How Geologic Materials Affect Shaking Intensity
	Based on ABAG "On Shaky Ground" Reports

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.

 

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.

The role of geologic materials in affecting the intensity of shaking has been known for almost 30 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 of On Shaky Ground.

 

Can more recent mapping of the flatlands geology materials in the Bay Area's valleys be used to improve the quality of shaking hazard maps?

Most of the people in the Bay Area live and work in the valleys. Materials in these valleys tend to amplify the strong ground shaking more than materials on hillsides. Revised and more accurate mapping should lead to better maps modeling past earthquake damage, and therefore the severity of shaking in future earthquakes.

William Lettis & Associates is working with the U.S. Geological Survey to improve the quality of mapping oof materials in these areas.

ABAG researchers and others have started to evaluate to what extent the newer mapping is a better predictor of past and potentially future shaking severity and damage patterns. The status of that effort is described in Appendix C of the "REAL Dirt on Liquefaciton" report.

FIGURE - Relationship Between Categories of Young Geologic Materials in Flat Areas Currently Used for Shaking Hazard Maps and Those Being Examined for Possible Use in Future Revisions

Young Geologic Materials in Flat Areas Used Through 2003 Source: Ref. 14

Young Geologic Materials in Flat Areas for Possible Future Use Source: Knudsen and others, 2000

Qu - Undivided Quaternary alluvium af - Artificial fill Qhsc - Holocene stream channel deposits Qhbm - Holocene Bay mud Qhs - Holocene beach and windblown sand Qhaf - Holocene fine-grained alluvium; fan and plain (basin) deposits Qhafs - Holocene fine-grained alluvium; fan and plain (basin) deposits - salt-affected Qhac - Holocene coarse-grained alluvium; fan and basin deposits Qham - Holocene medium-grained alluvium; fan and plain deposits Qpa - Pleistocene alluvium, undivided Qps - Pleistocene sand, including Merritt sand Qpea - Early Pleistocene alluvium Qpmt - Pleistocene marine terrace deposits

Qha - Undivided Quaternary alluvium af - Artificial fill Qhc - Modern stream channel deposits Qhbm - Holocene Bay mud deposits; includes channels Qhs - Holocene dune and beach sand Qhb - Holocene basin deposits Qht - Holocene terrace deposits Qhf - Holocene alluvial fan deposits Qhl - Holocene alluvial fan levee deposits Qa - Late Pleistocene to Holocene alluvium, undifferentiated Qf - Late Pleistocene to Holocene alluvial fan deposits Qpa - Pleistocene alluvium, undifferentiated Qpf - Late Pleistocene alluvial fan deposits Qps - Late Pleistocene dune and beach sand, including Merritt sand Qmt - Pleistocene marine terrace deposits Qoa - Early or middle Pleistocene alluvial deposits

jbp 10/15/03