Earthquake Engineering: Fundamental Concepts, Tectonics, Measurement, Visual Screening, Dynamics, and Time-History Analysis

A comprehensive vocabulary review deck covering fundamental earthquake engineering principles, plate tectonics, fault mechanisms, seismic wave measurement, FEMA P-154 rapid visual screening, single- and multi-degree-of-freedom structural dynamics, and time-history analysis.

Earthquake Engineering

An interdisciplinary branch of civil engineering that focuses on understanding seismic hazards and analyzing, designing, constructing, and maintaining structures to withstand earthquake ground motion, protect human life, and ensure operational functionality.

Earthquake Hazard

The potential occurrence of physical earthquake-related phenomena—such as ground shaking, surface fault rupture, liquefaction, landslides, or tsunamis—that have the capacity to cause harm or structural damage.

Exposure

The population, buildings, civil infrastructure, and economic assets located in areas subject to seismic hazards.

Vulnerability

The susceptibility of exposed elements to damage, governed by structural system, age, materials, construction quality, structural irregularities, detailing, maintenance, and site conditions.

Earthquake Risk

The potential adverse consequences—such as fatalities, injuries, structural collapse, and economic losses—resulting from the interaction of seismic hazard, exposure, and vulnerability ($\text{Risk} = \text{Hazard} + \text{Exposure} + \text{Vulnerability}$).

Ductility

The capacity of a structure or structural component to undergo substantial inelastic deformation beyond its yield point while retaining significant load-carrying capacity without brittle collapse.

Redundancy

The provision of multiple independent load paths and structural components capable of redistributing and sharing lateral earthquake forces if localized damage occurs.

Ruby Tower

A six-story commercial and residential reinforced concrete building in Binondo, Manila, whose total collapse during the August 2, 1968 Casiguran Earthquake resulted in $268$ deaths and prompted major advancements in Philippine seismic building codes.

Metropolitan Manila Earthquake Impact Reduction Study (MMEIRS)

A comprehensive seismic assessment study that evaluated earthquake risk in Metro Manila, predicting that a magnitude $7.2$ rupture on the West Valley Fault could cause heavy damage to approximately $40\%$ of residential buildings and tens of thousands of casualties.

Lithosphere

The rigid outer mechanical layer of the Earth, consisting of the crust and uppermost mantle, broken into continuously moving tectonic plates.

Divergent Plate Boundary

A tectonic boundary where two adjacent lithospheric plates move away from each other under tensional stress, typically causing crustal extension, normal faulting, and volcanic activity.

Convergent Plate Boundary

A tectonic boundary where two plates collide or move toward each other under compressional stress, resulting in subduction, reverse or thrust faulting, mountain building, and the generation of large earthquakes.

Transform Plate Boundary

A tectonic boundary where plates slide past one another horizontally under shear stress, predominantly characterized by strike-slip faulting.

Fault

A fracture or zone of fractures in the Earth's crust along which opposing blocks of rock have undergone relative displacement.

Hanging Wall and Footwall

The two blocks of rock adjacent to an inclined fault plane: the hanging wall is the block situated vertically above the fault plane, whereas the footwall is the block situated vertically below it.

Normal Fault

A dip-slip fault formed under tensional stress in which the hanging wall moves downward relative to the footwall, resulting in crustal extension.

Reverse Fault

A dip-slip fault formed under compressional stress in which the hanging wall moves upward relative to the footwall, resulting in crustal shortening.

Strike-Slip Fault

A fault governed by shear stress in which opposing rock blocks displace horizontally past each other, classified as left-lateral or right-lateral based on the apparent direction of motion of the opposite block.

Elastic Rebound Theory

The geological concept explaining that tectonic forces apply stress to locked fault segments, causing rocks to deform elastically until accumulated strain overcomes frictional resistance, causing sudden slip, rebound to an undeformed state, and release of energy as seismic waves.

Hypocenter (Focus) and Epicenter

The hypocenter (focus) is the point inside the Earth's crust where fault slip begins; the epicenter is the geographic point on the Earth's surface directly vertically above the hypocenter.

Philippine Fault Zone (PFZ)

A major active tectonic structure approximately $1{,}200\,\text{km}$ in length, characterized as an arc-parallel, left-lateral strike-slip fault traversing northwestern Luzon to southeastern Mindanao.

Active Fault

According to DOST-PHIVOLCS, a fault that has exhibited movement within the last $10{,}000$ years (the Holocene epoch) and is considered capable of generating future earthquakes.

Primary (P) Waves

Longitudinal, compressional body waves that travel fastest through rock, propagate through solids, liquids, and gases, and induce push-pull particle motion parallel to the direction of wave travel.

Secondary (S) Waves

Transverse shear body waves that arrive after P-waves, propagate exclusively through solids, and oscillate particles perpendicular to the direction of wave propagation, producing strong lateral shear demands on structures.

Surface Waves

Seismic waves—specifically Love and Rayleigh waves—that travel along the Earth's surface with larger amplitudes and lower frequencies than body waves, causing severe and prolonged structural shaking.

Seismograph vs. Seismogram

A seismograph is the physical instrument used to detect and record ground motion; a seismogram is the resulting graphical or digital trace of wave arrivals, ground displacement, velocity, or acceleration over time.

Earthquake Magnitude

A quantitative, instrument-based measurement of the total energy released at the earthquake source, having a single invariant numerical value for a specific event regardless of observation location.

Earthquake Intensity

A qualitative, observation-based assessment of the severity and physical effects of earthquake shaking at a specific site, varying based on epicentral distance, local soil conditions, and structural types.

Moment Magnitude ($M_w$)

The international standard magnitude scale derived from seismic moment ($M_0 = \text{rigidity} \times \text{slip} \times \text{fault area}$), which accurately measures energy release across all earthquake sizes without suffering from magnitude saturation.

Magnitude Saturation

The physical limitation of certain magnitude scales (such as Surface-Wave Magnitude, $M_s$) where values plateau near $8.0$, failing to increase proportionally with energy released during very large or great earthquakes.

Philippine Earthquake Intensity Scale (PEIS)

The 10-level scale adopted by DOST-PHIVOLCS (ranging from Intensity I - Scarcely Perceptible to Intensity X - Completely Devastating) used to describe observed shaking effects on people, structures, and landforms in the Philippines.

Rapid Visual Screening (RVS)

A preliminary, cost-effective sidewalk-survey procedure based on FEMA P-154 that identifies observable building features, structural systems, and irregularities to prioritize existing buildings for detailed engineering evaluations.

Soft Story Irregularity

A vertical structural irregularity occurring when a story has significantly lower lateral stiffness than the stories above it—often caused by open ground-floor parking or commercial storefronts—concentrating lateral drift and damage.

Weak Story Irregularity

A vertical irregularity characterized by a substantial deficiency in lateral strength (load-carrying capacity) in a particular floor compared to adjacent floors.

Vertical Setback

An elevation irregularity caused by abrupt changes in building width or plan dimensions between consecutive upper stories, altering load paths and causing local stress concentrations.

Floating Columns

Columns that terminate on transfer girders or intermediate beams rather than extending continuously down to the foundation, creating a discontinuous vertical load path.

Mass Irregularity

A vertical irregularity defined as occurring when the effective mass of any story exceeds $150\%$ of the effective mass of an adjacent story, significantly altering dynamic inertia force distribution.

Re-entrant Corner

A plan irregularity found in L-, T-, or U-shaped buildings where interior plan projections cause concentrated torsional and shear stresses during multidirectional ground motion.

Torsional Irregularity

A plan irregularity arising from eccentricity between a building's center of mass and center of rigidity, inducing rotational twisting about the vertical axis during translational ground shaking.

Structural Dynamics

The branch of structural analysis concerned with analyzing the movements, internal forces, and deformations of structural systems subjected to time-varying loads such as wind gusts or earthquake ground motion.

Degree of Freedom (DOF)

The number of independent spatial coordinates necessary to completely describe the position, displacement, or configuration of a vibrating structural system at any given instant.

Single-Degree-of-Freedom (SDOF) System

An idealized analytical model that represents a structure using a single lumped mass, elastic spring, and viscous damper, described completely by one kinematic displacement coordinate.

Natural Circular Frequency ($\omega_n$)

The angular rate at which an undamped SDOF system naturally oscillates when disturbed from its equilibrium position, given by $\omega_n = \sqrt{\frac{k}{m}}\,\text{rad/s}$.

Natural Period ($T_n$)

The time required for an undamped system to complete one full cycle of free vibration, expressed mathematically as $T_n = \frac{2\pi}{\omega_n} = 2\pi\sqrt{\frac{m}{k}}$.

Critical Damping ($c_c$)

The threshold value of viscous damping coefficient that permits an SDOF system to return to its resting position as rapidly as possible without oscillatory motion, defined as $c_c = 2m\omega_n = 2\sqrt{km}$.

Damping Ratio ($\zeta$)

A dimensionless measure of energy dissipation calculated as the ratio of actual damping coefficient to critical damping ($\zeta = \frac{c}{c_c}$, where $0 < \zeta < 1$ for underdamped oscillatory systems).

SDOF Equation of Motion

The governing differential equation expressing dynamic force equilibrium among inertia force ($m\ddot{u}$), damping force ($c\dot{u}$), and elastic restoring force ($ku$): $m\ddot{u} + c\dot{u} + ku = F(t)$.

Resonance

A dynamic condition occurring when the frequency of an applied cyclic excitation approaches or matches the natural frequency of a structure ($\omega \approx \omega_n$), resulting in severe amplification of vibration amplitudes.

Shear Building Model

An idealized multi-degree-of-freedom representation of a multistory building assuming floors are infinitely rigid in bending, mass is lumped at floor levels, joint rotations are zero, and lateral deflection is governed purely by shear frame action.

Time-History Analysis

A rigorous step-by-step dynamic analysis procedure that subjects a structural model to a recorded or synthetic earthquake accelerogram, tracking responses (displacements, velocities, accelerations, and member forces) across consecutive discrete time steps.

Accelerogram

A recorded or processed plot of ground acceleration versus time produced by a strong-motion accelerograph during an earthquake.

Peak Ground Acceleration (PGA)

The maximum absolute amplitude of ground acceleration recorded in an accelerogram, defined as $\text{PGA} = \max|a_g(t)|$, commonly expressed in $\text{m/s}^2$ or as a fraction of gravitational acceleration ($g$).

Relative Displacement vs. Absolute Displacement

Absolute displacement ($u$) measures the total movement of a structural mass relative to a fixed spatial reference frame, whereas relative displacement ($x$) measures deformation of the mass with respect to the moving ground ($u(t) = x(t) + u_g(t)$).

Story Drift

The relative lateral displacement between two consecutive floor levels separated by story height, used to evaluate structural ductility, member deformations, and nonstructural damage.

Base Shear

The total maximum horizontal inertia force transferred across the structural interface at the foundation level during an earthquake.

Nonlinear Time-History Analysis

An advanced dynamic response analysis that directly incorporates material yielding, hysteretic energy dissipation, stiffness degradation, and second-order geometric effects under ground-motion acceleration time series.