R. Gok

Sn attenuation in the Anatolian and Iranian plateau and surrounding regions

An edited version of this paper was published in Geophysical Research Letters by the American Geophysical Union (AGU). Copyright 2003, AGU. See also: http://www.agu.org/pubs/crossref/2003/2003GL018020.shtml; http://atlas.geo.cornell.edu/turkey/publications/Gok-et-al_2003.htm

Tomographic imaging of Lg and Sn propagation in the Middle east

Abstract — Observations based on relatively limited data recorded by sparsely distributed stations have indicated that regional seismic phase propagation (Lg and Sn) is very complex in the Middle East. Accurate characterization of regional seismic wave propagation in this region necessitates the use of a large number of seismic stations. We have compiled a large data set of regional and local seismograms recorded in the Middle East. This data set comprises approximately four years of data from national short-period networks in Turkey and Syria, data from temporary broadband arrays in Saudi Arabia and the Caspian Sea region, and data from GSN, MEDNET, and GEOFON stations in the Middle East. We have used this data set to decipher the character and pattern of regional seismic wave propagation. We have mapped zones of blockage as well as inefficient and efficient propagation for Lg, Pg, and Sn throughout the Middle East. Two tomographic techniques have been developed in order to objectively determine regions of lithospheric attenuation in the Middle East.¶We observe evidence of major increase in Lg attenuation, relative to Pg, across the Bitlis suture and the Zagros fold and thrust belt, corresponding to the boundary between the Arabian and Eurasian plates. We also observe a zone of inefficient Sn propagation along the Dead Sea fault system which coincides with low Pn velocities along most of the Dead Sea fault system and with previous observations of poor Sn propagation in western Jordan. Our observations indicate that in the northern portion of the Arabian plate (south of the Bitlis suture) there is also a zone of inefficient Sn propagation that would not have been predicted from prior measurements of relatively low Pn velocities. Mapped high attenuation of Sn correlates well with regions of Cenozoic and Holocene basaltic volcanism. These regions of uppermost mantle shear-wave attenuation most probably have anomously hot and possibly thin lithosphere.

Regional wave propagation in Turkey and surrounding regions

This paper was published by the American Geophysical Union (AGU). Copyright 2000, AGU. See also: http://www.agu.org/journals/gl/gl0003/1999GL008375/pdf/1999GL008375.pdf; http://atlas.geo.cornell.edu/turkey/publications/Gok-et-al_2000.htm

Shear wave splitting in a young continent‐continent collision: An example from Eastern Turkey

An edited version of this paper was published by the American Geophysical Union (AGU). Copyright 2003, AGU. See also: http://www.agu.org/pubs/crossref/2003.../2003GL017390.shtml; http://atlas.geo.cornell.edu/turkey/publications/Sandvol-et-al_2003b.htm

Seismogenic zones in Eastern Turkey

An edited version of this paper was published by the American Geophysical Union (AGU). Copyright 2003, AGU. See also: http://www.agu.org/pubs/crossref/2003.../2003GL018023.shtml; http://atlas.geo.cornell.edu/turkey/publications/Turkelli-et-al_2003.htm

The Scaling of Seismic Energy With Moment: Simple Models Compared With Observations

The scaling between small and large earthquakes remains an unresolved issue in seismology. The predominant hypothesis is the rupture process is self-similar, leading to predictions that source parameters such as apparent stress are the same for all earthquakes. As digital broadband data has become widely available, a number of published empirical studies have challenged self-similarity, though the evidence remains mixed. Using simple point source models in the time and frequency domains, we review the predicted scaling behavior of earthquake energy and other source parameters, under self- and non-self-similar assumptions. The models show self-similar scaling leads to some testable hypotheses, including the constancy of apparent stress and the invariance of spectral shape under a particular frequency transformation, regardless of the true (and perhaps unknown) source time function. We also review the problems posed by measurement errors in determining seismic energy and the limited magnitude ranges of events within given studies to answering the scaling question. To address these problems we apply multiple techniques to the 1999 Hector Mine California earthquake sequence. For two regional wave types, direct Lg and scattered coda waves, we examine spectral scaling using both seismic energy, and source shape invariance. The results show the Hector Mine sequence exhibits non-self-similar scaling with apparent stress increasing with moment approximately as M o 0.14 . Finally we briefly present four general scaling models, one self-similar with high variance, the others non-self-similar, which appear to be consistent with the earthquake apparent stress behavior that has been observed to date.

Source Mechanisms of Mine-Related Seismicity, Savuka Mine, South Africa

We report full moment tensor solutions for 76 mine tremors with moment magnitudes (Mw) between 0.5 and 2.6 recorded by a network of 20 high-frequency geophones in a deep gold mine in South Africa. Source mechanisms convey important information on how in-mine stresses are relaxed, and understanding the nature of such mechanisms is essential for improving our assessment of rock mass response to mining. Our approach has consisted of minimizing the L2 norm of the difference between observed and predicted P, SV, and SH spectral amplitudes, with visually assigned polarities, to constrain all six independent components of the seismic moment tensor. Our results reveal the largest principal stresses in the mine are com- pressive, oriented near vertically, and relaxed through a mix of volumetric closure and normal faulting, consistent with a gravity-driven closure of the mined-out areas. Pre- vious moment tensor studies in deep mines had suggested that the distribution of seis- mic sources in terms of the volumetric-shear mix was bimodal. A bimodal distribution is compatible with our moment tensor solutions only for moment magnitudes above 2.2. Events in the 0:5 <Mw <2:2 moment magnitude range display a continuous distribution of their volumetric-shear mix. Online Material: Focal parameters for mine tremors at Savuka.

An Application of the Coda Methodology for Moment-Rate Spectra Using Broadband Stations in Turkey

A recently developed coda magnitude methodology was applied to selected broadband stations in Turkey for the purpose of testing the coda method in a large, laterally complex region. As found in other, albeit smaller regions, coda envelope amplitude measurements are significantly less variable than distance-corrected direct wave measurements (i.e., L{sub g} and surface waves) by roughly a factor 3-to-4. Despite strong lateral crustal heterogeneity in Turkey, they found that the region could be adequately modeled assuming a simple 1-D, radially symmetric path correction. After calibrating the stations ISP, ISKB and MALT for local and regional distances, single-station moment-magnitude estimates (M{sub W}) derived from the coda spectra were in excellent agreement with those determined from multistation waveform modeling inversions, exhibiting a data standard deviation of 0.17. Though the calibration was validated using large events, the results of the calibration will extend M{sub W} estimates to significantly smaller events which could not otherwise be waveform modeled. The successful application of the method is remarkable considering the significant lateral complexity in Turkey and the simple assumptions used in the coda method.

Earthquake stress via event ratio levels: Application to the 2011 and 2016 Oklahoma seismic sequences

We develop a new methodology for determining earthquake stress drop and apparent stress values via spectral ratio asymptotic levels. With sufficient bandwidth, the stress ratio for a pair of events can be directly related to these low and high frequency levels. This avoids the need to assume a particular spectral model, and derive stress drop from cubed corner frequency measures. The method can be applied to spectral ratios for any pair of closely related earthquakes, and is particularly well suited for coda envelope methods that provide good azimuthally-averaged, point-source measures. We apply the new method to the 2011 Prague and 2016 Pawnee earthquake sequences in Oklahoma. The sequences show stress scaling with size and depth, with the largest events having apparent stress levels near 1 MPa, and smaller and/or shallower events having systematically lower stress values.

Seismic Event Location Calibration Using the Eastern Turkey Broadband Seismic Network: Analysis of the Agri Dam Explosion

A 12-ton controlled source explosion took place in eastern Turkey on 5 June 2001 and was recorded by 18 stations of the Eastern Turkey Seismic Experiment (ETSE) PASSCAL broadband network. This is a unique recording obtained for the first time in this region. Due to the blasting type and extremely high Lg and Sn attenuation in eastern Turkey, the blast is only observed out to a distance of about 300 km. We have used travel-time data from this explosion to obtain average crustal structure and site correction terms for the stations. The explosion was located using two new regional velocity models and the IASP91 velocity model to test the location capabilities of the ETSE network. We found that for surface focus events, the ETSE network is able to locate events to within 1–2 km of the true epicenter.