Shallow earthquakes, in general, produce more intense surface shaking and frequently result in the highest loss of life and property damage. Shallow earthquakes are frequently connected with destructive edges, when the subducting plate drops at a modest inclination, causing tensions near the surface. These may trigger a shallow earthquake anywhere along the plate boundary.
The most destructive earthquakes on record have been caused by faults that reach the surface. These are known as great (Mj) or giant (Mw) earthquakes. Faults that reach the surface but do not cause much damage at ground level are called deep-seated. Deep-seated faults may cause greater damage than shallow ones because they can release large amounts of energy over a long distance.
Greater damage results from two factors: larger magnitudes and deeper locations. The 2004 Indian Ocean tsunami was triggered by an M9.1 earthquake off the coast of Indonesia's Sumatra island. It was one of the most powerful earthquakes ever recorded and resulted in about 250 deaths. The death toll from the Japanese earthquake and tsunami was estimated to be around 10,000 people.
Damage occurs because the energy released during an earthquake is converted into motion which causes parts of the earth's surface to rise and fall with respect to each other. At the surface, this movement creates waves which travel through the soil and rock below the surface until they reach an obstruction such as a valley or cliff.
Stresses from the collision of two plates induce deformation in the overriding plate and, as a result, shallow earthquakes. Shallow earthquakes can also occur on the subducting slab when a locked zone ruptures (orange line in Figure 12.20). These events are often too small to be felt by humans but may be detectable by seismic instruments.
Larger earthquakes occur where two plates converge at a diverging boundary, such as along an oceanic trench. Trench earthquakes typically reach depths of less than 10 km but can extend down to 3-4 km below sea level. They can have magnitudes greater than 8 on the Richter scale.
Collisional convergence between plates can produce large earthquakes as well. The San Andreas Fault is an example of a diverging boundary. Collisions between the Pacific Plate and the North American Plate along the San Andreas Fault generate large earthquakes from time to time. The largest recorded earthquake in California was the Loma Prieta Event of October 9, 1989. It had a magnitude of 6.9 on the Richter Scale. Many people died or were injured because they lived in low-lying areas when it struck.
Other examples of converging boundaries include the Java Sea-Sunda Shelf Boundary and the Mid-Atlantic Ridge. Large earthquakes occur near these boundaries as well.
According to the US Geological Survey, most earthquakes occur at shallow depths and do more damage than deeper earthquakes. However, larger deep earthquakes do happen from time to time. In fact, deep earthquakes account for about 15% of the total number of earthquakes but they can be more damaging because there is less surface disturbance which reduces the impact that these events have on people's lives.
The size of an earthquake depends on several factors such as the distance between the center of mass of the earth and ocean floors where the energy is released, the type of rock involved, and the location on the planet or in the ocean. Because seismic waves travel faster in a medium with a high density such as water or soil than in air, small earthquakes usually have smaller magnitudes than large ones. Seismic waves also are reflected and refracted when they encounter obstacles such as buildings or fault lines. The size of these effects depends on the height of the obstacle relative to the wavelength of the seismic waves. Deep earthquakes occur at depths greater than 50 miles and their magnitude decreases with depth. However, even at shallow depths deep earthquakes can cause significant damage because the energy is released over a large area.
Deep earthquakes can be more destructive than shallow ones because they often affect areas that are not directly under the site of rupture.
Deeper earthquakes are often less devastating since their energy dissipates before reaching the surface. The current earthquake in New Zealand is likely to have happened at a shallower depth than the one last year.
The more deeply buried an earthquake is, the less damage it will do. Deep earthquakes occur below the reach of most people; those deep enough to cause serious damage are so rare that they happen only once in several hundred years or more. Shallow earthquakes, by contrast, can be felt everywhere around them. They are much more common and affect everyone within their radius.
Because shallow earthquakes are frequent events, they cause a lot of damage and kill many people every year. Deep earthquakes, by contrast, are rare events that happen only once in a few centuries. When they do, they can be very destructive. But because they are far down under the earth's surface, they cannot harm anyone nearby.
Surface processes, including as erosion and sedimentation, have been proven by researchers to cause shallow earthquakes (less than five kilometers deep) and to facilitate the rupture of big deep earthquakes up to the surface. Erosion removes material from beneath a site's surface causing stress to be released on the surrounding area. This can lead to an earthquake if the removal of material causes parts of the Earth's surface to collapse.
In addition to removing material, erosion also changes the shape of the ground underneath it. If the depth that is removed during an erosive event is enough to change the direction of the underlying stress, this can trigger a seismic event. Erosion that produces horsts and grabens - large, flat-topped hills or valleys, respectively-can trigger small earthquakes along their boundaries. The motion at a place like this can be very small, but when repeated over many years it can add up to a significant level of strain in the rock below the surface.
Finally, erosion can cause rocks under pressure to suddenly release this pressure, which can then result in an earthquake. When a section of rock breaks away due to erosion, it leaves a void behind. If the gap is large enough, it may not exert much pressure on the remaining rock above it, so would not be noticed by people or animals living near the surface.