Technical University ‘Gh. Asachi’, Faculty of Civil EngineeringIasi, Romania

Large earthquakes of the world. Case study: Kobe, Japan, 1995, January 17

Atasiei Raul ConstantinGroup: 3404

Study Year: IVSection: Civil Engineering

Facts:

The earthquake took place on 17th January 1995, at 5:46 am (local time).It had a magnitude of 7.2 (Richter scale) and lasted for 20 seconds.Epicenter: 34.6 N 135.0 E Awajishima Island, 20 miles south of Kobe : population 1.5 million, Japan's sixth largest city, world's sixth largest port .

Tectonic Setting of the Kobe Earthquake:

The following map shows the location, plate tectonic setting, and earthquake history, for the January 1995, Kobe earthquake.

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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.

In the northern part of Awajishima Island, it has been confirmed that an earthquake fault with large movement was formed. This fault, 9 km long, is a consequence of the movement of an existing fault known as the "Nojima Fault" which runs along west coast of Awajishima Island from the north end to middle.

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Earthquake faults, damages to roads and railways, liquefaction at reclaimed lands, fires, crushed houses, landsides were all manifested in this very destructive seismic shift.

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.

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;

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settlement; and the triggering of aftershocks and additional earthquakes.

Seismogram from Caltech

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.

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.

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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.

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

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even for its greater weight. The proportionally heavier tile roofs on wooden houses also might have been a factor.

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.

This elevated highway formed an inverted pendulum that the supporting columns were not able to restrain under shear during seismic shaking. 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.

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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.

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.

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

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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.

In Short: Direct and Indirect Costs:

$200 Billion in damages (4% of Japan's GDP) $100 Billion to restore basic functions $50 Billion in losses due to economic dislocation and business interruption $50 Billion in losses of private property

Structural Damage (Buildings):144,032 Buildings destroyed by ground shaking 7,456 Buildings destroyed by fire 82,091 Collapsed buildings 86,043 Severely damaged buildings

Structural Damage (Highways/ Bridges/Ports):All Kobe ports shut down to international shipping Damage to containing loader piers All access to Kobe via highway and railway blocked

Miscellaneous Facts:Largest peak accelerations 0.8g to greater than 1g 300,000 People were left homeless

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Biography: http://www.seismo.unr.edu

http://vathena.arc.nasa.gov/curric/land/kobe.html

http://www.dis-inc.com/kobe.htm

http://www.vibrationdata.com/earthquakes/kobe.htm

http://www.ce.berkeley.edu/geo/research/Kobe/Somerville/qnews.html

http://www.ce.berkeley.edu/geo/research/Kobe/KobeReport/contents.html

http://www.kananet.com/kobequake/3e-links.htm