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8.4 EARTHQUAKES AT CONVERGENT BOUNDARIES

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

How do earthquakes create refugees?

People who’ve lost their homes in a large earthquake in Pakistan live in a refugee camp, which appears as tents in the photo. Despite suffering the loss of their homes, material possessions, and sometimes loved ones, refugees are often most damaged by the fear that another earthquake could strike. With many people, each aftershock brings renewed terror.

Convergent Plate Boundaries

Earthquakes at convergent plate boundaries mark the motions of subducting lithosphere as it plunges through the mantle ( Figure below ). Eventually the plate heats up enough deform plastically and earthquakes stop.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

This cross section of earthquake epicenters with depth outlines the subducting plate with shallow, intermediate, and deep earthquakes.

Convergent plate boundaries produce earthquakes all around the Pacific Ocean basin.

Ocean-Ocean: Japan

Earthquakes in Japan are caused by ocean-ocean convergence. The Philippine Plate and the Pacific Plate subduct beneath oceanic crust on the North American or Eurasian plates. This complex plate tectonics situation creates a chain of volcanoes, the Japanese islands, and as many as 1,500 earthquakes annually.

In March 2011 an enormous 9.0 earthquake struck off of Sendai in northeastern Japan. This quake, called the 2011 Tōhoku earthquake, was the most powerful ever to strike Japan and one of the top five known in the world. Damage from the earthquake was nearly overshadowed by the tsunami it generated, which wiped out coastal cities and towns ( Figurebelow ). Several months after the earthquake, about 22,000 people were dead or missing, and 190,000 buildings had been damaged or destroyed. Aftershocks, some as large as major earthquakes, have continued to rock the region.

A map of aftershocks is seen here:http://earthquake.usgs.gov/earthquakes/seqs/events/usc0001xgp/ .

Here is an interactive feature article about the earthquake:http://www.nytimes.com/interactive/2011/03/11/world/asia/maps-of-earthquake-and-tsunami-damage-in-japan.html .

 

 

Destruction in Ofunato, Japan, from the 2011 Tōhoku Earthquake.

Ocean-Continent: Cascades

The Pacific Northwest of the United States is at risk from a potentially massive earthquake that could strike any time. The subduction of three small plates beneath North America produces active volcanoes, the Cascades. As with an active subduction zone, there are also earthquakes. Surprisingly, large earthquakes only hit every 300 to 600 years. The last was in 1700, with an estimated magnitude of around 9. A quake of that magnitude today could produce an incredible amount of destruction and untold fatalities.

An image of earthquakes beneath the Pacific Northwest and the depth to the epicenter is shown here: http://pubs.usgs.gov/ds/91/ .

Elastic rebound at a subduction zone generates an earthquake in this animation:http://www.iris.edu/hq/files/programs/education_and_outreach/aotm/5/AOTF5_Subduction_ElasticRebound480.mov .

Continent-Continent: Asia

Massive earthquakes are the hallmark of the thrust faulting and folding when two continental plates converge ( Figure below ). The 2001 Gujarat earthquake in India was responsible for about 20,000 deaths, and many more people became injured or homeless.

 

 

 

 

 

 

 

 

 

 

 

 

Damage from the 2005 Kashmir earthquake.

In Understanding Earthquakes: From Research to Resilience, scientists try to understand themechanisms that cause earthquakes and tsunamis and the ways that society can deal with them (3d) : http://www.youtube.com/watch?v=W5Qz-aZ2nUM (8:06).

 

 

Summary

  • Earthquakes occur all along the subducting plate as it plunges into the mantle.

  • All three types of convergent plate boundaries produce massive earthquakes.

  • Subduction zones around the Pacific Rim are responsible for many of the world’s earthquakes.

Practice

Use this resource to answer the questions that follow.

1. How do convergent plate boundaries occur?

2. What is formed by the continental-continental plate boundaries?

3. Where are these type of boundaries found?

4. What is formed at oceanic-continental plate boundaries?

5. Where are active volcanoes found?

Review

1. Why does a subducting plate produce so many earthquakes and what type of quakes does it produce?

2. What caused the most destruction from the 2011 Japan earthquake and why?

3. Why do you think the Pacific Northwest has such infrequent but exceptionally massive earthquakes? There are several possible reasons.

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