Seismic Envelopes of Coda Decay for Q-coda Attenuation Studies of the Gargano Promontory (Southern Italy) and Surrounding Regions

Here, we describe the dataset of seismic envelopes used to study the S-wave Q-coda attenuation quality factor Qc of the Gargano Promontory (Southern Italy). With this dataset, we investigated the crustal seismic attenuation by the Qc parameter. We collected this dataset starting from two different earthquake catalogues: the first regarding the period from April 2013 to July 2014; the second regarding the period from July 2015 to August 2018. Visual inspection of the envelopes was carried out on recordings filtered with a Butterworth two-poles filter with central frequency fc = 6 Hz. The obtained seismic envelopes of coda decay can be linearly fitted in a bilogarithmic diagram in order to obtain a series of single source-receiver measures of Qc for each seismogram component at different frequency fc. The analysis of the trend Qc(fc) gives important insights into the heterogeneity and the anelasticity of the sampled Earth medium.

Coda-attenuation imaging of the North Anatolian Fault Zone, northern Turkey

<p>The North Anatolian Fault (NAF), a right-lateral strike-slip fault spanning 1500 km in length, stretches across northern Turkey and it marks the boundary between the Eurasian and Anatolian plates. Nucleating in the east, at the Karliova triple junction and reaching the Aegean Sea at the west, it is a particularly active fault zone with a series of migrating high-magnitude earthquakes. Using 6445 Z-component waveforms from a temporary seismic network in the area (Dense Array for North Anatolia – DANA), this study aims to investigate the western part of the NAF, which splays into a northern and southern branch. Coda attenuation imaging is utilised for imaging the absorption characteristics of the area, as it can be used as a marker for source and dynamic Earth processes due to its higher sensitivity to small variations of lithospheric properties compared to seismic velocity. The absorption structure is recovered by inverting for the coda attenuation quality factor, Q<sub>c</sub>, at frequencies between 3-18 Hz, using sensitivity kernels. The extensive seismicity in the area, as well as the density of the seismic stations, provide high-resolution models of 0.04-0.05 degrees in spacing. The scattering structure of the region is imaged using peak-delay time, which is used as a direct measure for multiple forward scattering. Preliminary results show a clear change in scattering between the Istanbul and Sakarya zones, north and south of the fault respectively, with the scattering increasing from north to south at lower frequencies and decreasing at higher frequencies. At a smaller scale, absorption and scattering anomalies appear to outline contrasting geological units beneath the DANA network.</p>

CodaQback

Attenuation study of a province is considered as a basic quantity for seismic hazard assessment. It has already been established that the study of two physical processes, namely the seismic sources and propagation of the waves, is essential for seismic-hazard mapping. Additionally, attenuation plays an important role towards scaling seismic hazard. Accordingly, a computational tool entitled CodaQback is presented. Based on back scattering model, this versatile software is equipped with user-friendly graphical user interface. It also allows quick picking of phases for computing coda attenuation parameter. All outputs after each execution step in CodaQback are efficiently exported step-wise into a separate folder in Excel and text formats. To validate the computing tool, it is tested in real data analysis and there is found to be good matching of computed values with already established ones. It is envisioned that this package will enable user to derive quick and reliable estimation of coda attenuation parameter irrespective of geological and geo-morphological units.

Seismic attenuation tomography using body-wave energy normalised by the heterogeneous coda

<p>Seismic waves lose energy during propagation in heterogeneous Earth media. Their decrease of amplitude, defined as seismic attenuation, is central in the description of seismic wave propagation. The attenuation of coherent waves can be described by the total quality factor, <em>Q</em>, and it is defined as the fractional energy lost per cycle, controlling the decay of the energy density spectrum with lapse time. The coda normalization (CN) method is a method to measure the attenuation of P- or S-waves by taking the ratio of the direct wave energy and late coda wave energy in order to remove the source and site effects from P- and S-wave spectra. One of the main assumptions of the CN method is that coda attenuation, i.e. the decay of coda energy with lapse time measured by the coda quality factor <em>Q<sub>c</sub></em> is constant. However, several studies showed that Q<sub>c</sub> is not uniform in the crust for the lapse times considered in most attenuation studies. In this work, we propose a method to overcome this assumption, measuring coda attenuation for each source-station path and evaluating the effect of different scattering regimes on the corresponding imaging. The data consists of passive waveforms from the fault network in the Pollino Area (Southern Italy) and Mount St. Helens volcano (USA).</p>

Attenuation of Coda Waves in the Central Region of the Gulf of California, México

Coda waves were analyzed from events recorded by NARS seismic network deployed along both margins of the Gulf of California, Mexico, to estimate coda attenuation Qc. Sato’s (1977) single scattering model was used for a coda window of 20 to 25 s beginning at twice the S-wave travel time. Events recorded from 2003 to 2007 located in the central region of the Gulf of California were analyzed. Source-to-receiver distances are between 40 and 500 km. Assuming a power law of the form QC (f) = QO f a, QC values were averaged and a value of QO = 83±3 and a frequency-dependence α value of 1.06±0.03 in the frequency range from 1 to 7 Hz was obtained. QO value and the high frequency dependency agree with the values of other regions characterized by a high tectonic activity. Based on source-station distribution two subregions (north and south) were defined. QC values were calculated and correlated with tectonics and morphology of each area. The observed higher attenuation in the south region can be attributed to the fact that south region is more fractured since the greater earthquakes occur in central to south Gulf of California and the oceanic crust is reported to be thinner in the southern region.