Science Earthquakes

What is weathering?

The breakdown of rocks into smaller pieces at or near Earth’s surface.​

What is erosion?

The movement of weathered materials by water, wind, ice, or gravity.​

Where do most faults and earthquakes occur?

Most faults and earthquakes occur around plate boundaries, where plates interact.​

Why are earthquakes and volcanoes stronger in some places?

They are stronger where plates converge or slide past each other, especially in subduction zones and major transform boundaries.​

Normal fault definition

A fault where the hanging wall moves down relative to the footwall due to tension, usually at divergent boundaries.​

Reverse fault definition

A fault where the hanging wall moves up relative to the footwall due to compression, usually at convergent boundaries.​

Strike-slip fault definition

A fault where rocks on either side move horizontally past each other due to shear stress, usually at transform boundaries.​

Plate boundary and stress: normal fault

Plate boundary and stress: reverse fault

Reverse fault: convergent plate boundary and compression stress.​

Plate boundary and stress: strike-slip fault

Strike-slip fault: transform plate boundary and shear stress.​

What is tension (in faults)?

A pulling-apart force that stretches rock, associated with divergent boundaries and normal faults.​

What is compression (in faults)?

A pushing-together force that squeezes rock, associated with convergent boundaries and reverse faults.​

What is shear (in faults)?

A side-by-side sliding force that causes rocks to move horizontally past each other, associated with transform boundaries and strike-slip faults.​

What is an anticline?

The upward fold in rock layers formed by compression, like the top of a wave.​

What is a syncline?

The downward fold in rock layers formed by compression, like a valley between folds.​

What is an earthquake?

A sudden release of built-up energy when rocks on either side of a fault break and slip, sending out seismic waves.​

How is energy released during an earthquake?

Energy stored as stress along a fault is suddenly released as seismic waves when rocks rupture and move.​

What are aftershocks?

Smaller earthquakes that occur after the main earthquake as crust adjusts to new positions.​

Why are large earthquakes hard to predict?

They are rare and depend on complex stress buildup and rock strength deep in the crust.​

What is a seismograph?

An instrument that detects, measures, and records earthquake waves.​

What is a seismogram?

The recorded trace or graph produced by a seismograph showing seismic wave motion.​

What is a seismologist?

A scientist who studies earthquakes and seismic waves.​

What is amplitude on a seismogram?

The maximum height of the wave from its resting position on a seismogram.​

What are P waves (primary waves)?

Fast body waves that arrive first, move through solids and liquids, and shake material in a push–pull (back-and-forth) motion.​

What are S waves (secondary waves)?

Slower body waves that arrive after P waves, move only through solids, and shake material side-to-side or up-and-down.​

Why do P waves arrive before S waves?

P waves travel faster than S waves, so they reach seismic stations first.​

What are surface waves?

Seismic waves that travel along Earth’s surface, arrive after P and S waves, and cause most earthquake damage.​

Two main types of seismic body waves

P waves (primary, compressional) and S waves (secondary, shear).​

How do seismic waves show Earth’s interior?

By tracking which waves arrive and where, scientists infer which layers are solid or liquid, since S waves stop in liquids and P waves bend.​

What happens to S waves at the outer core?

They are absorbed because the outer core is liquid and S waves cannot travel through liquid.​

What is the shadow zone in seismology?

An area on Earth where certain seismic waves are not detected or are very weak due to refraction and absorption in the core.​

Layers of Earth: crust

The thin, rocky, brittle outer layer of Earth.​

Layers of Earth: mantle

The thick layer of solid but plastic-like rock beneath the crust.​

Layers of Earth: outer core

A molten (liquid) layer of iron and other metals beneath the mantle.​

Layers of Earth: inner core

A dense, solid ball of iron and other metals at Earth’s center.​

What is the focus of an earthquake?

The point inside Earth where the rock actually breaks and seismic energy is first released.​

What is the epicenter of an earthquake?

The point on Earth’s surface directly above the focus.​

Which point is the actual movement of rock in an earthquake?

The focus is where the rock movement that causes the earthquake actually occurs.​

Which seismic wave arrives first at a station, and why?

P waves arrive first because they travel faster than S and surface waves.​

How is the distance to an epicenter found from a seismogram?

By measuring the time difference between P and S wave arrivals and using a graph to convert that time difference to distance.​

What information is on a P–S wave travel-time graph?

The difference in arrival times of P and S waves plotted against distance to the epicenter.​

What is a nomogram (earthquakes)?

A chart used to determine earthquake magnitude by aligning distance to epicenter with wave amplitude.​

How do you use a nomogram for magnitude?

Draw a straight line between the distance and amplitude values; where it crosses the middle scale gives the magnitude.​

What is earthquake magnitude?

A measure of the amount of energy released by an earthquake, often expressed on a logarithmic scale.​

What is the Moment Magnitude Scale?

A modern magnitude scale that measures earthquake strength based on energy released, related to fault area and slip.​

How does earthquake magnitude increase between whole numbers?

Each increase of one whole number on the magnitude scale represents a large exponential increase in energy released.​

What is a tsunami?

A series of large ocean waves generated by an underwater earthquake, landslide, or similar event that displaces huge amounts of seawater.​

How fast can a tsunami travel in open ocean?

A tsunami can travel at hundreds of miles per hour in deep water, comparable to a jet airliner’s speed.​

Why are tsunamis dangerous to coasts?

As waves reach shallow water they slow down, grow taller, and can flood and destroy coastal areas.​

What is liquefaction?

A process where shaking turns loose, water-saturated soil into a fluid-like material that can cause buildings to sink or tilt.​

Why is liquefaction dangerous?

It undermines the ground beneath structures, leading to severe damage or collapse during earthquakes.​

Why was the Haiti earthquake so deadly?

It struck a densely populated area with many poorly built structures and limited preparedness, causing massive casualties.​

What are base isolators?

Devices placed at the base of buildings that allow them to move independently during shaking, reducing damage.​

How do base isolators help in earthquakes?

They absorb and redirect seismic energy, reducing how much shaking reaches the building above.​

Key items for earthquake preparedness

Emergency kit, plan to communicate with family, secure shelves and heavy objects, and know safe spots to shelter.​

Where is a safe place during an earthquake indoors?

Under sturdy furniture or in doorways away from windows and heavy objects.​

What usually causes injuries in earthquakes?

Falling or flying objects and collapsing structures rather than the ground crack itself.​

What is a transform fault?

A fault marking the boundary where two plates slide horizontally past each other, often linking mid-ocean ridge segments.​

How are transform faults and strike-slip faults related?

Transform faults are large plate-boundary strike-slip faults; both involve horizontal sliding motion.​

What are mid-ocean ridges?

Undersea mountain chains where two oceanic plates diverge and new crust forms.​

Why are small earthquakes common but large ones rare?

Small quakes release small amounts of stress frequently; large quakes need a long time and lots of stress to build up before rupturing.​

How can three seismic stations locate an epicenter?

By drawing circles with radii equal to their distances to the epicenter; the point where all three circles intersect is the epicenter.​

Why are at least three stations needed to find an epicenter?

One circle gives many possible points, two give two intersections, but three circles give a single location where all overlap.​

What does “fault scarp” mean?

The exposed step or cliff formed on Earth’s surface where movement along a fault has occurred.​

What is a fault plane?

The surface along which there is slip during an earthquake.​

What is the hanging wall?

The block of rock above the fault plane.​

What is the footwall?

The block of rock below the fault plane.​