This is the FAQ (Frequently Asked Questions) page. We encourage suggestions for improvement in the existing answers or suggestions for other questions that you think ought to be answered on this page. The questions should be related to the probabilistic ground motion maps, their calculation, the inputs used, the use of the results, the ground motion parameters, and relationships to other maps.

Glossarys

• Simple explanations for earthquake terms (text only glossary)
  U. S. Geological Survey, Earthquake Hazards Program
• Simpler explanations for kids (picture glossary)
  U. S. Geological Survey, Earthquake Hazards Program
• Kids' illustrations/descriptions and adult explanations for earthquake terms
  U. S. Geological Survey, Earthquake Hazards Program

General questions about earthquakes can probably be best answered by the Earthquake Hazards Program FAQ website.

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General questions!

1. I am having problems logging into your FTP site. Is this my problem or yours?

2. I am publishing a book (journal article, thesis, pamphlet, poster, postcard, T-shirt, etc.), and I wish to use one of the NEHRP national hazard maps as an illustration. Who do I contact to obtain permission to do this? Or, are these maps "in the public domain" so I don't need permission?

Miscellaneous map questions!

1. For a basic description of seismic hazard maps and how they are used see USGS Fact Sheet-183-96 1996 "Hazard maps help save lives and property"

Whose fault is it?

1. Where do I need to look to find a fault map of the US? Is one available in GIS format?

2. How do I find fault or hazard maps for California?

3. Where are the fault lines in the United States east of the Rocky Mountains?

4. I am trying to locate the nearest active faults to location X, for the purpose of determining the "Near Source Factor" Nv & Na of the '97UBC. Can you steer me to a web address for this information?

5. I am looking to buy land near the location of a large historical earthquake. I am wondering where the fault line runs. What is the seismic activity in the area today? How did the quake change the contours and elevations of the area?

What is probabilistic ground motion?

1. I need a simple description of what the maps are about.

2. How do I use these maps?

3. I am trying to calculate the ground motion effect for a certain location in California. I obtained the design spectrum acceleration from your site, but I would like to identify the soil type of this location, how can I reach it?

4. What is the likelihood of a large earthquake at location X?

Why aren't these maps 0, 1, 2, 3, 4 Zone maps?

1. Why are the maps not simple 0, 1, 2, 3, 4?

2. Where do I find a 0, 1, 2, 3, 4 map?

3. How do I get zone maps for locations outside of the US?

4. What is a seismic zone?

5. What zone is location X in?

6. What is the zone number for Federal Site Z?

What do the ground-motion parameters on these maps mean?

1. What is "peak acceleration" or "peak ground acceleration" (PGA)?

2. What is "% g"? What is the relation to building damage?

3. If I were to use an accelerometer to measure seismic events, how could I convert the acceleration data (in g's) to the magnitude numbers you folks use?

4. What is "probability of exceedance" or PE?

5. What is "spectral acceleration" or SA?

6. Your map is for ground motions having a 2% probability of exceedance in 50 years. Are those values the same as those for 10% in 250?

7. A certain ground motion has an x percent probability of being exceeded in Y years. What is the probability, w that that same ground motion is exceeded in Z years? See the next FAQ.

8. Your map values show ground motions that have a probability of being exceeded in 50 years of 10, 5 and 2 percent. What is the probability of their being exceeded in one year (the annual probability of exceedance)? What is the return period of the ground motions?

9. What is the relationship between peak ground acceleration PGA and "effective peak acceleration," Aa, or between peak ground velocity and "effective peak velocity," Av, as these parameters appear on building code maps?