Earthquakes are scary, that is not new information but it always seems to escape the public’s mind how dangerous they are until another one happens. Take a look at the earthquake that shook the city if San Francisco in 1989. A magnitude 6.9 earthquake erupted in the Bay Area at 5:04pm on October 17th and lasted for only 15 seconds but took the lives of 67 people in that time frame, the majority of which were crushed under the collapse of the top level of the San Francisco-Oakland Bay Bridge onto the lower deck. Over 3000 were injured during this time and many buildings and infrastructure collapsed under the quake’s violent shaking, especially San Francisco’s Marina district which was leveled during the chaos. This 15 second event cost the city over $5 billion in damages.
The 2011 Tohoku earthquake in Japan shows just how horrific an earthquake can become with a high magnitude and a reactive subduction zone. It became the most powerful earthquake ever in Japan and fourth largest since 1900. On March 11 at 2:46 pm local time, a 9.1 magnitude earthquake decimated many cities (especially coastal) in Japan, lasting for 6 minutes and destroying everything in it’s rocking. It’s reported than the earthquake was so powerful that it shifted the entire main island of Japan, Honshu, 2.4 meters east. over 120,000 buildings fully collapsed with 280,000 partially collapsed and another 699,000 partially damaged. The following tsunami also resulted in millions being left without power, water, food, or shelter. It even caused a nuclear disaster at the Fukushima Daiichi Nuclear Power Plant, causing one of the reactors to explode and spew out radiation to the surrounding area. The World Bank estimated the cost of the damage to be up to $235 billion, making it the most expensive natural disaster ever, 47 times more expensive than the 1989 San Francisco earthquake. The number of casualties officially recorded was 15,898 but with the inclusion of earthquake related deaths that number shoots up to 19,575!
So, what’s the cause behind the horror? What makes these earthquakes so dangerous? It’s all boils down to plate tectonics and waves; specifically body and surface waves. But let’s start with earth’s crust.
Earth’s crust is comprised of tectonic plates, a.k.a. what the continents sit on and what is below the ocean. There are two kinds of plates for each of these scenarios as well: continental plates and oceanic plates, both of which are made of the lithosphere and can shift in accordance with other plates. When plates do move in relation to each other though they do so at three different types of plate boundaries. The first plate boundary is a divergent plate boundary which is where two plates will move away from each other, forming rift valleys and mid oceanic ridges. The second type of plate boundary is known as a transform plate boundary, where two plates will slide against each other without actually moving away or closer to each other, causing faults. This kind of boundary can be found here in the Bay Area with the Hayward Fault. But the third type of plate boundary is generally the cause for many of Earth’s strongest, most relentless earthquakes. Convergent plate boundaries. At these boundaries, one plate will slide under another plate, causing the area of sliding to be called a subduction zone. These can cause trenches, volcanic mountains and islands, and mountain ranges and are the only boundaries that tsunamis can occur at.
Now, although earthquakes can happen at all three plate boundaries, there are different kinds of waves that can happen as a result of these earthquakes and these will determine the damage done to a location. When an earthquake first hits, it is P and S waves that will initially attack. P waves are the fastest and will arrive first. Known as compressional waves, P waves can move through all layers of the Earth from the mantle down to the liquid outer core and even further into the solid inner core. As is in their name, P waves force all materials in their way to contract and release while staying level relative to the land. S, or Shear, waves follow P waves and the second wave of attack in an earthquake. Since S waves are slower than P waves though, they can only penetrate the mantle of the Earth, unable to move through the liquid outer core or solid inner core. They also move differently than P waves. Where a P wave moves horizontally through compressions, an S wave will move in actual vertical waves. The third type of wave though is the most dangerous kind. Surface waves arrive last but cause the most damage during an earthquake, moving in a rolling motion like a wheel on a car. As the earthquake moves deeper though, surface waves become less of an issue.
As a study in plate tectonics, let’s look at the 2004 Indian Ocean earthquake and tsunami which was felt in Indonesia (which suffered the greatest destruction), Bangladesh, India, Malaysia, Myanmar, Thailand, Sri Lanka, and the Maldives and registered a magnitude of 9.1-9.3.
First, the earthquake which took place on December 26th. It is estimated that a 1600 km long fault line ruptured along the subduction zone of the Burma Plate and Indian, the later of which subducted under the other. The plate movement caused the longest ever recorded response of 8 to 10 minutes of shaking felt above ground. So, when the P waves first hit it was violent and sudden but the resulting S waves continued to hit Indonesia and surrounding countries following as well. When the surface waves occurred last though they obliterated entire cities and villages mainly in Indonesia but also in other Southeast Asian countries. The large displacement of the ocean from the subduction of the Indian Plate under the Burma Plate is what resulted in the tsunami moving out and toward the coastlines of Indonesia and surrounding Indian Ocean countries with waves reaching high points of 51 m!
This earthquake became the third most powerful one ever, one placement higher than Japan, but also gave the worst recorded tsunami in history. The total death count from this incident is thought to be as high as 227,898 with the majority being Indonesian but fatalities as far away as Kenya, Yemen, and South Africa. The economic cost was $15 billion which, although lower on the spectrum, fails to highlight the horrors experienced by all and the years of rebuilding that could never replace what was taken from the hundreds of thousands misplaced.
Oceans in Motion 