Local Earthquakes Research Articles

Fault Rheology Control on Rupture Propagation and Aftershocks Distribution during the 2016–2017 Central Italy Earthquakes

Abstract The stress released by the large coseismic ruptures and related aftershock sequences is strongly a heterogeneous process. We show highly resolved images of the normal fault system ruptured during the 2016–2017 central Italy earthquake sequence, as obtained by the high-resolution local earthquake tomography allowing relocation of a massive set of aftershocks. We get evidence that lateral changes of elastic properties on the fault planes account for the complexity in the rupture processes during the two Mw > 6 earthquakes. We observe an emergent phase in the first part of the Mw 6.5 rupture, and the coseismic slip becomes large when the rupture breaks through high Poisson ratio portions of the fault. Mainshocks break the fault portions that, although limited and segmented by inherited structural complexity, were dynamically interfering during the faulting episodes. The close repetition of slip on the same relatively high Poisson ratio patch suggests a dynamic weakening of the fault and/or an incomplete stress release during the first mainshock.

Geothermal and structural features of La Palma island (Canary Islands) imaged by ambient noise tomography

La Palma island is located in the NW of the Canary Islands and is one of the most volcanically active of the archipelago, therefore the existence of geothermal resources on the island is highly probable. The main objective of this work is to detect velocity anomalies potentially related to active geothermal reservoirs on La Palma island, by achieving a high-resolution seismic velocity model of the first few kilometres of the crust using Ambient Noise Tomography (ANT). The obtained ANT model is merged with a recent local earthquake tomography model. Our findings reveal two high-velocity zones in the island’s northern and southern parts, that could be related to a plutonic intrusion and old oceanic crust materials. Conversely, four low-velocity zones are imaged in the southern part of the island. Two of them can be related to hydrothermal alteration zones located beneath the Cumbre Vieja volcanic complex. This hypothesis is reinforced by comparing the S-wave velocity model with the seismicity recorded during the pre-eruptive phase of the 2021 Tajogaite eruption, which revealed an aseismic volume coinciding with these low-velocity zones. Another low-velocity zone is observed in the southern part of the island, which we interpret as highly fractured rocks which could favour the ascent of hot fluids. A last low-velocity zone is observed in the central part of the island and associated with loose deposits generated by the Aridane valley mega landslide.

Nature of the Himalayan Seismicity Belt in the Garhwal–Kumaun Segment and Its Tectonic Implications

Abstract High-quality data recorded by a dense network of 53 seismic stations in the Garhwal–Kumaun Himalaya between February 2017 and December 2021 is analyzed. A total of 813 local earthquakes are relocated using a newly developed regional 1D velocity model incorporating station corrections. In addition, focal mechanism solutions of M ≥ 3.8 events are estimated using waveform inversion. The relocated seismicity patterns along with the focal mechanism solutions are utilized to present a seismotectonic scenario of the region. Almost 95% of the relocated seismicity is found to be clustered along the Himalayan seismic belt (HSB), down to ∼24 km depth. Seismicity in this belt is interpreted to be caused due to interseismic stress loading associated with the ongoing India–Eurasia collision tectonics. A few scattered hypocenters in the deeper crust between 30 and 50 km depth attest the strength of the downgoing Indian plate. Focal mechanisms in the seismogenic upper crust reveal thrusting of the Indian plate beneath the Lesser Himalaya, with compression normal to the strike of the Main Central Thrust (MCT). The north-dipping thrust mechanisms can be associated with a near-horizontal Main Himalayan Thrust (MHT). In addition, more steeply dipping faults above it define the Lesser Himalayan duplex systems, similar to those in western and Nepal Himalaya. A prominent ∼50 km wide seismicity gap region observed within the HSB is probably due to (1) a locally varying locking width of the MHT; (2) an unruptured, ductile segment at the eastern end of the rupture zone of the great 1803 earthquake (Mw 7.8 ± 0.2); and (3) a slab tear in the MHT, similar to those in subduction zones.

High‐Resolution Local Earthquake Tomography Beneath the Zagros Simply Folded Belt (SFB): Implications for an Inhomogeneous Low‐Velocity Layer and Diapirism in the Upper Crust

AbstractHormuz salt formation is considered the origin of the evaporated salt deposits in the Zagros and its ductile behavior has been known as the reason for the inhomogeneous deformation in the Zagros. However, our knowledge about this formation has been limited to the salt domes on the surface. In our study, local earthquakes recorded by a temporary dense seismological network of 17 stations, deployed in the southern margin of the Zagros Simply Folded Belt (SFB) in southwestern Iran, have been used for seismic imaging. The high‐resolution earthquake local tomography revealed the first geophysical evidence about the Hormuz salt layer and its extension at depth. There is an uneven layer located at a depth of 8–12 km as the origin of extruded salt in this region through a shear fault zone and the production of the Dashti Salt Dome at the Kuh‐e‐Namak Mountain. Based on the obtained results, this layer is characterized by low seismic velocity volumes with significant Vp/Vs ratio variations. The seismic image is also consistent with a major NW‐trending NE‐dipping reverse fault, probably responsible for the 9 April 2013 Kaki earthquake. At depth of around 4 km, smaller scale high velocity anomalies, characterized by a high Vp/Vs ratio, may be related to the fluid saturated sediments in the uppermost sedimentary layer.

Variations of intrinsic and scattering seismic attenuation with depth in western Tibet

In this study, the depth-dependent seismic wave attenuation beneath western Tibet have been investigated using 628 local earthquakes recorded at Y2 network operated by US-China collaboration group from July 2007 to May 2011. Initially, we have calculated the attenuation of shear wave (Qs−1) using coda normalization method and coda waves (Qc−1) using single isotropic scattering model. Thereafter, we have estimated the quantitative values of scattering (Qsc−1) and intrinsic (Qi−1) attenuation using Wennerberg approach. The observations are made at five different central frequencies (1.5, 3, 6, 12, and 18 Hz) and five lapse time windows (30, 40, 50, 60, and 70 s). The results show frequency and lapse time dependence of Qc−1, Qsc−1 and Qi−1 values for western Tibet. Qc−1 and Qi−1 shows similar trend suggesting that the key mechanism is intrinsic dissipation for the decay of coda wave in the medium. Qi−1 is found to be as a dominant mechanism over Qsc−1 at all the investigated depth beneath western Tibet. Interestingly, the values of Qc−1 and Qi−1 decrease and get closer to Qsc−1 with increasing lapse time window/depth. It signifies the impact of ongoing underthrusting Indian lithosphere at deeper depth. A stronger intrinsic attenuation at shallow depth reflects the effect of the mid crustal low-velocity zone (LVZ) below Tethyan Himalaya and the southern Lhasa terrane. The obtained results in this study are also in good agreement with other reported results from seismically active regions in the world.

Cost Estimation of Structural Work for Residential Building with Seismic Design Consideration

The Sumatra-Andaman earthquakes had triggered local earthquakes in Malaysia by reactivation of ancient inactive faults. Previously on 5thJune 2015, Ranau, a region located in Sabah, Malaysia, had experienced a moderate earthquake of Mw6.1. The structural failures occurred because all existing buildings only designed for gravity load without any seismic provision. Recent research work exhibits the seismic designs’ impact on the cost of material and its parameters thatimpact the cost. There are two types reinforced concrete residential buildings called Type 1 and Type 2 for two storey and four storeywhich had been used as models. This research applied four seismicity levels to the reference peak ground acceleration value, αgR= 0.07g, 0.10g, 0.13g & 0.16g, and two soil types: Soil Types B and D. The result shows that for twostorey reinforced concreteresidential buildings on soil types B and D, seismic design increases structural work costs, which is around 0.62% to 1.31% and 0.61% to 2.16%, respectively,for Type 1 modelcompared to non-seismic design. Besides,model Type 2, the increment is around 0.24% to 1.22% and 0.20% to 1.71%, respectively.Otherwise, for reinforced concreteresidential building with four storeyon soil types B and D,the result shows that seismic design tends to have a higher structural work’s costaround 0.41% to 2.48% and 0.98% to 11.23%, respectively, for Type 1 model. Besides, for model Type 2 the increment is around 1.80% to 2.05% and 2.34% to 8.53%, respectively, compared to non-seismicdesign.

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.

Spatial variation in the crustal deformation pattern in the Koyna-Warna region, western India using shear wave splitting

SUMMARY This study investigates crustal anisotropy beneath the Koyna-Warna region (KWR) using waveforms of local earthquakes of magnitude ML ≥ 1.0, recorded at 24 surface and 6 borehole seismic stations during September 2005–February 2020. Waveforms of 1402 local earthquakes that fulfil the splitting criteria (ic < 45°) are used for shear wave splitting analysis, yielding 738 measurements. The study examines the spatial, temporal and depth variations in the fast polarization azimuths (FPAs) and delay times (δt). The average delay times in the Koyna and Warna regions are 0.035 and 0.050 s, respectively, which did not show any significant correlation with depth. The FPAs are predominantly oriented in the NNW–SSE direction in the KWR, commensurate with the in situ maximum horizontal stress (SHmax) obtained using data from a 3-km-deep scientific borehole and individual focal mechanism solutions. The anisotropy beneath KWR is due to the alignment of fluid-filled cracks and pore throats along the SHmax, and is attributed to stress-induced anisotropy. A considerable variation in the orientation of FPAs from NW to NNW–SSE beneath KWR could be associated with heterogeneity in the lithological properties due to variation in the fracture orientation under regional stress. The non-parallel orientation of FPAs with SHmax or faults at some stations is attributed to structure-induced anisotropy, which seems to be associated with surface topography. Measurements at stations, even near the reservoirs, did not show any significant temporal variation in the splitting parameters due to the fluctuation in the water levels.

Automatic determination of focal depth with the optimal period of Rayleigh wave amplitude spectra at local distances

SUMMARY Focal depth of earthquakes is essential for studies of seismogenic processes and seismic hazards. In regions with dense seismic networks, focal depth can be resolved precisely based on the traveltime of P and S, which is less feasible in case of sparse networks. Instead, surface waves are usually the strongest seismic phases at local and regional distances, and its excitation is sensitive to source depth, thus theoretically important for estimating focal depth even with a limited number of seismic stations. In this study, short-period (0.5–20 s) Rayleigh waves are explored to constrain focal depths. We observe that the optimal period (the period corresponding to the maximum amplitude) of Rayleigh waves at local distances (≤200 km) shows an almost linear correlation with focal depth. Based on this finding, we propose an automated method for resolving the focal depth of local earthquakes using the linear regression relation between the optimal period of Rayleigh wave amplitude spectra and focal depth. Synthetic tests indicate the robustness of this method against source parameters (focal mechanism, source duration and non-double-couple component) and crustal velocity structure. Although the attenuation (Q factor) of shallow crust can introduce complexities in determining focal depth, it can be simultaneously estimated if a sufficient number of stations are available. The proposed method is applied to tens of small-to-moderate earthquakes (Mw 3.5–5.0) in diverse tectonic settings, including locations in the United States (Oklahoma, South Carolina, California, Utah, etc.) and China (Sichuan, Shandong). Results demonstrate that reliable focal depth, with uncertainty of 1–2 km, can be determined even with one or a few seismic stations. This highlights the applicability of the method in scenarios characterized by sparse network coverage or historical events.

An Optimum 1D Velocity Model for the Garhwal–Kumaun Himalaya Using Monte Carlo Style Inversion

Abstract A Monte Carlo based algorithm is developed to improve the 1D velocity inversion routines and minimize bias due to the choice of a starting model. Using this algorithm, a well-resolved six-layer minimum 1D velocity model, down to ∼24 km depth, is determined for the Garhwal–Kumaun Himalaya. A total of 4765 P- and 4724 S-phase travel times of local earthquakes recorded at 53 broadband seismic stations are used for this purpose. The travel time–distance curves from these carefully analyzed phase data of events are used to subsequently derive a prior 1D velocity model. Forward modeling of the travel time–distance curve yields an average Moho depth of ∼46 km and bulk crustal P- and S-wave velocity values of 7.60 and 4.47 km/s, respectively. To circumvent the subjectivity due to manual intervention in the inversion and automate the process, we propose a Monto Carlo style semirandom generation of initial trial velocity models, guided by the initial values derived from forward modeling. The estimated minimum 1D model reveals P- and S-wave velocities increasing from 5.17 to 6.85 km/s and 3.12 to 3.82 km/s, respectively, from the surface to a depth of 26 km. Subsequently, an optimum model is constructed for the region by incorporating the Moho layer in the minimum 1D model.

Revisiting Vrancea (Romania) Intermediate-Depth Seismicity: Some Statistical Characteristics and Seismic Quiescence Testing

Background: The intermediate-depth seismicity in the Vrancea region (Romania) is characterized by localized and persistent earthquake activity that culminates about two or three times in a century with the occurrence of a large event (M ≥ 6.5). Here we have revisited some important seismicity characteristics, using earthquake catalog data spanning two different time periods: 1960–1999 and 2005–2013. Methods: we have determined the b-value of the frequency-magnitude distribution of earthquakes, using a maximum likelihood procedure, and estimated the parameter β to quantify anomalous seismicity rate decreases and increases. Results: by using data from the first period, we have confirmed the existence of a decreased b-value in the deepest part of the seismogenic zone; by using data from the second period, we have statistically confirmed the seismic quiescence that preceded the occurrence of the 1977 M7.4 Vrancea earthquake. Conclusions: the decreased b-value has been interpreted either in terms of an increased lithostatic stress with depth or as an indicator of the depth range where the next major Vrancea earthquake may occur. The time variation of the seismicity parameter β may reveal anomalous seismic quiescence and increased earthquake rates that may precede the occurrence of large earthquakes.

Evaluating how well active fault mapping predicts earthquake surface-rupture locations

Abstract Earthquake surface-fault rupture location uncertainty is a key factor in fault displacement hazard analysis and informs hazard and risk mitigation strategies. Geologists often predict future rupture locations from fault mapping based on the geomorphology interpreted from remote-sensing data sets. However, surface processes can obscure fault location, fault traces may be mapped in error, and a future rupture may not break every fault trace. We assessed how well geomorphology-based fault mapping predicted surface ruptures for seven earthquakes: 1983 M 6.9 Borah Peak, 2004 M 6.0 Parkfield, 2010 M 7.2 El Mayor–Cucapah, 2011 M 6.7 Fukushima-Hamadori, 2014 M 6.0 South Napa, 2016 M 7.8 Kaikoura, and 2016 M 7 Kumamoto. We trained geoscience students to produce active fault maps using topography and imagery acquired before the earthquakes. A geologic professional completed a “control” map. Mappers used a new “geomorphic indicator ranking” approach to rank fault confidence based on geomorphologic landforms. We determined the accuracy of the mapped faults by comparing the fault maps to published rupture maps. We defined predicted ruptures as ruptures near a fault (50–200 m, depending on the fault confidence) that interacted with the landscape in a similar way to the fault. The mapped faults predicted between 12% to 68% of the principal rupture length for the studied earthquakes. The median separation distances between predicted ruptures and strong, distinct, or weak faults were 15–30 m. Our work highlights that mapping future fault ruptures is an underappreciated challenge of fault displacement hazard analysis—even for experts—with implications for risk management, engineering site assessments, and fault exclusion zones.

Changes in well water level and rock damage zones in a shallow aquifer before and after local earthquakes

Changes in well water level and rock damage zones in a shallow aquifer before and after local earthquakes

Seismicity Analysis and Velocity Structure of Two-phase Geothermal Field in Southern Bandung

Over a duration of six months, we conducted a microearthquake analysis to characterize the subsurface conditions in the two-phase geothermal field located in Southern Bandung. Throughout the study, a total of 754 microearthquake (MEQ) data were recorded from at least 15 monitoring stations. After selection process of the dataset, 362 events were selected based on their azimuthal gap angle being less than 180°. Initially, the earthquake locations were determined using the Geiger method. Subsequently, we updated the hypocenter locations through simultaneous inversion, incorporating 1D velocity structure and 3D tomographic inversion. Our analysis revealed three primary seismicity clusters, which likely correspond to the injection and production activities within the geothermal field. The southern cluster aligns with the injection wells, extending from a depth of 1 km to 8 km, suggesting that the MEQ events were induced by injection activity. In the production area, a higher concentration of events is observed, densely distributed between depths of 1 km and 3 km. We suspect that the third cluster is associated with the development activity of a steam-dominated geothermal field located east of the main field. Notably, low Vp/Vs values near the surface are detected and exhibit increased thickness towards the north. These characteristics are interpreted as indicative of the steam zone, as the anomaly's location aligns closely with the production area. The thickening of the low Vp/Vs zone towards the north suggests the presence of steam as the impact of fluid extraction in the region, leading to a decline in pressure.

The attenuation of macroseismic intensity in the volcanic island of Ischia (Gulf of Naples, Italy): comparison between deterministic and probabilistic models and application to seismic scenarios

In this paper, we tackle the problem of the intensity attenuation at Ischia, a critical parameter in a high seismic risk area such as this volcanic island. Starting from the new revised catalogue of local earthquakes, we select a dataset of 118 macroseismic observations related to the four main historical events and analyse the characteristics of the intensity attenuation according to both the deterministic and probabilistic approaches, under the assumption of a point seismic source and isotropic decay (circular spreading). In the deterministic analysis, we derive the attenuation law through an empirical model fitting the average values of ΔI (the difference between epicentral intensity I0 and intensities observed at a site IS) versus the epicentral distances by the least-square method. In the probabilistic approach, the distribution of IS conditioned on the epicentre-site distance is given through a binomial-beta model for each class of I0. In the Bayesian framework, the model parameter p is considered as a random variable to which we assign a Beta probability distribution on the basis of our prior belief derived from investigations on the attenuation in Italy. The mode of the binomial distribution is taken as the intensity expected at that site (Iexp). The entire calculation procedure has been implemented in a python plugin for QGIS® software that, given location and I0 (or magnitude) of the earthquake to be simulated, generates a probabilistic seismic scenario according to the deterministic or probabilistic models of attenuation. This tool may be applied in seismic risk analyses at a local scale or in the seismic surveillance to produce real-time intensity shake-maps for this volcanic area.

New Insight into the Subducted Indian Plate beneath Central Myanmar Based on Seismic Activity and Focal Mechanisms Analysis

Abstract Myanmar is governed by the oblique subduction of India beneath the Burma microplate. Accurate earthquake location and focal mechanism data are crucial for understanding the detailed geometric features and dynamics of this subduction zone. We located 1433 subduction-related earthquakes and analyzed the focal mechanisms of 114 events using seismic data recorded by a recently deployed dense array in central Myanmar. The precisely located earthquakes show an east-dipping Wadati–Benioff zone beneath the Burma microplate at latitudes of 20.5°–25.0° N. In general, the subducting Indian plate imaged by seismicity has a dip angle of ∼18°–25° down to a depth of 80 km and then abruptly steepens to ∼33°–45°. The average thickness of the east-dipping seismic layer is ∼15 ± 10 km in the shallow-intermediate depth range (10–80 km) and gradually becomes ∼21 ± 10 km thick. By comparing the seismicity with the seismic velocity models revealed by receiver function and local tomography, intermediate-depth earthquakes appear to be mainly confined to the Indian lower continental crust at depths of 40–100 km. These earthquakes may be triggered by partial eclogitization reactions of the subducted lower continental crust. Stress inversion analysis of focal mechanism results grouped into 40–75, 75–90, and 90–130 km indicates that the principal compressional axes are essentially along the margin between the India and Burma microplate, implying resistance to the northward movement of the Indian plate. The principal tensional stress orients approximately east–west with its plunge systematically increasing with depth, implying the deformation may be caused by the negative buoyancy of the previously subducted oceanic slab.

On the trail of fluids in the northernmost intracontinental earthquake swarm areas of the Leipzig-Regensburg fault zone, Germany

The Leipzig-Regensburg fault zone is documented as a band of seismic activity extending northwards from the earthquake swarm region NW-Bohemia/Vogtland at the Czech-German border area and is intersected by several Hercynian fault zones. Along the fault zone, there are several earthquake swarm areas, the northernmost of which are Schöneck and Werdau. In this study, we investigate the presumably fluid-induced earthquake swarm activity of the Schöneck and Werdau area. For this purpose, we apply two methods: local earthquake tomography and receiver functions to identify the structural composition of the crust, the areas affected by fluids and the origin of the fluids. We detected potential fluid paths characterised by high Vp/Vs ratios and granite intrusions nearby the swarms characterised by low Vp/Vs anomalies. Receiver function analysis yields the Moho at 25 to 33 km depth and two seismic discontinuities at 55 km and 68 km depth.

Seismogenic Structure of the 2014 M6.5 Ludian Earthquake From Three‐Dimensional Joint Inversion of Magnetotelluric Data and Seismic Arrival Times

AbstractOn 3 August 2014, a destructive earthquake with a magnitude of 6.5 occurred in the Ludian region in Yunan, China, causing heavy casualties. However, the seismogenic structure of the Ludian earthquake is still unclear. To address this issue, we have developed a three‐dimensional joint inversion algorithm for magnetotelluric data and seismic body wave arrival times based on the cross gradient structural similarity constraint, which utilizes the complementary advantages of the two different datasets. We use arrival times of P‐ and S‐waves from 6,369 local earthquakes recorded by 30 seismic stations and magnetotelluric impedances from 127 MT stations to perform the joint inversion to construct a combined 3‐D P‐wave velocity (Vp), S‐wave velocity (Vs), and resistivity models and the locations of relocated aftershocks in the source region of the Ludian earthquake. Our models show significant low Vp/Vs ratios and low resistivity in the upper crust of the main shock zone, and the relocated aftershocks form an L‐shaped zone. Based on petrophysical studies and analysis of the cross‐plots of different physical properties, we infer the existence of cracks/fractures and fluids as well as high quartz contents in the source region. These fluids may play a key role in promoting the Ludian earthquake by reducing the effective normal stress of the fault zone, and the existence of high quartz contents further makes the crustal rocks in the source region more brittle. Our study provides a new evidence how the properties of the crust in the focal area have some control on the genesis of moderate to large earthquakes.

Time History Analysis of G+15 RCC Building at Different Earthquake Location

Abstract: Now a days multi-storey buildings is the most common word in the construction source, then study of earthquake and analysis of building using earthquake data essential for future safety. In the present paper study of nonlinear dynamic analysis of G+15 storied RCC building considering different locations is carried out and seismic responses of such building are studied. The building under consideration is modelled with the help of Staad Pro software. Three different earthquake data have been used considering at different locations for establishment of relationship between time vs acceleration graphs. The results of the study Shows similar variations pattern in Seismic responses such as base shear and storey displacements, base shear value calculate manually and compare with software value From the study it is recommended that analysis of multi storied RCC building using Time History method becomes necessary to ensure safety against earthquake force.

Seismic activity at the eastern Pyrenean termination

Although the seismic activity at the eastern Pyrenees is nowadays moderate and sparse, with events usually not exceeding magnitudes 4.5, this area has been affected in the past by the most destructive event occurred in the Pyrenees, reaching an intensity of IX, whose seismogenic structure is not well understood and still under debate. In order to progress in the knowledge of these structures, we present here the results derived from a 14 months-long broad-band seismic deployment focused on the Cerdanya Basin, but encompassing the eastern termination of the Pyrenees. The dense station coverage has allowed us to obtain accurate hypocentral locations, as well as up to 23 focal mechanisms from local low-magnitude earthquakes. In addition to a relatively sparse seismicity, several clusters of seismic events located in well-defined, small areas and depth ranges have been identified. The results show a few low-magnitude seismic events located in the southern limit of the Cerdanya Basin that could be related to oblique secondary faults within the footwall of the Têt Fault, the major tectonic structure in the area. Our data shows also that the Capcir Fault has associated seismicity, with some of the events located out of the fault plane, perhaps on secondary fault branches. To the east, a cluster of low-magnitude events is detected in the epicentral area of two relatively large earthquakes occurred recently. Further west, the Maladeta Massif has a sustained seismic activity, although its origin does not seem to be related to the most relevant structure in the area, the North Maladeta Fault. Regarding focal mechanisms, most of them show normal fault solutions with nodal planes NW-SE oriented, which agree with the extensional regime perpendicular to the axis of the chain derived from the seismic and geodetic data.