# Volcano-tectonic interaction at Santorini. The crisis of February 2025. Constraints from geodesy

Pierre Briole, Athanassios Ganas, Anna Serpetsidaki, François Beauducel, Vassilis Sakkas, Varvara Tsironi, Panagiotis Elias

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Pierre Briole, Athanassios Ganas, Anna Serpetsidaki, François Beauducel, Vassilis Sakkas, et al. Volcano-tectonic interaction at Santorini. The crisis of February 2025. Constraints from geodesy. Geophysical Journal International, 2025, 242 (3), 10.1093/gji/ggaf262. hal-05402086

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Submitted on 6 Dec 2025

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<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
<figure><img src="image_2.png" /><figcaption>**Figure 1.** Relocated seismicity from 2024 July 1 to 2025 March 4 and focal mechanisms (from the National Observatory of Athens catalogue https://bbnet.gein.noa.gr/HL/) of the eight events of $M_w \ge 5$. The colours indicate the dates of the events. Black and grey triangles indicate GNSS and seismological stations, respectively. Dashed lines indicate active faults from the NOA faults database (https://doi.org/10.5281/zenodo.13168947).</figcaption></figure>

& Wadge (1998) for the topographic corrections. Eqs (1) and (2) define the forward problem for a source at depth $f$ with volume change $\Delta V$, allowing us to compute the radial ($U_r$) and vertical ($U_z$) displacements at a given point located at a horizontal radial distance $r$ from the source, and an altitude $h$ above sea level. The Poisson's ratio, $\nu$, is set to 0.25 for an isotropic medium. For the selected observation time window, we determine the linear displacement trend for each component and station, along with the associated standard deviation.

$$U_r = \frac{(1 - \nu) \Delta V}{\pi} \frac{r}{((f + h)^2 + r^2)^{3/2}} \quad (1)$$

$$U_z = \frac{(1 - \nu) \Delta V}{\pi} \frac{f + h}{((f + h)^2 + r^2)^{3/2}} \quad (2)$$

The inverse problem is formulated within a Bayesian framework, where the likelihood function, given by eq. (3), incorporates an L1-norm misfit based on the data and their uncertainties.

$$P(d|m) = P_0 \exp \left( - \sum \frac{|d_{\text{obs}}^i - d_{\text{cal}}^i|}{\sigma^i} \right) \quad (3)$$

The model space is explored using a grid search method. The optimal model corresponds to the maximum likelihood estimate. The time-series are analysed and plotted in the WebObs environment (Beauducel et al. 2020). Fig. 3 and Fig. S5 (Supporting Information) present the observed and modelled displacements and Table S3 (Supporting Information) the parameters of the point source model for phase 1 and the other phases. The modelled source is located 1 km east of Thirasia island at $3.1 \pm 0.3$ km depth with a volume change of $7.7 \pm 0.6 \times 10^6$ m$^3$.

### 3. DEFORMATION ASSOCIATED WITH THE SEISMIC CRISIS OF FEBRUARY 2025

The coordinate time series show strong variations between around 2025 January 27 and February 24 especially at Santorini (Fig. S6, Supporting Information), with maximum intensity between 2 and 18 February, and progressive increase and decrease towards both ends of the period. The Anydros station only captures the second half of the crisis with displacements reaching 7 mm d$^{-1}$ vertically and westwards. The shape of the deformation at all stations fits well with a hyperbolic tangent of the form of eq. (4), with A amplitude, $t_0$ central time and T decay time.

$$D = Atanh \left( - \left( \frac{t - t_0}{2T} \right) \right) \quad (4)$$

Table S4 (Supporting Information) indicates the best fitting values of those parameters for each station. The average values of $t_0$ and T are 10/2/2025 and 3.5 d with small differences between stations. Fig. S7 (Supporting Information) shows the existence of a strong temporal correlation between the energy released seismically and the motion at SANT.

A planar dislocation model provides a good fit for both the amplitude and orientation of the motions. The fit is robust due to the good azimuthal coverage. Table S5 (Supporting Information) lists the parameters of our best models for the two candidate antithetic planes and Table S6 (Supporting Information) the modelled displacements. Both planes give fairly similar fits, with, in both cases, a 18 km long fault located between the Kolumbo volcano and the
<figure><img src="image_3.png" /><figcaption>**Figure 2.** Time-series (East, North, Up) of daily coordinates of the GNSS stations 048A, SANT and SNTR from 2024 January 1 to 2025 May 26 corrected for the tectonic trend (calculated with data before the unrest). Grey areas indicate phase 1 (dark grey), phase 2 (light grey) and phase 3 (dark grey), respectively. See Fig. S3 (Supporting Information) for the other stations.</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>**Figure 3.** Map of Santorini showing GNSS displacements and model for phase 1. Horizontal view (left, in UTM35) and North–South vertical profile (right, in m). Displacements and uncertainties (black vectors with large tips and uncertainty ellipses), and modelled displacements (red vectors with small tips). Probability of an isotropic point source location expressed as maximum normalized likelihood function (red shaded contours).</figcaption></figure>

north-west edge of Anydros with a tip at 7.5 km depth. The angular parameters of the model are consistent with the focal mechanisms of the main earthquakes (Fig. 1) calculated by various agencies and distributed by the Euro-Mediterranean Seismological Centre (https://www.emsc-csem.org/). The modelled seismic moment is $17.5 \times 10^{18}$ Nm which corresponds approximately to a $M_w = 6.8$ earthquake.

Fig. 4 shows the location of the south-east dipping fault model and the observed and calculated horizontal displacements.

Fig. 5 shows that this plane fits with the location and geometry of the Ios West normal fault while the antithetic plane does not correspond to any known structure. Figs 5 and 6 show that most of the earthquakes occur in the hanging wall and at the eastern side of the inferred fault, accounting for less than ten per cent of the seismic moment of the inferred slow slip.

Fig. S8 (Supporting Information) shows the ascending Sentinel-1 interferogram predicted by our model and Fig. S9 (Supporting Information) observed and modelled interferograms over Anydros. The interferograms show large gradients for both orbits, consistent in amplitude with our models with better azimuthal fit for the south-east dipping case, the residual misfit possibly due to a bending of the fault at its eastern edge.

## 4. TEST OF MODELS INVOLVING THE MOVEMENT OF MAGMA

The b-values of the seismicity during the above two phases (Fig. S10, Supporting Information) are close to 1 which is typical for tectonic events. However, according to Zahradnik et al. (2025) and Fountoulakis & Evangelidis (2025), several moment tensors show a component of opening. We therefore tested models involving the movement of magma.

Given the subsidence recorded by the Santorini and Anydros GNSS stations during phase 2, we first tested a deflation point source model. Fig. S11 (Supporting Information) shows the poor fit of the best fitting model, which cannot account for both vertical and horizontal displacements simultaneously.

We then tested two dyke injection models. In the first case, we assume that our south-east dipping fault model continues upwards for 3 km in the form of a vertical dyke (see Fig. 5). Fig. S12 (Supporting Information) shows the fit of the GNSS displacements when varying the inflation or deflation of the hypothetical dyke, inverting for the depth and slip of the fault. The fit is optimal for a contraction of 0.11 m, that is, $-5.9 \times 10^6 \text{ m}^3$ and remains within 10 per cent of its minimum value for dyke thickness in the range $-0.6$ to $+0.4$ m, thus corresponding to the volume range
<figure><img src="image_5.png" /><figcaption>**Figure 4.** Observations (black vectors with uncertainty ellipses) and model (red vectors) for phase 2 (map scale in km; UTM35). The numerical values are given in Tables S4 and S6 (Supporting Information). The grey rectangle shows the projection of the modelled fault (parameters in Table S5, Supporting Information) at the surface, and the dashed line the intersection of the modelled fault plane with the surface. The thick black line indicates the Ios West fault trace.</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>**Figure 5.** Section along the profile C–D (see Fig. 1) showing seismicity, location and geometry of the best fitting fault (blue line), surface trace and inferred location of the Ios West fault (dashed blue line) and Anydros fault (dashed green line). The alternative best fitting model, north–west dipping, is plotted in dashed black. The location of a hypothetical dyke injection discussed in Section 4 is plotted in red. Diamonds correspond to the stronger earthquakes (see Fig. 1) plotted with colour corresponding to their time occurrence.</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>**Figure 6.** Spatiotemporal distribution along the A–B profile (N36°E; Fig. 1) of the seismic moment release per kilometre corresponding to our modelled slow-slip earthquake (horizontal segments) and that of the observed seismicity. The moment release of the stronger earthquakes (see their focal mechanisms in Fig. 1) is indicated with colour corresponding to the occurrence date.</figcaption></figure>

−32.4 to 21.6 × 10⁶ m³. The modelled slip increases with the dyke opening. This indicates that there is no competition between the two sources which is further evidence against a primarily magmatic hypothesis.

The second model involves injecting magma into our normal fault. Fig. S12 (Supporting Information) shows a fit that degrades more quickly, +10 per cent from the optimal being obtained with an opening of 0.2 m which corresponds to 50 × 10⁶ m³.

## 5. CONCLUSIONS

During summer 2024, an inflation began at Santorini, comparable to that of 2011–2012 with a faster magma supply rate (0.42 m³ s⁻¹ instead of 0.27). In February 2025, a second and stronger phase occurred ~30 km north-east of Santorini.

Our analysis of the GNSS data suggests that, during this second phase, between around 2025 January 27 and February 24 and culminating between 2 and 18 February, a slow-slip earthquake, of equivalent magnitude 6.8, occurred along a SE-dipping normal fault.

The recorded seismicity accounts for less than ten per cent of the equivalent seismic moment of the inferred event and is mostly located at the eastern termination of that fault, around the Anydros islet.

Our fault model is consistent with the deep part (7.5 to 15 km depth) of the Ios West fault, one of the major tectonic structures of the Santorini–Amorgos graben (Preine et al. 2022). Is the upper segment of the Ios West fault likely to rupture seismically in the future? Can this hypothesis be minimized on account of a less brittle rheology there?

At the transition from phase 1 to 2, the migration of the seismicity (Fig. S1, Supporting Information) suggests that the processes involved in the two phases are coupled. The residuals of the phase 2 modelling (Fig. S13, Supporting Information) indicate a dominant radial residual at Santorini, that can be modelled by a deflation of the caldera corresponding to −6.8 × 10⁶ m³, close to the 7.7 × 10⁶ m³ stored during phase 1. This suggests that a dyke or a sill was injected at shallow depth during the transition period. However, there is no clear evidence for such injection in the GNSS data that fits well with just the combination of the fault model and the additional model of Fig. S13 (Supporting Information) with r.m.s. residuals below 10 mm.

Some seismological observations suggest that magma may be involved during phase 2. This is not incompatible with the GNSS data, whose resolution does not allow detecting a source with volume below 30–50 × 10⁶ m³, which would degrade the fit of the joint fault-dyke model by no more than 10 per cent of the minimum obtained with the single fault.

However, the GNSS data is incompatible with the alternative hypothesis of large dyke injection, because the vertical displacements at Santorini and Anydros would be uplift, while subsidence is observed.

The GNSS time series from March to May 2025 suggest that the current third phase is a relaxation.

It is important to document the transient motions associated with this unrest also because they bias the geodetic measurements of the secular velocity field of the area.

Finally, we detected a peculiarity in the GNSS time series during the period September 2020 to September 2021, particularly significant at the station SANT (Fig. S14, Supporting Information). The
signal can be modelled (Fig. S15, Supporting Information) with a
12 × 10⁶ m³ inflation of a point source located 8 km below the
Kolumbo volcano.

## Additional Supporting Information

- CSV file with the time-series of daily coordinates of the GNSS stations.

## ACKNOWLEDGMENTS

We are grateful to the Hellenic Positioning System (HEPOS) service and to companies and universities (HxGn Smartnet, URANUS, NTUA, AUTH, UPAT, NOANET) who provided the GNSS data. The seismological data is from NOA https://bbnet.gein.noa.gr/HL/databases/database. We used the NOAFAULTs database available at https://doi.org/10.5281/zenodo.13168947. We acknowledge Costas Synolakis, Stavros Meletlidis and Simon Bufferal for the fruitful discussions. Support for GNSS installations in South Cyclades was provided by the project SAT4GAIA (European Space Agency Contract No. 4000145364/24/NL/Ffi) to AG under the framework of the Greek National Satellite Space Project: Axis 3 Land Monitoring Service. The WebObs open-source code used to display and model the GNSS data is available at https://ipgp.github.io/webobs. We are grateful to Philip England, Jeff Freymueller and an anonymous reviewer for their insightful reviews.

## SUPPORTING INFORMATION

Supplementary data are available at *GJIRAS* online.

Figure S1. Temporal evolution of the seismicity.

Figure S2. Cross-sections of the seismicity.

Figure S3. Coordinates time series of the GNSS stations.

Figure S4. Santorini GNSS points motion during phase 1.

Figure S5. Source model for phase 1.

Figure S6. Santorini GNSS points motion during phase 2.

Figure S7. Seismic moment versus ground deformation.

Figure S8. Synthetic ascending interferogram corresponding to our fault model.

Figure S9. Observed and modelled interferograms of Anydros.

Figure S10. *b*-value of the seismicity.

Figure S11. Trial of a deflation source model for phase 2.

Figure S12. Evolution of the fit in the hypothesis of dyke intrusion.

Figure S13. Point source model for the residuals of phase 2.

Figure S14. Time-series of SANT, SANU and 048A since 2015.

Figure S15. Observations and model for the 2020–21 fluctuation.

Table S1. Coordinates of the GNSS stations.

Table S2. Observed displacements for phase 1.

Table S3. Parameters of the point sources estimated for the various phases.

Table S4. Observed displacements for phase 2.

Table S5. Parameters of the fault model of phase 2.

Table S6. Modelled displacements for phase 2.

Please note: Oxford University Press is not responsible for the content or functionality of any supporting materials supplied by the authors. Any queries (other than missing material) should be directed to the corresponding author for the paper.

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