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Tuzla earthquake swarm in Turkey

Komut Tolga, Baysal Rıza

Acta Geophysica

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2022

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Vol. 70, no 3

1037--1045

EN

Long-lasting earthquake clusters are common in western Anatolia. One of them has been active in the southwestern part of the Biga peninsula. We had studied this cluster in terms of strain energy produced over time and revealed its interesting characteristics. The seismicity in this clustering region is not normal in terms of both the number of earthquakes, duration of the activity, magnitude/frequency and mainshock/aftershock relations, and the strain energy produced. An abnormal seismic activity with a set of medium-sized earthquakes without a mainshock, which are indistinguishable in size occurred in this clustering region in 2017. Interesting features that are characteristic of earthquake swarms are probably related to the abnormal physical properties of the crust. The 2017 Tuzla activity, where there is also no notable aftershock activity, could be associated with a phase of the swarming phenomenon itself. For this, we approached the 2017 activity in terms of the fact that it may not be part of the active tectonic system. Apart from the presence of high geotherm and hot springs in the region, the crust has been weakened by pre-existing intense fault zones that have developed in previous deformation regimes since the paleo-tectonic periods. Historical and instrumental period large earthquakes have caused loss of life and property in Biga peninsula due to an existing active fault zone. The Tuzla region probably is in this zone extending NE-SW in the peninsula. However, it is very difficult or impossible to distinguish possible foreshocks or precursory phenomenon of a future large earthquake from the background activity of the earthquake swarms.

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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.

3

Microearthquake distribution in the Lake Aswan area, Egypt

Hassoup A.

Acta Geophysica Polonica

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2001

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Vol. 49, nr 3

317-330

EN

The temporal evolution of the Lake Aswan seismicity in Egypt over a 13-year period, from 1982 to 1995, is characterized by two separate sequences which occurred in August 1982 and June 1987 and a nearly continuous low-level activity over the remainder of the period. This study presents details of the space-time analyses of the two sequences. A nonlinear inversion method was used to relocate 200 events. The distribution of August 1982 foci exhibits asperity deformation; its center is located at 23.58°N, 32.57°E and 17 km depth. The foci distribution forms a zone dipping generally to the southwest. The June 1987 foci are distributed in a nearly vertical zone between 5 and 11 km depth, with epicentral coordinate centered at 23.57°N and 32.68°E. The August 1982 sequence is of foreshock-mainshock-aftershock type with a b-value of 0.8 and an aftershock decay rate of 0.78. The June 1987 activity is characterized by a b-value of 0.6 and is made up of two successive swarm bursts, which took place on June 17 and 19. The aftershock decay rates of these bursts are calculated as 1 and 0.99, respectively. The August 1982 cluster is of small amplitude and associated with a high background level. The June 1987 anomaly is of larger amplitude originating in a nearly zero background level. Our results suggest the Lake Aswan seismicity consists of the seismic zones with different characteristics.