A recent study has uncovered the geological risks posed by mudstone intrusions underneath the urban landscapes of southwestern Taiwan, indicating these underground structures can significantly affect surface deformation and create geological hazards. Researchers at National Cheng Kung University (NCKU) conducted core drilling at the hotspot of active mud diapirism to understand the origins and mechanics of these mud intrusions, which are associated with upward movement of deep mud, potentially threatening local infrastructure.
PUBLISHED on March 5, 2025, the research highlights findings from multi-disciplinary analyses, confirming the presence of sediment originating from mudstone located at depths of at least three kilometers. This study combines geological data with sophisticated imaging techniques, including electrical resistivity tomography (ERT), to visualize the subsurface features and evaluate their impact on the Earth's surface.
Earthquakes are typically seen as the main threat to buildings and infrastructure, especially in tectonically active regions such as Taiwan. Residents often associate structural damage with seismic activity, particularly shaking, soil liquefaction, and surface ruptures. Yet, during investigations, researchers discovered alarming signs of damage not directly correlated to active fault lines.
Geodetic records have indicated ground deformation rates approaching 10 mm per year within the study site. More precisely, surface uplift and other forms of deformation are being attributed not solely to seismic events but rather to the effects of these mudstone intrusions.
The investigation focused on areas along the Tainan and Chungchou anticlines, where numerous public and residential buildings have sustained damage. While there was previously little seismic activity documented at depths less than 12 kilometers, the observed structural issues prompted researchers to probe the underlying geology. The paradigm shift lies in the realization these mudstone structures might be harboring substantial pressures, able to push through the earth's crust and deform the surface.
Core drilling at the hotspot revealed both significant uplift and unusual sediment characteristics. Investigators drilled two boreholes: the primary site reaching 100 meters deep at AT1, and a comparative site reaching just 20 meters at BH3. The findings suggested the upper section of sediments at AT1 are relatively young, dating between 5,940 and 25,820 years before the present, contrasting with much older sediments—approximately 40,000 years—that were located at depths between 20 and 102 meters.
Research findings also emphasized the necessity of using ERT to visualize the complex layering within the ground. The method revealed low-resistivity zones indicative of mud intrusions, likely stemming from the Gutingkeng Formation at considerable depths. ERT surveys conducted across the study area mapped the resistivity variations, highlighting patterns consistent with mud intrusion hotspots.
From the geochemical sampling, the sedimentary cores contained evidence supporting the hypothesis of upward moving mud from deep formations, blending with younger sediment layers. Evidence from particle size analyses and radiocarbon dating indicated the pressure buildup within these layers could lead to heightened risks of geological hazards as urban construction progresses.
The researchers posit, "these mud diapiric intrusions create structural highs beneath the surface, shaping the landform." This shearing caused by mud uplift potentially incurs damage to buildings, bridges, and roads—a serious risk for community safety.
Significantly, the study denotes the first direct evidence correlatively linking underground mud movement with surface deformation and structural integrity concerns. Reports of failures, such as visible cracks and displacement between the ground and buildings, showcase the need for urban planners to reconsider development protocols, especially within known geological stress points. Electrical resistivity tomography revealed scattered mud intrusions causing localized uplift, identifying high-risk zones for construction.
The study's conclusions suggest potential widespread influences of mud intrusions, indicative of likely similar occurrences globally, where geological dynamics are complex, and the interaction between mud and other geological elements poses risks to urban settlements.
Moving forward, this research emphasizes the importance of multidisciplinary approaches, where continued studies and technological innovations—like enhanced ERT imaging—could provide pivotal information for urban safety and resilience. Confirming the origins of sediment and their upward trajectories, this study contributes to a growing body of knowledge on geohazards, encouraging proactive strategies to safeguard communities from the unnoticed threats of subsurface geological activities.
With more refined exploration methods and structural engineering techniques, even if construction cannot sidestep these hotspots, the slow deformation rates suggest targeted approaches can effectively mitigate potential damages influenced by subsurface mud intrusion phenomena.