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Chapter 10: Problem 4

Explain how the vertical and horizontal components of earthquake motion are detected on a seismometer. (C)

Short Answer

Expert verified
Seismometers detect both vertical and horizontal components of earthquake motion using a mass, frame, and spring arrangement. For vertical ground motion, the seismometer is placed upright, and the spring stretches or compresses as the ground shakes. Horizontal ground motion detection involves orienting the seismometer horizontally, and setting up two perpendicular directions for east-west (EW) and north-south (NS) components. The spring's movement is converted into an electrical current, producing a seismogram. Analyzing these seismograms enables seismologists to understand earthquake characteristics, such as epicenters, depth, and intensity.

Step by step solution

01

Introduction to Seismometers

A seismometer is an instrument specifically designed to measure the ground motions caused by earthquakes. The basic structure of a seismometer consists of a reference mass, a frame, a spring, and a damper. It detects both vertical and horizontal components of earthquake motion, providing important information about the earthquake, such as magnitude and location.
02

Sensing Vertical Ground Motion

Vertical ground motion is detected by placing a seismometer in an upright position, allowing the reference mass and spring arrangement to respond to vertical shaking. In this configuration, when the ground shakes up and down, the frame moves with the ground, while the mass, due to its inertia, tries to remain stationary. This relative motion between the frame and mass causes the spring to stretch or compress, converting the vertical ground motion into a measurable force.
03

Sensing Horizontal Ground Motion

Horizontal ground motion can be detected using the same principle as vertical motion. The seismometer is simply oriented in a horizontal manner. A horizontal component seismometer can be set up in two perpendicular directions to detect both horizontal components, usually referred to as east-west (EW) and north-south (NS) components. Just like with vertical ground motion, the mass tries to resist motion due to its inertia, while the ground and attached frame move. This motion stretches or compresses the spring, and the force can be measured, indicating the horizontal ground motion.
04

Recording the Motion

In most modern seismometers, this stretching or compressing of the spring either moves a coil through a magnetic field or moves a magnet within a coil. This movement induces an electric current, which is then recorded and converted into a seismogram – the graphical representation of ground motion over time, showing the vertical and horizontal components of an earthquake.
05

Interpreting the Seismograms

By analyzing these seismograms, seismologists can better understand the characteristics of earthquakes, including their epicenters, depth, and intensity. Comparing the different components of ground motion can provide valuable insights into the type and size of the earthquake, as well as the underlying geology and tectonic setting. In conclusion, seismometers detect both the vertical and horizontal ground motion of earthquakes using a combination of a mass, frame, and spring. By orienting the instrument vertically or horizontally, and interpreting the resulting seismograms, seismologists are able to investigate the causes and characteristics of various earthquakes.

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Most popular questions from this chapter

How are long-term and short-term earthquake predictions made? What is the basis for determining a recurrence interval, and what does a recurrence interval mean? What is an earthquake early warning system? (H)

On the seismogram of an earthquake recorded at a seismic station in Paris, France, the S-wave arrives 6 minutes after the P-wave. On the seismogram recorded at a station in Mumbai, India, for the same earthquake, the difference between the P-wave and S-wave arrival times is 4 minutes. Which station is closer to the epicenter? From the information provided, can you pinpoint the location of the epicenter? Explain. (D)

Describe elastic-rebound theory and the concept of stickslip behavior. (A)

What is a tsunami, and why does it form? (G)

Explain the differences among the scales used to describe the size of an earthquake. (E)
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