Simulation of seismic triggering and failure time perturbations associated with the 30 October 2020 Samos earthquake (Mw 7.0)

The 30 October 2020 Samos earthquake (Mw = 7.0) ruptured a north-dipping offshore normal fault north of the Samos Island with an extensional mechanism. Aftershocks mainly occurred at the western and eastern ends of the rupture plane in agreement with the Coulomb static stress changes. Mechanism of aftershocks located west of the rupture supported activation of the neighboring strike-slip fault almost instantly. In addition, a seismic cluster including events with Mw similar to 4 has emerged two days later at the SE side of Samos Island. This off-plane cluster displays a clear example of delayed seismic triggering at nearby active faults. In this study, numerical simulations are conducted to mimic the instant and delayed seismic triggering observed after this event and evaluate resultant seismic cycle perturbations at adjacent faults and near Izmir, where amplified ground motions caused heavy damage. For this purpose, Coulomb static stress changes and seismic waveforms recorded by strong-motion stations are combined as static and dynamic triggers on a rate-and-state friction dependent quasi-dynamic spring slider model with shear-normal stress coupling. According to our results, earthquakes with Mw = 4 events noticeably advance in failure time. However, instant triggering occurs only when static stress loading is very high, and the fault is close to fail, explaining the delayed triggering observed SE of Samos Island. Simulations also revealed that the shear-normal stress coupling increases static loading but does not affect the dynamically controlled failure time advances observed at the end of the seismic cycle. After the earthquake, some of the faults adjacent to the rupture are more likely to fail, especially the long strike-slip fault segment capable of generating large earthquakes at the western edge. On the other hand, the Samos earthquake induced no significant dynamic triggering on far away faults near Izmir

3-D crustal structure along the North Anatolian Fault Zone in north-central Anatolia revealed by local earthquake tomography

3-D P-wave velocity structure and Vp/Vs variations in the crust along the North Anatolian Fault Zone (NAFZ) in north-central Anatolia were investigated by the inversion of local P- and S-wave traveltimes, to gain a better understanding of the seismological characteristics of the region. The 3-D local earthquake tomography inversions included 5444 P- and 3200 S-wave readings obtained from 168 well-located earthquakes between 2006 January and 2008 May. Dense ray coverage yields good resolution, particularly in the central part of the study area. The 3-D Vp and Vp/Vs tomographic images reveal clear correlations with both the surface geology and significant tectonic units in the region. We observed the lower limit of the seismogenic zone for north-central Anatolia at 15 km depth. Final earthquake locations display a distributed pattern throughout the study area, with most of the earthquakes occurring on the major splays of the NAFZ, rather than its master strand. We identify three major high-velocity blocks in the mid-crust separated by the Izmir-Ankara-Erzincan Suture and interpret these blocks to be continental basement fragments that were accreted onto the margin following the closure of Neo-Tethyan Ocean. These basement blocks may have in part influenced the rupture propagations of the historical 1939, 1942 and 1943 earthquakes. In addition, large variations in the Vp/Vs ratio in the mid-crust were observed and have been correlated with the varying fluid contents of the existing lithologies and related tectonic structures

Extensional neotectonic regime through the NE edge of the outer Isparta Angle, SW Turkey: New field and seismic data

The Akşehir-Afyon Graben (AAG) is a 4-20-km-wide, 130-km-long NW-trending depression that separates central Anatolia in the NE and the Isparta Angle (IA) in the SW. Its southwestern margin-bounding fault determines the northeast edge of the outer IA that was previously interpreted as a compressional neotectonic structure, whereas our field evidence and recent seismic data substantiated that it is an oblique-slip normal fault characterising an extensional neotectonic regime. The AAG has an episodic and asymmetrical evolutionary history. This is indicated by two superimposed graben infills and structures. The older infill is folded, thrust-faulted and Early-early Middle Miocene in age. The younger infill is undeformed (nearly horizontal), Plio-Quaternary in age, and overlies the older infill with angular unconformity. Total throw amounts accumulated on both SW and NE margin-bounding faults, namely the Akşehir Master Fault (AMF) and the Karagöztepe Master Fault (KMF) since the Late Pliocene and Early Pleistocene, are 870 m and 200 m, respectively. Assuming a uniform motion, these values indicate motion rates of 0.3 mm/yr and 0.2 mm/yr, respectively, and the asymmetrical nature of the AAG. Kinematic analysis of surface slip data of both the AMF and KMF showed an oblique-slip motion with a minor right-lateral strike-slip component, and a NE-SW-directed extension. They also fit well with results of focal mechanism solutions of two recent seismic events, namely the 2000 December 15 Sultandaǧi (Mw=6.0) and the 2002 February 3 Çay (Mw=6.5) earthquakes. They have been sourced from the reactivation of the Akşehir-Pinarkaya and Sultandaǧi-Maltepe sections of the AMF. The Çay earthquake caused devastating damage to structures and loss of life in the region. The Çay earthquake has also led to the development of ground ruptures and surface deformation. The geometry of the ground ruptures and focal mechanism solutions of both earthquakes proved once more that the southern margin-bounding fault of the AAG or the northeastern edge of the IA is an oblique-slip normal fault. Consequently, all of these field and seismic data reveal an extensional neotectonic regime through the northeast edge of the outer IA despite the previously reported compressional neotectonic regime

Investigation of Dynamic and Static Effects on Earthquake Triggering Using Different Rate and State Friction Laws and Marmara Simulation

The clock of an earthquake can be advanced due to dynamic and static changes when a triggering signal is applied to a stress-loading fault. While static effects decrease rapidly with distance, dynamic effects can reach thousands of kilometers away. Therefore, earthquake triggering is traditionally associated to static stress changes at local distances and to dynamic effects at greater scales. However, static and dynamic effects near the triggering signal are often nested, thus identifying which effect dominates, becomes unclear. So far, earthquake triggering has been tested using different rate-and-state friction (RSF) laws utilizing alternative views of friction without much comparison. In this study, the analogy of an earthquake is simulated using single degree of freedom spring-block systems governed with three different RSF laws, namely "Dieterich", "Ruina" and "Perrin". First, the fault systems are evolved until they reach a stable limit cycle and then static, dynamic and their combination are applied as triggering signals. During synthetic simulations, effects of the triggering signal parameters (onset time, size, duration and frequency) and the fault system parameters (fault stiffness, characteristic slip distance, direct velocity and time dependent state effects) are tested separately. Our results indicate that earthquake triggering is controlled mainly by the onset time, size and duration of the triggering signal but not much sensitive to the signal frequency. In terms of fault system parameters, the fault stiffness and the direct velocity effect are the critical parameters in triggering processes. Among the tested RSF laws, "Ruina" law is more sensitive than "Dieterich" law to both static and dynamic changes and "Perrin" is apparently the most sensitive law to dynamic changes. Especially, when the triggering onset time is close to an unperturbed failure time (future earthquake), dynamic changes result the largest clock advancement, otherwise, static stress changes are substantially more effective. In the next step, realistic models will be established to simulate the effect of the recent (26 September 2019) Marmara earthquake with Mw=5.7 on the locked Kumburgaz fault segment of the North Anatolian Fault Zone. The triggering earthquake will be simulated by combining the static stress change computed via Coulomb law and the dynamic effects using ground motions recorded at broadband seismic stations within similar distances. Outcomes will help us to better understand the effects of static and dynamic changes on the seismic cycle of the Kumburgaz fault segment, which is expected to break soon with a possibly big earthquake causing damage at the metropolitan area of Istanbul in Turkey

M-Split: a graphical user interface to analyze multilayered anisotropy from shear wave splitting

Shear wave splitting analysis are commonly used to infer deep anisotropic structure. For simple cases, obtained delay times and fast-axis orientations are averaged from reliable results to define anisotropy beneath recording seismic stations. However, splitting parameters show systematic variations with back azimuth in the presence of complex anisotropy and cannot be represented by average time delay and fast axis orientation. Previous researchers had identified anisotropic complexities at different tectonic settings and applied various approaches to model them. Most commonly, such complexities are modeled by using multiple anisotropic layers with priori constraints from geologic data. In this study, a graphical user interface called M-Split is developed to easily process and model multilayered anisotropy with capabilities to properly address the inherited non-uniqueness. M-Split program runs user defined grid searches through the model parameter space for two-layer anisotropy using formulation of Silver and Savage (1994) and creates sensitivity contour plots to locate local maximas and analyze all possible models with parameter tradeoffs. In order to minimize model ambiguity and identify the robust model parameters, various misfit calculation procedures are also developed and embedded to M-Split which can be used depending on the quality of the observations and their back-azimuthal coverage. Case studies carried out to evaluate the reliability of the program using real noisy data and for this purpose stations from two different networks are utilized. First seismic network is the Kandilli Observatory and Earthquake research institute (KOERI) which includes long term running permanent stations and second network comprises seismic stations deployed temporary as part of the “Continental Dynamics-Central Anatolian Tectonics (CD-CAT)” project funded by NSF. It is also worth to note that M-Split is designed as open source program which can be modified by users for additional capabilities or for other applications

Crustal structure and seismic anisotropy near the San Andreas Fault at Parkfield, California

P>Receiver functions (RFs) from station PKD located similar to 3 km SW of the San Andreas Fault (SAF) samples the Salinian terrane near Parkfield. Crustal multiples indicate a 26-km-thick crust with a V-P/V-S of 1.88, which is slightly lower (1.83) for the upper and middle crust in the west. For the mid-crust, arrivals are observed at times corresponding to recently imaged seismic reflectors and may correspond to a layer of metasedimentary rocks below the base of the granitic batholith exposed at the surface. For the lower crust, RFs display strong polarity reversals with backazimuth and a change in Moho amplitude that require strong seismic anisotropy (> 15 per cent) in a low velocity, high V-P/V-S, possibly serpentinite or fluid filled schist layer that has a ENE dipping (similar to 35 degrees) rock fabric. Similar patterns of amplitude variations and polarity reversal observed in RFs for some southern California stations located west of the SAF support the hypothesis that the cause of these data characteristics is a regionally prevalent lower crustal anisotropy. The orientation of this anisotropic fabric is inconsistent with the recent San Andreas sense of shear and is most likely a fossilized fabric of past eastward-directed (Farallon Plate) subduction

Subsurface signature of North Anatolian Fault Zone and its relation with old sutures: New insight from receiver function analysis

The North Anatolian Fault Zone (NAFZ) is an active continental transform plate boundary that accommodates the westward extrusion of the Anatolian plate. The central segment of NAFZ displays northward convex surface trace which coincides partly with the Paleo-Tethyan suture formed during the early Cenozoic. The depth extent and detailed structure of the actively deforming crust along the NAF is still under much debate and processes responsible from rapid uplift are enigmatic. In this study, over five thousand high quality P receiver functions are computed using teleseismic earthquakes recorded by permanent stations of national agencies and temporary North Anatolian Fault Passive Seismic experiment (2005-2008). In order to map the crustal thickness and Vp/Vs variations accurately, the study area is divided into grids with 20 km spacing and along each grid line Moho phase and its multiples are picked through constructed common conversion point (CCP) profiles. According to our results, nature of discontinuities and crustal thickness display sharp changes across the main strand of NAFZ supporting a lithospheric scale faulting that offsets Moho discontinuity. In the southern block, crust is relatively thin in the west (∼35 km) and becomes thicker gradually towards east (∼40 km). In contrast, the northern block displays a strong lateral change in crustal thickness reaching up to 10 km across a narrow roughly N-S oriented zone which is interpreted as the subsurface signature of the ambiguous boundary between Istanbul Block and Pontides located further west at the surface