Earth Science? I am SOOO confused. Help with this one question?

[mark v]

Okay, assuming that they aren't interacting...

The local geology of the area affected by a seismic event changes the way that seismic energy is transmitted to a structure.

For instance, if conditions are right, a phenomenon called "liquefaction" can occur. This makes loose sediments suddenly go from behaving like a solid rock to behaving like a liquid. Obviously, if you try to put a building on a liquid, it isn't going to stand up very well. Even if you just put it on squishy mud, that won't be good for it (which is why the Leaning Tower of Piza is falling over - the only reason that it isn't destroyed by now is that people keep adding heavy counterweights).

The terrain, together with geology, will affect the damage. If you're in a mountainous area, you could theoretically have a large landslide damage the city.

Construction of the city itself can be a cause of destruction. If an older infrastructure is in place, gas mains will more easily be broken, with more chance of starting fires - fires which often cannot be put out, because the fire hydrants are useless, because the water mains are broken as well.

Surface waves are far more devastating than body waves (P, S). Rayleigh and Love waves are the two types of surface waves. Rayleigh wave energy causes a complex heaving or rolling motion, while Love wave energy causes a sideways movement. The combination of Rayleigh and Love waves results in ground heave and swaying buildings. Surface waves cause the most devastating damage to buildings, bridges, and highways.

The orientation of the fault, direction of fault movement, and size of an earthquake can be described by the fault geometry and seismic moment. These parameters are determined from waveform analysis of the seismograms produced by an earthquake. The differing shapes and directions of motion of the waveforms recorded at different distances and azimuths from the earthquake are used to determine the fault geometry, and the wave amplitudes are used to compute moment. The seismic moment is related to fundamental parameters of the faulting process. All of that means that you can have a smaller earthquake, but with different forces acting on each different wave type - at Point A, the surface wave types (the more damaging) may be amplified more than than a larger magnitude earthquake at Point B (perhaps more energy, but in less dangerous waves).

The increase in the degree of surface shaking (intensity) for each unit increase of magnitude of a shallow crustal earthquake is unknown. Intensity is based on an earthquake's local accelerations and how long these persist. Intensity and magnitude thus both depend on many variables that include exactly how rock breaks and how energy travels from an earthquake to a receiver. These factors make it difficult for engineers and others who use earthquake intensity and magnitude data to evaluate the error bounds that may exist for their particular applications.

An example of how local soil conditions can greatly influence local intensity is given by the catastrophic damage in Mexico City from the 1985, magnitude 8.1 Mexico earthquake centered some 300 km away. Resonances (shaking) of the soil-filled basin under parts of Mexico City amplified ground motions for periods of 2 seconds by a factor of 75 times. This shaking led to selective damage to buildings 15 - 25 stories high (same resonant period - resonance is ringing like a bell; the buildings "rang"), resulting in losses to buildings of about $4.0 billion and at least 8,000 fatalities.

The occurrence of an earthquake is a complex physical process. When an earthquake occurs, much of the available local stress is used to power the earthquake fracture growth to produce heat rather that to generate seismic waves. Of an earthquake system's total energy, perhaps 10 percent to less that 1 percent is ultimately radiated as seismic energy. So the degree to which an earthquake lowers the Earth's available potential energy is only fractionally observed as radiated seismic energy.

[linlyons]

maybe point A is landfill, and the ground is far less stable.

http://www.google.com/search?&q=loma+prieta+earthquake+marina+district

[sharon]

Because the bigger the number the higher the magnitude so the more damage done. A earthquake with a magnitude of 5.9 is stronger than a earthquake with a magnitude of 5.0.

[Kowalski]

Point A is in the center of a city. Point B is in the center of a prairie wheat field.