Frequently Asked Questions About ABAG’s Shaking Scenario Maps

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1. How likely are these earthquakes?

On the basis of research conducted since the 1989 Loma Prieta earthquake, the U.S. Geological Survey (USGS) (USGS) has published a report concluding that there is a 63% probability of at least one magnitude 6.7 or greater quake, capable of causing widespread damage, striking the San Francisco Bay region before 2032. Thus, a major quake is about twice as likely to happen as not to happen in the next 30 years.

This overall regional probability is broken down by fault system on the adjacent map, with probabilities for individual fault systems shown in the smaller boxes. As is shown, many earthquake faults realistically generate these large earthquakes and the faults are located throughout the Bay Area. Probabilities for individual fault scenarios are available from USGS report on earthquake probabilities.

The California Geological Survey and USGS have also produced a probabilistic shaking hazard map of California that shows the probability of a variety of shaking accelerations being exceeded over the next 50 years throughout California.

2. What’s the difference between the shaking intensity maps and the geologic materials shaking amplification map?

The shaking intensity maps show how violently the ground will shake if there is an earthquake on the particular fault specific to that map. The geologic materials shaking amplification map shows how much the ground will amplify the shaking caused by an earthquake on any fault. In most cases, the shaking intensity map is the appropriate map for you to use.

3. Why don’t you have a north arrow and a scale on these maps?

The interactive map has both a north arrow and a scale when viewed or printed. The software used to create the maps that are not interactive does not have this capability. Because the rasters used in the model (the small squares visible on the maps) are 100 meters by 100 meters, you can easily determine one kilometer by counting 10 raster-widths. The up-down border of these rasters is also exactly north-south.

4. Why do you say that these maps are neighborhood, not site specific, when I think I can find my house?

First, the intensity maps are typically gradational in nature. The difference between being on one side of the dark-red to light-red border is really not significantly different than being a couple of blocks into either zone. The one exception to this may be areas of Bay mud.

Second, the maps are based on a theoretical model that has been tested and calibrated using damage data from past Bay Area earthquakes. Although the models used to generate these maps are statistically better at replicating damage patterns in past earthquakes than earlier models, they are far from perfect.

Next, the streets themselves are not perfectly registered.

Finally, the model itself is run on 100 meter by 100 meter (one hectare or 2.471 acre) rasters. These are the small square areas visible on the city maps. These are the ultimate resolution of the maps.

5. Does ABAG have any data on the relationship between shaking and building damage or road closures?

Yes. There is some information on building damage in various intensity categories based on data from past earthquakes in the On Shaky Ground report. Additional information on housing losses for scenarios is on the Housing Losses page. Additional information on road closures for these scenarios is on the Infrastructure Losses page.

6. What is the difference between the Interactive maps and the regular maps?

The interactive software allows you to zoom by city, county, or even an address. A scale and north arrow automatically appear. Once you have zoomed into the area you want, you can “print” the map. When this option is selected, the software creates a pdf of the map that you can save to your computer or print. If the zoom capability does not work, it is probably because you are not allowing pop-ups for the ABAG site. The pop-ups are required by the software to display the maps properly, as well as to create the pdf map prints. We do not put ads on this site.

The regular maps use an older software that only allows you to look up pre-selected maps for cities or the entire Bay Area. In larger cities, such as Oakland and San Jose, the resulting maps can be difficult to read. However, this viewing format is better option for those with slow internet connections. Sometimes traffic on ABAG’s website is significant and the interactive maps do not display properly. During those times these maps also are an option.

Last updated: 09.17.2014