Scientists at the Caltech Tectonics Observatory have started
analyzing the devastating earthquake (magnitude 7.9) that struck China's Sichuan
province on May 12, 2008. This tragic event is one of a series of
earthquakes in the earthquake-prone region and is likely to
be repeated. Here we examine the underlying physics.
The earthquake occurred in an area that is deforming as a
result of the collision between two tectonic plates, the Indian
plate and the Eurasian (comprising the continents of Europe and Asia) plate. Figure 1 shows the motion of
India (large blue arrow pointing northward) relative to that
of Eurasia (three smaller blue arrows pointing eastward). The
white star indicates the location of the 2008 Sichuan earthquake.
The seismicity recorded since 1964 for earthquakes of magnitude
4.6 and greater is shown by the circles.
Figure 1. Map showing the location of the
2008 Sichuan earthquake (white star) as well as the location
of all earthquakes occurring between 1964 and 2004 with magnitudes
between 4.6 and 9.0 (colored circles). Larger red circles represent
the largest magnitude quakes, and smaller yellow circles represent
lesser magnitude quakes. Areas of dense circles indicate boundaries
between plates. This map was constructed using data from seismometers
located at various places over the surface of the earth. The
blue arrows show the northerly motion of India and the resulting
easterly motion of Tibet. The size of the arrows indicates the
relative speed of plate motion.
This collision, which has been going on for 50 million years,
is the cause of the high mountains and widespread seismicity
observed throughout central Asia. The area of dense circles
between India and Asia covers a wide region that is undergoing
large strain and deformation. It is this strain that led to
the Sichuan quake and will lead to others. India has been moving
northward at a rate of about 4 cm/year (2 inches/yr), which
is about as fast as fingernails grow, pushing into central
Asia and thus pushing Tibet eastward, out of its way. The 2008
Sichuan earthquake occurred where the eastern part of Tibet,
forced further eastward, overrides the Sichuan basin at a rate
of about 4 mm/year (an eighth of an inch/yr). This is the cause
of the ongoing rise of the Longmen Shan mountain range that
marks the eastern border of Tibet.
The motion of the land masses is shown in more detail in Figure
2. This velocity data is from Global Positioning System (GPS)
stations located in the region. These stations enable us to
measure surface velocity to within a fraction of a millimeter
per year. The relatively fast northerly motion of India is
evident, as is the somewhat slower easterly motion of Tibet.
The blue star indicates the location of the 2008 Sichuan earthquake.
Figure 2. Velocity data from GPS stations
located in the region. The blue star indicates the location of
the 2008 Sichuan earthquake.
Analysis of seismological measurements indicates that the
2008 Sichuan earthquake reached a magnitude of about 7.9, rupturing
the front of the Longmen Shan fault which marks the eastern
edge of Tibet where the steep front of the Longmen Shan mountain
range meets the Sichuan basin.
A closer look at this region is shown in Figure 3. The rupture
of the fault started in the mountains northwest of the city
of Chengdu (yellow star in Figure 3) and then, over the next
50 seconds, traveled at least 200 km (100 miles) toward the
northeast, tearing apart the land along the front of the mountain
range.
Figure 3. Map of fault region showing the
epicenter of the first quake (yellow star) as well as the location
of aftershocks (yellow circles) occuring within 5 days after
the quake. Note that the aftershocks (USGS quick report) occur
mainly along and in the vicinity of the fault line ruptured by
the original May 12 quake. The white arrows show the horizontal
motion of GPS stations located on the ground at those places,
indicating the motion of the land. The arrows on the left-hand
side are longer than those on the right-hand side, indicating
that the land mass on the left is overtaking that on the right.
There have been many daily aftershocks following the initial
rupture, as shown by the yellow circles in Figure 3. Note that
the aftershocks are concentrated along the same ruptured fault,
as well as in the vicinity. Aftershocks are expected to continue
and may do so for months or even years afterward, as happened
in the recent December 26, 2004 Sumatra earthquake (magnitude 9.1), though the frequency decreases
with time.
Model results for the earthquake rupture are shown in Figure
4, where the length of the blue rectangle shows the length
of the rupture along the surface and the width shows the depth
along the plane of the fault. Colors indicate the amount of
vertical slippage.
Figure 4. Model results of the ruptured
fault line (narrow rectangle) superimposed on the actual fault
line (modeling of the seismological data by Anthony Sladen).
The length and depth of the rupture zone are indicated by the
sides of the rectangle. Colors within the rectangle indicate
the length of slip on the fault plane, i.e., the amount by
which the land on the northwest side of the fault moved with
respect to the land on the southeast side. Note that the location
of the largest slip, indicated by dark red, is northeast of
the epicenter (red star). The elevation of the terrain is also
indicated by color. Note that the fault lies at the boundary
between the high mountains and the low plains. Open circles
indicate the aftershocks located by the USGS-NEIC in the 36
hours following the mainshock.
In some places, the vertical slippage along the fault line
was as large as 12 meters (39 feet), shown by red-shaded areas
in Figure 4. Note that this maximum vertical slippage did not
occur at the epicenter but at a location about 60 km northeast
of the epicenter.
Another view of the amount of slippage as a function of position
along the fault (same rectangle as in Figure 4) is shown in
Figure 5. Note that at the epicenter (red star), the depth
of the rupture was only about 10 km (6 miles) below the surface.
The amount of shaking correlates with rupture depth. The closer
the rupture is to the surface, the stronger the shaking.
Figure 5. Model results for the amount of
vertical slippage as a function of location and depth along the
ruptured fault (modeling of seismological data by Anthony Sladen).
This rectangle corresponds to that in Figure 4, though it has
been rotated so that the epicenter (star) is on the right. The
color scale is a bit different, with the maximum slip being indicated
by orange rather than red. Note that at the epicenter (red star)
the rupture originated about 10 km (6 miles) below the surface.
Motion along the southern edge of the fault was predominantly
dip slip (i.e., land on one side of the fault moving under
land on the other side), leading to an increment of uplift
of the mountain range and subsidence of the Sichuan basin (see
Figure 6).
Figure 6. Schematic of a thrust fault (dip
slip fault with landmasses coming together). The two landmasses
are moving toward each other, as indicated by the red arrows,
one landmass sliding under the other (animation).
The effect of the seismic shaking within both the basin and
the mountain range has been devastating. Heavy sedimentation
in the basin, where close to 100 million people live, amplifies
the shaking. As the seismic wave from either the original quake
or subsequent aftershocks travels across the basin, the effect
of the sediment is two-fold: the waves slow down, so that they
spend more time in the area, and their amplitude, or strength,
increases. Both effects compound the damage.
Shaking in the mountain range triggers landslides within the
range. Tragically, a number of towns have been reported to
be completely buried. Rescue efforts were also hindered by
slides in the mountain passes, which blocked several of the
main roads into the area. Such landslides also block the flow
of water, causing natural dams to form, which later can breach
or overflow. This occurred for an earlier earthquake in the
same region, the 1933 Diexi earthquake, estimated at magnitude
7.5. That earthquake, located further within the mountain range,
caused dramatic landslides and catastrophic draining of lakes
behind landslide-dams.
Can we expect more earthquakes in the future? Historically
this has always been an area of frequent earthquakes. Figure
7 shows the numerous faults in the region. Some of these faults
now have higher levels of stress and thus will have been brought
closer to rupture because of the 2008 Sichuan earthquake.
Figure 7. Three types
of faults in this region. Red star indicates location of the
2008 Sichuan quake. Note large number of faults that are building
up strain. This is an area of high earthquake risk.
Thus, it is likely that more earthquakes will occur in this
region. In the absence of prediction and remediation, the best
we can do is be prepared for them.
More
detailed information on the seismological analysis
More on the science behind the earthquake:
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