International Seismological Centre Research Articles

A new moment tensor catalogue of light to moderate earthquakes in Palu-Koro and Matano faults, Sulawesi, based on the regional network in Indonesia: Fault analysis and insight

Starting from 2009, substantial improvements in the regional seismic network operating in Sulawesi, as part of the Meteorological, Climatological, and Geophysical Agency (BMKG) station deployment, allowed us to determine earthquake mechanism for 140 shallow (5–60 km) light to moderate size (3.8 ≤ Mw ≤ 6.1) earthquakes between 2009 and 2018 along the Palu-Koro and Matano faults, through moment tensor inversion. Ten (10) out of the 140 solutions obtained in this study were found in the International Seismological Centre Bulletin and used as validation. The double-couple mechanism of the best solutions (variance reduction ≥0.4 and condition number ≤ 7.5) is further used to analyze the Palu-Koro and Matano segments. We observed that the Donggala segment of the Palu-Koro fault shows variable earthquake mechanism types. Whereas the two subsequent segments, the Palu and Saluki segments, show predominate oblique strike-slip with normal and reverse mechanisms. The Matano fault is characterized by strike-slip mechanisms, with normal mechanisms also observed. The normal mechanisms depict the pull-apart system in the Matano segment, manifested as Matano Lake. The results of this work improve our understanding of regional tectonics.

Teleseismic Indication of Magmatic and Tectonic Activities at Slow- and Ultraslow-Spreading Ridges

Magmatic and tectonic processes in the formation of oceanic lithosphere at slow–ultraslow-spreading mid-ocean ridges (MORs) are more complicated relative to faster-spreading ridges, as their melt flux is overall low, with highly spatial and temporal variations. Here, we use the teleseismic catalog of magnitudes over 4 between 1995 and 2020 from the International Seismological Center to investigate the characteristics of magmatic and tectonic activities at the ultraslow-spreading Southwest Indian Ridge and Arctic Gakkel Ridge and the slow-spreading North Mid-Atlantic Ridge and Carlsberg Ridge (total length of 14,300 km). Using the single-link cluster analysis technique, we identify 78 seismic swarms (≥8 events), 877 sequences (2–7 events), and 3543 single events. Seismic swarms often occur near the volcanic center of second-order segments, presumably relating to relatively robust magmatism. By comparing the patterns of seismicity between ultraslow- and slow-spreading ridges, and between melt-rich and melt-poor regions of the Southwest Indian Ridge with distinct seafloor morphologies, we demonstrate that a lower spreading rate and a lower melt supply correspond to a higher seismicity rate and a higher potential of large volcano-induced seismic swarms, probably due to a thicker and colder lithosphere with a higher degree of along-axis melt focusing there.

Global Rayleigh Wave Attenuation and Group Velocity from International Seismological Centre Data

This paper presents a study of global Rayleigh wave attenuation and group velocity at a period of around 20 s using data from the International Seismological Centre (ISC) bulletin. Rayleigh waves at this period are sensitive to the crustal structure beneath continents and the uppermost mantle beneath oceans. Tomographic imaging reveals strong continental-ocean contrasts due to this. Oceanic group velocities are high but vary with seafloor depth, while oceanic attenuation shows mid-ocean ridges. Subduction zone regions display high attenuation but little velocity reduction, indicating scattering attenuation. Low attenuation regions are associated with the Earth’s major cratonic regions, but there are no associated velocity changes. This implies that intrinsic attenuation is low and scattering dominates. Cratonic crustal scatterers have been annealed. A new surface wave magnitude scale is constructed that is valid from near-source to near-antipode distances.

Seismic Hazard Assessment in Maluku Province Using PSHA

On 10 January 2023, a strong earthquake with a moment magnitude (Mw) of 7.5 occurred in Banda Sea, Maluku Province. This earthquake caused casualties around the epicentre due to the building collapse which was not an earthquake-resistant structure. A high level of seismic vulnerability and non-standard infrastructure often result in large losses when earthquakes occur. Therefore, mapping of earthquake-prone areas needs to be carried out to optimize the mitigation efforts based on Peak Ground Acceleration (PGA) and Spectrum Acceleration (SA) values. In this paper, mitigation efforts are carried out by mapping earthquake-prone areas using Probabilistic Seismic Hazard Analysis (PSHA) which considers the potential of each complex earthquake source in Maluku Province. Input data was obtained from several earthquake catalogues such as Advanced National Seismic System (ANSS), International Seismological Center (ISC), and United States Geological Survey (USGS) which were processed and analyzed using the Matlab, ZMap, and R-CRISIS programs and mapped using the ArcMap program. The results of PSHA show that Maluku Province is a region with varying levels of earthquake vulnerability. The research results show that the distribution hazard value on PGA in Maluku Province reaches 0.02g to 0.48g for a return period of 500 years and 0.04g to 0.79g for a return period of 2,500 years and on SA for periods of 0.20 seconds and 1.00 second reaches 0.04g to 0.89g and 0.01g to 0.22g for a return period of 500 years and 0.07g to 1.48g and 0.02g to 0.35g for a return period of 2,500 years, respectively. The highest level of vulnerability is in the northern region including Seram Island, Way Apu Island and Ambon Island and the lowest level of vulnerability is in the eastern region including Aru Island.

Integrated Earthquake Catalog of the Ossetian Sector of the Greater Caucasus

This article is the continuation of a study by authors to create the most complete and representative earthquake catalogs with a unified magnitude scale. The catalog created of the Ossetian sector of the Greater Caucasus (the territory of the Republic of North Ossetia–Alania and adjacent areas) was formed by the aggregation of all available data from Soviet, modern Russian, and Georgian catalogs, as well as the data from the International Seismological Centre. The integration was carried out using the author’s approach based on the modified nearest neighbor method. The integrated catalog of the Ossetian sector of the Greater Caucasus contains 16,285 events for the period 1962–2022. For all events, magnitude estimates are reduced to a unified “proxy-MW” scale. The integration of data from various sources made it possible to significantly replenish the beginning of the aftershock sequence of the Racha earthquake with MW = 7.0, which occurred on 29 April 1991. There has been a change in the level of registration over time. Thus, there is a significant lack of events for the periods 1967–1970 and 1988–1991; starting from 1995, the catalog is complete for magnitude 3.2, and since 2005 for magnitude 2.2. The integration of Soviet and modern Russian and Georgian catalogs made it possible to significantly increase the completeness and representativeness of seismic events in the studied Ossetian sector of the Greater Caucasus. This once again demonstrates both the fundamental importance of merging seismic data from global, national, and regional catalogs and the effectiveness of the author’s developed method.

Generalized Extreme Value Distribution for Modeling Earthquake Risk in Makran Subduction Zone Using Extreme Value Theory

The long-term pattern of severe incidents is one of the most crucial and fascinating topics of seismic events. This work aims to analyze the maximum annual earthquake magnitude in the Makran subduction zone using extreme value theory by implementing the block maxima method. The seismic data utilized for the current study was collected from the International Seismological Center (ISC) ranging from 1934 to 2022. The extreme parameters have fitted utilizing the generalized extreme value distribution. Numerous plots of the generalized extreme value distribution approach gave the accuracy of the used model when fitted to seismic data of the Makran subduction zone. Using the profile likelihood approach, the shape parameters (?) calculated is 0.29. According to the model fit, the Fréchet distribution is the best model for predicting the annual maximum earthquake magnitude in the Makran subduction zone. The estimated return levels for different return periods 10, 20, 50, and 100 are 6.35, 6.81, 7.58, and 8.31, respectively, indicating that an earthquake's maximum magnitude is increasing across the future 100 years. We also computed the profile likelihood to achieve a precise confidence interval. Thus, the 1945 earthquake of the Makran region with magnitude 8.1(Mw) was one of the most significant events in this area and occurred once every 100 years. The significance of this research is to inform decision-makers to implement suitable risk-mitigation methods.

The Cause of the Earthquake (4.7 mb) and Recent Stress Regime Occurring in Al-Refaei Area, Mesopotamian plain, August 18th, 2017

Al-Refaei area is considered one of the highest seismically activity in the Mesopotamian Plain according to its seismic history. Depending on the International seismological Center, four seismic swarms were diagnosed in this area; these are: August 2004, January 2013, May-Jun 2017, and August 2017. Based on three focal mechanism solutions of magnitude greater than 4 mb, the stresses in the area were analysed. The results of focal mechanisms suggest that the faults movements in the study area show reverse movements, which is generated by compression force. To determine the directions of principal stress axes that affecting the faults in the area, the Rotational Optimization method were used. The directions of the three stress axes are: 02°/32°, 10°/302°, and 80°/136° for σ1, σ2, and σ3, respectively. The collision between the Arabian and Iranian plates is the main cause of seismic activity in Al-Refaei area. Therefore, Al-Refaei active fault is responsible for most the earthquakes in the area, and responsible for the earthquake that occurred in August 18th, 2017.

Relocation of the Seismicity of the Caucasus Region

ABSTRACT Our objective is to improve the view of the seismicity in the Caucasus region using instrumental data between 1951 and 2019. To create a comprehensive catalog, we combine the bulletins of local agencies and the International Seismological Centre, and use an advanced single-event location algorithm, iLoc, to obtain better locations. We show that relocations with iLoc, using travel-time predictions from the 3D upper mantle velocity model, Regional Seismic Travel Time, improve the locations. Then, using the iLoc results as initial locations and the ground-truth events identified in the iLoc results as fix points, we apply Bayesloc, a multiple-event location algorithm, to simultaneously relocate the entire seismicity of the Caucasus region. We demonstrate that the simultaneous relocation of the seismicity with Bayesloc clarifies the location and geometry of major active structures accommodating ongoing convergence between the Arabian and Eurasian continents between the Black and Caspian Seas. Among our major findings is the confirmation of widespread seismicity in the mantle beneath the northern flank of the Greater Caucasus and central Caspian, resulting from north-dipping subduction of the Kura and South Caspian basins and the identification of a discrete band of crustal seismicity beneath the southern flank of the Greater Caucasus.

Homogenizing instrumental earthquake catalogs – a case study around the Dead Sea Transform Fault Zone

The creation of a homogenized earthquake catalog is a fundamental step in seismic hazard analysis. The homogenization procedure, however, is complex and requires a good understanding of the heterogeneities among the available bulletins. Common events within the bulletins have to be identified and assigned with the most suitable origin time and location solution, while all the events have to be harmonized into a single magnitude scale. This process entails several decision variables that are usually defined using qualitative measures or expert opinion, without a clear exploration of the associated uncertainties. To address this issue, we present an automated and data-driven workflow that defines spatio-temporal margins within which duplicate events fall and converts the various reported magnitudes into a common scale. Special attention has been paid to the fitted functional form and the validity range of the derived magnitude conversion relations. The proposed methodology has been successfully applied to a wide region around the Dead Sea Transform Fault Zone (27N-36N, 31E-39E), with input data from various sources such as the International Seismological Centre and the Geophysical Institute of Israel. The produced public catalog contains more than 5500 events, between 1900 and 2017, with moment magnitude Mw above 3. The MATLAB/Python scripts used in this study are also available.

Catalogue-based spatio-temporal analysis and evaluation of seismic scenarios in the Talysh zone (Azerbaijan)

In this paper, we simulate ground motion shaking of the Talysh zone based on the hybrid stochastic-deterministic approach using catalogue-based spatiotemporal analysis for 1970—2019 from the various local and international sources and the fault source pattern. Data for a homogenous and reliable catalogue are obtained from the International Seismological Center (ISC), European Mediterranean Seismological Center, National Earthquake Information Center, and Republican Center of Seismological Survey at the Azerbaijan National Academy of Science. To evaluate the ground shaking and to characterize its spatial variability, an intensity map was plotted depending on the selected seven active faults and lineaments tracing in the Talysh zone, considering the location, size and length of faults, using empirical relations between magnitude and intensity. The mapping results are presented in terms of intensity distribution. The plotted map shows that shakings with an intensity of VII are observed mainly in the central and western parts of the zone and are predicted to be the highest in the study area. The ground shaking with an intensity VI covers most of the Talysh zone. The magnitude computation results show that a maximum earthquake with Mw=6.2 can be generated by the West Caspian fault.
 This study applied various seismic input parameters and different approaches to give the basic information to evaluate the range of intensity distribution in seismic scenarios. The results will contribute to implementing more solid analyses for advancing earthquake countermeasure plans and seismic hazard analysis.

Modelling of Earthquake b-and a-Values Using Least Squares and Maximum Likelihood Estimate Methods in Different Tectonic Regions of the World

Aims: This study modelled a- and b-values of earthquakes employing the least squares regression and maximum likelihood estimate methods. Methodology: Data used in the study were obtained from the International Seismological Centre (ISC), an earthquake catalogue of the United Kingdom. The time window was from 1st January 1988 to 31st December 2010 (30 years) with earthquake focal depth of 0-700km and magnitude Mb ≥ 1.3. Ten different locations were selected and a total of 149,965 events were used. The acquired data were processed and analysed using Microsoft Excel and the hypothesis was tested using independent t-test statistics with the aid of Statistical Software for Social Sciences (SPSS) version 23.0. Results: The findings of the study revealed that the b- and a-values calculated using the least squares regression method were higher than the ones obtained using the maximum likelihood estimate method. Also, the hypothesis revealed that there is a significant difference between the use of the least squares regression method and the maximum likelihood estimate method in the determination of b- and a-value of earthquakes in a given region. Conclusion: The maximum likelihood estimate gives a better estimate of b- and -a values than the least squares regression method.

Enhanced crustal and intermediate seismicity in the Hindu Kush-Pamir region revealed by attentive deep learning model

The Hindu Kush-Pamir region (HKPR) is characterized by complex ongoing deformation, unique slab geometry, and intermediate seismic activity. The availability of extensive seismological data in recent decades has prompted the use of deep learning algorithms to extract valuable insights. In this study, we present a fully automated approach for augmenting earthquake catalogue within the HKPR. Our method leverages an attention mechanism-based deep learning architecture to simultaneously detect events, perform phase picking, and estimate magnitudes. We applied this model to a ten-month dataset (January 2013–October 2013) from 83 stations in the region. Utilizing a robust criterion to evaluate the model's probabilities, we associated phases at different stations and pinpointed earthquake locations in the region. Our results demonstrate a significant enhancement, revealing nearly four and a half times more earthquakes than previously documented in the International Seismological Center (ISC) catalogue. A notable portion of these newly detected events falls within the category of very low-magnitude earthquakes (<3), which were absent in the ISC catalogue. Notably, our spatiotemporal analysis reveals a concentration of crustal seismicity along poorly mapped neotectonic north and northeast-oriented faults in the western Pamir, as well as the Vakhsh Thrust System and the Darvaz Karakul Fault. These findings underscore potential sources of future seismic hazards. Furthermore, our expanded earthquake catalogue facilitates a deeper understanding of the interplay between crustal and intermediate seismic activity in the HKPR, shedding light on the deformation and active faulting resulting from Eurasian-Indian plate interactions.

Slab tearing and lithospheric structures in Luzon island, Philippines: constraints from P- and S-wave local earthquake tomography

Luzon Island is a complex setting of seismicity and magmatism caused by the subduction of the South China Sea lithosphere and the presence of a major strike-slip fault system, the Philippine Fault. Previous studies of the structure of this subduction zone have suggested that a ridge subduction system resulted in a slab tearing along the ridge. On the other hand, the Philippine Fault plays an important role in understanding how major strike-slip faults deform and displace at a continental scale. To constrain the lithospheric geological structure in the area and refine the slab tearing model, we performed a P- and S-wave seismic tomography travel time inversion using local earthquakes. The dataset has been combined from seismic phases reported by the International Seismological Centre and new pickings from six broadband seismic stations in northern Luzon. The three-dimensional P- and S-wave velocity models in Luzon Island were analyzed by applying the LOTOS package with a one-dimensional velocity model obtained from the VELEST program. Our tomographic images indicate contrasting velocity structures across the Philippine Fault to a depth of 60 km. Therefore, we suggest that the Philippine Fault might be a lithospheric structure that displaces both the crust and the upper mantle. The results also indicate regions of low-velocity slab windows from a depth of 40 km, which are interpreted as the sites of slab tearing. Compared with focal mechanisms and earthquake occurrence in this region, we propose that slab tearing extends from the fossil ridge and creates regional kinematic perturbations. The tearing produces shallow upwelling magma to stay in the chambers beneath the crust, which is in contrast to the magmatic system observed in other regions.

Unified earthquake catalogue and mapping of Gutenberg–Richter parameters for the East African Rift System

BackgroundThe initial phase of earthquake hazard assessment involves the consolidation of diverse magnitude scales, thereby requiring the homogenization of various magnitudes. Moment magnitude (Mw) emerges as the preferred descriptor for a range of magnitudes, encompassing local magnitude (ML), teleseismic magnitude (e.g., mb and MS), duration magnitudes (MD), and other magnitude proxies. Unlike alternative scales, Mw does not exhibit saturation at high magnitudes, enhancing its reliability. To achieve uniformity in magnitude representation, diverse regression techniques are employed, with the General Orthogonal Regression (GOR) method being widely regarded as the most dependable, accounting for uncertainty in both independent and dependent variables.MethodsThis study utilized the International Seismological Centre (ISC) Catalogue (http://www.isc.ac.uk/) to compile an array of events related to the East Africa Rift System (EARS). Subsequently, the General Orthogonal Regression method was applied to merge and harmonize the collected data. Furthermore, the research computed Gutenberg-Richter b-values using the newly unified magnitude.ResultsNotably, the conversion relationships between magnitude proxies, including MS-mb, mb-Mw, MS-Mw, and ML-Mw, exhibited robust correlations, with coefficients of 0.86, 0.80, 0.88, and 0.94, respectively. In contrast, the relationship between ML and mb proxies revealed a notably weaker correlation, registering a coefficient of 0.54. Ultimately, the study identified a magnitude of completeness and a b-value of 3.8 and 0.71, respectively, for the EARS region, providing valuable insights for earthquake hazard assessment in this area.ConclusionGenerally, the homogeneous catalogue is a step forward in seismicity assessment and geodynamic activities in the EARS. Hence, developing the empirical equations for the area is essential for future studies on seismic hazards and engineering applications due to the peculiarity of EARS’s geological and tectonic characteristics.

Early Modeling of Seismic Velocity Structure P, S, and Vp/Vs Ratio in Bengkulu Region and its Surroundings

Bengkulu is located in the southern region of Sumatra, where the Indo-Australian and Eurasian plates are subducted. As a consequence of the subduction, seismicity in the area increases, as well as volcanic activity and the appearance of structures such as the Sumatran and the Mentawai faults. Seismic velocity structures were reconstructed using earthquake tomography in Bengkulu and the surrounding areas using data from the International Seismological Center (ISC) and the Meteorology, Climatology, and Geophysics Agency (BMKG). The earthquake data collection period was from January 1, 2010 to December 31, 2018. 19 stations recorded 1721 earthquakes. LOTOS-12 is used to do a tomography inversion, starting with a 1D velocity model and wadati’s Vp/Vs ratio and iteratively inverting for Vp, Vs, and Vp/Vs ratios. The bending algorithm is used to calculate the arrival time, and the LSQR method is used to invert it. The Checkerboard Resolution Test (CRT) yielded Tomography results. The results revealed the existence of zones with low-velocity anomalies and low-values of Vp/Vs ratios, indicating the presence of weak zones caused by Sumatra faults, Mentawai faults, and other local faults. Low-velocity anomalies with high-values of Vp/Vs ratios are related to volcanic activity, hereas high-velocity anomalies are associated with the existence of subduction plates and compact rocks. The low-velocity anomalies in the fore-arc at 30-50 km depths coincide with fluid circulation caused by slab dehydration.

The Boundary Areas Structure of the Malko-Petropavlovsk Fracture Zone from Local Seismic Tomography and Earthquake Foci Mechanisms Data

Abstract —This study continues analysis of the new seismic tomographic structure of the suprasubduction complex of the central zone of Kamchatka, obtained from the dense local networks data of 2018–2020, and is devoted to the analysis of the velocity structure in the Malko-Petropavlovsk fracture zone margins and around them. The seismic tomographic model involves about 98,000 P- and S-wave travel times from 2963 local earthquakes from August 2018 to July 2020. The resolution of this model makes it possible to trace the feeding systems of volcanoes of the South Kamchatka and East Volcanic Belt to the slab surface, as well as to identify subvertical structural faults. To construct the orientations of the compression and extension axes we used the foci mechanisms of 41 earthquakes with М ≥ 4.5 from the catalog of the International Seismological Center for the period 1979–2019. Along the Malko-Petropavlovsk fracture zone, the Avacha transform fault is clearly traced in the geometry and mutual arrangement of velocity anomalies almost throughout the entire depth of the model. Comparison of seismic anomalies with a map of the directions of the compression and extension axes distribution from the earthquake foci mechanisms showed the correlation between the change in the value of the velocity anomalies along the Avacha transform fault with the axes direction change by almost 180°. A near-surface low-velocity anomaly to the depths of 25–35 km was found along the western border of the Malko-Petropavlovsk zone under the southern tip of the Sredinny Ridge. This anomaly probably marks the axes junction zone boundary of the ancient volcanic front along the Sredinny Ridge and the modern active Eastern Volcanic Belt, which formed as a result of the Kronotsky paleoarc accretion. To the west from the Sredinny Ridge southern tip, another low-velocity anomaly was revealed. This anomaly was traced to a depth of ~150 km, has a contrasting southern boundary confirmed by the distribution of the compression and extension axes directions by the earthquake foci mechanisms and apparently marks the southern boundary of the West Kamchatka block.

Seismic monitoring in Greece, 1899–2014: catalogue completeness 1966–2014

SUMMARY Instrumental earthquake monitoring in Greece started in 1899–1906 with the first five seismic stations of the National Observatory of Athens, Institute of Geodynamics (NOA). Subsequent upgrades and expansions led to today’s Unified National Seismic Network, which includes almost all permanent seismic stations in Greece and provides waveform and parametric data. We examine the detection capabilities of the Greek seismic networks using phase, location and magnitude data from the International Seismological Centre (ISC) and the various National Institutes. We apply two methods to measure the network performance. In one, we form a grid, and find the 50 per cent probability detection threshold for each station-grid cell pair for different times. In the other, we find the probability-based magnitude of completeness grid for every three months from 1970 to 2014. Both methods show that in 1990 the detection threshold improved significantly in the north part of Greece. A much greater improvement took place in 2010 throughout Greece, yielding a completeness magnitude of $M_p\, \sim$ 1.

ANALYZING EARTHQUAKE ACTIVITY LEVELS IN NORTH SULAWESI USING MAXIMUM LIKELIHOOD METHOD AND GUTENBERG – RICHTER LAW

Tectonic earthquakes are disasters that often cause damage to buildings and loss of life. The Province of North Sulawesi and its surroundings are earthquake-prone areas because they are located at the confluence of the major plates. This study aims to analyze the level of earthquake activity in the Province of North Sulawesi and its surroundings using the maximum likelihood method and the Guttenberg-Richter law. The distribution of the earthquake studied for the period 1941-2021 in the Province of North Sulawesi and its surroundings with a Mag ≥3.0 Mw and a depth ≤60 km. The data used comes from the Meteorological, Climatological, and Geophysical Agency Indonesia and ISC (International Seismological Center). The level of earthquake activity was reviewed based on nine groups of year intervals, three groups of depth levels, and the overall research area. The results show that the level of earthquake activity, rock fragility, and local stress at certain year intervals is sometimes high or low for the 1941-2021 earthquake period in North Sulawesi and its surroundings. The same condition for certain earthquake depths. On the other hand, the level of earthquake activity is quite high and the level of rock fragility is moderate in the research area as a whole.

The coulomb stress changes for correlation analysis of tectonic earthquake activity and volcanic of the 2001–2010 Merapi eruption based on focal mechanism data

The incident of increased seismic activity of the Merapi volcano was initiated by several tectonic earthquake phenomena. Analysis of the mechanism of tectonic earthquakes can be studied based on the combination of static stress on the rock. This paper describes the distribution of the Coulomb stress changes due to the interaction that occurs in tectonic earthquakes with volcanic activity. The data in the calculation of the Coulomb stress distribution are secondary in the form of tectonic seismicity data downloaded from the ISC (International Seismological Center), USGS (United States Geological Survey), and Global CMT (Global Centroid Moment Tensor) catalogs between 2001 and 2010. Calculation results and data analysis is the occurrence of an increase in Coulomb stress response in the Merapi Volcano area with a value range of 0.2–10 ​kPa at a depth of 2–4 ​km below the peak of Merapi Volcano. The Coulomb stress changes distribution is thought to indicate the accumulation of stress due to tectonic earthquakes that downward at the top of the volcano, thereby affecting the stability of the magma chamber and causing volcanic activity at the Merapi volcano.

Hydroacoustic Study of a Seismic Swarm in 2016–2017 near the Melville Transform Fault on the Southwest Indian Ridge

ABSTRACT Hydroacoustic monitoring has become particularly efficient for studying the low-magnitude seismicity occurring at mid-ocean ridges. In 2016–2017, a seismic swarm occurred near the Melville transform fault of the ultraslow-spreading Southwest Indian Ridge in the Indian Ocean. It comprised 258 events in the land-based International Seismological Centre catalog, extending from June 2016 to March 2017. We examined this seismicity using hydroacoustic records from three to nine hydrophones moored in the southern Indian Ocean from the Hydroacoustic Observatory of Seismicity and Biodiversity in the Indian Ocean (OHASISBIO) temporary network and the International Monitoring System of the Comprehensive Nuclear-Test-Ban Treaty Organization. We detected 27,624 hydroacoustic events spanning 298 days (1 June 2016–25 March 2017) with lower localization and origin time errors. These include several energetic, short-duration impulsive events, which we interpret as due to lava–water interactions on the seafloor. The spatio-temporal distribution of all the detected events does not show clear tectonic mainshock–aftershock sequences and suggests a magmatic origin of the swarm with bursts of seismicity likely caused by dike emplacements.