Posted by: drazizul | January 14, 2010

Lecture 8: Earthquake

Effects of soil liquefaction during the 1964 Niigata earthquake

Earthquake
An earthquake (also known as a tremor or temblor) is the result of a sudden release of energy in the Earth’s crust that creates seismic waves.
Earthquakes are recorded with a seismometer, also known as a seismograph.
At the Earth’s surface, earthquakes manifest themselves by shaking and sometimes displacing the ground.
When a large earthquake epicenter is located offshore, the seabed sometimes suffers sufficient displacement to cause a tsunami.
The shaking in earthquakes can also trigger landslides and occasionally volcanic activity.

Human Impacts
Earthquakes may lead to disease,
Lack of basic necessities,
Loss of life,
Higher insurance premiums,
General property damage,
Road and bridge damage, and
Collapse or destabilization (potentially leading to future collapse) of buildings.
Earthquakes can also precede volcanic eruptions, which cause further problems; for example, substantial crop damage

Seismographs and Seismograms

Sensitive seismographs are the principal tool of scientists who study earthquakes. Thousands of seismograph stations are in operation throughout the world, and instruments have been transported to the Moon, Mars, and Venus.
Fundamentally, a seismograph is a simple pendulum. When the ground shakes, the base and frame of the instrument move with it, but intertia keeps the pendulum bob in place. It will then appear to move, relative to the shaking ground.
As it moves it records the pendulum displacements as they change with time, tracing out a record called a seismogram.
One seismograph station, having three different pendulums sensitive to the north-south, east-west, and vertical motions of the ground, will record seismograms that allow scientists to estimate the distance, direction, Richter Magnitude, and type of faulting of the earthquake. Seismologists use networks of seismograph stations to determine the location of an earthquake, and better estimate its other parameters. It is often revealing to examine seismograms recorded at a range of distances from an earthquake:

Seismologists use a Magnitude scale to express the seismic energy released by each earthquake.

Richter Earthquake Magnitudes Effects,..
Less than 3.5 Generally not felt but recorded
3.5-5.4 Often felt, but rarely causes damage.
Under 6.0 At most slight damage to well-designed buildings. Can cause major damage to poorly constructed buildings over small regions.
6.1-6.9 Can be destructive in areas up to about 100 kilometers across where people live.
7.0-7.9 Major earthquake. Can cause serious damage over larger areas.
8 or greater Great earthquake. Can cause serious damage in areas several hundred kilometers across

Factors those Effect Earthquake
The effects of any earthquake depend on a number of factors. These factors are all of:
Intrinsic to the earthquake – its magnitude, type, location, or depth;
Geologic conditions where effects are felt – distance from the event, path of the seismic waves, types of soil, water saturation of soil; and
Societal conditions reacting to the earthquake – quality of construction, preparedness of populace, or time of day (e.g.: rush hour).

There are two :
1)Direct and
2)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.

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.

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.

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.

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.

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 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.

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.

Many elevated structures were simply pulled apart by differential movements, here leaving the welded rails and ties suspended

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

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.

Sandblow or Sandboil
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.

Entire levees, dams, and other water-saturated embankments can liquefy and flow apart during strong shaking

Tsunamis are long-wavelength, long-period sea waves produced by the sudden or abrupt movement of large volumes of water. In the open ocean the distance between wave crests can surpass 100 kilometers, and the wave periods can vary from five minutes to one hour. Such tsunamis travel 600-800 kilometers per hour, depending on water depth. Large waves produced by an earthquake or a submarine landslide can overrun nearby coastal areas in a matter of minutes. Tsunamis can also travel thousands of kilometers across open ocean and wreak destruction on far shores hours after the earthquake that generated them.
Ordinarily, subduction earthquakes under magnitude 7.5 on the Richter scale do not cause tsunamis, although some instances of this have been recorded. Most destructive tsunamis are caused by earthquakes of magnitude 7.5 or more.

Preparedness against Earthquake
1.Earthquake engineering,
2. Earthquake preparedness,
3. Seismic retrofit (including special fasteners, materials, and techniques),
4. Seismic hazard
5.Mitigation of seismic motion, and
6. Earthquake prediction.

Earthquake engineering is the study of the behavior of buildings and structures subject to seismic loading. It is a subset of both structural and civil engineering.

The main objectives of earthquake engineering are:
Understand the interaction between buildings or civil infrastructure and the ground.
Foresee the potential consequences of strong earthquakes on urban areas and civil infrastructure.
Design, construct and maintain structures to perform at earthquake exposure up to the expectations and in compliance with building codes[1].
A properly engineered structure does not necessarily have to be extremely strong or expensive.


Earthquake preparedness measures can be divided into:

Retrofitting and earthquake resistant designs of new buildings and lifeline structures (e.g. bridges, hospitals, power plants).
Response doctrines for state and local government emergency services.
Preparedness plans for individuals and businesses.
Building design and retrofitting
Personal preparedness

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