Lesson 3 – Earthquakes: Key Vocabulary

A comprehensive set of vocabulary flashcards covering essential earthquake terminology, fault mechanics, seismic waves, measurement tools, hazard effects, mitigation technologies, and Philippine earthquake-related laws.

Earthquake

A violent, abrupt shaking of the ground caused by the sudden release of strain energy along faults due to movement of tectonic plates.

Hypocenter (Focus)

The point within the Earth where an earthquake rupture starts.

Epicenter

The point on Earth’s surface directly above the focus; usually where the greatest damage occurs.

Fault

A fracture or zone of fractures in Earth’s crust along which blocks of rock move relative to each other.

Footwall

The block of rock located below the fault plane in a cross-sectional view.

Hanging Wall

The block of rock located above the fault plane in a cross-sectional view.

Fault Plane

The planar surface that represents the fracture along which displacement occurs.

Heave

The horizontal component of displacement along a fault.

Throw

The vertical component of displacement along a fault.

Hade

The angle a fault plane makes with the vertical, measured perpendicular to strike.

Normal Fault

A fault in which the hanging wall moves downward relative to the footwall; produced by tensional (pull-apart) stress and common at divergent boundaries.

Reverse Fault

A fault in which the hanging wall moves upward relative to the footwall; produced by compressional stress and common at convergent boundaries. (Called a thrust fault if dip is small.)

Strike-Slip Fault

A nearly vertical fault where blocks move horizontally past each other due to shear stress; typical at transform boundaries (e.g., San Andreas Fault).

Seismic Waves

Energy waves generated by an earthquake that travel through or along Earth.

Body Waves

Seismic waves that travel through Earth’s interior (P-waves and S-waves).

Primary Wave (P-wave)

A compressional, longitudinal body wave that travels fastest and passes through solids, liquids, and gases.

Secondary Wave (S-wave)

A transverse body wave that moves particles perpendicular to wave direction; slower than P-waves and travels only through solids.

Surface Waves

Seismic waves that travel along Earth’s surface (Love and Rayleigh waves); slower but most destructive.

Love Wave (L-wave)

A surface wave that shears the ground side-to-side, causing severe horizontal shaking; generally the most damaging wave type.

Rayleigh Wave (R-wave)

A surface wave that produces rolling, up-and-down motion similar to ocean waves.

Seismograph

An instrument that records ground motion during an earthquake.

Seismometer

The internal sensing component of a seismograph, often a mass-spring or pendulum system.

Seismogram

The graphical record of seismic waves produced by a seismograph.

Magnitude

A quantitative measure of the energy released at the earthquake focus (e.g., Richter or Moment Magnitude).

Intensity

A qualitative measure of an earthquake’s effects on people and structures at a specific location (e.g., Modified Mercalli Scale).

Richter Scale

A logarithmic scale that quantifies local magnitude (ML); each whole number represents a tenfold increase in wave amplitude.

Moment Magnitude Scale (Mw)

A modern magnitude scale based on seismic moment; more accurate for very large earthquakes.

Modified Mercalli Scale

A twelve-level scale that describes earthquake intensity based on observed effects, from I (not felt) to XII (total destruction).

Liquefaction

Loss of soil strength and stiffness during shaking, causing ground to behave like a fluid.

Tsunami

A series of large sea waves generated mainly by underwater earthquakes, landslides, or volcanic eruptions; can exceed 500 mph in the open ocean.

Foreshock

A smaller earthquake that precedes the largest shock (mainshock) in a seismic sequence.

Mainshock

The largest earthquake in a sequence.

Aftershock

A smaller earthquake that follows the mainshock as the crust readjusts; can occur minutes to years later.

Base Isolation

An earthquake-resistant technology that decouples a building from ground motion using flexible bearings, such as lead-rubber bearings.

Structural Shock Absorber (Damper)

A device that converts seismic kinetic energy into heat, reducing building vibrations.

Pendulum Power (Tuned Mass Damper)

A large mass suspended inside a building that swings opposite to structural sway, dissipating seismic energy.

Replaceable Fuses

Sacrificial structural elements that deform and absorb seismic energy, sparing primary members; replaced after a quake.

Rocking Core-Wall

A reinforced concrete core designed to rock and self-center, dissipating seismic forces while protecting the main frame.

PHIVOLCS

Philippine Institute of Volcanology and Seismology; agency monitoring earthquakes, volcanoes, and tsunamis in the Philippines.

National Building Code (PD 1096)

Philippine law that prescribes earthquake-resistant design and construction standards.

Republic Act 10121

The Philippine Disaster Risk Reduction and Management Act of 2010, strengthening preparedness and response, including earthquake drills.

Seismic Shadow Zone

An area on Earth’s surface where direct P-waves or S-waves are not recorded due to refraction or inability of S-waves to pass through the liquid outer core.

Convergent Boundary

A plate boundary where two plates collide; site of compressional stress, reverse faults, and powerful earthquakes (e.g., Himalayas).

Divergent Boundary

A plate boundary where two plates move apart; site of tensional stress, normal faults, and mid-ocean ridges or rift valleys.

Transform Boundary

A plate boundary where plates slide horizontally past each other; characterized by strike-slip faults and shear stress (e.g., San Andreas Fault).