Earthquake Zones - Some locations are prone to earthquakes and some are not. Nearly 95% of all earthquakes take place along one of the three types of plate boundaries. Scientists use the location of earthquake epicenters to draw the boundaries of the plates because earthquakes frequently occur along plate boundaries.
The region of the planet with the most earthquakes is the area around the Pacific Ocean. About 80% of all earthquakes strike this area. This region is called the Pacific Ring of Fire because most volcanic eruptions occur there as well. The Pacific Ocean is surrounded by convergent and transform plate boundaries.
About 15% of all earthquakes take place in the Mediterranean-Asiatic belt. This is where convergent plate boundaries are shrinking the Mediterranean Sea and causing the Himalayas to grow. The remaining 5% of earthquakes are scattered around the other plate boundaries with a few occurring in the middle of a plate, away from plate boundaries.
All three types of plate boundaries have earthquakes. Enormous and deadly earthquakes occur at transform plate boundaries. Because the slabs of lithosphere slide past each other without moving up or down, transform faults have shallow focus earthquakes.
The most notorious earthquake fault in North America is the San Andreas Fault that runs through California. The 1,300 kilometer (800 mile) long fault is the transform boundary between the northeastward-moving Pacific plate and the southwestward-moving North American plate. The San Andreas is a right-lateral strike-slip fault.
The largest earthquake on the San Andreas Fault in historic times occurred in 1906 in San Francisco. This earthquake likely measured magnitude 7.8, which is a very large earthquake. The earthquake and the subsequent fire is still the most costly natural disaster in California history. An estimated 3,000 people died and about 28,000 buildings were lost, mostly in the fire.
In 1989, the Loma Prieta earthquake struck near Santa Cruz, California. The magnitude 7.1 quake resulted in 63 deaths, 3,756 injuries and left more than 12,000 people homeless. The property damage was estimated at about $6 billion. In 1994, an earthquake on a blind thrust fault struck near Los Angeles, California in the neighborhood of Northridge. It registered 6.7 on the moment magnitude scale. Seventy two people died, 12,000 more were injured and damage was estimated at $12.5 billion.
There are many other faults spreading off the San Andreas, which together with the main fault produce around 10,000 earthquakes a year (Figure 7.31). While most of those earthquakes cannot even be felt by people nearby, occasionally one is massive. In the San Francisco Bay Area, the Hayward Fault was the site of a magnitude 7.0 earthquake in 1868.
Convergent plate boundaries also produce massive and deadly earthquakes. Earthquakes mark the motions of subducting lithosphere as it plunges through the mantle. The earthquakes can be shallow, intermediate or deep focus. Convergent plate boundaries produce earthquakes all around the Pacific Ocean basin.
The Philippine plate and the Pacific plate subduct beneath Japan creating a chain of volcanoes and as many as 1,500 annual earthquakes. The great Kanto earthquake of 1923 is thought to have killed 140,000 people, many in the subsequent fire. In Yokohama, 90% of houses were damaged or destroyed and 60% of Tokyo’s population became homeless. In the Great Hanshin (Kobe) Earthquake of 1995, 6,434 people died.
Subduction is also taking place along the Cascades Mountains in the Pacific Northwest as part of the Pacific Ring of Fire. The Juan de Fuca plate is plunging beneath the North American plate and forming volcanoes that extend south into northern California. The Cascades volcanoes are active and include Mount Saint Helens, which had a large eruption in 1980. Mount Lassen, Mount Shasta, and Medicine Lake volcano in northeastern California are the three southernmost volcanoes in the Cascades chain.
Yet the Cascadia subduction zone is one of the world’s quietest subduction zones, with relatively few earthquakes. Though they don’t happen often, they are extremely powerful when they hit. The last major earthquake on the Juan de Fuca occurred in 1700, with a magnitude estimated at between 8.7 and 9.2. The geologic history of the area reveals that major earthquakes occur here about every 300 to 600 years. Since it has now been more than 300 years since the last earthquake in the area, the Pacific Northwest is at risk from a potentially massive earthquake that could strike any time.
The thrust faulting and folding that result from the convergence of continental plates creates massive earthquakes. The region in and around the Himalaya, for example, is the site of many earthquakes. The 2001 Gujarat, India earthquake is responsible for about 20,000 deaths with many more people injured or made homeless.
Earthquakes also occur at divergent plate boundaries. At mid-ocean ridges, these earthquakes tend to be small because the plates are young and hot. The earthquakes are shallow because the new plates are thin. Since divergent plate boundaries in the oceans are usually far from land, they have little effect on peoples’ lives. On land, where continents are rifting apart, earthquakes are larger and stronger.
About 5% of earthquakes take place within a plate; that is, away from plate boundaries. A large intraplate earthquake occurred in 1812 when a magnitude 7.5 earthquake struck near New Madrid, Missouri. The earthquake was strongly felt over around 50,000 square miles, and altered the course of the Mississippi River. Because very few people lived here at the time, only 20 people died.
However many more people live here today and the New Madrid Seismic Zone continues to be active. A similar earthquake today would undoubtedly kill many people and cause a great deal of property damage. Intraplate earthquakes are caused by stresses due to plate motions acting in solid slabs of lithosphere.
EmoticonEmoticon