Wyniki wyszukiwania - BazTech

1

Seismotectonic implications of the 2020 Samos, Greece, Mw 7.0 mainshock based on high resolution aftershock relocation and source slip model

Karakostas V., Tan O., Kostoglou A., Papadimitriou E., Bonatis P.

Acta Geophysica

|

2021

|

Vol. 69, no. 3

979--996

EN

The 30 October 2020, Mw 7.0 Samos mainshock took place in the ofshore north of Samos Island in eastern Aegean area, previously struck in 1904 with a comparable magnitude earthquake ofshore the southern coastline. The investigation of the aftershock seismicity evolution and the properties of the activated fault network was accomplished with aftershock relocation performed with the double-diference and cross-correlation techniques. The highly accurate relocated seismicity illustrates a well-defned E–W activated structure located deeper than 5 km with an average depth of~12 km. Moment tensor solutions indicate mostly normal faulting with an average T-axis~ 185ο . Strong-motion waveform modeling revealed a N-dipping fault plane with a coseismic slip patch of 36 km × 22 km and a maximum slip equal to 1 m at 12 km depth. The slip is mainly concentrated in a single asperity implying a rupture mode of asperities breaking in isolated earthquakes rather than to cooperate to produce a larger rupture. Coulomb stress calculations unveil increased positive static stress changes values at the locations of the majority of the aftershocks and activation of minor fault segments by stress transfer.

2

Relocation of the 2018 Zakynthos, Greece, aftershock sequence: spatiotemporal analysis deciphering mechanism diversity and aftershock statistics

Karakostas V., Kostoglou A., Chorozoglou D., Papadimitriou E.

Acta Geophysica

|

2020

|

Vol. 68, no. 5

1263--1294

EN

An Mw 6.8 earthquake occurred on October 25, 2018, 35 km ofshore from the southwest coastlines of Zakynthos Island. The aftershock sequence appeared remarkably productive with six aftershocks of M≥5.0 in the frst month and tens of aftershocks with M≥4.0 during the study period. The GCMT solution for the main shock suggests a very low angle plane (dip=24°) for a dextral strike–slip faulting (rake=165°). A similar solution is suggested for the largest aftershock (Mw 5.9) that occurred 5 days afterward. The proximity of the main shock location with the dextral active boundary of Kefalonia Transform Fault Zone (KTFZ) along with the Hellenic Subduction front supports this oblique faulting. The aftershock activity is comprised mostly in depths 5–12 km and forms eight distinctive clusters that accommodate regional strain and evidence strain partitioning. The role of stress transfer and statistical analysis are combined for detailing the highly productive aftershock sequence. Earthquake networks analysis reveals their random structure soon after the main shock, which became small-world structure after the frst 200 days. Time series analysis constructed from the aftershock frequency and seismic moment release and manifested signifcant correlation among the eight seismicity clusters.

3

Analysis of microseismicity associated with the 2017 seismic swarm near the Aegean coast of NW Turkey

Mesimeri M., Kourouklas C., Papadimitriou E., Karakostas V., Kementzetzidou D.

Acta Geophysica

|

2018

|

Vol. 66, no. 4

479--495

EN

A thorough spatiotemporal analysis of the intense seismic activity that took place near the Aegean coast of NW Turkey during January–March 2017 was conducted, aiming to identify its causative relation to the regional seismotectonic properties. In this respect, absolute and relative locations are paired and a catalog consisting of 2485 events was compiled. Relative locations are determined with high accuracy using the double-difference technique and differential times both from phase pick data and from cross-correlation measurements. The spatial distribution of the relocated events revealed a south-dipping causative fault along with secondary and smaller antithetic segments. Spatially, the seismicity started at the westernmost part and migrated with time to the easternmost part of the activated area. Temporally, two distinctive periods are observed, namely an early period lasting 1 month and a second period which includes the largest events in the sequence. The investigation of the interevent time distribution revealed a triggering mechanism, whereas the ETAS parameters show a strong external force (l[1), which might be attributed to the existence of the Tuzla geothermal field.

4

The 2014 Kefalonia Doublet (Mw6.1 and Mw6.0), Central Ionian Islands, Greece: Seismotectonic Implications along the Kefalonia Transform Fault Zone

Karakostas V., Papadimitriou E., Mesimeri M., Paradisopoulou P., Gkarlaouni Ch.

Acta Geophysica

|

2015

|

Vol. 63, no. 1

1--16

EN

The 2014 Kefalonia earthquake sequence started on 26 January with the first main shock (Mw 6.1) and aftershock activity extending over 35 km, much longer than expected from the causative fault segment. The second main shock (Mw 6.0) occurred on 3 February on an adjacent fault segment, where the aftershock distribution was remarkably sparse, evidently encouraged by stress transfer of the first main shock. The aftershocks from the regional catalog were relocated using a 7-layer velocity model and station residuals, and their distribution evidenced two adjacent fault segments striking almost N-S and dipping to the east, in full agreement with the centroid moment tensor solutions, constituting segments of the Kefalonia Transform Fault (KTF). The KTF is bounded to the north by oblique parallel smaller fault segments, linking KTF with its northward continuation, the Lefkada Fault.

5

A detailed analysis of microseismicity in Samos and Kusadasi (Eastern Aegean Sea) areas

Tan O., Papadimitriou E. E., Pabuccu Z., Karakostas V., Yoruk A., Leptokaropoulos K.

Acta Geophysica

|

2014

|

Vol. 62, no. 6

1283--1309

EN

A detailed investigation of microseismicity and fault plane solutions are used to determine the current tectonic activity of the prominent zone of seismicity near Samos Island and Kusadasi Bay. The activation of fault populations in this complex strike-slip and normal faulting system was investigated by using several thousand accurate earthquake locations obtained by applying a double-difference location method and waveform cross-correlation, appropriate for areas with relatively small seismogenic structures. The fault plane solutions, determined by both moment tensor waveform inversions and P-wave first motion polarities, reveal a clear NS trending extension direction, for strike slip, oblique normal and normal faults. The geometry of each segment is quite simple and indicates planar dislocations gently dipping with an average dip of 40-45°, maintaining a constant dip through the entire seismogenic layer, down to 15 km depth.

6

Statistical properties of aftershock rate decay: Implications for the assessment of continuing activity

Adamaki A, Papadimitriou E.E., Tsaklidis G.M., Karakostas V.

Acta Geophysica

|

2011

|

Vol. 59, no. 4

748-769

EN

Aftershock rates seem to follow a power law decay, but the assessment of the aftershock frequency immediately after an earthquake, as well as during the evolution of a seismic excitation remains a demand for the imminent seismic hazard. The purpose of this work is to study the temporal distribution of triggered earthquakes in short time scales following a strong event, and thus a multiple seismic sequence was chosen for this purpose. Statistical models are applied to the 1981 Corinth Gulf sequence, comprising three strong (M = 6.7, M = 6.5, and M = 6.3) events between 24 February and 4 March. The non-homogeneous Poisson process outperforms the simple Poisson process in order to model the aftershock sequence, whereas the Weibull process is more appropriate to capture the features of the short-term behavior, but not the most proper for describing the seismicity in long term. The aftershock data defines a smooth curve of the declining rate and a long-tail theoretical model is more appropriate to fit the data than a rapidly declining exponential function, as supported by the quantitative results derived from the survival function. An autoregressive model is also applied to the seismic sequence, shedding more light on the stationarity of the time series.

7

Seismicity patterns before strong earthquakes in Greece

Karakostas V.

Acta Geophysica

|

2009

|

Vol. 57, no. 2

367-386

EN

The spatial distribution of the stress field around a fault, before its failure, depends on the focal mechanism of the ensuing rupture. In the preseismic stage, the main fault is locked and background seismicity is distributed in the surrounding area along small faults due to the raise of the stress level. This distribution is well correlated with changes of the stress field, when these are calculated considering the fault slipping on the opposite sense (back-slip model), before the incoming strong event. Studies of the seismicity around faults with known geometry, dimensions and slip properties could contribute in the seismic hazard assessment. In the present work, the spatio-temporal distribution of the smaller magnitude seismic activity before the occurrence of the four most recent (1995-2008) strong M ≥ 6.4) shallow earthquakes in Greece is compared with the stress pattern necessary for the generation of the strong earthquakes. Studying the annual rate of occurrence it is found that in areas of positive pre-stress changes a sharp increase of the number of earthquakes is observed several years before the occurrence of the main shock. This increase lasts for a few years and then for several years before the main shock the occurrence rate declines but usually remains higher than it was before the sharp increase. Seismicity distribution in the respective areas of negative pre-stress changes shows a rather stable occurrence rate.