The location of almost all earthquakes in the Earth is related to the movement of large lithospheric plates. This is the theory of plate tectonics.

The mountain belt that runs along the long axis of Italy is called the Apennines

The Apennines are there because one plate is sliding beneath another along the axis of these mountains. In fact that is why there are mountains and uplift. The earthquake is due to the sliding of one plate beneath the other. The CRUSH ZONE is the mountain chain.

The earthquake in Italy on 8/24 was a 6.2 magnitude, normal faulting earthquake. Basically, it happened due to east-west extension in this area, which means that the region is being pulled apart in a east-west direction due to the plate tectonics there. While a 6.2 magnitude earthquake might not be considered very large by some standards, the amount of shaking (and resulting damage) has a lot to do with the geology of the area (i.e. what type of rocks the ground is made of) and what the buildings are made of.

The Mercalli Intensity scale is a scale used to measure the shaking from an earthquake, from 1 (no shaking felt, no potential damage) up to 10+ (extreme shaking, very heavy damage). The shaking nearest to the epicenter of the Italy earthquake was rated 8, or severe shaking and moderate/heavy damage. This, coupled with the fact that many of the buildings appear to be made out of stone bricks, would cause a lot of damage.

If you are trying to make a building that is earthquake proof, building it out of stone bricks is one of the worst materials, it does not hold up well when shaken and twisted. Brick walls will not bend or sway when met with seismic waves, instead they will just break and fall over. That is why there are so few brick buildings in southern California. This is likely the reason that there was so much damage from the 6.2M earthquake. The buildings were just not made to withstand an earthquake of that size.

As blocks of crust slowly move against each other, the stress that builds up is occasionally released very quickly along a fault to produce an earthquake. The earthquake in central Italy happened along a fault in the Apennines Mountains and had a hypocenter of 10 km (6.2 miles). That means that the location of the fault that ruptured was 10 km below Earth’s surface. The surface experienced stronger ground motion because the earthquake originated at a relatively shallow depth.

Buildings and roads that are older or less ‘earthquake proof’ generally receive more damage than newer buildings that are designed to withstand earthquakes. Many of the buildings damaged during the earthquake were built hundreds of years ago, when construction of buildings was very different than today.

The earth is covered with huge rocky plates that float on the hot stuff in the center of the earth. When two of these plates bump into each other, there's an earthquake; but we don't know when earthquakes will happen.

Most buildings aren't built strong enough to stand up in a big earthquake. When I was riding the train in Japan, I liked the beautiful blue tile roofs on the houses in the towns; but the blue tile roofs were heavy, and the houses fell down in a big earthquake.

People are learning how to build stronger buildings that can stand up in bigger earthquakes, but lots of buildings are old; and they were built before people knew how to build strong buildings.

The short answer is that, through the process of continental drift, Africa is ramming northward into Europe creating the Alps and closing up the Mediterranean, and the strain that this puts on the ground creates earthquakes at shallow depth.

This earthquake was not actually that powerful - the 1906 earthquake that devastated San Francisco for example was probably about a thousand times stronger. I'm not enough of an engineer to tell you what made this one so destructive, beyond the fact that sound waves (which are basically what earthquake waves are) can combine in ways that intensify or dampen them locally, which can increase or decrease damage.