How does the landscape affect an earthquake?

Question

My friend insists on the fact that the landscape affects an earthquake, but I can not see how.

So can somebody please explain to me how the landscape affects an earthquake? (and not how an earthquake affects the landscape, how the landscape affects an earthquake)

cheers,

Demo

Answer 1

http://www.seismo.unr.edu/ftp/pub/louie/...

Most news reports of quakes use the Richter scale, which measures total energy released by an earthquake.  The Mercalli scale, on the other hand, estimates the intensity of a quake by the amount of damage in a given area.

http://www.seismo.unr.edu/ftp/pub/louie/...

Here's a link to some Mercalli maps, with special attention to the 1906 San Francisco quake.

http://www.seismo.unr.edu/ftp/pub/louie/...

As the previous poster alluded to, the amount of shaking and damage experienced by a given area can be mitigated by the type of bedrock an area is built on.  Areas built on bedrock will be better off.  Areas built on loose soil will be the worst off.

Answer 2

Mountains are much heavier than plains and can add pressure to the underlying rock strata, which if unstable, can trigger an earth-quake.

But in general, the mountains have been there for so long that all the instabilities have usually settled down long ago and the landscape has little to none effect.

It's only when mankind builds dams which add a huge amount of weight in a relatively short time, that the underlying tensions can become unbalanced and re-adjust in the form of an earth quake.

To a lesser extent, mining can have a similar effect.

Landscape also affect earthquakes by either absorbing or amplifying the traveling 'wave' the earth-quake sends through the ground.  Hard rock transmits the wave very well.  Softer rock like sandstone can absorb the wave to some extent and deep sandy soil can absorb it totally.

Answer 3

Sorry, but your friend is right! The landscape can and does affect earthquakes! Well, to be more specific, it affects earthquake INTENSITY, but it does not affect earthquake MAGNITUDE. Intensity is the strength of the shaking you feel at a particular location. Magnitude is an absolute measure of the energy released by the earthquake and is independent of the landscape. Maybe that's where your confusion is stemming from.

When you deal with earthquakes, you deal with things like wave amplitudes and frequencies. These factors depend heavily on the properties of the medium through which the waves are traveling. In fact, the medium through which the waves are propagating will tend to amplify or dampen the wave energy, depending on the dynamic properties of the material. Soil and other "soft" material tend to amplify the low frequency waves and dampen high frequency waves, while competent rock tends to amplify high frequency waves and dampen low frequency waves. What does this mean to you, the person hiding beneath a table in the middle of the earthquake? If the landscape on which you were standing was composed of soft soils such as clays and bedrock is deep, then you would likely feel strong rolling motions, like waves beneath a boat. This is because the low frequency waves are being amplified. However, if you were close to rock, you would likely feel quick herky-jerky motions. This is because the rock is amplifying the high frequency motions.

Finally, topography, or the shape of the landscape, has a significant effect on earthquake intensity as well. If you are located near the margins of a valley, then you will probably feel greater shaking then you would if you were in the middle of the valley. This is because surface waves begin to pile ontop of each other due to the decreasing bedrock depth. This is called the basin effect.

Answer 4

Landscapes don't affect earthquakes.  Earthquakes affect landscapes. Continental plate movements affect earthquakes.