Slow-Moving Earthquakes

The scientific theory of plate tectonics (a word derived from the Greek word tectonicus which means ‘pertains to building’) describes the large-scale motion of the seven or eight major plates (the number depends on how the plates are defined) in the Earth’s lithosphere and the movement of the multitude of smaller plates. Plates typically move relative to each other anywhere from zero to 100mm each year. The motion between the plates at their boundaries determines the type of boundary. Boundaries can be convergent, divergent, or transforming.

A convergent plate boundary is one where the two plates are moving towards to each other, often resulting in one place sliding beneath the other (subduction). As the plates collides the boundary edge of one or both plates may fold up to form a mountain range. Alternatively, one of the plates may bend down to form a deep trench. Convergent boundaries destroy lithosphere. Chains of volcanoes are often formed along these boundaries and they are also the location of powerful earthquakes.

A divergent plate boundary is one where the plates are moving away from each other. This process occurs above rising convection currents. This happens above rising convection currents, which pushes up and lifts the lithosphere while flowing underneath it. The plate is then dragged in the same direction as the flow. Divergent boundaries create new lithosphere. Earthquakes commonly occur and magma rises to the Earth’s surface from the mantle along these boundaries.

A transform plate boundary is one where two plates side past each other. Transform boundaries do not create or destroy lithosphere. Fault lines are created along these boundaries, resulting in the common occurrence of earthquakes.

The movement and friction between these plates shape the landscape that we see and determine the intensity of the natural disasters that we experience. Earthquakes and volcanic activity occur along the plate boundaries, or fault lines.

Global positioning systems track the movement of earthquakes, providing researchers with a vast fount of information regarding earthquakes. However, slow-moving earthquakes have largely remained a mystery. The sliding of the plates against each other causes slow-moving earthquakes, or slow slip events, which occur over a period of many weeks. The movement is so slow that humans are not aware of the earthquake at all. These slow-moving events are also called slow slip events, or slow-moving earthquakes—sliding that occurs over weeks at a time. However, these slow-moving earthquakes are occurring all around the world, at all points in time.

Researchers from the department of geophysics in Stanford’s School of Earth, Energy & Environmental Sciences (Stanford Earth) hypothesized variation in friction explain how fast rock slips in the fault. With that in mind, they assumed slow slip events started as earthquakes, with a type of friction known as rate-weakening that makes sliding fundamentally unstable. But many laboratory friction experiments contradicted that idea. Instead, they had found that rocks from slow slip regions display a more stable kind of friction known as rate-strengthening, widely thought to produce stable sliding. The new computer simulations resolved this inconsistency by showing how slow slip can arise with contrary-seeming rate-strengthening friction. Rocks are made of a porous material, which means they are solid structures filled with pores or voids. Faults occur in rocks that are saturated with fluid. Consequently, the rocks are poroelastic in nature, which means that the pores naturally found in the rock give the rock the ability to expand and contract, changing the fluid pressure within the pores.

The research helps to explain the movement of the earthquakes. Adjusting the simulations to account for the porous nature of rocks provided the researchers with the understanding that as rocks are squeezed the fluid found in the pores cannot escape and therefore the pressure increases. As the pressure increases, friction decreases, causing a slow-moving earthquake.

Further information: Elías R. Heimisson et al. Poroelastic effects destabilize mildly rate-strengthening friction to generate stable slow slip pulses, Journal of the Mechanics and Physics of Solids (2019). DOI: 10.1016/j.jmps.2019.06.007