Earthquake Shaking Comes from Elastic Deformation
An earthquake is the shaking caused by what happens when rock ruptures (breaks), causing rocks on one side of a fault to move relative to the rocks on the other side. Although it is often thought that an earthquake is the vibrations from rocks sliding past each other and grinding against each other, it’s not that simple. In fact, you could even say that an earthquake happens after the rocks have done most of the sliding. Think about it this way- if rocks slide a few centimetres or even metres along a fault, should that motion alone cause the incredible damage that we see with some earthquakes? You might expect some motion, like the jolt if you were sitting in a car that suddenly accelerated then stopped, but would you expect buildings to vibrate back and forth until they shake themselves to pieces, train tracks to buckle and twist in s-shapes, and roads to roll up and down like waves on the ocean? What’s happening is that the rock isn’t just slipping. It’s also vibrating like a guitar string.
Even though we tend to think of rocks as being rigid, when stress is applied, rocks can be deformed in ways other than breaking. Up to a point, rocks stretch rather than breaking (Figure 12.2 a,b). If there hasn’t been too much stretching, the rock can snap back to its original shape when the force is removed. This kind of reversible deformation is called elastic deformation. In Figure 12.2c, the rock has ruptured. The rupture happens when the rock has been stressed beyond it’s ability to stretch. The rest of the rock on either side of the rupture has gone back to its original shape, and that is a result of elastic deformation. The snapping back of the rock returning to its original shape causes the rock to vibrate, and this is what causes the shaking during an earthquake.
The rupture that forms might be a new break, or it might be a place where the rock on either side of the fault is locked together because of friction and irregularities on the fault surface, and comes apart again.
Rupture Surfaces Are Where the Action Happens
Images like 12.2c are useful for illustrating elastic deformation and rupture, but they can be misleading. The rupture that happens doesn’t occur as in 12.2c, with the block being ruptured through and through. The rupture and displacement only happen along a subsection of a fault, called the rupture surface. In Figure 12.3, the rupture surface is the dark pink patch. It takes up only a part of the fault plane (lighter pink). The fault plane represents the surface where the fault exists, and where ruptures have happened in the past. Although the fault plane is drawn as being flat in Figure 12.3, faults are not actually perfectly flat.
Within the rupture surface, the amount of displacement varies. In Figure 12.3, the larger arrows indicate where there has been more displacement, and the smaller arrows where there has been less. Beyond the edge of the rupture surface there is no displacement at all. Notice that this particular rupture surface doesn’t even extend to the land surface of the diagram.
The size of a rupture surface and the amount of displacement along it will depend on a number of factors, including the type and strength of the rock, and the degree to which it was stressed beforehand. The magnitude of an earthquake will depend on the size of the rupture surface and the amount of displacement.
A rupture doesn’t happen all at once along a rupture surface. It starts at a single point and spreads rapidly from there. Figure 12.4 illustrates a case where rupturing starts at the heavy blue arrow in the middle, then continues through the lighter blue arrows. The rupture spreads to the left side (green arrows), then the right (yellow arrows).
Depending on the extent of the rupture surface, the propagation of failures (incremental ruptures contributing to making the final rupture surface) from the point of initiation is typically completed within seconds to several tens of seconds. The initiation point isn’t necessarily in the centre of the rupture surface; it may be close to one end, near the top, or near the bottom.
Foreshocks and Aftershocks Are Caused by Shifting Stress
Earthquakes don’t occur in isolation. There is usually a sequence in which smaller earthquakes occur prior to a larger one, and then progressively smaller earthquakes occur after. The largest earthquake in the series is the mainshock. The smaller ones that come before are foreshocks, and the smaller ones that come after are aftershocks. These descriptions are relative, so it can be necessary to re-label an earthquake. For example, we might label the strongest earthquake in the series as the main shock, but if another even bigger one comes after it, the bigger one is called the main shock, and the first one gets re-labelled as an aftershock.
In the previous section we saw that the rupture surface doesn’t fail all at once. A rupture in one place leads to another, which leads to another. Aftershocks and foreshocks represent the same thing, except on a much larger scale. In Figure 12.5, the rupture illustrated in Figures 12.3 and 12.4 reduced stress in one area, but in doing so, it transferred stress to another. Imagine a frayed rope breaking strand by strand. When a strand breaks, the tension on that strand is released, but the remaining strands must still hold up the same amount of weight. If another strand breaks under the increased burden, the remaining strands have an even greater burden than before. In the same way that the stress causes one strand after another to fail, one rupture causes subsequent ruptures nearby.
Figure 12.6 shows the distribution of immediate aftershocks associated with the 1989 Loma Prieta earthquake. Panel (b) is a section in the Earth along the San Andreas Fault. Each red dot is a specific aftershock that was recorded on a seismometer. The hexagon labelled “main earthquake” represents the mainshock. That stress released by the main shock was transferred to other nearby parts of the fault, and contributed to a cascade of smaller ruptures over an area approximately 60 km long and 15 km wide.
Stress transfer isn’t necessarily restricted to the fault along which an earthquake happened. It will affect the rocks in general around the site of the earthquake and may lead to increased stress on other faults in the region. The effects of stress transfer don’t necessarily show up right away- in the case of aftershocks, they can be delayed for hours, days, weeks, or even years. Segments of faults are typically in some state of stress, and the transfer of stress from another area is only rarely enough to push a fault segment beyond its limits to the point of rupture. The stress that is added by stress transfer accumulates along with the ongoing buildup of stress from plate motion and eventually leads to another earthquake.
Because stress transfer affects a region, not just a fault, and because there can be delays between the event that transferred stress and the one that was triggered by the transfer, it can sometimes be hard to be know whether one earthquake is actually associated with another, and whether it should be assigned as a foreshock or aftershock for a particular mainshock.
Episodic Tremor and Slip
Episodic tremor and slip (ETS) is periodic slow sliding along part of a subduction boundary. It does not produce recognizable earthquakes, but does produce seismic tremor (rapid seismic vibrations on a seismometer). It was first discovered on the Vancouver Island part of the Cascadia subduction zone by Geological Survey of Canada geologists Herb Dragert and Gary Rogers.
The boundary between the subducting Juan de Fuca plate and the North America plate can be divided into three segments (Figure 12.7). The cold upper part of the boundary is locked (locked zone). There the plates are stuck and don’t move, except with very large earthquakes that happen approximately every 500 years (the last one was M8.5+ in January 26, 1700). The warm lower part of the boundary (continuous slip zone) is sliding continuously because the warm rock is weaker. The central part of the boundary (ETS zone) isn’t cold enough to be stuck, but isn’t warm enough to slide continuously. Instead it slips episodically approximately every 14 months for about 2 weeks, moving a few centimetres each time.
You might be inclined to think that it’s a good thing that there is periodic slip on this part of the plate because it releases some of the tension and reduces the risk of a large earthquake. In fact, the opposite is likely the case. The movement along the ETS part of the plate boundary acts like a medium-sized earthquake and leads to stress transfer to the adjacent locked part of the plate. Approximately every 14 months, during the two-week ETS period, there is a transfer of stress to the shallow locked part of the Cascadia subduction zone, and therefore an increased chance of a large earthquake.
Since 2003, ETS processes have also been observed on subduction zones in Mexico and Japan.
- Rogers, G. and Dragert, H. (2003). Episodic tremor and slip on the Cascadia subduction zone: The chatter of silent slip. Science, 300, 1942-1943. ↵