The Problem:

This is a seismograph from the relatively recent earthquake in NZ. The arrival time for the P and S waves are indicated (there are two different S waves but you can use the first one). This record of seismic wave arrival was recorded at South Karori in New Zealand. Assume that the speed of the P wave is 5 km/s and the S wave is 3 km/s.

Here are some questions about this plot:

• What is the correct "thunder and lightning rule" for this country (s/mile)?
• What are the units on the scales shown? The horizontal axis is not labeled but explain your thoughtful educated guess.
• Show me your equations that describe the time it takes for the P and S waves to arrive at South Karori from Christchurch. Show me that the units of each term are appropriate and lead to a time in seconds.
• What does the plot say the difference in the arrival times for the P and S waves is?
• How far away from Christchurch (the epicenter of the quake) is South Karori (solve the math)? I'm tempted to make you hunt for it on a map.