Publication Date:
2023
Description:
Due to increasing the population in metropolitan regions such as Tehran and the existence of the underground constructions, the importance of seismic investigation is evident to reduce damages caused by probable earthquakes. Accordingly, the precise detection of micro to medium earthquakes is effective tool for tracking fault dynamics in seismic cycles, as well as for earthquake prediction and seismic hazard assessment. In this study, the recorded ambient noise at Tehran Disaster Mitigation and Management Organization (TDMMO) and Road, Housing and Urban Development Research Center (BHRC) networks, as an accelerometer network installed in Tehran city, have been used on the point of characterizing the noise spectrum for each station as a function of time for obtaining the detection threshold of these networks. Therefore, an indirect approach based on the signal-to-noise ratio (SNR) in the time domain, with parameterization in the frequency domain is applied. Based on SNR method, the source signature is simulated by a simple source model called a circular fault model. So the signal is estimated via the Brune function as a most common models for circular faults. While, to determine the noise, the real data of 13 accelerometer stations of the TDMMO and 7 joint stations with the BHRC were used. In this respect, the Power Spectral Density (PSD) of noise was calculated using PQLX software in the frequency domain and then transferred to the time domain by the Parsville theorem. Eventually, the SNR value was acquired for each station by dividing these two quantities. As a result, the minimum detectable magnitude in at least five stations with an SNR larger than five is 3.0 for S-waves and 3.3 for P-waves, which frequently occurs in the center of the network.Another finding of this studies is to analyze the effect of spatial variations of the noise on the detection ability. For this, a constant noise was allotted to all stations, lowest observed noise level, as a result of which, the smallest magnitude detectable is 1.7 for S waves and 2.2 for P waves.At last, the sensitivity of the detection capability to 3 fundamental parameters, including stress drop, focal depth and reduced time, which were assumed as a constant value within the network, were investigated. In fact, these parameters are strongly affected by uncertainty and aren't absolute values. Consequently, the impact of their changes was studied. In our case, implied that the variation in the stress drop has no effect on the detection threshold, but the focal depth and the reduced time are effectual. A 15 km variations in the focal depth, the detectable magnitude changes by 0.3 units, and by changing the reduced time from 0.015 to 0.035 s, the detectable magnitude varies by 0.4 units in Mw.
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