Foundation, Concrete and Earthquake Engineering

Searching Faults: As Earthquake Geologic Evidence

Geologic evidence for earthquake hazard analysis is mainly focused on pointing out of faults. The techniques that are introduced by geologist are:

a. Remote sensing imagery (infrared photography)

b. Examining air photos

c. Reviewing published literature

d. Field reconnaissance with trench log

e. Boring and test pits

f. Geophysical 
Evidence of earthquake ground movement by rippling of rails
Evidence of earthquake ground movement by rippling of rails

Reiter suggested following features for identifying faults:

1. Indication of fracturing on the surface and clearly noticeable fractures on the surface which includes:

a. Slickensides

b. Fault gauge

c. Fault breccias

We know these terms are new to you; the explanations are 

Disruption of ground surface due to earthquake
a. Disruption ground surface

b. Evidence of ground movement

c. Both sides of fault is grinded

2. We know geologic strata are an essential portion of geologic map. We can study on such strata and look for presence of

a. Dissimilar materials

b. Repeated or missing strata

c. Truncation of structures or strata

The rest features will be discussed in next part where we will include:

a. Geomorphic and topographic indicators
Both sides of fault is grinded during earthquake
b. Secondary geologic characteristics

c. Remote sensing imagery

d. geophysical indicators

e. Geodetic indicators

In important and larger project with geotechnical investigation it is also done.

We are now discussing rest features as continuation of previous post as follows:

3. Geomorphic and topographic indicators. Land form is concerned in geomorphic indicator. These indicators include:

a. Offsets in drainage or stream 
Earthquake geologic evidence; offset stream channel by strike slip faulting
b. Change or tilt in elevation of shorelines or terraces

c. Sag ponds which includes water accumulation near faults (strike-slip) in depression
d. Anomalous gradient in streams

e. Topographic scraps
San Andreas Fault: Offset stream as earthquake geologic evidence
f. Triangular corner or ridges

4. Secondary geologic characteristics:

These includes

a. Juxtapositions of hot springs
Earthquake geologic evidence; Juxtapositions of hot springs
b. Changes in chemical composition, alignment and gradient of


-volcanic vents

5. Features of imagery found from remote sensing device which includes, contrast in

a. vegetation

b. topography

c. tone

6. Geophysical indicators which includes subsurface faulting. The indicators are

a. offsets on earthquake reflection horizons

b. difference in earthquake wave velocity

c. steep magnetic gradient or linear gravity

7. Geodetic indicators which include fault movement appeared as changes in distance or tilting from previous points in geodetic surveys.

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