PHK Seismic Lexicon

Base isolation is a seismic protection strategy that decouples a structure from ground motion by inserting flexible isolators between the superstructure and its foundation. This shifts the structure's fundamental period to a long value, reducing seismic demands and protecting both the structure and its contents.

Base shear is the total horizontal force that a structure's foundation must resist during an earthquake, representing the sum of all inertial forces developed throughout the building. It is the primary design force in equivalent lateral force procedures and the starting point for seismic design of most buildings.

Bayesian inference is a statistical framework that updates the probability of a hypothesis as new evidence becomes available, using Bayes' theorem to combine prior knowledge with observed data. It provides a principled approach to decision-making under uncertainty, widely used in earthquake engineering, SHM, and model updating.

Computer vision (CV) is a field of artificial intelligence that enables computers to extract, analyze, and understand information from digital images and videos. In earthquake engineering, it is increasingly used for automated crack detection, post-earthquake damage assessment, and structural component recognition.

Damage detection is the process of identifying the presence, location, and severity of damage in a structure using measurements of its response or properties. It is the primary objective of structural health monitoring and is applied across a wide range of civil, mechanical, and aerospace systems.

A damage index is a scalar or vector quantity that quantifies the extent of damage in a structure or structural element, typically by comparing a measured or computed response to a reference (undamaged) state. It provides a single measure for assessing severity and supporting decision-making.

The damping ratio (ζ) is a dimensionless measure of how much a structure dissipates vibrational energy, expressed as a fraction of critical damping. It determines how quickly free vibrations decay and how strongly the structure amplifies response at resonance, with typical values ranging from 2% for steel structures to 10% for structures with significant energy dissipation devices.

Ductility is the ability of a structure or structural element to undergo large inelastic deformations without significant loss of strength. It is the fundamental property that allows structures to dissipate seismic energy and is the basis for the response modification factors used in modern seismic design codes.

An earthquake is the sudden release of accumulated elastic strain energy in the Earth's crust, typically along a fault, that generates seismic waves and causes ground shaking. Earthquakes range from imperceptible microtremors to catastrophic events that reshape cities and claim hundreds of thousands of lives.

The Finite Element Method (FEM) is a numerical technique for solving partial differential equations by dividing a complex domain into smaller, simpler subdomains called finite elements. It is the dominant computational tool in structural engineering for analyzing stresses, deformations, and dynamic response of structures under arbitrary loading.

A fragility curve is a probabilistic function that gives the likelihood of a structure or component reaching or exceeding a specific damage state as a function of a ground motion intensity measure. It is a fundamental tool in performance-based earthquake engineering and seismic risk assessment.

Incremental Dynamic Analysis (IDA) is a nonlinear response history analysis method in which a structure is subjected to a suite of ground motions, each scaled to multiple intensity levels, to develop a comprehensive picture of its performance across the full range of seismic demands — from elastic response to collapse.

Liquefaction is the phenomenon in which saturated cohesionless soils lose strength and stiffness during earthquake shaking, temporarily behaving as a liquid. It is a major cause of ground failure, foundation damage, and building collapse in seismic regions worldwide.

Loss estimation is the process of quantifying the expected consequences of earthquakes — in terms of repair costs, casualties, downtime, and economic disruption — by combining hazard, exposure, and vulnerability models. It provides the quantitative basis for risk-informed decision-making.

Machine learning (ML) is a field of artificial intelligence that develops algorithms capable of learning patterns from data and making predictions without being explicitly programmed. In earthquake engineering, it is increasingly used for damage detection, fragility modeling, rapid post-earthquake assessment, and surrogate modeling.

Magnitude is a logarithmic measure of the size of an earthquake, quantifying the energy released at its source. Unlike intensity, which varies from place to place, magnitude is a single value assigned to each earthquake, independent of where it is observed.

The natural period is the time required for a structure or structural element to complete one full cycle of free vibration. It is the single most important dynamic property of a structure, governing how it responds to earthquake ground motion and determining whether resonance effects will amplify the response.

Operational Modal Analysis (OMA), also called output-only modal analysis, is the identification of a structure's modal properties (natural frequencies, mode shapes, damping ratios) using only response measurements, without measuring the input excitation. It is the primary modal identification technique for full-scale structures under ambient or operational conditions.

Peak Ground Acceleration (PGA) is the maximum absolute acceleration experienced by the ground surface during an earthquake, expressed as a fraction of gravitational acceleration (g). It is the most widely used intensity measure for characterizing the severity of ground shaking at a site.

Peak Ground Velocity (PGV) is the maximum absolute velocity experienced by the ground surface during an earthquake, typically expressed in cm/s. It is a key intensity measure that correlates strongly with structural damage in the intermediate-period range and is widely used in seismic hazard assessment and damage prediction.

Performance-based design (PBD) is a seismic design philosophy that explicitly defines performance objectives for a structure at multiple hazard levels and uses analytical procedures to demonstrate that the structure achieves them. It is the modern successor to prescriptive code-based design.

Probabilistic Seismic Hazard Analysis (PSHA) is a methodology that estimates the probability of exceeding a given level of ground shaking at a site within a specified time period, accounting for all possible earthquake scenarios and their associated uncertainties. It is the foundation of modern seismic design codes and risk assessment.

Pushover analysis is a nonlinear static analysis method in which a structure is subjected to a monotonically increasing lateral load pattern until a target displacement or collapse is reached. It provides insight into the structure's sequence of yielding, its lateral capacity, and its failure mechanism.

Resilience is the ability of a community, system, or structure to prepare for, absorb, recover from, and adapt to disruptive events such as earthquakes. It encompasses both the reduction of damage during an event and the speed and effectiveness of recovery afterward.

A response spectrum is a plot of the peak response (acceleration, velocity, or displacement) of a single-degree-of-freedom system as a function of its natural period, for a given ground motion and damping ratio. It is the fundamental tool for characterizing earthquake ground motion and forms the basis of most seismic design spectra in modern building codes.

Seismic risk is the potential for loss — in lives, property, economic activity, or functionality — resulting from the interaction of seismic hazard, exposure, and vulnerability. It is distinct from seismic hazard, which describes the physical phenomenon alone, and is the quantity that risk mitigation seeks to reduce.

Site response analysis is the calculation of how soil deposits modify earthquake ground motion as it propagates from bedrock to the ground surface. It is used to develop site-specific design spectra and to evaluate the effects of local soil conditions on ground shaking.

Soil-structure interaction (SSI) is the mutual influence between a structure and the soil supporting it during dynamic loading. It modifies the structure's effective period, damping, and deformation pattern, and is especially important for stiff, heavy, or embedded structures on soft soil.

Structural Health Monitoring (SHM) is the process of implementing a damage detection and characterization strategy for engineering structures using continuous or periodic measurements. It combines sensing, data acquisition, signal processing, and decision-making to assess the condition of a structure over its lifetime.