On 14 July 2025, an Mw 5.3 earthquake occurred in the northeastern Alboran Sea, Spain. Its epicenter was close to the Eastern Betics Shear Zone, a region that has historically experienced major destructive earthquakes, including events with macroseismic intensity VIII–IX (EMS-98) in Vera (1,518), Alhama de Almería (1,522), and Dalías (1,804). However, in the area closest to the epicenter, offshore from the Almería coast, no previous earthquakes of this magnitude or greater have been recorded, and the nearest mapped active faults, the Carboneras Fault and the Palomares Fault, are both approximately 50 km away. To investigate the source responsible for this earthquake, we compute its seismic moment tensor and relocate the subsequent seismic sequence using a three-dimensional Earth model in a non-linear probabilistic approach and a double-difference relocation. Additionally, to better understand this offshore area, we apply the same methodology to study three similar recent seismic sequences along the Carboneras Fault in 2008, 2010, and 2012, west of the 2025 event. The mainshock moment tensor solution shows a strike-slip faulting mechanism consistent with the regional tectonics, and the relocated sequences form dense, elongated clusters, mainly following a NW–SE trend, perpendicular to the Carboneras Fault. The depth distribution shows that hypocenters in the 2025 seismic series are deeper, ranging from 5 to 20 km and slightly increasing to the northwest, compared with the previous sequences, whose hypocenters are mostly shallower (<10 km) and show no clear dipping direction. The relocation results improve depth convergence and accuracy of earthquake locations of the northeastern Alboran seismicity, providing a better-constrained earthquake catalog that helps to improve our understanding of this complex offshore region.
NE Alboran Seanon-linear probabilistic locationdouble-difference locationmoment tensor inversionfocal mechanismThe author(s) declared that financial support was received for this work and/or its publication. This work has been partially supported by the project PID2020-114682RB-C32 funded by MICIU/AEI/10.13039/501100011033 and the National Geographic Institute (IGN, Ministry of Transport and Sustainable Mobility, Spanish Government).section-at-acceptanceSolid Earth GeophysicsIntroduction
The Alboran Sea is an active tectonic region and one of the most seismically active zones of the western Mediterranean (Gràcia et al., 2019; Stich et al., 2020; Sanchez-Roldán et al., 2024; Lozano et al., 2025). The regional dynamics are controlled by the oblique NW-SE convergence between the African plate and the Iberian microplate, at a slow rate of 4–6 mm/year (DeMets et al., 2015; Palano et al., 2015). This convergence has led to the development of large transcurrent structures, along with reverse faults that cross the Alboran basin (Gràcia et al., 2006; Gràcia et al., 2012; Gràcia et al., 2019; Pedrera et al., 2010; Moreno et al., 2016; Perea et al., 2018; Gómez de la Peña et al., 2022; Canari et al., 2025). These structures have been recently proposed to delimit the main crustal domains in the Alboran Sea (Gómez de la Peña et al., 2018).
This study focuses on the northeastern Alboran Sea. In contrast to the widely studied recent seismic series in the South Alboran Basin (Buforn et al., 2017; Gràcia et al., 2019; Stich et al., 2020; Perea et al., 2022; Sanchez-Roldán et al., 2024; Lozano et al., 2025; Bouhali et al., 2025), the NE Alboran seismicity has been less studied due to the low instrumental seismic activity detected in this area (Grevemeyer et al., 2015).
The NE sector of the Alboran Sea mainly comprises the East Alboran Basin (EAB) and the southeasternmost part of the Iberian margin, the so called Palomares margin. The former is a ∼18–20 km thick magmatic arc crust (Gómez de la Peña et al., 2018; Gómez de la Peña et al., 2020), and the latter a thinned continental crust intruded by volcanism, which runs in the transition to the back-arc oceanic crust of the Algero-Balearic Basin to the east (Giaconia et al., 2015; Gómez de la Peña et al., 2016; Gómez de la Peña et al., 2018) (Figure 1). This region is affected by the present-day compressive tectonic regime of the Western Mediterranean, with the NW-SE shortening direction oblique to the margin. Dominant strike-slip faults in the area, such as the Carboneras Fault (CF), are proposed to transfer the deformation between ‘en-echelon’ thrusts and folds that mainly absorb the convergence (Giaconia et al., 2015). The recent review of the active tectonics and stress regimes across Iberia from moment tensor focal mechanism analysis conducted by Olaiz et al. (2025) confirms a predominant strike-slip regime with a normal component in the easternmost part of the northern Alboran Sea. The reported focal mechanisms are mainly strike-slip, with occasional reverse and normal faulting earthquakes (Stich et al., 2006; Olaiz et al., 2025). In this region, the main tectonic feature is the CF, running from the southernmost onshore part of the Eastern Betic Shear Zone (EBSZ) (Bousquet, 1979; Sanz de Galdeano, 1990; Alfaro et al., 2002) into the offshore EAB with a general NE-SW trend. The CF is a left-lateral transpressive strike-slip structure, active since the Late Miocene, with an estimated slip rate of 1.3 mmyr-1 (Moreno et al., 2015; Gómez de la Peña et al., 2016). Its submarine portion continues further into the EAB in a NE-SW trending zone of deformation of about 90–100 km long and 0.5–2 km wide and shows changes in the strike and dip directions along its northern and southern segments (Gràcia et al., 2006; Moreno et al., 2016). It is formed by several subvertical traces, probably extending at least up to 15–20 km depth and reaching the Moho (Pedrera et al., 2010; Moreno et al., 2016). In contrast to what has been suggested in previous works (De Larouzière et al., 1988), some studies reported a clear termination of the CF at both ends and no evident connection between the EBSZ with the North-African Rif (Gràcia et al., 2006; Moreno et al., 2016). To the south of the CF, two NW-SE right-lateral transcurrent structures are observed, the Yusuf and Averroes faults, defining a widely distributed deformation area (Moreno et al., 2016). To the north and east, the onshore CF ends at the convergence point between the E-W Polopos reverse fault zone and the NNE-SSW Palomares left-lateral strike-slip fault. Although the offshore continuation of the Palomares fault is controversial, several recent studies (Gràcia et al., 2006; Gómez de la Peña et al., 2016; Moreno et al., 2016) found no evidence of its continuation at sea and suggest that the deformation in the area is accommodated by onshore structures.
Seismotectonic framework of the NE Alboran Sea region. Historical (white dots) and instrumental (red dots) M > 3 seismicity from the IGN seismic catalog and active (blue lines) and debated (purple lines) faults from QAFI4 database (IGME, 2022; García-Mayordomo et al., 2012): Adra Fault (AF), Al-Idrissi Fault (AIF), Alboran Ridge North and South Faults (ARNF, ARSF), Averroes Fault (AVF), Carboneras Fault (CF), Palomares Fault (PF), Polopos Fault Zone (POFZ), Yusuf Fault (YF). Inset (top left): context map of studied region (in red).
Topographic map of the East Alboran Basin and surrounding regions showing earthquake epicenters as red and white circles of varying sizes representing magnitudes, major faults in blue, and key dates marked for notable seismic events. Inset map locates the Alboran Sea between Iberia and Africa.
Seismic activity in this region is generally classified as low to moderate (Buforn et al., 1995; Stich et al., 2003; 2010; 2019; Grevemeyer et al., 2015), however, significant destructive earthquakes have occurred. The most remarkable examples are the 1,518 Vera, 1,522 Almería and 1804 Dalías earthquakes, with maximum intensity VIII-IX (EMS 98). Important events include also the 1,487 Almería and 1804 and 1910 Adra earthquakes, with maximum intensity VIII and VII-VIII (EMS-98). Present day seismic activity in the NE Alboran Sea shows low seismicity, with swarms of small to moderate (M < 5) earthquakes mainly concentrated offshore in the Almería-Adra margin, to the north of Chella Bank and the surroundings of Campo de Dalías. In this area, the 1993–1994 Berja-Adra seismic series occurred, representing the most intense earthquake activity recorded in the last decades, with a maximum magnitude of M5 and intensity of VI-VII (Stich et al., 2001; Gràcia et al., 2006; Pedrera et al., 2012). Although several earthquakes occur in the vicinity of CF, very few or none of the events can be associated with it (Stich et al., 2010; Grevemeyer et al., 2015). In fact, most of the seismicity recorded in the NE Alboran region occurs outside the magmatic arc crust of the EAB and is nucleated within the crust at shallow depths (<15 km) (Grevemeyer et al., 2015; Gómez de la Peña et al., 2020).
Within this context, an Mw 5.3 earthquake occurred on 14 July 2025, at 05:13:27 UTC. Its epicenter was located offshore, approximately 30 km southeast of the Almería coast (Figure 1). This earthquake represents the largest seismic event recorded in this area in the instrumental record. According to the Instituto Geográfico Nacional (IGN) seismic catalog (doi:10.7419/162.03.2022; https://www.ign.es/web/resources/sismologia/tproximos/informes/INFORME_2025-07-14_Mw5.3_CABO_DE_GATA.pdf), it was widely felt across 586 localities in southeastern Spain, with maximum observed intensities reaching IV (EMS-98) in several localities of Almería and Murcia. The hypocenter was located at a shallow depth of 3 km, and the largest aftershocksrecord. According to the Instituto were two events of Mw 3.4 approximately 5 min and Mw 3.5 nearly 1 month after the mainshock. The seismic series includes more than a hundred aftershocks in the subsequent 2 months, mostly M < 2 events distributed almost perpendicular to the coastline and located at depths of less than 25 km. Preliminary focal mechanism results from the main shock reveal strike-slip dextral movement with nodal planes with an NNE-SSW and E-W orientations (IGN seismic catalog, doi:10.7419/162.03.2022).
This earthquake occurs in a region with very scarce seismicity during the instrumental era, where most significant historical earthquakes are located to the west, in the surroundings of Campo de Dalías and Chella Bank. It is the first recorded case of a moderate magnitude (M > 5) earthquake in this area and provides a unique opportunity to improve our understanding of the stress regime of the NE Alboran Sea where no evidence of significant offshore active structures has been found except for the CF (Gómez de la Peña et al., 2016; Gómez de la Peña et al., 2020; Moreno et al., 2016). Along the northern offshore segment of this fault, west of the 2025 series, several M4 earthquakes have also occurred in the last two decades with associated seismic sequences, in 2008, 2010, and 2012. Although smaller in magnitude, these series also include several M3 events and span approximately between 2 weeks and 2 months (see Table 1).
Main characteristics of the seismic series and data sets used for the hypocentral relocation. The maximum magnitude (Max. Mag.) corresponds to Mw from IGN seismic catalog.
2025 seismic series
2012 seismic series
2010 seismic series
2008 seismic series
Period
2025/07/14–2025/08/31
2012/04/16–2012/05/24
2010/07/04–2010/07/20
2008/10/20–2008/12/24
# eq M ≥ 2
46
23
54
55
# eq M ≥ 3
3
4
4
8
Max. Mag
5.3
4.2
4.3
4.3
#P , S arrivals/eq
≥20
≥10
# stations
116
74
Taking advantage of this exceptional event, our study contributes with novel results of focal mechanism and precise relocations to further characterize the seismicity in this region and to investigate the possible responsible sources. In this work, we obtain the focal mechanism for ten M ≥ 3.5 earthquakes through moment tensor inversion and absolute and relative relocations, through non-linear probabilistic approach with a regional 3D velocity model and double-difference algorithm, of a selected set of M ≥ 2 earthquakes from the major 2025 seismic series in the area and previous smaller series that occurred in 2008, 2010, and 2012 west of the 2025 series. The use of a 3D velocity model combined with relative locations significantly enhances the quality of the relocated seismicity and enables a more accurate interpretation of the seismicity. This procedure has been successfully applied in previous studies (Waldhauser et al., 2020; Raggiunti et al., 2023; Suárez et al., 2023; Lozano et al., 2022; Lozano et al., 2025) and allows us to provide a better constrained earthquake catalog and new insights into the active tectonics of the NE Alboran Sea.
Data and methods
Data used correspond to good-quality waveforms from BB seismic stations and P- and S-wave arrivals from the IGN catalog recorded by permanent and temporary broadband (BB) and strong-motion stations, mainly from the Spanish Digital Seismic Network (ES, doi:10.7914/SN/ES), the Western Mediterranean network (WM, doi:10.14470/JZ581150), the Andalusian Institute of Geophysics and Earthquake Disaster Prevention network (IG), the Portuguese National Seismic Network (PM, doi:10.7914/SN/PM), the Morocco Seismic Network from the Institut National de Géophysique, Rabat (CNRST) (MO) and the University of Alicante and Institut Cartogràfic Valencià (VB, doi: 10.7914/SN/VB). We analyzed a selection of well-recorded earthquakes corresponding to the 2025 seismic series and previous smaller series in 2008, 2010, and 2012 occurred in the NE Alboran Sea.
Moment tensor inversion
The deviatoric moment tensor (MT) inversion was performed in the time domain using the software package Computer Programs in Seismology (Herrmann, 2013), fitting complete velocity waveforms. Although we attempted MT inversion for M > 3 events, only those with M ≥ 3.5 produced stable and reliable solutions. The broadband recordings were sampled at 100 Hz, assigning equal weights to all components during the inversion process. The instrumental response was removed, and the seismograms were rotated into radial and transverse components. The data were resampled to 5 samples per second and filtered with a band-pass between 0.02 and 0.05 Hz for an adequate signal-to-noise ratio.
Considering the magnitudes of the events, we used stations with epicentral distances between 35 and 395 km, depending on the event. The number of stations contributing to each inversion ranged from 7 to 28, with a maximum azimuthal gap of 230°. Synthetic seismograms were computed using Green’s functions calculated by the wavenumber integration method (Herrmann, 2013), with a sampling interval of 0.2 s. We used the wavenumber integration method and assumed a parabolic source time function with a base of 0.8 s, considering the 1D velocity model from Rueda and Mezcua (2005).
For each inversion, we selected 90 s time windows starting 5 s before the P-wave arrival, allowing a time shift of 0.4 s to optimize waveform fitting. Additionally, we performed a grid search over focal depth at 2 km intervals to identify the optimal solution in terms of waveform misfit and double-couple percentage. Figure 2 shows an example of the MT inversion results for the largest-magnitude earthquake.
Moment tensor inversion results for the main event of the 2025 series. Top left: Inversion results at different depths. Directly below, the focal mechanism is shown. Bottom: the seismic stations used in the inversion and all retrieved source parameters. Right panel: observed (black) and synthetic (green) seismograms corresponding to the best-fitting moment tensor solution.
Seismic event analysis figure includes: a depth misfit graph, double-couple beachball diagram, epicenter location map with colored triangles, event metadata, moment tensor decomposition, nodal plane parameters, and waveform comparison plots for vertical, radial, and transverse components at multiple stations, showing observed vs. modeled data in green and black.Hypocentral relocation
We applied a common procedure based on an initial absolute location with a probabilistic non-linear approach and a subsequent relative relocation based on double differences.
For the absolute relocation, we applied NonLinLoc (NLL) location algorithm (Lomax et al., 2000; Lomax et al., 2014) to estimate the maximum likelihood hypocenter and posterior probability density function (PDF) of each event by means of a nonlinear global search in a 3D space. For travel-time calculation we considered a local 3D P-wave velocity model for the Ibero-Maghrebian region (mEM) (El Moudnib et al., 2015), which allows to account for propagation effects in heterogeneous media. We computed theoretical P-wave travel times at each station within the 3D velocity grid using a finite-differences eikonal-equation algorithm (Podvin and Lecomte, 1991). To consider S arrivals in the location procedure, we assumed constant vP/vS ratios derived from the Wadati diagram (Wadati, 1933) between 1.74 and 1.75 (Supplementary Figure S1 in Supplementary Material). To derive the PDF, we used the Oct-tree sampling algorithm (Lomax and Curtis, 2001), which converges stably in complex velocity structures, and the equal differential-time (EDT) likelihood function (Zhou, 1994; Lomax, 2005), which is highly robust in the case of outlier data (Husen et al., 2003; Lomax et al., 2014). During NLL performance, the program computes station corrections which were included in a second run to compensate for nonmodeled propagation effects, reducing the RMS misfit. Besides, arrivals with residuals larger than 2 s were removed to minimize the influence of outlier data.
Furthermore, we computed relative locations of the seismic series using the double-difference algorithm HypoDD (Waldhauser and Ellsworth, 2000; Waldhauser, 2001). Thus, greater detail can be obtained on the relative positions and earthquake clustering, enhancing the interpretation of seismicity in relation to its possible sources. This algorithm minimizes the residual travel-time differences (double-differences, DD) for pairs of earthquakes at each station by means of a weighted least squares method. We used the NLL hypocentral solutions obtained in the previous stage as initial locations and first P and S phase arrivals from the IGN seismic catalog (doi:10.7419/162.03.2022) to compute travel-time differences from catalog data. To optimize the solutions during HypoDD inversion, we tested different parametrizations and weighting schemes, depending on the available data for each data set. We only considered event pairs with a hypocentral distance lower than 6–8 km, a minimum of 8 links per event pair, and stations located within 300–400 km distance from the cluster centroid. The solutions were obtained following the singular value decomposition (SVD) method which provides a good estimation of the least-square errors and is recommended for small sets of earthquakes (Waldhauser, 2001). HypoDD uses 1D velocity models for the inversion, and in this study, we used the 1D continental-crust velocity model for the Iberian Peninsula from Rueda and Mezcua (2005) and the calculated constant vP/vS ratios. In this case, the use of a 1D velocity model instead of a 3D model is adequate since its influence is expected to be minimal due to the short interevent distances and differential travel times.
This double-step relocation, using a 3D velocity model for the absolute location in NLL and a 1D model with HypoDD, has been successfully applied in previous studies (Waldhauser et al., 2020; Raggiunti et al., 2023; Suárez et al., 2023; Lozano et al., 2022; Lozano et al., 2025). Following this procedure for each seismic series, we analyzed a set of M ≥ 2 earthquakes in the IGN seismic catalog (doi:10.7419/162.03.2022) to ensure the homogeneity of all the analyzed seismicity and the availability of a sufficient amount of good-quality data. The data selected consists of earthquakes with a minimum number of first P- and S-wave arrivals to ensure well-constrained solutions and recorded at stations located within the region covered by the 3D velocity grid (mEM) (Table 1). The station coverage is irregular, with an azimuthal gap between 65° and 260°, a higher density of stations on the southern Iberian Peninsula and a more scattered distribution to the north of Africa (Figure 3).
Seismic stations (dark grey triangles) within the 3D velocity grid (pink rectangle area in the inset figure) used for the absolute relocation in this study. The colored diamonds correspond to the IGN seismic catalog location of each seismic series: 2008 (yellow), 2010 (red), 2012 (purple), and 2025 (blue).
Map showing the seismic stations (black triangles) located in southern Iberian Peninsula and northern Africa, used for the absolute relocation of 2008, 2010, 2012, and 2025 seismic series (colored diamonds labeled by years); inset indicates coverage area labeled Iberia and North Africa.ResultsMoment tensor inversion
The results of the MT inversion are shown in Figure 4 and Table 2 (all the calculated MT parameters are available in Supplementary File S1 in the Supplementary Material). The quality of the solutions was evaluated using the misfit and the percentage of the compensated linear vector dipole (CLVD), the azimuthal gap (<230°) and the number of used stations (≥7). All events show good overall quality, with misfit values ranging from 0.25 to 0.51 and CLVD components below 32%, indicating that the source mechanisms are dominated by the double-couple (DC) component. The presence of minor non-DC components may also arise from limited data coverage, noise, or modelling errors rather than from the true source process, especially for small-magnitude earthquakes (Dahm and Krüger, 2014; Hu et al., 2023; Yang and Wang, 2025). Moment magnitudes (Mw) range from 3.6 to 5.3, and centroid depths vary between 3 and 17 km. The lack of correlation between misfit and CLVD percentage indicates that the solutions are not biased by poorly constrained solutions or inversion convergence issues.
HypoDD relocated hypocenters (coloured circles in map view and NS-EW depth sections) and focal mechanisms obtained in this study. The grey solid lines represent main quaternary active faults from the QAFI v.4 database (IGME, 2022; García-Mayordomo et al., 2012). The thick black arrows indicate the shortening axis estimated for the northern Alboran Sea (Olaiz et al., 2025; Madarieta-Txurruka et al., 2026). Inset figure (right bottom): P-axes obtained from the calculated focal mechanisms.
Scientific map and diagrams showing earthquake event locations along a coastal region, with colored clusters labeled 2008, 2010, 2012, and 2025, focal mechanisms, depth versus location plots, and two bold black arrows highlighting movement directions.
Moment tensor parameters obtained in this study for M ≥ 3.5 earthquakes in the analyzed seismic series.
Date Time
Strike/Dip/Rake (o)
M0 (Nm)
Mw
CLVD (%)
Misfit (0–1)
h (km)
# stations
2025/08/08
02:06:42
358
83
−1
2.8e14
3.6
31
0.30
13
21
2025/07/14
05:13:27
9
76
6
1.0e17
5.3
6
0.41
7
28
2012/04/18
09:47:25
19
70
−26
1.8e15
4.1
14
0.51
5
14
2010/07/05
10:41:10
32
45
7
2.2e15
4.2
4
0.25
4
11
2008/11/07
11:02:51
28
85
−42
3.2e15
4.3
23
0.33
1
8
2008/10/26
00:51:22
42
51
12
1.1e15
4.0
5
0.49
17
9
2008/10/21
12:50:56
45
27
25
5.1e14
3.8
24
0.39
3
7
2008/10/21
05:55:46
25
74
−7
2.1e15
4.2
11
0.28
3
9
2008/10/21
03:28:53
24
72
−20
2.3e15
4.2
17
0.34
10
9
2008/10/20
09:46:11
21
87
−39
3.2e14
3.6
16
0.36
4
7
Most of the solutions correspond to strike-slip mechanisms. The nodal planes for the two earthquakes in the 2025 series are predominantly N-S and E-W oriented, while those for the 2008, 2010, and 2012 series show NNE-SSW and WNW-ESE orientations. The MT solutions for the 2008 series agree with the solution obtained by Stich et al. (2010). These mechanisms are broadly consistent with the maximum pressure axes (P-axes) aligning with the expected regional tectonic stress direction (Stich et al., 2019; Olaiz et al., 2025; Madarieta-Txurruka et al., 2026).
Hypocentral relocation
The relocated hypocenters obtained from the relative location with HypoDD are shown in Figure 4. The NLL maximum likelihood hypocenters can be found in Supplementary Figure S2 in Supplementary Material. All the calculated hypocentral parameters from the probabilistic and double-difference relocations for each seismic series are available in Supplementary Files S2, S3 in the Supplementary Material.
The 2025 hypocenters are located offshore and show a NW-SE trending epicentral pattern, roughly perpendicular to the CF (Figure 4). Within this overall trend, smaller clusters of activity can be distinguished. This distribution of the epicenters represents a rupture area about 20 km long and 5–6 km wide, with the main shock located at the southern end of the epicentral region, about 40 km SE of the CF. In the central part of this distribution, a denser cluster is observed, including most of the aftershocks and the M3 events. Regarding the depth distribution, our results indicate that most 2025 hypocenters range between 10 and 18 km, showing a slight increase in depth toward NW, with Mw 5.3 event located at approximately 10 km depth.
Concerning the rest of the seismic series, a clear NNW-SSE alignment is also observed for the 2010 earthquakes, located offshore and west of the 2025 series. In this case, epicenters follow a thinner distribution of about 12–15 km x 2–3 km, nearly perpendicular to the northern segment of the CF, and with most earthquakes located east of the fault trace. The 2012 and 2008 epicenters are also located offshore but further southwest, near the joint area with the southern segment of this fault. These earthquakes do not show such a clear and well-defined trend as the previous cases, but are instead distributed in smaller clusters, between 6 and 7 km long and 4 km wide. The 2012 events are grouped on the CF, and the 2008 cluster centroid is located about 8 km south of this fault. Overall, the depth of the different sequences significantly decreases to the west, with 2008, 2010, and 2012 earthquakes located up to 10 km depth, clearly shallower than the 2025 events. Moreover, all these previous series display a nearly vertical distribution with no clear systematic variation with depth.
The absolute location uncertainties estimated with NLL indicate well-constrained solutions for the 2025 seismic series with small and well-defined ellipsoid density scatter plots (Supplementary Figure S3 in Supplementary Material). The estimated vertical and horizontal Gaussian errors derived from the PDF samples (90% confidence ellipsoid) range below 5 km (2.1 ± 1.5 km and 2.8 ± 0.8 km mean values, respectively). The previous series are worse constrained, displaying larger and more elongated scatter clouds than 2025 series and uncertainties ranging between 2 and 8 km (3.5 ± 1.1 km and 5.3 ± 1.5 km mean values, respectively). In all cases, the location errors and travel-time residuals have been significantly reduced in this study compared to the IGN solutions, with an overall average reduction of 0.5 km in depth and 2 km in horizontal uncertainties. Concerning the relative errors, these are much smaller than the absolute values but can never be considered as absolute errors in earthquake location. The relative vertical and horizontal uncertainties remain below 2 km for the 2025 seismic series and below 1 km for the previous series (Supplementary Figure S4 in Supplementary Material).
Discussion
The seismicity patterns and focal mechanisms obtained in this study provide new insight into the active tectonics of the NE Alboran Sea, revealing a complex distribution in this region where only limited activity had been recorded prior to the 2025 sequence. The 2025 offshore Mw 5.3 earthquake and its aftershocks represent the first instrumentally documented moderate-magnitude activity in this sector, underscoring its tectonic significance. In contrast, earlier, smaller seismic sequences in 2008, 2010, and 2012, also studied in this work, occurred slightly farther west, off the coast near the area where most of the historical and instrumental seismicity is concentrated. The potential of this region for earthquake and tsunami hazard to the coasts of Spain and North Africa has been often underestimated mainly owing to the lack of well-recorded historical or instrumental seismicity along the offshore CF (Gràcia et al., 2006; Grevemeyer et al., 2015; Moreno et al., 2016). However, previous studies (Gràcia et al., 2006; Moreno et al., 2016) have pointed out the potential of this fault to generate large events, including rupture scenarios that led to maximum moment magnitudes (Mw) up to 7.2 for the offshore portion of the CF. Hence, the occurrence of the recent seismicity analyzed in this study should be considered in any seismic and tsunami hazard re-evaluations.
In the following, we first analyze the hypocentral distribution of the different seismic series to assess the reliability of the locations and characterize the overall spatial pattern of the seismic activity, providing the necessary context for the study of the 2025 sequence. We then focus on this latest sequence to characterize the seismic source and its relation to local tectonics of the NE Alboran Sea.
Hypocenter distribution analysis of the seismic series
The first remarkable feature observed in the 2025 and 2010 seismic series is the more widespread activity, in contrast with the 2008 and 2012 sequences. In the case of 2025 earthquakes, this distribution is broader and roughly follows a NW-SE direction, almost perpendicular to CF, while in the case of 2010 earthquakes, display a narrower and elongated alignment oriented in an NNW-SSE direction. To analyze whether these trends are real or consequence of location uncertainties, we performed an S-P analysis at a reference seismic station for the 2025 and 2010 series. For each earthquake in the seismic series, we computed the S-P arrival time differences at a given station aligned with the epicentral trend and located at an epicentral distance <1.5° to ensure direct Pg and Sg phases in the crust. Subsequently, we computed the S-P distance (DS−P from the arrival time differences tS−tP, assuming a homogeneous crust with constant P and S-wave velocities (vP,vS, applying the following equation:DS−P=tS−tP×vP/vPvS−1
For vP, a mean P-wave velocity of 6.15 km/s in the upper crust (first 11 km) from the 3D velocity model grid (mEM) was considered. We assumed the same constant vP/vS ratio 1.74–1.75 used for the relocation (Supplementary Figure S1 in Supplementary Material). By representing the S-P distances with the epicenter-station distances, we observed a linear relation between both parameters (Supplementary Figures S5, S6 in Supplementary Material). In fact, the distance between the farthest event and the closest event to a common station is about the same length as the relocated cluster (about 23–30 km long for 2025 series and 26–29 km long for 2010 series). These results confirm that the observed elongation in the earthquake distribution is genuine and not an artifact of the station configuration.
The second feature to highlight, considering the results obtained for all the series, is the sharp increase in depth towards the east. As shown in Figure 4, hypocenters of the 2008, 2010, and 2012 series along the marine segment of the CF are clearly shallower west of −2.3° longitude, with foci constrained in the first 10 km of the crust. However, the 2025 sequence is located towards northeast approaching the Palomares continental margin, and, in this case, hypocenters deepen up to 20 km with hardly any event above 10 km depth. It is remarkable that these differences in depth are not only observed from the hypocentral relocations (Supplementary Figures S2, S3 in the Supplementary Material) but are also consistent with most of the centroid results of MT inversion for the larger-magnitude earthquakes (Table 2). A recent study by Gómez de la Peña et al. (2020) found crustal thickness around ∼20 km for the EAB, deepening toward the basin margins to the north and south (Moho depth around 28 km beneath Morocco and between 28 and 23 km in the South Iberian margin, shallowing toward the Alboran basin). These authors pointed out that seismicity in this area is constrained to crustal depths with earthquakes mainly nucleating in upper and middle crust (<15 km depth). Moreover, Grevemeyer et al. (2015) estimated a depth ∼10 km for the 450 °C isotherm in the EAB and deeper depths when approaching the shoreline and on land, increasing the seismogenic layer. This isotherm defines the maximum depth of seismogenic behavior, below which no earthquakes occur in the lithosphere. Our hypocentral relocations support these results, showing focal depths <10 km in the NE Alboran region and deeper foci when approaching the continental margin. Of particular importance for understanding the seismotectonics of the region, the accurate relocation obtained in this study confirms that seismic activity can also occur in the EAB crust, and that low-to-moderate magnitude earthquakes (up to magnitudes exceeding Mw 5) can be generated in this region, despite previous studies indicating low seismicity (Grevemeyer et al., 2015).
Insights into the causative source of the 2025 seismic series
The main observation from the moment tensor inversion of the largest events analyzed in this study is that solutions reveal a dominance of strike-slip mechanisms, consistent with NW-SE compression direction and the regional stress regime (Stich et al., 2019; Olaiz et al., 2025; Madarieta-Txurruca et al., 2026). However, small differences between the sequences are observed: the 2025 Mw 5.3 and Mw 3.6 earthquakes show nodal planes oriented N-S and E-W, while the largest events of previous sequences also display strike-slip mechanisms with nodal planes roughly oriented NNE-SSW and WNW-ESE (Figure 4). These variations in focal mechanisms suggest a slight rotation of the principal axes on the 2025 series and, given that it is located only ∼40 km east of the previous ones, it highlights the complexity of the local stress field in this region.
All the studied earthquakes are located in the vicinity of the offshore CF northern segment. This fault is characterized in the QAFI database (IGME, 2022; García-Mayordomo et al., 2012) as a left-lateral vertical strike-slip fault with an average azimuth and dip of 48° ± 9° and 90° ± 10°, respectively. Two of the earthquakes from the 2008 seismic series show fault planes consistent with this strike orientation (2008/10/21 and 2008/10/26, Table 2); however, most of the events that occurred between 2008 and 2012 display fault planes with slightly lower azimuths, with an average strike of 25° ± 5° and a dip of 72° ± 15°. This discrepancy suggests that although part of the seismicity may be directly associated with the main CF, a significant proportion of the events likely ruptured subsidiary fault planes.
The 2025 sequence is located eastern, deeper and slightly farther from the CF and its results follow a different and more complicated pattern. On one hand, the two analyzed earthquakes show vertical strike-slip focal mechanisms with fault planes oriented N-S and E-W, clearly discrepant with the kinematics of the CF. On the other hand, the general NW-SE trend of the epicentral distribution of the aftershocks does not coincide with any of the nodal planes as can be seen in the map view and N-S and E-W fault planes-oriented cross-sections of Figure 4. These observations, together with the distribution of the earthquakes into different clusters covering a relatively wide rupture area, suggests that the seismicity likely occurred along different small-scale fault segments with N-S or E-W orientation.
High-resolution bathymetry studies and multi-channel seismic profiles in this region show a rough bathymetry with large canyons, pressure ridges, ‘en-echelon’ folds, and submarine volcanic ridges (Gràcia et al., 2006; Pedrera et al., 2010; Gómez de la Peña et al., 2016) but a lack of active structures with surface expression except for the CF. Thus, we suggest that most of the analyzed seismicity may have occurred in complex fault systems of small ‘en-echelon’ unmapped faults and dominated by the regional stress regime, rather than propagating along a single well-defined fault.
Taking into account that all the obtained local P-axes are consistent with the shortening axis estimated for the northern Alboran Sea (N154°E) (Olaiz et al., 2025; Madarieta-Txurruka et al., 2026) and thus, with NW-SE compression direction and the regional stress regime, we can conclude that the seismic sequences occurred in this area are the response of an inhomogeneous crust to the regional stress field. In this context, the complex fault system, including both CF and smaller subsidiary structures, would accommodate deformation over a wide area, reflecting distributed slip rather than activity along a well-defined fault plane.
Conclusion
The NE Alboran Sea has shown scarce seismicity of small to moderate (M < 5) earthquakes in the instrumental record but for the 1993–1994 Berja-Adra seismic series in the proximity of Campo de Dalías where most of the activity occurs. Also, few events have been associated with the major active mapped structure in this area, the CF. The July 2025 Mw 5.3 earthquake is the first recorded case of M5 earthquake in this region during the instrumental record. This earthquake was located offshore several kilometers away from the CF and its aftershock sequence was distributed in a general NW-SE trend nearly perpendicular to this fault. Along the northern offshore segment of the CF, southwest of the 2025 series, several smaller seismic series have also occurred in 2008, 2010, and 2012, including a few M4 earthquakes. Our study provides valuable new insights into the spatial and depth distribution and faulting style of these seismic sequences, contributing to the understanding of offshore tectonics and seismicity in western Mediterranean.
The focal mechanisms derived from moment tensor inversion and the precise seismic relocation performed for a selected set of M > 3.5 and M > 2 earthquakes from the major 2025 seismic series and previous smaller series in 2008, 2010, and 2012, occurred in the vicinity of the northern offshore segment of the CF, have enhanced our understanding of the tectonic context and seismic characteristics of the NE Alboran Sea region:
Focal mechanisms of our study reveal a predominance of strike-slip faulting, with N-S to NNE-SSW and E-W to WNW-ESE nodal planes orientations and near-vertical planes, not consistent with CF kinematics. Our results indicate maximum pressure axes (P-axes) along an approximate NNW-SSE direction, consistent with the local shortening expected for the Alboran Sea and southeast Iberian margin and with the general NW-SE compression direction and the regional tectonic regime.
•The hypocentral relocations obtained in this study show an overall NW-SE trend for the 2025 seismic series, roughly perpendicular to the CF. The distribution of aftershocks appears to be clustered in certain areas, resulting in a rupture zone several kilometers wide. A similar pattern is observed in the previous 2010 series, displaying a narrower and elongated alignment oriented in an NNW-SSE direction, in contrast with the more clustered 2008 and 2012 series. In all cases, there is no coincidence between the epicentral distribution and any of the calculated fault planes.
•In depth, the relocated seismicity is constrained to shallow depths, with foci deepening to the north as they approach the continental margin, confirming a seismogenic layer of 10–20 km in the NE Alboran crust. Our results indicate that seismic activity can also occur in the magmatic-arc crust of the EAB domain, and low-to-moderate magnitude earthquakes could be nucleated.
•Our focal mechanism and relocation results suggest that all this seismicity responds to the regional stress regime. We suggest that most of the 2025 seismicity may have occurred in complex fault systems of small ‘en-echelon’ unmapped faults, rather than propagating along a single well-defined fault. A similar cause may apply in the case of the previous series although in that case, given their proximity to the northern offshore segment of the CF, could be related to this structure.
•The analyzed seismic sequences in the NE sector of the Alboran Sea may be the response of an inhomogeneous crust to the regional stress field. In this context, the complex fault system, including both CF and smaller subsidiary structures, would accommodate deformation over a wide area, reflecting distributed slip rather than activity along a well-defined fault plane.
Although seismicity in the NE Alboran Sea has previously appeared to concentrate in the vicinity of the Campo de Dalías and Chella Bank in relation to nearby structures, the recent 2025 Mw 5.3 earthquake represents the first recorded M5 event in the northeastern end of this region. The occurrence of this new seismicity, together with previous smaller series in 2008, 2010 and 2012, along the offshore northern section of the CF, suggests that the potential of this area to generate moderate events has been underestimated and should therefore be integrated in any seismic and tsunami hazard assessments.
Regarding future work, analysis of the slip and stress drop distribution on the 2025 main shock (Coulomb stress transfer) would greatly contribute to better assessing its link to the aftershock series and previous seismicity. Besides, more accurate geological and high-resolution bathymetric studies may contribute to obtain a clearer picture of the geometry at depth and to identify possible active structures or fault features that could explain the seismicity of this region.
Data availability statement
The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding author.
Author contributions
LL: Conceptualization, Data curation, Formal Analysis, Methodology, Writing – original draft, Writing – review and editing. CL-S: Conceptualization, Data curation, Formal Analysis, Methodology, Writing – original draft, Writing – review and editing. CF: Conceptualization, Funding acquisition, Methodology, Writing – original draft, Writing – review and editing. MM: Data curation, Formal Analysis, Methodology, Writing – original draft. JC: Conceptualization, Project administration, Supervision, Writing – review and editing.
Acknowledgements
The authors are grateful to A.Villaseñor (ICM-CSIC, Spain) for providing the digital 3D P-wave velocity model grid of Iberia and Morocco used for the absolute relocation; J. Barco and R. Antón from the National Geographic Institute (IGN, Spain) for their help with the waveform and instrumental databases. The authors also acknowledge the Instituto Andaluz Universitario de Geofísica y Prevención de Desastres Sísmicos (Universidad de Granada, Spain), the Real Observatorio de la Armada de San Fernando (Cádiz, Spain), the Instituto Português do Mar e da Atmosfera, the Instituto Dom Luiz (Universidade de Lisboa), the Institut National de Géophysique (CNRST, Rabat, Morocco), the University of Alicante and Institut Cartogràfic Valencià, the Universidad Complutense de Madrid (UCM, Spain) and the Centre de Recherche en Astronomie, Astrophysique et Geophysique of Algeria for their kindness in sharing their data. The authors would like to acknowledge both reviewers for their valuable suggestions and comments.
Conflict of interest
The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
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Supplementary material
The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/feart.2026.1773588/full#supplementary-material
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Edited by:Mourad Bezzeghoud, Universidade de Évora, Portugal