Earthquakes - Extreme Events: A Physical Reconstruction and Risk Assessment - page 151

Hence the age of this event is bounded by the ages of units 4 and 3. Likewise, a
third earthquake event is evident in the stratigraphy and it terminates at unit 1.
There have been no earthquakes since deposition of unit 1 began, but when the
next earthquake occurs it too will experience surface disruption. The older the
unit the more earthquakes it will have experienced and the more deformed it
will be compared to younger units.
When reconstructing these event stratigraphies it is important to determine
that the downward increases in deformation were derived from sudden, brittle
surface rupturing processes associated with large earthquakes rather than from
gradual fault movements (Lienkaemper et al ., 1999). Fumal et al .(2002a)found
that upward fault terminations may not always be a very reliable indicator
of past earthquakes. They suggest that folded sedimentary units, where they
can be mapped across a relatively broad area, may be more useful. Fumal et al .
(2002a)used fault scarps, colluvial wedges, in-filled fissures and increased tilting
of sediment layers downsection to identify earthquake horizons.
Studies using microfaulting and associated deformation features have shown
detailed histories of major earthquakes along large fault systems in California,
USA and elsewhere globally. The Hayward fault in northern California, which
lies just east of the San Andreas fault, for example, has experienced seven
or more surface faulting earthquakes over approximately the last 2000 years
(Lienkaemper et al ., 1999). The average recurrence interval between these events is
<270 years. The last major earthquake occurred here after AD 1640 but there
have been no large earthquakes since AD 1776. This suggests that the next large
earthquake in this region is relatively imminent.
Asimilar conclusion was drawn by Fumal et al .(2002b)for segments of the
San Andreas fault, California. Four and probably five large earthquakes occurred
on the Mission Creek strand of this major fault over the past 1200 years. The
most recent large earthquake occurred around AD 1675 and the palaeoseismic
record suggests that these events have a mean recurrence interval of around
215 years. This suggests that over 300 years have elapsed since the last major
earthquake and the region is overdue for the next event.
Liquefaction features
Liquefaction features, or sand blows, occur when ground shaking dur-
ing an earthquake liquefies a subsurface layer of sand which penetrates into
and/or through an overlying, otherwise impenetrable layer. The overlying
layer is intruded via a sand dike and if this layer is the uppermost strati-
graphic unit the liquefied sand can penetrate to the ground surface and
be expressed as a mound or area of sand. The original ground surface is