Exposure is the inventory of people, buildings, infrastructure, and economic assets located in an area subject to seismic hazard. It is one of the three fundamental components of seismic risk, alongside hazard and vulnerability, and it determines who and what is at stake.
Exposure is the inventory of assets β people, buildings, infrastructure, economic activity, and environmental resources β located in an area that could be affected by a hazard. In earthquake engineering, exposure is one of the three fundamental components of seismic risk, along with hazard (the physical phenomenon) and vulnerability (the susceptibility of the exposed assets to damage). The relationship is often summarized as: Risk = Hazard Γ Exposure Γ Vulnerability. A region with high exposure (dense population, extensive infrastructure, valuable property) but low hazard may face moderate risk; a region with low exposure but very high hazard may also face moderate risk; but a region with both high exposure and high hazard β such as Tehran or Tokyo β faces the highest risk.
Exposure is characterized through several dimensions. Population exposure describes the number, distribution, and characteristics of people in the hazard zone β including demographic factors (age, income, mobility) that affect vulnerability and evacuation capacity. Building exposure describes the number, type, age, and value of buildings β typically characterized by building type (e.g., reinforced concrete moment frame, unreinforced masonry, steel frame), construction era (which determines the applicable building code), occupancy (residential, commercial, industrial), and replacement value. Infrastructure exposure describes the critical networks β water, power, transportation, communications, healthcare, and emergency services β that communities depend on. Economic exposure describes the economic activity, employment, and value at risk in the hazard zone. Environmental exposure describes the natural and cultural resources that could be affected. Modern exposure databases β such as those developed by GEM (Global Exposure Database), FEMA (HAZUS), and national agencies β organize this information at various spatial scales, from individual buildings to census tracts to regions.
Exposure is dynamic and challenging to characterize accurately. The building stock changes over time through new construction, demolition, retrofit, and modification β meaning that an exposure inventory can become outdated quickly. Construction quality and code compliance can vary widely, even within a single city or neighborhood, making exposure characterization inherently uncertain. Population distribution changes daily (commuting, migration) and seasonally (tourism), and long-term trends (urbanization, aging) alter exposure over decades. In data-poor regions β which include many high-hazard areas β exposure information may be limited to coarse estimates derived from satellite imagery, census data, or expert judgment. Key challenges in exposure modeling include: (1) maintaining up-to-date inventories as the built environment evolves; (2) characterizing the distribution of building types and construction quality within a region; (3) accounting for time-dependent changes in population and economic activity; (4) integrating exposure data from multiple sources (satellite imagery, census, cadastral records, field surveys); and (5) characterizing the uncertainty in exposure estimates, which propagates through to risk estimates. In Iran, exposure is a major concern: the country has a large, rapidly urbanizing population, with many cities located in high-hazard zones. Tehran alone has over 9 million residents and an estimated 2.5 million buildings, many of which are vulnerable to seismic damage. Exposure studies for Iranian cities have been conducted by the International Institute of Earthquake Engineering and Seismology (IIEES), the Building and Housing Research Center (BHRC), and university research groups, often in collaboration with international partners. The 2003 Bam earthquake β in which an estimated 26,000 people died β highlighted both the high exposure of Iranian cities and the importance of accurate exposure data for emergency planning and risk reduction.