Earthquakes

Q: What are the types of earthquake waves? Discuss them

Q: World largest earthquake was assigned a magnitude of 9.5 by the United States Geological
Survey on 22 May 1960 in southern Chile Valdivia. What do you know about earthquake? Also
explain shallow-focus and deep focus earthquakes.?

Definition:
An earthquake is the sudden shaking or movement of the Earth's surface caused by the release of
energy from the Earth's crust. This energy radiates as seismic waves, often leading to ground
shaking, structural damage, and sometimes tsunamis.

Key Points:
 Focus (Hypocenter): The point within the Earth where the earthquake
originates.
 Epicenter: The point directly above the focus on the Earth's surface.
 Seismic Waves: Vibrations that travel through the Earth during an earthquake.
 Fault: A crack or fracture in the Earth's crust where movement has occurred.

Theories of Earthquakes
Natural Cause:

Elastic Rebound theory:

 Most earthquakes are produced by the rapid release of elastic

energy stored in rock that has been subjected to great forces.
 When the strength of the rock is exceeded, it suddenly
breaks, causing the vibrations of an earthquake.
 Rupture occurs and the rocks quickly rebound to an unreformed shape
 Energy is released in waves that radiate outward from the fault

Plate Tectonics Theory:

 Lithosphere is broken up into what is called plate tectonics.

 7 major plates.
 Occurs at plate boundaries due to the movement of Earth's tectonic plates.
 Types:

 Convergent Boundaries: Plates collide

(e.g., Himalayas, Japan).
 Divergent Boundaries: Plates move apart (e.g., Mid-Atlantic Ridge).
 Transform Boundaries: Plates slide past each other (e.g., San Andreas Fault).

 This movement causes earthquakes, but not

volcanic activity, as no new crust is created or
destroyed. These boundaries are also known as
strike-slip faults

Anthropogenic cause:

o Reservoir-Induced Seismicity: The weight of water in large dams increases

pressure on faults.
o Mining Activities: Extraction of minerals can weaken the crust.
o Underground Nuclear Tests: Can trigger minor tremors.
Seismic Waves:
Definition: “an elastic wave in the earth produced by an earthquake or other means.”

These waves are categorized into Body Waves and Surface Waves, each having distinct
characteristics.

1. Body Waves:

Body waves travel through the interior of the Earth and are faster
than surface waves. They are divided into two main types: Primary Waves
(P-waves) and Secondary Waves (S-waves).

Primary waves:

Primary Waves (P-waves)

• Nature: These are compressional waves that travel through solids, liquids, and gases.
• Motion: P-waves cause particles to move back and forth in the same direction as the
wave, similar to a slinky moving back and forth.
• Speed: P-waves are the fastest seismic waves, so they are the first to be detected by
seismographs. They typically travel at about 5-8 km/s through the Earth's crust.
• Propagation: They can travel through all layers of the Earth: the solid crust, the semi-
liquid mantle, and the liquid outer core.

. Secondary Waves (S-waves)

 • Nature: These are shear waves that can only travel through solids.
• Motion: S-waves move the ground perpendicular to the direction of the wave
propagation (side-to-side or up-and-down).
• Speed: S-waves are slower than P-waves, with speeds ranging from 3-5 km/s in the
Earth's crust.
• Propagation: Unlike P-waves, S-waves cannot travel through liquids. They are stopped
by the Earth's liquid outer core, providing evidence for the Earth's internal structure.
2. Surface Waves:

“Surface waves travel along the Earth's surface, and they are slower than body waves but
typically cause the most damage during an earthquake due to their larger amplitude and slower velocity.
Surface waves are divided into Love Waves and Rayleigh Waves.”

Love Waves

• Nature: Love waves are horizontal shear waves that move the ground side to side in a
horizontal direction.
• Motion: They cause particles to move in a horizontal, transverse motion, similar to a
snake's motion.
• Speed: Love waves are slower than P-waves and S-waves but faster than Rayleigh
waves.
• Propagation: Love waves move along the Earth's surface and cannot travel through
liquids or gases, which is why they are confined to the Earth's crust.

b. Rayleigh Waves

• Nature: Rayleigh waves are a combination of both longitudinal and transverse motions.
They are similar to ocean waves, causing both vertical and horizontal ground motion.
• Motion: The ground moves in an elliptical motion, both in the direction of wave
propagation and perpendicular to it, resembling the motion of water waves.
• Speed: Rayleigh waves are the slowest of all seismic waves, with speeds ranging from 2-
4 km/s.
• Impact: They are usually responsible for much of the ground shaking during an
earthquake and cause extensive damage because of their large amplitude and slower
speed.
Measurements:
Instruments:

Seismograph (or Seismometer):

 A device that detects and records seismic waves.

 Produces a seismogram, which shows the strength and duration of the earthquake.

2. Scales for Measuring Earthquakes:

A. Richter Scale

 Developed by Charles F. Richter in 1935.

 Measures magnitude (the amount of energy released).
 Logarithmic scale: each whole number increase represents 10 times more amplitude and
about 32 times more energy.
 Range: Usually from 1 to 10.
o Minor: Less than 3.0 (generally not felt).
o Light: 3.0–4.9 (often felt, but rarely causes damage).
o Moderate: 5.0–5.9 (can cause minor damage).
o Strong: 6.0–6.9 (may cause significant damage in populated areas).
o Major: 7.0–7.9 (serious damage over larger areas).
o Great: 8.0 or higher (catastrophic damage over vast regions).
 Example: A 6.0 magnitude earthquake releases 32 times more energy than a 5.0.
B. Moment Magnitude Scale (Mw)

 A modern and more accurate version of the Richter Scale.

 Measures the actual energy released using seismic data and fault size.
 Used by seismologists worldwide today.

C. Modified Mercalli Intensity Scale (MMI)

 Measures intensity, i.e., the effects of the earthquake on people, buildings, and land.
 Based on observations, not instruments.
 Scale ranges from I (not felt) to XII (total destruction).
 Subjective but helpful in estimating damage and human experience.

Shallow-Focus and Deep-Focus:

Earthquakes can be classified based on their depth, and they fall into different categories: shallow-focus,
intermediate-focus, and deep-focus earthquakes. The distinction between shallow-focus and deep-focus
earthquakes lies in the depth at which they occur beneath the Earth's surface.

Shallow-Focus Earthquakes:

• Definition: “Shallow-focus earthquakes are those that occur at a depth of less than 70
kilometers (about 43 miles) beneath the Earth's surface.”
• Characteristics: These earthquakes are the most common and tend to be the most
destructive due to their proximity to the Earth's surface.
• Cause: They occur when tectonic plates interact near the Earth's surface, typically at
plate boundaries where plates are colliding, pulling apart, or sliding past one another.
• Impact: Since these earthquakes are closer to the surface, the seismic waves they
generate travel less distance, resulting in more intense shaking and greater damage to
buildings and infrastructure. These quakes are often felt by a large number of people.
• Examples: Many significant earthquakes, such as the 2011 Tōhoku earthquake in Japan
(magnitude 9.0), occurred at shallow depths.

Deep-Focus Earthquakes:

• Definition: Deep-focus earthquakes occur at depths greater than 300 kilometers (about
186 miles) beneath the Earth's surface, often extending into the mantle.
• Characteristics: These earthquakes are less frequent but can be extremely powerful. The
seismic waves they generate can travel much farther than those from shallow-focus
earthquakes, meaning that they are sometimes felt over a much larger area.
• Cause: They are usually associated with the subduction zones where one tectonic plate is
forced beneath another (subduction). As the subducting plate descends deep into the
mantle, stress builds up and is released through earthquakes.
• Impact: While deep-focus earthquakes are powerful, they tend to be less damaging at the
surface than shallow-focus earthquakes because the seismic waves have to travel a much
greater distance. The shaking felt at the surface is often less intense.
 • Examples: The 1960 Valdivia earthquake (Chile), which is the largest earthquake ever
recorded, had a magnitude of 9.5 and was caused by a subduction zone event, with a deep
focus (around 590 kilometers deep).

Famous Examples of Earthquakes:

• 1960 Valdivia Earthquake (Chile): The largest earthquake ever recorded, with a
magnitude of 9.5. It occurred at a shallow depth and caused widespread destruction in
southern Chile.
• 2004 Indian Ocean Earthquake (Sumatra, Indonesia): A magnitude 9.1-9.3
earthquake at a shallow depth off the coast of Sumatra triggered a massive tsunami,
affecting countries around the Indian Ocean.
• 2011 Tōhoku Earthquake (Japan): A 9.0 magnitude earthquake that occurred at a
shallow depth near Japan's east coast. It caused a devastating tsunami and nuclear disaster
at Fukushima.

26 October 2015 Earthquake in Pakistan

• Magnitude: 7.5 on the Richter Scale.

• Epicenter: The epicenter was located in the Hindu Kush region of Afghanistan, near the
Badakhshan province.
• Locus (Focus): The earthquake had a depth of 212 kilometers (131.7 miles) below the
Earth's surface, making it a deep-focus earthquake.

Impact in Pakistan:
• Affected Regions : Tremors were felt across Pakistan, including major cities such as
Islamabad, Lahore, Karachi, Peshawar, and Quetta.
• Casualties and Damage:
 Over 300 people lost their lives in Pakistan, with Khyber Pakhtunkhwa (KP)
being the most affected province.
 Thousands of homes, schools, and buildings were damaged, particularly in the
northern areas.