Scientific- Research Quarterly of Geographical Data (SEPEHR)

Scientific- Research Quarterly of Geographical Data (SEPEHR)

Presenting attenuation relationships observed in near-field and far-field earthquakes of Zagros seismotectonic region using two different simulation methods

Document Type : Research Paper

Authors
Ali Hasankhani 1
Mahdi Modiri 2
Ahmad Naghavi 3
1 Master's Degree in Geophysics-Earthquake Engineering
2 Professor of Malek Ashtar University of Technology
3 Master’s Degree in Artificial Intelligence
10.22131/sepehr.2023.1987998.2949
Abstract
Extended Abstract
Introduction
Unfortunately, seismic data recorded globally during the last fifty years does not include every type of wave propagation conditions in the environment, types of construction, the rupture process on the fault, and the geometrical relationship between the construction and the fault. This is especially seen in near-field regions. Before the 1999 Chi-chi earthquake in Taiwan and the 1999 Izmit earthquake in Turkey, there were only about 20 records of earthquakes with a magnitude greater than 7 at a distance of less than 20 kilometers from the fault.
The Turkish earthquake added 5 records and the Thai earthquake added 65 records to this collection, but only two fault rupture scenarios were added to our knowledge, while thousands of other possible scenarios may occur. Thus, seismologists and earthquake engineers have tried to estimate parameters related to strong near-field motions of the earth with an acceptable confidence using various experimental and theoretical simulation methods.           
Methods
In earthquake engineering and seismology, earthquake phenomenon and the resulting movements are generally investigated and analyzed using dynamic and kinematic methods. Seismological models and problems are thus divided into two categories: Kinematic models which are based on slip distribution and do not take the state of stress on the fault into account. Dynamic models deal with the physics of fault rupture and its causes. Simulation methods are also divided into three main categories: deterministic (low frequencies), stochastic (high frequencies) and hybrid (broad band) methods.
Generally speaking, simulating strong ground motion plays an important role in the estimation of related parameters especially in regions lacking such data. Accelerographs are used to simulate strong ground motions. The present study has introduced, investigated and validated two methods: decisive simulation models (Discrete-Wave Number and Finite Fault) and Finite Fault models. It also explains how the simulated recording are produced for near-field (less than 20 km to a seismogenic fault) and far-field events, presents attenuation relationships for the Zagros seismotectonics region, and predicts parameters of strong ground motions.
Results & Discussion
Due to the special geological conditions and the existence of many active faults in Iran, our country is considered to be located in an earthquake-prone region. Zagros region is considered to be the most earthquake-prone region of Iran. Finite fault modeling combines various aspects of plate source with the ground motion model based on point source. Since previously mentioned limitations are not naturally present in finite fault modelling, the method takes geometry of the fault and the directivity effect into account. Time delay method and the sum of accelerations recorded in maps of a two-dimensional network are used for simulation in finite fault model. The fault plate is divided into various elements and a minor event is simulated for each one. The overall seismic acceleration equals the sum of the effects of these minor events. The strong ground motions in each micro-fault are calculated using the random point source method and then summed up at the desired point with an appropriate time delay to obtain the ground motion of the entire fault.
Previous geological and seismic studies of each seismic region are used to determine the key parameters of the simulation input. To produce a comprehensive database, a significant number of stations are taken into account around the fault based on different hypotheses and artificial accelerograms are produced in accordance with the seismological parameters of the region. A suitable function is then selected and an attenuation relationship is fitted. The simulation results and the resulting attenuation relationship are then compared with valid global attenuation relationships and their consistency (compliance percentage) is investigated.
 Conclusion
The present study has produced a wide range of simulated records (about 20 thousand records) for Zagros seismotectonics region. Thus, the resulting relationships will hopefully have sufficient accuracy and efficiency to be used in structure designing and urban development. It worth noting that the regression correlation coefficient (R-Square) was above 0.95 in all fits.
These attenuation relationships can provide a new perspective on site selection, and help us in understanding the dynamic behavior of structures, and the development of various infrastructure. They also help urban managers to predict and reduce earthquake damages.
Keywords
Earthquake
Seismological models
Strong ground motion simulation
Finite fault
Attenuation relationships
Zagros
Site selection
Subjects
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