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  • Seismological Society of America (SSA)  (3)
  • American Geophysical Union (AGU)  (1)
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  • 1
    Publication Date: 2014-06-12
    Description: We provide a complete description of the characteristics of excitation and attenuation of the ground motion in the Lake Van region (eastern Turkey) using a data set that includes three-component seismograms from the 23 October 2011 M w  7.1 Van earthquake, as well as its aftershocks. Regional attenuation and source scaling are parameterized to describe the observed ground motions as a function of distance, frequency, and magnitude. Peak ground velocities are measured in selected narrow frequency bands from 0.25 to 12.5 Hz; observed peaks are regressed to define a piecewise linear regional attenuation function, a set of excitation terms, and a set of site response terms. Results are modeled through random vibration theory (see Cartwright and Longuet-Higgins, 1956 ). In the log–log space, the regional crustal attenuation is modeled with a bilinear geometrical spreading characterized by a crossover distance at 40 km: fits our results at short distances ( r 〈40 km), whereas is better at larger distances (40〈 r 〈200 km). A frequency-dependent quality factor, Q ( f )=100( f / f ref ) 0.43 (in which f ref =1.0 Hz), is coupled to the geometrical spreading. Because of the inherent trade-off of the excitation/attenuation parameters ( and ), their specific values strongly depend on the choice made for the stress drop of the smaller earthquakes. After choosing a Brune stress drop Brune =4 MPa at M w =3.5, we were able to define (1) an effective high frequency, distance- and magnitude-independent roll-off spectral parameter, eff =0.03 s and (2) a size-dependent stress-drop parameter, which increases with moment magnitude, from Brune =4 MPa at M w  3.5 to Brune =20 MPa at M w  7.1. The set of parameters mentioned here may be used in order to predict the earthquake-induced ground motions expected from future earthquakes in the region surrounding Lake Van.
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences , Physics
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  • 2
    Publication Date: 2017-03-01
    Description: Both the geometry and the depth of the seismogenic zone of the North Anatolian Fault under the Marmara Sea (the Main Marmara Fault (MMF)) are poorly understood, in part because of the fault's undersea location. We recorded 10 months of microseismic data with a dense array of ocean bottom seismographs and then applied double-difference relocation and 3-D tomographic modeling to obtain precise hypocenters on the MMF beneath the central and western Marmara Sea. The hypocenters show distinct lateral changes along the MMF: (1) both the upper and lower crust beneath the Western High are seismically active and the maximum focal depth reaches 26 km; (2) seismic events are confined to the upper crust beneath the region extending from the eastern part of the Central Basin to the Kumburgaz Basin; and (3) the magnitude and direction of dip of the main fault change under the Central Basin, where there is also an abrupt change in the depth of the lower limit of the seismogenic zone. We attribute this change to a segment boundary of the MMF. Our data show that the upper limit of the seismogenic zone corresponds to sedimentary basement. We also identified several seismically inactive regions within the upper crust along the MMF; their spatial extent beneath the Kumburgaz Basin is greater than beneath the Western High. From the comparison with seafloor extensometer data, we consider that these regions might indicate zones of strong coupling that are accumulating stress for release during future large earthquakes.
    Print ISSN: 2169-9313
    Electronic ISSN: 2169-9356
    Topics: Geosciences , Physics
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  • 3
    Publication Date: 2014-05-20
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences , Physics
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  • 4
    Publication Date: 2016-10-08
    Description: Reliable moment magnitude estimates for seismic events in the Middle East region can be difficult to obtain due to the uneven distribution of stations, the complex tectonic structure, and regions of high attenuation. In this study, we take advantage of the many new broadband seismic stations that have become available through improved national networks and numerous temporary deployments. We make coda envelope-amplitude measurements for 2247 events recorded by 68 stations over 13 narrow frequency bands ranging between 0.03 and 8 Hz. The absolute scaling of these spectra was calculated based on independent waveform modeling solutions of the moment magnitudes for a subset of these events to avoid circularity. Using our 1D path calibrations, we determined coda-based magnitudes for a majority of the events. We obtain fairly good agreement with waveform-modeled seismic moments for the larger events ( M w 〉4.5) at low frequencies (〈0.7 Hz). As expected, the coda-derived source spectra become increasingly scattered at higher frequencies (〉0.7 Hz) because of unaccounted 2D path effects, as well as mixing of both Sn coda and Lg coda, which have different attenuation behavior. This scatter leads to increased variance in the magnitudes estimated for smaller events in which low-frequency amplitudes are below the noise levels and the higher frequencies are the only signals available. We quantify the expected variance in coda envelope amplitudes as a function of frequency using interstation scatter as our metric. The net results of this study provide thousands of new 1D coda magnitude estimates for events in the broad region, as well as the necessary initial starting model for use in a new related 2D coda study ( Pasyanos et al. , 2016 ). Online Material: Table of site terms and moment magnitudes.
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences , Physics
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