The interaction among the Cocos, Nazca, and Panama plates at the Panama Triple Junction (PTJ) generates diverse seismic sources that remain poorly characterized. Using data from the National Seismological Network of Costa Rica and the Chirinet network of Panama, we analyzed four recent earthquake sequences (Mw 5.6-6.2, 2019-2021) near the Burica Peninsula and relocated 32 significant events (Mw 5.6-6.7) that occurred between 2000 and 2025. The results reveal: (1) right-lateral strike-slip faulting along the Panama and Balboa fracture zones; (2) right-lateral faulting on their inland projections, including the 2019 Mw 6.0 Canoas Fault earthquake; (3) reverse faulting during the 2008 (Mw 5.6) and 2020 (Mw 5.6) North Burica sequences, associated with the Media Fault Zone of the upper Panama Plate; (4) left-lateral strike-slip faulting within the subducting Cocos Plate, where at least five Mw 5.8-6.5 events occurred between 2002 and 2019; and (5) a seismogenic Cocos-Panama interplate zone extending southeast of the Burica Peninsula. We further reinterpret the 1934 Ms. 7.6 Armuelles tsunamigenic earthquake as a large megathrust rupture on this interplate boundary. Together, these findings refine the geometry of the seismic sources near the PTJ improving our understanding of the tectonic processes shaping the region.

The dominant forces shaping the unique geometries of flat slabs are still not fully understood. Knowing how the stress field changes with respect to the shape of the slab allows inferences of the dominant forces acting on the slab. In this study we calculated new models of the slab geometry and the intraslab stress field in the Pampean flat slab region of the Chile-Argentina Subduction Zone (latitude similar to 25 degrees 36 degrees S) where the Nazca Plate subducts together with the aseismic Juan Fernandez Ridge. To build the models, we used a catalogue of 1 059 well-located slab earthquakes recorded by the SIEMBRA and ESP temporary seismic arrays and calculated 411 new focal mechanisms that were analysed together with 407 focal mechanisms from other catalogues. Our results confirmed slab seismicity features such as a reverse dip (i.e. opposite to the subduction direction) of the seismicity band within the flat slab, two bands of descending seismicity and two regions with an absence of earthquakes. These seismicity patterns express the shape of the slab and its hydration state, with more localized slab dehydration along the inland path of the Juan Fernandez Ridge relative to the surroundings. In one of the regions without earthquakes, the slab is most likely continuous and dry, while in the other one the slab is missing, in agreement with previous works that proposed a hole in the slab visible with other methods. A comparison between the stress field and the local slab dip from both our new model and a previous one (Slab2) indicates that the dominant forces acting on the flat slab are the slab pull and the ridge buoyancy. Finally, the shape of the flat slab is controlled by the geologic migration of the Juan Fernandez Ridge, making the flat slab four times wider than the ridge offshore, and by the competing forces of the slab pull and the ridge buoyancy that creates a notable flexure (bulge) resembling the geometry of the outer rise near the trench.

Este estudio presenta 139 nuevos mecanismos focales elaborados en el marco del resumen anual de la sismicidad del año 2024. En total, la Red Sismológica Nacional (RSN) localizó 5378 eventos, cuya energía conjunta equivale a un sismo de magnitud momento (Mw) 6,6. Solo el ~3,8 % (204) del total de sismos fue percibido por la población. Los mecanismos focales y las localizaciones de estos eventos permitieron agrupar su origen en cinco categorías: fallamiento en las placas cabalgantes Caribe y Panamá, zona interplacas de la subducción de la placa Coco, deformación interna de la placa Coco subducida, fallamiento del límite entre las placas Coco y Nazca y fallamiento de la placa Coco previo a la subducción. Los sismos superficiales (< 35 km) en las placas cabalgantes, al igual que los asociados con el límite Coco-Nazca, ocurrieron principalmente en fallas de desplazamiento de rumbo, mientras que los relacionados con subducción fueron de tipo inverso. Los diez sismos sentidos más relevantes en 2024 alcanzaron magnitudes Mw 5,5-6,2. La intensidad máxima observada fue de V, en áreas muy reducidas cerca de los epicentros de seis eventos (Mw 4,9 y 6,2).

The subducted Nazca plate flattens at ~100 km depth beneath the south-central Andes (29˚S-32˚S), known as the Pampean flat slab. This flat slab results in a volcanic gap, active basement-cored uplifts of the overriding South American Plate, and a high rate of crustal seismicity at depths below 50 km distinct from the subducting slab seismicity. Previous studies suggest extensive slab dehydration and partial eclogitization of the overriding plate’s lower crust. In this study, we analyze seismic data recorded by the SIMEBRA and ESP projects (deployed between 2007 and 2010) with a modified spectral decomposition method to constrain the source spectra of hundreds of earthquakes in the study area. We further estimate the earthquakes’ corner frequencies and stress drops, revealing similar stress drops between the slab and overriding-plate events, with little depth dependence in the region. In addition, we observe low stress drops in the slab at ~150 km depth, coinciding with a low-velocity and high Vp/Vs zone, potentially indicating slab dehydration.

En esta investigación se estudian las fallas Jaris y Candelaria, en el antearco de Costa Rica, a partir de la fotointerpretación, el trabajo de campo y la relocalización de sismos registrados por la Red Sismológica Nacional de Costa Rica. Presentamos evidencias que confirman que ambos sistemas son de tipo dextral y de rumbo noroeste-sureste. Además, determinamos una longitud total de 35 y 65 km, para los sistemas Jaris y Candelaria, respectivamente, lo que implica un potencial sísmico de magnitud momento (Mw) 6.9 y 7.2. Postulamos que el Terremoto de Damas del 2004 (Mw 6.4) está asociado con el sistema Candelaria y, además, que estas fallas, junto con otras fallas dextrales del centro del país facilitan el escape tectónico del Bloque Antearco Centroamericano desde una zona mucho más al sureste de lo propuesto anteriormente. Debido a su cercanía con la Gran Área Metropolitana de San José (15-35 km), las fallas estudiadas son peligrosas para la zona más poblada de Costa Rica.

Central America is a seismically active region located in a tectonic setting dominated by the subduction zone between the Cocos and Caribbean plates, transform boundaries between the North American and Caribbean plates, and local or crustal faulting with some of the most important fault systems aligned with the volcanic arc. Combining seismic data from various Central American seismic agency catalogs covering the period from 1520 to 2020, we present an updated regional earthquake catalog for the region. Fourteen databases containing seismic events from local and regional agencies were collected for different time periods, homogenized to moment magnitude (Mw), and subsequently unified using a prioritization criteria approach. We analyzed to the data to identify and remove duplicate earthquakes, prioritizing records with the lowest RMS value, depth consistent with their location based on the region’s crustal thickness, and magnitudes in accordance with historical reports or bibliographic sources. Additionally, significant seismic events (Mw ≥ 6.0) were carefully reviewed based on their epicentral locations and magnitudes, according on reliable publications. The earthquake catalog compiled includes a total of 260 548 earthquakes, for which we conducted a descriptive, spatiotemporal statistical analysis, as well as estimations of the magnitude of completeness (Mc) and declustering. Among the most important results, we highlight recent completeness periods for magnitudes Mw < 5.0. Geographically, seismic zones with better Mc are directly related to either good seismic network coverage or high seismicity rates in the region. As regards declustering, the Reasenberg declustering method considers several main shocks with 76

Los 153 sismos sentidos en Costa Rica durante el 2023 pueden agruparse en cuatro categorías de acuerdo con su origen: fallamiento en las placas cabalgantes Caribe y Panamá, zonas interplacas de la subducción de Coco y Caribe, deformación interna de la placa Coco subducida y fallamiento del límite entre las placas Coco y Nazca. La sismicidad sentida representa solo el ~3 % del total de 4438 eventos localizados por la Red Sismológica Nacional (RSN) en el 2023, los cuales, equivalen en conjunto a la energía liberada por un terremoto de magnitud momento (Mw) 6,6. Los sismos ocasionados por fallamiento superficial ocurrieron en fallas de todos los tipos y geometrías, mientras que los ocasionados por subducción fueron de tipo inverso y los asociados con el límite Coco-Nazca de desplazamiento de rumbo. Los mecanismos focales en los sismos por deformación interna de la placa Coco fueron oblicuos con componentes normales o inversas. Los sismos sentidos de mayor tamaño del 2023 fueron cuatro eventos de Mw 6,4-6,5 con epicentros en países vecinos, por lo que se percibieron de forma leve en Costa Rica. La intensidad máxima observada en el 2023 fue de VI, debido a dos sismos (Mw 5,5 y 5,2) originados por fallamiento en las placas cabalgantes.

Nestled between the Cocos, Nazca, Caribbean, and South American plates, the Panama microplate represents an area of rapidly evolving tectonics throughout the past ~10 m.y. Past and current studies have observed a notable amount of seismicity throughout this region, in particular the Caribbean coast of Costa Rica, which experienced a Mw 7.7 earthquake in 1991 CE. We investigated the crust and upper mantle structure of this region using the receiver function methodology and report two results: (1) first-order lateral constraints on the position of the Panama microplate boundary near the intersection between the Central Costa Rica Deformed Belt (onshore) and North Panama Deformed Belt (offshore), and (2) an impedance contrast south and east of these belts, supporting that the Caribbean plate currently subducts beneath the Panama microplate. Observed local seismicity is a consequence of the recently (ca. 14 Ma) initiated Caribbean plate subduction beneath the overlying Panama microplate. Our results are also consistent with a doubly convergent subduction margin dominating southern Costa Rica tectonics, uplifting the Talamanca Cordillera, and causing the cessation of southern Costa Rica volcanism over the past ~10 m.y.

AB4STRACTCosta Rica is located at the boundary of four tectonic plates where the regularity of destructive earthquakes highlights the necessity of seismic hazard estimations. This study contains the most recent Probabilistic Seismic Hazard Assessment (PSHA) for Costa Rica, calculated with the largest and the most updated earthquake database from both—the Earthquake Engineering Laboratory and the National Seismological Network of the University of Costa Rica. For the PSHA, we updated the seismicity parameters for the upper plate, subduction interplate, and intraslab tectonic domains, characterized the upper-plate zones by percentages of fault types, and used weighted ground-motion models for each of the tectonic domains. The resulted maps of peak ground acceleration (PGA) at return periods of 475 yr (PGA-475) and 2475 yr, as well as the spectral accelerations, show geographic trends that allow for the division of the country in four seismic hazard levels: (1) extremely high for the Nicoya, Osa, and Burica peninsulas, situated directly above the subduction interplate, where the PGA-475 could be 0.55–1.20g; (2) very high for most of the Guanacaste Province, where the PGA-475 may be 0.55–0.70g; (3) high for most of the country (∼41%) with PGA-475 values of 0.40–0.55g, including Central Costa Rica and the capital city of San Jose; and (4) moderate for the Talamanca Cordillera and Northern Costa Rica, with PGA-475 up to 0.40g. These ground-motion values are 0.1–0.6g higher than the previous PSHA for the Pacific peninsulas, Guanacaste, and the southeastern Caribbean. Further, hazard curves, uniform hazard spectra, and a hazard disaggregation indicate that the seismic hazard is lower but more complex in San Jose than in Liberia—the largest city in Guanacaste.

Durante el año 2022, la Red Sismológica Nacional (RSN) localizó 4763 sismos locales, lo cual equivale a la energía sísmica liberada por un terremoto de magnitud momento (Mw) 7,0. Los epicentros se concentraron principalmente en nueve zonas con 25-400 temblores. Se documentaron 184 sismos (~4 % del total) sentidos por la población, incluyendo 19 con Mw entre 5,0 y 6,7. La mayoría de estos eventos fueron superficiales (~87 % < 30 km), de magnitud baja (~52 % Mw < 3,9) y fueron originados en fallas de las placas Caribe y Panamá (~52 %). El sismo de mayor tamaño del año, de Mw 6,7, se originó por la subducción de la placa Nazca y generó la intensidad más alta reportada, de V en Burica. Todos los sismos de Mw ≥ 5,5 tuvieron sus epicentros mar adentro y alejados de centros poblados, por lo que no se reportaron intensidades altas.

A noticeable decrease in seismic noise was registered worldwide during the lockdown measures implemented in 2020 to prevent the spread of COVID-19. In Central America, strong lockdown measures started during March of 2020. In this study, we used seismic stations from Costa Rica, Guatemala, El Salvador, and Nicaragua to study the effects of these measures on seismic records by characterizing temporal variations in the high-frequency band (4–14 Hz) via spectral and amplitude analyses. In addition, we studied the link between the reduction in seismic noise and the number of earthquake detections and felt reports in Costa Rica and Guatemala. We found that seismic stations near the capitals of Costa Rica, Guatemala, and El Salvador presented a decrease in their typical seismic noise levels, from 200 to 140, from 100 to 80, and from 120 to 80 nm, respectively. Our results showed that the largest reduction of ∼ 50 % in seismic noise was observed at seismic stations near main airports, busy roads, and densely populated cities. In Nicaragua, the seismic noise levels remained constant (∼ 40 nm), as no lockdown measures were applied. We suggest that the decrease in seismic noise levels may have increased earthquake detections and the number of felt reports of low-magnitude earthquakes. However, the variations observed in several seismic parameters before and after the lockdown are not significant enough to easily link our observations or separate them from other contributing factors. Our results imply that the study of seismic noise levels can be useful to verify compliance with lockdown measures and to explore their effects on earthquake detection and felt reports.

La Red Sismológica Nacional (RSN) localizó 3745 sismos locales durante el año 2021, lo cual es equivalente a la energía sísmica liberada por un evento de magnitud (Mw) 6,9. La sismicidad se localizó principalmente en siete zonas con 50-350 temblores. Hubo 209 sismos (~6% del total) sentidos por la población, incluyendo 31 con Mw entre 5,0 y 6,7. La mayoría de estos sismos fueron superficiales (~88% < 30 km), de magnitud baja (~52% Mw < 3,9) y fueron originados en fallas de las placas Caribe y Panamá (~69%). El sismo mayor del año, de Mw 6,7, se originó en la Zona de Fractura de Panamá, sin generar daños ni víctimas. Todos los sismos de Mw ≥ 5,5 tuvieron sus epicentros mar adentro y alejados de centros de población por lo que no se reportaron intensidades altas.

Between August and November 2018, a seismic sequence took place in the vicinity of Golfito, a city in the Dulce Gulf in Southeastern Costa Rica. The main shock had a moment magnitude (Mw) of 6.1 and was widely felt in Costa Rica and Western Panama, with maximum Modified Mercalli intensities of VI. In this region, the oceanic Cocos Ridge, riding on top of the Cocos Plate, subducts beneath the Panama Microplate. Using the seismic records from the National Seismological Network of Costa Rica, in this work the seismicity is relocated using the double-difference technique, and an analysis of its temporal and geographic distribution together with the focal mechanism and intensities of the strongest events are presented. The results show that the sequence occurred at the interplate seismogenic zone, within the rupture area of the 1983 Golfito earthquake (7.4 Mw), between 12 and 27 km depth, in a cluster dipping 35º northeast underneath the Dulce Gulf. Based mainly on these results and on previous seismic sequences, it is here proposed that the seismogenic zone in Southeastern Costa Rica has an extension of ~160 x 45 km. Further, during the Golfito sequence, the rupture of an inverse fault (5.9 Mw) took place within the Cocos Plate beneath the Dulce Gulf, as well as of dextral strike-slip faults (4.6-5.6 Mw) in the Panama Microplate, 50 km away of the Dulce Gulf. The analysis of the interseismic interplate seismicity contributes to a better understating of the dynamics of the seismogenic zone. This is of particular relevance in Southeastern Costa Rica, where at least six damaging earthquakes of Mw > 7 have occurred since 1803, implying the impending risk of the next big earthquake in this region.

Costa Rica is a seismically active region located in a subduction zone. Combining earthquake data from the National Seismological Network and several Central American catalogs from 1522 to 2020, we present a new estimation for the Gutenberg-Richter relationship. The seismic catalog compiled with similar to 122,000 earthquakes was studied by calculating the magnitude of completeness (Mc) and by applying a space-time window declustering method. The residual catalog and a previously proposed seismic zonation were used to determine the b-value, the maximum magnitude (Mmax), and the mean recurrence interval (MRI). Our results show a temporal and geographic variation in the Mc, decreasing from 7.0 since 1793 to 3.0 since 1995, and with the lowest estimate (2.5) in Central Costa Rica. The b-values of 0.85 for the entire catalog and similar to 0.83 for the interplate zones, are similar to other regions worldwide with young subducting slabs. There is a general trend of higher and more variable b-values among the upper-plate (average 0.90) and intraslab (1.14) zones as compared to the interplate regions (0.85). The upper-plate variability is explained in terms of the diversity in geological units and faulting style, whereas in the interplate and the subducting slab is connected to the stress level imposed by different seafloor morphologies and hydration states along the margin. Our data suggest a seismic potential of moment magnitude (Mw) 7.9-8.0 for the Nicoya and Southern Nicaragua interplate zones and for the Limon region. The Gutenberg-Richter distribution shows that a Mw 7.0 has a longer MRI for the intraslab zones (similar to 72 years) than for the subduction interplate region (similar to 15 years) and plate and microplate boundaries (similar to 40 similar to 45 years). Interplate earthquakes have occurred recently (1991 Mw 7.7 and 2012 Mw 7.6) but no major intraslab since 1948 (Mw 7.0).

El Caribe Sur de Costa Rica fue en 1991 el escenario del ultimo gran terremoto (Mw 7,7) entre la placa Caribe y la microplaca de Panama. Pese a la alta sismicidad y avances en la cobertura de las redes sismicas, esa zona carece de imagenes tomograficas detalladas. Usando los paquetes VELEST y SIMULPS, presentamos los modelos de velocidad unidimensional y tridimensional resultantes de la inversion de 1208 sismos registrados entre 1998 y 2020 por la Red Sismologica Nacional (RSN). Esta nueva tomografia muestra una franja de bajas velocidades que se inclina desde el Caribe hacia el suroeste hasta los 50 km de profundidad debajo de Talamanca, incluyendo el hipocentro del terremoto de Limon y sismicidad hasta los 30 km. Se interpreta esta configuracion como la subduccion de la placa Caribe debajo de la microplaca de Panama y el terremoto de Limon como un evento de zona sismogenica interplacas. La geometria determinada proporciona un nuevo marco para interpretar la compleja tectonica del sureste de Costa Rica.

We analyze the performance of a prototype earthquake early warning system deployed at the National Seismological Network of Costa Rica in collaboration with the Swiss Seismological Service by presenting the real-time performance during six earthquakes (Mw 5.1-6.4) that took place during 2018 and 2019. We observe that, despite only limited efforts to optimize the existing network of 158 stations, for EEW purposes, the network density allows fast determination of source parameters using both the Virtual Seismologist and the Finite Fault Rupture Detector algorithms. Shallow earthquakes on or near-shore are routinely identified within 11–20 s of their occurrence. The warning times for the capital city of San Jose are of 43 s for epicenters located at 220 km, like for the Mw 6.4 Armuelles earthquake. On the other hand, during the time analyzed, the EEW system did not provide positive warning times for earthquakes at distances less than 40 km from San Jose. Even though large (Mw > 7) distant historical earthquakes have not caused heavy damage in San Jose, there is potential for developing an EEW system for Costa Rica, especially for the purposes of rapid earthquake notifications, disaster response management, and seismic risk mitigation.

Data sources, details of data analysis methodology, and additional diagrams and maps of shear wave splitting measurements.