426 TECTONICS/Neotectonics
sea-level rise, etc.), because it focuses on crustal movements that can be expected to recur within a future
interval of concern to society. Contemporary crustal
movements may be discerned in Earth surface processes and landforms, such as in the sensitivity of
alluvial rivers to crustal tilting. In addition, geomorphological and geological studies are important in
recording the surface expression of Earth movements
such as earthquake ground ruptures, which, due to
their subtle, ephemeral, or reversible nature, are unlikely to have been preserved in the geological record.
However, active tectonics also employs an array of
high-technology investigative practices; prominent
among these are the monitoring of ongoing Earth
surface deformation using space-based or terrestrial
geodetic methods (tectonic geodesy), radar imaging
(interferometry) of ground deformation patterns produced by individual earthquakes and volcanic unrest,
and the seismological detection and measurement
of earthquakes (seismotectonics). These techniques
are applied globally via the World-Wide Standardized Seismograph Network and regionally via local
seismographic coverage.
The modern snapshots of tectonism can be pushed
back beyond the twentieth century through the analysis of historical accounts and maps to infer past land
surface changes or to deduce the parameters of past
seismic events (historical seismology). In addition,
earthquakes can leave their mark in the mythical
practices and literary accounts of ancient peoples,
recorded in the stratigraphy of their site histories and
in the damage to their buildings (archaeoseismology).
The time covered by such human records varies markedly, ranging from many thousands of years in the
Mediterranean, Near East, and Asia to a few centuries
across much of North America. Generally records
confirm that regions that are active today have been
consistently active for millennia, thereby demonstrating the long-term nature of crustal deformation,
but occasionally records reveal that some regions that
appear remarkably quiet from the viewpoint of
modern seismicity (such as the Jordan rift valley) are
capable of generating large earthquakes. In reality, the
distinction between neotectonics and active tectonics
is artificial; the terms simply describe different time
slices of a continuum of crustal movement. This
continuum is maintained by the persistence of the
contemporary stress field, which means that inferences of past rates and directions of crustal movement
from geological observations can be compared directly with those measured by modern geodetic and
geophysical methods.
Although the terms ‘neotectonic’ and ‘active’ are
somewhat blurred and are often used interchangeably,
societal demands (for instance, regulatory authorities
for seismic hazard, nuclear safety, etc.) often require the
incidence of tectonic movements to be defined strictly.
For instance, in the United States, under California law,
an ‘active fault’ is presently defined as one that has
generated surface-rupturing earthquakes in the past
11 000 years (i.e., the time period was established to
relate to the time when the Holocene was considered
to have begun). Other regulatory bodies recognize
a sliding scale of fault activity: Holocene (activity
in the past 10 000 years), Late Quaternary (activity in
the past 130 000 years), and Quaternary (activity in the
past 1.6 million years). Neotectonic faults, by comparison, are simply those that formed during the imposition
of the current tectonic regime. ‘Real’ structures, of
course, are unconstrained by such legislative concerns.
Many modern earthquakes rupture along older (i.e.,
palaeotectonic) basement faults. Indeed, it is important
to recognize that any fault that is favourably oriented
with respect to the stress currently being imposed on it
has the potential to be activated in the future, regardless
of whether it has moved in the geologically recent past.
Global Tectonics
A useful way to differentiate styles and degrees of
neotectonic activity is in terms of tectonic strain rate,
which is a measure of the velocity of regional crustal
motions and, in turn, of the consequent tectonic
strain build-up. Crustal movements are most vigorous, and therefore most readily discernible, where
plate boundaries are narrow and discrete. In these
domains of high tectonic strain, frequent earthquakes
on fast-moving (>10 mm year 1) faults ensure that a
century or two of historical earthquakes and a few
years of precise geodetic measurements are sufficient to capture a consistent picture of the active tectonic behaviour. Intermediate tectonic strain rates
characterize those regions where plate–boundary
motion is distributed across a network of slower
moving (0.1–10 mm year 1). Examples of such broad
deforming belts are the Basin and Range Province of
the western United States or the Himalayan collision
zone, where earthquake faults rupture every few hundred or thousand years, ensuring that the Holocene
period is a reasonable time window over which to
witness the typical crustal deformation cycle. In contrast, areas with low strain rates ensure that intraplate
regions, often referred to as ‘stable continental interiors’, are low-seismicity areas with slow-moving
(<0.1 mm year 1) faults that rupture every few tens
(or even hundreds) of thousands of years, making the
snapshot of human history an unreliable guide to the
future incidence of tectonic activity.