# Homework Help: Analyzing a seismogram

1. Nov 18, 2015

### RJLiberator

1. The problem statement, all variables and given/known data
A recent earthquake occurred in SE Mexico and was recorded in Ohio.

1. Which Component (BHZ: Up-Down, BHE: East-West, BHN: Noth-South) of hte seismogram should we look at for the best recording of the P wave?

2. Which Component (BHZ: Up-Down, BHE: East-West, BHN: Noth-South) of hte seismogram should we look at for the best recording of the SH wave?

2. Relevant equations

I know that P waves propagate in every direction and SH waves are the s waves that are horizontal.

3. The attempt at a solution

From the given seismograms I choose for P waves BHE and for SH waves I choose BHZ.
I understand this problem is dictated by location from earthquake in SE Mexico to Ohio, but I can't understand how to look at it. If that were the case, wouldn't SH wave be best East-to-west (BHE) instead of BHZ?

2. Nov 19, 2015

### davenn

yes, the P waves do propagate al all directions, BUT their amplitude in a given direction is very dependant on the rupture direction of the fault
relative to where the recorder is. Consider these 2 fault motions in Mexico and the propagation of the P waves
which event is going to produce the largest P waves in Ohio ?

considering the latitude difference and angle separation between source and recorder, you didn't choose the other axis

but it gets a little more complicated than that and looking at ray diagrams gives a better indication .....

for any recorder at significant distance from the quake, what does that diagram tell you about the
orientation of the waves arriving at the P and S or PP and SS locations ?
and which sensor will record the incoming waves the best at those locations ?

cheers
Dave

3. Nov 19, 2015

### RJLiberator

Hm. Wouldn't they be the same still? Horizontal and vertical, if the P-waves propagate in all directions.

4. Nov 19, 2015

### davenn

no, look at the angle the P and S waves are arriving at the P, S and PP, SS locations. Which sensor is going to record the waves the best ?
Z, E or N ?

Hint, at a reasonable distance, around 500km or more, the near surface P and S waves are going to be considerably diminished in amplitude. Instead, those following those ray paths are going to be dominant.
I see this time and again in seismograms from my own and other stations recordings.

Dave

Last edited: Nov 19, 2015