How To Use Figure 4.11 To Sketch A Typical Seismogram Like A Pro In 3 Easy Steps

Ever stared at a textbook diagram and thought, “How on earth do I turn that into a real‑world seismogram?”
You’re not alone. Most students can point out the P‑wave, S‑wave and surface wave on a sketch, but when the professor says “use Figure 4.11 to draw your own seismogram,” a lot of heads go blank. The short version is that the figure is a roadmap, not a copy‑and‑paste. Grab a pencil, a ruler, and let’s walk through what that page is really trying to teach you.

What Is a Seismogram, Anyway?

A seismogram is simply a record of ground motion over time, captured by a seismometer. Think of it as the heartbeat of the Earth: each wiggle tells a story about where the quake started, how fast the waves traveled, and what kind of rocks they passed through.

When you look at a classic seismogram you’ll see a flat baseline, a sharp first spike (the P‑wave), a broader hump (the S‑wave), and then a long, noisy tail (the surface waves). Figure 4.11 in most introductory geophysics texts lays out exactly those pieces, labeling travel times, amplitudes, and the “arrival” points you need to copy That's the part that actually makes a difference. Practical, not theoretical..

The Parts You’ll Sketch

• Baseline – the zero‑line where nothing’s happening.
• P‑wave onset – the first, fastest arrival, usually a tiny, high‑frequency bump.
• S‑wave onset – a slower, larger‑amplitude bump that follows the P‑wave.
• Surface‑wave envelope – a long‑lasting, often sinusoidal tail that dominates the later part of the record.

That’s it. The rest of the figure is just context: distance‑time curves, velocity models, and a few sample amplitudes. All you really need to reproduce are the shape, relative timing, and rough height of each segment.

Why It Matters – Beyond the Classroom

Understanding how to sketch a seismogram isn’t just a quiz‑night trick. In practice, a quick hand‑drawn trace can help you:

1. Estimate earthquake depth – the time gap between P and S arrivals tells you how far the source is beneath the surface.
2. Identify wave types – if you can see the surface‑wave envelope, you know the event will cause strong shaking.
3. Communicate with non‑technical folks – a clean sketch is a universal visual when you’re explaining risk to a community board.

Missing any of those steps can lead to misreading the data, which in the field could mean under‑estimating a quake’s impact. That's why real‑world seismologists still start with a rough sketch before they feed data into a computer model. So mastering the art of “Figure 4.11 to seismogram” is actually a foundational skill.

How to Do It – Step‑by‑Step Guide

Below is the practical workflow you can follow with just a sheet of graph paper and a ruler. Grab a fresh page and let’s get those waves on paper.

1. Set Up Your Axes

• Horizontal axis = Time (seconds) – Figure 4.11 usually marks 0 s at the left edge and goes out to about 120 s.
• Vertical axis = Amplitude (mm or arbitrary units) – The scale isn’t absolute; just keep the relative heights consistent.

Draw a light baseline across the page. Use a ruler so your time intervals are even; most textbooks split the axis into 10‑second blocks.

2. Plot the P‑Wave Arrival

• Locate the P‑wave start – In Figure 4.11 it appears at roughly 12 s.
• Mark a small, sharp spike – The amplitude is low, maybe 1‑2 mm on your scale.
• Add a short tail – A quick decay back to baseline within 2‑3 seconds.

Why the tiny size? P‑waves are compressional; they move fast but don’t shake the ground as hard as the later waves.

3. Add the S‑Wave

• Find the S‑wave onset – Usually around 22 s in the figure, about 10 seconds after the P‑wave.
• Draw a broader hump – Height about 3‑4 mm, width ~10 s.
• Include a gentle decay – Let it taper off slowly, not as sharp as the P‑wave.

The gap between P and S (the “S‑P interval”) is the key number you’ll use later for distance calculations.

4. Sketch the Surface‑Wave Envelope

• Start the envelope – Around 35 s, the surface waves kick in.
• Make it sinusoidal – Think of a series of waves that gradually increase in amplitude, then slowly die out after 80‑90 seconds.
• Peak amplitude – Can be 5‑6 mm, the highest point on your whole trace.

Surface waves dominate the shaking felt on the surface, so they get the most visual emphasis.

5. Label Everything

Write “P”, “S”, and “Surface” near the respective sections. Add the time markers (e.g., “12 s” under the P‑wave) and a short note on amplitude if you like. A clean label set makes the sketch instantly readable.

6. Double‑Check Against Figure 4.11

• Timing – Does your P‑to‑S gap match the figure?
• Relative size – Is the surface wave taller than the S‑wave?
• Shape – Are the P‑wave spike and surface‑wave sinusoid recognizable?

If anything feels off, adjust the scale slightly; the goal is a faithful relative reproduction, not an exact copy.

Common Mistakes – What Most People Get Wrong

1. Over‑scaling the P‑wave – Newbies often make the first spike too tall, confusing it with the S‑wave. Remember: P‑waves are the quiet kids in the class.
2. Ignoring the S‑P interval – Skipping the 10‑second gap throws off any later distance estimate.
3. Flattening the surface wave – A flat tail looks like noise, not a surface‑wave envelope. Keep those sinusoidal wiggles.
4. Messy axes – If your time axis isn’t evenly spaced, the whole sketch loses credibility.
5. Copy‑pasting the figure – Trying to trace directly from the textbook can lead to a “photo‑realistic” look that defeats the purpose of a quick, conceptual sketch.

Avoid these pitfalls and your seismogram will look like something a seasoned field technician would sketch on a napkin during a drill.

Practical Tips – What Actually Works

• Use a light pencil for the baseline, then go darker for the waves. It helps you keep the zero line clean.
• Keep a ruler handy for the time intervals; a 1 cm = 5 s rule of thumb works on standard graph paper.
• Practice with different magnitudes – Try drawing a “small” quake (shorter surface wave) and a “large” one (big, long tail). The variation trains your eye.
• Add a tiny “noise” line after the surface wave to mimic real recordings. It shows you understand that real data isn’t perfectly smooth.
• Digitize later – If you need a clean image for a report, scan the sketch and trace it in a vector program. The hand‑drawn feel still shines through.

FAQ

Q: Do I need to know the exact velocity model to sketch the seismogram?
A: No. Figure 4.11 already incorporates the typical velocities (≈6 km/s for P, 3.5 km/s for S). Just follow the timing shown.

Q: How precise does the amplitude have to be?
A: Only relative size matters. As long as the surface wave is the tallest, the S‑wave is medium, and the P‑wave is the smallest, you’re good.

Q: Can I use a digital tool instead of paper?
A: Absolutely. Apps like Sketchpad or even PowerPoint work, but the manual method forces you to think about each component, which is great for learning.

Q: What if my textbook uses a different figure number?
A: Look for the same three‑wave pattern: a tiny early spike, a larger mid‑spike, and a long sinusoidal tail. The numbers may shift, but the shape stays consistent.

Q: How do I convert the S‑P interval into distance?
A: Use the simple formula Distance = (Δt × Vp × Vs)/(Vp – Vs), where Δt is the S‑P time, Vp and Vs are the P‑ and S‑wave velocities. It’s a quick back‑of‑the‑envelope calculation The details matter here..

That’s it. You’ve turned a textbook diagram into a usable, hand‑drawn seismogram you can show off in class—or on a field notebook. Which means 11,” you’ll know exactly where to start, what to avoid, and how to make the sketch actually mean something. Here's the thing — the next time someone says “use Figure 4. Happy drawing, and may your waves always arrive on time.