The distribution of earthquakes across the globe is shown in Figure 3.6. It is relatively easy to see the relationships between earthquakes and tectonic plate boundaries. Along divergent boundaries like the mid-Atlantic ridge and the East Pacific Rise, earthquakes are common but restricted to a narrow zone close to the ridge and consistently the hypocenters are commonly at less than a 30-kilometre depth. Shallow earthquakes are also common along transform faults, such as the San Andreas Fault. Along subduction zones, earthquakes are very abundant, and they are increasingly deep on the landward side of the subduction zone.

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, and the Lake Baikal area of Russia.

Earthquakes at Divergent and Transform Boundaries

Figure 3.7 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 2, 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.

Earthquakes at Convergent Boundaries

The distribution and depths of earthquakes in the Caribbean and Central America areas are shown in Figure 3.8. 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 3.8, 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.

There are also various divergent and transform boundaries in the area shown in Figure 3.8, and as we’ve seen in the mid-Atlantic area, most of these earthquakes occur along the transform faults.

The distribution of earthquakes with depth in the Kuril Islands of Russia in the northwest Pacific is shown in Figure 3.9. This is an ocean-ocean convergent boundary. The small red and yellow dots show background seismicity over several years, while the larger white dots are individual shocks associated with an M 6.9 earthquake in April 2009. The relatively large earthquake took place on the upper part of the plate boundary between 60 kilometres and 140 kilometres 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 — M 9 or higher — take place at subduction boundaries because there is the potential for a greater width of the rupture zone on a gently dipping boundary than on a steep transform boundary. The largest earthquakes on transform boundaries are in the order of M 8.

The background seismicity at this convergent boundary and other similar ones is predominantly near the subducting plate’s upper side. 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-kilometre 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 kilometres away from the plate boundary.

The distribution of earthquakes in the area of the India-Eurasia plate boundary is shown in Figure 3.10. This is a continent-continent convergent boundary. It is generally assumed that there is no actual subduction, although the India Plate continues to move north toward the Asia Plate. 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, adjacent parts of China, Pakistan and Afghanistan. Many of the earthquakes are related to the transform faults on either side of the India Plate. 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 in Figure 3.10 are related to the thrust faults shown in Figure 3.11 (and hundreds of other similar ones that cannot be shown at this scale). T

There is a very significant concentration of both shallow and deep (greater than 70 kilometres) earthquakes in the northwestern part of Figure 3.10. This is northern Afghanistan, and at depths of more than 70 kilometres, many of these earthquakes are within the mantle as opposed to the crust. It is interpreted that these deep earthquakes are caused by the northwestward subduction of part of the India Plate beneath the Asia Plate in this area.

Exercise 3.1 Earthquakes in British Columbia

Figure 3.12 shows the incidence and magnitude of earthquakes in British Columbia over a one-month period in March and April 2015.

1. What is the likely origin of the earthquakes between the Juan de Fuca (JDF) and Explorer Plates?
2. The string of small earthquakes adjacent to Haida Gwaii (H.G.) coincides closely with the rupture surface of the 2012 M7.8 earthquake in that area. How might these earthquakes be related to that one?
3. Most of the earthquakes around Vancouver Island (V.I.) are relatively shallow. What is their likely origin?
4. Some of the earthquakes in B.C. are interpreted as being caused by natural gas extraction (including fracking). Which of the earthquakes here could fall into this category?

 

Image descriptions

Figure 3.12 image description: The incidence and magnitude of earthquakes in British Columbia over a one-month period in March and April 2015: There were a few dozen smaller earthquakes spread out around Vancouver Island and the sunshine coast with a magnitude of 2. Farther west along the Explorer Plate, which is between the North American plate, the Juan de Fuca Plate, and the Pacific Plate, there were quite a few earthquakes with a magnitude of 3 and at least one earthquake with a magnitude of 4. Between the North American Plate and the Pacific Plate off the south-west coast of Haida Gwaii, there was a large cluster of earthquakes with magnitudes of 2. Along the Alaskan panhandle, there was a collection of 2- and 3-magnitude earthquakes. In addition, there were two 3-magnitude earthquakes west of Fort St. John in northern British Columbia and one or two 2-magnitude earthquakes. In total, this map shows one hundred and forty-nine earthquakes with a magnitude less than 2, ninety-seven earthquakes with a magnitude of 2, thirty-nine earthquakes with a magnitude of 3, and two earthquakes with a magnitude of 4 [Return to Figure 3.12]

Media Attributions

• Figure 3.6: Global Earthquakes © Dale Sawyer, Rice University. Used with permission.
• Figure 3.7: Earthquakes Around the Mid-Atlantic Ridge © Steven Earle after Dale Sawyer, Rice University.
• Figure 3.8: Earthquakes Around the Central-American Region © Steven Earle after Dale Sawyer, Rice University.
• Figure 3.9: Earthquakes Around the Kuril Islands. © Steven Earle after Gavin Hayes, from data from the USGS [PDF].
• Figure 3.10: Earthquakes Around the India Plate © Steven Earle after Dale Sawyer, Rice University.
• Figure 3.11: India-Asia Convergent Boundary © Steven Earle based on D. Vouichard, from a United Nations University document.