Under the coupling of multiple agents and mutual feedback among multiple spheres, urban construction and operation and the human living environment face severe geological safety risks, mainly manifested in the following aspects:

(1) Urban geological disasters are diverse in type, widely distributed, and severe in impact, seriously threatening urban residential safety.

(2) The continuous expansion of urban construction scale, complex agents, and strong effects have caused geological safety risks to surge.

(3) Within the basin,endogenic and exogenic agentsaccumulate and superimpose, interact hierarchically, and couple from multiple sources, making urban disaster risk prevention and control severe.

Therefore, the foundation of urban safety lies in geological safety, and the key to urban geological safety lies in basin sphere interactions. Guided by Earth system science, in-depth study of the intrinsic connection between basin sphere structure and urban geological safety is of great significance for scientifically assessing urban geological disaster risks, rationally planning urban layout, and improving urban geological disaster prevention and control, thereby effectively safeguarding urban residential environment safety and the implementation of major national strategies.

Figure 1 Distribution of urban agglomerations and urban geological disasters in China and typical disaster events

1 Composition and structure of basin spheres

The basin sphere structure system is open, interconnected, and dynamically balanced; thus, the five spheres of the basin have good dynamic mechanism linkages. The interrelationships among the basin's spheres and their mutual feedback mechanisms are closely and intrinsically connected to urban geological safety risks. Specifically: basement tectonic sphere → disaster-prone tectonic environment → regional instability risk; cover structure sphere → disaster-prone key strata zones → local clustered disaster risk; hydrological cycle sphere → disaster-prone hydrological conditions → urban subsidence and fissuring risk; underground space sphere → disaster-prone interaction zones → surrounding rock deformation risk; surface engineering sphere → disaster-prone clustered spaces → shallow surface catastrophic risk. Research on sphere interactions aims to reveal how geological phenomena or problems in a given sphere are related to the physical mechanisms of deeper or shallower spheres, which is of profound and major significance for urban geological safety risk prevention and control.

Figure 2 Composition and constituent elements of basin spheres

Figure 3 Research framework for urban geological safety based on key geological risks of basin sphere interactions

2 Dynamics of basin spheres and urban geological safety risks

2.1 Basement tectonic sphere and regional instability risk

2.2 Cover structure sphere and local clustered disaster risk

2.3 Hydrological cycle sphere and urban subsidence and fissuring risk

2.4 Underground space sphere and surrounding rock deformation risk

2.5 Surface engineering sphere and shallow surface catastrophic risk

3 Mechanisms linking basin sphere mutual feedback and urban geological safety

Conclusion

The interaction and stability of basin spheres seriously affect urban safety; early warning, prevention, and control of urban geological safety risks are important guarantees for urban construction and development. From bottom to top, the basement tectonic sphere is the tectonic cornerstone of urban geological safety, affecting the stability of regional geological bodies; the cover structure sphere is the material basis of urban geological safety, affecting the stability of engineering geological bodies; the hydrological cycle sphere is an important medium of urban geological safety, affecting the stability of engineering rockand soil masses; the surface engineering sphere and the underground space sphere are the concrete manifestation of urban geological safety, affecting the stability of engineering structures. Basic research on the interactions among the five spheres—surface engineering sphere, underground space sphere, hydrological cycle sphere, cover structure sphere, and basement tectonic sphere—is crucial for urban construction and operation and the implementation of major national strategies.

Outlook

(1) Research on the geological safety response mechanisms and catastrophic effects of multi-sphere cross-scale mutual feedback in basin mega-urban agglomerations: Multi-sphere cross-scale mutual feedback is a hot topic in geoscience in recent years, whose core is the disaster-breeding and disaster-causing mechanisms of geological processes responding at the shallow surface. Research on multi-sphere cross-scale mutual feedback in basins is an important prerequisite for ensuring urban geological safety and sustainable development. However, research in this area is still very lacking, and many issues remain inadequately clarified.

(2) Research on the influence mechanisms of material cycles, stress cycles, and water cycles among basin spheres on urban geological safety: Urban geological safety involves multiple spheres, and material and energy cycles are important manifestations of sphere mutual feedback processes, among which the most important links are material cycles, stress cycles, and water cycles. However, research on the influence of material and energy cycles among spheres on urban geological safety is likewise very lacking, yet such research is an important basis for quantitative analysis of the mechanisms linking sphere mutual feedback and urban safety.

(3) Research on dynamic evaluation methods for urban geological safety based on the stable state, critical state, and unstable state of basin multi-spheres: The soul of engineering geology and disaster geology is "stability," based on which the concept of the "three states" of basin spheres is proposed, namely the stable state, critical state, and unstable state. Urban engineering construction must conform to the law of coordinated development between humans and nature; when basin spheres are in the stable state,urban sustainable developmentcan be achieved, and environmental safety and ecological balance can be safeguarded. Accurate identification of the stable, critical, and unstable states of basin multi-spheres is the prerequisite; a multi-scale risk analysis and assessment model driven by both physics and data can be constructed, combined with intelligent analysis technology for accurate discrimination, which requires systematic and long-term monitoring as support. Meanwhile, the key among the "three states" is the critical state; once the critical state is reached, urban geological safety is already facing major threats, yet research on the driving conditions and critical thresholds of the critical state is scarce. Adopting a systems theory approach can deepen the understanding of the "three states" of basin spheres. Whether the basin sphere system for urban geological safety is stable depends on the balance of three aspects, namely external material and energy input, the system's own self-absorption and self-repair, and the output of material and energy within the system.