Seismic velocity structure along the North Anatolian Fault beneath the Central Marmara Sea and its implication for seismogenesis

Summary The offshore part of the North Anatolian Fault (NAF) beneath the Marmara Sea is a well-known seismic gap for future M > 7 earthquakes in the sense that more than 250 years have passed since the last major earthquake in the Central Marmara region. Although many studies discussed the seismic potential for the future large earthquake in this region on the basis of historical record, geodetic, and geological observations, it is difficult to evaluate the actual situation on the seismic activity and structure along the NAF beneath the Marmara Sea due to the lack of ocean bottom seismic observations. Using ocean bottom seismometer observations, an assessment of the location of possible asperities that could host an expected large earthquake is undertaken based on heterogeneities in the microseismicity distribution and seismic velocity structure. Specifically, seismic tomography and precise hypocenter estimations are conducted using offshore seismic data whose recording period is 11 months. About five times more microearthquakes are detected with respect to events recorded in a land-based catalog. A comparison with previously published results from offshore observation data suggests that the seismicity pattern had not changed from September 2014 to May 2017. The location accuracy of microearthquakes is greatly improved from only the land-based earthquake catalog, particularly for depth direction. There are several aseismic and inactive zones of microearthquake, and the largest one is detected using land-based seismic observation, whereas other zones are newly detected via offshore observations. The obtained velocity model shows a strong lateral contrast, with two changing points. The western changing point corresponds to a segmentation boundary, where the dip angle of the NAF segments changed. High-velocity zones from tomographic images are characterized by low seismicity eastward of the segment boundary. To the east of 28.50° E, the high-velocity zone becomes thicker in the depth direction and is characterized by low seismicity. Although the low seismic activity alone could be interpreted as both strong coupling and fully creeping, the high-velocity features at the same can be concluded that these zones are consist of brittle material and strong coupling. From comparison with other geodetic and seismic studies, we interpret these zones as locked zones that had been ruptured by the past large earthquakes and could be ruptured by future ones. These zones might accumulate strain since the mainshock rupture associated with the May 1766 Ms7.3 earthquake, the latest major earthquake in this region.

What Is Fling Step? Its Theory, Simulation Method, and Applications to Strong Ground Motion near Surface Fault Ruptures

ABSTRACT We present the theory of the fling step and a theoretical method for simulating accurately the near-fault strong motions, and apply it to reproduce various strong-motion records near surface faults. Theoretically, the fling step is the contribution of the static Green’s function in the representation theorem (Hisada and Bielak, 2003), and we show that this theory holds for any seismic velocity structure. We first demonstrate the validity of this theory using theoretical solutions of a circular fault model in a homogeneous full-space. Next, we apply the theory to layered half-spaces, present a theoretical method based on the wavenumber integration method, and introduce various techniques to simulate the near-fault ground motions including fling steps with high accuracy. Finally, we demonstrate the effectiveness of the method by reproducing various strong-motion records near surface fault ruptures and discuss the characteristics of near-fault strong motions including the fling step and the forward directivity pulse. We made all of the software and data used in this article available on the internet.

A Possible Roll-Over Slab Geometry Under the Caroline Plate Imaged by Monte Carlo Finite-Frequency Traveltime Inversion of Teleseismic SS Phases

The shape of a subducting slab varies as a function of trench motion. Two end-members of subduction modes are geodynamically possible: roll-back mode underneath neighboring plates and roll-over mode underneath the plate itself. Whereas most of major slabs seem to roll back while the Pacific plate shows a slab piling behavior down to ∼1,000 km depth under the Mariana trench, no clear evidence of slab roll-over in nature has been reported so far. Here we show a possible roll-over slab beneath the Caroline microplate, revealed from its three-dimensional seismic velocity structure derived by analyzing teleseismic reverberating SS phases. We suggest that slab roll-over is driven by at least two factors: 1) the overall buoyancy and fragility of the Caroline microplate at the surface, induced by a thin hot mantle plume that rises from depths ≥800 km; and 2) the pushing force of the Pacific plate acting on the trailing edge of the Caroline plate.

Investigating seismic properties of the NEGIS onset region using ice-drilling noise as a seismic source

<p>Investigating the physical conditions underlying and enabling fast glacier flow is crucial to understanding the future stability of ice sheets, as well as their impact on future sea-level rise. Seismic surveys have been widely used to measure material properties of the ice and substrate, including seismic velocity structure, anisotropy, and bed properties. While traditional seismic surveys rely on natural seismicity or man-made sources such as explosives, anthropogenic noise generated through ice-core drilling can also be used as a seismic source. Placing geophones around an ice-core drilling site therefore presents an exciting opportunity to complement and extend measurements from ice cores to the surrounding area.</p><p>Here, we present preliminary results from a seismic investigation conducted using noise generated by ice-core drilling activities at the East Greenland Ice Core Project (EGRIP) site. The EGRIP site is located near the onset region of the Northeast Greenland Ice Stream (NEGIS), which drains over 10% of the Greenland Ice Sheet. The ice-core drilling process creates a variety of semi-continuous (e.g., generator-induced) and impulsive (e.g., core break) seismic source signals. As drilling progresses through the ice column, the corresponding variation in seismic signals can be used to generate a vertical profile of seismic properties. In the summer of 2019, nine 3-component surface geophones were deployed at 0, 300, 750, 1500 and 3000 m distance from the drill site along two lines corresponding to the along- and cross-flow directions of the ice stream. The network recorded at a sampling frequency of 400 Hz for 28 days, during which drilling progressed between 1920 and 2110 m depth below the surface. Both continuous and impulsive sources related to the drilling process were recorded at all stations. Impulsive arrivals were identified using STA/LTA phase-picking across multiple components and stations. Because the depth of the drill head at any given time is known, the move-out of each event could then be used to determine the integrated seismic velocity structure along the source-receiver ray path.</p><p>Additionally, sporadic passive microseismic signals resulting from ice stream motion over the bed were observed at all stations. Both individually distinguishable icequakes and 3-5 minute-long “gliding” tremors were recorded, indicative of stick-slip motion at the bed of NEGIS. Further work will concentrate on modelling these tremors to resolve the shear modulus of the substrate, and on incorporating continuous drill-generated noise into our overall analysis. Our approach demonstrates the added value of opportunistic seismic networks as a complement to ice drilling operations.</p>