Answers to Exploration Questions: Earthquakes

ANSWER: Geologists wanted to map the stiffness of the soil throughout the city to understand the damage patterns from the past earthquake. The type of soil affects the seismic vibrations and damage caused by an earthquake. Softer soils amplify the ground shaking, while stiffer soils can muffle it. They also wanted to help guide rebuilding efforts and prepare for the next earthquake. They use sledgehammers to help them measure shear wave velocity, which is related to soil stiffness. They hit the ground with sledgehammers to produce surface waves and then they used sensors to measure the speed of these surface waves—the shear wave velocity.

ANSWER: Geologists in the Pacific Northwest are studying layers of sediment along the banks of tidal creeks to study the geologic record of the area. This record goes back thousands of years and contains evidence that several earthquakes have occurred in that time. For example, the layer of sand between salt marsh peat and tide flat mud is an indication that a tsunami occurred.

3. Explain how each of the following tools or methods helps scientists reconstruct the geologic history of a fault:
• LANDSAT imagery and aerial photography
• Trenching
• Radiocarbon dating

ANSWER: LANDSAT imagery and aerial photography are used to locate faults that have been buried under sedimentary deposits. Trenching is the process of digging into the sediment to expose the fault. Radiocarbon dating is used to determine the age of peat- and charcoal-rich sediment layers to determine when earthquakes occurred.

All photos, © AMNH.