Under the ocean, the crust is about 5 kilometres thick, but under the continents, its mean thickness is about 35 kilometres The earth's crust is mainly composed of basalt and granite. Basalt is the main component of the earth's crust under the oceans, whereas granite is the main component of the continental crust. The point on the earth's surface directly above the focus of an earthquake is called the epicentre. Faults are fractures or breaks between rocks where movements occur. They are characterized geologically by the kinds of movements that occurred along them.
Earthquakes tend to reoccur along previous faults because they are weak zones.
The magnitude of an earthquake is the measure of the quantity of energy released by the earthquake. The magnitude is estimated from the amplitude of the seismic waves that are recorded by sensitive instruments called seismographs. The magnitude relates to the energy released at the focus. Earthquakes with magnitude of 2.0 or less are usually called micro-earthquakes and are normally too small to be felt. Earthquakes with a magnitude of about 5 or greater are strong enough to be recorded by seismographs all over the world and earthquakes of 6 or greater can do damage.