What difficulty will you encounter if you only have data from two recording station?

If you have only two data from two recording stations then you will be having a hard time finding the correct location of the epicenter. This is because triangulation method requires 3 recording station. If you have 2 recording station, the 2 circles will intersect at 2 points giving you 2 locations that could possibly be the epicenter.

The seismic data are often recorded using seismograph, that records the arrival of the P and S waves during an earthquake. This seismograph is used to locate the epicenter of an earthquake. During an earthquake, the seismographs placed in the nearby regions records the data regarding these seismic waves, and generates a circle.  

When three seismographs placed at three different locations forms three circles due to an earthquake, it intersect at a common point. This intersecting point is the approximate location of the epicenter. It is a very much efficient and are commonly used.

But when there are only two recording stations, then it becomes difficult to determine the epicenter because two seismographs at two different location can form two circles after recording the data and this two circle will intersect each other at two different point. As a result of which the difficulties will arise in locating the epicenter.

we need to know the time interval during which the force acts. impulse of force = momentum change