UNAM Researchers Use Fiber-Optic Technology to Reveal Rainfall's Role in Mexico City Microseisms
Mexico City sits atop a complex geological foundation that makes it particularly vulnerable to seismic activity. The city's location in a former lake bed amplifies ground motion during earthquakes, a fact well known to residents and scientists alike. In recent years, researchers at the Universidad Nacional Autónoma de México (UNAM) have turned to an innovative tool to better understand the subtle seismic signals that occur daily beneath the metropolis. Their work with distributed acoustic sensing, or DAS, technology deployed on existing telecommunications fiber-optic cables has uncovered a surprising connection between intense rainfall, changes in the city's aquifers, and the generation of microseisms.
DAS works by transforming ordinary fiber-optic cables into dense arrays of seismic sensors. A laser interrogator sends pulses of light down the fiber and measures tiny changes in the backscattered light caused by vibrations or strain in the cable. This allows scientists to record seismic waves with spatial resolution on the order of meters over tens of kilometers, far denser than traditional seismometer networks. In Mexico City, the UNAM team tapped into dark fiber segments already laid beneath the streets, turning infrastructure that carries internet and phone traffic into a powerful scientific instrument without disrupting service.
The Study's Scope and Methodology
The research spanned approximately 15 months of continuous recordings. Scientists connected DAS interrogators to two telecom fibers crossing the city and analyzed the resulting data to locate nearly 100 local microseismic events with unprecedented precision. Microseisms are low-amplitude seismic signals often generated by natural processes such as ocean waves, wind, or, in this case, interactions between water and faults. By combining the high-density DAS measurements with meteorological records and aquifer level data, the team identified clear patterns linking surface water infiltration to subsurface seismic activity.
Step by step, the process involved first calibrating the DAS system to convert optical phase changes into ground motion estimates. Researchers then applied seismic interferometry techniques to extract velocity changes in the shallow subsurface. Finally, they cross-referenced these observations with rainfall data from the end of the dry season, when aquifer levels are typically at their lowest. This timing proved critical: heavy rains after prolonged drought periods appeared to trigger moderate microseismic events that in turn influenced slow slip on perpendicular fault systems beneath the city.
Key Findings on Hydroseismic Interactions
The data revealed that intense rainfall events coincide with measurable increases in microseismic activity. When water rapidly infiltrates the ground after a dry period, it increases pore pressure along faults, effectively lubricating them and allowing small slips that generate detectable seismic signals. These events then appear to influence neighboring fault segments oriented perpendicularly, creating a cascade of hydroseismic interactions unique to Mexico City's geology.
UNAM researchers noted that the phenomenon is most pronounced when aquifers are depleted. The contrast between dry conditions and sudden heavy precipitation maximizes the pressure change, making the effect easier to observe. This finding adds a new dimension to understanding seismic risk in megacities where groundwater extraction and climate-driven rainfall variability are both significant factors.
Implications for Seismic Risk Assessment
Traditional seismic monitoring in Mexico City relies on a network of conventional seismometers operated by institutions including UNAM's own seismological service. While effective for larger events, these stations provide limited spatial resolution for microseisms. The DAS approach offers a complementary, high-resolution view that can help refine models of fault behavior and site amplification.
City planners and emergency managers may eventually use such data to improve early-warning systems or to identify zones where rainfall-induced microseismicity could interact with larger tectonic events. The study underscores the value of repurposing existing infrastructure for scientific monitoring, a cost-effective strategy that could be replicated in other urban centers worldwide.
Connections to Aquifer Management and Urban Planning
Beyond seismology, the research highlights the interplay between water resources and seismic processes. Mexico City has long struggled with groundwater depletion due to over-extraction for its growing population. The UNAM findings suggest that managing aquifer recharge and rainfall infiltration could have secondary benefits for seismic stability.
Urban green spaces, permeable pavements, and improved stormwater systems might help moderate the rapid pressure changes that appear to trigger microseisms. Integrating seismic monitoring with hydrological data could inform more holistic approaches to sustainable urban development in seismically active regions.
Broader Context of DAS Technology in Mexico and Beyond
DAS is gaining traction globally for applications ranging from earthquake detection to infrastructure monitoring. In Mexico, UNAM's work builds on earlier experiments that demonstrated the technology's potential in urban environments. The Mexico City deployment stands out for its focus on environmental triggers and its use of operational telecom fibers rather than dedicated dark fiber.
Similar projects in other countries have shown DAS can monitor traffic noise, ocean waves, and even permafrost changes. The UNAM team's success in linking meteorological and hydrological variables to seismic signals opens possibilities for multi-hazard monitoring systems that serve both scientific and public-safety goals.
Photo by David Trinks on Unsplash
Future Outlook and Potential Applications
Looking ahead, expanding the DAS network across more fiber routes in Mexico City could provide even finer-grained data. Integration with machine-learning algorithms might enable real-time detection and characterization of microseismic events, improving the speed and accuracy of hazard assessments.
International collaborations could further advance the field. Researchers from institutions in the United States and Europe have already partnered on DAS projects, and Mexican universities are well positioned to lead similar efforts in Latin America. Continued investment in fiber-optic sensing infrastructure promises to enhance both scientific understanding and practical resilience in one of the world's largest and most seismically complex cities.
Conclusion
The UNAM fiber-optic seismic study represents a significant step forward in understanding the subtle but important connections between weather, groundwater, and seismic activity in Mexico City. By leveraging everyday telecommunications infrastructure, researchers have demonstrated a powerful, scalable method for monitoring microseisms and their environmental drivers. As climate patterns shift and urban populations grow, such innovative approaches will become increasingly valuable for protecting communities and managing resources sustainably.