7.3.3: Earthquake

Earthquakes 

Earthquakes are the Earth’s way of rapidly releasing built-up tectonic energy. When rock masses along a fault finally slip after being "stuck" by plate motions, they release a burst of energy through the lithosphere that we feel as ground shaking.

Notable Seismic Events

• 1960 Valdivia, Chile: The strongest ever recorded (M_w 9.5).

• 2004 Sumatra: A M_w 9.1 event that triggered a massive Indian Ocean tsunami.

• 2011 Tohoku, Japan: A M_w 9.0–9.1 event that led to a tsunami and a nuclear crisis.

Defining the Location

When an earthquake occurs, we define its location using two specific points:

• Focus (Hypocenter): This is the precise point beneath the Earth's surface where the rock actually ruptures and the energy is first released (shown in Figure \(\PageIndex{1}\)).

• Epicenter: This is the geographic point on the surface directly above the focus illustrated in Figure \(\PageIndex{1}\).

Figure \(\PageIndex{1}\): Normal Fault with Hypocenter and Epicenter. (Public Domain; via USGS).

Figure \(\PageIndex{1}\) illustrates the relationship between an earthquake, a fault plane, the hypocenter (focus), and the epicenter.

A sloping fracture through the rock is labeled the fault plane. This is the surface along which movement occurs during an earthquake.

Deep below Earth's surface, a star symbol located on the fault plane is labeled hypocenter (also called the focus). The hypocenter is the point within the Earth where the fault first ruptures and where seismic energy is initially released.

A dashed vertical line extends upward from the hypocenter to the ground surface. The point where this line meets the surface is labeled the epicenter. The epicenter is the location directly above the hypocenter on Earth's surface.

The diagram shows rock layers that have been displaced along the fault plane, illustrating movement on the fault. The ground surface includes a stream and vegetation, providing a reference for the location of the epicenter relative to the landscape.

The figure explains the key parts of an earthquake. Movement along a fault begins at the hypocenter, where stored stress is released. Seismic waves then radiate outward through the Earth. The epicenter marks the surface location directly above the hypocenter and is commonly used to identify the geographic location of an earthquake. The diagram helps distinguish between the subsurface origin of an earthquake (hypocenter) and its corresponding surface position (epicenter).

Measuring Magnitude

Magnitude is a numerical representation of the energy released during a quake, typically measured by seismometers shown in Figure \(\PageIndex{2}\).

Figure \(\PageIndex{2}\): Diagram of a Seismometer. (Public Domain; via USGS).

• The logarithmic scale: Magnitude scales are logarithmic. For every whole number increase on the scale, the amplitude of ground motion increases 10 times, while the actual energy released increases by approximately 32 times.

• The Richter Scale : Created in 1930, this is primarily used for smaller magnitude earthquakes.

• Moment Magnitude Scale (M_w): Used by the USGS for more precise calculations. It determines energy based on the seismic moment, which accounts for the area of the fault rupture, the rigidity of the rock, and the distance of the displacement.

Seismic Waves

The energy from an earthquake moves through the Earth as seismic waves. These waves are the primary tool scientists use to map the Earth's interior, as their speed changes based on the density and elasticity of the material they encounter.

Figure \(\PageIndex{3}\): P and S waves in the crust. (Public Domain; via USGS).

Figure \(\PageIndex{3}\) figure compares the two primary types of seismic waves generated during an earthquake: P-waves (primary waves) and S-waves (secondary waves).

Top diagram: P-Waves - The upper block is labeled P Waves. Arrows along the sides point alternately toward and away from each other, indicating that the material is being compressed and stretched in the direction of wave travel. The rock within the block shows zones that are squeezed together and zones that are spread apart.

The ground surface remains mostly level, but objects and materials experience a back-and-forth motion parallel to the direction the wave moves. P-waves are compressional waves that push and pull particles in the same direction that the wave travels. They are the fastest seismic waves and therefore the first to arrive at a seismic station. P-waves can travel through solids, liquids, and gases.

Bottom diagram: S-Waves - The lower block is labeled S Waves. The ground surface is shown with a wavelike shape, indicating up-and-down motion. The rock layers beneath are bent into a series of alternating ridges and troughs. Unlike the P-wave diagram, the movement of material is perpendicular to the direction of wave travel.

S-waves are shear waves that move particles at right angles to the direction the wave travels. This side-to-side or up-and-down motion causes stronger shaking than P-waves. S-waves travel more slowly than P-waves and arrive later. They can travel only through solids and cannot pass through liquids.

Body Waves (Internal) - These waves travel through the interior of the planet:

• Primary waves (P-waves): These are fast, compressional "push-pull" waves. They are unique because they can travel through solids, liquids and gases. In the crust, they move at about 6–7 km/s.
• Secondary waves (S-waves): These are slower (3.5 - 4 km/s) and can only move through solids. 

Surface Waves (External) - Waves like Love and Rayleigh waves travel along the Earth’s crust rather than through the interior. While they are the slowest (3–4 km/s), they are responsible for the most significant structural damage during an earthquake.

Geographic Patterns of Faults and Earthquakes

The fracturing and movement of the Earth's surface have been linked to stresses created by movements of molten rock within the asthenosphere. Convection currents within this subsurface region tug at crustal plates, driving them into or past one another, or pulling them apart. Thus, the patterns of earthquakes and faults are closely associated with the boundaries of plates.

Figure \(\PageIndex{4}\): New Madrid Seismic Zone. (CC BY-SA 4.0; Michael E Ritter via The Physical Environment).

Though faulting and earthquakes are common along plate boundaries, they also occur in zones of weakness within the interior of a plate. The New Madrid Seismic Zone shown in Figure \(\PageIndex{4}\), is the site of one of the largest earthquakes to strike North America (1811-1812). Though it had relatively little effect on human populations due to its location in a sparsely populated area at the time, it had a significant impact on the natural environment. The largest earthquakes caused the Mississippi River to flow backwards and low waterfalls were formed from the fracturing of the earth. 150,000 acres of forest ere destroyed and Reelfoot Lake, KY was formed. Now, several large population centers could be affected by the seismic zone.