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Earthquake swarms and complex seismic sequences driven by transient forcing in tectonic and volcanic regions

Earthquake swarms are characterized by a complex temporal evolution and a delayed occurrence of the largest magnitude event. In addition, seismicity often manifests with intense foreshock activity or develops in more complex sequences where doublets or triplets of large comparable magnitude earthquakes occur. The difference between earthquake swarms and these complex sequences is subtle and usually flagged as such only a posteriori. This complexity derives from aseismic transient forcing acting on top of the long-term tectonic loading: pressurization of crustal fluids, slow-slip and creeping events, and at volcanoes, magmatic processes (i.e. dike and sill intrusions or magma degassing). From an observational standpoint, these complex sequences in volcanic and tectonic regions share many similarities: seismicity rate fluctuations, earthquakes migration, and activation of large seismogenic volume despite the usual small seismic moment released. The underlying mechanisms are local increases of the pore-pressure, loading/stressing rate due to aseismic processes (creeping, slow slip events), magma-induced stress changes, earthquake-earthquake interaction via static stress transfer or a combination of those. Yet, the physics behind such processes and the ultimate reasons for the occurrence of swarm-like rather than mainshock-aftershocks sequences, is still far beyond a full understanding.

This session aims at putting together studies of swarms and complex seismic sequences driven by aseismic transients in order to enhance our insights on the physics of such processes. Contributions focusing on the characterization of these sequences in terms of spatial and temporal evolution, scaling properties, and insight on the triggering physical processes are welcome. Multidisciplinary studies using observation complementary to seismological data, such as fluid geochemistry, deformation, and geology are also welcome, as well as laboratory and numerical modeling simulating the mechanical condition yielding to swarm-like and complex seismic sequences.

Co-organized by NH4/TS4
Convener: Luigi Passarelli | Co-conveners: Simone Cesca, Federica LanzaECSECS, Francesco Maccaferri, Maria MesimeriECSECS
| Mon, 23 May, 10:20–11:48 (CEST), 13:20–14:05 (CEST)
Room D3

Mon, 23 May, 10:20–11:50

Chairpersons: Luigi Passarelli, Francesco Maccaferri, Federica Lanza


Miriam Christina Reiss et al.

Deciphering the nature of seismicity in regions of active magmatic and tectonic areas is critical when examining the interplay between faulting, magmatism and magmatic fluids. Here, we present a rich seismic data set from a 15-month temporary network from the Natron basin of the East African Rift System, which provides an ideal location to study these processes owing to its recent magmatic-tectonic activity and ongoing active carbonatite volcanism at Oldoinyo Lengai. We report seismicity, seismic swarms and their fault plane solutions which we use to constrain the complex volcanic plumbing system and long-term tectonic processes.

Between March 2019 and May 2020, we locate ~10 000 earthquakes with ML -0.85 to 3.6. These are related to ongoing magmatic and volcanic activity in the region, as well as regional tectonic extension. We observe seismicity down to ~17 km depth north and south of Oldoinyo Lengai and shallow seismicity (3 - 10 km) beneath the inactive shield volcano Gelai, including two likely fluid driven swarms. The deepest seismicity (down to ~20 km) occurs above a previously imaged magma body below Naibor Soito volcanic field. These seismicity patterns reveal a detailed image of a complex volcanic plumbing system, supporting potential lateral and vertical connections between shallow- and deep-seated magmas, where fluid and melt transport to the surface is facilitated by intrusion of dikes and sills.

Focal mechanisms vary spatially and are a strong indicator for differences between magmatic and tectonic forces. T-axis trends reveal dominantly WNW-ESE extension near Gelai, while strike-slip mechanisms and a radial trend in P-axes are observed in the vicinity of Oldoinyo Lengai. These observations support local variations in the state of stress, resulting from a combination of volcanic edifice loading and magma-driven stress changes imposed on a regional extensional stress field. Our results indicate that the southern Natron basin is a segmented rift system, in which fluids preferentially percolate vertically and laterally in a region where strain transfers from a border fault to a developing magmatic rift segment.

How to cite: Reiss, M. C., Muirhead, J., Laizer, A., Kazimoto, E., Ebinger, C., Link, F., and Rümpker, G.: The nature of seismicity in a complex volcanic rift setting, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5154, https://doi.org/10.5194/egusphere-egu22-5154, 2022.

Martina Raggiunti et al.

An increase of fluid pressure can induce fault slip and therefore lead to the occurrence of earthquakes. The aim of our works is to investigate this phenomenon from a seismic point of view.

We analyzed the EAGLE seismic database, that includes the earthquakes that occurred in the Northern Main Ethiopian Rift (NMER) from October 2001 to February 2003, with the aim of achieving accurate earthquake locations that show subsurface fault structure and temporal behavior. The earthquakes in the database were relocated with a number different methods including double difference relative relocation following waveform cross correlation. We focus on the Fentale-Dofan magmatic segment, an area involved in the active rifting process with a widespread seismicity and with the presence of surface hydrothermal deposits that suggest ongoing hydrothermal activity. The earthquakes were first relocated with NonLinLoc using a non-linear method and the velocity model from controlled source seismology. The events relocated with NonLinLoc was divided in four distinct clusters, with three clusters in the rift and one cluster on the western border fault. Each cluster was then relocate separately with HypoDD double-difference location algorithm, including implementation of waveform cross correlation. From the earthquake magnitudes, b-values and seismic moment were also computed. Seismic data was interpreted with hydrothermal surface data obtained from automated remote mapping from Landasat 8 images.

The analysis of the temporal-spatial distribution of earthquakes shows that some of the clusters are strongly concentrated in time and in space, and therefore swarm-like. These swarms are characterized by events with similar waveforms. There is direct correlation between the increase of seismic rate in the cluster and the presence of families of similar earthquakes. The values found for the seismic moment suggest that the events are originated from activation of rift related structures. This is supported by the N to NE elongation strike of seismic clusters highlighted by the HypoDD location, in accordance with the tectonic setting of the area. The events are mostly localized in the top 15 km of the crust. The b-values calculated for the clusters are smaller than 1, with the exception for the cluster localized near Dofan volcanic complex. The hydrothermal deposits mapped by us are mainly focused in two areas: on the western side of Dofan volcanic complex, in an area intense faulted by NNE-SSW faults; and around the Fentale volcano with a circular pattern on southern side of volcanic edifice.

The no clear correlation between seismicity and mapped hydrothermal deposits suggesting that seismicity is not driven by shallow hydrothermal fluid flow. It is possible to conclude that these earthquakes have a component fluid induced, but the origin of these fluids are deeper than the fluids that feed the hydrothermal systems.

How to cite: Raggiunti, M., Keir, D., Pagli, C., and Lavayssière, A.: Analysis of fluid induced earthquake swarms in Northern Main Ethiopian Rift, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5869, https://doi.org/10.5194/egusphere-egu22-5869, 2022.

Vaibhav Vijay Ingale et al.

Seismic clusters of volcanic and tectonic events along mid-oceanic ridges are inherent to seafloor spreading. Due to the rapid attenuation of seismic waves in the solid Earth, land-based seismic networks lack the low-level seismicity associated with such clusters. However, regional studies using autonomous underwater hydrophones overcome this difficulty due to their sensitivity to low-frequency hydroacoustic waves, known as T-waves, that travel in the SOund Fixing And Ranging (SOFAR) channel over very long distances with little attenuation. Using hydroacoustic records from the temporary OHASISBIO network and permanent stations of the CTBT Organization, we have examined a seismic cluster near the Melville Fracture Zone (FZ) at 61°E along the ultraslow spreading Southwest Indian Ridge (spreading rate: 14-15 mm/yr).

Near 61°E, 259 events were reported in the International Seismological Center (ISC) catalogue between 9th June 2016 and 25th March 2017 in the region of 3 x 3 degrees in latitude and longitude around Melville Transform. Out of them, 17 events display normal faulting mechanisms parallel to the ridge axis (Global Centroid Moment Tensor (GCMT) solutions).

In the preliminary analysis, we have detected 4273 hydroacoustic events between 9th June and 11th July 2016, vs 28 events in the ISC catalogue, so with ~150-fold increase in the event detections. These events are mostly aligned parallelly to the ridge axis near its intersection with the Melville FZ. The event median uncertainties are ~4.7 km in latitude and longitude, and ~1.4 s in origin time. Their median acoustic magnitude or Source Level (SL) is 225.26 dB.

This seismic cluster includes several highly energetic and short duration (~10 s) impulsive events, located on the slopes of seamounts near the FZ at 61.2°E. These events are interpreted as thermal explosions resulting from direct magma supplies on the seafloor. Also, most of the hydroacoustic events are clustered around the same seamounts. There is no evidence for long mainshock-aftershock sequence at the onset of this seismic cluster. These observations point to a magmatic origin for this seismic cluster with an active source located near a chain of seamounts in the vicinity of Melville FZ.

How to cite: Ingale, V. V., Bazin, S., and Royer, J.-Y.: Hydroacoustic observations of a seismic cluster at Melville Fracture Zone along the Southwest Indian Ridge in 2016-17, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-462, https://doi.org/10.5194/egusphere-egu22-462, 2022.

Simone Cesca et al.

A swarm of ~85,000 volcano-tectonic earthquakes started in August 2020 at the Bransfield Strait, between the South Shetland Islands and the Antarctic Peninsula. The Bransfield Basin is a unique back-arc basin, where the past active subduction slowed down dramatically ~4 Ma, leaving a small remnant of the former Phoenix plate incorporated in the Antarctic plate. Today there is no clear evidence for recent normal seafloor spreading. Continental crust is thinning to develop oceanic crust and the current extension is either attributed to the Phoenix Block subduction and rollback or to shear between the Scotia and Antarctic plates. The 2020 seismicity occurred close to the Orca submarine volcano, previously considered inactive. Geodetic data reported a transient deformation with up to ~11 cm northwestward displacement over King George Island. We use a wide variety of geophysical data and methods to reveal the complex migration of seismicity, accompanying the intrusion of 0.26-0.56 km3of magma off the Orca seamount at ~20 km depth. Deeper, clustered strike-slip earthquakes mark the magmatic intrusion at depth, while shallower normal faulting events are induced by the growth of a lateral dike, extending ~20 km NE-SW. Seismicity abruptly decreased after the largest Mw 6.0 earthquake, suggesting the magmatic dike lost pressure with the slipping of a large fault and the opening of upward paths. A seafloor eruption is likely, but not confirmed by sea surface roughness or temperature anomalies. The unrest documents episodic magmatic intrusion in the Bransfield Strait and provides unique insights into active continental rifting.

How to cite: Cesca, S., Sugan, M., Rudzinski, Ł., Vajedian, S., Niemz, P., Plank, S., Petersen, G., Deng, Z., Rivalta, E., Vuan, A., Plasencia Linares, M. P., Heimann, S., and Dahm, T.: A massive earthquake swarm driven by magmatic intrusion at the Bransfield Strait, Antarctica, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3468, https://doi.org/10.5194/egusphere-egu22-3468, 2022.

Zilin Song and Yen Joe Tan

Seismic swarms at volcanic regions are important manifestations of volcanic unrest. While they are often inferred to be related to fluid or magma movements, their underlying process remains an active research topic. In particular, quantifying the proportion of seismic swarms that are related to magma movement can potentially improve their utility for eruption forecasting. To better understand the relationship between seismic swarms and magma movement, we focus on the Akutan volcano where episodic inflations have been recorded every 2-3 years since 2002. We first applied template matching on continuous seismic waveforms between 2005-2017 to improve the earthquake catalog’s magnitude of completeness. We further classified the events as long-period (LP) or regular volcano-tectonic (VT) events based on their frequency content. After waveform-based double-difference relocation, we find that the VT and LP events are concentrated above and below the shallow magma reservoir respectively. We clustered the VT and LP events based on their spatiotemporal evolution and find that most clusters are swarm-like with no clear mainshock-aftershock sequences. Based on their temporal relation to the inflation episodes, we infer that the LP swarms are related to ascending magma into the shallow reservoir, which sometimes triggers VT swarms through stress transfer.

How to cite: Song, Z. and Tan, Y. J.: Relationship between seismic swarms and episodic inflations at Akutan Volcano in Alaska, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3364, https://doi.org/10.5194/egusphere-egu22-3364, 2022.

Philippe Danre et al.

Natural earthquake swarms occur in various geological contexts, and are usually interpreted as driven by fluid pressure diffusion. However, little is known about their fluid-driving processes, as no direct observations of either fluid and deformation are possible at such depths. To improve our understanding of the processes involved in swarms, we develop a quantitative comparison between natural and injection-induced swarms. Fluid injections in the crust, for instance geothermal reservoir development or wastewater storage, are accompanied by a prolific seismicity, that can be related to the fluid-pressure perturbation and potentially in association with aseismic slip at depth. It is well-accepted that the released seismic moment scales with injected fluid volume, but proposed relations usually not consider the contribution of aseismic deformation. Constraining such a relation might provide information on what happens at depth during natural earthquake swarms. Indeed, based on the numerous similarities observed between natural and injection-induced swarms, we confirm that both types of sequences seem to obey the same physics. In our work, we establish a framework to relate seismic observables to the fluid volume circulating at depth. This allows us to quantify aseismic slip for all types of swarms, but also to estimate the volume of fluids circulating at depth during natural earthquake swarms. By focusing on several natural swarms, this sheds a new light on the processes driving swarms of seismicity.

How to cite: Danre, P., De Barros, L., and Cappa, F.: Injection-induced sequences give us insights about what is happening at depth during natural earthquake swarms, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1068, https://doi.org/10.5194/egusphere-egu22-1068, 2022.

Dalija Namjesnik et al.

In post-mining environments, seismic hazard is still not very well understood, as number of research studies remains limited. Seismicity is often considered in post-mining risk mitigation procedures as a precursory of failure initiation in rocks within the mining works leading to ground instabilities. However, flooding of the mines can also lead to perturbations of stress states and pore pressures within the rock mass leading to failure of pre-existing faults, which may have more important impact on public safety due to a potentially longer period of activity and possibly higher magnitudes of the induced seismic events depending on the fault size.  

In a former coal mine in Gardanne, France, which was abandoned in 2003 and flooded afterwards, seismicity started appearing and raising concerns since 2010, when flooding reached the center of the mining basin. The seismic activity has been occurring approximately every two years in the form of crises. Events were also felt by the local population. A sparse temporary monitoring network has been installed in 2013 in this seismically active area. Based on research results so far, seismicity originates from the reactivation of faults underlying the mining excavations and is influenced by flooding, pumping of the water, and seasonal meteorological conditions. 

We investigate the clustering behavior and multiplet occurrences within the seismic events recorded by the sparse temporary microseismic network between 2014 and 2017. Detailed cluster analyses, the spatio-temporal distribution, recurrence time patterns, and source parameters help to characterize seismically active structure(s) below the mining works. The triggering of the seismic activity in each cluster appears to be differently influenced by the hydro-meteorological conditions, with some clusters being more affected by rainfall, while other by dry period. The variations of the pumping rate strongly affect the rate of seismicity in this area as well. The analysis is complemented by incorporating a new dataset recorded by an enhanced monitoring network during 2019, which allows to follow the evolution of the cluster activity. 

How to cite: Namjesnik, D., Niemz, P., Kinscher, J., Cesca, S., Contrucci, I., Dominique, P., and Aochi, H.: Clustering and event similarity based fault characterization of post mining induced seismicity of Gardanne mine, France, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7757, https://doi.org/10.5194/egusphere-egu22-7757, 2022.

Beata Orlecka-Sikora et al.

Water Reservoir Impoundment (WRI) can trigger swarms and strong earthquakes under favorable geological conditions. Although many studies have investigated the relationship between the pore pressure changes due to WRI and the observed seismicity, hydromechanical models that explain the observed processes are rare. We investigated the role of hydromechanical interactions in producing earthquake swarm bursts under pore pressure changes, using the Song Tranh 2 Water Reservoir Impoundment (WRI) in Vietnam as an example. Our work contributes to the investigation of the physical mechanisms responsible for earthquake swarms. We find that the seismic swarms accompanying WRI represent the shearing of a damage fault zone composed of multiple interfering surfaces. The source parameters of seismic swarms image the quasi-dynamic weakening of the fault damage zone. Fault weakening during the propagation of seismic rupture is a key process governing the earthquake rupture dynamics and energy partitioning. Quasi-dynamic weakening evolution means here that it captures histories of fault zone slip, including the seismic slip phases within this zone, and slip weakening shows a memory effect that fades with time. Based on the calculated traction evolution within the damage zones in ST2 we estimate the effective slip-weakening distance , which is a significant parameter for characterizing a fault-weakening process. The observed quasi-dynamic weakening process is fluid driven at slower migration velocity of the order of meters/day but over short duration the migration of seismicity accelerates to velocities of kms/day. We therefore conclude that the seismic swarms are driven by a combination of fluid pressurization and stress perturbation through aseismic slip induced by pore pressure changes.

This work was partially financed by National Statutory Activity of the Ministry of Education and Science of Poland No 3841/E-41/S/2021 (BOS, ŁR, TS, GL), and Polish National Science Centre grant No UMO-2017/27/B/ST10/01267 (GL), and co-financed by the European Union and the Polish European Regional Development Fund grant No POIR.04.02.00-14-A003/16 (DO)

How to cite: Orlecka-Sikora, B., Lizurek, G., Rudziński, Ł., Olszewska, D., and Shirzad, T.: Insight into the mechanics of seismic swarms triggered by water-reservoir impoundment, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8100, https://doi.org/10.5194/egusphere-egu22-8100, 2022.

Andrew Delorey et al.

Seismicity both at The Geysers geothermal field (northern California) and in north-central Oklahoma is heavily influenced by industrial activities related to energy production, though the mechanism in which earthquakes are induced or triggered is different. At The Geysers, much of the seismicity is linked to thermoelastic stresses caused by injecting cold water into hot rocks, while in Oklahoma the seismicity is linked to a reduction of confining stress on faults due to increasing pore pressure resulting from wastewater injections. Here we show that these contrasting conditions are also evident in tidally-triggered earthquakes. At The Geysers, earthquakes preferentially occur during maximum extensional strain, which does not occur at the same time as maximum shear strain on optimally oriented faults in the regional stress field. In Oklahoma, earthquakes preferentially occur during maximum shear strain on optimally oriented faults, rather than maximum extensional strain. The magnitude of tidal extensional strain is naturally much greater than tidal shear strain. However, in a fluid saturated environment, pore pressure responds to changes in volume, which can counteract or reduce the effect of the applied stress. The difference in behavior at these two sites is indicative of the level of coupling between applied stress and pore pressure, corresponding to unsaturated conditions at The Geysers and high pore pressure in Oklahoma.

How to cite: Delorey, A., Ma, X., and Chen, T.: Triggered Earthquakes Reveal Hydraulic Properties of the Subsurface, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9312, https://doi.org/10.5194/egusphere-egu22-9312, 2022.

Pierre Dublanchet

Earthquake swarms are generally interpreted as resulting from the redistribution of stresses within the crust. Swarms develop in response to fluid flow and poro-thermo-elastic stresses in reservoirs, aseismic slip on major faults, or during magmatic events in volcanic areas. However, our ability to quantify stress changes at depth from the observation of earthquake swarms is still very limited.  In his seminal study (Dieterich, 1994) was able to develop a model leading to a quantitative relationship between stress and seismicity rate. This model, based on non-interacting spring-and-slider systems undergoing rate-and-state friction was successful in determining stress conditions from seismicity rate in several active areas involving both tectonic and magmatic processes. This approach nevertheless relies on very strong assumptions, one of them being that no stress redistribution occurs following an earthquake. Stress redistributions are however known to drive earthquake sequences as observed during foreshock aftershock sequences. Ignoring this contribution might lead to wrong estimations of stress conditions at depth from seismicity rate.
In order to evaluate the role of stress redistribution in earthquake swarm dynamics, I present a new physics based earthquake simulator extending Dieterich's model. It consists of a set of planar rate-and-state frictional faults distributed in a 3D homogeneous elastic medium, and loaded by a prescribed stress history. Faults can have any size and orientation. Stress redistributions are thus fully accounted for.
The model is then used to investigate the relationship between seismicity rate and stressing history under different loading conditions (constant tectonic stressing, periodic loading) and fault properties (initial stress, frictional properties, relative distance between faults). In many cases, Dieterich's theory ignoring stress transfers captures many features of the seismicity rate patterns. This is particularly true for periodic loading, which generates frequency dependent seismicity modulation: at low frequency, seismicity rate scales exponentially with the loading stress, while at higher frequencies it tracks the stressing rate. The period separating the two modulation regimes is correctly predicted by Dieterich's theory. Under constant loading, seismicity rate is also constant (as predicted by Dieterich's theory) if the sequences are analysed over long enough time series involving several seismic cycles on each fault. At a shorter time scale however, significant clustering (not predicted by Dieterich's approach) arises, in particular for compact fault distributions enhancing the stress redistributions. 
More generally, the approach presented here allows to define the mechanical conditions leading to a significant contribution of stress transfers in the development of earthquake swarms.

How to cite: Dublanchet, P.: What is the contribution of stress redistribution in earthquake swarm dynamics?, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10369, https://doi.org/10.5194/egusphere-egu22-10369, 2022.

Alexis Sáez and Brice Lecampion

Seismic swarms are often interpreted to be driven by natural fluid pressurization in the Earth’s crust, when seismicity is observed to spread away from a common origin and follows approximately a square-root-of-time pattern of growth. On the other hand, a growing body of literature suggests that aseismic fault slip seems to be a frequent result of fluid injections and may trigger seismicity due to the stress transfer of quasi-statically propagating ruptures in critically stressed regions. Although in some conditions a nominal pore pressure perturbation front may evolve proportionally to the square root of time, much less is known about the temporal patterns of fluid-driven aseismic slip fronts. The latter hinders efforts to distinguish whether some seismic swarms are driven by aseismic slip episodes or not. In this contribution, we provide an extensive set of physics-based solutions that describes the evolution of fluid-driven aseismic slip fronts for a wide range of conditions in terms of in-situ stress state and fluid flow. Our solutions show that fluid-driven aseismic slip fronts may result in many different patterns of propagation, depending on the characteristics of the fluid source (e.g., constant-pressure source, constant-rate source, among others) and also if simplified 2-D or fully 3-D elasticity is considered. Other parameters such as the initial stress state and fault hydraulic properties are also relevant in the propagation of the slip fronts. Our family of solutions includes cases in which aseismic slip fronts propagate following a square-root-of-time dependence, a linear expansion with time, power laws of time with exponents lower than ½, and some other more complex evolutions. These results are based on the model of a fluid-driven frictional shear crack that propagates on a planar fault interface characterized by a constant friction coefficient and a constant permeability, embedded in an infinite linearly elastic medium with an initially uniform state of stress. Although the basic assumptions of the model are simple, it results in a significant amount of complexity in terms of possible spatio-temporal patterns of rupture propagation. Since a constant friction coefficient corresponds to a fault interface with zero fracture energy, we show by analyzing the rupture-front energy balance of fluid-driven aseismic slip transients with non-zero fracture energy, that an asymptotic regime in which the fracture energy is negligible is always ultimately reached. This regime is approached asymptotically when the rupture has propagated over a distance larger than a characteristic length-scale depending on the frictional fracture energy and the in-situ stress state. We expect our results to provide a simple means to interpret observations of seismic swarms for which fluid-driven aseismic slip transients are thought to be a relevant mechanism in the triggering of seismicity.

How to cite: Sáez, A. and Lecampion, B.: Spatio-temporal patterns of fluid-driven aseismic slip transients: implications to seismic swarms, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11943, https://doi.org/10.5194/egusphere-egu22-11943, 2022.

Mon, 23 May, 13:20–14:50

Chairpersons: Maria Mesimeri, Simone Cesca

Xiao Ge Liu et al.

Understanding the nature of foreshock evolution is important for earthquake nucleation and hazard evaluation. Aseismic slip and cascade triggering processes are considered to be two end-member precursors in earthquake nucleation processes. However, to perceive the physical mechanisms of these precursors leading to the occurrence of large events is challenging. In this study, the relocated 2021 Yangbi earthquake sequences are observed to be aligned along the NW-SE direction and exhibit several evident spatial migration fronts towards the hypocenters of large events including the mainshock. An apparent static Coulomb stress increase on the mainshock hypocenter was detected, owing to the precursors. This suggests that the foreshocks are manifestations of aseismic transients that promote the cascade triggering of both the foreshocks and the eventual mainshock. The temporal depth of the brittle-ductile transition exhibit deepening, followed by shallowing during the foreshock-mainshock-aftershock sequence. By jointly inverting both InSAR and GNSS data, we observe that the mainshock ruptured on a blind vertical fault with a peak slip of 0.8 m. Our results demonstrate that the lateral crustal extrusion and lower crustal flow are probably the major driving  mechanisms of mainshock. Additionally, the potential seismic hazards on the Weixi-Weishan and Red River faults deserve further attention

How to cite: Liu, X. G., Xu, W. B., He, Z. L., Fang, L. H., and Chen, Z. D.: Aseismic slip and cascade triggering process of foreshocks leading to the 2021 Mw 6.1 Yangbi Earthquake, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6917, https://doi.org/10.5194/egusphere-egu22-6917, 2022.

Hasbi Ash Shiddiqi et al.

Parts of northern Norway, located between the rifted Mid Norwegian margin and the Northern Scandinavian mountains, are seismically active despite being situated in a stable continental region. Previously, seismic swarms have been observed in different places along the coast, but detailed studies on the swarms could not yet be carried out due to sparse seismic networks. During the last decade, the number of seismic stations has increased significantly, allowing for a more detailed study of the seismicity. Here, we develop a machine-learning-based earthquake catalog from eleven years of continuous data (2010-2021) and combine it with the earthquake catalog from the Norwegian National Seismic Network. To improve accuracy, we perform relative earthquake relocation using differential times, and clustering analysis based on waveform cross-correlation. The relocation results reveal distinct clusters of possibly repeating events and several swarm sequences. A prominent seismic swarm occurred in the Jektvik area between 2014 – 2016 with the largest magnitude of ML 3.2. We compare the spatio-temporal distribution, b-value, seismic moment rate, and seasonal variation of each sequence. The Jetkvik swarm exhibits a diffusive pattern, which together with a low VP anomaly found by a previous tomography study suggests that fluids may play a role in the source process. We find that the possibly repeating clusters are not as diffuse in space, and mostly spread along the vertical axis. These earthquake clusters may be attributed to fault intersections, and fluids may not be a major factor in their generation. 

How to cite: Shiddiqi, H. A., Ottemöller, L., Halpaap, F., and Rondenay, S.: Earthquake swarms and clusters in stable continental regions: a case study from Northern Norway , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10517, https://doi.org/10.5194/egusphere-egu22-10517, 2022.

Gian Maria Bocchini et al.

The Armutlu Peninsula, bounded between two major sub-branches of the North Anatolian Fault (NAF) at the eastern Sea of Marmara, hosts the only onshore NAF segment along the Marmara seismic gap. It also hosts intense seismic and hydrothermal activity and documented episodes of aseismic slip. Here, we investigate the spatio-temporal distribution of seismicity in the northern Armutlu Peninsula to identify primary deformational mechanisms (i.e. seismic vs aseismic) and investigate the processes driving the seismicity. We employ multi-station matched-filter techniques to generate an enhanced seismicity catalog using up to 30 seismic stations, including regional permanent stations augmented by temporary stations from the SMARTnet network. We detect 7,677 events between 2019.01.25 and 2020.02.10, and successfully relocate 4,182 of them using double-difference methods. The enhanced seismicity catalog reveals four week-long sequences with up to ~> 200 events per day alternating in month-long periods with only < 10-20 events per day. Earthquakes primarily concentrate within a narrow region of ~80 km2 between 40.540°-40.600° N and 28.920°-29.025° E, forming linear structures striking from NW-SE to N-S at 5-12 km depth. Nearest-neighbor cluster analysis shows a gradual decrease of the ratio between swarm-like and burst-like activity, accompanied by a decrease of the background activity rates from the first to the fourth seismic sequence. Periods with predominantly swarm-like behavior and increased background activity exhibit a higher b-value. We invert focal mechanism solutions of background seismicity and obtain an extensional stress regime for the broader Armutlu Peninsula and a transtensional stress regime for the narrow, most seismically active region. Within the narrow seismically most active region the minimum compressive stress (σ3) is approximately horizontal and well defined, while the maximum (σ1) and intermediate (σ2) compressive stresses are close in magnitude and less well constrained. Moreover, in the most seismically active region, we observe that the principal stress orientations obtained from aftershocks is similar to that estimated from background seismicity. In contrast, the respective orientations of σ1 and σ2 inferred from foreshocks switch from vertical and horizontal to horizontal and vertical. Clusters of both normal faulting and strike-slip events identified through waveform based clustering analysis are optimally oriented with respect to the regional stress field, where normal faulting kinematics are predominant. We observe negligible seismic activity associated with the onshore segment of the NAF in the Marmara seismic gap. In contrast, we observe seismicity at 5-12 km depth that highlights the geometry of a major normal fault structure, the Waterfall fault, in the northern Armutlu Peninsula. The seismicity distribution and stress-field orientation suggest that the Waterfall fault exerts a primary control in the deformation of the northern Armutlu Peninsula.

How to cite: Bocchini, G. M., Martínez-Garzón, P., Verdecchia, A., Harrington, R. M., Bohnhoff, M., Turkmen, T., and Nurlu, M.: Spatio-temporal distribution of seismicity in the northern Armutlu Peninsula (northwest Turkey) , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2319, https://doi.org/10.5194/egusphere-egu22-2319, 2022.

Georgios Michas et al.

In 2020, a pronounced earthquake sequence occurred at the Perachora peninsula, at the eastern edge of the active continental Corinth Rift (Greece). The sequence evolved as a swarm over the course of four months, with the largest magnitude event (Mw=3.7) occurring approximately 2 months after its initiation. The sequence was widely felt by the local population, rising public concern regarding its evolution and a possibly impending stronger and damaging event. Herein, we use seismic waveform data from the Hellenic Unified Seismic Network (HUSN) to decipher the spatiotemporal evolution of the sequence and to investigate the possible triggering mechanisms. We use a custom velocity model for the area and apply the double-difference algorithm to relocate earthquake hypocenters at the East Corinth Rift for the period January 2020 – June 2021. Although the area lacks a local dense network, the herein analysis is able to reduce the relative location uncertainties and to enhance the spatial resolution of the catalogue, providing clues on the activated structures at depth. The spatiotemporal evolution of the sequence presented distinct characteristics of earthquake migration. The Perachora earthquake swarm initiated at shallow depths at the easternmost side of the activated area and progressively migrated towards greater depths to the northwest and then west. The observed seismicity migration pattern is consistent with an expanding parabolic front of hydraulic diffusivity D=2.8 m2/s and an average velocity of 0.22 km/day, indicating pore-fluid pressure diffusion as the primary triggering mechanism. This result is further supported by the relatively high diffusion exponent of the sequence (α=0.89±0.06), which is consistent with anomalous fluid transport phenomena in heterogeneous and fractured media. Overall, the analysis and results demonstrate that the sequence was triggered by fluid overpressures. The source of fluids is likely the down-going flux of meteoric water, possibly combined with fluids of hydrothermal affinity due to the area’s proximity to the Sousaki geothermal system. The activated structures are linked with the Pisia Fault Zone, a major tectonic feature in the area that was activated during the 1981 Alkyonides earthquakes; a series of three Mw > 6 events within a period of few days, which caused severe damage and fatalities in the broader area, including Athens.          


The research project was supported by the Hellenic Foundation for Research and Innovation (H.F.R.I.) under the “2nd Call for H.F.R.I. Research Projects to support Post-Doctoral Researchers” (Project Number: 00256).

How to cite: Michas, G., Kapetanidis, V., Spingos, I., Kaviris, G., and Vallianatos, F.: Spatiotemporal evolution of the 2020 Perachora peninsula earthquake sequence (East Corinth Rift, Greece) and its association with pore-fluid pressure diffusion, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5645, https://doi.org/10.5194/egusphere-egu22-5645, 2022.

Vincenzo Convertito et al.

The Thessaly seismic sequence (TSS) in Central Greece, started on 3 March 2021 with a Mw 6.3 event that struck an area located about 25 km WNW of the Larissa town. In the following days, TSS was affected by other two major events: An Mw 6.0 on March 4, localized about 7 km to the northwest of the first one, and a Mw 5.6 on March 12, located 12 km further towards the northwest of the second one. A large number of smaller events have been also recorded until mid-April when the sequence decreased in frequency and magnitude. The TSS represents the largest seismic sequence affecting a continental extensional domain in Greece that has been monitored by modern geodetic techniques. Thanks to the short satellite revisit time, InSAR measurements made it possible to isolate each contribution of the three major earthquakes of the sequence, thus allowing the study of their interactions. In addition, available geological data indicate that the northern sector of Thessaly represents a large seismic gap. This may be a direct consequence of the limited size of the faults (less than 20 km) and their intrinsic capability to originate earthquakes of small-to-moderate magnitude only. TSS, which finally filled the gap, confirmed this hypothesis.

We modelled the available InSAR deformation maps to retrieve the parameters characterizing some finite dislocation sources, which were used to perform a Coulomb stress transfer in order to investigate possible faults interactions. To constrain the geometry and location of the main fault structures involved during the TSS, we considered 1853 earthquakes occurred in the area from 28 February 2021 to 26 April 2021 with magnitude ranging between 0.2 and 6.3. Our model shows that the TSS has nucleated at shallow depths (<12 km) and is related to the activation of several blind, previously unknown, faults; moreover, the seismic sequence developed in a sort of domino effect involving a complex interaction among the normal faults within the activated crustal volume. As for the temporal evolution of the sequence, the delayed triggering of the Mw 6.0 earthquake can be explained by the distribution of the events occurred earlier, which encircle the asperities that will fail in the subsequent event together with a fluid diffusion in the seismogenic volume.

Finally, we highlight the key role played by the configuration of the Thessaly Basin characterized by blind faults interconnected at depth, particularly interesting from the neotectonics point of view. The used approach can help improving our knowledge on the seismic potential of the Thessaly region and refine the associated seismic hazard.

How to cite: Convertito, V., De Novellis, V., Reale, D., Adinolfi, G. M., and Sansosti, E.: March 2021 Thessaly, central Greece, seismic sequence:  domino effect of a complex normal fault system, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13176, https://doi.org/10.5194/egusphere-egu22-13176, 2022.