Tectonic earthquakes, such as those occuring along th San Andreas fault, are the result of the shifting of the Earth’s tectonic plates.
Earthquakes occur when the planet’s plates move against one another. This movement can create stress that causes the Earth’s exterior shell, the lithosphere, to shift or break. There are several types of earthquakes, each characterized by the force that causes the plates to move. The most common form of earthquake is the tectonic earthquake; it happens when the shifting of the Earth’s plates is driven by geological force. Understanding how these earthquakes function includes grasping both the scientific causes and human effects.
The lithosphere (the Earth’s crust) is comprised of a collection of unyielding, asymmetrically shaped plates known as tectonic plates. The solid-rock plates range in thickness from four to forty miles and are diverse sizes and shapes. Tectonic plates cover the entire surface of the earth, with those located below the ocean being thinner than the plates located beneath the continents. Plates often contain both land and sea within their individual boundaries. There are exceptions, however; the Pacific Plate is entirely under the ocean and the Turkish-Aegean Plate is entirely under land. There are nine major plates: the Antarctic, Indo-Australian, Eurasian, South American, North American, African, Cocos, Zazca, and the Pacific. There are also a variety of smaller, though no less significant, plates located between the major plates. While the division of the plates is based on size, a designation of “small” does not refer to the effect of the plate. For example, the Juan de Fuca Plate (a small plate) has been the primary culprit in volcanic and earthquake related activities in the northwest United States. In addition, tectonic plate movement anywhere on the planet will have effects on the other plates. For instance, the growing divide between the South American and African plates is causing the Atlantic Ocean to expand but is also affecting the ocean floor above the Pacific Plate.
When tectonic plates move, they develop new boundaries or change existing boundaries between regions. Boundaries can be divergent, convergent or transform. Divergent boundaries occur when plates move apart and new crust begins to form in the newly created space between the plates. Convergent boundaries involve the destruction and salvaging of the existing crust into the Earth’s interior as one plate moves under another. Convergent boundaries are often noted by the presence of volcanoes or mountains at the boundary site. Transform boundaries, also known as fault lines, are where two plates move past each other horizontally. They are commonly found under the ocean and are the cause for what is known as “shallow earthquakes.” Some, such as the San Andreas fault boundary in California, are on land and visible in exposed areas.
While some earthquakes can be minor, barely affecting day-to-day human life due to location or level of force, others have been devastating. In 1906, a century of stress was released along the San Andreas fault causing a short but powerful 8.3 earthquake in San Francisco. The earthquake cost more than seven hundred people their lives and 250,000 their homes due to fires caused by the quake. In 1960, a massive 9.5 Richter scale –rated earthquake led to 20,000 deaths. Over four decades later, the 2004 earthquake near Indonesia and Sumatra topped the Richter scale at 9.3 causing a major tsunami in the Indian Ocean and fatalities of more than 300,000. The 2011 8.9 magnitude earthquake in Japan also caused a major tsunami, which laid waste to human life and caused extensive property damage.
Other Types of Earthquakes
While tectonic plate shifts are the cause of the majority of earthquakes on Earth, there are three other types of earthquakes that can cause movement of the planet’s surface. Volcanic earthquakes are the result of volcanic activity. Collapse earthquakes are minor quakes caused by the collapse of underground areas such as caves and mines. Explosion earthquakes are man-made and occur due to nuclear or chemical explosions.