Calculating hypocenters of microseismic events. At least some of the example embodiments are methods including: detecting seismic waves from a microseismic event, the detecting by a sensor array disposed on a seafloor, the microseismic event originating in a formation below the sensor array, and the detecting creates received data; separating the received data into up-going data and down-going data, the down-going data indicative of detected seismic waves after a first reflection of the seismic waves at a sea surface; and calculating a hypocenter of the microseismic event using both the up-going data and the first down-going data.