Sources of Information, Earthquakes & Seismic Waves

3.1 The Earth We Cannot Visit

The Earth's radius is roughly 6,378 km. No human has reached the centre of our planet, drawn samples from the deep mantle, or photographed the core. Yet textbooks confidently describe the structure, temperature and even the chemistry of every layer down to the very middle. How do we know? This chapter answers that question. It tells the story of how geologists, by combining patient surface work with the remarkable behaviour of seismic waves, have built up a layer-by-layer X-ray of an unreachable interior.

3.2 Why the Interior Matters

The shape of the surface of the Earth — every mountain, plateau, ocean basin, volcano and earthquake zone — is largely a product of processes operating inside the planet. Two great families of processes work on the landscape constantly: exogenic processes (driven by the Sun and gravity acting at the surface, like rivers and weathering) and endogenic processes (driven by heat and pressure deep within the Earth, like volcanism and earthquakes). A complete understanding of any landscape needs both. To know why the Earth shakes during a quake or how a tsunami wave is generated, we must know how the planet is built layer by layer — from the outermost crust down to the central core.

3.3 Sources of Information About the Interior

Most of what we know about the interior is built up from estimates and inferences. Yet a part of the information also comes from direct observation and analysis. Geologists divide their evidence into two broad streams.

3.3.1 Direct Sources

The most easily available solid earth material is the surface rock — the rocks we walk on, and those exposed in cliffs, road cuttings and quarries. Rocks brought up from mining areas add to the picture. The deepest gold mines of South Africa reach depths of 3 to 4 km; below this, work becomes nearly impossible because of intense heat. To go deeper, scientists run drilling projects. Two major international efforts — the Deep Ocean Drilling Project and the Integrated Ocean Drilling Project — recover cores from below the seafloor. The deepest land borehole, the Kola superdeep borehole in the Arctic, has reached a depth of about 12 km. Together these wells supply enormous quantities of rock samples for laboratory work.

Volcanic eruptions are another direct source. When molten material — magma — is hurled onto the surface during an eruption, it cools into rock that geologists can analyse straight away. The catch is that we cannot easily tell the exact depth from which a particular magma rose, so volcanic samples answer some questions but raise others.

3.3.2 Indirect Sources

Most of the planet is far beyond drill or mine. To reach those depths geologists rely on indirect evidence — measurements made at the surface that, when interpreted carefully, betray the conditions inside.

The single most powerful indirect source is seismic activity — the propagation of earthquake waves. Because seismic waves change speed and direction as they cross materials of different density and physical state, every quake acts like a giant X-ray of the planet. The rest of this lesson follows that idea in detail.

3.4 The Earthquake — Anatomy of a Shake

An earthquake, in the simplest sense, is the shaking of the Earth. It is a natural event caused by the sudden release of energy that travels outwards in all directions as waves. The study of these waves — seismology — gives us our most complete picture of the layered interior.

3.4.1 Why Does the Earth Shake?

The release of energy occurs along a fault — a sharp break in the crustal rocks. Rocks on either side of a fault tend to move in opposite directions, but the friction between them, increased by the weight of the overlying strata, locks them together. Stress builds for years, decades or centuries. At some point, the tendency of the blocks to slip overcomes the friction; the rocks deform and then suddenly slide past one another. The pent-up energy is released and travels outwards as seismic waves.

The point underground where the rupture begins — where the energy is first released — is called the focus^?, also known as the hypocentre. The point on the surface directly above the focus is called the epicentre^?; it is the first surface point to feel the shaking.

3.5 Earthquake Waves

All natural earthquakes occur in the lithosphere^? — the rigid outer shell of the Earth. (You will study this in the next part of the chapter; for now, it is enough to know that the lithosphere extends roughly to a depth of 200 km from the surface.) An instrument called a seismograph records the waves as they reach the surface. Its trace shows three distinct sections, each representing a different family of waves. Earthquake waves are basically of two kinds — body waves and surface waves.

Body waves are generated at the focus and travel through the body of the Earth in all directions. When body waves reach the surface, they interact with surface rocks and generate a new set of waves called surface waves, which travel along the surface. The velocity of waves changes as they travel through materials of different density: the denser the material, the higher the velocity. Their direction also changes — through reflection (rebound) and refraction (bending) — whenever they cross from one material to another.

3.5.1 Body Waves: P-waves and S-waves

There are two kinds of body waves. They are called P-waves^? and S-waves^?.

• P-waves (Primary waves) move faster and are the first to arrive at the surface — hence the name "primary". They are similar to sound waves and can travel through gaseous, liquid and solid materials. P-waves vibrate parallel to the direction of propagation, exerting alternating push and pull on the material; they create density differences as they stretch and squeeze the rock.
• S-waves (Secondary waves) arrive after P-waves with some time lag. The crucial fact about S-waves is that they can travel only through solid materials. Their vibration is perpendicular to the direction of propagation in the vertical plane, creating troughs and crests in the rock through which they pass. This single property — "S-waves do not travel through liquids" — has helped scientists infer the existence of a liquid outer core.

3.5.2 Surface Waves

Surface waves are the last to register on a seismograph and are considered the most destructive. They cause displacement of rocks at the surface and the collapse of buildings. Three of the four wave types in a seismograph trace (S-waves and the two principal kinds of surface waves) vibrate perpendicular to the direction of propagation; only the P-wave vibrates parallel to it.

3.6 Emergence of the Shadow Zone

Earthquake waves are recorded by seismographs at far-off locations. However, there are some specific zones on the Earth where the waves are not reported. Such a zone is called the shadow zone^?. The study of many earthquakes shows that for each earthquake there is an entirely different shadow zone — its position depends on the location of the epicentre.

Detailed analysis revealed the following pattern.

• Seismographs located within 105° of the epicentre record the arrival of both P-waves and S-waves.
• Seismographs located beyond 145° from the epicentre record only the arrival of P-waves — they never receive S-waves.
• The zone between 105° and 145° from the epicentre is the shadow zone for both kinds of waves; neither P nor S arrives there.
• The entire region beyond 105° receives no S-waves.

The shadow zone of P-waves appears as a band around the Earth between 105° and 145° away from the epicentre. The shadow zone of S-waves is much larger — it covers a little over 40 per cent of the Earth's surface. From these two patterns geologists drew a remarkable inference: somewhere inside the Earth there must be a layer that blocks S-waves entirely. Since S-waves cannot pass through liquids, that layer must be liquid. This is precisely how the existence of a liquid outer core was first deduced — without ever seeing it.

3.7 Types of Earthquakes

Not every quake has the same trigger. Five recognised types appear in the literature.

3.7.1 Measuring Earthquakes — Magnitude vs Intensity

Earthquakes are scaled either by the magnitude of the shock or by its intensity.

• The Richter scale measures magnitude — that is, the energy released. It is expressed in numbers from 0–10.
• The Mercalli scale, named after the Italian seismologist Mercalli, measures intensity — that is, the visible damage. Its range is 1–12.

Quakes of magnitude 8 and above are quite rare — they occur roughly once in every 1–2 years — whereas "tiny" quakes occur almost every minute somewhere on the Earth.

3.8 Effects of Earthquakes

The earthquake is a natural hazard. Its immediate hazardous effects fall into about a dozen categories.

The first six categories shape landforms; the others endanger life and property. Tsunamis are waves generated by a tremor whose epicentre lies below oceanic waters and whose magnitude is sufficiently high (usually more than 5 on the Richter scale). A tsunami is therefore caused by an earthquake — not the earthquake itself. The actual quaking lasts only seconds, yet its consequences can be devastating for hours or days afterwards.