The Ojos del Salado volcano is the highest active volcano in the world, located in the southern Puna Plateau in the high Andes, and at the southern end of the Central Andean Volcanic Zone. Ojos del Salado has erupted in the Holocene and appears to be geothermally active as it is the most likely source of the hot springs that feed into the nearby Laguna Verde. Despite the Ojos del Salado’s size and recent activity, it is not closely monitored and little is known about the current state of this quiet giant. To remedy this, we deployed a passive network of 29 geophones on the flanks of Ojos del Salado and down to the Laguna Verde to record seismic activity at and near the volcano for the month of February, 2024. This data provides insight into the level of seismic activity, and from this we can draw some conclusions about the volcanic activity present at Ojos. We used machine learning techniques to detect events, these were then manually checked and compiled into a SEISAN catalogue. Initially 345 local events and 129 regional events were detected. Taking only events that were recorded on at least eight stations and could be relocated within the network we found that there remained 152 local events within the network. These relocated events formed two main clusters, one on the western flank of the summit and one just north of the summit, between the Laguna Verde and the Ojos del Salado. The magnitude of completeness of this relocated catalogue was -0.3ML, and the local magnitudes ranged from -1ML to 2.8ML. The locations and fault plane solutions of the events at the summit suggest north south extension and generally follow mapped faults in the area, and agree with the main regional stress axes. The smaller cluster just north of the volcano are oriented differently, we suggest that this cluster, which mainly occurred as a swarm on February 8^th following two days of heavy rainfall, may be due to stresses caused by an increase in geothermal fluids supplied in part by the heavy rains joining the geothermal system and increasing the local pore pressure. The passive data collection has also allowed us to analyze the continuous seismic signal which shows a periodicity of 12 hours and 24 hours in frequency bands from 1 to 10Hz. This could be an indication of the effect of solid earth tides, temperature, or even rainfall on the seismicity at the Ojos del Salado volcano.

How to cite: Murray-Bergquist, L., Thorwart, M., Garcia Pina, A., Ulloa Correa, C., van Ginkel, J., and Beniest, A.: Linking Environmental Controls to Seismicity at Ojos del Salado Volcano, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16491, https://doi.org/10.5194/egusphere-egu26-16491, 2026.

This study focuses on a deep crustal earthquake swarm that has been occurring since February 2025 in northern Yamaguchi, southwestern Japan. The swarm is located at depths of 25–35 km, approximately 20 km deeper than typical earthquake swarms in Japan. Because direct geophysical observations in the lower crust are limited, deep crustal earthquake swarms provide an important observation for investigating seismogenesis in the deep crustal environment.

We performed hypocenter relocation of 3,886 earthquakes with M ≥ 0.0 that occurred between February and June 2025, using initial hypocenters from the Japan Meteorological Agency. The 3,871 relocated hypocenters show two north–south–aligned planar clusters over a spatial scale of ~5 km. The two clusters are separated by a ~1 km-wide low seismicity zone. Within each cluster, hypocenters are heterogeneously distributed, with localized dense and sparse regions.

To grasp the spatiotemporal characteristics in the swarm, we first investigated seismicity migrations along the strike and dip directions of each planar cluster. In both clusters, seismicity exhibits an overall migration from deeper to shallower areas. Along strike, migration is not simple but shows zigzag-like fluctuations, whereas along dip, seismicity typically migrates upward and downward at a rate of ~1 km/day.

Because seismicity migration appears to be active at ~14 days intervals, we qualitatively compared the timing of seismicity migration with tidal normal stress variations on the fitted planes. As a result, we found that periods of large temporal variations in normal stress, particularly between low tides, corresponded to episodes of seismicity migrations.

We further analyzed the multiple seismicity migrations. To evaluate multiple migration episodes, we treated each earthquake as a spatiotemporal origin and analyzed subsequent events within a fixed time window of several days. Almost all of the migration sequences can be explained by an isotropic pore-fluid pressure diffusion model. Estimated diffusivities of diffusive migrations range from 1.0 to 5.0 m²/s, comparable to values reported for volcanic earthquake swarms.

The Moho depth beneath the swarm area is estimated to be ~40 km, indicating that the swarm occurs in the lower crust just above the Moho. The swarm is located just beneath the Abu monogenic volcano group, where petrological studies suggest partial melting of the lower crust. Considering that the most recent eruption occurred ~8,800 years ago, fluids separated from a magma reservoir may persist and migrate upward. The complex deep to shallow seismicity migration, relatively large diffusivities, and tidal modulation suggest that this deep crustal earthquake swarm is driven by highly pressurized, low-viscosity fluids moving through a structurally and hydraulically heterogeneous swarm area in the lower crust, particularly during periods of tidal normal stress variations.

How to cite: Amezawa, Y., Yabe, S., Sawaki, Y., Imanishi, K., Miyazawa, M., Nishikawa, T., Nishimura, T., Nagaoka, A., Miyamachi, R., and Ohmi, S.: Deep crustal earthquake swarm and complex seismicity migrations in northern Yamaguchi, southwestern Japan, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15500, https://doi.org/10.5194/egusphere-egu26-15500, 2026.

In subduction zones, seismicity exhibits pronounced space–time heterogeneity that has not been fully explained. While many possible mechanisms, such as frictional heterogeneities, plate geometry, and stress distribution, can lead to contrasting slip behaviors, the precise origin of these slip heterogeneities and their manifestation in space–time distribution of seismicity remain open questions. This is partly due to observational resolution limitations that hinder the detection of fine-scale processes.

For this reason, the north-central zone of Chile is of particular interest: due to the heterogeneities present at the plate interface, associated with the Juan Fernandez Ridge and the Challenger Fracture, where both bathymetric anomalies affect seismic coupling and stress distribution. It is a structurally complex area with abundant seismicity, the occurrence of large interplate earthquakes in recent decades (e.g., the 2015 Mw 8.3 Illapel earthquake and the 1971 Mw 7.8 La Ligua earthquake), and the presence of multiple seismic sequences and persistent seismic activity.

In this study, we conduct a detailed spatiotemporal analysis of seismicity in the North-Central Chile subduction zone, with emphasis on the identification and characterization of seismic clusters using onland seismic time series  from the National Seismological Center (CSN), and from the S5 onland temporary seismic networks , and offshore Distributed Acoustic Sensing (DAS) data , obtained from submarine telecommunications cables (Abyss network), we investigated the nature of these clusters and their possible classification as (foreshock–)mainshock–aftershock sequences, seismic swarms, or repetitive characteristic earthquakes.

By applying deep-learning techniques for seismic phase detection, together with unsupervised learning methods for seismic clustering, we explore the temporal evolution of seismicity and evaluate the existence of unique or recurrent characteristic events through time. Our results aim to elucidate why seismicity in this region is highly heterogeneous and to identify the physical processes that control this variability.

How to cite: Flores, M. C., Peralta, T., Potin, B., Baillet, M., Ambrois, D., Rivet, D., and Ruiz, S.: Seismic heterogeneity and earthquake clustering in the North-Central Chile subduction zone , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-19158, https://doi.org/10.5194/egusphere-egu26-19158, 2026.

The ~900 km long Queen Charlotte Fault (QCF), which separates the Pacific and North American plates, is the fastest-slipping oceanic–continental transform fault on Earth. Since the early 20th century, six major earthquakes with moment magnitudes greater than 7.0 have struck along the QCF, posing significant hazard threats to western America and Canada. The most recent event, the 2013 Mw 7.5 Craig earthquake, ruptured the central segment of the QCF and has been proposed as the first reported oceanic interplate earthquake exhibiting supershear rupture (Yue et al., 2013, JGR, 10.1002/2013JB010594), attracting widespread attention within the seismological community. Nevertheless, owing to the lack of long-term near-field monitoring, the seismic behavior of this region remains poorly understood.
In this study, we analyzed data from a dense ocean-bottom seismometer (OBS) network (network code YI) deployed along the central QCF from late August 2021 to early September 2022. The network consists of 25 OBS stations with an average spacing of ~15 km, providing an exceptional opportunity to characterize microseismicity along the central QCF. Using PickBlue, a machine-learning–based phase picker trained on OBS data, we constructed a high-resolution seismicity catalog comprising 502 well-located earthquakes with moment magnitudes ranging from 1.0 to 3.3. Our catalog delineates a steeply dipping (75°–80°) subvertical fault plane and reveals distributed seismicity within the Pacific plate, suggesting that transpressive convergence along the central QCF is largely accommodated by slip on the dipping fault plane and by offshore deformation of the Pacific plate.
Furthermore, along the central QCF, a highly segmented seismic behavior was revealed. Two primary earthquake clusters were detected in the southern section near the epicenter of the 2013 Mw 7.5 Craig earthquake, whereas the northern section remains nearly aseismic. The most active cluster was located at the margin of the main coseismic rupture area and coincides with a slightly curved fault segment, which may have decelerated northward rupture propagation during the 2013 Craig earthquake while accommodating most deformation. Further south, in the largest coseismic slip region, an additional cluster is observed within the area of maximum coseismic slip, suggesting progressive stress reloading on the previously ruptured fault plane. To better understand the stress evolution of the 2013 Craig earthquake, we also relocated a 21-day local aftershock catalog recorded ~4 months after the mainshock (Walton et al., 2019, EPSL, 10.1016/j.epsl.2018.11.021). Notably, the spatial distributions of aftershocks and interseismic events display a pronounced complementary pattern in the largest coseismic slip region, with interseismic events distributed at the center of the rupture zone and aftershocks beautifully surrounding it. Together, these observations illuminate the stress evolution of the 2013 Craig earthquake from the postseismic to the interseismic period and provide new insights into understanding the seismic cycle.

How to cite: Liu, Y., Lange, D., and Grevemeyer, I.: Post- and inter-seismic behavior in the central Queen Charlotte fault: implications for earthquake cycle, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7435, https://doi.org/10.5194/egusphere-egu26-7435, 2026.

The Christiana-Santorini-Kolumbo volcanic field is the most volcanically active segment of the Aegean Arc, encompassing both the large, caldera system of Santorini, and the smaller, highly active submarine volcano Kolumbo to the northeast. In early 2025, the region experienced a strong seismic swarm, with activity concentrated in the Anydros area, between the islands of Santorini and Amorgos. The most widely accepted cause for the swarm so far is a dyke intrusion beneath the Anydros Ridge (e.g. 1, 2).

As part of the HYDROMOX project, the RRS Discovery conducted a multidisciplinary research cruise in March 2025, acquiring passive seismic data alongside heat flow measurements, hydrothermal fluid and gas samples, and ROV imagery across Santorini, Kolumbo, and the Anydros Ridge. Here, we present results from a local network of 25 short-period and broadband ocean bottom seismometers (OBS), deployed for approximately three weeks within the Santorini-Amorgos area during the waning phase of the seismic swarm in March 2025. Despite the swarm having peaked prior to deployment, the OBS array recorded over 140,000 seismic events across 23 days, resulting in ~20,000 well-located events. This local microseismic dataset allows the recording of smaller magnitude events with better spatial coverage than the land networks alone, particularly at shallow depths. Integrating these results with regional structural information and seismic velocity models allows us to further investigate the after-effects of the hypothesised dike intrusion and the activation of shallow fault networks. Our findings provide new insights into the post-peak dynamics of this complex volcano-tectonic seismic swarm in a densely populated and seismically active region of the Aegean.

References:

[1] Isken, M. P. et al. (2025). Volcanic crisis reveals coupled magma system at Santorini and Kolumbo. Nature 2025 645:8082, 645(8082), 939–945.

[2] Lomax, A. et al. (2025). The 2025 Santorini unrest unveiled: Rebounding magmatic dike intrusion with triggered seismicity. Science, 390(6775). 

How to cite: Gregory, E., Bayrakci, G., Tary, J.-B., and Yeo, I.: Resolving the final phase of the 2025 Santorini-Amorgos seismic swarm using a local ocean bottom seismometer array, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-20161, https://doi.org/10.5194/egusphere-egu26-20161, 2026.

Deep low-frequency earthquakes (DLFs) have been observed at a depth of about 30 km near the Moho around active volcanoes. Source process of DLFs is thought to be related to the deep magma supply system (e.g., Nakamichi et al. 2003) and the number of them sometimes increase several months before an eruption. Therefore, DLFs is a key information for understanding the magma supply process from deep to shallow (e.g., Shapiro et al. 2017). The Iwate volcano, which located in Northeastern part of Japan, has experienced a volcanic unrest such as abnormal crustal deformation and increase of seismicity including DLFs since February 2024. In this study, we investigated the seismicity and focal mechanisms of DLFs in the Iwate volcano based on the matched filter method (e.g., Gibons and Ringdal, 2006) and waveform inversion.

We performed matched filter analysis for the period from January 2019 to December 2025. We used 175 template events relocated by Kurihara and Obara, (2021). This analysis detected approximately 8000 events over a 6-year period. The Iwate volcano has three DLF clusters: 10 km depth just beneath the volcano, 30 km depth of northern and southern parts of it. Among them, seismicity of DLFs in the north cluster has extremely increased since August 2024 and the number of them has also increased in the shallow cluster since three months after the activation of northern cluster. This result suggests the possibility that magma supply from deep to shallow areas has continued.

Then, we estimated source time functions and moment tensor components for template events based on the procedure of Aso and Ide (2014). Obtained moment tensors have various orientations and significant compensated linear vector dipole component, which is consistent with the previous study of DLFs in the Iwate volcano (Nakamichi et al. 2003). However, comparing focal mechanisms and seismicity, we found that DLF activity is mainly composed of events detected from specific templates. This result suggests the existence of a dominant local stress field such as the shape of stagnated magma near the Moho. As a preliminary interpretation, activation of DLFs can be caused by the volumetric deformation associated with intrusion into the magma.

How to cite: Oikawa, G.: Seismicity and focal mechanisms of deep low-frequency earthquakes in the Iwate Volcano, Northeast Japan, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8624, https://doi.org/10.5194/egusphere-egu26-8624, 2026.

We have investigated the spatial and temporal variations in stress drop during a series of earthquakes, from the swarm to the early aftershocks in the Noto Peninsula, central Japan. The swarm began in 2018 and lasted more than five years until the devastating 2024 Mw 7.5 Noto earthquake. We estimated stress drop from a comparison between the observed and theoretical Frequency Index (FI). Employing a commonly used source model and the relations between corner frequency and stress drop, the theoretical FI is a function of S-wave velocity, attenuation factor, and stress drop. By assuming a S-wave velocity and using the separately measured attenuation factor, the theoretical FI depends solely on stress drop. We estimate the stress drop that minimizes the difference between the observed and theoretical FI. We validated the method by comparing the obtained stress drop with that from the ordinary method to measure the corner frequency. The result is consistent, though our method gives slightly higher stress drop than a one-to-one relationship. As long as we discuss the relative values of stress drop, this difference has little effect on subsequent observations and interpretations.

We estimated stress drop for 3,490 earthquakes with magnitudes from 1.5 to 4.0. The obtained stress drop shows an apparent spatial variation: Low-Stress-Drop Events (LSDEs) are dominant in the southern part of the swarm area, whereas both LSDEs and High-Stress-Drop Events (HSDEs) coexist in the northern part. Previous studies classified the swarm area into four subareas of S (southeast), W (southwest), N (northwest), and E (northeast). The swarm areas temporarily expanded, including subareas in this order. Our results show that earthquakes in the S subarea, where the swarm started, mostly have low stress drop. Some events with extremely low stress drop exhibit a unique waveform with a low-frequency band and a decaying amplitude over time, resembling volcanic low-frequency earthquakes. Previous studies have suggested that crustal fluids contribute to seismogenesis in the area. Our results give further and strong support for this suggestion. HSDEs mainly occurred in subarea N, with the highest seismicity, and in subarea E, which hosted many large swarm earthquakes. They are located sandwiched between and around the band of LSDEs. During the early aftershock stage, HSDEs are absent in these locations. The rupture of the mainshock originated in these subareas and was "quiet," with only minor moment release. Previous studies suggested that these subareas experienced strain release preceding the mainshock due to the swarm. The spatio-temporal variation of HSDEs is consistent with the interpretation. On the other hand, LSDEs in subarea S occurred during both the swarm and the aftershock sequences, implying continuous fluid supply from the anticipated fluid source beneath the swarm area. Our method of stress drop estimation does not aim for high accuracy, but rather to obtain estimates for many earthquakes with acceptable accuracy. The results of this study indicate that the approach works well to investigate the seismogenesis of complex earthquake sequences.

How to cite: Kosuga, M. and Maeda, T.:  Spatio-temporal evolution in stress drop during earthquake sequences from the swarm to aftershocks in the Noto Peninsula, central Japan, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8786, https://doi.org/10.5194/egusphere-egu26-8786, 2026.

How to cite: Allas, H.-R., Jenkins, J., Winder, T., Ágústsdóttir, T., Guðnason, E. Á., Heimisson, E. R., Brandsdóttir, B., and Rawlinson, N.: Seismic Network in the Underexplored Central Iceland (SNUCI): Investigating Local Seismicity in the Langjökull and Hofsjökull volcanic systems , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14712, https://doi.org/10.5194/egusphere-egu26-14712, 2026.

How to cite: Kraft, T., Simon, V., and Toledo, T.: Replacing spatial distance with waveform similarity in nearest-neighbour earthquake clustering , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8019, https://doi.org/10.5194/egusphere-egu26-8019, 2026.

Tectonic earthquake swarms deviate significantly from the spatio-temporal evolution characteristic of mainshock-aftershock sequences. While earthquake sequences often begin with a dominant event called a mainshock, followed by a decaying rate of aftershocks governed by the Omori-Utsu law, earthquake swarms are defined by a gradual escalation of seismic activity lacking a singular, triggering large earthquake at the start of the cluster. In these sequences, peak magnitudes often emerge mid-sequence or later, frequently accompanied by distinct spatial migration and episodic bursts. This complex clustering evolution is driven by the interaction between steady tectonic loading and transient, short-term forcing mechanisms. Identifying these phenomena requires robust, unsupervised methodologies—a need that has spurred the development of various detection algorithms over recent decades.

This research provides a systematic evaluation of prevalent cluster-detection techniques and sequence characterization via the release of seismic moment over time. We apply four well-known (de-)clustering algorithms to identify clusters in space-time-magnitude space; subsequently, we evaluate each cluster using the statistical moment of the cluster source time function (i.e., the release of seismic moment over time). By utilizing thousands of synthetic catalogs generated through Epidemic-Type Aftershock Sequence (ETAS) modeling with time-varying background rates, we simulate realistic swarm behavior to test these tools. This synthetic framework allows us to define parametric boundaries that robustly differentiate swarms from mainshock-aftershock clusters. We then validate our findings against well-documented real-world datasets, including the 2010–2014 Pollino sequence and the Húsavík-Flatey transform fault in Northern Iceland. Additionally, show that the cluster classification to distinguish swarms from mainshock-aftershock sequences via the proposed statistics depends on the type of (de-)clustering algorithm used and, most importantly, on the cluster duration.  Accordingly, our results highlight that real-world application remains sensitive to algorithm choice and catalog completeness, suggesting that human oversight is still essential for precise swarm characterization and interpretation.

How to cite: Passarelli, L., Petersen, G., Mizrahi, L., and Cesca, S.: Detecting and characterizing swarm-like seismicity, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6631, https://doi.org/10.5194/egusphere-egu26-6631, 2026.

The East Eifel Volcanic Field (EEVF) in Germany is a densely monitored yet dormant distributed volcanic field comprising hundreds of Quaternary volcanoes, including the most recent eruption of the Laacher See Volcano ~13,000 years ago. The GFZ's Central European Volcanic Province Observatory (CVO) integrates data delivered by multiple partners: Seismic data, GNSS, a superconducting gravimeter, and fluid monitoring sites to detect subtle signals of volcanic and tectonic activity. This study focuses on microseismic swarms observed within the EEVF since January 2020. We present a multidisciplinary analysis of selected swarm sequences, with particular emphasis on their spatial proximity to recently mapped crystal velocity anomalies, interpreted as potential melt reservoirs. Notably, the suspected locations of partial melt reservoirs beneath the EEVF correlate well with the location of the DLF earthquakes. Most recently, a swarm of approximately 120 locatable events (Mw < 1.6) occurred near the Laacher See Volcano in October 2025, prompting considerable public and media interest. Preliminary moment tensor inversion and cross-correlation-based clustering indicate a highly self-similar sequence with oblique normal faulting to strike-slip faulting mechanisms in agreement with the regional stress field. Although the EEVF is not typically characterized by extensive swarm activity, our analysis reveals tens of tiny swarm sequences over the past six years. The Oct-2025 swarm illustrated the feasibility of a quick, multidisciplinary assessment of the EEVF; during this swarm, no co-seismic changes in ground deformation, fluid properties, or gravity were detected. In our ongoing work, we explore several hypotheses linking the microseismic swarms to fluid-driven processes near potential melt reservoirs.

How to cite: Petersen, G., Laumann, P., Isken, M., Dahm, T., Deng, Z., Woith, H., Voigt, C., Zimmer, M., Hensch, M., Zeckra, M., Schmidt, B., and Zhang, H.: Microseismicity swarm activity in the East Eifel Volcanic Field - A response to magmatic processes?, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2824, https://doi.org/10.5194/egusphere-egu26-2824, 2026.