What do we know 48 hours after the Samos earthquake?

The Aegean Sea island of Samos is struck by another large earthquake after 116 years.
 

By Margarita Segou, Ph.D., British Geological Survey
 

Citation: Segou, M., 2020, What do we know 48 hours after the Samos earthquake?, Temblor, http://doi.org/10.32858/temblor.132
 

 

On Oct. 30, 2020, a lethal magnitude-7.0 earthquake hit 14 kilometers (9 miles) offshore of the north coast of Samos Island in the East Aegean Sea, Greece. Damage on the island occurred in historical, unreinforced masonry buildings from the early 1900s. The earthquake likely occurred on the offshore Samos Fault, a normal fault oriented approximately east-west, and was followed by a tsunami that swept 300 meters (980 feet) inland in the port of Samos, Vathi and Izmir in Turkey. The tsunami reached the port of Kos Island, 100 kilometers (60 miles) to the south with a recorded sea-level change of half a meter. Even in Athens, 350 kilometers (220 miles) to the west, I felt shaking.

The last time an earthquake of this magnitude struck the east Aegean Sea was in 1904. That earthquake, a magnitude-6.8, likely ruptured another similar normal-oblique fault near the south coast of the island. In the years following 1904, the broader East Aegean-Turkey margin was struck by other large earthquakes, although whether there is a direct connection with the 1904 Samos earthquake is uncertain (Paradisopoulou et al., 2010).

 

Aftershocks in the complex Aegean-Anatolia tectonic margin

Following a large earthquake, the U.S. Geological Survey (USGS) and other institutes around the world estimate how much and in what direction the fault slipped. These analyses inform scientists about possible patterns of expected aftershocks, and are critical in disaster response because it is not uncommon that aftershocks cause more extensive damage than the mainshock.

Preliminary data analysed by seismologists shows some ambiguity as to whether this offshore normal fault was dipping towards the north or south, but the available models show that a large amount of slip occurred close to the epicenter together with a second, shallower, slip patch. Since aftershocks usually do not occur on fault patches that slipped during the mainshock, but instead tend to fill the space outside the rupture patch on the primary fault plane, the former observation perhaps hints that expected aftershocks may happen at shallower parts of the fault, closer to the north coast of Samos.

Aftershocks are considered a stress-balance mechanism for the Earth’s crust that also influences the conditions for failure of surrounding faults. The “stress-transfer hypothesis” is the main principle upon which scientists rely to improve our understanding of earthquake triggering processes, and therefore improve our future earthquake forecasts (Segou, 2020). Preliminary data — available within the first 30 hours after the strong earthquake — show that Friday’s earthquake further loaded some faults, making them more likely to slip in a future earthquake, whereas others are moving away from failure.

 

 

Faults brought closer to failure

Friday’s earthquake brought most of the main faults surrounding Samos closer to failure. Faults immediately to the north of Izmir were brought farther from failure, but only marginally so. These estimates may reflect a first qualitative assessment of the aftershock triggering potential but as high-resolution seismic data become available, updated mathematical models that tie seismic evidence with continuum mechanics principles, will quantify the expected aftershock rates over the broader region. Past studies (Parsons et al., 2014) show that active extensional environments, including Greece, are susceptible to other driving mechanisms, such as delayed dynamic triggering due to the passage of the seismic waves through the shallow crust, that may play a role in triggering earthquakes.

Often, early results and interpretations suffer from uncertainties found in preliminary earthquake information, but in this case, they also clearly show that the faulting patterns dominating the east Aegean Sea should be further investigated in the aftermath of this catastrophic earthquake. The aftershocks will continue for months to come, as is expected for an earthquake of this magnitude. This highlights the importance of dense seismic monitoring in both countries.

 

Further reading

Paradisopoulou P.M., Papadimitriou E.E., Karakostas V.G., Taymaz T., Kilias A., Yolsal S. (2010) Seismic Hazard Evaluation in Western Turkey as Revealed by Stress Transfer and Time-dependent Probability Calculations. In: Savage M.K., Rhoades D.A., Smith E.G.C., Gerstenberger M.C., Vere-Jones D. (eds) Seismogenesis and Earthquake Forecasting: The Frank Evison Volume II. Pageoph Topical Volumes. Springer, Basel. https://doi.org/10.1007/978-3-0346-0500-7_12

Segou, M. (2020). The Physics of Earthquake Forecasting, Seismol. Res. Lett. 91, 1936–1939, doi: 10.1785/0220200127.

Parsons, T. M.Segou, W.Marzocchi (2014), The global aftershock zone, Tecto., 618, 1-34,
https://doi.org/10.1016/j.tecto.2014.01.038.

Mainshock location source: http://www.gein.noa.gr/en/
Past earthquake locations source: ISC-GEM catalog, International Seismological Centre (20XX), ISC-GEM Earthquake Catalogue, https://doi.org/10.31905/d808b825

Finite fault model taken from the USGS Event Page (source: https://earthquake.usgs.gov/earthquakes/eventpage/us7000c7y0/finite-fault).

Faults in map are taken from GEM Global Active Faults Map (source: https://blogs.openquake.org/hazard/global-active-fault-viewer/).