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Short term clustering modeling of seismicity in Eastern Aegean Sea (Greece): a retrospective forecast test of the 2017 Mw =6.4 Lesvos, 2017 Mw =6.6 Kos and 2020 Mw =7.0 Samos earthquake sequences

Kourouklas Christos, Mangira Ourania, Console Rodolfo, Papadimitriou Eleftheria, Karakostas Vassilios, Murru Maura

Acta Geophysica

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2021

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Vol. 69, no. 3

1085--1099

EN

Short-term earthquake clustering properties in the Eastern Aegean Sea (Greece) area investigated through the application of an epidemic type stochastic model (Epidemic Type Earthquake Sequence; ETES). The computations are performed in an earthquake catalog covering the period 2008 to 2020 and including 2332 events with a completeness threshold of Mc=3.1 and separated into two subcatalogs. The frst subcatalog is employed for the learning period, which is between 2008/01/01 and 2016/12/31 (N=1197 earthquakes), and used for the model’s parameters estimation. The second subcatalog from 2017/01/01 to 2020/11/10 (1135 earthquakes), in which the sequences of 2017 Mw=6.4 Lesvos, 2017 Mw=6.6 Kos and 2020 Mw=7.0 Samos main shocks are included, and used for a retrospective forecast testing based on the constructed model. The estimated model parameters imply a swarm like behavior, indicating the ability of earthquakes of small to moderate magnitude above Mc to produce their own ofsprings, along with the stronger earthquakes. The retrospective evaluation of the model is examined in the three aftershock sequences, where lack of foreshocks resulted in low predictability of the mainshocks, with estimated daily probabilities around 10–5. Immediately after the mainshocks occurrence the model adjusts with notable resemblance between the expected and observed aftershock rates, particularly for earthquakes with M≥3.5.

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Spatio-temporal evolution of aftershock energy release following the 1989, MW6.9, LOMA Prieta earthquake in California

Gospodinov D.

Acta Geophysica

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2017

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Vol. 65, no. 3

565--573

EN

We apply a stochastic model to study Benioff strain release after the Mw6.9 October 18, 1989 Loma Prieta strong earthquake in north California, USA. The model is developed, following a compound Poisson process and contours the evolution of strain release during the aftershock sequence following the main shock occurrence. First, the temporal evolution of the aftershock decay rate was modeled by the Restricted Epidemic Type Aftershock Sequence (RETAS) model and after that the recognized best fit model is integrated into the strain release stochastic analysis. The applied stochastic model of Benioff strain release empowers a more detailed study by detecting possible deviations between observed data and model. Real values of the cumulative Benioff strain release surpass the expected modeled ones, indicating, that large aftershocks cluster at the beginning of the Loma Prieta sequence immediately after the occurrence of the main shock. Strain release spatial analysis reveals release patterns, which change during the aftershock sequence.

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Fault complexity associated with the 14 August 2003 Mw6.2 Lefkada, Greece, aftershock sequence

Karakostas V.G., Papadimitriou E.E.

Acta Geophysica

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2010

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Vol. 58, no. 5

838-854

EN

The Mw6.2 Lefkada earthquake occurred on 14 August 2003 beneath the western coastline of Lefkada Island. The main shock was followed by an intense aftershock activity, which formed a narrow band extending over the western coast of the Island and the submarine area between Lefkada and Kefalonia Islands, whereas additional off fault aftershocks formed spatial clusters on the central and northwestern part of the Island. The aftershock spatial distribution revealed the activation of along-strike adjacent fault segment as well as of secondary faults close to the main rupture. The properties of the activated segments were illuminated by the precisely located aftershocks, fault plane solutions determination and the cross sections performed parallel and normal to their strike. The aftershock focal mechanisms exhibited mainly strike slip faulting throughout the activated area, although deviation of the dominant stress pattern is also observed. The results help to emphasize the importance of the identification of activated nearby fault segments possibly triggered by the main rupture. Because such segments are capable to produce moderate events causing appreciable damage, they should be viewed with caution in seismic hazard assessment in addition to the major regional faults.