Determine the fault responsible for the Samos earthquake

Preliminary data indicates that the fault that erupted in the Samos earthquake slid 6 feet (1.8 meters).

Written by Athanasios Janas, National Observatory of Athens, Geodynamic Institute, Panagiotis Elias, National Observatory of Athens, Institute of Astronomy, Astrophysics, Space Applications and Remote Sensing Pierre Prioll, Normale Superior de Paris, PSL Research University, Geology Laboratories, Varvara Seroni, Athens National Observatory Institute of Geodynamics; Department of Geology, University of Patras, Sotiris Valkaniotis, Coronidos Street, Greece Javier Escartin, National Observatory of Athens, Institute of Astronomy, Astrophysics, Space Applications and Remote Sensing, Electra Carasante, National Observatory of Athens, Institute of Geodynamics Irene Evestathio, National Observatory of Athens, Institute of Geodynamics

Citation: Ganas, A., Elias, P., Briole, P., Tsironi, V., Valkaniotis, S., Escartin, J., Karasante, I., and Efstathiou, E., 2020, Fault responsible for the Samos earthquake Identified, Temblor, http://doi.org/10.32858/temblor.134

Editor’s Note: The abstract is written for a general audience. Those interested in technical details should read the full article.

Summary

On October 30, 2020, at 11:51 UTC, a strong, shallow earthquake struck the eastern Aegean Sea. The epicenter was located off the coast of the Greek island of Samos, about 160 miles (260 kilometers) east of Athens. The earthquake was magnitude 6.7 on the Richter scale, according to the National Observatory of Athens (NOA). The effects of the earthquake were devastating in Greece and Turkey. In Greece, two children were killed when a wall collapsed in the town of Vathy, east of Samos. In Turkey, more than 100 people were killed in several building collapses in the city of Izmir, 40 miles (60 kilometers) north of the epicenter.

Here we present the first analysis of geodetic data collected and processed as of November 4, 2020 at 15:00 UTC. Initial modeling indicates that the rupture occurred in a fault 23 miles (37 km) north (“normal”) off the northern shore of Samos.

InSAR is a form of image that is collected by repeating the passage of a satellite over a specific area. This technique is used to measure how much the Earth is moving roughly vertically between each lane of a satellite and can give scientists an indication of how much sliding has occurred on a fault beneath the surface. In this case, the fault ruptured in the Samos earthquake slid 6 feet (1.8 meters). Rupture of the upper rim of the fault – the part that slid from the fault closest to the surface, 0.9 miles (1.5 kilometers) deep near the northern shore of Samos.

Introduction

The Samos region is located in the eastern part of the delicate Aegean (Eurasia) Plate, an extended region with a posterior arch known behind the Hellenic subduction (McKenzie, 1978; Ganas and Parsons, 2009). The kinematics of plate motions are determined by the subduction of the African oceanic plate under the Aegean Sea and the westerly movement of the Anatolian plate. Crust stretching is accommodated by a combination of natural sliding and beating sliding movements along active faults, especially in the central Aegean and western Anatolia (Mascle and Martin, 1990; Taymaz et al. 1991; Tan et al. 2014). In terms of stress, the amount of cortical expansion between Samos and Western Anatolia (wider Izmir region) is 7.4 mm / year according to Vernant et al. (2014) based on GNSS data modeling.

The Mw = 7.0 earthquake occurred on October 30, 2020 at 11:51 UTC north of Samos Island (Fig.1), along a normal error hitting electronic warfare as evidenced by momentary tensor solutions for both regional and tele-data (compiled by EMSC). During 2 November 2020, more than 776 aftershocks (with 2.0≤ML≤5.2) were recorded by EMSC (Fig.2). Three hours after the main tremor, a medium-sized aftershock M = 5.2 struck at 15:14 UTC. The torque tensor solutions of the main shock indicate a normal fault in the EW to the ESE-WNW in agreement with the expansionary regional tectonics. The aftershock sequence extends over 70 km between east and west with most of the events occurring to the east of the main earthquake (Figure 2). EMSC aftershocks data (length versus time plot; in days from the main shock) show that the sequence is spatially restricted between 26.4 ° E – 27.2 ° E (Fig. 2 middle panel).

Figure 1: Site map showing the shaded terrain / bathymetry, the focus mechanism (beach ball; GCMT solution) and the epicenter of the Samos earthquake on October 30, 2019. The triangles indicate the locations of the permanent GPS (GNSS) stations.

Figure 2. Spatiotemporal evolution of the aftershocks sequence of SAMOS (data source: EMSC). The colors of the circle correspond to depth, and volume with volume. The bottom panel shows the frequency of occurrence with respect to the longitude (east-west).

Seninel-1 Interferogram

We used the ascending images obtained from the European Sentinel-1 satellite on October 24 and October 30, 2020 on Route 131. Interference diagram (Fig. 3) was performed on the Geohazards Exploitation Platform using SNAP software. Digital Elevation Model (DEM) used for processing is the Shuttle Radar Topography Mission (SRTM) 1 Arc-Second Global. We improved the signal to noise ratio by applying the Goldstein and Werner (1998) adaptive power spectrum filter with a coherence threshold of 0.3. The quality of the interference pattern is good, both in terms of coherence and tropospheric noise. The intersection diagram shows the opposite edges of the deformation of the land on the shore of Samos. The absolute value of the interferometric terminals is estimated by correlating the SAMO and SAMU stations of the GNSS system (Fig. 1) that captured the common seismic displacement. All cilia correspond to the movement towards the satellite except for the short northern part near the coast where the movement is further from the satellite. We extracted line-of-sight (LOS) displacements by selecting the six limbs in Fig. 3 at 64 locations in total. The rise is explained by the combined seismic movement along a natural fault offshore, electronic warfare operation and diving in the north.

Figure 3. A coseismic interference diagram (enveloping phase, cropped patch) over Samos Island for photo pair from October 24 to October 30, 2020.

Joint seismic motion of GNSS stations

We analyzed data for eleven GNSS stations belonging to two private Greek networks, SmartNet and Uranus, and from the Turkish network CORS (Fig.1). Processing was performed with two different Precision Positioning (PPP) programs: the GIPSY / OASIS II program (version 6.4) developed by the Jet Propulsion Laboratory (JPL), and the Canadian online processing PPP service. The coseismic displacements are listed on the tab. Figure 1 and Figure 4 show the time series in SAMO (Karlovasi). We were unable to see evidence of GNSS data on any rapid deformation after the earthquakes, and in particular no compensation at the time of the large aftershock on October 30, 2020 at 15:14 UTC (M = 5.2). Thus, given that a second Sentinel-1 image was acquired on the afternoon of 30 October, shortly after the event, we assume that the interference pattern (Fig.2) contains only an anosexual signal with little slip of the post-earthquake fault.

Figure 4. Time-series position (E, N, top) of the SAMO station (see location in Fig. 1). The common seismic compensation is the following: dE = -6 cm, dN = -37 cm and dU = +8 cm. The red vertical lines indicate the timing of the main shock.

Error form

We use the InSAR los offset and GNSS offsets to estimate error parameters assuming a rectangular source buried in a homogeneous semi-elastic space and a homogeneous slip. The reflection approach finds the geometry and kinetics (hit and dip angle) of the most appropriate error model. Together, we reflect los InSAR ground motions and GNSS common displacements using an inverse symbol 6 (Briole, 2017). The modeling allows us to restrict seven parameters: the three-dimensional position of the center of the highest error, the characteristic of the error, the length and width and the amount of slip. We assume a pure normal error and do not reflect one of the rake components, as this parameter is associated with the characteristic of the error in the reflection of the geodesic data. We also fix the wrong slope angle of 37 ° given by the moment seismic tensor, as the geodetic data does not have the ability to solve this angle. Fault width is only loosely restricted because there are no GNSS points in the near field on the Turkish shore in front of Samos. Our best fit model is a malfunction of 36 km long and 18 km wide, hitting N276 ° E, (Fig.5).

When analyzing the area of ​​the solution, we find that all lengths between 32 and 42 km are possible, all lengths between 14 and 19 km, and all slides between 1.5 and 2.2 meters, with the product of the three being constant (for compatibility with the moment-derived geodesic). The potential range of fault base slip depths is 10 to 13 km, and thus slightly deeper than 10 km that was found for the Kos – Bodrum earthquake Mw = 6.6 July 2017 (Ganas et al. 2019). We also verified the impossibility of the Southern dip-error model, as this model could not co-fit the GNSS vectors and InSAR terminals.

Figure 5: Synthetic interference diagram corresponding to our best fit error model (error projection in black). The direction of the rift retraction is northward and so the Samos forms a foot wall block. White arrows indicate the observed GNSS offsets and black arrows in the model.

Acknowledgments: We thank Brendan Crowell, Simon Buferal, Nicolas Chamot-Rooke, Marco Meschis, Tuncay Taymaz, Diego Melgar, Evi Nomikou, Margarita Segou and Efthimios Lekkas for the comments and discussions. We are indebted to ESA, Geohazards Lab, and Terradue to provide access to the Geohazards Exploitation Platform (GEP) for InSAR cloud processing. GNSS data was provided by Hexagon SmartNET and Uranus (Tree) in Greece. We thank Tuncay Taymaz and Semi Ergintav for sharing Turkish GNSS data.

References

Briole, P. 2017. Modeling seismic slip by inverting GNSS and InSAR data assuming a homogeneous elastic medium. Zenodo, http://doi.org/10.5281/zenodo.1098399Ganas, A., Parsons, T., 2009. 3D model of the deformation of the Hellenic Arc and the origin of the Cretan Ascension. Geophysics. Res: Solid Earth 114 (B6) https://doi.org/10.1029/2008JB005599. Ganas, A., et al, 2019. The M6.6 Kos earthquake of July 20, 2017: Seismic and geodetic evidence for an active north – dipping a normal fault in The western end of the Gulf of Zhukova (southeast of the Aegean), Pure and Applied Geophysics, 176 (10), 4177-4211 https://doi.org/10.1007/s00024-019-02154-yGoldstein, RM; Werner, CL 1998. Radar interference pattern filtering for geophysical applications. Geophysics. Precision. Lett. 25 (21), 4035-4038 Mascle, J., and L. Martin, 1990. Shallow structure and recent development of the Aegean: a synthesis based on continuous reflection patterns, Marine Geology, 94, 4, 271-299. McClusky, S., et al. 2000, Global Positioning System Limitations on Plate Kinematics and Dynamics in the Eastern Mediterranean and Caucasus, J. Geophys. Precision. 105, B3, 5695-5719, DOI: 10.1029 / 1999JB900351.McKenzie, D. 1978. Active Tectonic Movement of the Alpine Belt – Himalayas: Aegean Sea and Surrounding Areas, Geophysics. Ji Roy. Aster. 55, 1, 217-254, DOI: 10.1111 / j.1365-246X.1978.tb04759.x.Tan, O., Papadimitriou, EE, Pabucçu, Z. et al. 2014. Detailed analysis of the fine excursion in Samos and Kusadasi (Eastern Aegean Sea). Acta Geovese. 62, 1283-1309. https://doi.org/10.2478/s11600-013-0194-1Taymaz, T., J. Jackson, and D. McKenzie 1991. Tectonic dynamics of the northern and central Aegean, geophysics. J. Int. 106, 2, 433-490, DOI: 10.1111 / j.1365-246X.1991.tb03906.xVernant, P.; R. Reilinger, S. 018 SAR images obtained by SENTINEL-1 satellites are routinely distributed by the European Space Agency (ESA) free of charge. GNSS data from stations surrounding the epicenter were provided by Greek GNSS Private Networks (SmartNet and Uranus) and Turkish CORS Network.

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