Answer
If a single disturbance (e.g., an earthquake) suddenly emits both transverse and longitudinal waves, and if these waves travel at 2 different known speeds, then a distant observer can measure the time difference between the 2 arrivals, and calculate the distance to the source. The longer the delay, the farther the distance.
If several observers use this procedure to determine the distance to the disturbance from their own locations, the data may be combined to triangulate the source of the disturbance (in the case of an earthquake, the epicenter).