When thinking about the effects of an earthquake, imagine a pebble dropped into a pond. It hits the water creating a circular ripple that weakens the farther it moves from the center.
An earthquake behaves differently. 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. And these soil characteristics can vary wildly within small areas.
This is why 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. Site effects are based on two general characteristics:
- The softness of the soil or rock
- The total thickness of the sediment above the bedrock
The earthquake’s effects vary with the softness and thickness 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 National Earthquake Hazards Reduction Program (NEHRP) defined six different soil and rock types based on their shear-wave velocity, in order to determine amplification effects:
- Type A, hard rock (igneous rock)
- Type B, rock (volcanic rock)
- Type C, very dense soil and soft rock (sandstone)
- Type D, stiff soil (mud)
- Type E, soft soil (artificial fill)
- Type F, soils requiring site-specific evaluations.
The earlier in the alphabet, the harder the soil. Type A soil — igneous rock — is the hardest and results in the least wave amplification. Type E soil is the opposite — the softest soil with the most amplification. Type F is a catch-all category for soils which don’t match any other type.
Although soil type is a big 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 “hotspots” unique to each earthquake.
Do you know what type of soil your risks are standing on? Turn to WSRB's PropertyEDGE™ software, which provides you with soil type, maps, and liquefaction data (illustration below). To find out more about PropertyEDGE and how it can help you to assess risk, contact us.