Frequently Asked Questions (FAQs) About ABAG's Shaking Hazard Maps

1. How likely are these earthquakes?

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

3. Why don't you have a north arrow and a scale of some sort on these maps?

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

5. The legend on these maps has changed since 1995. Are these a different type of map?

6. Where is the boundary between the North Oakland and East Oakland maps when I view the earthquake scenario maps by city?

7. Where are the boundaries between the four San Jose maps when I view the earthquake scenario maps by city?

8. Does it do any good for me to complain about a particular unincorporated area not being available?

1. How likely are these earthquakes?

On the basis of research conducted since the 1989 Loma Prieta earthquake, the U.S. Geological Survey (USGS) has published a report concluding that there is a 62% 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 sytems 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 as part of a larger 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.
3. Why don't you have a north arrow and a scale of some sort on these maps?
Because the rasters used in the model (the small squares visible on the city 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. The NEW interactive (GIS) maps contain both a north arrow and a scale when printed. North is "up" on the inactive display.
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. Thus, 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. The KEY (legend) on these maps has changed since 1995. Are these a different type of map?
No, the maps still show the same type of shaking hazard. The legend on current maps describes shaking severity level. This differs from the legend on maps published as part of the original On Shaky Ground report in 1995, which described damage level. Click here for more information about this change.
6. Where is the boundary between the North Oakland and East Oakland maps when I view the earthquake scenario maps by city?
North Oakland includes the Montclair, Piedmont, and Lake Merritt areas, and everything northwest of Lincoln and Fruitvale avenues. East Oakland includes Joaquin Miller Park area and everything southeast of Lincoln and Fruitvale avenues. There is some overlap among the maps. The NEW interactive (GIS) maps do not subdivide cities, so this problem has been eliminated.
7. Where are the boundaries between the four San Jose maps when I view the earthquake scenario maps by city?
North and Central San Jose includes downtown, Santa Clara, Milpitas and everything west of 680 and north of 280. Northeast San Jose includes everything east of 680 and north of Tully Road. Southeast San Jose includes everything east of the Almaden Expressway and south of Tully Road. Southwest San Jose includes Los Gatos, Campbell, Monte Sereno and all of San Jose west of the Almaden Expressway and south of 280. There is some overlap among the maps. The NEW interactive (GIS) maps do not subdivide cities, so this problem has been eliminated.
8. Does it do any good for me to complain about a particular unincorporated area not being available for the city scenario maps?

The NEW interactive (GIS) maps can zoom into any part of the Bay Area, so this problem has been eliminated.

ABAG, the Association of Bay Area Governments, is the regional planning and services agency for the nine-county San Francisco Bay Area.
This file was last updated on 7/1/08 by jbp.