Hazard (noun) [ C ]

 US  /ˈhæz·ərd/

 (DANGER) - something dangerous and likely to cause damage:

a health/fire hazard

Source: http://dictionary.cambridge.org/us/dictionary/english/hazard

A volcanic hazard refers to any potentially dangerous volcanic process (e.g. lava flows,

pyroclastic flows, ash). A volcanic risk is any potential loss or damage as a result of the volcanic

hazard that might be incurred by persons, property, etc. or which negatively impacts the

productive capacity/sustainability of a population. Risk not only includes the potential monetary

and human losses, but also includes a population's vulnerability. 

Source: http://www.geo.mtu.edu/volcanoes/hazards/primer/

Pyroclastic FlowsPyroclastic flows are fluidized masses of rock fragments

and gases that move rapidly in response to gravity.

Pyroclastic flows can form in several different ways. They

can form when an eruption column collapses, or as the

result of gravitational collapse or explosion on a lava dome

or lava flow (Francis, 1993 and Scott, 1989). These flows

are more dense than pyroclastic surges and can contain

as much as 80 % unconsolidated material. The flow is

fluidized because it contains water and gas from the

eruption, water vapor from melted snow and ice, and air

from the flow overriding air as it moves downslope (Scott,

1989). The image on the right shows the formation of

pyroclastic flows during a 1980 eruption of Mount St.

Helens (photo courtesy of J.M. Vallance).

Ignimbrites and nuees ardentes are two types of

pyroclastic flows. An ignimbite contains mostly vesiculated material whereas a nuee

ardente contains denser material (Francis, 1993). Nuee ardente means glowing cloud and was

named for the pyroclastic flows seen at Mount Pelee. These flows were often accompanied by a

cloud of ash elutriated from the flow. When the incadescent ash particles are observed at night,

the flow looks like a glowing cloud moving away from the volcano.

Pyroclastic flows can move very fast. Small pyroclastic flows can move as fast as 10 to 30 m/s

while larger flows can move at rates of 200 m/s (Bryant, 1991). Nuees ardentes have been

known to extend 50 kilometers from their source and Ignimbrites, because of the lighter weight

material that they carry, can extend 200 km from their source (Bryant, 1991 and Scott, 1989). At

Mount Pinatubo in the Philipines, pyroclastic flow deposits were 220 m thick in some valleys but

averaged 30 to 50 m thick in others (Wolfe, 1992). Pyroclastic flows have been known to top

ridges 1000 m high (Bryant, 1991).

Pyroclastic SurgesPyroclastic surges are low density flows of pyroclastic material. The reason they are low density

is because they lack a high concentration of particles and contain a lot of gases. These flows

are very turbulent and fast. They overtop high topographic features and are not confined to

valleys. However, this type of flow usually does not travel as far as a pyroclastic flow.

Pyroclastic surges can travel up to at least 10 kilometers from the source (Scott, 1989).

There are three types of pyroclastic surges: 1) base surge, 2) ash cloud surge, and 3) ground

surge. A base surge is usually formed when the volcano initially starts to erupt from the base of

the eruption column as it collapses. It usually does not travel greater than 10 kilometers from its

source. A ground surge usually forms at the base of a pyroclastic flow. An ash cloud

surge forms when the eruption column is neither buoying material upward by convection or

collapsing. Such deposits can be formed before, after, and during, the formation of pyroclastic

flows (Francis, 1989).

Pyroclastic surges are very hazardous. Such surges can bury, burn, and destroy things upon

impact. These surges contain lots of gases that can asphyxiate people. Many people have been

killed by pyroclastic surges. Probably the greatest number of people killed by pyroclastic surges

was in 1902 near Mount Pelee in the town of St. Pierre when 30,000 people lost their lives

(Scott, 1989).

Debris Avalanches, Landslides, and Tsunamis

Volcanic structural collapse in the form of avalanches, rock fall, or landslides can be almost any

size ranging from a few loose rocks falling from the crater rim of a volcano to large avalanches

such as the one at Socompa Volcano in Chile that covers 500 square kilometers (Francis,

1993). Large scale debris avalanches are associated with many volcanic areas including

Augustine Volcano in Alaska, Bandai-san in Japan, and Mount St. Helens in Washington. Large

scale landslide deposits have been found off the coast of several Hawaiian islands including

Molakai and the big island of Hawai'i.

A debris avalanche is formed when an unstable slope collapses and debris is transported away

from the slope. Large scale avalanches normally occur on very steep volcanoes. There are two

general types of debris avalanches: those that are "cold" and those that are "hot". A cold debris

avalanche usually results from a slope becoming unstable whereas a hot debris avalanche is

the result of volcanic activity such as volcanic earthquakes or the injection of magma which

causes slope instability. For several months before Mount St. Helens erupted in 1980, magma

was injected into the north flank of the volcano creating a bulge that was extremely unstable. An

earthquake triggered the movement of land in this area and the result was a fast moving debris

avalanche (see image above).

Geological evidence shows that the larger a debris avalanche is, the faster it moves. The

reason for this is due to energy stored in material within the avalanche. Some large avalanches

have been known to carry blocks as large as three kilometers in length, several kilometers from

their source. Such avalanches can travel close to 300 km/hr (Francis, 1993)!

Landslide is a general term for mass movement. It implies a gradual movement rather than the

more sudden movement of an avalanche. The perfect example of possible large scale

landsliding currently taking place is on the south side of the island of Hawai'i. The dominant

feature on this side of the island is Kilauea Volcano. On the south flank of Kilauea there are

large fault scarps that are as high as 500 m (1,500 ft). Movement along these fault scarps is

causing the south side of Hawai'i to fall into the sea. Studies, using GLORIA side scanning

radar and bathymetry maps, on other Hawaiian Islands, such as Molakai, show large areas of

landslide deposits covering the sea floor. This indicates that large scale landsliding has occured

in the past and may occur in the future (Moore and Normark, 1994).

The hazards that avalanches and landslides can produce are numerous. Both processes can

travel large distances and can wipe out everything in their paths. They can dam rivers and lakes

and produce flooding. It is also possible that a landslide or avalanche can lead to a decrease in

pressure and cause a volcanic explosion. The mixture of debris from a landslide or avalanche

with water may produce lahars which can affect people living in valley areas far away from the

volcano's summit (Francis, 1993).

Another important hazard that can be produced from avalanches and landslides are tsunamis.

Tsunamis are large sea waves that have long wave periods. When these waves reach coastal

areas, they can go far inland. If such a wave were produced by the collapse of a large portion of

the south side of the island of Hawai'i then it would put people in danger who were living on the

coast of Hawai'i and surrounding islands. It could even travel across the Pacific to coastal areas

thousands of miles away. Tsunamis aren't always produced by collapsing land masses. They

can also be produced by volcanic earthquakes and explosions, atmospheric shock waves due

to rapidly moving volcanic material, and lahars or pyroclastic flows that have entered the sea. In

1883, when Krakatau erupted, the main cause of death was due to tsunamis. These great

waves reached as high as 35 meters and killed greater than 30,000 people (Scott, 1989). On

the Hawaiian Island, Lanai, a large scale landslide produced a tsunamis that deposited debris

300 m above sea level (Tilling and Lipman, 1993).

Not much can be done to stop avalanches or landslides from occuring. The best way to prevent

disaster from happening is to be aware of what has happened in the past and what is happening

in the present. Geologists and Geophysicists spend a great deal of time deciphering the

geologic past and present. Their efforts can make people aware of the hazards in various areas.

As for tsunamis, steps have been taken in the Pacific to establish a warning system. This can

be done since tsunamis have a unique wave frequency. For those living in coastal areas that

may be affected by tsunamis, it is important to be aware of what a tsunami is and where to go

when such a wave arrives.

Lava FlowsLava flows are the least hazardous of all processes in volcanic eruptions. How far a lava flow

travels depends on the flows temperature, silica content, extrusion rate, and slope of the land. A

cold lava flow will not travel far and neither will one that has a high silica content. Such a flow

would have a high viscosity (a high resistance to flow). A basalt flow like those in Hawai'i have

low silica contents and low viscosities so they can flow long distances. Such a flow can move as

far away as 4 km from its source and have a thickness of 10 m (Bryant, 1991). These flows can

move at rates of several kilometers per hour (Scott, 1989). More silica-rich flows can move as

far away as 1.3 km from their sources and have thicknesses of 100 m (Bryant, 1991). These

flows can move at rates of a few to hundreds of meters per hour (Scott, 1989). If a lava flow is

channelized or travels underground in a lava tube then the distance it travels is greatly

extended.

Lava flows as you can see don't move very fast so people rarely get killed by them. However,

lava flows are very hot (between 550 degrees C and 1400 degrees C) and can therefore cause

injuries. People have burnt their skin, charred their eyebrows, and melted the soles of their

boots from being near or on a hot lava flow. Lava flows don't cool instantaneously. It can take

days to years for a lava flow to completely cool.

Volcanic GasesAn erupting volcano will release gases, tephra, and heat into the atmosphere. The largest

portion of gases released into the atmosphere is water vapor. Other gases include carbon

dioxide (CO2), sulfur dioxide (SO2), hydrochloric acid (HCl), hydrogen fluoride (HF), hydrogen

sulfide (H2S), carbon monoxide (CO), hydrogen gas (H2), NH3, methane (CH4), and SiF4.

Some of these gases are transported away from the eruption on ash particles while others form

salts and aerosols. Volcanic gases are also produced when water is heated by magma. Gases

also escape from pyroclastic flows, lahars, and lava flows, and may also be produced from

burning vegetation.

Acid rain can be produced when high concentrations of these gases are leached out of the

atmosphere. When Katmai erupted in 1912, acid rain damaged clothes that were drying outside

on a line 2000 km away from the erupting volcano in Vancouver, British Columbia (Bryant,

1991). High concentrations of CaF2 can burn vegetation and other material on contact. Fluoride

and chloride can contaminate water. Livestock have died from drinking such contaminated

water. Fluoride and chloride can also be irritating to the skin and eyes of animals, and can

damage clothes and machinery. Carbon monoxide and carbon dioxide are usually produced in

small amounts. However, large amounts of these gases will sometimes build up in low lying

areas and can asphyxiate livestock and harm vegetation (Bryant, 1991 and Scott, 1989).

One of the world's most photogenic volcanoes

By Leslie Patrick, for CNN

Source: http://edition.cnn.com/2014/09/11/travel/photogenic-volcanoes/

The last thing he ever saw: Dead hiker's posthumous photograph captures Japanese volcano's ash cloud sweeping towards him moments before it claimed his life Read more: http://www.dailymail.co.uk/news/article-2779616/The-thing-saw-Hiker-s-photograph-captures-Japanese-volcano-s-ash-cloud-sweeping-moments-claimed-life.html#ixzz4HxUGIGyc Follow us: @MailOnline on Twitter | DailyMail on Facebook

These haunting photographs capture the huge cloud of ash from a Japanese volcano that swept

towards a hiker just seconds before killing him.

The images - which were taken last Saturday on Mount Ontake - show the moments

immediately after the volcano erupted, sending dense plumes of gas and ash high into the sky

and leaving at least 47 people dead, with a further 16 people still unaccounted for.

Among the victims of the volcano was 59-year-old hiker Izumi Noguchi, whose body was found

near Mount Ontake's summit shrine compound. 

Search and rescue teams recovering his body discovered his camera and among the

photographs he had taken were images of the a huge cloud of ash creeping ever closer to him

following the eruption. His wife Hiromi has now opted to make the images public as a tribute to

Mr Noguchi's memory. 

The images emerged as doctors determined that almost all of those killed on Mount Ontake

died of injuries relating to rocks flying out of the volcano. 

Evaluation of BiomechanicalProperties of Human SkinCHRISTOPHER EDWARDS, PhD

The tensile strength of skin (ultimate load divided

by cross-sectional area) ranges from 5 to 30 N/mm2,

with the mean showing a maximum of about 21

N/mm2 at 8 years, declining to about 17 N/mm2 at 95

years.

The ultimate modulus of elasticity (calculated from

the end portion of the stress-strain curves) ranges

from about 15 to about 150 N/mm2. The mean shows a

maximum value of about 70 N/mm2 at age 11, with a

shallow decline to about 60 N/mm2 at 95 years.

Picture power: Tragedy of Omayra SanchezThe World Press Photo foundation celebrates the 50th anniversary of its annual photographic competition this year.

In the second of five pieces by photographers talking about their award-

winning work, Frank Fournier describes how he captured the tragic

image of 13-year-old Omayra Sanchez trapped in debris caused by a

mudslide following the eruption of a volcano in Colombia in 1985.

Red Cross rescue workers had apparently repeatedly appealed to the

government for a pump to lower the water level and for other help to

free the girl. Finally rescuers gave up and spent their remaining time

with her, comforting her and praying with her. She died of exposure

after about 60 hours.

The picture had tremendous impact when it was published. Television

cameras had already relayed Omayra's agony into homes around the

world.

When the photo was published, many were appalled at witnessing so intimately what transpired to be the

last few hours of Omayra's life. They pointed out that technology had been able to capture her image for

all time and transmit it around the globe, but was unable to save her life.

I arrived in Bogota from New York about two days after the volcanic eruption. The area I needed to get

to was very remote. It involved a five-hour drive and then about two and a half hours walking.

The country itself was in political turmoil - shortly before the explosion,

there had been a takeover of the Palace of Justice in Bogota by leftist

M-19 guerrillas. Many people had been killed and this had a big impact

on the way people in the remote town of Armero were helped. The army,

for example, had been mobilised in the capital.

I reached the town of Ameroyo at dawn about three days after the explosion. There was a lot of confusion

- people were in shock and in desperate need of help. Many were trapped by debris.

I met a farmer who told me of this young girl who needed help. He took me to her, she was almost on her

own at the time, just a few people around and some rescuers helping someone else a bit further away.

'Eerie silence'She was in a large puddle, trapped from the waist down by concrete and other debris from the collapsed

houses. She had been there for almost three days. Dawn was just breaking and the poor girl was in pain

and very confused.

 Dawn was just breaking

and the poor girl was in pain and very confused 

Frank Fournier's winning photo

Enlarge Image

All around, hundreds of people were trapped. Rescuers were having

difficulty reaching them. I could hear people screaming for help and then

silence - an eerie silence. It was very haunting. There were a few

helicopters, some that had been loaned by an oil company, trying to

rescue people.

Then there was this little girl and people were powerless to help her.

The rescuers kept coming back to her, local farmers and some people

who had some medical aid. They tried to comfort her.

When I took the pictures I felt totally powerless in front of this little girl,

who was facing death with courage and dignity. She could sense that

her life was going.

I felt that the only thing I could do was to report properly on the courage

and the suffering and the dignity of the little girl and hope that it would mobilise people to help the ones

that had been rescued and had been saved.

I felt I had to report what this little girl had to go through.

PowerfulBy this stage, Omayra was drifting in and out of consciousness. She even asked me if I could take her to

school because she was worried that she would be late.

I gave my film to some photographers who were going back to the airport and had them shipped back to

my agent in Paris. Omayra died about three hours after I got there.

At the time, I didn't realise how powerful the photograph was - the way

in which the little girl's eye connect with the camera.

The photograph was published in Paris Match magazine a few days

later. People were very disturbed by it because Omayra's plight had

been captured by television reporters and relayed around the world.

Then my picture of her in the last few hours of her life was published

after she had died.

People were asking: "Why didn't you help her? Why didn't you get her out?" But it was impossible.

There was an outcry - debates on television on the nature of the photojournalist, how much he or she is a

vulture. But I felt the story was important for me to report and I was happier that there was some reaction;

it would have been worse if people had not cared about it.

I am very clear about what I do and how I do it, and I try to do my job with as much honesty and integrity

as possible. I believe the photo helped raise money from around the world in aid and helped highlight the

irresponsibility and lack of courage of the country's leaders. There was an obvious lack of leadership.

 There was an outcry -

debates on television on the nature of the photojournalist - how much he or she is a vulture 

 People were asking:

'Why didn't you help her? Why didn't you get her out?' But it was impossible 

There were no evacuation plans, yet scientists had foreseen the catastrophic extent of the volcano's

eruption.

People still find the picture disturbing. This highlights the lasting power of this little girl. I was lucky that I

could act as a bridge to link people with her. It's the magic of the thing.

There are hundreds of thousands of Omayras around the world - important stories about the poor and the

weak and we photojournalists are there to create the bridge.

The question of the power of the press is more important today that it ever has been because it is so

much under pressure from the business side of things. 

Source: http://news.bbc.co.uk/2/hi/4231020.stm

When volcanic ash stopped a Jumbo at 37,000ftA plume of volcanic ash from

Iceland has led to flights across the

UK being grounded. The events

around one British Airways flight in

1982 reveal the potential dangers

of this sort of dust.

When all four engines on the

Boeing 747 being flown by Captain

Eric Moody shut down at 37,000ft,

he hadn't a clue why.

It wasn't until later, when Capt Moody, his crew and the 247

passengers on board the flight, were safely back on the ground, that he

discovered the cause of the narrowly averted catastrophe - volcanic

ash.

Airports are being closed across the UK after dust which spewed from a

volcano in Iceland, began drifting southwards. The experience of Capt

Moody, almost 30 years ago, shows the potential danger clouds of

volcanic ash present to modern jet aircraft.

There had been no hint of trouble when flight BA 009 took off from

Kuala Lumpur in Malaysia on the evening of 24 June, 1982.

The passenger jet effectively

turned into a glider

Heading for Perth, Australia, the

weather forecast for the five-hour

journey was good and the crew

were anticipating an uneventful

flight.

The first sign of trouble came as

the plane, which had hit cruising

height, headed past Java over the

south-eastern Indian Ocean.

Capt Moody, who had left the

cockpit for a stroll, was summoned back to the flight deck. As he

climbed the stairs of the Jumbo he noticed puffs of "smoke" billowing

from the vents in the floor and detected an acrid smell.

When he opened the door to the cockpit he saw the windscreen ablaze

with a St Elmo's fire - a discharge of static electricity.

But that alone wasn't enough to cause alarm, Capt Moody says,

recalling the events when he spoke to BBC's Good Morning Scotland

on Thursday.

"That's not unusual in high wispy cloud. But it developed into something

more than we'd ever seen before."

Looking out the side windows of the cockpit, the crew noticed the front

 It's not unusual [too see fire

like that] in high wispy cloud. But it developed into something more than we'd ever seen before 

Capt Eric moody

Volcanic ash halts all UK flights

of the engines were glowing as if lit

inside.

Then Capt Moody's flight engineer

detailed the impact the dust was

having on the aircraft itself.

"Engine failure number four...

engine failure number two," he said.

"Three's gone… They've all gone."

Within a few moments, a passenger

jet powered by four Rolls Royce

engines had become a glider.

Needing time to calmly consider his

options, Capt Moody used autopilot

to put the plane into a gentle

descent, and instructed his first officer to issue a mayday call.

While the crew on the flight deck were frantically trying to figure out the

cause of this freak failure, many passengers were largely unaware that

anything was wrong.

But eventually, when the passenger oxygen masks dropped as the

plane steepened its descent, the news had to be announced.

"Good evening ladies and gentlemen. This is your captain speaking.

We have a small problem. All four engines have stopped. We are all

doing our damnedest to get them going again. I trust you are not in too

much distress."

Eventually, after quarter of an hour without any power, the engines

were brought back to life. Ash had clogged the engines, which only

restarted when enough of the molten ash solidified and broke off.

"We glided from 37,000ft to 12,000ft before we got [the engines] going

again," recalls Capt Moody.

The plane headed back to Jakarta where it landed safely, though even

then one of the engines had failed again.

Capt Moody accepting an award

after his safe handling of the crisis

It was two days before investigators

confirmed that volcanic ash had

been responsible for the near

disaster. The plane had flown into a

cloud of dust spewed out by an

eruption of Mount Galunggung, 110

miles south east of Jakarta.

A close examination of the plane

revealed the damage a plume of

these tiny particles can do to an

engine - the tips of the turbine blades had been ground away. The

findings were eventually incorporated into a report on the dangers of

volcanic ash to aircraft.

Reflecting on the chilling events of that flight 28 years later, Capt

Moody, who lives in Camberley in Surrey, shows the sort of

understatement characteristic of those in his profession.

"It was, yeah, a little bit frightening."

Source: http://news.bbc.co.uk/2/hi/uk_news/magazine/8622099.stm

Electrolytic Tiltmeter BasicsThe words tiltmeter, inclinometer and clinometer all refer to an inertially referenced device that measures angularrotation with respect to a vertical gravity vector, the most stable of all external references. At Jewell Instruments wedenote our highest precision instruments as tiltmeters. We use the name clinometer for our general-purpose 900-Seriesproducts.All Jewell tiltmeters and clinometers include full signal conditioning electronics that produce stable output signals over awide range of input voltages. This important feature means you can be confident that your measurements representactual movement and not power supply variations. The high-level voltage, current, and serial outputs provided by ourelectronics assure reliable data delivery over long cables and wireless data links. As an added bonus, all of our tiltmetersinclude temperature measurement in their output.

Clare Wickett, a flight attendant on

BA 009

We divide our tiltmeters into four different Series based on measurement resolution and angular range. Certaininstruments report resolution in microradians or even nanoradians: a reflection of their high level of sensitivity.

Electrolytic Tilt SensorsJewell Instruments tiltmeters and clinometers each containone or more electrolytic tilt sensors. These sensors consist ofa sealed glass or alumina vial that is partially filled withelectrolytic liquid. This conductive fluid covers three or fiveinternal electrodes according to the sensor type. ReferenceFigure 1. The sensor’s operation is based on thefundamental principle that an enclosed bubble, suspendedin a liquid, always orients itself perpendicular to the verticalgravity vector.

Waves, Seismograms, and Seismometers

Contents

Waves : Wavelength and Period | Multiple Frequency Waves Seismic Signals Seismograph Systems : Analog and Digital Records | Seismometers | Seismometer Responses Seismograms : Broad-band, Long- Short-Period | Acceleration Seismic Networks

Introduction

The fundamental observations used in seismology (the study of earthquakes) are seismograms which are a record of the ground motion at a specific location. Seismograms come in many forms, on "smoked" paper, photographic paper, common ink recordings on standard paper, and in digital format (on computers, tapes, CD ROMs). Careful observation of ground vibrations during the last 80 years or so have lead to our understanding these vibrations, which are caused by seismic waves. We'll discuss waves in more detail in the next section, for now we need a few basics so that we can understand the variety of seismic signals and instruments.

Waves

A wave is a disturbance that transfers energy through a medium.

Waves are very common in nature: light is a wave, sound is a wave, ocean surf is generated by waves, and even matter has wave-like properties. The "disturbance" can be an alternating electromagnetic field strength (light), a variation in water height (ocean waves), a variation in material density (sound waves), or a distortion of the shape of the ground (seismic waves).

If you've felt Earth shake during an earthquake or explosion then you've felt seismic waves. These vibrations travel outward in all directions from their source. Waves generated by large earthquakes can be detected throughout the world and are routinely recorded and analyzed by seismologists.

Seismic waves are generated by many different processes:

Earthquakes Volcanoes Explosions (especially nuclear bombs) Wind Planes (supersonic) People Vehicles

The range of ground motion amplitudes that are of interest in earthquake studies is very large and seismometers we use are very sensitive. They can detect motions that are much smaller than the thickness of a sheet of paper or as tall as a room. We can detect ground motion in Missouri caused by increased surf activity as a hurricane or large storm system approaches the eastern coast of the lower 48 states.

Wavelength and Period

To understand the difference between these seismic recording instruments, we need to discuss a little about the waves. First, we use the term wavelength the refer to the peak-to-peak distance on a wave measured at a single time - like in a snapshot. To measure the wavelength directly, we would need a group of instruments that measure the amplitude of the wave at the same time but at different locations. If we record the ground motion at a single location for a range of time, we can measure the time between peaks in the motion, which we call the wave period. Another important term

is frequency, which is the inverse of the period, or one divided by the peak-to-peak time between wave crests.

Waves are energy transmitting phenomena that have an amplitude and a wavelength. The upper panel shows a snapshot of the wave at a single time. The lower panel can be thought of as the motion of a single point for an interval of time.

The period and the wavelength are related by a simple expression connecting the two with the speed of the wave

wavelength = speed x period .

Note that the units of the quantities on both sides of the equation balance: wavelength is a measure of distance, such as kilometers, speed is usually specified in terms of kilometers/second, and period is measured in seconds. Checking the physical units of equations is an important way to make sure that you've got the mathematics correct.

Multiple-Frequency Signals

The example waves sketched in the cartoon above show a monochromatic, or single-frequency wave, but most interesting signals are actually composites of many frequencies. In many ways seismic vibrations are analogous to light and sound waves so I'll discuss these more familiar waves first.

Light

Light is actually just one of a range of electromagnetic waves that we use each day. Other related waves, which differ primarily from visible light in their wavelength, include microwaves, radar, AM and FM radio, x-rays, etc.

Different colors are actually light waves with different frequencies. For example, red light has a longer wavelength than violet light. The wavelength for red light is about 650 nanometers (billionth of a meter), violet is about 400 nanometers, an x-ray is about 1/3 of a nanometer. FM radio and TV waves have wavelengths around 2-12 meters, AM radio signals have a wavelength of about 30 meters. Perhaps you've heard waves with wavelengths longer than red called infrared and those just shorter than violet called ultra-violet. These terms derive from the wavelength of the colors. TV, VCR, Stereo, etc. remote controls transmit signals in the infrared and high-energy signals like x-rays and ultra-violet radiation are in the part of the spectrum beyond violet.

White light is composed of all visible colors and sunlight contains a large range of the visible spectrum. We could use prism to split the light into it's different color components. The same thing happens when water vapor in the atmosphere is aligned with the rays of light arriving from the sun in just the right geometry, the water vapor acts like a prism and splits the sunlight into a rainbow.

Observed seismograms like white light, or sunlight are composites of waves of many different frequencies.

Sound

Another common example of a multiple-frequency signals are ever day sounds. Humans can hear frequencies in the ranges of 20 to 20,000 hertz and the musical note "middle A" is around 440 hertz. We can calculate the wavelength of sound waves that we can hear using the formula wavelength = velocity / frequency. In air sound travels at about 340 meters per second (the precise value depends on temperature, humidity, and pressure). We hear (measured in air) range from about two-thirds of an inch to 50 feet.

Seismic Signals

Much like sunlight contains different colors that can be split apart with a prism, seismic waves contain many different "frequencies" that we can record with specially "tuned" seismometers. The idea is completely analogous with light and sound

Seismic Light SoundShort-period Blue Treble

Long-period Red Bass

The range of ground motions that are interesting to seismologists is very large because the process of earth deformation occurs at many different rates and scales.

The amplitude range of interesting signals in earthquake studies as a function of frequency compared with a similar range of physical dimensions of some common items. Since I am comparing the "spectral" amplitude as a function of frequency with physical dimensions of the common items, the analogy is not perfect, but

the range of variation in size is well represented. "D" represents the distance from the earthquake. We usually specify large distances over Earth's surface in units of degrees and 1 degree = 111.19 km.

The large range of amplitudes we are interested in exists because we are interested in all the processes occurring in Earth, from small rock fractures that form in mines to the great earthquakes that occur each year. The amount of energy released by these different processes is enormous, and the large range of interesting amplitudes reflects this.

Seismographs

Seismographs, which generally consist of two parts, a sensor of ground motion which we call a seismometer, and a seismic recording system. Modern seismometers are sensitive electromechanical devices but the basic idea behind measuring ground movement can be illustrated using a simpler physical system that is actually quite similar to some of the earliest seismograph systems.

 

A simple mechanical system that illustrate the basic ideas behind of seismic recording systems.

The physics behind the sensor is Newton's Law of Inertia:

"A body in motion tends to stay in motion unless acted upon by a force, and a body at rest tends to remain at rest unless acted upon by another force."

The sensor and the recorder do not have to be located next to one another. For example, the recording "drum" in the lobby of the Macelwane Hall records signals from a sensor in the basement. On the 3rd floor of Macelwane Hall, we record signals from sensors distributed throughout the central United States (the center hallway on 3rd floor is open for public viewing 8:30 AM-3:30 PM weekdays).

Seismometers are spread throughout the world, but are usually concentrated in regions of intense earthquake activity or research. These days, the recording system is invariably a computer, custom designed for seismic data collection and harsh weather. Often they are also connected to a satellite communication system. Such systems enable us to receive seismic signals from all over the world, soon after an earthquake.

A real-time seismic recording system with digital storage and satellite communications. Ground vibrations are detected by the sensor, digitally recorded, and then transmitted via satellite.

Classic Seismograms

For most of the last century, seismograms were recorded on sheet of paper, either with ink or photographically. We call such records "analog" records to distinguish them from digital recordings. These records are read just like a book - from top-to-bottom and left-to-right.

The classic paper seismogram is read like a book, from left-to-right and top to bottom. A continuous record is constructed by drawing the line as a sheet of paper fastened to a rotating drum constantly moves horizontally on a threaded attachment. When the ground vibrates the pen moves up or down creating th seismic record of the vibrations. Seismograph station and component, date and start time are recorded on the upper left of this paper.

One problem with these mechanical systems was the limited range of ground motion that could be recorded - vibrations smaller than a line thickness and those beyond the physical range of the ink pen were lost. To circumvent these limitations we often operated high and low-gain instruments side-by-side, but that was neither as efficient nor effective as the modern digital electronic instruments. However, modern "digital"

or computerized instruments are relatively new, only about 15-20 years old, and most of our data regarding large earthquakes are actually recorded on paper (or film). Additionally, we still use paper recording systems for display purposes so we can see what is going on without a computer.

Digital Seismograms

Today, most seismic data are recorded digitally, which facilitates quick interpretations of the signals using computers. Digital seismograms are "sampled" at an even time interval that depends on the type of seismic instrument and the interest of the people who deploy the seismometer. The same principle is used to provide "digital" sound on compact disks. The motion of the ground is continuous, but we can pick only certain positions and reconstruct the motion (within certain limits).

A digital seismogram is a record of the ground movement stored as an array of numbers which indicate the time and the movement of the ground for a range of times and are easily analyzed using computers. The principle is the same as that used for digital audio signals that are stored on Music CD's.

Also, since with live in a three-dimensional space, to record the complete ground motion, we must record the motion in three directions. Usually, we usually choose:

Up-down North-south East-west

With three records of ground motion in three directions, a single seismic station that records about 20 samples per second must store or transmit about

3 x 1.7 x 10^6 samples/day ~ 5 x 10^6 samples/day

which equals about 5 megabytes per day. For each 200 seismometers of this type around the world, we are recording about

1,000 mbytes/day ~ 1 gigabyte/day

Although we can do better using compression algorithms to save storage, keeping up with this flux of data is a challenge.

Seismometers

Before technological advances in the last few decades, to record seismic signals we developed and deployed many different kinds of seismometers. In the 1960's, as part of an effort to verify underground nuclear test threshold treaties, two seismometer models became standard for global earthquake analyses, the world-wide network long-period and short-period instruments. These instruments have been replaced by "broad-band" instruments which can detect ground motions over large ranges, or "bands", of periods. However, since much of our historical data are recorded on the older "narrow-band" short-period and long-period instruments, they remain important sources of data. Different seismometers record different frequencies (or periods) of ground motion and are analogous to different colors in a picture.

Seismometer Response Curves

Seismometers are usually designed to record signals over a specified range of frequencies (or periods) so it is convenient to discuss instruments based on the range of vibration frequencies that they can detect. Thus one way to characterize seismometers is to describe the range of vibration frequencies that they can detect. A plot of the amplification as a versus frequency is called an instrument response. An instrument is sensitive to vibrations at frequencies for which the "response" curve is relatively large. Five sample instrument response curves are shown below. The frequency of is shown along the horizontal axis, the equivalent period (period = 1/frequency) is shown along the top horizontal axis. The vertical axis shows the ground-motion amplification factor.

To characterize an instrument, what's really important is the range of amplitudes, not the specific amplification, which is usually adjusted depending on the location of the seismometer. I have used numbers around one to illustrate the differences between the response curves for different instruments but actual amplification factors are usually much larger than those shown.

The broad band instrument senses most frequencies equally well; the long-period and short period instruments are called "narrow" band, because they preferentially sense frequencies near 1/(15 s) and 1 hertz respectively. The yellow region is the low end of the frequency range audible to most humans (we can hear waves around 20 hertz to 20,000 hertz).

The left panel is a comparison of a modern broadband seismometer response and the classic World-Wide Standard Seismic Network (WWSSN) long- and short-period instruments. The same broad-band response is shown in the right panel, to compare the response with a special short-period instrument, the Wood-Anderson, and an accelerometer. The Wood-Anderson short-period instrument was the one that Charles Richter used to develop his magnitude scale for southern California. The accelerometer is an instrument designed to record large amplitude and high-frequency shaking near large earthquakes. Those are the vibrations that are important in building, highway, etc. design.

Seismograms

The figure below shows the results of different recording instruments on the measurements of ground motion (displacement) for an earthquake that occurred in Texas, in 1995. The observations were recorded on a broad-band instrument and the

signals that would have been recorded on the WWSSN instrument types were simulated using a little mathematics since all the vibrations that would be detected by the long- and short-period seismometers are also recorded by the broadband seismometer.

The above diagram shows the ground displacement observed near Tucson, Arizona, caused by an earthquake in southwestern Texas. The top panel shows the vibrations measured using a broad-band seismometer, the middle panel shows the vibrations as they would be detected by the long-period sensor, and the bottom panel the vibrations that would be sensed by a short-period sensor (scaled by a factor of 10 so we can see them better). The displacements are shown in microns, which are 1x10^-6 meters.

Accelerometers

Another important class of seismometers was developed for recording large amplitude vibrations that are common within a few tens of kilometers of large earthquakes - these are called strong-motionseismometers. Strong-motion instruments were designed to record the high accelerations that are particularly important for designing buildings and other structures. An example set of accelerations from a large earthquake that occurred in near the coast of Mexico in September of 1985 are shown in the diagram below.

The above diagram shows the ground displacement recorded at a strong-motion seismometer that was located directly above the part of a fault that ruptured during the 1985 Mw = 8.1, Michaocan, Mexico earthquake.

The left panel is a plot of the three components of acceleration (one vertical and two horizontal). From the curve we can see that strong, high-frequency shaking lasted almost a minute in the region. The peak acceleration was about 150 cm/s^2. Often we will report such numbers as a fraction of the gravitational acceleration at Earth's surface, which we call "g" and which is about 980 cm/s^2. Thus the peak acceleration in this region was about 150/980 g, or about 0.15g, or equivalently 15% of g.

One place with which we are all familiar with acceleration changes is an elevator. The acceleration that you experience in an elevator is about 2 m/s^2 or about 0.2g. However, in an elevator the transition from 0g (not moving) to 0.2g is smooth and comfortable. During the earthquake, you can see that the ground accelerations were varying between -0.1g to +0.1g several times each second, for at least 10-15 seconds. That is not very gentle shaking.

The middle panel shows the velocity of ground movement, which we can calculate using calculus - the velocity is the integral of the acceleration. The peak velocity for this site during that earthquake was about 20-25 cm/sec. And if we integrate the velocity, we can compute the displacement, which is shown in the right-most panel. From the displacement plot, we can see that the permanent offsets near the seismometer were up, west, and south, for a total distance of about 125 centimeters (since the ground displacement is a vector and the seismograms show the three components of that vector, we must use the vector length to specify ground offset, which is the square root of (50^2 + 50^2 + 100^2)).

Seismometer Networks

Seismic recording systems monitor the ground movements continuously and we have records of the motion of the ground at some places dating back nearly 100 years. Many institutions have been monitoring earthquake activity since the development of seismometers and for example, Caltech and UC Berkeley have relatively long histories of monitoring California seismic activity. At Saint Louis University, Father Macelwane and his students operated a seismic station on campus since around 1911 and we have preserved seismograms for many of the large earthquakes that occurred since then. In 1974, Father Stauder (now one of the University vice presidents) initiated a regional monitoring program to study seismic activity in the central US in general, and the New Madrid region in particular. That work continues and we are about to upgrade the 1970's instruments with modern, broader-band digital equipment.

International monitoring of earthquakes also has a long history. I mentioned the WWSSN that was deployed by the US Government in the 1960's to monitor nuclear explosions and help verify nuclear test treaties since the 1960's. Several smaller networks were deployed before then and today's global seismic network is a cooperative, international effort that consists of more than 100 seismic recording stations.

Locations of seismic recording stations that are part of the Global Seismic Network - a cooperative federation of international seismology organizations that share data. Click here to go to the Incorporated Institute for Seismology (IRIS) WWW site where the original map and many details about each station can be accessed.