scholarly journals 20 July 2017 Bodrum-Kos Earthquake (Mw:6.6) in southwestern Anatolia, Turkey

2021 ◽  
Vol 25 (3) ◽  
pp. 309-321
Author(s):  
Semir Över ◽  
Süha Özden ◽  
Esra Kalkan Ertan ◽  
Fatih Turhan ◽  
Zeynep Coşkun ◽  
...  
Keyword(s):  
Fault Plane ◽  
Regional Scale ◽  
Aegean Sea ◽  
Horizontal Stress ◽  
Focal Mechanisms ◽  
Stress Axis ◽  
Small Scale ◽  
Normal Type ◽  
Fault Plane Solutions ◽  
Stress Regime

In the Aegean Sea, the western part of Gökova Gulf, Kos and Bodrum were struck by a 6.6 (Mw) earthquake on July 20, 2017. The fault plane solution for the main shock shows an E-W striking normal type fault with approximately N-S (N4°E) tensional axis (T-axis). Fault plane solutions of 33 aftershocks show two groups of normal type fault with E-W and NE-SW to ENE-WSW orientations. The inversion of the focal mechanisms of the aftershocks yields two different normal faulting stress regimes: one is characterized by an approximately N-S (N5°E) σ3 axis (minimum horizontal stress axis). This extension is obtained from 13 focal mechanisms of aftershocks with approximately E-W direction. The other is characterized by approximately NW-SE (N330°E) σ3 axis. The latter is calculated from 21 seismic faults of aftershocks with approximately NE-SW direction. These aftershocks occurred on relatively small-scale faults that were directed from NE-SW to ENE-WSW, and possibly contributed to expansion of the basin in the west. The 24 focal mechanisms of earthquakes which occurred since 1933 in and around Gökova Basin are introduced into the inversion analysis to obtain the stress state effective in a wider region. The inversion yields an extensional stress regime characterized by an approximately N-S (N355°E) σ3 axis. The E-W directional metric faults, measured in the central part of Gökova Fault Zone bordering the Gökova Gulf in the north, also indicate N-S extension. The NE-SW extension obtained from NE-SW aftershocks appears to be more local and is responsible for the expansion of the western part of the asymmetric Gökova Basin. This N-S extension which appears to act on a regional-scale may be attributed to the geodynamic effects related to the combined forces of the southwestward extrusion of Anatolia and the roll-back process of African subduction beneath Anatolia.

scholarly journals A functional tool to explore the reliability of micro-earthquake focal mechanism solution for seismotectonic purposes

2021 ◽  
Author(s):  
Guido Maria Adinolfi ◽  
Raffaella De Matteis ◽  
Rita De Nardis ◽  
Aldo Zollo
Keyword(s):  
Focal Mechanism ◽  
Fault Plane ◽  
Focal Mechanisms ◽  
Seismic Network ◽  
Fault System ◽  
Focal Mechanism Solution ◽  
Fault Plane Solutions ◽  
Stress Regime ◽  
Focal Mechanism Solutions ◽  
Earthquake Focal Mechanism

Abstract. Improving the knowledge of seismogenic faults requires the integration of geological, seismological, and geophysical information. Among several analyses, the definition of earthquake focal mechanisms plays an essential role in providing information about the geometry of individual faults and the stress regime acting in a region. Fault plane solutions can be retrieved by several techniques operating in specific magnitude ranges, both in the time and frequency domain and using different data. For earthquakes of low magnitude, the limited number of available data and their uncertainties can compromise the stability of fault plane solutions. In this work, we propose a useful methodology to evaluate how well a seismic network used to monitor natural and/or induced micro-seismicity estimates focal mechanisms as function of magnitude, location, and kinematics of seismic source and consequently their reliability in defining seismotectonic models. To study the consistency of focal mechanism solutions, we use a Bayesian approach that jointly inverts the P/S long-period spectral-level ratios and the P polarities to infer the fault-plane solutions. We applied this methodology, by computing synthetic data, to the local seismic network operated in the Campania-Lucania Apennines (Southern Italy) to monitor the complex normal fault system activated during the Ms 6.9, 1980 earthquake. We demonstrate that the method we propose can have a double purpose. It can be a valid tool to design or to test the performance of local seismic networks and more generally it can be used to assign an absolute uncertainty to focal mechanism solutions fundamental for seismotectonic studies.

scholarly journals A functional tool to explore the reliability of micro-earthquake focal mechanism solutions for seismotectonic purposes

Solid Earth ◽  
2022 ◽  
Vol 13 (1) ◽  
pp. 65-83
Author(s):  
Guido Maria Adinolfi ◽  
Raffaella De Matteis ◽  
Rita de Nardis ◽  
Aldo Zollo
Keyword(s):  
Focal Mechanism ◽  
Fault Plane ◽  
Normal Fault ◽  
Focal Mechanisms ◽  
Seismic Network ◽  
Fault System ◽  
Fault Plane Solutions ◽  
Stress Regime ◽  
Focal Mechanism Solutions ◽  
Earthquake Focal Mechanism

Abstract. Improving the knowledge of seismogenic faults requires the integration of geological, seismological, and geophysical information. Among several analyses, the definition of earthquake focal mechanisms plays an essential role in providing information about the geometry of individual faults and the stress regime acting in a region. Fault plane solutions can be retrieved by several techniques operating in specific magnitude ranges, both in the time and frequency domain and using different data. For earthquakes of low magnitude, the limited number of available data and their uncertainties can compromise the stability of fault plane solutions. In this work, we propose a useful methodology to evaluate how well a seismic network, used to monitor natural and/or induced micro-seismicity, estimates focal mechanisms as a function of magnitude, location, and kinematics of seismic source and consequently their reliability in defining seismotectonic models. To study the consistency of focal mechanism solutions, we use a Bayesian approach that jointly inverts the P/S long-period spectral-level ratios and the P polarities to infer the fault plane solutions. We applied this methodology, by computing synthetic data, to the local seismic network operating in the Campania–Lucania Apennines (southern Italy) aimed to monitor the complex normal fault system activated during the Ms 6.9, 1980 earthquake. We demonstrate that the method we propose is effective and can be adapted for other case studies with a double purpose. It can be a valid tool to design or to test the performance of local seismic networks, and more generally it can be used to assign an absolute uncertainty to focal mechanism solutions fundamental for seismotectonic studies.

scholarly journals The 1991 Sierra Madre earthquake sequence in southern California: Seismological and tectonic analysis

1994 ◽  
Vol 84 (4) ◽  
pp. 1058-1074 ◽  
Author(s):  
Egill Hauksson
Keyword(s):  
Los Angeles ◽  
Fault Plane ◽  
B Value ◽  
Tectonic Stress ◽  
Aftershock Sequence ◽  
Strike Slip ◽  
Focal Mechanisms ◽  
Stress Axis ◽  
The North ◽  
Sierra Madre

Abstract The (ML 5.8) Sierra Madre earthquake of 28 June 1991 occurred at a depth of 12 km under the San Gabriel Mountains of the central Transverse Ranges. Since at least 1932 this region had been quiescent for M ≧ 3. The mainshock focal mechanism derived from first-motion polarities exhibited almost pure thrust faulting, with a rake of 82° on a plane striking N62°E and dipping 50° to the north. The event appears to have occurred on the Clamshell-Sawpit fault, a splay of the Sierra Madre fault zone. The aftershock sequence following the mainshock occurred at a depth of 9 to 14 km and was deficient in small earthquakes, having a b value of 0.6. Twenty nine single-event focal mechanisms were determined for aftershocks of M > 1.5. The 4-km-long segment of the Clamshell-Sawpit fault that may have ruptured in the mainshock is outlined by several thrust focal mechanisms with an east-northeast-striking fault plane dipping to the north. To the west, several thrust aftershocks with east-striking nodal planes suggest some complexity in the aftershock faulting, such as a curved rupture surface. In addition, several strike-slip and normal faulting events occurred along the edges of the mainshock fault plane, indicating secondary tear faulting. The tectonic stress field driving the coexisting left-lateral strike-slip and thrust faults in the northern Los Angeles basin is north-south horizontal compression with vertical intermediate or minimum principal stress axis.

State of stress in the northern Tabas block, east-central Iran, as inferred from focal mechanisms of the 1978 Tabas earthquake sequence

2011 ◽  
Vol 3 (1) ◽  
Author(s):  
G. Behzad Zamani
Keyword(s):  
Sedimentary Cover ◽  
Focal Mechanisms ◽  
Central Iran ◽  
Stress Axis ◽  
Low Grade ◽  
Thrust Faults ◽  
Stress Regime ◽  
East Central ◽  
State Of Stress ◽  
Tabas Earthquake

AbstractIn this paper, the state of stress in the northern Tabas block in east-central Iran is analyzed based on the systematic inversion of aftershock focal mechanisms from the 1978.09.16 Tabas earthquake, to characterise the stress regime that controls most earthquakes in this area. Here, stress inversions of double-couple focal mechanisms of earthquakes recorded during the 30 days following the main shock have been carried out. The calculated average stress regime indicates dominant major 226° to 237° trending compression for the Tabas region. The dominating regime in east-central Iran is thrusting with a minimum stress axis, σ 3, close to vertical. The reconstruction of the main seismotectonic stress in east-central Iran with a NE-SW compression is consistent with independent information of the active plate convergence related to Arabia-Eurasia convergence. Most earthquakes in the mentioned area occur near or around concealed Quaternary thrust faults with their activity being controlled by the NE-SW compression. Where ϕ, the ratio of principal stress differences, is 0.5, a small difference between σ 2; σ 3 and σ 1 and small amounts of deviatoric stress is indicated. Therefore, for small deviatoric horizontal σ 1 it is not possible to increase and reactivate small sections of basement thrust faults and create secondary basement aftershocks. Reconstructed stress regimes in this study for sedimentary cover (237) and basement (226) of Tabas are similar. Therefore, it seems that the basement and cover were coupled together, possibly along the 2–4 km of upper Precambrian low-grade metamorphic rocks. Then these segments of the fold-and-thrust belt were involved in similar seismic activity under a similar stress regime.

scholarly journals RECENT SEISMIC ACTIVITY IN CENTRAL GREECE REVEALING LOCAL SEISMOTECTONIC PROPERTIES

2017 ◽  
Vol 43 (4) ◽  
pp. 2075
Author(s):  
Ch. K. Karamanos ◽  
V. G. Karakostas ◽  
L. Seeber ◽  
E. E. Papadimitriou ◽  
A.A. Kilias
Keyword(s):  
Fault Zone ◽  
Fault Plane ◽  
Focal Mechanisms ◽  
Normal Faulting ◽  
Seismic Excitations ◽  
Lateral Component ◽  
Fault Plane Solutions ◽  
Central Greece ◽  
Source Characteristics ◽  
The Town

The December 2008, M=5.2 earthquake occurred in the Voiotikos–Kifissos basin near the town of Amfikleia in Central Greece and was followed by an intense sequence with hundreds of earthquakes. Mainshock source characteristics derived from the recordings of the Greek National Seismological Network are consistent with previous known earthquakes as well as with the current nearly N–S extensional regime. The adequate azimuthal coverage and the calculated time residuals at each seismological station ensure high location accuracy, whereas the stations operated close to the seismic excitations constrained 80% of the focal depths between 8 and 12km. Distances from the mainshock epicenter to the 10 closest seismological stations vary from 15 to 75 km. Hypoinverse and HypoDD were used for locations, and FPFIT was used for fault plane solutions of events with an adequate number of clear first arrivals. The hypocenters and focal mechanisms illuminate a ≈10km–long fault zone striking nearly E–W with oblique normal faulting and a small left lateral component. The Voiotikos–Kifissos basin is bordered in the south by two left–stepping en echelon segments known as the Pavliani fault zone and the Parnassos detachment, which strike NW and dip NE. In our preferred interpretation, the Amfikleia mainshock ruptured a previously recognized south–dipping fault antithetic to the basin border faults. This fault may be associated with the left step on the border fault, which would be releasing if that fault had a sinistral component.

scholarly journals Fault-plane solutions from moment-tensor inversion and preliminary Coulomb stress analysis for the Emilia Plain

2012 ◽  
Vol 55 (4) ◽  
Author(s):  
Angela Saraò ◽  
Laura Peruzza
Keyword(s):  
Elastic Stress ◽  
Fault Plane ◽  
Moment Tensor ◽  
Focal Mechanisms ◽  
Structural System ◽  
Fault Plane Solutions ◽  
Stress Changes ◽  
Intermediate Distance ◽  
Complex Structural ◽  
Coulomb Stress Analysis

<p>We investigate the seismicity occurred in the Po area, in the period July 2011-June 1012, by means of moment tensor and we use our set of revised focal mechanisms - computed for M&gt; 3.7 earthquakes - to evaluate Coulomb elastic stress changes in order to detect potential intermediate-distance faults interaction, and the main features of this complex structural system.</p>

scholarly journals Present-day stress state in northwestern Syria

2020 ◽  
Vol 59 (4) ◽  
pp. 299-316
Author(s):  
Mohamad Khir Abdul-Wahed ◽  
Mohammed ALISSA
Keyword(s):  
Fault Plane ◽  
Eastern Mediterranean ◽  
Active Faults ◽  
Plate Boundary ◽  
Shear Zones ◽  
Stress Pattern ◽  
P Wave ◽  
Fault System ◽  
Fault Plane Solutions ◽  
Stress Regime

Northwestern Syria is a key area in the eastern Mediterranean to study the active tectonics and stress pattern across the Arabia-Eurasia convergent plate boundary. This study aims to outline the present-day stress regime in this region of Syria using the fault plane solutions of the largest events recorded by the Syrian National Seismological Network from 1995 to 2011. A dataset of fault-plane solutions was obtained for 48 events having at least 5 P-wave polarities. The tectonic regime for most of these events is extensional and produces normal mechanisms in agreement with the local configurations of the seismogenic faults in the region. Strike-slip mechanisms are more scarce and restricted to certain areas, such as the northern extension of the Dead Sea fault system. The results of the current study reveal the spatial variations of SHmax orientation across the northwestern Syria region. This spatial variation of the present-day stress field highlights the role of main geometrically complex shear zones in the present-day stress pattern of northwestern Syria. However, these results show, regardless of the relatively small magnitudes of the studied events, they provide a picture of the local stress deviations that have currently been taking place along the local active faults.

The tectonic stress and tectonic motion direction in the Pacific and Adjacent areas as calculated from earthquake fault plane solutions

1965 ◽  
Vol 55 (1) ◽  
pp. 147-152 ◽  
Author(s):  
A. E. Scheidegger
Keyword(s):  
Fault Plane ◽  
Horizontal Stress ◽  
Tectonic Stress ◽  
Pacific Basin ◽  
Motion Direction ◽  
Fault Plane Solutions ◽  
Earthquake Fault ◽  
The Pacific ◽  
Tectonic Motion ◽  
Consistent Picture

Abstract The best P and T axes as well as the best normals to the null directions were calculated for groups of earthquake fault plane solutions belonging to 29 areas of the Pacific Basin and vicinity. The method employed was one developed in an earlier paper of the writer; it is based on a calculation of the eigenvectors of a quadratic form. It is shown that the principal horizontal stress (PHS) directions obtained in this fashion are in excellent agreement with those obtained from other evidence. In the Western Pacific Basin and vicinity the calculations were sufficiently dense to determine PHS trajectories; the latter are shown and yield a consistent picture of the area in question.

Fault kinematic investigations along the Panchkula-Morni region, NW Himalaya, India

2020 ◽  
Author(s):  
Ajay Kumar ◽  
Soumyajit Mukherjee ◽  
Mohamedharoon A. Shaikh ◽  
Seema Singh
Keyword(s):  
Fault Plane ◽  
Regional Scale ◽  
Fault Slip ◽  
Fault System ◽  
Inversion Algorithm ◽  
Stress Regime ◽  
Kinematic Indicators ◽  
Kinematic Evolution ◽  
Slip Planes ◽  
Paleostress Inversion

&lt;p&gt;The Morni hills located in the north-western Himalaya in Panchkula district, Haryana has undergone poly-phase deformation owing to its complex tectonic history. In order to better understand the kinematic evolution of study area, detailed structural analyses of the fault system at regional-scale is carried out. We perform paleostress analyses on the collected fault-slip data to derive the paleostress tensors. The fault-slip data includes attitudes of fault planes and slickenside lineations, and the sense of slip along the fault plane determined by observing various kinematic indicators. The study area mainly exposes compacted, fine- to medium-grained calcareous sandstones belonging to the lower Siwalik formation in the Himalayan foreland basin. The exposed sandstones contain numerous striated slip planes of varying slip-sense. As the fault planes are intra-formational and exposed in uniform lithology, sense of slip cannot be determined through offset markers. In such cases, the sense of slip of the fault plane is determined solely by observing various slickenside kinematic indicators and fracture types developed on the faulted surface. The slickenside kinematic indicators e.g., calcite mineral steps were found useful in deciphering the sense of movement of each of the slip plane. The paleostress inversion of fault-slip data was carried out by applying the open source software T-Tecto studio X5 to obtain the reduced stress tensor. The Paleostress inversion algorithm called the Right Dihedral Method (RDM) is executed to estimate the principal stress axes orientations. Temporally, the slip planes may have reactivated multiple times preserving multiple slickenside orientations superimposing one another. Such fault-slip data are called heterogeneous and therefore, multiple stress states are deduced to explain the heterogeneous fault-slip data. The paleostress analysis results indicate stress regime index (R&amp;#8217;) range 1.25&amp;#8211;2.25 and 0.20&amp;#8211;1.00 suggesting pure strike-slip to transpressive and pure extensive to transtensive stress regime respectively prevailing in the study area.&lt;/p&gt;

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