2. What types of earthquakes exist?

Earthquakes can only be differentiated with respect to their origin:

Tectonic earthquakes

Large faults within the Earth’s crust accommodate differential or shear motion produced by the inner forces (called tectonic forces) responsible for the plate movements. When the energy accumulation along one or more faults becomes too high, rocks break suddenly, releasing energy through a rapid motion (the earthquake). Such faults can be very large (many kilometers) and they are responsible for the adjustment of movements between the various plates composing the Earth’s crust.

Different types of Plate Boundaries. This image is in the public domain because it contains materials that originally came from the United States Geological Survey, an agency of the United States Department of Interior. For more information, see the official USGS copyright policy. Source: Wikipedia

Volcanic earthquakes

Earthquakes can also occur in active volcanic areas as they are continuously deforming: such deformation is mainly produced through large and little fractures. In general, volcanic earthquakes release low energy and have a shallow focal depth.

Most earthquakes associated with young volcanoes are related to volcanic processes and may indicate that a quiet volcano is becoming active. Although large eruptions are often preceded by several significant earthquakes and many small rock breaking quakes, there is also a continuous release of seismic energy associated with the underground movement of magma. Volcanic tremor is a seismic vibration caused by the pulsing of pressurized magma and gas. The seismogram is of longer duration and more continuous than rock-breaking earthquakes of the same amplitude. Earthquake swarms recorded by seismometers and the ground deformation monitored by tiltmeters, help scientists determine the depth and location of flowing magma beneath the volcano, which in turn, provides scientists with information to issue hazard warnings. Source: USGS - http://gallery.usgs.gov/videos/660

Anthropogenic earthquakes

Anthropogenic earthquakes occur where human activity has played some part in bringing the system to failure. They can be classified in:

a) Induced earthquakes, where human activities produce stress changes, which are sufficiently large as to produce an earthquake. The rocks may not necessarily have been in a stress-state that would have led to an earthquake in a reasonably near future (in a geological sense!).

b) Triggered earthquakes, where a small perturbation generated by anthropogenic activities has been sufficient to move the rocky system from an almost-critical state to an unstable state. The seism would have eventually occurred anyway although probably at some later time. That is, these activities have advanced the earthquake clock. In this case, the additional perturbation stress is often very small in comparison with the pre-existing stress system. In recent years there has been an increasing attention toward earthquakes related to some kinds of human activities, such as traditional drillings for oil exploitation, the hydraulic fracturing (known as “fraking”) procedure to extract oil from shales or the over-exploitation, through years, of deep underground water. There is still no clear evidence of correlation between high magnitude earthquakes and such human activities; nevertheless, it is known that any kind of surface and / or underground perturbation is able to produce seismic waves, although of small intensities.

Example of how "fracking" works. Source: http://savesomegreen.co.uk/definition-frack/