Earthquakes

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It’s well-known that Japan is susceptible to earthquakes. It is located in the Pacific Ring of Fire, an area encircling the Pacific Ocean that is prone to earthquakes and volcanic activity due to Earth’s tectonics. Just offshore of Japan is the Boso Triple Junction, where three tectonic plates meet (usually only two plates meet), and another source of earthquakes. If you have lived in Japan for any length of time you have most likely experienced an earthquake.

Earthquakes can be really severe, as the recent Kumamoto earthquakes and the 2011 Tohoku earthquake demonstrate. What makes the earthquakes so destructive? Ground shaking leads to building collapse, fire outbreak, tsunami, and other destruction. I’m going to take a look at how we measure the ground shaking and the destructive nature of these natural phenomena.

3.11 Tohoku Earthquake

On May 11, 2011, a major earthquake shook the Tohoku 東北 region – that’s the northeastern part of the main island of Japan, including Akita, Aomori, Fukushima, Iwate, Miyagi, and Yamagata prefectures. The earthquake waves were felt across most of the eastern part of the main island, and destruction occurred even in Tokyo.

The main earthquake was magnitude 9.0 and occurred just before 2:30 PM. There were a number of foreshocks (before the main event) and aftershocks (after the main event) of very large magnitude. Luckily, my friends and their families were not affected by loss of life or loss of home during this earthquake. However, almost 16,000 people lost their lives, and many thousands more were permanently displaced.

The earthquake occurred offshore, and the displacement of water in the ocean caused a large tsunami (or tidal wave) that caused further destruction. It wiped out entire towns along the shoreline and took many lives. The tsunami affected not only Japan, but also hit other countries as it traveled across the Pacific Ocean. Debris from the tsunami destruction washed out to sea and ended up on foreign shores.

2016 Kumamoto Earthquakes

Just a few weeks ago in April, three major earthquakes shook the western part of Japan, centered in Kumamoto prefecture. A strong foreshock of magnitude 6.2 on April 14th and a 6.0+ foreshock on April 15th preceded a magnitude 7.0 earthquake at just before 1:30 AM on April 16th. There were also a number of smaller foreshocks and aftershocks. The damage was more localized, but severe damage occurred in Kumamoto and Oita prefectures.

The loss of life was far less and the damage less widespread, but the shaking caused extensive building damage and left many homeless. Unfortunately, several of my friends were left homeless by the damage. Landslides and flooding also contributed to property destruction. With many people evacuated and homeless, it will be difficult to rebuild quickly.

Fuji Sengen Shrine

Shindo 震度 vs. Magnitude

The difference in magnitude between the Tohoku earthquake (magnitude 9.0) and the Kumamoto earthquake (7.0) is based on the moment magnitude scale and gives you the log-based magnitude of the quake at the epicenter (the location of the earthquake in the ground). Thus, the M 9.0 quake caused much more widespread destruction than the M 7.0 quake in Kumamoto.

The differences go further, because the Tohoku earthquake occurred deep in the ocean, 30 km below the sea floor and quite far offshore, while the Kumamoto earthquakes were quite shallow, at about 10 km below ground but centered directly underneath the Kumamoto region. The Japanese Meteorological Agency seismic intensity scale measures the shindo 震度 or degree of shaking. The intensity of an earthquake is not determined entirely by its magnitude, but varies with the depth and distance of the area measured from the event. Thus, the shindo may vary from city to city for the same event.

Copyright Japan Meteorological Agency

Kyushu Shindo Map 2016 April 14 – Copyright Japan Meteorological Agency

The shindo for both the Tohoku earthquake and the M 6.2 Kumamoto foreshock was 7, while the main shock of the M 7.0 Kumamoto registered as a shindo 6+. Thus, although the magnitude was greater in the Tohoku than the Kumamoto main shock, the intensity of shaking (shindo) was very high for both. So, while the size of the earthquake “bomb” that went off in the Tohoku earthquake was much larger and affected a larger area, it was also offshore and deeper underground. The Kumamoto earthquake “bombs” were more localized, but at a shallow depth directly under the Kumamoto area caused intense shaking and quite a bit of damage.

In short…

Although most of the world measures the magnitude, which gives the total power of the earthquake “bomb explosion,” Japan provides for each city a measure of the intensity of the shaking (shindo), accounting for the depth and distance of the “explosion” from that locality. The shindo measurement can improve an understanding of the destructive power of the earthquake at a certain location.

Have you ever experienced an earthquake, big or small? Have you lived in a place with a lot of earthquakes? I’d love to hear your experiences!

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2 thoughts on “Earthquakes

  1. We moved to Japan in early 2012 so we just missed the big one and left Japan a couple of months before the Kumamoto quakes. Interesting luck we had there. During those three years we felt many smaller shakes and saw that they occur incredibly often. It was always at the back of my mind that it could happen. We lived in San Diego, California before Japan and felt a few while living there. It is definitely scary.

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