عنوان مقاله [English]
Kuhbanan fault is in southeast margin of micro continent of Iran between Tabas and Yazd blocks. It directed from northwest of Kerman to northwest of Bahabad east of Bafgh (Yazd). Its length is about 300 km and its overall direction in Iran is in southeast – northwest. This fault has caused many historical deadly Earthquake and is recently more active at its southeast part near Zarand city. The most resent (2005) Earthquake of this fault costs 600 dead.
We tried in this work to use specific parameters of this fault and stresses related to its Earthquakes for an attempt to Earthquakes prediction. Various methods have historically been used to find a way to Earthquake prediction. This has caused many researches to use different parameters in different ways. Although, there has been no considerable success except one or two cases, but attempts have opened many windows to search about.
In this work, with respect to sensitivity of fiber optics to changes in physical parameters such as pressure and temperature, as a new approach we have theoretically investigated the possibility of using fiber optics in fault structure to find a precursor to seismic activities. In a special way, using Coulomb failure diagram and Mohr circle we could estimate stress distribution with time before the Earthquakes. The stress distribution around the focus of the quakes toward the surface inside the faults, where the cable may or can be used, was investigated. We fond from previous studies that there is no considerable differences between stresses at focal point and at points near surface.
Depending on sensitivity of fiber optics to changes of pressure acting on and sensitivity of the cable to resulting changes of its length or its diameter, we found Bragg grating glass coated fiber is more useful to use in this range of stress changes. In such designed fibers the applied stresses will cause changes in the length of the fiber that is more effective in its sensitivity than changes in its diameter.
As a reality, pre hazard stresses of the 2005 Zarand Earthquake related to the Kuhbanan fault structure, northern part of Kerman province (Iran), have been used to act on this kind of fibers to estimate changes in wavelength of output wave. This 2005 Earthquake of 6.4 magnitude, in Richter scale, has occurred two years and four months after the 2002 Earthquake of 5 magnitude in the same small area of Zarand. We could determine the increasing stresses between these two quakes in different times up to two days before the 2005 Earthquake.
The stresses were applied to a glass coated fiber with a wave of 1550 nm wavelength passing through. As the stress is increasing during the time between the two quakes changes in wavelength (Δλ) increase toward a maximum value of 6.8 nm at the time of 2005 Earthquake. This value of Δλ is 6.57 nm 22 days and 6.16 nm about two months before the Earthquake, which are easily readable. That means in Kuhbanan fault structure when changes in output wavelength pass through 6.16 nm one should be careful.