EARTHQUAKE HAZARD MAPS
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Part B: Back Ground Information
EARTHQUAKE HAZARD MAPS |
Read over the following material to gain a better understanding of the earthquake hazard maps that you will be using throughout this lesson.
The San Francisco Bay Area is in "earthquake country." In some earthquakes, the surface of the ground can rupture along a fault -- or a landslide can be triggered -- or underground sand layers may flow (liquefy) -- or a tsunami ("tidal" wave) may be generated in water. But in ALL earthquakes, the ground shakes. In large magnitude earthquakes, more ground shakes, and it shakes longer, than in small magnitude earthquakes. Ground shaking causes damage tens of miles away from the fault source.
When the ground shakes, damage occurs to buildings, facilities and their contents. People can be injured or killed. People find that they may no longer be able to sleep in their homes, or even have access to their belongings. Businesses can't function and segments of the economy suffer. Hazardous materials are released which can be damaging to people and the environment.
Various options are available to avoid, reduce or otherwise mitigate these results. What YOU do to prepare for shaking can minimize or eliminate these effects.
Most earthquake damage is caused by the shaking of the ground itself. Yet, at the same time, many existing local and State government hazard reduction programs and regulations focus on other earthquake hazards. Our purposes in preparing this booklet are to expose ground shaking as a significant hazard, to show (using maps) the areas with the strongest expected shaking, and to suggest ways to mitigate shaking damage.
Intensity is a measure of the effect of the earthquake at a specific location.
An earthquake has one moment magnitude, but a range of intensities. The most commonly used intensity scale is the modified Mercalli intensity scale (MMI scale). The intensity of ground shaking at a site varies for any particular earthquake based on several factors:
"Just as a light bulb above my desk is 100 watts regardless of where I'm sitting, and the intensity of the light varies with where I am in my office, an earthquake has a single moment magnitude and a variety of intensities distributed throughout the region."
Jeanne Perkins
The epicenter is the point on the surface above the location where the fault begins the slip which generates the earthquake. There is a common myth that most damage will occur near the epicenter of the earthquake, or that the epicenter is synonymous with "ground zero." However, the earthquake epicenter is typically not the point at which most damage occurs. The fault rupture can be tens of miles long and waves are generated along the entire length of the fault.
In general, areas closer to the source fault will be shaken more than areas further away.
Thus, predictions of ground shaking intensities are not based on distances from possible epicenters, but on distances from known faults, or segments of faults, on which large earthquakes are anticipated.
Intensity decreases ("attenuates") with distance from the fault. But the critical distance is not simply the nearest distance to the fault. Seismologists have come to realize that earthquake sources radiate energy at depth; thus, the distance used to attenuate expected shaking must be measured between the site and this underground source. However, rupture propagates both upward from this underground source and along the fault axis. (This "directivity" effect is described in the next paragraph.) Thus, there is significant amplification of shaking within a mile of these major fault zones.
Directivity, or focusing of energy along the fault in the direction of rupture, is a significant factor for most large earthquakes in the Bay Area, including the Loma Prieta earthquake. Shaking intensity decreases ("attenuates") much more rapidly perpendicular to the fault rupture plane (or surface fault trace) than along the fault axis. Thus, San Francisco and Oakland, in line with the fault axis, felt stronger shaking than expected in the Loma Prieta earthquake, while San Jose, perpendicular to the fault, felt weaker shaking. The directivity varies with the location of the epicenter. The maps show an "average" directivity since we do not know the location of the epicenter prior to an earthquake. (See Appendix A and Note below.)
The final factor affecting the change of intensity with distance from the fault is the magnitude of the earthquake. The intensity boundaries extend further from the fault source for larger magnitude earthquakes. Thus, a site 20 miles from the fault source will experience stronger and longer shaking from an earthquake with a moment magnitude of 7 than from an earthquake with a moment magnitude of 6. Even though the energy released in an earthquake is over thirty times as great in a magnitude 7 quake than a magnitude 6 quake, the shaking is not 30 times as intense. Rather, a larger area is exposed to strong shaking.
Note: One additional factor in the recent Loma Prieta earthquake was the reflection of the seismic waves from the Moho. (The "Moho" is short for the Mohorovicic discontinuity, the boundary between the earth's crust and mantle, and is named for the Croatian scientist who discovered it.) This "bounce" resulted in stronger shaking which ranged from 45 to 60 miles from the fault trace and amounted to somewhere between one-half and one intensity increment level increase over what might have been expected. Both Oakland and San Francisco were within this distance band. However, there are insufficient data to reliably calculate such increases for future earthquakes. Because the Loma Prieta earthquake began deeper than is typical for Bay Area earthquakes, this Moho-related increase was probably closer to the fault source than would be expected in future Bay Area earthquakes. Thus, the increase, if it occurs, will be in areas with lower baseline shaking levels and should result in small or insignificant increases in damage.
All ground in the Bay Area was not created equal. A critical factor affecting intensity at a site is the geologic material underneath that site. Deep, loose soils tend to amplify and prolong the shaking. The worst such soils in the Bay Area are the loose clays bordering the Bay -- the Bay mud -- and the filled areas. The type of rock that least amplifies the shaking is granite. The remaining materials fall between these two extremes, with the deeper soils in the valleys shaking more than the rocks in the hills. Most development is in the valleys.
The role of geologic materials in affecting the intensity of shaking has been known for at least twenty years. Several researchers at the U.S. Geological Survey clearly demonstrated this relationship when they examined data from the 1906 San Francisco earthquake in 1975. Other researchers have expanded this effort by examining the relationship between intensity and geologic materials. Although the categories of geologic materials are the same as used in earlier ABAG maps , the extent to which these materials modify the shaking intensity has been changed slightly. These susceptibility categories are quite similar, but not identical, to the categories recently developed for use in site-dependent building code provisions.
The distance-based intensities mapped for each scenario earthquake are increased or decreased based on the shaking amplification potential of each geologic material to produce the final intensity map for each scenario. The extent of these changes ("intensity increments" or fractional changes in intensity units) is listed in Appendix B of On Shaky Ground.
If you compare two houses, both of which are the same distance and orientation to the earthquake source, the one on Bay mud will experience stronger and longer shaking than the one on rock.
Any future modifications or updates of these maps are likely to be available on the Internet prior to "hard copy" publication.
These intensity maps are not intended to be site-specific. Rather, they depict the general risk within neighborhoods, and the relative risk from community to community. Individual intensities can easily be incorrect by plus or minus one intensity unit.
Several cities have copies of the critical earthquake scenario intensity maps for their area. You can also examine or obtain copies of maps for individual areas through ABAG.
Source: Association of Bay Area Governments
http://www.abag.ca.gov/bayarea/eqmaps/doc/contents.html
Copyright © 1997 Regents of the University of California
