6: Slopes, Landslides, and Discontinuities

Stress

A force applied to a material, such as rock from plate movements or overlying weight, resulting in deformation.

Strain

The amount of deformation (change in length) of a material divided by its original length, resulting from applied stress.

Compressive Stress

Stress that occurs where tectonic plates converge, pushing rocks together.

Tensional Stress

Stress that occurs where tectonic plates diverge, pulling rocks apart.

Shear Stress

Stress that occurs where tectonic plates slide past each other, causing shearing deformation.

Elastic Deformation

Reversible deformation where material returns to its original shape after stress is removed.

Plastic Deformation

Permanent, non-reversible deformation where material does not return to its original shape.

Brittle Deformation (Fracture)

Deformation where material breaks with little to no plastic deformation, such as jointing or faulting.

Joint (Fracture)

A break in rock where there is no movement of the rocks on either side of the break.

Fault

A boundary within Earth's crust where rocks on either side have moved relative to each other.

Earthquake

The shaking of the ground caused by the rupture and displacement of rocks along a fault.

Reverse Fault

A fault where the hanging wall moves upward relative to the footwall, caused by compressional stress.

Normal Fault

A fault where the hanging wall moves downward relative to the footwall, caused by extensional stress.

Strike-Slip Fault

A fault where rocks slide horizontally past each other, caused by shear stress; can be right-lateral or left-lateral.

Graben

A down-dropped block of crust bounded by normal faults, forming a valley.

Horst

An uplifted block of crust bounded by normal faults, forming a ridge or plateau.

Thrust Fault

A low-angle reverse fault where older rock is pushed over younger rock over large distances, due to intense compression.

Hypocenter (Focus)

The point within the Earth where an earthquake rupture initiates.

Epicenter

The point on the Earth's surface directly above the hypocenter.

Main Shock

The largest earthquake in a sequence of seismic events on a fault.

Foreshock

A smaller earthquake that precedes the main shock on the same fault.

Aftershock

A smaller earthquake that follows the main shock, caused by stress redistribution along the fault.

Episodic Tremor and Slip (ETS)

A periodic, slow movement along part of a subduction zone that generates tremors but not strong earthquakes, and can transfer stress to locked zones.

Seismic Waves

Waves of energy released during an earthquake that travel through or along the Earth's surface.

Body Waves

Seismic waves that travel through the Earth's interior; includes P-waves and S-waves.

P-Wave (Primary/Compression Wave)

A fast body wave that compresses and expands material in the direction of travel; can move through solids and liquids.

S-Wave (Secondary/Shear Wave)

A slower body wave that moves material perpendicular to the direction of travel; can only move through solids.

Surface Waves

Seismic waves that travel along the Earth's surface; cause most earthquake damage; includes Love and Rayleigh waves.

Love Wave

A surface wave that moves the ground side-to-side horizontally.

Rayleigh Wave

A surface wave that moves the ground in a rolling, elliptical motion, similar to an ocean wave.

Earthquake Magnitude

A quantitative measure of the energy released at the source of an earthquake (e.g., Moment Magnitude Scale).

Earthquake Intensity

A qualitative measure of the shaking and damage experienced at a specific location (e.g., PHIVOLCS Earthquake Intensity Scale - PEIS).

Seismic Importance Factor (I)

A building code factor that increases design forces for structures like hospitals and emergency centers based on occupancy category.

Soil Profile Type

A classification of site soil conditions (e.g., from hard rock SA to soft soil SE) that affects ground shaking amplification.

Seismic Zone Factor (Z)

A coefficient in building codes representing the level of seismic hazard in a geographic zone.

Ground Rupture

The deformation and displacement of the ground surface directly along a fault during an earthquake.

Ground Shaking

The disruptive vibration of the ground during an earthquake, a primary cause of structural damage.

Liquefaction

A process where saturated, loose sediments lose strength and behave like a liquid during strong ground shaking.

Earthquake-Induced Landslide

The downslope movement of soil, rock, and debris triggered by seismic shaking.

Tsunami

A series of large ocean waves generated by the sudden displacement of water, often due to an undersea earthquake.

Stress Transfer

The redistribution of stress along a fault after an earthquake, which can trigger aftershocks or increase stress on adjacent locked segments.

Rupture Surface (Fault Plane)

The area within a fault where slip occurs during an earthquake; displacement is highest at the center and decreases to zero at the edges.

Boudin

A structure where a stronger, brittle layer (like chert) within a ductile rock (like limestone) stretches and breaks into segments.

McConnell Thrust

A classic example of a thrust fault in the Canadian Rockies where older Cambrian rock was pushed over younger Cretaceous rock.

PHIVOLCS

The Philippine Institute of Volcanology and Seismology, the agency responsible for monitoring earthquakes and volcanoes in the Philippines.

National Structural Code of the Philippines (NSCP)

The set of standards governing structural design in the Philippines, including provisions for seismic-resistant design