I attempt each quarter to get this through the heads of each and every one of my students: always pay attention to any geologic hazards before purchasing a property. Look at the land around you, and the type of environment you are in. There are some places that most people should just not live in!
1) Settlement due to loose fill soils, quick clays, organic-rich soils, etc. As the organics in the soil underneath the house decay, the soil will settle, causing damage to the house. This is hard for the layperson to detect; your best bet is to talk with realtors and neighbors, especially in areas with a lot of new construction. Having seen the crap most contractors will try to use as structural or non-structural fill, I'll always be careful looking at houses built on or near steep artificial slopes.
However, it's not impossible to remediate a settling house. There are a variety of reinforcement measures, including pin piling, helical anchors, and cast piles. However, all are expensive to install.
2) Earthquake-induced liquefaction. If you live on the West Coast anywhere in the flood plain or on reclaimed land (I.E. the Marina district of San Francisco), you could be at risk of having the ground liquefy and settle underneath you in an earthquake. Basically, the water in the underlying soil helps to contribute to the overall stability of the site. Shake it, the water gets driven off, and the soils, without the benefit of pore water pressure, will settle into a more dense configuration, taking your house with it. Areas with shallow water tables and loose sediments are most vulnerable.
Most states, counties, and cities are required by law to have an inventory of liquefaction and seismic hazards in their jurisdiction. Contact your local Planning and Urban Development department for more information.
3) Earthquake damage (non-liquefaction). Not much you can do about this. As usual, the West Coast is at the highest risk; however, the New Madrid Fault zone runs across the central Mississippi Valley, through Arkansas, Missouri, Tennessee, southern Illinois, and western Kentucky. If you're in a high-risk seismic area (once again, check your local planning office, or consult a geologist), seriously consider purchasing earthquake insurance, if you can get it.
4) Steep slopes. People have a habit of building next to very steep slopes. There are reasons that slope is there; they are called erosion and mass wasting. I personally would avoid any house near a significant slope. However, if you're set on it, look for the following things:
a. Is there any disturbance on the slope? Downed trees, 'pistol-butting', raw exposed soil, etc.
b. Where is the water? Is is streaming out of cracks or springs on the slope? That's bad.
c. What is the toe of the slope like? Is it stable, or is it next to the ocean or a river. Undercutting of the toe is a common cause of landslides.
d. How far back is the house from the edge of the slope break? Most cities and counties have a requirement for the minimum distance a house must be from the slope break (setback).
If you choose to buy a house near or on a steep slope, you MUST take the following steps to ensure long-term stability.
a. Keep water off the slope! Make sure all roof gutters and runoff from impermeable surfaces is tightlined to the storm sewer, and not drained onto the slope. Make sure your property is graded so that water runs away from the slope break, instead of towards it.
b. Keep the slope well-vegetated! Vegetation, especially a combination of extensive ground cover with deep-rooted trees, has a very positive effect on slope stability.
c. If you're on septic, maintain the system. Don't allow any sort of a pressure head to build up on the upslope side.
d. During periods of extended or heavy rain, keep an eye on the slope. Just in case.
e. Don't let the downslope neighbors mess with the toe of the slope. Ever. Even if you have to take them to court.
Once again, check with your local government officals to determine what they consider a steep slope, where the problem areas in your neighborhood are, and what the legal setback requirements are.
5) Landslides and rockfall. This one is a pretty easy one. Don't buy a house next to steep cliffs of punky looking rock or underneath an avalanche chute. Avoid houses on existing mapped landslides, even if the seller tells you it's been 'remediated'. Once again, contact the city / county for the location of known mapped landslides.
6) Wildfire. This used to be a distinctly Western US problem, but I've heard of it in the South too. I lived in Southern California, and I know people who have been burned out by brush fires. Wildfire really becomes a problem when you live in rugged terrain that dries out during the summer. Often, it's tough to get in and out to fight the fire. Plus, fires move fast, and there's always a chance you won't have time to escape. Talk to the local fire department during the home shopping process. Get their feedback about what areas are more hazardous to live in from a wildfire standpoint.
Finally, if you do choose to live in an area where wildfires are a potential hazard, I'd recommend the following:
a. Always have 'go' bags with essential supplies such as clothes, medicines, and copies of important documents.
b. Always keep the area around your house free and clear of brush and combustible debris. Take whatever the local fire department suggests as a suitable buffer zone and double it.
7) Radon. Somebody already mentioned this.
8) Sinkholes and Karst Terrane. Karst occurs when acidic groundwater dissolves away limestone bedrock. It results in a landscape pitted with shallow lakes, caves, and occasional collapse sinkholes. You can't get insurance (as far as I know) to cover this, so be careful where you buy. Karst is common throughout the South, from east Texas across to Georgia. Much of Florida is also underlain by limestone, which is vulnerable to dissolution.
The good news is that if you're really worried about karst, it's fairly easy to detect whether or not there is a problem. Call your local consulting geologist or geophysicist. We can come out with various instruments (ground-penetrating radar, microgravity, electrical imaging), and give a fairly confident 'thumbs-up', generally in less than an hour. Costs money, tho. Once again, contacting the county or city would be a good move before buying.
9) Floods. Don't buy houses within the 100-year floodplain. Unless you're ready to take the risk of getting flooded out. It's that simple.
The Federal Emergency Management Agency has published a series of flood hazard maps that insurance companies use to gauge premiums for flood insurance. Most libraries have copies of these maps. If not, the city or county planner will have some. It's a good thing to look at before you close.
Keep in mind that many of the events described above occur infrequently, and maybe not at all in your lifetime. However, they can be catastrophic, and are generally not covered by standard homeowners insurance. It's become more and more difficult (and expensive!) to obtain earthquake and slide insurance, as when claims do occur, they not only occur in bunches, but also tend to be for total losses.
Finally, if you have the money to spend, there are professionals out there who specialize in pre-purchase site assessments (I'm not one of them...I mostly worked construction). Costs range from $500 - $1500, depending on time involved and the desired end product. If you're looking at a $300,000 - $500,000+ house, $500 may be a good investment. YMMV.
Note: I am not a licensed geologist in any state yet, so this is for information purposes only. Still a git (G.I.T)....