Earthquakes

A set of practice flashcards covering core concepts from Chapter 8 on Earthquakes, including definitions, causes, waves, scales, belts, effects, and notable events.

What is an earthquake?

A tremor below the Earth's surface that causes shaking of the crust due to stresses that build up and release suddenly.

What are the four main causes of earthquakes listed in the notes?

Volcanoes, Plate Tectonics, Folding and Faulting, and Man-made causes.

How are volcanic eruptions connected to earthquakes?

Volcanic eruptions push gases and magma that can move the crust, triggering earthquakes; volcanic activity and seismic events are interrelated.

What are tectonic (tectonic) earthquakes and where are they most frequent?

Earthquakes caused by movement of tectonic plates; most frequent along plate boundaries, especially around the circum-Pacific belt (Ring of Fire).

What are folding and faulting in the context of earthquakes?

Internal horizontal and vertical movements due to compressional and tensional forces that cause rock displacement and earthquakes.

Give examples of human-made causes of earthquakes mentioned in the notes.

Construction of large dams, nuclear explosions, blasting of rocks, mining, near fault zones.

Where do most earthquakes originate geographically?

At plate boundaries; the Circum-Pacific Belt accounts for about 70% of earthquakes; others occur along the Mid-world Mountain Belt and the Mid-Atlantic Ridge.

What are the three main types of seismic waves?

P-waves (Primary), S-waves (Secondary), and L-waves (Surface) — Love and Rayleigh waves are types of L-waves.

Describe P-waves.

Primary (push) waves; compressional; particles vibrate in the direction of travel; can move through solids, liquids, and gases; arrive first on seismograms.

Describe S-waves.

Secondary (shear) waves; distortional; particles vibrate perpendicular to the wave direction; travel only through solids and arrive after P-waves.

Describe L-waves.

Surface (long) waves; travel along the Earth's surface and are usually the last to arrive; include Love and Rayleigh waves.

What is the seismic focus?

The origin point within the Earth from which seismic waves emanate.

What is the epicenter?

The point on the Earth's surface directly above the seismic focus.

What is the typical depth range for most seismic focus points?

Most earthquakes have a seismic focus between about 8 and 35 kilometers below the surface.

What is a seismograph and what does it measure?

An instrument that records ground motion with a pen on moving paper; it measures arrival times of P- and S-waves to determine distance to the epicenter.

What does the Richter scale measure?

Magnitude of an earthquake; scale typically ranges from 1 to 9 and represents energy release with intensity increasing by factors of 10 per unit.

What does the Mercalli scale measure?

Earthquake intensity based on observed effects, on a 12-point scale.

What is the moment magnitude (Mw) scale?

A scale that measures the total energy released, especially for large earthquakes; based on the same principles as the Richter scale and is considered more accurate for big events.

Name the main earthquake belts and their approximate share of global earthquakes.

The Circum-Pacific Mountain Belt (~70%), The Mid-world Mountain Belt (Europe-Asia), The Mid-Atlantic Ridge (along Atlantic ridges and nearby islands).

What is a tsunami and where does the term come from?

A large ocean wave generated by a disturbance that rapidly displaces mass of water, usually an undersea earthquake; the term comes from Japanese, meaning harbor (tsu) and wave (nam).

What typically generates a tsunami?

Disturbances such as undersea earthquakes, volcanic eruptions, or landslides that rapidly displace large volumes of water.

What was the 2004 Indian Ocean earthquake and its impact?

A massive undersea earthquake near Sumatra that triggered devastating tsunamis, killing about 275,000 people.

What is an aftershock and provide an example from the notes?

A smaller earthquake following the main event; e.g., the Nepal 2015 sequence included a major aftershock on May 12, magnitude 7.3.

What is an example of a human-caused earthquake mentioned in the notes?

The Koyna dam in Maharashtra, where dam construction near fault zones is linked to seismic activity.

Why can a lower-magnitude earthquake cause more damage than a higher-magnitude one?

Damage depends on factors like depth, population density, and construction quality; for example, the 2001 Gujarat earthquake (magnitude 7.7) caused more damage than some higher-magnitude quakes.

What is the difference between magnitude and intensity in earthquakes?

Magnitude measures the total energy released by the earthquake; intensity measures the observed effects at a specific location.

What is the order of arrival of earthquake waves at a location?

P-waves arrive first, followed by S-waves, and then L-waves (surface waves) arrive last.