4.1 Earthquake mechanisms and seismic waves

Earthquakes are Earth's way of releasing built-up energy. They happen when rocks suddenly break along faults, sending out seismic waves that shake the ground. Understanding how earthquakes work is central to predicting and preparing for them.

Seismic waves come in different types, each behaving uniquely as they travel through Earth. By studying these waves, scientists can determine an earthquake's size, location, and potential impact.

Plate Tectonics and Fault Types

Plate Tectonic Theory

Earth's outer layer, the lithosphere, is broken into several rigid plates that float on the asthenosphere, a hotter, more ductile layer beneath. These plates are constantly moving relative to one another, and the boundaries where they interact are where most geologic action happens.

Plate boundaries fall into three categories:

• Divergent boundaries: plates move apart from each other (e.g., the Mid-Atlantic Ridge)
• Convergent boundaries: plates push toward each other (e.g., the Cascadia Subduction Zone)
• Transform boundaries: plates slide horizontally past each other (e.g., the San Andreas Fault)

The driving force behind plate motion is mantle convection: hot material rises from deep in the mantle, spreads laterally, cools, and sinks back down. This circulation drags the plates along. Earthquakes, volcanic eruptions, mountain building, and ocean trench formation all concentrate along plate boundaries as a direct result of these interactions.

Fault Types and Characteristics

A fault is a fracture in rock where blocks on either side have moved relative to each other. The type of fault depends on the stress acting on the rock.

• Normal faults form under tensional (pulling-apart) stress. The hanging wall drops down relative to the footwall. You'll find these in extensional settings like the Basin and Range Province in the western U.S.
• Reverse faults form under compressional (squeezing) stress. The hanging wall is pushed up relative to the footwall. The Himalayan Mountains are a classic example of reverse faulting at a convergent boundary. A thrust fault is a special case of a reverse fault with a shallow dip angle.
• Strike-slip faults form under shear stress. Two blocks slide past each other laterally, with no significant vertical movement. The San Andreas Fault is the most well-known example.

To describe a fault's geometry, geologists use three measurements: strike (the compass orientation of the fault line on the surface), dip (the angle of the fault plane relative to horizontal), and slip (the distance and direction the blocks have moved).

Earthquake Locations

Two terms you need to keep straight:

• The hypocenter (also called the focus) is the point inside Earth where the fault rupture begins and seismic waves first originate.
• The epicenter is the point on Earth's surface directly above the hypocenter.

Seismologists determine earthquake locations by comparing the arrival times of seismic waves at multiple stations and triangulating the source. You need data from at least three stations to pinpoint an epicenter.

Hypocenter depth tells you a lot about what's going on tectonically. Shallow earthquakes (less than ~70 km deep) typically occur along all types of plate boundaries. Intermediate and deep earthquakes (down to ~700 km) are almost exclusively associated with subduction zones, where one plate dives beneath another.

Seismic Waves

Types of Seismic Waves

Seismic waves are energy waves generated by earthquakes (or explosions, volcanic eruptions, etc.) that travel through and along Earth. They divide into two broad categories:

• Body waves travel through Earth's interior.
• Surface waves travel along Earth's surface.

Body waves include P-waves and S-waves. Surface waves include Rayleigh waves and Love waves.

Body Waves: P-Waves and S-Waves

P-waves (primary waves) are the fastest seismic waves and the first to arrive at a seismic station.

• They are compressional: particles oscillate parallel to the direction the wave travels, like a slinky being pushed and pulled.
• P-waves can travel through solids, liquids, and gases, which is why they pass through every layer of Earth's interior.

S-waves (secondary waves) are slower and arrive after P-waves.

• They are shear waves: particles oscillate perpendicular to the direction the wave travels, like shaking a rope side to side.
• S-waves can only travel through solids. This is a critical detail: S-waves cannot pass through Earth's outer core, which told scientists the outer core must be liquid. That S-wave "shadow zone" on the far side of Earth was one of the key pieces of evidence for a liquid outer core.

Surface Waves

Surface waves travel along Earth's surface and are typically the most destructive because they carry large amplitudes and their energy stays concentrated near the surface where structures are built.

• Rayleigh waves cause particles to move in an elliptical, rolling motion (both vertical and horizontal components). Think of ocean waves rolling across the ground.
• Love waves cause particles to move side-to-side, perpendicular to the wave's travel direction, with no vertical motion.

Compared to body waves, surface waves have longer wavelengths and lower frequencies. Their velocity depends heavily on near-surface material properties: loose sediments slow them down and can amplify shaking, while solid bedrock transmits them more quickly with less amplification.

Measuring Earthquakes

Earthquake Magnitude Scales

The Richter Scale was one of the first widely used magnitude scales. It's logarithmic, meaning each whole-number increase represents a tenfold increase in wave amplitude and roughly a 32-fold increase in energy released. So a magnitude 7 earthquake releases about 32 times more energy than a magnitude 6, and about 1,000 times more than a magnitude 5.

The Richter scale has a significant limitation: it saturates for very large earthquakes (roughly above magnitude 7), meaning it underestimates their true size. It also doesn't account for the frequency content of seismic waves.

The Moment Magnitude Scale ($M_w$) has replaced the Richter scale as the standard used by seismologists. It's based on the seismic moment, which accounts for three physical factors:

1. The area of the fault that ruptured
2. The average slip (how far the two sides moved)
3. The rigidity (stiffness) of the rock

Because it's tied to the actual physics of the rupture, $M_w$ doesn't saturate and gives reliable measurements across all earthquake sizes. For small to moderate earthquakes, Richter and moment magnitude values are often similar, but they diverge for large events.

Seismographs and Seismograms

A seismograph is the instrument that detects and records ground motion. The basic design relies on inertia: a heavy mass is suspended (often by a spring) so that when the ground shakes, the mass stays relatively still while the frame moves around it. That relative motion is recorded.

The output is a seismogram, a visual record of ground motion over time. On a seismogram, you can identify:

• The arrival of P-waves (first, smaller wiggles)
• The arrival of S-waves (larger wiggles arriving later)
• The arrival of surface waves (the largest, longest-period oscillations, arriving last)

The time gap between P-wave and S-wave arrivals increases with distance from the epicenter. Seismologists use this S-P time interval from multiple stations to triangulate the earthquake's location and calculate its magnitude.

Earthquake-Induced Liquefaction

Liquefaction occurs when water-saturated, loosely packed sediments (typically sand or silt) temporarily lose their strength during strong shaking and behave like a liquid. Under normal conditions, the grains in these sediments are in contact with each other and support the weight of whatever sits on top. During an earthquake, intense shaking increases the water pressure between grains, pushing them apart and eliminating that grain-to-grain contact. The ground essentially turns to quicksand.

The 1964 Niigata, Japan earthquake is a classic example: entire apartment buildings tilted and sank into liquefied ground while remaining largely intact structurally.

Several factors control how likely liquefaction is at a given site:

• Sediment type: sand and silt are most susceptible; clay and gravel are more resistant
• Water saturation: the sediment must be saturated (water table near the surface)
• Shaking intensity and duration: stronger, longer shaking increases liquefaction risk

Mitigation strategies include ground improvement (compacting loose sediments, installing drainage to lower the water table) and foundation design (using deep piles that reach down to stable layers, or reinforced mat foundations that spread loads).