If the arrival time difference between P and S waves is zero, the epicenter of the earthquake is directly beneath the seismograph station.

Here's why:

* P waves (primary waves) travel faster than S waves (secondary waves).

* The difference in arrival time between these waves increases with distance from the earthquake's epicenter.

* If the difference is zero, it means both waves arrived simultaneously, indicating the epicenter is right below the seismograph.

However, it's important to note that this scenario is highly unlikely in real-world situations. Earthquakes rarely occur directly below a seismograph station.