Downhole microseismic detection for passive ranging to a target wellbore

A method can include measuring microseismic activity in a relief wellbore, thereby detecting a microseismic event in an earth formation penetrated by the relief wellbore, and determining a location of an influx into a target wellbore, based on the microseismic event detecting. A microseismic ranging system for use with a subterranean well can include at least one microseismic sensor in a relief wellbore that penetrates an earth formation, with the microseismic sensor detecting a microseismic event in the formation, the microseismic event being caused by an influx into a target wellbore. Another method can include measuring optical scattering in an optical waveguide positioned in a relief wellbore, thereby detecting a microseismic event in an earth formation penetrated by the relief wellbore, and determining a location of an influx into a target wellbore, based on the microseismic event detecting.