Jan. 17, 1995 Hyogo-Ken Nanbu Earthquake
The worst earthquake catastrophe in years occurred on western Honshu Island early this year. More than 5000 people perished in southern Hyogo prefecture, most in the city of Kobe, Japan's most important port. The loss of so many lives, in a country where so much effort had been made to prepare for earthquakes, shocked observers worldwide. However, the magnitude of this catastrophe is probably due to a terrible coincidence of a few simple seismological and societal factors, which may become clear in the photos below.
(from the USGS) Kobe is located farther than many other cities in Japan from
the dangerous intersection of three tectonic
plates: the Pacific, Eurasian, and Philippine. This triple junction
is a junction of three compressive subduction zones. The red-hatched
areas above are the parts of the subduction fault that had aleady broken in great
earthquakes in 1944 and 1946. Kobe is also somewhat off the Median Tectonic
Line, a zone of strike-slip faults.
(from the Earthquake Research Institute, Tokyo) This map shows the epicenters
of the earthquake's aftershocks within the first two days afterward. Decades of
observation show that the most reliable way to locate the fault that broke in any
earthquake is to observe where aftershocks are concentrated. This map shows
that the earthquake fault obliquely cut the north side of Awaji Island, and
cossed the bay to run along the Honshu coast directly below the city of Kobe.
Probably the most important coincidence leading to the mass casualties was
this ``direct hit'' of the city by such a large faulting event. There was no
intervening distance to mitigate the effects. The Northridge area of Los Angeles
suffered a similar coincidence in January 17, 1994.
Direct Effects
There are two classes of earthquake effects: direct, and secondary. Direct effects are solely those related to the deformation of the ground near the earthquake fault itself. Thus direct effects are limited to the area of the exposed fault rupture. Many earthquake faults (such as at Northridge) never break the surface, ruling out direct effects. In the Hyogo-Ken Nanbu event, surface rupture of the fault was observed only in a rural area of Awaji Island, with displacements of up to 3 meters. Few structures were near enough the fault to be damaged by the displacement, although underground utilities, fences, and irrigation ditches were cut. Rice paddies were thrown far out of level.
(taken by the
Geographical Survey Institute of Japan;
used by permission.)
Aerial view of the fault rupture on northern Awaji Island, taken on January
18th, the day after the event.
From left to right along the rupture, a landslide
from the rupture covers a road; a fault scarp across a rice paddy; a right-lateral
offset in a dirt road (inset); and three more pointers to the scarp. Note how little
damage there apparently is to homes even very close to the fault.
(from a report by J.-P.
Bardet at USC and
others at Gifu Univ.; used by permission)
View along the fault scarp
on Awaji Island. The section of rice paddy to the right has been uplifted by more
than one meter. Note the cut road in the foreground.
It is often possible to measure the displacement and length of the exposed fault
rupture to estimate the slip and area of the subsurface fault,
providing an independent estimate of the earthquake's
magnitude.
Photo from the Japanese edition of Newsweek showing the fault scarp.
Note the horizontal as well as vertical offset shown by the dike in the
rice field. Well-built structures often escape major damage even so close
to a seismogenic fault.
Elastic rebound, the
permanent deformation of the ground due to the fault rupture, will extend many
kilometers from the fault itself, and is often measurable even where the rupture
itself remains buried. In the past geodesists have had to make painstaking
and expensive surveys, visiting hundreds of field sites, to measure deformation
of an area struck by an earthquake.
(analyzed by the
Geographical Survey Institute of Japan and the National Space Development
Agency of Japan; used by permission.)
Lately planetary geophysicists have
developed a quick way to make a map image of deformation using satellite
radar interferometry. They compare satellite microwave-radar
images of a region taken before and after the event. Any area displaced toward
or away from the spacecraft's positions will form an interference pattern,
tracing out contours of equal displacement.
The map image above of Awaji Island shows eight or more colored interference
fringe lines approaching the fault, at 11 cm of vertical displacement per fringe
contour, demonstrating almost 1 meter of uplift by the earthquake.
At left, two fringes parallel the coast through the city of Kobe, showing about
20 cm of displacement across the city from the buried fault.
Secondary Effects
Most of the damage done by earthquakes is due to their secondary effects, those not directly caused by fault movement, but resulting instead from the propagation of seismic waves away from the fault rupture. Secondary effects result from the very temporary passage of seismic waves, but can occur over very large regions, causing wide-spread damage. Such effects include: seismic shaking; landslides; liquefaction; fissuring; settlement; and the triggering of aftershocks and additional earthquakes.Seismic Shaking
(from the Architecture Dept. of Tokyo Metro. Univ.) Since seismic waves spread out from their source just like ripples on a pond, they get weaker the farther you get from the earthquake. The ground acceleration versus distance plot at left shows this effect near Kobe. An acceleration of 1000 gal for seismic waves in the ground would be equal to the acceleration of gravity; so if it acted straight up it would be able to throw any object into the air. These accelerations are mostly from side-to-side, so even at only 50% of the acceleration of gravity are capable of toppling anything standing. Note that the recorded acceleraction can vary by a factor of two or three, especially near the fault.
(compiled by the Earthquake Research Institute, Tokyo)
The great differences in secondary effects even between adjacent localities
is shown by this map of instrumentally-recorded ground accelerations and
velocities. Although the measurements do fall with distance from the
epicenter, adjacent sites can vary by more than 50%.
Such variation is usually caused by variation in the soil conditions.
(from the Architecture Dept. of Tokyo Metro. Univ.) The seismograms above were
recoded at two different sites near Kobe. On the left are three records of the
sharp pulse, lasting less than 15 s, recorded at a station founded in relatively
solid rock. On the right are three records of the strong and extended shaking,
lasting two or even three minutes, at sites near the coast having soft, thick,
water-saturated soils. The geological conditions right at a particular site
play a crucial role in the strength, and length, of seismic shaking that can be
experienced there. In all earthquakes, low-lying areas having soft, water-saturated
soils experience by far the most damage.
(from a report by J.-P.
Bardet at USC and
others at Gifu Univ.; used by permission)
Since most seismic shaking is side-to-side, a shaken structure will
undergo shear as this house front in Kobe did. Shear is the bending
of right angles to other angles. As it is much more difficult to shear a triangle
than a rectangle, effective seismic design requires triangular bracing for shear
strength.
(from Kobe University) This wooden house collapsed during the seismic shaking.
It is likely that its heavy roof of ceramic tile created more shear force than
its wood frame was built to resist. Tile roofs are popular in Japan.
(from a report by J.-P.
Bardet at USC and
others at Gifu Univ.; used by permission)
Behind this completely collapsed wood-frame house is a house of
reinforced concrete that suffered no structural damage. The number of wood
versus masonry buildings that collapsed in Kobe astonished most observers,
as wood-frame structures are usually thought to be much better at resisting
shear forces. Possibly the concrete house was better-designed and stronger
even for its greater weight. The proportionally heavier tile roofs on wooden
houses also might have been a factor.
(from a report by J.-P.
Bardet at USC and
others at Gifu Univ.; used by permission)
Another anomaly was the large number of about 20-year-old
high rise buildings that collapsed at the fifth floor. The older version of the
code they were built under allowed a weaker superstructure beginning at the fifth floor.
(from Kobe Univ.) This photo demonstrates the extreme danger of being in the
street during seismic shaking. Signs, windows, and the entire fronts of buildings
collapsed into the street. During an earthquake, it is usually much safer to find
shelter under strong furniture inside than to run out of a building. Exit carefully
after the shaking stops.
Effects on Lifelines
Debris choking streets was just one of the coincidences that made this earthquake so deadly. Almost all utilities, roadways, railways, the port, and other lifelines to the city center suffered severe damage, greatly delaying rescue efforts. Most lifelines in Kobe were constructed 20-30 years ago, before the most modern construction standards were put into practice.
(from a report by J.-P.
Bardet at USC and
others at Gifu Univ.; used by permission)
This elevated highway formed an inverted pendulum that the supporting
columns were not able to restrain under shear during seismic shaking.
(from a report by J.-P.
Bardet at USC and
others at Gifu Univ.; used by permission)
The columns above show a failure typical of somewhat older reinforced concrete
structures throughout the world. The vertical steel rods can hold the
weight of the structure just fine when that weight is exerted straight down,
as usual. During seismic shaking much more steel wound around
the rods horizontally can keep the column from breaking apart under the shear forces.
Stronger columns are more expensive to build.
(from a report by J.-P.
Bardet at USC and
others at Gifu Univ.; used by permission;
and from Japanese TV)
Large sections of the main Hanshin Expressway toppled over. This was particularly
likely where the road crossed areas of softer, wetter ground, where the shaking
was stronger and lasted longer.
(from a report by J.-P.
Bardet at USC and
others at Gifu Univ.; used by permission)
Many elevated structures were simply pulled apart by differential
movements, here leaving the welded rails and ties suspended.
(from a report by J.-P.
Bardet at USC and
others at Gifu Univ.; used by permission)
Below one intersection a subway station collapsed, leaving the road
above to sink unpredictably for months until it could be excavated.
Fire
The destruction of lifelines and utilities made it impossible for firefighters to reach fires started by broken gas lines. Large sections of the city burned, greatly contributing to the loss of life.
(from the Univ. of Texas)
Most of the destruction of San Francisco from the 1906 earthquake was also due to fire. The city installed an entirely independent water system for firefighting, with its own reservoirs. The 1989 earthquake broke a firefighting water main near the Mission Street Post Office, draining the entire system in less than 15 minutes. Fortunately most damage and fires were confined to low-lying districts of the city near the Bay, and fireboats were available to pump bay water as much as one mile inland. Only a few blocks were lost.
Liquefaction
One of the reasons that areas of soft, water-saturated soil are hazardous is their potential to liquefy during strong seismic shaking. The shaking can suspend sand grains in waterlogged soil so that they loose contact and friction with other grains. Soil in a state of liquefaction has no strength and cannot bear any load.
(from a report by J.-P.
Bardet at USC and
others at Gifu Univ.; used by permission)
Commonly a soil layer on the side of a hill will liquefy during seismic shaking
and flow as a landslide or mudflow, as above.
(from a report by J.-P.
Bardet at USC and
others at Gifu Univ.; used by permission)
A liquefied sand layer can shoot to the surface through cracks, forming a
sandblow or sandboil, and depositing a characteristic lens of
sand on the ground with a volcano-like vent in the center. With all the material
in the layer forced to the surface, the surrounding area sinks unevenly.
(from a report by J.-P.
Bardet at USC and
others at Gifu Univ.; used by permission)
Entire levees, dams, and other water-saturated embankments can liquefy and
flow apart during strong shaking.
(from Kobe Univ.)
Buildings founded on liquefied ground will lean or topple.
(from a report by J.-P.
Bardet at USC and
others at Gifu Univ.; used by permission)
The Kobe port, having been constructed on two artificial islands made of relatively
loose fill, and always water saturated, suffered widespread liquefaction and
settlement, and was incapacitated for two months. Shipping was disrupted
worldwide.
(from a report by J.-P.
Bardet at USC and
others at Gifu Univ.; used by permission)
On the port islands settlement was so pervasive that any structure built on
deep pilings, like this elevated roadway, appeared to have risen a full meter.
The world's longest suspension bridge, under construction but having such
foundations, was hardly damaged at all.
