Hydrometeorological phenomena detected through a citizen-science seismic network in Los Cabos (2024-2025)
Fenómenos hidrometeorológicos detectados mediante una red sísmica de ciencia ciudadana en Los Cabos (2024-2025)
Roberto Ortega1,*, Dana Carciumaru2
, Luis Quintanar3, Victor H. Espíndola3, Jesus Pérez Santana3, Raúl Ochoa Terán4, Graciela Tiburcio Pintos5, Ignacio Leal Orozco5, José Luis Escalante Arriola5, Francisco E. Patiño Velis5, Greg Noela5, Estela Figueroa5
1 Unidad Académica La Paz, Centro de Investigación Científica y de Educación Superior de Ensenada. C. Miraflores 334, Bella Vista, 23050 La Paz, Baja California Sur, México.
2 Unidad Académica La Paz, Secretaría de Ciencias, Humanidades, Tecnología e Innovación (SECIHTI), Centro de Investigación Científica y de Educación Superior de Ensenada. C. Miraflores 334, Bella Vista, 23050 La Paz, Baja California Sur, México.
3 Instituto de Geofísica, Universidad Nacional Autónoma de México. Circuito de la Investigación Científica, Ciudad Universitaria, Coyoacán, 04510, CDMX, México.
4 Orbis Consultores en Geología y Geofísica. Carretera a CD. Sahagún Bodega 4 Ext. 527, Industrial La Paz, 42186 Mineral de la Reforma, Hidalgo, México.
5 Organización para la Sustentabilidad y la Conservación del Medio Ambiente (SyCOMA). Ernesto Chánez Chávez Local 5, Ampliación Santa Rosa, 23428 San José del Cabo, Baja California Sur, México.
* Corresponding author: (R. Ortega) This email address is being protected from spambots. You need JavaScript enabled to view it.
How to cite this article:
Ortega, R., Carciumaru, D., Quintanar, L., Espíndola, V. H., Pérez Santana, J., Ochoa Terán, R., Tiburcio Pintos, G., Leal Orozco, I., Escalante Arriola, J. L., Patiño Velis, F. E., Noela, G., & Figueroa, E. (2026). Hydrometeorological phenomena detected through a citizen-science seismic network in Los Cabos (2024-2025). Boletín de la Sociedad Geológica Mexicana, 78(2), A271125. https://doi.org/10.18268/BSGM2026v78n2A271125
Manuscript received: August 1, 2025. Corrected manuscript received: November 15, 2025. Manuscript accepted: November 27, 2025.
ABSTRACT
In 2024-2025, a prolonged seismic swarm unfolded in the Los Cabos region of Baja California Sur, Mexico, within one of the country’s most densely developed coastal corridors. Residents and scientists rapidly identified the sequence as a sustained swarm with no mainshock. A collaborative deployment of low-cost seismic stations—installed in private homes with citizen support—enabled near-real-time monitoring and waveform analysis. Over 8000 events were recorded between August 2024 and May 2025, revealing episodic bursts of seismicity separated by calm intervals. A strong rainfall event, delivering over 47 mm of precipitation in just three hours, preceded the onset of a highly localized seismic cluster west of the San José fault. Analysis of this cluster showed upward depth migration and radial spreading over a ~20-day period. Statistical evaluation of the seismicity within this zone revealed anomalously high b-values, ranging from 1.6 to 2.6, with a preferred value of b = 2.0 for Mc = 1.6. These results deviate significantly from tectonic backgrounds (b ≈ 1.0), supporting the hypothesis of fluid-induced seismicity driven by pressure diffusion. Diffusive modeling of the space-time envelope yielded a best-fit hydraulic diffusivity of D ≈ 1.7 m²/s, consistent with values reported in other natural and anthropogenic fluid-driven swarms. We emphasize the importance of integrating meteorological, hydrological, and seismological observations and show that community-based, ethically grounded monitoring—where data collection is transparent, conducted with informed consent, and locally co-designed— can overcome institutional and logistical barriers that often limit long-term seismic and hydrological surveillance.
Keywords: seismic swarm, hydraulic diffusivity, fluid‑induced seismicity, citizen science, Los Cabos, Mexico.
RESUMEN
Durante 2024-2025, se desarrolló un enjambre sísmico prolongado en la región de Los Cabos, Baja California Sur, México, dentro de uno de los corredores costeros más densamente urbanizados del país. Residentes e investigadores identificaron rápidamente la secuencia como un enjambre sostenido sin sismo principal. Una red colaborativa de estaciones sísmicas de bajo costo—instaladas en viviendas privadas con apoyo ciudadano—permitió el monitoreo casi en tiempo real y el análisis de formas de onda. Se registraron más de 8000 eventos entre agosto de 2024 y mayo de 2025, revelando estallidos episódicos de sismicidad separados por intervalos de calma. Un evento de lluvia intensa, con más de 47 mm de precipitación en solo tres horas, precedió al surgimiento de un clúster sísmico altamente localizado al oeste de la falla de San José. El análisis de este clúster mostró migración ascendente en profundidad y expansión radial durante un periodo de ~20 días. La evaluación estadística de la sismicidad en esta zona reveló valores b anómalamente altos, entre 1.6 y 2.6, con un valor preferido de b = 2.0 para Mc = 1.6. Estos resultados difieren significativamente de los valores tectónicos típicos (b ≈ 1.0), lo que respalda la hipótesis de una sismicidad inducida por fluidos mediante difusión de presión. La modelación difusiva del envolvente espacio–tiempo arrojó una difusividad hidráulica óptima de D ≈ 1.7 m²/s, consistente con valores reportados en otros enjambres naturales o inducidos por actividades humanas. Se destaca la importancia de integrar datos meteorológicos, hidrológicos y sismológicos, al tiempo que se demuestra cómo el monitoreo ético —donde la recolección de datos es transparente, coordinada y responsable—, apoyado por la ciudadanía, puede superar limitaciones institucionales y logísticas.
Palabras clave: enjambre sísmico, difusividad hidráulica, sismicidad inducida por fluidos, ciencia ciudadana, Los Cabos, México.
1. Introduction
The Los Cabos region, located at the southern tip of the Baja California Peninsula, is geologically complex due to its position at the boundary of major tectonic plates (Figure 1).
The region is underlain by a geologically diverse basement that records a long and complex tectonic history (Figure 1a). In addition, we present the complex geological setting of the region in a simplified geological map (Figure 1b), emphasizing the lithological domains most relevant for understanding the seismic and hydrogeological context of this work.
The mapped units include Mesozoic metamorphic and sedimentary rocks, Jurassic intrusive bodies, and extensive Cenozoic sedimentary and volcanic deposits. In particular, the Oligocene–Miocene succession comprises conglomerates, sandstones, shales, rhyolitic tuffs, and volcanic flows that form the structural and hydrogeological framework of the Los Cabos corridor (Busch et al., 2011; Umhoefer et al., 2014). These deposits are laterally heterogeneous and rest on a rugged crystalline basement, producing marked contrasts in mechanical properties and permeability. This heterogeneity poses challenges for geophysical interpretation but also provides a favorable setting for the occurrence of localized seismic swarms triggered by transient stress or fluid-pressure changes.
In addition to its complex lithostratigraphy, the Los Cabos region is characterized by significant tectonic activity (Figure 1a), shaped by its proximity to the Gulf of California rift system (Busch et al., 2011; Umhoefer et al., 2014). This system represents a young and active plate boundary where continental extension and oceanic spreading are currently ongoing. Faults in the area, including the San José del Cabo fault system, exhibit evidence of Quaternary movement, and several are considered potentially active, with the capacity to generate moderate to strong earthquakes. These structures are often obscured beneath sedimentary cover, making their detection and delineation through geophysical methods particularly relevant
Although southern Baja California is not among the most seismically active regions of Mexico, the Los Cabos area has experienced several seismic events in recent decades, some of which have caused localized damage. For example, the 2006 swarm in Bahia Asuncion (Munguía et al., 2016) revealed that localized fault systems can release accumulated stress, likely associated with crustal deformation induced by plate interactions. The swarm-like seismicity patterns observed in many tectonic settings worldwide suggest that transient processes—such as fluid migration, pore-pressure diffusion, or aseismic stress transfer across fractured zones—can play a key role in swarm initiation and evolution. These mechanisms have been documented in both intraplate and plate-boundary environments, often revealing complex spatiotemporal migration and elevated b-values that depart from mainshock–aftershock behavior (Chen et al., 2012; Smirnov et al., 2022). Such observations underscore the importance of dense local monitoring networks, as they allow for high-resolution detection of swarm dynamics and a better understanding of the underlying physical processes.
These geodynamic conditions, combined with the heterogeneity of the subsurface, underscore the importance of high-resolution geophysical surveys for understanding both shallow and deep features. A local seismic network was installed in late August 2024 and remained operational throughout the year. Although its original purpose was to monitor microseismic activity and track the evolution of the swarm within the hotel zone of Los Cabos, it soon became a unique opportunity to investigate the hydrometeorological effects on the subsurface. Citizen participation played a central role in this effort: local volunteers provided shelter for the stations, assisted with data connectivity and maintenance, and contributed to the creation of a technically robust and professionally managed network. As a result, this initiative has evolved into a permanent seismic monitoring system with multiple applications, which is expected to expand in the coming years
The term seismic swarm lacks a universally accepted definition in seismological literature (Horálek et al., 2015). While generally understood as a cluster of earthquakes occurring within a confined space and time window without a single dominant mainshock, the boundary between swarms and other types of seismic sequences—such as aftershock series—remains imprecise. In some instances, swarms are defined by the sheer volume of low-magnitude events occurring over days or weeks, with no significant peak event. In other cases, they present ambiguous behavior that lies somewhere between classic mainshock–aftershock dynamics and fluid-triggered seismic bursts. A statistical criterion often employed is the analysis of the Gutenberg–Richter b-value, where values exceeding 1.2 are frequently associated with swarm-type behavior. These elevated b-values suggest a seismic regime dominated by small events and low stress drops, although variations in b across time, depth, and tectonic context limit its reliability as a universal swarm indicator (El-Isa and Eaton, 2014; Smirnov et al., 2022).
Despite the variability in definitions and seismic patterns, one commonality in most swarms—regardless of tectonic or volcanic setting—is their close association with fluid-related processes. Swarms frequently occur in response to transient changes in pore pressure, hydrothermal fluid migration, or slow aseismic slip, and they are increasingly recognized as markers of hydromechanical coupling within the crust. In volcanic settings, swarms are often linked to magmatic or geothermal processes; in tectonic environments such as Los Cabos, swarms may be induced by rainfall infiltration, local changes in groundwater level, or stress redistribution along critically stressed fault segments. Similar fluid-driven behavior has been documented in both natural and anthropogenic cases worldwide, including hydrothermal regions and fluid-injection zones. These observations emphasize the value of multiparameter monitoring—particularly the integration of seismic, hydrological, and atmospheric data—to better understand swarm initiation, evolution, and hazard potential (Passarelli et al., 2021; et al., 2013a; Terakawa et al., 2013b). Over the past decades, several studies have documented that intense rainfall can trigger or modulate seismicity in tectonically active regions. Husen et al. (2007) showed that an extreme rainfall event in the Swiss Alps triggered local seismicity through rapid water infiltration and the resulting increase in pore pressure, which facilitated the reactivation of pre-existing faults. Similarly, Rigo et al. (2008) analyzed the catastrophic storm of September 2002 in southern France and reported a marked increase in seismicity rate and transient wave-velocity variations, interpreted as evidence of stress-state changes caused by hydrological forcing. At a broader temporal scale, Johnson et al. (2017) modeled annual atmospheric and thermal loading cycles on California faults, showing that such cycles perturb background stress and modulate the temporal distribution of earthquakes. Together, these studies highlight the importance of considering hydrological-tectonic interactions when analyzing seismic swarms in arid regions and provide a solid conceptual framework for interpreting the observations presented in this work.
In Mexico, public and scientific attention has historically focused on the country’s large, destructive earthquakes—such as those of 1985 and 2017—which have shaped national awareness and disaster response frameworks. However, seismic swarms, both in volcanic and non-volcanic regions, have emerged as a growing challenge in recent decades. Episodes near volcanoes such as Popocatepetl, Tacana, and Chichon, as well as in tectonic settings such as Los Cabos, Chiapas, or Michoacán, demonstrate that swarms can generate sustained concern despite lacking a mainshock ( Jaimes and Suárez, 2025; Pacheco et al., 1999; Suárez and Jaimes, 2024). In the age of real-time communication, where social media platforms instantly amplify reports of even minor tremors, public perception can evolve rapidly—sometimes outpacing institutional responses. This dynamic underscores the need for a modernized approach to swarm monitoring and risk communication, one that is capable not only of detecting and interpreting seismic signals but also of engaging communities with transparency, scientific clarity, and empathy. In this context, initiatives that combine dense local networks, citizen participation, and multidisciplinary observation—such as the one described in this study—represent a crucial step toward closing the gap between geophysical knowledge and social resilience.
2. Network deployment and configuration
In June 2024, residents and workers along the Los Cabos tourist corridor began reporting a series of unusual “explosive” sounds and ground vibrations occurring during the early morning hours. While such sensations are not uncommon in this region—where ongoing urban development and construction activity frequently cause perceptible ground motion—these events stood out due to their timing, often between midnight and dawn, and their repetitive character. Initial reports, shared by concerned citizens through local networks and social media, were soon communicated to researchers at CICESE and the Instituto de Geofísica of the Universidad Nacional Autónoma de México (UNAM) and to various local institutions.
In response, the first phase of the investigation involved deploying a low-cost seismometer in private residences near the reported epicentral area. The goal was to characterize the waveform patterns of the detected signals and assess whether the source was anthropogenic or tectonic. Within a short time, the emergence of a coherent swarm pattern became evident: the signals exhibited typical swarm-like characteristics, including low magnitudes, high frequency of occurrence and lack of a dominant mainshock. Notably, this episode began exactly one year after a previous seismic swarm occurred in San José del Cabo albeit in a rural inland setting.
This time, however, the context was markedly different. The swarm unfolded in the heart of Mexico’s most heavily visited coastal corridor, where infrastructure density, economic significance, and public visibility are far greater. As a result, the event drew immediate attention from the public, authorities, and media, generating an unprecedented opportunity—and responsibility—to design and deploy a local seismic network with both scientific and social relevance.
By the end of September 2024, a total of five seismic stations had been deployed in the core swarm area (Figure 2), supplementing two preexisting stations located farther from the epicentral zone and therefore not optimal for swarm characterization. The installed network included a combination of low-cost and semi-professional instrumentation, chosen for rapid deployment, community integration, and waveform resolution. The central station (Figure 3), PALM (Palmilla), was equipped with a Güralp 40T broadband seismometer configured in HH [Z, E, N] channels (High Band, High Sensitivity, Channel), connected to a RJ (Raspberry Shake) digitizer and exported to national repositories via SeedLink2EW, coordinated by the National Seismological Survey (Servicio Sismológico Nacional-SSN) (Figure 3).
In addition to PALM (Palmilla), the other stations—LADE (Laderas SJC), PLAY (Playita), COLI (Colinas), and JOLL (La Jolla)—were configured with Raspberry Shake nodes, most operating with EH [Z, E, N] (Extremely short period, High-gain seismometer, [Z, E, N] components) or EHZ (Extremely short period, High-gain seismometer, Z vertical only) channel at sample rates ranging from 100 Hz. These units were primarily installed in private residences or small community centers with the support of local citizens, who also facilitated maintenance, connectivity, and real-time data sharing. All stations employed SeedLink2EW for integration with Earthworm-based acquisition systems, ensuring synchronized waveform archiving, event detection, and visualization. Despite modest instrumentation, the network demonstrated remarkable sensitivity and reliability, capable of detecting events below magnitude 1.0 and offering sufficient azimuthal coverage to constrain swarm hypocenters within the coastal corridor.
The greatest challenge, however, was to develop a system that—under very limited resources—could both alert the population and serve a valid scientific purpose, while also meeting the strict operational timelines imposed by national academic institutions, which often face federal administrative constraints limiting project duration. Moreover, the geographic isolation of the region made it unrealistic for a single institutional crew to handle the technical workload alone. The solution emerged organically through a collaboration with local citizens, who proposed hosting seismometers in their homes. These stations transmitted data to a central node in La Paz, from where the signals were relayed in real time via Earthworm, enabling the UNAM Seismological Service to leverage its full technical and human infrastructure for both data analysis and public communication. In this way, all parties benefited. Importantly, although researchers involved in this initiative were affiliated with multiple institutions—such as CICESE—the effort was conducted with professionalism, united solely by a shared commitment to scientific rigor and social responsibility.
3. Seismic catalog generation and data processing
The local seismic network recorded a total of 8008 earthquakes between August 1, 2024, and May 28, 2025, defining one of the most extensive swarm-type seismic episodes ever documented in the Los Cabos region. The seismic catalog was processed using waveforms streamed in real time via Earthworm and later integrated into the data analysis pipelines of the SSN. All event locations were computed using standardized regional velocity models, widely adopted by the SSN for crustal seismicity in Mexico. Despite the generalization inherent in such models, the solutions showed remarkably low location uncertainties, owing to the dense local network and high-quality arrivals. The average RMS error was 0.02 seconds, with a standard deviation of only 0.05, and most events had well-constrained depths around 10.3 ± 1.9 km, consistent with shallow tectonic activity along the crustal fault systems of the region.
The catalog exhibits a narrow magnitude distribution, with a mean magnitude of 1.77 (σ = 0.38) and a peak daily activity of 236 events/ day, highlighting the prolific and persistent nature of the swarm. As data accumulated, the spatial patterns of hypocentral distribution began to reveal distinct migratory behaviors and clustering along aligned structures, consistent with fluid-induced swarm mechanisms. It was during this phase of analysis—enabled by continuous, high-density data—that the true evolution of the swarm became visible, both in space and time. The transition from scattered epicenters to organized patterns was crucial for recognizing fault segment activation and temporal progression, features that would have remained obscure without such localized coverage. This catalog now serves as the base for ongoing waveform analysis, b-value tracking, and stress-inversion modeling efforts.
4. Observation of the hydrometeorological event
Meteorological conditions during 2024 were notable for their overall dryness, even within the context of the naturally arid desert climate of the Los Cabos region (Figure 4). While the area typically receives significant precipitation during the tropical cyclone season (May15-November30), thatyearwasexceptionally calm, with no hurricanes making landfall. Only a single noteworthy rainfall event occurred, during which over 47 mm of precipitation fell within approximately three hours—a volume well above average for such a short interval. This isolated event stands out not only for its intensity but also for its potential hydrological implications, particularly in triggering pore pressure changes in fractured crustal zones. Throughout the rest of the year, air temperatures remained remarkably stable, fluctuating seasonally between 19°C and 29°C, with only brief excursions, such as an isolated high of 35°C just days prior to the rainfall event. Wind speeds were also modest under normal conditions; however, on the day of the intense precipitation, wind gusts exceeded 35 km/h, aligning with the passage of a tropical system that remained offshore. Although this system did not qualify as a direct cyclone impact, its meteorological influence was sufficient to produce extreme localized rainfall, providing a valuable opportunity to examine the coupling between atmospheric forcing and seismic swarm activity.
Around 14 to 20 days after the intense rainfall, a new cloud of seismicity emerged west of the San José fault, at the Los Tules region, displaying notably shallow depths and a clear westward spatial migration (Figure 5c). This migration pattern also showed a slight upward trend in depth, culminating over approximately 20 days. Although no significant events followed this sequence, a considerable number of earthquakes continued to occur at various depths and times thereafter. The behavior observed strongly suggests a diffusive process, where the fluid pressure perturbation propagated through fractured pathways, driving late-stage seismicity. This hypothesis is reinforced by the spatial distribution and depth variability of the later events, especially those in the southwestern sector (Figures 5d-5e), which warranted a more detailed seismic analysis, including refined hypocentral locations and b-value estimations. These findings emphasize the importance of recognizing hydro-mechanical feedback in swarm dynamics, particularly in regions where localized rainfall may act as a natural trigger for fault reactivation. The temporal progression of the Los Cabos swarm reveals a highly episodic and clustered behavior, inconsistent with a homogeneous Poissonian process. Instead of a uniform distribution over time, the seismicity is marked by clearly defined bursts of activity interspersed with periods of near-quiescence, sometimes lasting several days (Figure 5). Before September 14, 2024, a total of 1391 events were recorded, with a mean depth of 10.71 km, indicating early-stage swarm nucleation. After that date, the swarm entered a more intense phase, with 6617 additional events occurring between September 14 and May 2025, at a slightly shallower average depth of 10.24 km. Notably, a surge in seismicity was observed immediately after October 3–5, with 4493 events post–October 3 and 4130 post–October 5, marking one of the most active clusters in the entire sequence (Figure 5d).
Spatially, the southwestern segment of the swarm area—defined by latitudes below 23.10°N and longitudes west of −109.70°—concentrated most of the seismicity, accumulating 7,639 events with an average depth of 10.40 km. The dense daily event distribution peaked at 236 earthquakes in a single day, yet this intense activity was interrupted by calmer phases, illustrating the non-stationary character of the swarm. Rather than decaying monotonically or behaving stochastically, the swarm appeared in episodic bursts of seismicity, each separated by relatively quiet intervals.
A focused analysis of the diffusive zone (as outlined by the Los Tules region in Figure 5) reveals an anomalously high distribution of b-values, with estimates consistently falling within the range of 1.8 to 2.6 (Figure 6b). The preferred magnitude of completeness for this subset was Mc = 2.0, with a characteristic b-value near 2.0, significantly elevated when compared to the typical values observed in tectonic environments. When contrasted with a benchmark b-value of 1.0—commonly associated with tectonic fault reactivation under stress-controlled regimes (Figure 6d)—the deviation is striking. Such elevated b-values strongly suggest a non-tectonic triggering mechanism, inconsistent with traditional models of stress accumulation and release along active faults.
Although previous studies have proposed b ≈ 2.3 as a possible signature of diffusive processes (Smirnov et al., 2022), this value of 2.3 likely arises from a possible misinterpretation of the b-parameter within the Darcy–Richards framework, where b has a diffusive meaning. What is well established is that b is significantly greater than one; a precise value of 2.3 still requires stronger theoretical support; there is no single threshold b-value that universally characterizes diffusion. Rather, what distinguishes these episodes is the fact that their b-values are significantly higher than those expected in tectonically dominated regions. The observed b = 2.0 is substantially higher than typical tectonic environments and, when considered alongside other diagnostic features—such as the clear spatial migration of events, their temporal alignment with intense rainfall, the generation of seismicity at extremely shallow depths, and the pattern consistent with pressure diffusion from a surface or near-surface Darcy-type front—provides compelling evidence for the role of overpressurized fluid propagation (Hummel and Shapiro, 2013; McKernon & Main, 2005; Zhai et al., 2019). These fluids appear to follow diffusion-controlled pressure gradients, giving rise to a temporally bounded seismic episode that can be quantitatively interpreted via space-time distributions measured from the initial pressure front location (Brown and Ge, 2018). Together, these features form a coherent and irrefutable signature of fluid-driven swarm behavior, distinguishing this zone from purely tectonic sources (Chen et al., 2012; Zhai et al., 2019).
In Figure 7, the seismic events are plotted regarding both depth and distance from the assumed pressure front origin, illustrating the spatial and temporal evolution typical of sismo-hydrological diffusion fronts. The graph also includes a time–distance relationship commonly associated with pressure diffusion in porous media. A clear concentration of seismicity near the origin point is evident, with a notable tendency for events to occur at slightly shallower depths than the initial nucleation level. Over time, the seismicity progressively migrates outward, reaching greater distances from the source, yet short-distance events continue to occur simultaneously.
This behavior represents the canonical signature of a diffusion-controlled process (Do Nascimento et al., 2005): as pressure propagates through a permeable medium, the highest seismic response typically remains close to the pressure front, while diffusion drives an expanding envelope of activation. The persistence of activity at short distances, coupled with systematic migration outward, reflects a stable and approximately constant diffusivity (D)—a hallmark of hydromechanical triggering. These observations reinforce the interpretation that the swarm dynamics in this zone are governed by fluid pressure diffusion, consistent with models where seismicity is modulated by a Darcy-type expansion of overpressured fluids within fractured rock volumes.
Figure 8 presents several modeled diffusion envelopes corresponding to different constant values of hydraulic diffusivity (D), overlaid on the spatial–temporal distribution of seismic events. Among these, a value of approximately 1.7 m²/s provides the best visual fit to the leading edge of the swarm migration, suggesting it represents a reasonable estimate of the effective diffusivity controlling the evolution of this seismo-hydrological system. The coherence between this envelope and the observed migration of seismicity, both in time and distance from the origin point, supports the interpretation of the swarm as being driven by fluid pressure diffusion through a fractured and permeable medium.
This value is consistent with those reported in similar studies of fluid-triggered seismicity in tectonic and volcanic contexts. For instance, (Shapiro et al., 2005) estimated diffusivities in the range of 0.2 to 2.0 m²/s for swarms associated with fluid injection and natural hydrothermal systems. Likewise, (Terakawa et al., 2013b) found diffusivities on the order of 1–10 m²/s in crustal fault zones affected by fluid migration. The convergence of our findings with these values reinforces the notion that the Los Cabos swarm represents a textbook case of seismicity modulated by pore pressure diffusion, with a diffusivity compatible with fractured upper crustal conditions. This adds to the growing body of evidence that diffusive processes play a central role in non-tectonic swarm generation, even in regions with no active volcanism or anthropogenic fluid injection.
5. Discussion
The seismic swarm observed in the Los Cabos region between August 2024 and May 2025 presents a clear example of non-tectonic, fluid-driven seismicity, both in its temporal behavior and spatial distribution. Although this activity was initially classified as a swarm, ongoing monitoring is needed to better define its nature. In October 2025, a probable Mw ≈ 5.3 earthquake occurred in the same area and was followed by many smaller events. The conventional distinctions used to define seismic swarms become less useful. However, because these episodes occur in the same location but are separated by about a year—and each year shows very different rainfall conditions—it is not straightforward to decide whether the behavior corresponds to a true swarm or to a mainshock-related sequence. Even so, several spatial and temporal patterns repeat from year to year, indicating that the activity is part of a recurrent process influenced by hydrological conditions rather than a single, isolated seismic episode.
The dense local network—deployed through an exceptional citizen–scientist collaboration—went far beyond the usual logistical support that accompanies temporary seismic deployments. In this case, it was the local citizens themselves who strongly advocated for the establishment of the network, participated actively in selecting the sites, provided shelter and infrastructure for the instruments, and even contributed to counting and identifying earthquakes during the monitoring period. Their involvement not only sustained the network’s operation but also shaped its evolution and focus, ultimately encouraging us to document these efforts formally in this article. This level of participation transformed what could have been a conventional temporary network into a true community-driven initiative, exceeding by far the common notion of ‘assistance’. This network enabled the detection of more than 8000 events, many with magnitudes below 2.0, providing a rare level of resolution for swarm analysis in a highly populated, economically sensitive corridor. The temporal clustering, depth migration, and southwest concentration of hypocenters are all consistent with transient changes in pore pressure, rather than stress accumulation from tectonic loading.
The analysis of b-values across the swarm area revealed significantly elevated values, particularly within the polygonal region identified as the core of a diffusive pressure front. With b-values consistently between 2.6 and 2.8 and a preferred estimate around 2.0 for Mc = 2.0, the swarm contrasts sharply with tectonic environments, where b ≈ 1.0 is typically observed. This deviation reinforces the interpretation of a fluid-controlled process, likely governed by pressure diffusion through a fractured medium. The classic space–time diffusion signature—early clustering near the source, followed by radial migration of activity—was observed clearly after the intense rainfall event, further suggesting that surface recharge may have acted as a triggering mechanism. The comparison to other studies reporting diffusivities in the range of 1–2 m²/s lends credibility to the modeled estimate of D ≈ 1.7 m²/s, as presented in Figure 8.
From a broader perspective, this study highlights the scientific and societal importance of understanding seismic swarms beyond their immediate magnitudes. Despite the absence of damaging earthquakes, the swarm raised substantial concern among residents and stakeholders, illustrating how low-magnitude seismicity can generate high levels of social anxiety, especially when it occurs in urbanized or economically strategic regions. The collaborative model of deploying instruments in private homes, supported by real-time streaming to national observatories, offers an ethically sound and operationally effective strategy for rapid response and scientific investigation. Most importantly, the results emphasize that rainfall-induced seismicity in non-volcanic arid regions is not an anomaly but a legitimate geophysical process that deserves deeper integration into hazard assessment frameworks, particularly under changing climatic conditions that may intensify extreme precipitation events.
6. Conclusions
1. The Los Cabos 2024–2025 seismic swarm represents a clear example of non-tectonic seismicity, driven by transient hydrological forcing. This interpretation does not imply that deformation occurs outside a faulted region, but rather that the major fault adjacent to the swarm has shown no tectonic activity for at least three decades. In contrast, such regional faults are well known to act as efficient hydrological barriers and conduits. In this context, the observed microseismicity is better explained by fluid-pressure variations within a structurally inherited but currently inactive fault zone, rather than by ongoing tectonic loading. The spatial–temporal distribution of events, migration patterns, and elevated b-values collectively support a fluid-driven pressure diffusion model, distinct from typical tectonic stress-release mechanisms.
2. The onset of seismicity approximately two weeks after a high-intensity rainfall event, with over 47 mm of precipitation in under three hours, reinforces the interpretation of a sismo-hydrological triggering process. The post-rainfall cluster west of the San José fault exhibited upward migration, shallow depths, and time–distance behavior characteristic of a diffusive front.
3. Statistical analysis revealed exceptionally high b-values within the core diffusive zone, ranging from 2.6 to 2.8, with a representative value of 2.0 for Mc = 2.0. These values deviate significantly from tectonic norms (b ≈ 1.0) and align with theoretical expectations for fluid-induced microseismicity. A hydraulic diffusivity of D ≈ 1.7 m²/s was estimated, consistent with literature on natural and induced seismic diffusion in fractured crust.
4. The seismicity exhibited episodic, non-Poissonian behavior, with intense clusters followed by relative quiescence, highlighting the need for models beyond stationary statistical assumptions. The swarm’s evolution reflects dynamic changes in subsurface pore pressure, possibly modulated by rainfall infiltration and lithological contrasts.
5. Citizen participation was critical to the success of this study. Local volunteers hosted seismometers in private residences, enabling rapid deployment and dense spatial coverage far beyond what institutional constraints would typically allow. This collaborative model improved the scientific outcome and represents a best-practice example of ethical and community-integrated science, surpassing response capacities seen in some developed nations with greater institutional resources.
6. The findings have implications for hazard awareness and public policy in arid, rapidly urbanizing regions exposed to intense rainfall. Low-magnitude, fluid-triggered swarms—though not destructive—can cause significant social concern and require better integration into seismic monitoring strategies, particularly under the influence of climate-driven hydrometeorological extremes.
Contributions of authors
(1) Conceptualization: RO; (2) Data analysis or acquisition: RO; (3) Methodological/technical development: RO, LQ; (4) Writing – original draft: RO, DC; (7) Fieldwork: RO, ROT, GTP, IL, JLE, FEP, GN, EF; (8) Interpretation: RO; (9) Funding acquisition: DC; (10) Other: Satellite Data Transmission: JPS; Seismic Location: VHE.
Funding
Financial support for this research was provided through the grants CF-2023- G-958 and 319664 from SECIHTI, as well as the “Investigadores por México” Project 1220.
Acknowledgments
We thank Drs. M. J. Schmitz and J. M. Gómez-González for their kind comments, which added valuable improvements in both quality and presentation during the review process. We gratefully acknowledge the support of the Civil Protection of Los Cabos. We also extend our thanks to the Coordinator of CICESE UALP and the members of the Seismology Department for the truly unparalleled experience of having to find on our own external channels of support —an adventure that, in its own special way, contributed to the eventual success of this project. The principal author RO gratefully acknowledges the Institute of Earth Physics of Bucharest for hosting a research stay during which this article was written.
Conflicts of interest
The authors declare that there are no conflicts of interest of any kind related to the content of this article.
Handling editor
Raúl Castro Escamilla.
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