I'm seeing lots of confusion between earthquake magnitude, shaking and damage. Here's a simple outline:
Earthquakes happen because of slip across a fault that produces shaking as one of its effects. The total energy released is the magnitude. One number per quake. 1/7
Earthquakes happen because of slip across a fault that produces shaking as one of its effects. The total energy released is the magnitude. One number per quake. 1/7
Every point on the quake's fault produces energy, so the bigger the fault, the bigger the quake. Magnitude depends on the fault length, width, and the amount of slip. Both width and slip tend to scale with length, so you can guess the length of the fault from the magnitude. 2/7
The fault starts to move at a hypocenter and then moves down the fault at ~3 km/sec. A 3 km fault (M~5) breaks in 1 sec. A 300 km fault (~M8) takes 100 sec to break. So the duration of the quake depends on magnitude. Bigger quakes last longer and affect a larger area. 3/7
Shaking at a site is described by MMI Intensity, written with Roman numerals to make it clear it's not the magnitude. It is a description of how much damage. MMI VII might be from 4 seconds of strong shaking from a nearby M6 or 30 seconds of weaker shaking from a distant M8 4/7
Shaking is strongest near the fault and dies off with distance - in any direction. If the fault is 40 km deep, it is at least 40 km away from any building. Deeper quakes are not weaker than shallow quakes, they are just farther away. 5/7
Damage is caused by shaking and by secondary hazards - landslides, ground slumps and liquefaction. Structures can withstand very strong shaking if we are willing to spend a little more to make them very strong. 6/7
Ground failure, especially liquefaction, causes a lot of damage to infrastructure like roads and pipes. The #anchoragequake produced strong shaking that caused liquefaction and buckled the roads. But most buildings did well because the followed the seismic building codes. 7/7
