Use Figure 4.11 To Sketch A Typical Seismogram

Here's how to use a reference seismogram to understand and sketch your own, focusing on key wave arrivals and characteristics. This will help you interpret earthquake data.

Understanding Seismograms: A Guide to Sketching and Interpretation

Seismograms are the visual records produced by seismographs, instruments that detect and record ground motion. They're essential tools for seismologists, providing crucial data about earthquakes, underground explosions, and other seismic events. Learning to interpret a seismogram, even through sketching, allows you to understand the types of seismic waves, their arrival times, and ultimately, the location and magnitude of an earthquake. This guide will walk you through the process, using a typical seismogram as a reference point.

The Anatomy of a Seismogram: Decoding the Data

Before we dive into sketching, let's break down the components of a typical seismogram:

• Time Axis: The horizontal axis represents time. Usually, time is marked in seconds, minutes, or even hours, depending on the duration of the recording.

• Amplitude Axis: The vertical axis represents the amplitude of the ground motion. Amplitude is a measure of the displacement of the ground, and it's directly related to the energy released by the seismic event.

• Background Noise: Even when there's no earthquake, seismograms aren't perfectly flat. They show small, irregular wiggles representing background noise caused by things like traffic, wind, and even ocean waves.

• Wave Arrivals: The most important features are the distinct arrivals of different types of seismic waves. The main types are:

  • P-waves (Primary Waves): These are the fastest seismic waves and are the first to arrive at the seismograph. They are compressional waves, meaning they cause the ground to move back and forth in the same direction the wave is traveling. On a seismogram, P-waves appear as relatively small, high-frequency (closely spaced) oscillations.

  • S-waves (Secondary Waves): These waves are slower than P-waves and arrive second. They are shear waves, meaning they cause the ground to move perpendicular to the direction the wave is traveling. S-waves cannot travel through liquids, which is a critical piece of evidence supporting the existence of the Earth's liquid outer core. On a seismogram, S-waves typically have a larger amplitude than P-waves and a lower frequency.

  • Surface Waves: These waves travel along the Earth's surface and are the slowest, but often the most destructive. There are two main types:

    • Love Waves: These are shear waves that move the ground horizontally, perpendicular to the direction of propagation.
    • Rayleigh Waves: These waves cause the ground to move in a rolling, elliptical motion, similar to waves on the ocean.

Using a Reference Seismogram (Figure 4.11)

Imagine you have a reference seismogram, labeled as Figure 4.11 (since a real figure can't be provided here). This seismogram shows a clear sequence of wave arrivals. Let's assume it displays the following characteristics:

1. Initial Flat Line: The seismogram starts with a relatively flat line, representing the background noise before the earthquake waves arrive.
2. P-wave Arrival: A small, high-frequency wiggle marks the arrival of the P-wave. The amplitude is small compared to later arrivals. Note the time of this arrival carefully.
3. S-wave Arrival: After a period of relative quiet, a larger amplitude, lower frequency oscillation marks the arrival of the S-wave. The time difference between the P-wave and S-wave arrivals is crucial for determining the distance to the earthquake's epicenter.
4. Surface Wave Arrival: Finally, the largest amplitude waves arrive. These are the surface waves (Love and Rayleigh waves). They are characterized by long, sustained oscillations and can have a much larger amplitude than the P- and S-waves.

Step-by-Step Guide to Sketching a Typical Seismogram

Now, let's use this understanding to sketch our own seismogram.

Materials:

• Pencil
• Paper
• Ruler (optional, but helpful for drawing straight lines)

Steps:

1. Draw the Axes:

   • Draw a horizontal line to represent the time axis. Mark it with appropriate time intervals (e.g., seconds, minutes).
   • Draw a vertical line to represent the amplitude axis. You don't need precise amplitude units; just a relative scale will do.

2. Background Noise:

   • Before any major wave arrivals, draw a slightly wavy line close to the time axis. This represents the background noise. Keep the amplitude small and irregular.

3. P-wave Arrival:

   • At a certain point on the time axis (let's say 1 minute), draw a small, sharp wiggle. This is your P-wave arrival. The wiggle should be relatively high frequency (the oscillations are close together) and low amplitude. Label this point "P".

4. S-wave Arrival:

   • After a short time interval (e.g., 2-5 minutes after the P-wave), draw a larger amplitude, lower frequency oscillation. This is your S-wave arrival. The amplitude should be noticeably larger than the P-wave, and the oscillations should be spaced further apart. Label this point "S".

5. Surface Wave Arrival:

   • After another time interval (e.g., 5-10 minutes after the S-wave), draw a series of very large amplitude, low-frequency oscillations. This represents the surface waves. These waves should dominate the seismogram at this point. Label this section "Surface Waves".

6. Tail Off:

   • As time progresses, the amplitude of the surface waves will gradually decrease. Draw the oscillations slowly diminishing until they return to a level closer to the background noise.

7. Labeling:

   • Label the axes (Time, Amplitude).
   • Label the P-wave, S-wave, and Surface Wave arrivals clearly.
   • Consider adding a scale for time (e.g., 1 minute per division).

Tips for Sketching:

• Exaggerate the Amplitudes: In reality, the difference in amplitude between the P-wave and surface waves can be significant. Don't be afraid to exaggerate the amplitudes in your sketch to make the different wave arrivals more distinct.
• Focus on Relative Timing: The relative timing between the P-wave, S-wave, and surface wave arrivals is crucial. Use your reference seismogram (Figure 4.11) as a guide to estimate the appropriate time intervals.
• Practice Makes Perfect: The more you sketch seismograms, the better you'll become at recognizing the different wave arrivals and their characteristics.

The Science Behind the Waves: A Deeper Dive

Why do these different waves exist, and why do they arrive at different times? The answer lies in the physics of wave propagation and the structure of the Earth.

• Wave Speed and Density: The speed of seismic waves depends on the density and elasticity of the material they are traveling through. Denser and more rigid materials generally allow seismic waves to travel faster. This is why P-waves, which can travel through both solids and liquids, are faster than S-waves, which can only travel through solids.
• Wave Refraction and Reflection: As seismic waves travel through the Earth, they encounter boundaries between different layers (e.g., the crust, mantle, and core). At these boundaries, the waves can be refracted (bent) or reflected, changing their direction of travel. This is similar to how light bends when it passes from air into water.
• The P-S Interval and Distance: The time difference between the arrival of the P-wave and the S-wave (the P-S interval) is directly related to the distance from the seismograph to the earthquake's epicenter. The larger the P-S interval, the farther away the earthquake. This relationship is used in a process called triangulation, where data from multiple seismographs are used to pinpoint the exact location of the earthquake.
• S-wave Shadow Zone: One of the most important discoveries in seismology was the observation that S-waves do not travel through the Earth's outer core. This observation led to the conclusion that the outer core is liquid, as shear waves cannot propagate through liquids. The absence of S-waves in a certain region on the opposite side of the Earth from an earthquake is called the S-wave shadow zone.
• Surface Wave Attenuation: Surface waves are confined to the Earth's surface, so they lose energy more slowly than body waves (P-waves and S-waves) that travel through the Earth's interior. This is why surface waves often have the largest amplitudes on seismograms, especially at large distances from the earthquake.

Advanced Seismogram Interpretation: Beyond the Basics

While sketching a basic seismogram is a great starting point, real-world seismograms can be much more complex. Here are some additional factors to consider:

• Earthquake Magnitude: The magnitude of an earthquake is related to the amplitude of the seismic waves recorded on a seismogram. Larger earthquakes produce larger amplitude waves. The Richter scale and the moment magnitude scale are two common measures of earthquake magnitude.
• Focal Mechanism: The focal mechanism (also known as a fault-plane solution or "beach ball" diagram) describes the type of faulting that occurred during an earthquake. It is determined by analyzing the pattern of first motions (the direction of ground movement at the arrival of the P-wave) at multiple seismographs.
• Local Geology: The local geological conditions at a seismograph station can significantly affect the amplitude and duration of seismic waves. For example, seismographs located on soft sediments tend to record larger amplitude waves than those located on hard bedrock. This phenomenon is known as site amplification.
• Seismic Noise: Real-world seismograms are often contaminated by various sources of seismic noise, such as traffic, construction, and industrial activity. Sophisticated signal processing techniques are used to filter out this noise and isolate the earthquake signals.
• Types of Seismographs: There are different types of seismographs designed to measure different aspects of ground motion. Some seismographs measure vertical motion, while others measure horizontal motion. Some are designed to record high-frequency waves, while others are designed to record low-frequency waves.

Applications of Seismology: Beyond Earthquake Studies

Seismology is not just about studying earthquakes. It has a wide range of applications in other fields:

• Exploration Geophysics: Seismology is used to image the subsurface structure of the Earth for oil and gas exploration. Controlled explosions or vibrations are used to generate seismic waves, which are then recorded by seismographs. The data is used to create 3D images of underground rock formations.
• Nuclear Test Monitoring: Seismology is used to monitor underground nuclear explosions. The seismic waves generated by an explosion can be detected by seismographs around the world, allowing scientists to estimate the size and location of the explosion.
• Volcano Monitoring: Seismology is used to monitor volcanic activity. Changes in the pattern of earthquakes around a volcano can indicate that an eruption is imminent.
• Structural Health Monitoring: Seismology is used to monitor the health of bridges, buildings, and other structures. Small sensors can be used to detect vibrations and identify potential structural problems.
• Glaciology: Seismology is used to study the dynamics of glaciers and ice sheets. The seismic waves generated by icequakes (earthquakes in glaciers) can provide information about the internal structure and flow of ice.

Common Questions About Seismograms

• Why are there so many wiggles on a seismogram? Many of the wiggles are due to background noise, reflections, and refractions of seismic waves, and the complex layering of the Earth.
• Can I tell how big an earthquake was just by looking at a seismogram? You can get a rough idea, but accurate magnitude determination requires more sophisticated analysis and data from multiple seismographs.
• What is a seismograph made of? Modern seismographs use sophisticated electronics to detect and amplify ground motion. They typically consist of a mass suspended by springs or a pendulum, a sensor to detect the motion of the mass, and a recorder to store the data.
• Are seismographs only used to detect earthquakes? No, they can detect any ground motion, including explosions, volcanic eruptions, and even the movement of heavy machinery.
• How many seismographs are there in the world? There are thousands of seismographs around the world, forming a global network that monitors seismic activity.

Conclusion: Your Journey into Seismology Begins

Sketching a seismogram is a valuable exercise for understanding the basics of earthquake seismology. By learning to recognize the different wave arrivals and their characteristics, you can gain a deeper appreciation for the information contained within these wiggly lines. While real-world seismograms can be complex, the fundamental principles remain the same. So, grab a pencil and paper, and start sketching! You're on your way to becoming a seismogram sleuth.