Earth Observatory Blog
Slipping Towards a Better Understanding of How Earthquakes are Generated
Earthquakes continue to cause tremendous damage and casualties around the world. Contrary to other geophysical hazards, such as storms and floods, seismic hazards still elude short-term prediction. This is due, on the one hand, to our limited understanding of how rocks deform and break; and on the other hand, by the difficulty of probing Earth's interior to determine the physical parameters of a given fault.
To improve our understanding of how earthquakes are generated, a useful approach is to confront our hypotheses with a combination of laboratory experiments, field observations, and theoretical predictions.
The monitoring of plate boundaries with seismometers and GPS instruments, together with the development of increasingly sophisticated laboratory experiments, have revealed that Earth's faults can break in a variety of styles. A convenient way to classify these different ruptures is by how fast they move.
On the fast end of the spectrum, we find typical earthquakes. During earthquake ruptures, the particles close to the fault move at about 4 km/h (2.5 mph) for a few seconds or sometimes a few minutes, depending on the earthquake’s magnitude. This rapid movement radiates seismic waves that eventually cause the destruction we see on the Earth’s surface.
Fault motion is caused by the relative movement of tectonic plates, usually of the order of 10 mm/yr. This doesn't seem like much. But 1 mm/yr is also 1 km/Myr, so this motion can have dramatic consequences over geological time scales; like the formation of mountains, the opening of seas, and the breaking apart of continents.
What we have discovered in the last decade is that faults can break in isolated events with speed ranging anywhere from a tectonic rate (~mm/yr) to an earthquake rate (~km/h). Slow-slip events therefore are like earthquakes, only with much smaller slip rates and much longer durations, sometimes lasting several months!
Understanding the spectrum of fault slip is important because not all rupture styles create the same seismic hazards. Only insignificant seismic waves emanate from slow-slip events and these are not directly associated with shaking. Also, they reduce the local tension on the fault, thereby reducing the overall occurrence of local earthquakes. However, the regions prone to slow-slip events are usually found next to more seismogenic parts of a fault so it is possible that slow-slip events may in fact trigger large earthquakes at different fault locations. There is also the possibility – although yet to be verified – that slow-slip events may herald an impending earthquake. If this hypothesis is confirmed, we could monitor slow-slip to help predict the occurrence of strong and destructive earthquakes. We would also like to know if earthquake ruptures could propagate in regions of slow-slip, and increase the earthquake magnitude in the process.
Slow-slip events are found on virtually all subduction zones around the Rim of Fire, implying that subduction processes, in particular regional metamorphism, create favourable conditions for this phenomenon. This paper, published in Scientific Reports on 18 April 2018, proposes a hypothesis connecting the emergence of slow-slip events to their geological context. It suggests that serpentinisation, a type of mantle rock metamorphism process involving water and high pressure, creates ideal conditions for slow slip to occur.
At low pressure conditions of the continental crust, serpentinite is usually stable. So if serpentinite is found on the San Andreas Fault, for example, it would make the plates creep (to slip steadily at rates close to the relative plate motion). However, the deformation of serpentinite is unstable at higher pressures and higher temperatures near the crust-mantle boundary (the Mohorovicz discontinuity, or simply, the Moho). Simultaneously, serpentinite will start to deform much like a highly viscous fluid near Moho depth.
As slow-slip events are often found near the continental Moho in subduction zones, it is possible that slow slip is caused by the semi-brittle behaviour of serpentinite. The authors of the paper propose a physical model that reconciles recent laboratory results for serpentinite with field observations of slow-slip events that constrain their durations, slip speeds, and locations. The model shows that long-term slow-slip events (the ones that are very slow and undetectable with seismic instruments) are likely promoted by semi-brittle serpentinite in the mantle wedge corner. As the degree of serpentinisation reduces with depth, the sluggish, viscous response of rocks disappears, and ruptures can proceed at faster speeds (in short-term slow-slip events, for example).
Earthquake prediction remains a long-term aspiration, but as we refine our understanding of rock mechanics and improve our means of making quantitative predictions about how faults break, we strive to produce more realistic scenarios of future seismicity.