Trenching




Trenching of a fault scarp is probably the most common and high potential technique for investigating an active fault with surface rupture. Two conditions have to be satisfied: 1) the location of the fault scarp at the site should be known with uncertainty of no more than a few meters, and 2) the geomorphic setting of the site should favor sedimentation to make more likely the preservation by burial of the geologic records of past earthquakes.


Figure 10. Excavation of a large fault trench, Point Conception, California.

Figure 10. Excavation of a large fault trench, Point Conception, California. ( modified from Keller, 1996).


Ways followed in TRENCHING

1 – Identification of active fault or active fault segment
2 – Site – selection of trenching
3 – Making connection with local authorities
4 – Getting safe precautions
            *Shoring of trench walls
            *Construction of fence surrounding trench site
5 – Preparedness of trench walls for mapping
6 – Gridding of trench walls
7 – Marking sites of datable material
8 – Marking sites of features be mapped with coloured nails
9 – Measuring displacement along trench wall
10 – Mapping trench walls
11 – Precisely marking end points of faults
12 – Sampling and packaging of datable material
13 – Backfilling of trench
14 – Sending samples to a reliable laboratory for C14- dating




Figure 11. Trench site on the Rodgers Creek strike slip fault  (North San Francisco bay) 
(from Pantosti et al, 1993)

The fault runs across this little alluvial basin (the main fault trace is indicated by arrows) and forms a little pressure ridge clearly cut by the two trenches on the left. In order to evaluate both the horizontal and vertical components of displacement on strike-slip or oblique-slip faults, it is often useful to excavate two or more trenches.


Two sets of trenches have been opened, parallel and perpendicular to the fault, to investigate the slip along the fault by reconstructing piercing lines and recognize the occurrence of repeated faulting events as recorded in the fault zone (Figure 12)



Figure 12. Evaluation of the horizontal component of displacement by detecting buried 
channel deposit, i.e., piercing point.


Important features are highlighted by painted nails or little colored flags. A further step for the preparation of a trench for its study is the setting of a horizontal and vertical grid formed by 1 m wide squares outlined by string or twine; these serve as a reference for logging trench walls and for correlating features to other trenches at the same site (Figure 13).




Figure 13. Detail of a trench wall with flagging and reference net.
In this trench across the Irpinia fault (Southern Italy) the fault zone
appears as a warping of sediments and ground surface
(from Pantosti et al, 1993)


Event horizon is known as the stratigraphic layer that was the ground surface at the time of the earthquake (Figure 14)




Figure 14. Log of a trench opened across the 1980, Irpinia (Italy) earthquake surface rupture (from Pantosti et al, 1993).



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