An earthquake swarm is a temporally concentrated sequence of seismic events occurring within a limited geographical area over a relatively short interval—ranging from days to years—in which no single event clearly dominates as a mainshock. This behavior contrasts fundamentally with a classic mainshock–aftershock sequence, where one large event is followed by a decaying cluster of smaller aftershocks. Documented multi‑year examples, such as the Noto swarm (2020–2024) and the 2003–2004 Ubaye sequence, demonstrate that swarms can persist as coherent, high‑productivity seismic episodes in specific regions.
Observational characteristics and monitoring limitations
High‑resolution monitoring of swarms often reveals intense temporal clustering: one two‑year chronology recorded over 16,000 detected events (daily counts shown as red bars), while only a subset—about 1,400 earthquakes—were sufficiently well recorded to be located and assigned magnitudes (plotted as magnitude–location symbols). This disparity reflects the distinction between detection and reliable location; small events frequently fall below the locating capability of local networks (for example, Sismalp could not locate many events below magnitude ~1), producing an apparent paucity of very small earthquakes on location‑based plots. Because the Gutenberg–Richter relation predicts an order‑of‑magnitude increase in counts per unit decrease in magnitude (M0 ≈ 10× M1), even modest detection/locating thresholds substantially bias observed swarm size distributions and inferred seismic productivity.
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Mechanisms, descriptors and measurement
Swarms may be generated by diverse processes—including fault slip, magmatic or hydrothermal activity associated with volcanism, and anthropogenic stress perturbations (induced seismicity)—so their interpretation requires integrating geologic, geodetic and seismic data. Standard seismological descriptors used in swarm analysis include epicenter and hypocenter locations, epicentral distance, shadow zones, and the character of seismic phases (notably P and S waves), which together inform source mechanism and subsurface structure. Instrumentation and metrics comprise seismometers for ground‑motion recording, magnitude scales (to quantify event size), and intensity scales (to describe local shaking effects); each plays a distinct role in documenting swarm behavior.
Context within seismology and forecasting
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Earthquake swarms form one element of a broader taxonomy of seismic phenomena (e.g., mainshocks, foreshocks, aftershocks, doublets, interplate and intraplate events, megathrusts, remotely triggered earthquakes, slow events, submarine events, supershear ruptures and tsunami‑generating earthquakes), providing a framework for classification and hazard assessment. Forecasting efforts—coordinated by institutional bodies such as earthquake prediction committees and implemented through formal forecasting programs—face particular challenges with swarms because the absence of a single mainshock complicates standard probabilistic models. Consequently, multidisciplinary techniques (including shear‑wave splitting analysis, geophysical modeling using relations like the Adams–Williamson equation, regional zoning such as Flinn–Engdahl regions, and considerations from earthquake engineering and seismite studies) are often required to characterize swarm sources and assess associated hazards.
History and generalities
Earthquake swarms—sequences of many earthquakes clustered in space and time without a single outstanding mainshock—have long been recognized in specific regions. The Ore Mountains (Erzgebirge) on the Czech–German border, documented since the 16th century, are a classical swarm province where episodes commonly last from weeks to months. The term Schwarmbeben was coined by Josef Knett in 1899 after his study of roughly one hundred felt events in western Bohemia/Vogtland in early 1824; Knett’s usage emphasized the tight spatial clustering of hypocentres, a pattern often likened to a swarm of insects when viewed in maps or cross sections.
Some swarms are extraordinarily intense and well recorded. Between 1965 and 1967 the Matsushiro sequence beneath a suburb of Nagano produced on the order of 1,000,000 earthquakes. Continuous monitoring from an observatory in a former military tunnel captured the escalation from undetectable shocks to peak days with thousands of events (6,780 in one day, of which 585 were felt), illustrating both the temporal concentration and the capability of modern instrumentation to resolve swarm dynamics. The Matsushiro activity has been interpreted as driven by magmatic uplift, potentially accelerated by the regional 1964 Niigata earthquake, demonstrating how regional seismic events can interact with magmatic systems to sustain prolonged swarms.
Spatially, swarms are characteristic of volcanic and hydrothermal settings—Japan, central Italy, the Afar depression, Iceland, and Quaternary volcanic provinces such as Vogtland and the Vosges—but they are not confined to plate‑boundary zones. Intraplate occurrences in places like Nevada, Oklahoma and parts of Scotland show that local crustal processes can also generate swarm behaviour. A unifying mechanism in many cases is migration of high‑pressure fluids in the crust: upward or lateral movement of magmatic, hydrothermal or overpressured fluids can trigger and control the spatio‑temporal evolution of seismicity.
Hydrological forcing is documented in some shallow swarms: the Hochstaufen sequence in Bavaria, with focal depths near 2 km, shows a clear correlation between seismicity and precipitation, providing direct evidence that surface water and pore‑pressure changes can modulate swarm activity. From a hazard perspective swarms pose particular challenges because their cessation is difficult to predict and a larger event can follow protracted swarm activity (for example, the Mw 6.3 L’Aquila mainshock in 2009 followed prior lower‑magnitude swarm activity). Although individual shocks in swarms are often moderate, their persistence can be disruptive and induce substantial public distress, complicating risk communication and emergency response.
Examples — case selection and rationale
The case studies were chosen to emphasize seismic swarms that exhibit distinctive and diagnostically informative behaviors: exceptionally high event counts, spatial–temporal coupling with larger nearby earthquakes, prolonged activity over months to years, and very shallow hypocenters. Each selected swarm therefore serves to illuminate a particular atypical facet of swarm dynamics and its geographic or tectonic implications rather than to provide a complete global catalog.
Swarms with complex interactions with larger shocks are used to explore fault linkage and stress transfer between clusters of small events and adjacent larger ruptures, shedding light on how seismicity organizes across neighboring fault segments or volcanic systems. High‑density clustering exposes fine‑scale seismogenic structures, aids delimitation of active fault zones, and permits analysis of migration or diffusion patterns of seismicity. Long‑lived swarms illustrate sustained seismic regimes and enable study of temporal evolution, cumulative slip, and processes such as slow‑slip or fluid‑driven deformation that can progressively modify regional stress fields. Ultra‑shallow swarms are included because their near‑surface hypocenters produce pronounced surface effects, demand close geological correlation, and bear directly on local hazard and geomorphological response.
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Some examples deliberately come from regions conventionally regarded as aseismic to prompt reassessment of local tectonic frameworks and to identify otherwise unrecognized seismic sources; together, the selected cases demonstrate the geographic variability of swarm triggers across different crustal settings. This compilation is therefore a curated set intended to highlight salient peculiarities of swarm behavior for comparative and regional analysis, not a comprehensive inventory of all global swarms.
India’s recent earthquake activity includes two notable, spatially and temporally distinct sequences that illustrate the complexity of seismic hazard in areas conventionally regarded as aseismic. Beginning 11 November 2018, the Dahanu area of Maharashtra has undergone a sustained swarm characterized by very high temporal frequency of small shocks—typically under magnitude 3.5, with some days yielding on the order of ten to twenty felt events—and a largest recorded event of M4.1 in February 2019. The sequence conforms to the operational definition of an earthquake swarm: numerous closely spaced earthquakes occurring over a short interval without a single dominant mainshock and aftershock series.
Although most Dahanu events are small, two shallow shocks produced surprisingly severe local effects, including structural damage and fatalities. The disproportionate impact is attributed to their very shallow focal depths, which concentrate seismic energy near the surface and therefore amplify ground shaking relative to deeper events of equivalent magnitude. A separate, long-lived episode has affected Bamhori village in Seoni district since February 2000; like Dahanu, Bamhori represents persistent, localized seismicity that repeatedly affects a defined settlement, but it is temporally distinct and longer in duration.
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Together, these cases underscore three important hazard and geomorphological lessons: (1) magnitude alone is an inadequate proxy for hazard because shallow foci can produce disproportionately severe local effects; (2) the occurrence of swarms in historically quiet regions implies active stress adjustment or localized faulting processes that warrant targeted geophysical monitoring; and (3) both short-term swarms (Dahanu) and protracted local sequences (Bamhori) highlight the need for detailed local seismic surveillance, community vulnerability assessment, and microzonation to inform risk-reduction measures for affected settlements.
The Philippines has experienced multiple earthquake swarms that vary in depth, magnitude and societal impact, underscoring both the complexity of local seismic processes and the vulnerability of populated areas.
The Batangas swarm (early April–mid August 2017) began with three shallow, co‑located shocks (Ms 5.5 on 4 April; Ms 5.6 and Ms 6.0 on 8 April) with hypocentres in the upper crust (7–28 km). A larger Ms 6.3 event on 11 April, however, was spatially and seismically distinct—its epicentre lay roughly 50 km away and its focal depth (177 km) qualified it as an intermediate‑depth earthquake in PHIVOLCS records—illustrating how a local shallow cluster and a deeper, apparently independent event can occur in temporal proximity, with potential triggering or interaction implications.
Panay Island has seen repeated shallow swarm activity that has affected urban centers. On 5 November 2018, a sequence of M 4.0–4.8 shocks struck Antique, Iloilo and Guimaras (including a M 4.7 at San Jose, Antique, and felt intensity IV in Iloilo City). A subsequent swarm on 15 October 2020 produced smaller M 2.5–4.5 events, again felt in Iloilo City, indicating recurrent low‑to‑moderate magnitude clustering near populated areas.
The October 2019 Mindanao sequence centered on Tulunan (Cotabato) demonstrates the high hazard potential when swarms include larger damaging shocks. Three major events (16 Oct Mwp 6.3; 29 Oct Mww 6.6; 31 Oct Mww 6.5) occurred within weeks, producing widespread intensity VII–VI effects across multiple provinces, numerous injuries and deaths, and structural collapses—highlighting the severe human and infrastructural consequences that can arise from temporally clustered seismicity.
A concentrated episode in Camarines Sur (14–18 October 2021) comprised at least 27 recorded earthquakes (M 1.7–4.3) with shallow hypocentres (1–40 km); ten were reported felt as far as neighboring provinces, documenting a brief but intense phase of shallow seismicity in the Bicol region. Collectively, these Philippine cases illustrate a spectrum of swarm behavior—from low‑magnitude, recurrent urban‑affecting sequences to complex interactions with deeper events and to clusters that generate major damage—underscoring the importance of continuous monitoring and rapid impact assessment by PHIVOLCS.
Iceland — Reykjanes Peninsula earthquake swarm (Oct 2023–Apr 2025)
A magmatic intrusion beneath the Reykjanes Peninsula initiated an intense earthquake swarm on 24 October 2023 that prompted evacuation of the coastal town of Grindavík and ultimately progressed to a sequence of eruptions beginning on 18 December 2023, with eruptive activity reported as continuing through April 2025. The swarm began with a rapid increase in shallow seismicity: by 30 October roughly 8,000 earthquakes had been detected, most at depths of 2–4 km, marking a sharp escalation from background rates earlier in October (about 700 events with a maximum magnitude near 3.3).
Seismicity intensified further in early November. Different monitoring reports recorded cumulative counts of about 20,000–22,000 earthquakes by 10 November and maximum events exceeding M5.1–5.2, reflecting rapidly rising rates and large individual events during the escalation. The Icelandic Meteorological Office (IMO) anticipated an eruption, noting that magma was likely to reach the surface over days rather than hours, and continued to report sustained daily activity on the order of 700–1,000 earthquakes by mid-November.
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Geophysical and geochemical observations defined the intrusion geometry and nearness of magma to the surface. IMO localized the principal swarm around Svartsengi north of Grindavík; the inferred horizontal reach of the intrusion extended toward the Sundhnúkur crater chain (≈3.5 km north of Grindavík), while the volcanic edifice of Hagafell (≈2 km north) was later identified as a particularly high-risk area. Sulphur dioxide was detected in the atmosphere on 14 November and subsequently by borehole sensors in Svartsengi on 16 November, signals consistent with magma lying within a few hundred metres of the surface.
Ground-deformation measurements documented extreme crustal movement. In situ sensors at Festarfjall and Svartsengi recorded roughly 1.20 m of horizontal opening, and satellite interferometry showed about 1 m of subsidence across an approximately 5 km by 2 km zone from the Sundhnúkur craters toward western Grindavík. The subsiding area formed a graben-like structure; volume estimates derived from these deformations put the magmatic intrusion at roughly 70 million cubic metres, with subsidence continuing at an estimated rate of ~4 cm per day during monitoring. A major crack that opened through Grindavík coincided with areas of greatest subsidence and, according to historical mapping and University of Iceland researchers, represents reactivation of a preexisting fault likely formed during the last Sundhnúkur eruption over 2,000 years ago.
Deeper magmatic processes were inferred from episodic uplift. A rapid 30 mm uplift in the Svartsengi area between 18 and 21 November 2023 was interpreted as upwelling from a deeper magma source at depths of five kilometres or more, consistent with multi-level magma migration prior to surface eruption. Together, the seismic, gas, and deformation records describe a classic progression from intrusion and shallow pressurization to surface eruption, with both shallow and deep magma transport contributing to the observed hazards.
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Czech Republic / Germany
The western Bohemia–Vogtland border zone, straddling the Czech–German frontier, is a long‑recognized centre of earthquake swarm activity, with systematic observations beginning in the late nineteenth century and continued seismicity into the twenty‑first. Swarms here recur episodically and can attain relatively large magnitudes for an intraplate environment (notable maxima include about M5.0 in 1908, M4.6 in 1985–86, M3.2 in 2000 and M3.8 in 2008), indicating concentrated strain release within the stable continental interior.
A paradigmatic recent example is the October 2008 Nový Kostel sequence, which persisted for roughly four weeks yet produced an extraordinarily dense catalogue of microevents—local monitoring by WEBNET registered on the order of 25,000 detections—demonstrating how brief intervals can host intense microseismic activity. Seismogenic structures in the area are typically steeply dipping faults; the 2008 activity was confined to a steep fault patch and exhibited a clear spatial progression of events.
Detailed spatial–temporal analysis of that patch revealed an overall upward migration of seismicity, with early shocks clustering near the patch base and later ones toward its top, a pattern consistent with vertical propagation of rupture or stress changes (and plausibly influenced by fluid–pressure variations). Taken together, the historical recurrence, high event rates captured by dense local networks, steep fault geometries and systematic upward migration identify the western Bohemia–Vogtland border as a distinct intraplate swarm domain in which repeated activation of localized fault patches and complex stress–fluid interactions drive short, intense earthquake sequences.
France — seismic swarm activity
The Ubaye Valley (Alpes‑de‑Haute‑Provence) constitutes the most active seismic province of the French Alps. Although the upper valley can produce classical mainshock–aftershock sequences (for example the damaging M 5.5 event of 1959), much of the seismic energy there is released through swarm behavior concentrated between Barcelonnette and the French–Italian border. Regional maps used to characterise these swarms employ a colour‑coded legend: white symbols for the 2003–2004 sequence, pink for the 2012–2015 sequence up to 6 April 2014, red for events recorded from 7 April 2014 onward, brown for the latest twenty events as plotted in July 2015, with symbol size scaled to magnitude and blue triangles marking the nearest seismic stations. The mapped alignments of pink and red symbols overlapping the white cluster mark the epicentres of two moderate shocks (26 Feb 2012, M 4.3; 7 Apr 2014, M 4.8) that each produced local damage and short aftershock sequences while sustaining an extended multi‑year swarm pattern.
The La Condamine–Châtelard area exemplifies this swarm‑dominated regime. An exceptional 2003–2004 sequence generated more than 16,000 events recorded by the local network, yet individual magnitudes remained low (maximum ≈ 2.7) and the activity was spatially confined to roughly an 8 km linear zone. After a quiescent interval the site underwent renewed swarming from 2012 into 2014–2015; this second cluster was offset by a few kilometres, extended to about 11 km, and was effectively initiated by the M 4.3 event of 26 February 2012 and later reactivated by the M 4.8 event of 7 April 2014, producing sustained swarm‑style activity at moderate magnitudes over nearly four years.
Hypocentral depths for the Ubaye swarms are predominantly between ca. 4 and 11 km and lie within the crystalline basement. Focal mechanisms indicate chiefly normal faulting with subsidiary strike‑slip components, implying a combination of extensional and lateral deformation accommodated within basement rocks rather than in shallow sedimentary cover.
Elsewhere in southeastern France, the Tricastin (lower Rhône Valley) area has a long historical record of swarm episodes, with damaging sequences noted in the 18th and early 20th centuries. Instrumental observations in 2002–2003 documented an unusually shallow swarm beneath a hamlet near Clansayes, with focal depths on the order of 200 m—an exceptional depth for tectonic events. The ruptures, located within an Upper‑Cretaceous reef‑limestone slab, produced audible explosion‑like sounds even for very small (M ≤ 1) events. The slab appears to rupture episodically over months to years for reasons that remain poorly understood.
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The Maurienne Valley has also experienced episodic swarm activity. A prolonged 19th‑century sequence lasted about five and a half years (December 1838–June 1844) and included damaging shocks near Saint‑Jean‑de‑Maurienne. More recently, a swarm that began in October 2015 near Montgellafrey showed a marked escalation on 17 October 2017, when over 300 events occurred within two weeks and two shocks reached M 3.7; the elevated activity continued for roughly another year, giving the overall episode a duration exceeding three years.
Greece — Anydros (Amorgos–Santorini) seismic episode
Since 26 January 2025 a pronounced episode of seismicity has been recorded in the maritime zone between Amorgos and Santorini, spatially concentrated on and around the island of Anydros in the central–southern Aegean (Cyclades). More than 7,700 events were detected during the monitoring interval; roughly 100 of these exceeded magnitude 4.0 (≈1.3% of the total), and the largest observed shock reached magnitude 5.2 on the Richter scale on the evening of 5 February 2025.
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The temporal and spatial clustering, together with the predominance of low‑to‑moderate magnitudes, conforms to the definition of a seismic swarm—numerous events occurring in a limited area and period without a single, clearly dominant mainshock. This pattern implies that the sequence is dominated by many smaller shocks with a minority of larger events rather than a mainshock–aftershock cascade.
Tectonically, the swarm locus lies within a region of complex crustal deformation and volcanic features, notably the Santorini caldera. Its position between island shorelines and submarine basins makes the distribution of hypocenters important for regional hazard appraisal and for interpreting how stress is transferred across the Cyclades.
To resolve the driving mechanisms and refine hazard assessments, higher‑resolution analyses are required: accurate hypocenter relocations including focal depths, determination of focal mechanisms, and characterization of the temporal evolution of magnitudes. Continued dense seismic monitoring is essential to discriminate volcanic from tectonic forcing, to map depth distribution, and to quantify ongoing stress changes in the area.
El Salvador — April 2017 seismic episode
In April 2017 the municipality of Antiguo Cuscatlán, a suburban district of San Salvador, experienced a concentrated, short-lived seismic episode comprising nearly 500 events over roughly 48 hours. Individual shocks ranged from about Mw 1.5 to Mw 5.1, with the largest reaching magnitude 5.1 while most were of lower magnitude. The strongest shock produced minor structural damage locally and was associated with one fatality, demonstrating that moderate-magnitude events can produce societal and infrastructural impacts in densely populated urban-peripheral settings.
Contemporaneous monitoring and expert review found no anomalous activity at nearby volcanoes and no clear magmatic signals accompanying the sequence. In that context the episode is best interpreted as a seismic swarm or tectonic cluster—an aggregation of many small-to-moderate earthquakes concentrated in time and space without a single dominant mainshock—rather than as an eruptive or magmatic event.
The location of the swarm within the San Salvador metropolitan area, where tectonic and volcanic hazards overlap, elevates short-term risk despite the moderate size of individual shocks. The episode highlights the importance of timely seismic monitoring and rapid expert assessment (including coordinated seismic and volcanic surveillance), clear public communication about swarm behavior, and preparedness for cumulative effects of repeated shaking—such as progressive structural damage and strain on emergency-response systems—even when no volcanic unrest is detected.
United States
Earthquake swarms in the United States exhibit diverse spatial patterns, magnitudes, durations and triggering mechanisms, ranging from concentrated low‑magnitude clusters to prolonged episodes of tens of thousands of events. In Nevada during 2008 a focused swarm near Reno produced roughly 1,000 small earthquakes between February and November, peaking in April when three events above M4 occurred within two days; the largest reached Mw 4.9 and caused only localized damage. Such concentrated outbreaks illustrate how swarms can involve frequent, modest shocks with occasional moderate events.
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The Yellowstone Caldera in northwestern Wyoming has been a recurrent site of strong swarms since the late 20th century, including several thousand events over months in 1985 and numerous smaller swarms thereafter. Many Yellowstone sequences show rapid‑fire behavior—tens to hundreds of small-to-moderate earthquakes in very short intervals—and commonly occur within the caldera boundary (notably swarms at the northwest of Yellowstone Lake in late 2007–early 2008 and the large sequence in January 2010 that produced more than 1,600 events). The U.S. Geological Survey attributes most Yellowstone swarms to slip on pre‑existing faults rather than direct magma intrusion or simple hydrothermal injections.
Anthropogenic triggering has been implicated in other U.S. swarms. The Guy–Greenbrier sequence in central Arkansas (August 2010–March 2011) displayed a linear distribution of epicenters with a southwestward migration of activity and a maximum computed magnitude of 4.7. Subsequent analyses linked the sequence to deep wastewater‑disposal operations, prompting regulatory measures including a moratorium on such injections. This case exemplifies how fluid injection can perturb stress on faults and induce seismicity.
Idaho’s 2017 Soda Springs swarm began on 2 September and produced five events between M4.6 and M5.3 within nine days. Seismologists framed public guidance cautiously—recalling both legal lessons from past earthquake forecasting controversies and regional seismic history (for example, a 1983 M6.9 earthquake)—noting that while an imminent larger mainshock was unlikely, a stronger event could not be entirely excluded.
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The prolonged Cahuilla swarm in Riverside County, California (early 2016–late 2019) comprised more than 22,000 events spanning magnitudes ~0.7–4.4, with the largest shock occurring in August 2018. High‑resolution analyses, including machine‑learning reconstruction of the fault zone, characterized the causative structure as a narrow (~≤50 m) fault ~4 km long. Early events originated near the fault base at ~9 km depth and then migrated upward to ~5 km while spreading along strike. The subsurface geometry includes horizontal channels and bends that overlie a pressurized fluid reservoir connected to the fault by a sealed conduit at ~8 km; breach of that seal in 2016 allowed fluid to enter the fault base and diffuse upward, producing sustained seismicity. These observations provide strong empirical support for a “fault‑zone valving” mechanism in which episodic fluid release into fault zones can generate prolonged swarms.
El Hierro, the smallest and southwesternmost of the Canary Islands, underwent a pronounced magmatic episode culminating in the 2011–12 eruption. Between July and October 2011, seismic monitoring networks recorded hundreds of low‑magnitude earthquakes clustered beneath and around the island, forming a sustained seismic swarm indicative of volcanic unrest. The accumulated seismic energy rose markedly on 28 September 2011, signaling a clear escalation of subsurface activity. Geophysical interpretation attributed the swarm to magma intrusion and migration at depth, with repeated brittle failure events accompanying magmatic movement prior to any surface expression. This period of unrest culminated in a submarine eruption adjacent to El Hierro on 9 October 2011, demonstrating that the magmatic system had breached the seafloor.
Indian Ocean — Mayotte earthquake swarm (2018–2019)
An intense seismic sequence began east of Mayotte on 10 May 2018, comprising thousands of closely spaced events many of which were felt by the island’s residents. The sequence included the largest-magnitude shock ever recorded in the Comoro region, a M 5.9 event on 15 May 2018. Temporarily deployed ocean‑bottom seismometers located the principal locus of activity roughly 10 km east of Mayotte and imaged hypocentres unusually deep in the oceanic lithosphere, primarily between ~20 and 50 km depth.
These observations conflicted with earlier interpretations that attributed the unrest to deflation of a magma reservoir inferred some 45 km east of Mayotte at ~28 km depth; the seafloor seismometer data therefore revealed both a lateral and depth discrepancy relative to that hypothesised source. Subsequent oceanographic mapping in May 2019 documented substantial seafloor modification: a newly formed submarine edifice about 50 km east of the island with ~800 m of vertical relief. Temporally, the swarm’s rate declined between August and November 2018 before an anomalous 11 November 2018 event that generated globally observable surface waves yet produced no clear P or S phases at global stations. Despite the late‑2018 lull and the singular character of the November signal, elevated seismicity and volcanic activity persisted through 2019, indicating prolonged unrest in the eastern Mayotte offshore region.
Pacific Ocean — Santa Cruz Islands (January–February 2013)
Between January and February 2013 the Santa Cruz Islands, part of the Solomon Islands archipelago in the southwest Pacific, experienced an intense seismic swarm that culminated in a major rupture on 6 February 2013. In the seven days immediately preceding the principal event there were more than 40 earthquakes of magnitude 4.5 or greater, an unusually dense concentration of moderate shocks within a short temporal window. The sequence also contained multiple strong events: seven earthquakes exceeded magnitude 6 and numerous additional shocks fell in the magnitude 5–6 range, indicating the swarm comprised several substantial ruptures rather than only minor foreshocks.
Tectonically, the Santa Cruz group lies within a complex convergent plate- boundary setting where the Australian and Pacific plates interact with intervening microplates. Subduction-related thrusting and the geometric and kinematic complexities of these plate interactions produce frequent clustered seismicity and episodic large earthquakes. The observed pattern of many M≥4.5 events concentrated in days, including several M>6 shocks followed by the M8.0 mainshock on 6 February, is consistent with stress accumulation and transfer along a subduction fault system and is best interpreted as a pronounced foreshock–mainshock sequence. The culminating M8.0 qualifies as a great earthquake and markedly elevates the potential for severe regional ground shaking and cascading hazards—tsunamis, coastal destruction, and landslides—emphasizing the high seismic risk for the Santa Cruz Islands and adjacent Solomon Islands regions.