Abstract Earthquake early warning (EEW) systems that deliver real-time alerts to the public are now operating in Guatemala, El Salvador, Nicaragua, and Costa Rica. An independent system is operated by national seismological agencies in each country. The EEW systems were developed through the Alerta Temprana de Terremotos en América Central (ATTAC) project, a multi-year collaboration (2016–2024) led by ETH Zurich and funded by the Swiss Development Agency. These landmark systems are unique as they provide operational EEW in several low- and middle-income countries. Each system uses the same ETHZ-SED SeisComP EEW software that implements the virtual seismologist (VS) and finite-fault rupture detector (FinDer) algorithms. The national seismic monitoring infrastructures are supported by 72 EEW-ready strong-motion stations. Public alerts are disseminated primarily via a mobile application that has over one million users. EEW performance is regionally consistent due to shared real-time seismic data and common system architecture. Our evaluation period started when the software was installed and included periods of network and software optimization. The VS algorithm achieved an overall accuracy of 89%. Including FinDer reduced accuracy to 77%, mainly due to false positives in Nicaragua and Costa Rica that occurred early in the evaluation window. First alerts are generally available 10 s after origin time for shallow onshore events, with location and magnitude errors typically below 30 km and 0.5 units, respectively. Mobile alerting has proven effective in providing low-latency alerts to increasingly large numbers of users, with most users receiving red alerts within 2–8 s after issuance, though some variability can occur. Maintaining and enhancing EEW in the region requires sustained investment in seismic infrastructure, trained personnel, and dissemination solutions that, for the short term at least, will include the mobile application. Long-term viability depends on stable governance, consistent funding, and strengthened and coordinated public guidance on how to respond to alerts.

Precise position and geometry of the subducted slab are crucial for deciphering subduction zone volcanism. This is particularly important in the southern Costa Rican subduction zone, where the Cocos Ridge subducted at similar to 2-3 Ma, coincident with the uplift of the Talamanca Range but later than the arc volcanism cessation (similar to 5-8 Ma). Here, we apply a refined Dip Direction Searching method to teleseismic receiver function waveforms from 17 broadband stations in this region. A northeast-dipping interface is imaged deep to similar to 110 and similar to 60 km in the northwest and southeast of the subduction zone, respectively, interpreted as the Moho of the Cocos slab that has possibly undergone tearing. A flat interface is identified at similar to 40-60 km depth exclusively beneath the Talamanca Range, which is most likely the Moho of the Nazca plate that has stagnated there since similar to 8 Ma, blocking melt upwelling and consequently stopping the arc volcanism. Plain Language Summary Subduction zones represent the most active plate boundaries on Earth, hosting most global volcanic eruptions and large earthquakes. In the southern Costa Rican subduction zone, the arc volcanos abnormally ceased eruption similar to 5-8 million years ago, spatially coincident with the Talamanca Range uplift and the arrival of the Cocos Ridge to the trench (since similar to 3-4 million years). However, the tectonic events are incompatible in time with the volcanic cessation, making the mechanism of volcanic cessation still enigmatic. Here we use seismic data from 17 seismometers to constrain the geometry and position of the Cocos slab. Our results show a pronounced along-strike difference in the Moho depth of the Cocos slab: reaching similar to 110 km depth in the north whereas only similar to 60 km depth in the south, potentially indicating lateral slab tearing. We also identify a flat interface at similar to 40-60 km depth exclusively beneath the Talamanca Range. Combining with previous studies, we interpret this interface as the Moho of the Nazca plate emplaced above the subducting Cocos slab. This stagnant Nazca plate may act as a physical barrier, impeding the mantle magma ascent and therefore halting the volcanism.

Slow slip events (SSEs) release tectonic strain without causing sudden ground shaking. SSEs have been observed at many subduction zones, some dynamically triggered by stress changes due to the passage of seismic waves. However, there are limited observations of SSEs induced by post-seismic deformation. Here, we report a significant increase in the recurrence rate of SSEs in the shallow portion of the Nicoya megathrust following the 2012 M-w 7.6 earthquake. These shallow SSEs occurred immediately updip of the large afterslip zone and their recurrence rate returned to pre-earthquake level 1.5 years after the earthquake. In contrast, deeper SSE recurrence rate remained unchanged. Coulomb Failure Stress modeling indicates the shallow SSE area experienced substantial stress perturbation during afterslip, while the deeper megathrust did not. We interpret this temporarily increased shallow SSE recurrence rate to be driven by static stress loading from large afterslip.

At the Osa Peninsula in southern Costa Rica, magnitude >7 earthquakes have been generated along the Middle American trench in 1904, 1941, and 1983 following a similar to 40-year recurrence interval, suggesting a rupture may be impending. However, regional interseismic coupling remains poorly constrained, largely due to sparse observations that are likely contaminated by aliasing effects of repeating shallow slow slip events (SSEs) that occur roughly every 4 years, but were only discovered recently. These SSEs, while likely reducing megathrust coupling near the trench, may load or trigger the next rupture of the 1983 asperity. Using new continuous Global Navigation Satellite System (GNSS) data from an updated and densified regional network, we derive inter-SSE rates of deformation and invert for slip deficit and megathrust coupling along the Middle American Trench, implementing block modeling to correct for the motion of the Panama microplate. We invert for slow slip and remove a time-averaged estimate of cumulative slow slip from our models. Our results indicate that the region of highest inter-SSE coupling (>0.8) corresponds with the spatial extent of SSE slip. We also find that SSEs are sufficient to release nearly all the elastic strain accumulated over their 4-year recurrence interval in localized regions. Accounting for this, in the region immediately downdip of the slow slip patch-the same region thought to have ruptured in the 1983 M-w 7.4 event-we estimate an interseismic coupling ratio of similar to 0.5-0.7 corresponding to similar to 1.75-2 m of accumulated slip deficit since 1983, sufficient to generate a similar magnitude rupture in the future. Plain Language Summary Megathrust coupling at the Osa Peninsula of Costa Rica has historically been poorly constrained due to sparse observations, but accurate coupling estimates are vital for understanding and forecasting seismic hazard in the region. The megathrust offshore southern Costa Rica has been host to a number of historical earthquakes, generating magnitude >7 events in 1904, 1941, and most recently 1983, following a similar to 40-year recurrence interval. This pattern of earthquakes suggests a rupture may be impending, emphasizing the urgent importance of understanding the location and behavior of frictionally locked asperities on the megathrust. In this paper we present new estimates of coupling based on data from an updated and densified regional geodetic observation network. We also account for the influence of slow slip events (effectively slow earthquake ruptures) on estimates of megathrust coupling. Overall, we find that enough strain has accumulated on the megathrust at the Osa Peninsula to generate an earthquake as large as the 1983 M-w 7.4 event.

Earthquake Early Warning (EEW) systems aim to alert users in advance of imminent shaking, enabling them to take action. In collaboration with local seismic agencies, the Swiss Seismological Service (SED), has developed national EEW systems across Central America. Public EEW alerts are now available, considering the frequent seismic activity and the vulnerability of the building stock, EEW has the potential to reduce casualties (i.e. fatalities and injuries). In this study, we build upon a probabilistic framework to quantify the potential benefits of EEW systems in reducing casualties. For each event generated in the stochastic catalog (100,000 event sets), we estimate the number of casualties in the absence of EEW. The framework evaluates the potential casualty reduction attributable to an operational EEW system, considering the expected warning times in each event at the target site, the subsequent actions taken upon receiving the alert, and system performance. For a return period of 475 years, the fatality reduction could reach ∼14% to 17% corresponding to hundreds fewer fatalities in Costa Rica and Nicaragua, and thousands fewer fatalities in El Salvador and Guatemala. From this baseline scenario, we explore strategies to improve casualty reduction: (1) increase warning time by densifying the seismic network; and (2) compare the effectiveness of Drop, Cover, And Hold On (DCHO) versus evacuation as recommended protective actions. Our results suggest that evacuation is a suitable strategy for reducing fatalities in this region, given the prevalence of single-story structures. Given the available warning time, evacuation is advised for occupants on the first floor, and those on upper floors should adopt DCHO. Our findings indicate that the implementation of EEW leads to a ∼10% reduction in average annual fatalities. A cost–benefit analysis reveals that the economic benefits of public EEW systems significantly outweigh the associated costs, making EEW a cost-effective mitigation strategy.

Central America has an elevated seismic risk, resulting from the vulnerability of the building stock and steady population growth. Earthquake Early Warning (EEW) aims to provide warning in advance of imminent shaking, allowing recipients to take action and reduce casualties during damaging motions. The Swiss Seismological Service (SED) has been collaborating with local seismic agencies to develop national EEW systems across Central America, which can potentially benefit nearly 47 million inhabitants. We conducted a public survey to comprehend the desire for EEW, the preferences for EEW attributes, and the current behaviour of people during earthquakes and the driving factors behind it. We recruited participants from Nicaragua (N = 513), Costa Rica (N = 1350), Guatemala (N = 559), and El Salvador (N = 491). In all four countries, participants consider it necessary to have an EEW system, are tolerant of false alerts, and are likely to react promptly to alerts. The desirable alert threshold is for low felt intensities, ranging between MMI III to IV. We found that a significant number of respondents already take protective action when earthquakes strike, and appropriate reactions are expected to increase when EEW is available. Our survey is unique in providing insights into the social dimension of EEW systems in low-income regions with high earthquake risk and where no operational EEW system yet exists.

Abstract Using new continuous geodetic time series, we identify five shallow slow slip events (SSEs) offshore and beneath the Osa peninsula in southern Costa Rica. An early event was detected by one station in 2013, and two events occurring in close succession in both 2018 and 2022 were detected by multiple stations, indicating a preliminary recurrence interval of ∼4–5 years. While SSEs have been observed to the northwest at Nicoya, this is their first documentation in southern Costa Rica. Modeled slip distributions of the 2018 and 2022 events indicate they likely ruptured the same or overlapping patches of the plate interface, near the trench, updip of the 1983 Mw 7.4 Osa event. Immediately offshore, estimated cumulative slip from the 2018 and 2022 events is sufficient to close the slip deficit from tectonic loading over the recurrence interval, potentially limiting the magnitude and spatial slip distribution of future large ruptures.

Abstract We show that a fixed smartphone network can provide robust Earthquake Early Warning for at least two orders of magnitude less cost than scientific‐grade networks. Our software and cloud‐based data architecture that we have constructed for the Alerta Sismica Temprana Utilizando Teléfonos Inteligentes (ASTUTI; Earthquake Early Warning Utilizing Smartphones) network in Costa Rica is easily scaled and exported. Implementation comprises provisioning and installing modern smartphones in judicious locations. Stand‐up time for regionally operational networks can be on the order of days. We evaluated a non‐parametric ground‐motion detection and alerting strategy that would alert the entire Costa Rican population of any event with a ground motion detection threshold of 0.55–0.65 %g at four neighboring stations. During a 6‐month evaluation period ASTUTI detected and alerted on five of 13 earthquakes with Mw 4.8–5.3 that caused felt Modified Mercalli Intensity shaking levels of 4.3–6. The system did not produce any false alerts and the undetected events did not produce wide‐spread or significant felt shaking. System latencies were less than or similar to scientific‐grade latencies. Alerts for all five detected events would have reached the capital city, San Jose, before strong S‐wave shaking. This would have afforded time for Drop Cover Hold On actions by most residents. Two of the five alerts were triggered by P‐waves suggesting that smartphone‐based networks could approach the fastest theoretical EEW performance, especially with future expected improvements in smartphone sensors and processing algorithms.

Abstract Constraining the long‐term variability and average of the Earth's magnetic field strength is fundamental to understanding the characteristics and behavior of the geomagnetic field. Questions remain about the strength of the average field, and the relationship between strength and reversal frequency, due to the dispersion of data from key time intervals. Here, we focus on the Cretaceous Normal Superchron (CNS; 121‐84 Ma), during which there were no reversals. We present new intensity results from 41 submarine basaltic glass (SBG) sites collected on the Nicoya Peninsula and Murcièlago Islands, Costa Rica. New and revised 40Ar/39Ar and biostratigraphic age constraints from previous studies indicate ages from 141 to 65 Ma. One site with an age of 135.1 ± 1.5 Ma (2σ) gave a reliable intensity result of 34 ± 8 µT (equivalent to a virtual axial dipole moment, VADM, value of 88 ± 20 ZAm2), three sites from 121 to 112 Ma, spanning the onset of the CNS, vary from 21 ± 1 to 34 ± 4 µT (53 ± 3 to 87 ± 10 ZAm2). These results from the CNS are all higher than the long‐term average of ∼42 ZAm2 and data from Suhongtu, Mongolia (46–53 ZAm2) and are similar to the Troodos Ophiolite, Cyprus (81 ZAm2, reinterpreted in this study). Together with the reinterpreted data, the new Costa Rica results suggest that the strength of the geomagnetic field was approximately the same both before and after the onset of the CNS. Therefore, the data do not support a strict correlation between polarity interval length and the strength of the magnetic field.

We analyzed continuous GPS data collected from 2002–2020 to characterize slow slip events (SSEs) in and near the Nicoya Peninsula, Costa Rica. These data are bisected by the 5 September 2012 M w 7.6 earthquake. The displacement time series contain multiple signals, including plate convergence, plate interface locking, coseismic and postseismic deformation, seasonal oscillations, SSEs, and noise. GPS‐measured coseismic and postseismic displacements associated with the M w 7.6 earthquake are modeled and removed by a step function plus multiple timescale relaxation processes with four characteristic times: 11, 94, 470, and 1,865 days. Seasonal oscillations are eliminated using a multichannel singular spectrum analysis (M‐SSA). Ten major SSEs ( M w > 6.6) are observed in the remaining time series, with a constant recurrence interval of 21.7 ± 2.6 months. SSEs occur in both shallow (~10 km) and deep (~35 km) portions of the plate interface, but the latter last longer and have larger magnitudes. There is minimum to no slow slip in the M w 7.6 seismic rupture area and a persistent slow slip patch beneath the Nicoya Gulf entrance. Despite strong earthquake‐related stress perturbations, the inter‐SSE locking status on the megathrust is very similar between the late and early stages of the earthquake cycle and includes locked patches that ruptured in the 2012 earthquake or continue to rupture via SSEs. Some locked patches offshore south of the Nicoya Peninsula did not rupture in 2012, do not participate in SSEs, and may be indicative of supercycle behavior, that is, strain accumulation over several seismic cycles. These areas warrant heightened monitoring.

The national seismic networks in Central America have been developing network-based early warning since 2016 for Nicaragua, 2018 for El Salvador and 2019 for Costa Rica. This effort is part of a project with the Swiss Seismological Service (ETH Zurich) including funds for accelerograph deployment. At each network, delay for first earthquake parameter estimations have been significantly reduced by optimizing data acquisition, metadata quality, and configuration of the EEW algorithms implemented in SeisComP3, i.e. Virtual Seismologist and the Finite fault rupture Detector. Issues remain with significant numbers of deployed instrumentation that for a variety of reasons, do not optimally contribute to the EEW systems. Building on our experience so far, we design national network upgrades that will optimize the earthquake early warning performance in the Central America region, mitigating the current issues with velocimeter clipping during large events, datalogger delays, and incomplete network coverage. The new instruments have been selected after testing all available EEW-capable accelerographs natively compatible with SeisComP3 including class A force balance accelerometers as well as MEMs. To justify our instrument selection, we summarize the performance of these different instruments. We model and discuss reference maps for performance expectations, and present planned instrument vaults. Our primary focus is on minimizing first alert times but we also wish to accentuate the broad value of the network upgrade for seismological monitoring showing changes in the magnitude of completeness in the region. We demonstrate the value of the network upgrade for earthquake early warning with real-time processing simulation using synthetic data for the maximum magnitude earthquake expected for the Central America subduction zone.

The Central Costa Rica Deformed Belt (CCRDB) is a diffuse faulting area that represents the western border of the Panama Microplate. Using the Markov Chain - Monte Carlo method and under three scenarios of the spatial distribution of the CCRDB, we analyzed the inter-seismic crustal deformation in Costa Rica and its surroundings based on the results of the Global Navigation Satellite System (GNSS) observations for Costa Rica, Nicaragua, and Panama. We assumed that the observed inter-seismic crustal deformation on the surface is a result of the kinematic effects of the rigid tectonic blocks motion, the elastic deformation due to the block interface interactions and the internal strain inside of the tectonic blocks. Assuming that the seismic moment in the subduction and inland interfaces is accumulated only as an elastic strain and is then released co-seismically, the resulting seismic moment accumulated rates reflect the capacity for producing earthquakes M-w > 8 along the Cocos Plate convergence and earthquakes M-w > 7 along the inland interfaces. Although these are the values that the modeling outputs, limited historical data suggests that this earthquake potential might be overestimated, but the historical seismicity in Costa Rica is still short for disesteeming higher earthquake potential levels.