MICROGRAVITY SURVEY TO EVALUATE AN EARTHQUAKE EFFECTS ON DERBENDIKHAN DAM SITE,NE IRAQ

Abstract

A microgravity survey includes 324 measuring points distributed on four profiles were carried out on Derbendikhan rockfill Dam Site, 230km NE Baghdad. The aim of the survey is to evaluate the possible effects resulted in a 7.3 magnitude earthquake hit the Iraqi-Iranian borders, 40km from the site, on Nov. 13, 2017. The study area lies within the high folded zone which is a part of the Western Zagros Fold-Thrust Belt, and the dam was constructed on Sirwan-Diyala River which is running on a fault zone. A gravimeter Model CG-5 was used in this survey, and all the necessary corrections were applied to the raw data to calculate the Complete Bouguer Anomaly map. A 2m upward continuation filter was applied on Bouguer map for regional field purpose that used to calculate the residual anomaly map. The residual map showed numerous negative and positive gravity anomalies that reflect subsurface heterogeneity in density distributions. Power spectrum analysis of Bouguer map showed six depth slices of gravity sources; 0.95m, 1.75m, 5.5m, 12.5m, 21.0 and 55.0m. The results showed that no obvious evidence regarding the evolution of subsurface channels after the earthquake was given. They also showed two prominent anomalies appear close to the right bank on approximately 21m depth; the first is a positive anomaly of 8m long, and the second is a negative anomaly of 12m long. These positive and negative anomalies suggest high and low- density zones coincide with different degrees of compression stresses resulted in the earthquake energy. A more detailed picture about subsurface, especially near-surface, distribution of densities, could be released from modeling the residual field. This modeling may interpret the near surface high (compact) or low (expanded) density zones.