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2 Acta Geophysica

2 2021

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The improvement of the earthquake and microseismic Horizontal-to-Vertical Spectral Ratio (HVSR) in estimating site efects

Sarmadi Mohammad Ali, Heidari Reza, Mirzaei Noorbakhsh, Siahkoohi Hamid Reza

Acta Geophysica

2021

Vol. 69, no. 4

1177--1188

The horizontal-to-vertical spectral ratio (HVSR) has been widely used to calculate near-surface site efects including funda mental resonance frequency and soil amplifcation. In this study, we investigated the features of HVSR in order to assess the reliability of this method. The main premise of Nakamura (1989; Nakamura, Y., 2000. Clear identifcation of fundamental idea of Nakamura’s technique and its applications. In: Proceedings of the 12th World Conference on Earthquake Engineer ing. Auckland, New Zealand.) HVSR method is to assume there are little to no amplifcation or de-amplifcation of motion recorded by the vertical component. In this study, we disregarded this assumption and used diferent proposed spectral ratios of earthquake accelerograms to examine and evaluate the accuracy of the HVSR technique. In addition, to increase the accuracy of the results, near-surface attenuation parameter Kappa (K), as an important spectral decay parameter which has infuence on the site efects, has been incorporated in proposed spectral ratios. The efciency of proposed spectral ratios was evaluated using 3948 accelerograms (Includes surface and borehole stations) of 496 earthquakes from 18 KiK-net accelerometery stations of the two regions of Japan with moment magnitudes ranging from 3 to 5. The two selected areas have diferent geological characteristics and diferent site efects parameters. According to the results, The HVSR method is under-determined at stations where the waves are mainly amplifed as they pass through the site. Moreover, in a single sta tion by calculating the amplifcation value on the vertical component, Site efects measurements obtained from surface and borehole data along with classical HVSR technique have a lower mean absolute error than those obtained by using HVSR technique alone.

Delineation of potential seismic sources using weighted K means cluster analysis and particle swarm optimization (PSO)

Sheikhhosseini Zohreh, Mirzaei Noorbakhsh, Heidari Reza, Monkaresi Hamed

Acta Geophysica

2021

Vol. 69, no. 6

2161--2172

Potential seismic sources play an important role in seismic hazard analysis. Identification of seismic sources is generally carried out on the basis of expert judgments, and in most cases, different and controversial results are obtained when several experts are consulted. In fact, the method of source identification is probably an important cause of uncertainty in the seismic hazard analysis. The main objective of this research is to provide an algorithm which combines the weighted K-means clustering analysis and Particle Swarm Optimization in order to automatically identify global optimum clusters by analysing seismic event data. These clusters, together with seismotectonic information, can be used to determine seismic sources. Two validity indexes, Davies–Bouldin's measure and Chou–Su–Lai's measure (CS), are used to determine optimum number of clusters. Study area is located at the longitude of 46°–48° E and latitude of 34°–36° N that is considered as the most seismically active part of Zagros continental collision zone, which has experienced large and destructive earthquakes due to movements of Sahneh and Nahavand segments of Zagros Main Recent Fault. As a result, 7-cluster model which is identified on the basis of DB validity index seems to be suitable for the considered earthquake catalogue, despite some limitations in partitioning.