Where Do Most Earthquakes Occur?

Most earthquakes in the world occur along the contacts between the large rigid plates that cover the earth; the rigid plates are called lithosphere. The lithosphere is made of the Earth's crust and the uppermost mantle which behaves like a rigid block. These plates are ~50-100 km thick for oceanic crust and ~150 km thick beneath continents albeit some studies have found continental lithosphere to be up to ~250km thick in places. These rigid plates "float" on the Earth's convecting mantle; the movement in the mantle "pushes" the plates around the Earth. In general continental plates are less dense (lighter) than oceanic plates. Because of this, oceanic plates are recycled into the Earth's mantle via a process called subduction while the cores of continental plates remain on Earth's surface (like pond scum). This is why the oldest oceanic plates are only about 200 million years old; the oldest oceanic plate is in the Mediterranean Sea where the Tethys ocean used to be is about 270-340 million years old. However, the continental plates have material that are billions of years old. Some of the oldest continental rocks on Earth can be found in North America and they are ~4 billion years old. This is very old considering the Earth was formed about 4.55 billion years ago!

There are different types of interaction between plates:

1. On the west coast of South and Central America, an oceanic plate and a continental plate collide. The oceanic plate is denser than the continental plate, so the oceanic plate is sinking into the Earth's mantle to be recycled into the Earth. This type of boundary is called a subduction zone. The largest earthquakes in the world occur in these areas with magnitudes have reached up to 9.5 Mw.
2. In the San Andreas fault the Pacific and North American plates move laterally next to each other like two cars driving past each other on a road. Earthquakes in these areas can be large, historically up to ~8.0 Mw such as the famous 1906 San Francisco earthquake which is estimated to have had a magnitude of ~7.8 Mw.
3. In the Himalayas, the two continental plates are colliding and forming the large mountain ranges in the region (including Mt Everest!). Earthquakes in these areas can also be large such as the 2015 Nepal earthquake that had a magnitude of 7.8 Mw.
4. In areas such as the mid-Atlantic ridge, where new oceanic plates are generated as two plates separate permitting new material from the uppermost mantle to rise to the surface as lava and cool into a new plate. Mid-ocean ridges are actually the largest volcanoes on Earth; some consider the mid-Atalantic ridge to be one ~70,000 km long volcano. The earthquakes that occur here are generally not very large or deep.