Geological

Geomorphological data : Faulted landforms

A landform created prior to the last surface-rupturing earthquake at a site may preserve a record of the rupture, such as:

ØStream terraces

Offset streams

ØMarine terraces

Stream terraces

Terraces consist of broad surfaces, sometimes datable, that can be particularly useful in studying paleoearthquakes.

Sequence (A) shows the development of two river terraces (1,2) that are subsequently faulted (A3). Sequence (B) is more complex.
Terrace 1 is faulted (B2). Following faulting, terrace 2 forms (B3), and finally the sequence is faulted again (B4).
Because faulting occurred at two specific times, the fault scarp for terrace 1 is higher than that for terrace 2.
This illustration (B4) shows a multiple-event scarp on terrace 1. (From McCalpin, 1987)

Strike-slip faults with little or no vertical component of motion will not cause large vertical deformation of terraces or sub-horizontal surfaces of other landforms. Figure 2 shows several offset or deflected streams along the Santa Cruz Island fault in Southern California. This fault is left-lateral and so the streams are displaced to the left, if you were to follow the stream, either upstream or downstream, you would have to turn to the left to follow the channel across the fault

Marine terraces

Like stream terraces, the broad, sub horizontal surfaces of marine terraces are useful for measuring vertical fault motions and estimating the age of paleoearthquakes. Figure 3. shows a series of five such terraces on Middleton Island, Alaska.All five terraces on a 1947 photograph predate the 1964 (M = 8.25) earthquake which uplifted the coast 3.5 m at that site and produced a sixth terrace.

Figure 3. Drawing from an aerial photograph (1947) of the southeastern end of Middleton Island, Alaska.
The island emerged approximately 4.9 ka, and the terraces were each formed by 5 m to 7 m coseismic uplift.
The 1964 earthquake caused additional uplift of approximately 3.4 m, forming a sixth terrace (from Keller, 1996)

Fault scarps are the direct manifestation of surface-rupturing earthquakes. They are produced almost instantaneously as an earthquake rupture propagates to the surface. Fault scarps are slopes and, as such, have a basic morphology common to many natural slopes (Figure 4)

Figure 5. Diagram showing change in slope elements (fault-scarp morphology)
through time for fault-scarp degradation in the Basin and Range.
(From Wallace, 1977)

Colluvium is unconsolidated material found at the base of steep slopes. Following an earthquake which produces a fault scarp, a colluvial wedge may form at the base of the fault scarp as the free face degrades to a debris slope (Figure 6)

Figure 6. Development of a three-event fault scarp. Each faulting event is followed
by the generation of a fault-scarp colluvial wedge (C1, C2, and C3)
(After McCalpin, 1987)

The clearest evidence of past earthquakes found in fault exposures is displaced strata. Figure 7 shows an idealized diagram of sand and gravel deposits that have been faulted by three splays or strands of a fault system.

Figure 7. Trench exposure showing displacement of sand and gravel deposits, buried fault scarps, and a surface fault scarp.
Fault 1 displaces only unit C. Fault 2 displaces B and C. and fault 3 displaces A. B. and C.
This stratigraphy. along with buried fault scarps and the sur- face fault scarp, suggests that three discrete faulting events occurred.
The oldest faulting event occurred on fault 1 and the youngest on fault 3.
( modified from Keller, 1996)

Sand-boil deposits, sometimes also called ''sand craters,'' have been associated with many earthquakes. At the surface, they are characterized by low mounds of sand that have been extruded from fractures. Figure 8 represents liquefied sand extruded onto the surface and later buried by other materials.

Figure 8. Idealized diagram showing how a sand boil may appear in the stratigraphic record following burial. It is important to note keep in mind, however, that sand boils are also produced by processes
other than earthquakes. ( modified from Keller, 1996)

Large earthquakes may form numerous fissures and cracks (Figure 9). Material from the surface and from the sides of the fissures soon fills them

Figure 9. Recent, open fissures and older, filled and buried fissures. As with sand boils, fissures are not absolute proof of earthquakes because they may be produced
by several other processes as well as earthquakes. (modified from Keller, 1996)