What determines earthquake intensity?
How the intensity of an earthquake is measured is often confused by the different scales. The most familiar is the Richter Scale, but this measures the amplitude of the seismic waves and it's limited to a more local area.
The Moment Magnitude Scale was created to measure the energy released from an earthquake.
Earthquake intensity is commonly identified using the Mercalli Intensity Scale.
But, the same magnitude earthquake can cause significantly less or more damage based on various factors that affect the intensity.
Some factors affecting intensity
When a fault ruptures, there are two main types of seismic waves that are generated. These are body and surface waves. Body waves travel through the interior, or body, of the earth and are known as primary waves and secondary waves, or P waves and S waves. They are named this because the P waves travel faster and are detected first. P waves are compressional waves and seldom do much damage. They transmit through both solid and liquid materials.
The S waves move through the earth in a side-to-side motion and are slower than P waves. However, the S waves can cause more damage. S waves travel only through solid material. Some of the early warning systems being developed are trying to leverage the detection of P waves.
The surface waves travel through the outer layers of the crust and are detected after the P and S waves. The two types of surface waves are Raleigh waves, which move in a rolling or circular motion, similar to an ocean wave, and Love waves that move horizontally, with side-to-side motions.
How damaging seismic waves are--the earthquake intensity--can be are affected by several factors. Three of these are the depth of the fault rupture, the ground composition of the event area, and the shape, or geology, of the quake zone.
Incidentally, the frequency of seismic waves are usually less than 20 Hertz, which is lower than what most humans can detect. If a P wave manages to refract out of the surface and into the air, and has a higher frequency, it could be heard as a low rumble. However, most of the rumbling noise heard during an earthquake is the result of buildings and object moving.
Generally speaking, the deeper the hypocenter, the point on the fault below the epicenter where the earthquake originates from, the less the effects are felt on the surface. The magnitude of a deep earthquake may be large, but the seismic waves have further to travel before they reach the surface, so they lose energy and destructive power. Deep earthquakes are often felt over a larger area, but they are less damaging.
Shallow earthquakes have less distance for the seismic waves to travel, which results in greater energy to shake things on the surface and a stronger earthquake intensity. Shallow depths are from the surface to about 40 miles (60 km) below.
Intermediate depths are considered from about 40 to 186 miles (60 to 300 km) below the surface.
Most earthquakes happen at depths less than 50 miles (80 km) from the surface.
Ground Composition and Geology
The composition of the ground and geology of the land are closely interrelated with how they can affect seismic waves and the intensity of the earthquake. The shape, density, material, and other factors of the earth’s crust and the surface each affect how the waves travel, and how long they last.
The shape of the underlying geology can also have an amplifying or dissipating effect on seismic waves. For example, if a city is built over a geologic basin the basin can act like a bell ringing, where seismic waves are amplified and propagated resulting in increased shaking.
Other topography, like mountains or rivers can affect ground shaking. Softer sediments can amplify seismic waves, where hard rock will dampen the waves.
Rock and sediment layers, particularly along boundary layers where the differing materials have dissimilar elastic, compressional, and density properties, can reflect, refract, or even diffract and scatter seismic waves. These differing boundaries are often related to geology and topography of the area. Reflection is the abrupt change of the wave’s direction. Refraction is the change in the wave passes through different material, such as through loose soil versus hard rock. Diffraction occurs when the wave spreads around obstacles.
Here's a short article from phys.org about the differences between shallow and deep quakes