Natural Hazards Midterm

Earthquake Prediction
future earthquakes will likely occur where past ones did
Earthquake Preparation:
mitigation’s last resort
What is an Earthquake?

            A subterranean event; Rapid, sheer slip across an approximately planar surface; Seismic waves

Where do Earthquakes occur?

At plate boundaries; At seduction zones; Spatially clustered

Seismometer
buried deeply in the ground to remove surface noise; pendulum principle;             translates the ground waves into volts via a computer
Seismograph
measures ground motion as a function of time
How many earthquakes are there every year?
Many small, few big
Avg. number of Eqs per year with magnitude greater than: 5, 6, 7, 8

8……………………………………1

                        7………………………10-15 (12)

                        6………………………….100-200

                        5…………………….1,000-3,000

Given the G-R seismicity, 30 yrs., when will the next mag5 EQ occur?

                        Sometime in the next 30 yrs.

Given the G-R seismicity , 30 yrs., when will the next mag6 EQ occur?


Sometime in the next 300 yrs.
When will an earthquake occur?
Random
Fault Creep
slow sliding of a fault; doesn’t generate seismic waves; safe
Stick slips
tension, followed by a rapid slip; earthquakes; generates waves

Earthquakes are hazardous because…

Seismic waves in the epicentral region cause ground shaking

Our buildings fall down on us. They aren’t as tough as we are.

Landslides can be triggered

Tsunamis can be generated

What can be more damaging in a metropolitan area than the earthquake?
Fire (SF 1906)
Regional Earthquakes

nearby, within a few hundred kilometers

 

San Francisco 1906 Facts 

7.8

            Ruptured ground surface along the San Andreas

            Ground shift of 4-5ft/sec.

San Francisco 1906 Damage

$400 mil in property damage ($8 billion today)

            3000+ dead

            26 minutes

            Fires 90% of damage

            Fires burned for 3 days and 3 nights

            Firebreaks backfired 50% of damage

            Flames at the melting point of iron

            Fires essentially burned itself out

San Francisco 1906 Aftermath

Insurance with fire only, so arson

            SEIC: first gvmt. Commissioned, scientific investigation into EQs

            Theory of Elastic Rebound= fault will become elastic and distort, then return to its original state?; faults cause EQs, rather than the reverse

            Building codes developed

Loma Prieta 1989 Facts

Santa Cruz Mountains of Cali

            15-20 sec.

            6.9 or 7.1?

            Caused by a 2 meter slip in the San Andreas fault

Loma Prieta 1989 Damage

62 dead

            $6 billion

            San Francisco Bay Bridge unusable for 1 month

            Balls make lives?

            Fires cause most damage

            Most devastation occurred over 60 miles away in San Francisco

Loma Prieta Aftermath

            First test of ATC-20

            Response time hindered by landslides that damaged highways 

Mercalli Intensity Scale

Attempts to use us and our surroundings to describe EQs

            Subjective

            People are unreliable about EQs

 

Great Hanshin EQ 1995 Facts

5:46 AM January 7, 1995

            7.2

            Shallow focus

            ìStrike-slip mechanismî

Great Hanshin EQ 1995 Damage

Over 5,000 killed

            Elevated highway collapse

            Older wood-frame houses as a liability because of the roof

            Seismic codes were mostly followed afterward

            90% of deaths occurred within 15 minutes

Great Hanshin EQ 1995 Aftermath

Most people were rescued by neighbors           

            Unexpected, so not many preparations

            Serious evacuation drills in schools since

            SRPA: not very successful

Volcano

A place where molten rock reaches the Earth’s surface

Magma

lava; molten rock

Strato
Volcano: usual; Mt. Fuji, Mt. St. Helens
Shield

Volcano: flatter; Iceland, Hawaii

Where do volcanoes occur?

Convergent plate boundaries

            Pacific Ocean basic is surrounded by trenches

            Ring of Fire: 2/3 of world’s active volcanoes

Volcanic Explosivity Index

 : ranges from 0 to 8 according to volume of material ejected

Viscosity
inversely related to temperature; directly related to silica content
Mafic magma
mid-ocean ridges and hot spots; fluid lava
Felsic magma

: adjacent to subduction zones; thick, pasty lava

Andesite Line

Boundary between mild oceanic eruptions and more explosive continental eruptions of the Pacific basin
Shield Volcanoes

Built by gentle outpouring

            When magma cools, it forms basalt

            Mostly oceanic

Lava Domes

Not all lava is created equal!

            Sometimes, it will be thick and move slowly

            New magma flows over cooled magma, making the dome gradually larger

            Magma that forms lava domes is usually rich in silicates

            Can be dangerous

            Build-up of pressure can lead to an explosion

Cinder Cones

 

Smallest and most common
Benefits of Volcanoes

Help begin the water cycle

            Help enrich soil

            Form the earth’s valuable resources

            Shaping the earth 

3 Types of Magma

Basalt:

            Andesite

            rhyolite

What type of Volcano lives in Hawaii?
Shield!
Local Dangers of Volcanoes
explosive blast,  magma rivers, tsunami waves

Regional Dangers of Volcanoes

fall-out from clouds of rock fragments and dust

 

Global Dangers of Volcanoes

 

: rock dust in atmosphere changes weather and climate

Can you prepare for landslides?
Yes
Can you predict landslides?
yes, once the local conditions are characterized
Can you prevent landslides?

yes, much can be done including some simple, inexpensive actions, such as vegetation and drainage control.

Can you promote landslides?

yes. An increasing number of landslides are caused or promoted by human activity.

Can you prevent Earthquakes?
no, though no one does that currently
What type of volcano are the Cascades?
Strato
What are landslides?

sliding of a mass of loosened rocks or earth down a hillside or slope

Where are landslides?

Landslides are associated with hilly or mountainous landscapes. They are also common along coastlines and river valleys. Landslides occur most frequently in regions where climate and precipitation, bedrock and soil conditions, and slopes are susceptible to failure.

 

 

How do we mitigate landslides?

Zoning laws

            Control water

            Promotion: How? Construction