Final answer:
Sandra should subtract the P-wave arrival time from the S-wave arrival time to calculate the time difference between the P-waves and S-waves. This time difference is crucial for determining the warning time before the stronger S-waves arrive and for calculating the distance to an earthquake's epicenter.
Step-by-step explanation:
To calculate the time difference between the P-waves and S-waves, Sandra should make the following calculation: S-wave arrival time minus P-wave arrival time (D). This is because P-waves (Primary waves) always arrive before the S-waves (Secondary waves), so the subtraction will give the time that residents have as a warning before the more serious shaking from S-waves begins.
As for seismographs, they measure the arrival times of earthquakes with a precision of 0.100 seconds. By comparing the arrival times of S- and P-waves, which travel at different speeds, geologists can determine the distance to the earthquake's epicenter. This is important because it can help in the detection of the source of seismic events, including underground nuclear tests, although there are limits to this method based on the precision of the wave speed measurements.