The SEM Theory sets mitigation alongside risk source and risk exposure as an element in its own right, and carries it through to a Space–Air–Ground monitoring workflow tested on an urban village in southern China.
Cities are open complex giant systems, and the populations, wealth and critical functions they hold are highly concentrated. Their safe, continuous operation therefore bears directly on national security and development. A paper in ENGINEERING Cities recasts what urban safety means: not the elimination of risk, but a city's ability to hold its essential functions inside an acceptable safety boundary while under disturbance, and to keep and defend that state afterwards. Qingrui Yue of the Research Institute of Urbanization and Urban Safety at the University of Science & Technology Beijing led the work.
Most of the paper is spent building the Risk Source–Risk Exposure–Mitigation Force Theory of Urban Safety, or SEM Theory, into something that can be written down mathematically. Risk Source is any hazard-producing entity, event or process that may threaten urban safety, and the authors give it three fundamental characteristics: coupling and cascading, urban specificity, and macro-environmental sensitivity. Risk Exposure is the people, assets and systems that take the effects; in cities it appears in four forms the authors call the "4H" patterns, namely high population density, high asset concentration, a high concentration of elements critical to national security, and high systemic coupling. Mitigation Force combines risk cognition, mitigation organization, mitigation technology and mitigation resources, and the paper places it at the same analytical level as the other two elements. That placement is the argument. For systems that depend on the intrinsic robustness, redundancy and recoverability of exposed entities, mitigation had been folded into wider ideas such as resilience and capacity rather than treated as an element in its own right.
Roles inside the theory are not fixed. Damaged infrastructure can turn into a secondary risk source. A protective facility that fails stops protecting and starts producing risk. The same physical thing may sit inside Risk Exposure in one scenario and inside Mitigation Force in another. On top of that comes a two-layer formulation: heterogeneous observations are converted into functional equivalents, and safety is read off a safety index S whose theoretical critical threshold is 0.5. Above 0.5 a system can generally maintain its functions. At 0.5 it sits at critical equilibrium; below 0.5 it is overloaded or functionally impaired.
"Mitigation Force is not the arithmetic sum of individual measures, resources, and capabilities," Yue said. "Rather, it emerges through the coordination of these elements across departments, systems, and levels of governance, and its intervention may be directed toward Risk Source, Risk Exposure, or the risk-evolution process." Modeling it independently, he argued, "frees proactive risk reduction from its status as an appendage of 'resilience' and 'capacity,' making it a quantifiable and optimizable element of urban safety assessment."
The second half of the paper turns the theory into a monitoring system. Spaceborne observation, airborne sensing and ground-based monitoring are placed in one spatiotemporal reference. The combination is then asked to answer three questions in order: how to sense across scales, at high precision and at high density; how to interpret what has been sensed by embedding multidimensional knowledge; and how to fuse information across scales into an assessment of safety performance. The workflow was tried on a self-built housing community in an urban village in southern China (180,000 m², more than 350 buildings). It ran in four steps. InSAR identified 26 buildings with abnormal settlement among the more than 350; air–ground integrated monitoring then examined roof loading, surface damage, surrounding engineering and ground collapse; a neural network narrowed the list to 15 suspected high-risk buildings; and ground-based cross-validation confirmed five of them as dangerous. The manual inspection workload fell by roughly 96%. The authors read the result as evidence that the chain from conceptual definition through theoretical modeling to operational monitoring holds together, and that a community can be screened quickly and accurately.
The case itself stops at building safety and at community scale. Extending it to lifeline systems such as energy, transport, and water supply and drainage would take definitions and calibration specific to each subsystem: functional units, monitoring parameters, safety thresholds, interdependency pathways. The paper closes with four trends it expects to shape that work: intensifying external disturbance under climate change; geopolitical change and armed conflict widening the scope of urban safety; the aging of both urban assets and populations; and industrial transformation broadening the risk spectrum.
Other authors are affiliated with the Shenzhen Technology Institute of Urban Public Safety, Beijing Normal University, and the Institute of Engineering Mechanics, China Earthquake Administration.
URL : https://journal.hep.com.cn/ec/EN/10.2738/ENGC.2026.0016
Subject of Research : City
Cite this article :
Qingrui Yue, Zhongqi Shi, Zhen Xu, Kai Liu, Lin Zhou, Nan Jin, Yuzhou Liu, Yuan Tian, Jianxin Zhang, Zhiwei Liu, Jiale Qian, Zhichao Lin, Donglian Gu, Yuxing Xie, Jian Ma. The Risk Source–Risk Exposure–Mitigation Force Theory of Urban Safety and Its Enabling Space–Air–Ground Integrated Monitoring Technologies. ENGINEERING Cities , 2026, 1 (1) : 41-70 DOI:10.2738/ENGC.2026.0016
About the Journal
ENGINEERING Cities focuses on key scientific questions and challenges in the field of urban science and engineering. It particularly emphasizes international frontier topics such as urban complex mega-systems, urban safety and resilience, urban renewal and sustainable development, climate change and the urban environment, digital twins and smart cities, and urban governance. The journal is committed to building a high-level international academic exchange platform, promoting deep interdisciplinary integration, encouraging the coordinated development of original theories, technologies, and engineering practices, advancing the understanding of urban systems and technological innovation, and fostering the global development of emerging interdisciplinary fields in urban science.
The Risk Source–Risk Exposure–Mitigation Force Theory of Urban Safety and Its Enabling Space–Air–Ground Integrated Monitoring Technologies
21-Oct-2026