Module 4—Effect of rock and soil on shaking

2min 44s Novice

Here, we examine the 1989 M6.9 Loma Prieta earthquake, that occurred south of San Francisco and its surrounding cities, known as the Bay Area. Shaking intensity was severe in Oakland and the Marina District in San Francisco, over 100 kilometers away. This intensity was higher in those two places than in most of the areas closer to the earthquake. But why?
 
The answer: local rock and soil conditions. Mud substrate under the Marina District and parts of Oakland caused earthquake waves to amplify by a factor of two. Seismic waves amplify when entering softer soils compared to hard bedrock.
 

Keypoints:

Learners will be able to:
 
  • Understand how local rock and soils can amplify earthquake waves.
  • Describe why areas farther away from an earthquake may experience stronger shaking.
  • Relate what soil types are prone to more intense shaking.