Earthquakes & Volcanic Studies (Kobe! Fuji!)

Earthquakes & a case study of the Great Hanshin Earthquake (Kobe) By Jasmine and Melody 310 ‘06

Kobe: The Story

Formally known as the South Hyogo Earthquake

Other names include Hyogo-ken Nanbu & Kobe

Struck western Japan at 0546 hours, January 17, 1995

Lasted about 20 seconds

Devastation was the worst in the port city of Kobe

7.2 magnitude quake toppled roadways, wrecked docks, severed communication lines and left the city in flames into the next day

killed almost 6500 people

Why? (Investigation of Cause)

Japan’s precarious position upon 3 plates , formed by molten magma from the Phillipines plate

Subducting Phillipines sea plate

Subducting Pacific plate

 Under Eurasian continental plate

Friction resulting from colliding on destructive margin

Seismic energy from collisions

is eventually released through

earthquakes

Subduction happening about

10 cm per year

Why? (Geological Conditions)

Kobe was located slightly off the Median Tectonic line, a zone of strike-slip faults

Focus was 16km below Awaji-shima, an island in the Japan Inland Sea, close to Kobe

Shallow focus and probable surface ruptures

Kobe, a low-lying port city, was situated on soft, wet ground

Constructed on 2 artificial islands of water-saturated loose fill

High potential of ground liquefying, suspending & loosening sand, with ground unable to bear weight & buildings collapsing

Why? (Human Conditions)

Kobe was an older city with residential infrastructure built with wood on the bottom and heavy tiles as roofs

Created more shear force than frame could resist (1.1, 1.2)

Old infrastructure codes allowed weaker superstructures at 5 th floor

Thus some buildings sunk in at 5 th floor (1.3)

Most infrastructure were old & built before strict seismic codes in 1980s

Lifelines constructed in the 60s and 70s, before modern construction practices

Debris choked streets & rescue work was delayed. Fires were widespread, making it more deadly

1.1 1.2 1.3

Why? (Cont’d) & Effects..

Older reinforced concrete structures

Vertical steel rods were unable to preserve structure from horizontal shaking forces (2.1)

The Hanshin Expressway fell on its side (2.2)

Soft soil, bad infrastructure?

Subways sunk in, luckily as it was early morning so nobody was killed (2.3)

Soft, saturated soil [liquefaction]

Broken gas lines

Fires ensued, killing those who survived the quake & slowed rescue work (2.4)

2.1 2.2 2.3 2.4

Ouch! (Some More Serious Effects)

300,000 made homeless, requiring shelter

People squeezed into public buildings like schools and town halls

Overcrowded, unsanitary

Lack of food, clean water & medical supplies

Jan 17 th – cold winter , at temp. of -2 degrees Celsius

Hanshin Expressway closed, 130 km of subway closed

Transport impaired, interrupting trade and commerce

Disrupted school, unemployment, etc

120 out of 150 cranes at shipyard fell, destroying goods and ships

3-5% of Japan’s economy located near/at Kobe

Shipbuilding, steelworks are just some that got affected

So What Next? (Coping With Problems)

Water, gas, electricity were restored within half a year

Railways back in service by August

134,000 housing units constructed, although some people still lived in temporary accommodation

New laws passed to make buildings more earthquake-proof

More instruments installed in area to monitor earthquake movements

However, Japanese government was heavily criticized

WE ARE POLITICALLY CORRECT & DIPLOMATIC HAHA

(slow, uncoordinated, badly equipped, refused foreign relief)

What is an earthquake?

A

Sudden,

Violent,

& (sometimes) Catastrophic

movement of a part of the Earth's surface

Causes of Earthquakes

Rocks in earth are elastic, may store mechanical energy same way a spring does

Rock gives way, cracks when forces are applied strong enough that they exceed the strength of the weakest part of the solid rock

These forces include convectional currents that cause our plate movements

Strain energy is relieved, forces are accommodated by sudden dislocation of rocks on either side of crack

Releasing of energy and dislocation of rocks is called ‘elastic rebound’

Causes of Earthquakes (cont’d)

At it’s source (or focus ), movement may be over in a few seconds

However, resulting huge amount of energy released travels through rocks of the crust in seismic waves

The crack marking the dislocation is known as a fault .

Following an earthquake (the displacement of the rocks), the rocks on either side come to rest in new positions

Causes of Earthquakes (cont’d)

These rocks are held in place as a result of friction acting on the rough surfaces of the rocks as well as other forces exerted upon them from the crust

The stresses and strains that caused the original earthquake continue, and eventually break the ‘frictional lock’ to cause another earthquake

Seismic Gap theory

A theory that was developed to explain the regular occurrence of earthquakes in certain places

For example, Parkfield, which has produced magnitude 6 earthquakes every 20-30 years

Idea – since plates are moving all the time at a steady rate, and since that is what causes the earthquakes;

… we should be able to predict the occurrence of earthquakes based on when and where they have occurred in the past.

Effects of Earthquakes

Varies from earthquake to earthquake

Mainly includes the ground shaking, and in more serious cases damage to and destruction of infrastructure

As well as tsunamis in port cities

Fires as a result of the collapse of buildings & broken gas pipes

Loss of life; especially in large cities

Divided into Primary & Secondary,

Primary ones - deformation of ground near the fault (immediate collapse of buildings, roads)

Secondary ones - result from seismic waves away from fault rupture (fire, aftershocks, homelessness)

Primary Effects

Formation of fault-scarps

Collapse of existing structures, both natural and man-made

Formation of ‘ sand boils’ , different types of shock wave vibrations can cause sand and water to spurt out in little cones that look like miniature volcanoes and may reach up to 30cm high

In rare cases, streams diverted (due to fault scarps)

Liquefaction –shaking increases underground water pressure and water invades all interstices on the sandy silt.

Buildings often sink

Common cause to countless lives lost in devastating earthquakes, e.g. San Francisco, Messina and Reggio di Calabria as well as Kobe.

Secondary Effects

Landslides

Changes in the level of the land

Flooding; whereby the debris blocks up drainage systems

Water-borne diseases as a result of flooding

Cholera, typhoid, dengue

Lack of basic necessities such as food, potable water and basic sanitary needs

Holdups in transport and communications

Leads to more deaths as it delays the rescue of people trapped in the debris

Measuring Earthquakes

The Richter Scale

Based on the size, or amplitude of the waves traced by the pen on a seismograph

Seismographs had to be standardized to allow for a fair comparison

Based on complex mathematical formulas

However it tells us little about the effect of earthquakes on human lives

Measuring Earthquakes (cont’d)

The Mercalli Scale: the intensity of an earthquake is assessed by subjective and qualitative observations of the landscape and environment

Unlike the Richter scale; not based on mathematical calculations

Calculates the degree of damage done (in a scale of I-XII) based on timing and site of occurrence

E.g. An earthquake of magnitude 8.4 (Richter) may cause relatively little damage in a sparsely populated area as compared to an earthquake of magnitude 5.8 that occurs in a town

Measuring Earthquakes (cont’d)

Other factors that affect intensity of the damage includes distance of the settlement in question to the epicenter

Settlements built on reclaimed land , or soft, water-saturated materials such as silt , would also be at higher risk as silt magnifies ground motion by as much as 75 times

Volcanoes & a case study of Fuji-san By Jasmine and Melody 310 ‘06

Fujiyama: The Story

Actually made up of three different volcanoes; Ko-mitake , Ko-Fuji (Older Fuji Volcano) and the present Shin-Fuji (Younger Fuji Volcano) which lie one upon the other.

Ko-Mitake – has been dormant since 100 thousand years ago.

Ko-Fuji – which formed the

base of the current Mount Fuji,

was active between 100,000 &

10,000 years ago.

Shin-Fuji – which is responsible for the mountain’s current shape, started to erupt about 10 thousand years ago continued erupting repeatedly over 100 times during a period of about 10 thousand years.

!!!! Flank resulted from last eruption. Fuji isn’t TOTALLY symmetrical.

Fujiyama: The Story (cont’d)

Studded with more than 100 parasitic cones and flank openings although most of them are too small to be seen.

Dimensions: about 3776 m above sea level, 50 km across the base with a circular crater of about 500m across

Archetype of the stratovolcano

Also known as composite volcanoes composed of lava flows & lahar in alternate layers.

Fuji erupted at least 16 times since 781 AD. Most of these eruptions were moderate to moderate-large in size

Two largest eruptions to date were in 1050 and 930 BC

Fujiyama: The 1707 eruption

Latest Eruption: 1707-1708, where 0.8 cubic km of ash, blocks, and bombs was ejected

Eruption started December 16 and ended about February 24, 1708

This flank eruption was explosive and generated mudflows

It caused damage, but according to

‘Volcanoes of the World’, there were

no fatalities.

What is a Volcano?

In basic terms, a volcano is a hole in the earth’s crust , on land or on the sea floor, from which materials are expelled naturally from below.

What is a Volcano? (cont’d)

These ( pyroclastic ) ‘materials’ may include hot lava, cinders, blocks, ash or pumice, cold rock fragments of all sizes, aerosols, steam and water.

Some materials cool and pile up around the hole, or chimney, and often form a cone shaped hill ( composite )

Other fine materials ( lahar ) can be blasted into the stratosphere, carried around the world, and may remain as aerosols that veil the sun for several years. ( Mt. Krakatoa )

The Mafic Cone (ooh, runny!)

Low silica content (~50%)

High Fe and Mg content

Relatively less viscous (hence the runny-ness)

Travels relatively faster, able to cover distance of 150km before stopping to solidify

Low-level of explosivity

Because less gas escapes and hence, little vesiculation

Low gradient due to slow drying of lava

The Felsic Cone (look at the gas!)

High silica content (~70%)

Low Fe and Mg content

Relatively more viscous

Hence moves at slower speeds

Often explodes violently, producing large amounts of aerosols and fahar

Gas escapes; causes viscosity to increase

Vesiculation is initiated; may trigger an explosive volcanic eruption

Steep gradient due to viscosity

Formation of a Volcano

Generated by eruption of magma through a planet's surface; molten rock welling up from the planet's interior

Magma wells up due to tectonic activity in the plates – e.g. when plates converge or diverge – causing either constructive , destructive or transform plate boundaries.

Formation (types of plate boundaries)

Constructive

The most common

The least visible (usually occur at considerable depths underwater)

Cause the formation of a mid-ocean ridge

E.g. the Mid-Atlantic Rift

May sometimes lead to volcanoes reaching the surface – e.g. St. Helena

Formation (types of plate boundaries)

Destructive

The most visible and well-known

Form above the subduction zones, where oceanic plates subduct under the lighter continental plates

Cause the formation of subduction volcanoes

E.g. the Nazca plate diving under the South-American plate caused the formation of the Andes range

Formation (Hotspots)

Originally generalization for volcanoes that didn't fit into one of the above two categories

Refers to more specific circumstance today - where an isolated plume of hot mantle material hits the underside of the crust .

Mantle plume can lead to volcanic center that’s not obviously connected with a plate margin

E.g. Hawaiian Islands

Generated by a hotspot underneath the oceanic crust of the Pacific

E.g. 2: Yellowstone

In this case involves continental crust

(Iceland may also be considered, but the coincidence of a hotspot intersecting a oceanic ridge constructive margin complicates things. )

Primary Effects

Phreatic eruptions (steam-generated eruptions)

Explosive eruption of high-silica lava (e.g., dacite & rhyolite)

Effusive eruption of low-silica lava (e.g., basalt)

Pyroclastic flows

Lahars (debris flow)

Fumarolic activity (gaseous emissions, e.g. H20, C02, S02, mainly, with lesser toxic gases HCl, HF, H2SO4, H2S)

Secondary Effects

Earthquakes

Hot springs

Fumaroles

Mud pots

Geysers

*Low-magnitude earthquakes often precede eruptions.

(that’s like Mother Nature’s forewarning)

Measuring Volcanoes

Volcanologists monitor the following phenomena to help forecast eruptions:

Seismicity

Seismic activity – or earthquakes etc, is observed

Gas Emissions

Magma nears surface,

pressure decreases, gas escapes

Sulphur dioxide (main

component of volcanic gases, increasing amounts of it herald arrival of increasing amounts of magma near surface)

Sampling volcanic gases in an evacuated flask.

Measuring Volcanoes

Ground Deformation

Swelling of volcano signals magma has accumulated near surface.

Tilt of volcanoes’ slope

Mapped, rate of swelling tracked in order to help predict eruptions

Increased rate of swelling, accompanied by increase in sulphur dioxide emissions & harmonic tremors is a high probability sign of eruption

Deformation of Mount St. Helens prior to the May 18, 1980 eruption is another classic example of deformation

Most cases of ground deformation are usually detectable only by sophisicated equipment used by scientists 

Eruption Styles

Four main eruption styles: Hawaiian, Strombolian, Vulcanian and Pelean, after best known volcanoes of each group

Mild eruptions

Limited to emissions of gas, steam, hot water, sulphurous fumes and bubbling mud from geysers, hissing holes and fissures.

May be common on dormant or dying volcanoes

Moderate eruptions

Expel lava and some gas – lava usually basic, basaltic

Occur on fissures

Most common kind of eruption – not usually very dangerous

Eruption Style (Hawaiian)

Magma rises in a central vent

Thin flows of basalt emerges and spreads over a wide area

Volcano ‘grows’ as each eruption piles on a new layer

Lava flows may also emerge from vents on either side of the summit, in flank eruptions (Fuji!)

Characteristically broad-based shape – ‘shield volcano’

Erupts relatively often

Usually non-explosive in nature

Destructive but slow-moving flows

Eruption Style (Strombolian)

Constant, but usually short lived moderate activity

Usually cinder cones with a shallow, bowl-shaped crater

Cinder cones : molten fragments settle on cones and embers cool – eventually many layers are created after constant repeating of this process

Dust and ash released into atmosphere ( lahar )

Lava flows are initially fast-moving, but eventually slow down

Eruption Style (Vulcanian)

Vigorous eruptions

More explosive than moderate eruptions as they often contain a strong element of gas or steam

Gases escape from rising magmas that are viscous and silicic – large black cloud of ash and steam

Sudden eruptions – quick successions of explosions shatter fragments off the volcano, shooting them from the chimney

May be followed by several days of total calm until eruptive spasm is over

Although mostly basaltic, lava flows are often viscous

Larger cinder cones than strombolian counterparts

Craters are large and deep

Eruption Style (Pelean)

Viscous, silicic magma rises

Gases in magma separate into bubbles and, near the top of the chimney, blow the magma into smithereens, forcing a lot of volcanic material of all sizes to be blown out