Extended Nucleation of the 1999 Mw 7.6 Izmit Earthquake

International audienceLaboratory and theoretical studies suggest that earthquakes are preceded by a phase of developing slip instability in which the fault slips slowly before accelerating to dynamic rupture. We report here that one of the best-recorded large earthquakes to date, the 1999 moment magnitude (Mw) 7.6 Izmit (Turkey) earthquake, was preceded by a seismic signal of long duration that originated from the hypocenter. The signal consisted of a succession of repetitive seismic bursts, accelerating with time, and increased low-frequency seismic noise. These observations show that the earthquake was preceded for 44 minutes by a phase of slow slip occurring at the base of the brittle crust. This slip accelerated slowly initially, and then rapidly accelerated in the 2 minutes preceding the earthquake. [ABSTRACT FROM AUTHOR

Shear-wave velocity structure in the northern Basin and Range province from the combined analysis of receiver functions and surface waves

A new method based on the joint inversion of receiver functions and surface-wave phase velocities results in well-determined shear-velocity structures that are consistent with the compressional-wave structure, gravity, heat flow, and elevation data in the northern Basin and Range. This new inversion method takes advantage of average-velocity information present in the surface-wave method and differential velocity information contained in the receiver function method, thus minimizing the nonuniqueness problem that results from the velocity-depth trade-off. An unusually thick (38 km) and relatively faster crust and upper mantle are found in central and eastern Nevada compared to the thin (28 to 34 km) and slower crust and upper mantle of the western Basin and Range. We interpret the regions of thicker and faster crust and upper mantle as zones that have undergone less Cenozoic extension relative to the surrounding regions to the west and north. The thick crust and consequently greater depth to the dense mantle material is consistent with the gravity low pattern in central and eastern Nevada. Simple gravity modeling shows both local and regional isostatic compensation occur within 40 km of the surface, indicating a near-classical Airy type of compensation in the province. We analyze in detail the shear-wave (S-wave) velocity model derived from the receiver functions at station BMN and compressional-wave (P-wave) velocity models derived from the 1986 PASSCAL experiment in northwestern Nevada. The most interesting feature of these models is the presence of negative-velocity gradients in the S-wave model with no corresponding velocity decrease in the P-wave models between depths of 10 and 24 km. This combined velocity model may be explained by high pore fluid pressures at these depths. This model favors a layered fluid porosity model proposed in the literature to explain extensive middle- to lower-crust continental seismic reflections and high electrical conductivity. An upper-mantle, gradational low-velocity zone is present between 32 and 38 km in the S-wave model. This upper-mantle, shear-wave, low-velocity zone is consistent with partial melt, which may be the source material for magmatic underplating in this region

The nucleation of the Izmit and Düzce earthquakes: some mechanical logic on where and how ruptures began

International audienceIn spite of growing evidence that many earthquakes are preceded by increased seismic activity, the nature of this activity is still poorly understood. Is it the result of a mostly random process related to the natural tendency of seismic events to cluster in time and space, in which case there is little hope to ever predict earthquakes? Or is it the sign that a physical process that will lead to the impending rupture has begun, in which case we should attempt to identify this process. With this aim we take a further look at the nucleation of two of the best recorded and documented strike-slip earthquakes to date, the 1999 Izmit and Düzce earthquakes which ruptured the North Anatolian Fault over ∼200 km. We show the existence of a remarkable mechanical logic linking together nucleation characteristics, stress loading, fault geometry and rupture speed. In both earthquakes the observations point to slow aseismic slip occurring near the ductile-to-brittle transition zone as the motor of their nucleation

Moho, crustal architecture and deep deformation under the North Marmara Trough, from the SEISMARMARA Leg 1 offshore–onshore reflection–refraction survey

Understanding further the nature and evolution at lithospheric scale of the Sea of Marmara on the North Anatolian Fault needs constraints on the deep crustal and Moho spatial variation. This has been probed here with offshore–onshore and OBS, Ocean Bottom Seismometer refraction seismics, in addition to coincident MCS, marine multichannel reflection seismic profiles over the whole North Marmara Trough. The diverse strikes of MCS profiles in a dense grid allow to avoid misinterpretation of late echoes in the deep basin as Moho reflections and attribute them to sidesweeps. Moho is instead positively identified from reversed observations of first-arrival head and refracted waves at the top of the mantle obtained at large offset by land stations. A significant and sharp reduction in its depth, on the order of 5 km occurs beneath both the eastern and western rims of the North Marmara Trough, with a more progressive crustal thinning from the south. The wide-angle reflections on OBS and land stations document in addition to Moho the top of a lower crustal reflective layer, which is also sampled by MCS profiles, and appears to follow Moho topography. The dense grid of MCS profiles along the southwestern margin of the North Marmara Trough reveals a dipping reflector through the upper crust with tilted basement blocks on top. This low-angle fault is suggested as a normal sense detachment extending in depth towards the reflective lower crust. The upwarp of the Moho and lower crustal layer towards the North Marmara Trough suggests that crustal thinning occurs mostly in the upper crustal part, with lateral transport of the material towards WSW in the footwall of the detachment, and possibly other features to the south, in the motion of Anatolia with respect to stable Eurasia oblique to the North Marmara Trough. Thinning can be accommodated in an asymmetric partitioning of the displacement on several branching faults at lithospheric scale.The SEISMARMARA seismic experiment was operated as a joint integrated project between Turkish and French scientists, research institutions and universities, technical facilities, and funding agencies, coordinated by TUBITAK in Turkey (Naci Görür) and INSU-CNRS in France (Rolando Armijo). N/O Nadir and the seismic source and streamer operated by IFREMER/GENAVIR were allocated in the frame of a special extension of deadline for the regular call for proposals for use of these French national facilities. The OBS provided and operated by ISV Hokkaido, Japan, as well as the land refraction stations of the INSU pool were funded by ACI CATNAT of the French Ministry of Education and Research, with additional support and personnel of these institutions and of Turkish ones coordinated by TUBITAK. R/V Sismik 1 of MTA was operated for TUBITAK, who also funded Turkish participation to all other aspects, in the frame of the cooperative Turkish–French program following the destructive earthquakes, coordinated by Naci Görür and Rolando Armijo. MCS Processing has been done with the Geovecteur software of CGG at the CNRS-IPGParis, PROMAX at the University of Pau. We thank the masters and crews of N/O Nadir, R/V Sismik, the scientific and technical teams of the MCS, OBS and land stations deployments, as well as data recovery and processing. The Shipboard Party comprised in addition S. Bazin, S. Cetin, S. Singh, A. Vigner, and R. Saatcilar. We acknowledge the critical comments of Pr. E. Flueh and anonymous journal reviewers IPGP contribution 2462.Peer reviewe

Submarine fault scarps in the Sea of Marmara pull-apart (North Anatolian Fault): Implications for seismic hazard in Istanbul

International audienceEarthquake scarps associated with recent historical events have been found on the floor of the Sea of Marmara, along the North Anatolian Fault (NAF). The MARMARASCARPS cruise using an unmanned submersible (ROV) provides direct observations to study the fine-scale morphology and geology of those scarps, their distribution, and geometry. The observations are consistent with the diversity of fault mechanisms and the fault segmentation within the north Marmara extensional step-over, between the strike-slip Ganos and Izmit faults. Smaller strike-slip segments and pull-apart basins alternate within the main step-over, commonly combining strike-slip and extension. Rapid sedimentation rates of 1?3 mm/yr appear to compete with normal faulting components of up to 6 mm/yr at the pull-apart margins. In spite of the fast sedimentation rates the submarine scarps are preserved and accumulate relief. Sets of youthful earthquake scarps extend offshore from the Ganos and Izmit faults on land into the Sea of Marmara. Our observations suggest that they correspond to the submarine ruptures of the 1999 Izmit (Mw 7.4) and the 1912 Ganos (Ms 7.4) earthquakes. While the 1999 rupture ends at the immediate eastern entrance of the extensional Cinarcik Basin, the 1912 rupture appears to have crossed the Ganos restraining bend into the Sea of Marmara floor for 60 km with a right-lateral slip of 5 m, ending in the Central Basin step-over. From the Gulf of Saros to Marmara the total 1912 rupture length is probably about 140 km, not 50 km as previously thought. The direct observations of submarine scarps in Marmara are critical to defining barriers that have arrested past earthquakes as well as defining a possible segmentation of the contemporary state of loading. Incorporating the submarine scarp evidence modifies substantially our understanding of the current state of loading along the NAF next to Istanbul. Coulomb stress modeling shows a zone of maximum loading with at least 4?5 m of slip deficit encompassing the strike-slip segment 70 km long between the Cinarcik and Central Basins. That segment alone would be capable of generating a large-magnitude earthquake (Mw 7.2). Other segments in Marmara appear less loaded