1. May 21, 2012

### Apple96

So, I have to find the location of the epicenter of seismic activity. Of course, our project does not intend to find that of real seismic activity. This is a school project, in which my teacher will drop an object at some unknown location in a large room (our school gym), and we have to find out the location, based on the data recorded by three-four sensors we can place anywhere we want.

I researched and figured out that the way scientists find the location of the actual epicenter of seismic activity is to measure the time difference between P and S wave, which will give distance (Based on a very easy formula, d=vt, we know v for each P and S and t for each P and S) for each three sensor, and draw the circle with its diameter corresponding to its distance from the epicenter, and triangulate the location.

But, this is not a real earthquake; it's a fake one created by an object free falling on the ground made of wood.

So my question was.. first of all, is it transverse wave created by the object?

And second, how am I supposed to find the distance? The object and the floor do not create P and S wave like earthquake does. Our teacher said he will drop the object twice to create an "After-shock," which I'm supposed to use to figure out the distance from the epicenter to each three sensor. But, it does not make any sense.. scientists do not use after-shock of the earthquake to figure out the location of the epicenter.. or do they? According to my teacher, yes, but I'm completely confused..

If I know how to figure out the distance from epicenter to each three sensor, it is possible, I think, to triangulate the location.
Please help me out. This is a really important project, and I really desperately need some guide. Thank you.

2. May 21, 2012

### davenn

hi apple96
welcome to the PF

there's a number of problems you are going to encounter with what is planned
I will try and address them in order as they appear in your text

1) a --- initially how are you going to record the thumps on the floor of the gym ?
do you have geophone sensors ? if not what was the planned sensor ?
b --- are the outputs from the sensors going to a digital recording system on computers ?

2) Yes we (us ones into recording earthquakes) use S - P time to determine the distance to the epicentre. As you have pointed out the thump your teacher is going to generate, isnt going to produce a P and a S wave. So you so you have no time difference to measure

3) The P waves of an earthquake will travel at a very different speed through the earth compared to the similar style pulsed wave travelling through a wooden floor. The pulse of the thump on the wooden floor will travel at or close to the speed of sound and in an avg. density wood thats going to be ~ 3500 metres / sec

4) over the very small distance of a floor area of a gym the time taken for the pulse to travel from the point of origin to any of the sensors is going to be a tiny fraction of a second
VERY VERY difficult to time accuractly

5) with just a single pulse generated you have no time difference to measure and calculate. the teacher producing 2 thumps isnt the same as simulating a P and a S wave as in an earthquake those 2 waves start radiating at the same instant. Unlike the teacher's double thump which is going to have a time difference.
Large below ground blasts, like in a quarry produce P and S waves. I used to live close to a couple of quarries and would on a weekly basis record blasts from one or the other of them. But at 3 km to the closest quarry, I could not see any difference in arrival time between the P and S waves

6) No, your teacher is incorrect, we dont use aftershocks to determine the location of the earthquakes. Each quake in a set of quakes, main shock and all the aftershocks, each get their own locations assigned.
Logically they are all going to be in the same region, else they wouldnt be called aftershocks of the main shock.
recording multiple aftershocks are great for determining the orientation of the fault plane that produced the main event

Location is done by triangulation using a minimum of 3 sensors spread well apart and not in a line with each other. The more sensors you have recording the quake, the more accurate you can get the location of the quake.

here's an example of a triangulation of the Kobe, Japan earthquake of the early 1990's

It shows 3 circles drawn around 3 recording stations. The radius of each circle represents the distance from the recording station as a function of the S-P travel time in seconds x the standard multiplication factor for regional quakes, in this case ~ 9.5.

the 3 stations from closest to the quake to furtherest is....

Tokyo, Japan ( yellow circle); Pusan, South Korea ( blue circle); Akita, Japan ( green circle)
You can see that the 3 circles cross very close to Kobe

any other questions

cheers
Dave

#### Attached Files:

• ###### GetTheirCirclesImage.jpg
File size:
32.2 KB
Views:
211
Last edited: May 21, 2012
3. May 21, 2012

### Apple96

I'm going to talk to my teacher about the p-s wave issue today. So let's assume I know how to measure the distance for each three sensor.

But do you still see this project impossible to conduct? Based on your observation, #5, it seems like, in a small gym, where three sensors are separated 50 meters at most, the three sensors will never perceive the time difference between p-s wave, considering your sensor, 3 km away from the center, did not perceive any.

Also, about the velocity of the wave, I think I use digital recording system on computers. So is it still impossible to measure the time it takes for the wave to reach the sensor accurately? Assuming I can start all three sensors at the same time?

4. May 21, 2012

### davenn

hi Apple96

to be able to record the travel time of a single pulse you would need the accurate start time of the pulse. Now maybe if you have a sensor at the point where the pulse is originating then you may be able to measure the difference between the start time and the arrival time. Over a few 10's of metres in your stadium, its going to be a very very short time difference ( as spoken about in last post) to measure.
It will be interesting to see if you can observe a time difference

cheers
Dave

5. May 22, 2012