Stress Inversion of Regional Seismicity in the Sea of Marmara Region, Turkey (original) (raw)
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Geophysical Journal International
Local rotations of the stress field may serve as an indicator to characterize the physical status of individual fault segments during the seismic cycle. In this study we focus on the pre-, aftershock-, and post-seismic phase of the 1999 Mw7.4 Izmit earthquake in northwestern Turkey. Using a compilation of focal mechanism data we applied a stress tensor inversion. By systematically considering error bounds of the resolved stress field orientation we find distinct temporal variations of the principal stress directions and of the relative stress magnitude along individual segments of the Izmit rupture that correlate with the local tectonic setting and along-strike variations of coseismic slip during the mainshock. The regional stress field prior to Izmit and following the three-month aftershock sequence reflects a well-resolved strike-slip regime with a ~N130E° trending subhorizontal maximum principal stress and is thus in accordance with the dextral EW-trending North Anatolian Fault Z...
Geophysical Journal International, 2014
We analysed locations and focal mechanisms of events with magnitude ≥3, which are recorded by 39 broad-band seismic stations deployed during the North Anatolian Passive Seismic Experiment (2005-2008) around central segment of the North Anatolian Fault (NAF). Using P-and S-arrival times, earthquakes are relocated and a new 1-D seismic velocity model of the region is derived. Relocated events in the area are mainly limited to a depth of 15 km and present seismicity in the southern block indicates widespread continental deformation. In the next step, focal mechanisms are derived from first motions (P, SH) and amplitude ratios (SH/P) using a grid-search algorithm in an iterative scheme. Analysis of our well-constrained focal mechanisms indicate mainly strike-slip motions apart from some normal and few thrust events that are related to complex local fault geometry. Calculated pressure/tension axes are mainly subhorizontal and maximum horizontal stress directions (SH max) are oriented predominantly in NW-SE direction which corresponds well with the slip character of NAF and its splays. In the east, E-W trending splays show right-lateral strike-slip mechanisms similar to the main strand whereas in the west, antithetic N-S trending faults show left lateral strike-slip motions. The seismic cluster that converged near Ç orum after relocation indicates a dominant right-lateral strike-slip mechanism along the E-W trending fault. These focal mechanisms are used to perform stress tensor inversion across the region to map out the stress field in detail. Overall, maximum (σ 1) and minimum (σ 3) principal stresses are found to be subhorizontal and the intermediate principle stress (σ 2) is vertically orientated, consistent with a dominant strike-slip regime. These directions point to the clockwise rotation of stress trajectories from N to S where NW-SE directed σ 1 in the north turns towards N-S in the south away from the NAF. Moreover, the 200-km-long Ezinepazar-Sungurlu Fault which is previously mapped as an active strike-slip fault is characterized by minor seismic activity and trends perpendicular to the computed maximum stress direction in the southwest away from the main strand of NAF suggesting that the Sungurlu segment is either compressional in nature or inactive.
We investigate spatiotemporal variations of the crustal stress field orientation along the rupture zones of the 1999 August Izmit M w 7.4 and November Düzce M w 7.1 earthquakes at the North Anatolian Fault zone (NAFZ) in NW Turkey. Our primary focus is to elaborate on the relation between the state of the crustal stress field and distinct seismotectonic features as well as variations of coseismic slip within the seismogenic layer of the crust. To achieve this, we compile an extensive data base of hypocentres and first-motion polarities including a newly derived local hypocentre catalogue extending from 2 yr prior (1997) to 2 yr after (2001) the Izmit and Düzce main shocks. This combined data set allows studying spatial and temporal variations of stress field orientation along distinct fault segments for the preand post-seimic phase of the two large earthquakes in detail. Furthermore, the occurrence of two M > 7 earthquakes in rapid succession gives the unique opportunity to analyse the 87-d-long 'inter-seismic phase' between them. We use the MOTSI (first MOTion polarity Stress Inversion) procedure directly inverting first-motion polarities to study the stress field evolution of nine distinct segments. In particular, this allows to determine the stress tensor also for the pre-and post-seismic phases when no stable single-event focal mechanisms can be determined. We observe significantly different stress field orientations along the combined 200-km-long rupture in accordance with lateral variations of coseismic slip and seismotectonic setting. Distinct vertical linear segments of the NAFZ show either pure-strike slip behaviour or transtensional and normal faulting if located near pull-apart basins. Pull-apart structures such as the Akyazi and Düzce basins show a predominant normal faulting behaviour along the NAFZ and reflect clearly different characteristic from neighbouring strike-slip segments. Substantial lateral stress field heterogeneity following the two main shocks is observed that declines with time towards the post-seismic period that rather reflects the regional right-lateral strike-slip stress field.
The Armutlu peninsula, located in the easternMarmara Sea, coincideswith the western end of the rupture of the 17 August 1999, İzmitMW 7.6 earthquake which is the penultimate event of an apparently westward migrating series of strong and disastrous earthquakes along the NAFZ during the past century. We present new seismotectonic data of this key region in order to evaluate previous seismotectonic models and their implications for seismic hazard assessment in the eastern Marmara Sea. Long term kinematics were investigated by performing paleo strain reconstruction from geological field investigations by morphotectonic and kinematic analysis of exposed brittle faults. Short term kinematics were investigated by inverting for the moment tensor of 13 small tomoderate recent earthquakes using surface wave amplitude spectra. Our results confirm previous models interpreting the eastern Marmara Sea Region as an active transtensional pull-apart environment associatedwith significant NNE–SSWextension and vertical displacement. At the northern peninsula, long termdeformation pattern did not change significantly since Pliocene times contradicting regional tectonic models which postulate a newly formed single dextral strike slip fault in the Marmara Sea Region. This area is interpreted as a horsetail splay fault structure associated with a major normal fault segment that we call the Waterfall Fault. Apart from the Waterfall Fault, the stress strain relation appears complex associated with a complicated internal fault geometry, strain partitioning, and reactivation of pre-existing plane structures. At the southern peninsula, recent deformation indicates active pull-apart tectonics constituted by NE–SW trending dextral strike slip faults. Earthquakes generated by stress release along large rupture zones seem to be less probable at the northern, but more probable at the southern peninsula. Additionally, regional seismicity appears predominantly driven by plate boundary stresses as transtensional faulting is consistentwith the southwest directed far field deformation of the Anatolian plate.
Seismotectonics of the Southern Marmara Region, NW Turkey
Bulletin of the Geological Society of Greece
The Southern Marmara Region is an active deformation area, which is a transition zone between the strike-slip tectonics manifested by the North Anatolian Fault System and the N-S extensional regime of the Aegean Region. We have reviewed tectonic and geological structure of the region, based onseismological studies. We have obtained a total of 37 earthquake moment tensor solutions between 1953 and 2015. In addition, stress tensor analysis has been carried out using 37 earthquake moment tensor solutions. Also long term seismicity were investigated and a,b, Mc values were calculated and mapped. Moment tensor solutions indicate that the source of these earthquakes are mostly NE-trending dextral strike-slip faults and some of them are E-W trending dip-slip normal faults. The stress tensor analysis shows that the direction of the regional compressive stress is NW-SE. The temporal and spatial distrubution of the large earthquakes (1944, 1953, 1964) indicate that the ruptures unilaterally p...
Geophysical Journal International, 2010
Major discrepancy between present-day fault models in the Marmara Sea is closely related to structural development of depression areas beneath the sea. Therefore, in this study, the Marmara Sea and especially its major basins are investigated by means of recent seismological observations consisting of microseismicity (1.2 ≤ M d ≤ 3.6; 2002 May-2006 May), focal mechanisms (2.4 ≤ M d ≤ 5.0; 1999-2006) and stress analysis based on the focal mechanisms. In the Ç ınarcık Basin where either pull-apart model or negative flower structure is proposed, northeastern margin is represented by strike-slip sources having a NW alignment, probably corresponding to a cross-basin strike-slip fault developing in a matured pull-apart basin. This suggestion is directly relevant to experiments of sand box analogue models. These experiments indicate that such a cross-basin strike-slip fault is produced after the pull-apart basins attain their mature stage, eventually resulting in less vertical displacements attributed to normal faults bounding the basin. This experimental decline may be corroborated by the paucity of normal mechanisms in the basin. Extensional motion in this region is more influential outside of the basin, especially to the area where an onshore seismic activity is monitored on the land. On the other hand, another strike-slip activity in the Ç ınarcık Basin is distributed on a possible strand, parallel to the northern coastline of the Marmara Sea in the Armutlu Peninsula. This may indicate that one or more short strands of the northern splay extend to the west by bounding the basin from the south. In the Central Basin, the existence of a right-lateral strikeslip faulting on an east-west trending fault is supported by similar strike-slip focal mechanisms located at both extremities of the basin. To the west, dominant style of deformation gradually shifts from strike-slip faulting to transpression along the western coastline of the Marmara Sea where the northern splay joins with the offshore continuation of the Ganos Fault. Finally, focal mechanisms in the Marmara Sea are inverted for the best fitting stress tensor, characterized by a subvertical σ 2-axis and two subhorizontal stress axes of σ 1 and σ 3 , oriented NW-SE and NE-SW, respectively. Conclusively, this study infers that the current tectonics of the Marmara Sea is mainly governed by a strike-slip regime, ascribed to both submarine splays of the NAF, and partly dominated by extensional and compressional features indicating shallow processes at local scales.
Active tectonic stress field analysis in NW Iran-SE Turkey using earthquake focal mechanism data
Active tectonic stress field analysis in NW Iran-SE Turkey using earthquake focal mechanism data., 2021
NW Iran-SE Turkey is a tectonically active zone related to the Arabia-Eurasia convergence, but the active stress state in this zone has not yet been clearly studied. To improve the knowledge of present-day stress state in this region, optimum reduced stress tensor was analysed. For this, a large number of earthquake focal mechanisms (277) were collected. The analyses show most mechanisms exhibit strike-slip to thrust faulting. These data indicate that this region is dominated by an N158° maximum horizontal compressive stress (S Hmax) belonging to a transpressional tectonic regime. In the scale of the study area, the relative magnitude of the intermediate and minimum principal stress axes do not differ much (ϕ = 0.09). Brittle deformation in this area is dominantly accommodated by a combination of strike-slip and thrust faulting (Aϕ = 1.82 to 2.30). The analyses reveal that two sets of faults show a high tendency to slip and reactivate. These sets contain NW-SE-striking right-lateral and NNE-SSW-striking left-lateral faults. The results of this study may help to study the active seismicity, tectonic activity, and seismic risk in this region.
Stress coupling between earthquakes in northwest Turkey and the north Aegean Sea
Journal of Geophysical Research, 1998
We have investigated the Coulomb stress interactions of 29 earthquakes (M s ≥ 6.0) that have occurred in the region of northwest Turkey and north Aegean Sea since 1912. Of these events, 23 may be related to earlier events, and 16 are clearly related to earlier events. All events after 1967 are related to previous events. Events in the early part of our time interval that show no correlation could be related to historical events as yet unidentified. In some cases, faults that have received a stress reduction from earlier events are prepared for an event by an earthquake occurring a few years before that creates a local Coulomb stress rise. Thus regions of Coulomb stress shadow can become regions where a damaging earthquake may occur. The relation between smaller events and the Coulomb stress distribution is less clear, but may be related to poor data quality and practical limitations of our modeling technique. Nonetheless, there are 4 times as many events per unit area in regions of enhanced stress than where stress is reduced. We discuss the contemporary distribution of Coulomb stress and argue that it is possible to identify the likely locations of future damaging earthquakes including identifying the most likely candidate faults.
Stress accumulation and increased seismic risk in eastern Turkey
Earth and Planetary Science Letters, 2002
Unlike the North Anatolian fault zone, which has produced 11 large earthquakes since 1939, the East Anatolian fault zone (EAFZ) has been relatively quiescent in the last century when compared to historical records and has therefore accumulated significant stresses along its length. Determination of the location and likely magnitude of a future probable earthquake along the EAFZ is of interest both because of this history of large earthquakes, (MW8), and the density of population in the area. Here we calculate stress evolution along the fault zone due to both seismic and tectonic loading since 1822. A sequence of 10 well constrained historical earthquakes is selected and the resulting stresses are calculated, summed with tectonic loading stresses and resolved onto the mapped active faults. We identify two areas of particular seismic risk, one of which might be expected to yield a large event. Our results are sensitive to the previous history of large earthquakes in the region and indicate a need for detailed investigations to constrain the exact rupture geometries of previous earthquakes on these segments.