Figure 11.2.1 General distribution of global earthquakes of magnitude 4 and greater from 2004 to 2011, color coded by depth (red: 0 to 33 kilometers, orange 33 to 70 kilometers, green: 70 to 300 kilometers, blue: 300 to 700 kilometers).

Earthquakes are also relatively common at a few intraplate locations. Some are related to the buildup of stress due to continental rifting or the transfer of stress from other regions, and some are not well understood. Examples of intraplate earthquake regions include the Great Rift Valley area of Africa, the Tibet region of China, the Lake Baikal area of Russia, and occasionally parts of Colorado.

Earthquakes at Divergent and Transform Boundaries

Figure 11.2.2 provides a closer look at magnitude (M) 4 and larger earthquakes in an area of divergent boundaries in the mid-Atlantic region near the equator. Here, as we saw in Chapter 10, the segments of the mid-Atlantic ridge are offset by some long transform faults. Most of the earthquakes are located along the transform faults, rather than along the spreading segments, although there are clusters of earthquakes at some of the ridge-transform boundaries. Some earthquakes do occur on spreading ridges, but they tend to be small and infrequent because of the relatively high rock temperatures in the areas where spreading is taking place.

Figure 11.2.2 Distribution of earthquakes of M4 and greater in the area of the mid-Atlantic ridge near the equator from 1990 to 1996. All are at a depth of 0 to 33 kilometers.

Earthquakes at Convergent Boundaries

The distribution and depths of earthquakes in the Caribbean and Central America area are shown in Figure 11.2.3. In this region, the Cocos Plate is subducting beneath the North America and Caribbean Plates (ocean-continent convergence), and the South and North America Plates are subducting beneath the Caribbean Plate (ocean-ocean convergence). In both cases, the earthquakes get deeper with distance from the trench. In Figure 11.2.3, the South America Plate is shown as being subducted beneath the Caribbean Plate in the area north of Colombia, but since there is almost no earthquake activity along this zone, it is questionable whether subduction is actually taking place.

Figure 11.2.3 Distribution of earthquakes of M4 and greater in the Central America region from 1990 to 1996 (red: 0 to 33 kilometers, orange: 33 to 70 kilometers, green: 70 to 300 kilometers, blue: 300 to 700 kilometers) (Spreading ridges are heavy lines, subduction zones are toothed lines, and transform faults are light lines.)

There are also various divergent and transform boundaries in the area shown in Figure 11.2.3, and as we’ve seen in the mid-Atlantic area, most of these earthquakes occur along the transform faults.

Figure 11.2.4 Distribution of earthquakes in the area of the Kuril Islands, Russia (just north of Japan) (White dots represent the April 2009 M6.9 earthquake. Red and yellow dots are from background seismicity over several years prior to 2009.)

The distribution of earthquakes with depth in the Kuril Islands of Russia in the northwest Pacific is shown in Figure 11.2.4. This is an ocean-ocean convergent boundary. The small red and yellow dots show background seismicity over a number of years, while the larger white dots are individual shocks associated with a M6.9 earthquake in April 2009. The relatively large earthquake took place on the upper part of the plate boundary between 60 kilometers and 140 kilometers inland from the trench. As we saw for the Cascadia subduction zone, this is where large subduction earthquakes are expected to occur.

In fact, all of the very large earthquakes — M9 or higher — take place at subduction boundaries because there is the potential for a greater width of rupture zone on a gently dipping boundary than on a steep transform boundary. The largest earthquakes on transform boundaries are in the order of M8.

The background seismicity at this convergent boundary, and on other similar ones, is predominantly near the upper side of the subducting plate. The frequency of earthquakes is greatest near the surface and especially around the area where large subduction quakes happen, but it extends to at least a 400 kilometer depth. There is also significant seismic activity in the overriding North America Plate, again most commonly near the region of large quakes, but also extending for a few hundred kilometers away from the plate boundary.

Figure 11.2.5 Distribution of earthquakes in the area where the India Plate is converging with the Asia Plate (data from 1990 to 1996, red: 0 to 33 kilometers, orange: 33 to 70 kilometers, green: 70 to 300 kilometers). (Spreading ridges are heavy lines, subduction zones are toothed lines, and transform faults are light lines. The double line along the northern edge of the India Plate indicates convergence, but not subduction. Plate motions are shown in millimeters per year).

The distribution of earthquakes in the area of the India-Eurasia plate boundary is shown in Figure 11.2.5. This is a continent-continent convergent boundary, and it is generally assumed that although the India Plate continues to move north toward the Asia Plate, there is no actual subduction taking place. There are transform faults on either side of the India Plate in this area.

The entire northern India and southern Asia region is very seismically active. Earthquakes are common in northern India, Nepal, Bhutan, Bangladesh and adjacent parts of China, and throughout Pakistan and Afghanistan. Many of the earthquakes are related to the transform faults on either side of the India Plate, and most of the others are related to the significant tectonic squeezing caused by the continued convergence of the India and Asia Plates. That squeezing has caused the Asia Plate to be thrust over top of the India Plate, building the Himalayas and the Tibet Plateau to enormous heights. Most of the earthquakes of Figure 11.2.5 are related to the thrust faults shown in Figure 11.2.6 (and to hundreds of other similar ones that cannot be shown at this scale). The southernmost thrust fault in Figure 11.2.6 is equivalent to the Main Boundary Fault in Figure 11.2.5.

Figure 11.2.6 Schematic diagram of the India-Asia convergent boundary, showing examples of the types of faults along which earthquakes are focused. The devastating Nepal earthquake of May 2015 took place along one of these thrust faults.

There is a very significant concentration of both shallow and deep (greater than 70 kilometers) earthquakes in the northwestern part of Figure 11.2.5. This is northern Afghanistan, and at depths of more than 70 kilometers, many of these earthquakes are within the mantle as opposed to the crust. It is interpreted that these deep earthquakes are caused by northwestward subduction of part of the India Plate beneath the Asia Plate in this area.

Media Attributions

• Figure 11.2.1: Global Earthquakes © Dale Sawyer, Rice University. Used with permission.
• Figure 11.2.2: Earthquakes Around the Mid-Atlantic Ridge © Steven Earle after Dale Sawyer, Rice University.
• Figure 11.2.3: Earthquakes Around the Central-American Region © Steven Earle after Dale Sawyer, Rice University.
• Figure 11.2.4: Earthquakes Around the Kuril Islands. © Steven Earle after Gavin Hayes, from data from the USGS [PDF].
• Figure 11.2.5: Earthquakes Around the India Plate © Steven Earle after Dale Sawyer, Rice University.
• Figure 11.2.6: India-Asia Convergent Boundary © Steven Earle based on D. Vouichard, from a United Nations University document.