The city of Iwaki, located in Fukushima Prefecture, Japan, faced water outage twice because of the Tohoku earthquake that occurred on March 11, 2011, as well as the earthquake that followed on April 11, 2011. It is rare that two large earthquakes, each with different mechanisms, occur within a short period of one month. This study focuses on the pipeline damage in Iwaki due to these earthquakes and tries to clarify the seismic vulnerability of pipe attributes on the basis of the earthquake damage analysis on the water-supply pipeline. As the result of the analysis on the seismic vulnerability index of the pipe material, pipe diameter, and its geomorphological classification, the seismic vulnerabilities to pipe diameter and material were very similar between the two earthquakes. The seismic vulnerability to pipe diameter was also similar to that in earthquakes in the past, whereas polyvinyl chloride pipes (VPs) were evaluated to be stronger. The geomorphological classification factor does not always need to add seismic vulnerability depending on the evaluation of the seismic motion.