Earthquakes - Extreme Events: A Physical Reconstruction and Risk Assessment - page 150

Fault termination

Unit 1

Unit 2

Event 3

Fault termination

Event 2

Unit 3

Fault termination

Unit 4 Unit 5

Event 1

Unit 6

Figure 6.2. Idealised sequence of upward fault terminations in sedimentary units.
Note the increased disruption of sedimentary units with depth. Three
palaeoearthquakes are evident in this stratigraphy.

microfaults (upward fault terminations)

Microfaults or upward fault terminations are small faults that occur
within sedimentary and rock units during major earthquakes. These faults are
usually examined in late Quaternary sedimentary sequences that straddle major
faults (Nikonov, 1995). Often these sedimentary sequences experience ongoing
sedimentation with time, so early earthquake events will produce microfaults in
the sediments that are present at the time. Subsequently deposited sedimentary
units will not record the early earthquake because they were not present dur-
ing that event. But these later sedimentary units can be faulted by subsequent
major earthquakes. In this fashion, the microfaults terminate upwards against
asedimentary unit that was deposited after that earthquake event. The number
of overlying sedimentary units displaying upward terminating faults provides
information on the number of prehistoric earthquakes at that site and the ages
of the lowermost faulted unit and overlying non-faulted unit bracket the timing
of the earthquake.
Figure 6.2 illustrates a hypothetical microfaulting scenario. Unit 6, the oldest
sedimentary unit is overlain by unit 5 and both are faulted. The fault does not
extend into unit 4 so this unit provides a minimum age for the event while unit
5provides a maximum age. In other words the earthquake occurred during depo-
sition of unit 5. Unit 4 is disrupted by another fault that terminates at unit 3.