Evaluation of the effect of subsidence on seismicity using satellite images in Tehran plain, a case study (Varamin plain and Shahryar Plain)

Due to the specific geographical and geological situation of Iran, the earthquake phenomenon imposes numerous human and financial losses on our country every year, and identifying areas prone to earthquakes and providing practical solutions are among the ways to reduce these losses. Also, by examining the impact of the subsidence phenomenon in the seismic process and reaching a comprehensive and practical model, it is possible to prevent and adjust the volume and scope of potential incidents rooted from this widespread hazard; noting that the relevant institutions have all the necessary regulations and standards for construction and safety regarding this process. The earthquake-prone areas which were subject to subsidence and earthquakes at the same time were reviewed. Conducting comprehensive and detailed studies in order to create a complete and scientific structure to identify and provide maps and updated information on the adaptation of the seismic situation and the subsidence of the country helps to prevent the occurrence of accidents and widespread loss of life and capital during these phenomena. In this regard, this study was planned aiming to investigate and assess the effect of subsidence on the seismicity trends of Varamin and Shahriar plains in Tehran region.

The current research aims to investigate the effect of subsidence rate changes on the seismicity trend using Sentinel-1 satellite data from 2014 to 2021. The studied area is in the southern part of the Alborz mountain range, namely Varamin Plain and Shahriar Plain which are located in the southern and western extremes of Tehran province, respectively. Due to the sensitivity of the research topic and to obtain more accurate results, a combination of digital information and hydrological data along with radar images of the area and field data were processed and analysed. Geological maps 1:50000, fault and seismicity 1:1000000 from the Geological Organization, topography 1:25000 from the National Mapping Organization, and seismic data from the years 1350 to 1400 were obtained from the Geophysics Institute of Tehran University and the International Research Institute of Seismology and Earthquake Engineering of Tehran. The data from 46 piezometer wells in the region from 2014 to 2021 were used for zoning and checking the underground water level. The satellite data of the Sentinel-1 sensor was used for interferometry to determine the subsidence rate of the region.

In the present study, the correction of the seismic catalog with standard techniques and the micro-seismic catalog showed that Varamin and Shahriar plains have subsidence rates of 65 and 54.5 cm per year, respectively, based on changes of 600,000 cubic meters per year in the volume of the aquifer. The studied case has led to the minimum stress changes in the area to the extent of 5 bar to 17 bar (equivalent to 500 to 1700 kilopascals). Also, the seismicity pattern of the area shows an increasing trend in the occurrence of subsequent earthquakes on one side, and a shortening of the seismic return period in the studied area on the other side.

The high consistency of the data trend in the distribution charts demonstrates the consistency of the subsidence trend and, subsequently, its effect on the seismicity trend of the studied area, which in turn indicates the effect of subsidence on the seismicity trend in the studied areas. The stress output resulting from loading shows a high increase in the stress of the area on one hand, and a decrease in pore pressure on the other hand. This is due to the decrease in the resistance of the soil in the studied area which manifests in the form of collapses and falling of structures and, with an emphasis on the extensive decrease in the strength of surface and subsurface layers, causes subsidence and intensification of subsidence in the region. To put it shortly, both of these phenomena will influence one another in an increasing cycle.