These faults are said to be nearly vertical, so if the epicenter of an aftershock is on the fault, the hypocenter should be within the fault.

If an earthquake is outside of the fault zone, it will not produce fault zone trapped waves. In this way, we can test our method of identifying FZTW because if we see FZTW in earthquakes that are off the fault, we know that we may be mistaking other wave packets for FZTW.