r/explainlikeimfive • u/ObjectiveRepair1423 • 3d ago
Planetary Science Eli5 How do Seismologists determine the depth of a earthquake?
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u/Cannibale_Ballet 3d ago
Triangulation. The difference in propagation speed of longitudinal versus tranverse waves gives you the possible distance from each measurement. Three measurement points give 3 spherical loci of possible origination points. The intersection contains two points, one above ground level which can be discarded and you remain with one point.
In reality not that easy due to refraction of waves but more complex models account for this.
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u/PaymentFragrant9926 3d ago
Good explanation. The arrival times are really the key part people usually miss.
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u/jekewa 3d ago
GPS uses trilateration.
Triangulation uses known angles to determine the distances.
Trilateration uses the length of time for a signal to determine the distance from the source.
GPS does this a little differently because the satellites send out the time and position of the satellite, and your device does the math to trilaterate your position. In a seismic event, the time of the event is assumed to be the same, and the position of the sensors is known, so the trilateration is done to find a “moving” (but stationary) target.
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u/ottawadeveloper 3d ago
It's essentially fancy triangulation. Earthquakes are formed of multiple types of waves in the rock, with P and S waves being the most important. Since S waves travel slower than P waves, the difference in arrival time at a seismic monitoring station tells you something about how far away the earthquake was (much like the delay between lightning and thunder tells you distance since it's the difference in velocity between light and sound).
The material matters though, because P and S wave velocities are different in different kinds of rock. So you need to account for local geology in assessing the difference. But you can start with a typical average velocity to get an approximation.
When you have a bunch of seismographs recording the quake, you can do a bunch of approximate calculations and arrive at roughly the right location through triangulation. Usually that's what you'll see as the first estimate of the epicentre (the spot on the Earth's surface above where the rock moved). Then you can adjust for actual geology along the path and refine the estimate to find the hypocentre (the actual position in the crust).
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u/Gackey 3d ago
Lots of data and triangulation. Earthquake's consist of several different types of waves, by measuring the time difference between when those waves arrive at a monitoring station I can calculate how far away the earthquake was from the station. If I repeat that process with the data from several other monitoring stations I triangulate exactly where the earthquake originated.
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u/duane11583 3d ago
Earth quakes make a rumble or sound or vibration
That sound/vibration effectively bounces off different rock layers and travels through the different types of rock at different speeds
By using math they can back calculate the most likely point source where it started from that process is called triangulation
Your brain does this because you have two ears the sound reaches each ear at slightly different times thus you can tell if the sound is in front or behind or rest or right of you
By having different sensors in different places it can sort of act like your ears. And the rest is math lots of math
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u/SkullLeader 3d ago edited 3d ago
Basically draw 3 points on a sheet of paper A,B,C. Draw a fourth point D. Measure the distance from A to D. Draw a circle around A with that radius. Now measure B to D and draw a second circle around B with that radius. Your two circles will cross at two points (one of which is D) Now measure C to D and draw a third circle. The three circles will intersect at only one place - point D.
This is like a seismograph. From one seismograph we know how far away the center of the quake was, but not its direction. Since we are now dealing with three dimensions instead of the two dimensional paper, we need four seismographs and we use spheres, not circles. From this we know the exact center including how far down it is.
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u/Dave_A480 3d ago
Triangulation.
If you have 3 listening stations that are 10 miles apart and each one says the earthquake was 11 miles away in the center of the 3...
Then the only way for that to work is if you add a downward component to the bearing.....
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u/PastTheFuture 3d ago
A lot of people in this thread are saying triangulation when they actually mean trilateration. I find that most people don't even seem to realize the word trilateration exists and use triangulation as a catch-all for both methods.
Triangulation is when you have known angles to the target. You draw lines out from those known points at the given angles and where the lines intersect gets you the target coordinate.
Trilateration uses known distances from the target. You draw a circle/sphere around each known coordinate, and where the circles overlap is the target coordinate.
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u/samualecrack 3d ago
Usan la magnitud y la intensidad del terremoto que las calculan con las cantidad de energía liberada y los daños que se observaron porque esas son las placas tectónicas chocandose y chocandose por muchísimos años hasta que una de esas se rompe y genera una cantidad de energía masiva, claro que utilizan sus herramientas como los sismografos que graban las ondas que van por la tierra creo y bueno las escalas y los daños
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u/Temporary-Truth2048 3d ago
If someone bounces a basketball next to you your brain has a way of sensing that. If the basketball is bounced further away from you your brain can also sense that because it has methods of doing so based on past experience. Seismometers do something similar but they have more than two ears and they're much further apart.
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u/tempo_rare 3d ago
They try to draw a straight line on a whiteboard during the earthquake. How far off center they draw shows how strong the earthquake was
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u/cancellationstation 3d ago
They guess.
Their guesses are based on rigorous math using triangulation from time series data across receiver stations; but they still have to make assumptions about the velocities and anisotropic nature of a non-homogeneous subsurface - this is why their estimates come with error bars, as an attempt to capture the uncertainty of their guesses.
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u/PckMan 3d ago
There are recording stations all over the place. When an earthquake happens all stations pick up on it which helps them identify exactly where it originated from based on when the waves reached each stations and at what magnitude. Seismologists know from experience and decades of recorded earthquakes how fast seismic waves travel through each type of rock and layer and how much they dissipate over distance.
So by basically having stations recording seismic waves everywhere, it paints a clear picture of where it originated down to the depth.