Faults and Earthquakes - Earthquake Engineering for Structural Design - page 205

6.4 INTRASLAB (INSLAB) EARTHQUAKES
Scientists now know that shallow earthquakes are caused by the slippage of giant
blocks of the Earth along faults, but the cause of deeper earthquakes remains a
mystery (Fitzpatrick, 1996).
Contrary to crustal earthquakes, around the world there is a large number of
earthquakes occurring at depth situated under the crust thickness. Figure 6.26
presents the location of large intraslab earthquakes. There are some continuous
zones where intraplate earthquakes occur: Mexican and South American coasts,
Western Pacific Ocean, and Eastern of Indian Ocean. At the same time, some
isolated intraslab earthquakes (as in Vrancea, Sicilian, Crete, Persian and
Himalayan zones) exist. The largest deep earthquake ever recorded had a
magnitude M 8.3 and occurred 600 km below Bolivia in 1994.
A slab is an oceanic crustal plate that subducts under a continental plate and it
is consumed by the Earth's mantle (Fig. 6.26), in a zone where the thermo-
dynamic, physical and chemical processes of Earth's interior are dominant. These
phenomena assure the planet's largest recycling system (Stern, 2002). Intraslab
earthquakes occur in the subduction slab in a zone situated under the contact
between the two plates and under the Earth's crust.
There are three categories of intraslab earthquakes (Seno and Yoshida, 2004,
Boudreau, 2001, Houston, 2004, Fox, 2007): shallow, produced in the slab in the
range of 30-70 km; intermediate, in the deeper range of 70 to 300 km; deep, when
deeper than 300 km (note that these ranges differ from one investigator to another).
The differences between these earthquake types consist in the influence of
temperature and in the rupture process. Figure 6.27 shows a variation of the
temperature with the depth of the slab and of the subducted plate. For the shallow
earthquakes, the temperature does not influence the rupture characteristics,
remaining a mechanical process, dominated by friction. In case of deep
earthquakes, high temperature and pressure have a great influence on the
characteristics of rupture. The causes of intermediate and deep earthquakes have
been a very controversial problem for the last decades, being an important and
“mysterious” class of earthquake. Understanding these earthquakes could be the
key for unlocking the remaining secrets of plate tectonics (Reid, 2002). Shallow
earthquakes represent the normal brittle failure of rocks. But the intraslab
earthquakes occur when a combination of extreme heat and pressure cause the
rocks to metamorphose and change into different forms, due to the various
chemical reactions, changing the plate into molten lava. As they change, the rocks
release water due to the dehydratation , which essentially lubricates the fault,
causing the fault to slip (Kirby et al, 1991, Tibi et al, 2002). This explanation of the
particular behavior of intraplate earthquakes is known as the Kirby's Theory