Deep earthquakes were discovered in the 1920s, but they remain a subject of contention today. The reason is simple: they aren't supposed to happen. Yet they account for more than 20 percent of all earthquakes.
Shallow earthquakes require solid rocks to occurmore specifically, cold, brittle rocks. Only these can store up elastic strain along a geologic fault, held in check by friction, until the strain lets loose in a violent rupture.
The Earth gets hotter by about 1 degree C with each 100 meters of depth on average. Combine that with high pressure underground and it's clear that by about 50 kilometers down, on average the rocks should be too hot and squeezed too tight to crack and grind the way they do at the surface. Thus deep-focus quakes, those below 70 km, demand an explanation.
Slabs and Deep Earthquakes
Subduction gives us a way around this. As the lithospheric plates making up Earth's outer shell interact, some are plunged downward into the underlying mantle. As they exit the plate-tectonic game they get a new name: slabs. At first the slabs, rubbing against the overlying plate and bending under the stress, produce shallow-type subduction earthquakes. These are well explained. But as a slab goes deeper than 70 km, the shocks continue. Several factors are thought to help:
- The mantle is not homogeneous but rather is full of variety. Some parts remain brittle or cold for very long times. The cold slab can find something solid to push against, producing shallow-type quakes, quite a bit deeper than the averages suggest. Moreover, the bent slab may also unbend, repeating the deformation it felt earlier but in the opposite sense.
- Minerals in the slab begin to change under pressure. Metamorphosed basalt and gabbro in the slab changes to the blueschist mineral suite, which in turn changes into garnet-rich eclogite around 50 km depth. Water is released at each step in the process while the rocks become more compact and grow more brittle. This dehydration embrittlement strongly affects the stresses underground.
- Under growing pressure, serpentine minerals in the slab decompose into the minerals olivine and enstatite plus water. This is the reverse of the serpentine formation that happened when the plate was young. It is thought to be complete around 160 km depth.
- Water can trigger localized melting in the slab. Melted rocks, like nearly all liquids, take up more space than solids, thus melting can break fractures even at great depths.
- Over a wide depth range averaging 410 km, olivine begins to change to a different crystal form identical to that of the mineral spinel. This is what mineralogists call a phase change rather than a chemical change; only the volume of the mineral is affected. Olivine-spinel changes again to a perovskite form at around 650 km. (These two depths mark the mantle's transition zone.)
- Other notable phase changes include enstatite-to-ilmenite and garnet-to-perovskite at depths below 500 km.
Thus there are plenty of candidates for the energy behind deep earthquakes at all depths between 70 and 700 kmperhaps too many. And the roles of temperature and water are important at all depths as well, though not precisely known. As scientists say, the problem is still poorly constrained.
Deep Earthquake Details
There are a few more significant clues about deep-focus events. One is that the ruptures proceed very slowly, less than half the speed of shallow ruptures, and they seem to consist of patches or closely spaced subevents. Another is that they have few aftershocks, only one-tenth as many as shallow quakes do. And they relieve more stress; that is, the stress drop is generally much larger for deep than shallow events.
Until recently the consensus candidate for the energy of very deep quakes was the phase change from olivine to olivine-spinel, or transformational faulting. The idea was that little lenses of olivine-spinel would form, gradually expand and eventually connect in a sheet. Olivine-spinel is softer than olivine, therefore the stress would find an avenue of sudden release along those sheets. Layers of melted rock might form to lubricate the action, similar to superfaults in the lithosphere, the shock might trigger more transformational faulting, and the quake would slowly grow.
Then the great Bolivia deep earthquake of 9 June 1994 occurred, a magnitude 8.3 event at a depth of 636 km. Many workers thought that to be too much energy for the transformational faulting model to account for. Other tests have failed to confirm the model. But not all agree. Since then, deep-earthquake specialists have been trying new ideas, refining old ones, and having a ball.