When you think about the effects of an earthquake, you might imagine a pebble dropped into a pond and the concentric circles of a ripple. You may think the earthquake's power weakens essentially evenly as you move away from the center.
However, earthquakes behave differently.
The importance of soil type
The earth’s land surfaces aren’t uniform like the water in a pond, and neither are the ripple effects after an earthquake. Some of the earth is covered in hard rock, some of it in dense soil, and some of it in mud or artificial fill. The distribution of these soil types can vary wildly within small areas.
As a result, two points the same distance from an earthquake’s epicenter can experience significantly different effects. One may suffer over ten times the impact of another due to geological variations known as site effects, which are based on two general characteristics:
- The softness of the soil or rock.
- The total thickness of the sediment above the bedrock.
How soil type affects earthquake damage
An earthquake’s effects vary with the softness of the sediment. As seismic waves travel through the ground, they move faster through hard rock than soft soil. When waves transition from hard to soft earth, they increase in amplitude (or size). A bigger wave causes stronger shaking.
The same principle also applies to sediment thickness. The deeper the sediment layer above bedrock, the more soft soil there is for the seismic waves to travel through. Soft soil means bigger waves and stronger amplification.
The earthquake damage to this building may have been influenced by the type of soil it's sitting on.
Soil type and site classifications
The National Earthquake Hazards Reduction Program (NEHRP) defined six different site classifications, based on the type of soil and rock in the area and their shear-wave velocity:1
A: hard rock (igneous rock).
B: rock (volcanic rock).
C: very dense soil and soft rock (sandstone).
D: stiff soil (mud).
E: soft soil (artificial fill).
F: soils requiring site-specific evaluations.
The earlier in the alphabet, the harder the soil. Site class A soil is the hardest and results in the least wave amplification. Site class E soil is the opposite — the softest soil with the most amplification. Site class F could contain a few types of soil, such as those vulnerable to potential failure during an earthquake, peat or some clays.
Although soil type is a significant predictor of an earthquake’s effects, it’s not the only factor. Other characteristics — like the fault’s orientation, irregularities in the rupturing fault surface, and dispersion of waves as they hit subsurface structures — can create spots of significant damage, and those hot spots are unique to each earthquake.
Find out more about different risk factors for property damage due to an earthquake in our blog post.
Joe Nolan is WSRB's Manager, Risk Data & Analytics. He has extensive experience providing data and technology solutions for the insurance sector as well as a strong analytics and Geographic Information Systems (GIS) background.
 NEHRP Recommended Provisions for Seismic Regulations for New Buildings and Other Structures, Federal Emergency Management Agency, https://www.fema.gov/media-library-data/20130726-1532-20490-4965/fema_450_1_provisions.pdf