Transform fault4/18/2023 ![]() ![]() The largest earthquakes on transform boundaries are in the order of M8. 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. As we saw for the Cascadia subduction zone, this is where large subduction earthquakes are expected to occur. The relatively large earthquake took place on the upper part of the plate boundary between 60 km and 140 km inland from the trench. 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. This is an ocean-ocean convergent boundary. The distribution of earthquakes with depth in the Kuril Islands of Russia in the northwest Pacific is shown in Figure 11.10. There are also various divergent and transform boundaries in the area shown in Figure 11.9, and as we’ve seen in the mid-Atlantic area, most of these earthquakes occur along the transform faults. Figure 11.9 Distribution of earthquakes of M4 and greater in the Central America region from 1990 to 1996 (red: 0-33 km, orange: 33-70 km, green: 70-300 km, blue: 300-700 km) (Spreading ridges are heavy lines, subduction zones are toothed lines, and transform faults are light lines.) In Figure 11.9, 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. In both cases, the earthquakes get deeper with distance from the trench. 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). The distribution and depths of earthquakes in the Caribbean and Central America area are shown in Figure 11.9. Figure 11.8 Distribution of earthquakes of M4 and greater in the area of the mid-Atlantic ridge near the equator from 1990 to 1996. 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. 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. Here, as we saw in Chapter 10, the segments of the mid-Atlantic ridge are offset by some long transform faults. Earthquakes at Divergent and Transform Boundariesįigure 11.8 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. 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. 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. Figure 11.7 General distribution of global earthquakes of magnitude 4 and greater from 2004 to 2011, colour coded by depth (red: 0-33 km, orange 33-70 km, green: 70-300 km, blue: 300-700 km) Įarthquakes are also relatively common at a few intraplate locations. Along subduction zones, as we saw in Chapter 10, earthquakes are very abundant, and they are increasingly deep on the landward side of the subduction zone. Shallow earthquakes are also common along transform faults, such as the San Andreas Fault. 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 at less than 30 km depth. It is relatively easy to see the relationships between earthquakes and the plate boundaries. The distribution of earthquakes across the globe is shown in Figure 11.7. ![]()
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