers, please refer to the official report.)

> **Southern Tokyo, Magnitude 7 (70% chance next 30 years, the worst case scenario)**

* ***194,400 damaged buildings***
82,100 by shaking
112,200 by fires
* ***6,100 dead***
3,600 by shaking
2,400 by fires
* ***93,400 injured***
83,400 by shaking
9,900 by fires
* ***2,993,700 evacuees***

> **Eastern Tama, Magnitude 7 (70% chance next 30 years)**

* ***161,500 damaged buildings***
70,100 by shaking
91,400 by fires
* ***4,900 dead***
3,000 by shaking
1,900 by fires
* ***81,600 injured***
74,300 by shaking
7,200 by fires
* ***2,755,500 evacuees***

> **Tachikawa Fault, Magnitude 7 (0.5-2.0% chance next 30 years)**

* ***51,900 damaged buildings***
16,000 by shaking
35,800 by fires
* ***1,400 dead***
700 by shaking
700 by fires
* ***19,200 injured***
16,600 by shaking
2,500 by fires
* ***590,100 evacuees***

> **Taisho Kanto, Magnitude 8 (0.0-6.0% chance next 30 years)**

* ***54,900 damaged buildings***
28,300 by shaking
26,600 by fires
* ***1,700 dead***
1,200 by shaking
500 by fires
* ***38,700 injured***
37,000 by shaking
1,600 by fires
* ***1,506,100 evacuees***

> **Nankai Trough, Magnitude 8-9 (70-80% chance next 30 years)**

* ***Expected maximum tsunami height is about 2.0-2.6 meters in city, and about 28 meters on islands***
* ***1,200 damaged buildings***
— by shaking (expected no damage due to shaking)
— by fires (expected no damage due to fires)
1,200 by tsunami
* ***900 dead***
— by shaking
— by fires
900 by tsunami

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#HOW TO MINIMIZE DAMAGE

Factors that help bring down building damage, death rates, and injury rates are:

* **Increasing rate of earthquake-resistant buildings**

Currently 92% of Tokyo’s residential buildings meet the 1981 building standards. They want to work towards 100% of 1981 standards, or start shifting towards the newer year 2000 building standards.

If this goal is reached, the number of destroyed buildings and deaths can decrease by 60%.

* **Increasing rate of people taking measures to make indoor furniture fall-proof**

Currently 57.3% of people have implemented ways to prevent furniture from falling. They want to work towards 75% or 100%, and make sure people implement them correctly. 23% of the furniture measures in the 1995 Kobe Earthquake were installed wrong and ineffective.

If these goals are reached, the number of deaths and injuries can decrease by 40-80%.

* **Increasing fire suppression measures**

Currently 8.3% have seismic breakers installed to prevent electrical fires, and 36.6% of fires suppressed when they start. They want 25-50% to have seismic breakers, and 60-90% of fires to be initially extinguished.

If these goals are reached, the number of destroyed homes and deaths can decrease by 70-90%.

Since the 2011 Great East Japan Earthquake, Tokyo wards have been implementing projects to make urban areas more fire-proof, such as splitting the city into firebreak zones using roads, rivers, rails, parks, and fire-resistant buildings. This will further reduce damage by fires.

* **Increasing awareness of tsunami evacuation**

Currently 38.7% would evacuate immediately, 58.0% would evacuate after some errands, and 3.3% would not evacuate. It is assumed that evacuation would start about 10 minutes after the disaster occurs in the early morning. They want to raise immediate evacuation to 100%, and everything else to 0%. They also want to decrease the response time down to 5 minutes, and increase evacuation speed by x1.5.

If these goals are reached, the number of deaths can be reduced to 0.

* **Increase and improve earthquake resistance and fire prevention to reduce economic damage**

The earthquake will damage homes and lifelines as well as businesses and supply chains. They predict about ￥95.3 trillion (or about USD $744.8 billion) in indirect and direct damage nationwide.

If buildings’ earthquake resistances and fire prevention measures are improved, economic damage can be reduced by ￥5.5-9.0 trillion (or USD $43.1-70.6 billion).

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https://www.reddit.com/r/japan/comments/v1j2ib/your_tldr_for_tokyos_worstcase_earthquake_scenario/