On Shaky Ground

During the last twenty years, ABAG, with funding from both the U.S. Geological Survey and the National Science Foundation, has developed a number of earthquake hazard maps for the nine-county San Francisco Bay Area. Those maps were last revised in the mid-1980s and resulted in the publication in 1987 of the first On Shaky Ground report. The Loma Prieta earthquake in 1989 and the Northridge earthquake in 1994 were devastating in their effects on northern and southern California. However, they have also provided us with valuable information to test the hypotheses forming a basis for those earlier maps and to develop a better understanding of the physical processes that occur in earthquakes. The maps described and shown on the following pages are the result of this research and better understanding. They are an updated version of the maps documented in the 1987 On Shaky Ground report.

This report documents ABAG's shaking hazard maps to encourage appropriate planning for and mitigation of earthquake hazards.

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.

The fact that a devastating earthquake occurred in 1906 -- the San Francisco earthquake -- is common knowledge. Larger earthquakes generally affect larger areas; the San Francisco earthquake caused extensive damage in Oakland, San Jose and Santa Rosa. More recently, the 1989 Loma Prieta earthquake caused extensive damage in the Santa Cruz Mountains, as well as in Oakland and San Francisco tens of miles away. But many moderate to great earthquakes (over magnitude 6.0) have affected the Bay Area; 22 such events have occurred in the last 160 years -- for an average of one every seven years.

Note that the level of earthquake activity in the last 15 years is closer to the period prior to the 1906 San Francisco earthquake, while the 1911 to 1979 period, when most of the Bay Area developed, is exceptionally quiet.

Earthquakes occur in the Bay Area when forces underground cause the faults beneath us to rupture and suddenly slip. If the rupture extends to the surface, we see movement on a fault (surface rupture). But strong earthquakes can occur when the fault rupture does not extend to the surface. The fault rupture of the ground generates vibrations or waves in the rock which we feel as ground shaking. Because faults are weaknesses in the rock, earthquakes tend to occur over and over on these same faults. Almost all of the major faults in the Bay Area are strike-slip faults where the rupture extends almost vertically into the ground and the ground on one side moves past the ground on the other side of the fault. Thrust faults, where ground moves over adjacent ground, are much more common in the Los Angeles area than the Bay Area because the San Andreas fault makes a large bend to the west there before heading northwest. Thrust faults in southern California are caused by this bending.

Magnitude is a measure of overall earthquake size.

Larger magnitude earthquakes generally cause a larger area of ground to shake hard, and to shake longer. This relationship is generally well understood. Thus, one principal factor in determining shaking hazard is the magnitude of the earthquake.

Seismologists now have several measures of earthquake magnitude in addition to the familiar Richter (or "local") magnitude. The Richter magnitude has difficulty differentiating the size of large and great (7-1/2+) magnitude earthquakes. To overcome this difficulty, modern seismologists use moment magnitude because it best reflects the energy released by the earthquake. The moment magnitude is proportional to the area of the fault surface that has slipped. Thus, it is directly related to the fault length. Because the models used to generate the shaking hazard maps in this report are based on fault length, they, in effect, bypass magnitude. (See Appendix A.)

Fault segments generate "characteristic" earthquakes. Some faults are weak and tend to generate earthquakes with moment magnitudes of 5 and 6. However, at least ten fault segments in the Bay Area are relatively strong and can store up enough energy to generate earthquakes of magnitude 7 or so. These stronger faults will generate these large earthquakes, not magnitude 5 and 6 events. The concept of "characteristic" earthquakes means that we can anticipate, with reasonable certainty, the actual damaging earthquakes that will occur on these fault segments. These anticipated events are the scenario earthquakes depicted in the color maps.

The probability of one of these scenario earthquakes occurring varies from fault segment to fault segment. The two Hayward fault segments and the peninsula segment of the San Andreas are felt to have, roughly, a probability of one in four of occurring in the next 30 years (Ref. 3). Other fault segments are less well understood; equivalent probabilities are being developed.

Note: No probability data is provided in Ref. 3 for the entire Hayward fault rupturing at once. However, experts at the U.S. Geological Survey working on a map to be included in the Uniform Building Code are now assuming that one in every four Hayward earthquakes will involve both segments of the Hayward rupturing at once, yielding a probability of 5-6% in 30 years (personal communication, Art Frankel).

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:

• the size (magnitude) of the earthquake (which is related to the length of the fault that ruptures);

• the distance from the site to the fault source for the earthquake;

• the directivity (focusing of earthquake energy along the fault axis rather than perpendicular to the fault); and

• the type of geologic material underlying the site, with stronger shaking occurring on softer soils

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 following table shows the 11 scenario earthquakes on 10 fault segments in the Bay Area for which intensity maps have been generated. (Both segments of the Hayward fault rupturing at once provides the eleventh scenario.) Approximate magnitudes calculated for the scenario earthquakes can aid in relating these scenarios to past earthquakes. All of these earthquakes result in areas of modified Mercalli intensities of V to X.

Moment Magnitude Based on Fault Length for Scenario Earthquakes

Source Fault                       Fault Length  (in     Moment  Magnitude of    
                                         km)                Characteristic       
                                      (See Note)              Earthquake         

Hayward                                  85.0                     7.3            

San Gregorio                             57.1                     7.1            

Healdsburg-Rodgers Creek                 56.5                     7.1            

Greenville                               53.9                     7.1            

Concord-Green Valley                     53.2                     7.1            

Peninsula Segment of the San             52.4                     7.1            
Andreas                                                                          

Northern Hayward                         49.3                     7.1            

Southern Hayward                         44.7                     7.0            

Northern Calaveras                       37.2                     6.9            

Maacama                                  32.3                     6.8            

West Napa                                24.1                     6.7            

Note : The formula used to estimate moment magnitude for each of these fault segments (from Ref. 48) is:Moment Magnitude = 5.16 + [1.12 x log (surface fault length in km)]

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.

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. (See Ref. 28.) 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. (See Refs. 25, 33, 34, 35, and 38.) 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. (See, for example, Ref. 45.) 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 map opposite groups the geologic materials in the region into eight categories, each with similar amplification in earthquakes.

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. (See Ref. 28.) Other researchers have expanded this effort by examining the relationship between intensity and geologic materials. (See Ref. 36.) Although the categories of geologic materials are the same as used in earlier ABAG maps (Refs. 41, 42, 43, and 44), 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. (See Ref. 26.)

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.

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.