test 2

• mean: 75, median: 79• multiple choice: 42 questions, 2 points each• short answer: 4 questions, 4 points each• 100 total: circled number inside front cover of

blue book• answer key to multiple choice is posted on

blackboard under ‘test materials’

Earthquakes and Related Phenomena

chapter 8

EQ terms• fault: break (fracture) in

a rock• rupture: breaking of

rock along a fault• hypocenter (focus):

rupture point of rock below surface

• epicenter: point on earth surface directly above hypocenter

• scarp: topographic (elevation) display of a ruptured fault

how to quantify the size of an event?• describes the energy released by an EQ• different types based on different parameters:– Modified Mercalli Scale: based on how much people felt the

ground shake• very subjective, but all we have for past events• scale from I to XII

– Richter magnitude: based on size (amplitude) of largest seismic wave• easy to measure• scale from 0 to ~9

– moment magnitude: based on amount of energy released by an event• more accurate because takes into account specific factors about the

fault (slip area, rigidity of rock, etc.)• scale from 0 to ~10

how do we record events?• observations, journals (Mercalli)• seismograms (Richter)– record movement (displacement)

of earth in three directions• vertical (up and down)• horizontal (left and right)• transverse (front and back)

• known information about specific fault (moment)

how to interpret the scales

• Richter magnitude (M) is predominantly used– for each 1 step up in M, there are ~1/10 of the

number of events from before– for each 1 step up in M, there is an ~10x increase

in ground shaking – for each 1 step up in M, there is an ~30x increase

in energy release

where do EQ’s happen?

where do EQ’s happen• plate boundary EQ’s:– transform boundaries

(California)– convergent boundaries

(Japan)• intraplate EQ’s– seismically active regions

not directly linked to plate boundaries (rifts)

– New Madrid (Missouri, Tennessee, Arkansas): Reelfoot rift

where do EQ’s happen?

fault types• strike slip– right lateral– left lateral

• reverse– hanging wall up

• normal– footwall up

fault activity• use geologic record to determine when was the

latest rupture on a fault• active faults: movement in the last 10000 years• partially active: movement in the last 1.65

million years• inactive: no evidence of movement within the

last 1.65 million years• public policy relationship: US Nuclear

Regulatory Commission: “active” faults are movement within the last 50000 years

activity measurements

• recurrence interval (how often a fault is active) found by:– paleoseismic data: average time between events,

according to geologic data– slip rate: average movement (displacement)

divided by movement per year– seismicity: averaging time between historical

earthquakes

seismic waves• actual displacement and shaking of earth is

due to seismic waves (energy release when fault ruptures)

• seismic waves travel several kilometers per second through the earth

• types:– P waves (primary, compressional)– S waves (secondary, shear)– surface waves

P waves• faster of seismic waves

types• can travel through

solids, liquids, and gasses

• moves in same direction as wave motion

• moves like pushing a slinky together and pulling it apart

S waves• slower than P waves• can only travel

through solids• moves perpendicular

to wave motion• moves like a cracking

a whip or a snake along the ground

surface waves

• slowest seismic wave type

• complex vertical and horizontal motion patterns

• cause most damage to buildings, etc.

how we record EQ’s (again)• each wave type has

a characteristic size• by looking at a

seismogram that recorded an EQ, we can see when the different wave types arrived

how do we know where EQ’s happen?• can analyze multiple

seismograms to tell where an EQ’s hypocenter was– we know the time the

difference between the amount of times the different wave types took to get to the seismogram

– we know how long it takes those wave types to travel a certain distance

ground material effects• different earth material respond differently to

seismic waves• amplitude: vertical movement of shaking• hard rocks resists shaking, unconsolidated

sediments are vulnerable to shaking

ground acceleration

• ground shaking is recorded as acceleration• how fast the shaking of the ground changes speed

(horizontally and vertically)• recorded in comparison to gravity acceleration

(9.8 meters/second2)– if an earthquake has 1g ground acceleration, that

means the shaking of the ground was increasing at 9.8 meters/second2

– an earthquake of M7.0 has ~25%g ground acceleration

EQ cycle

• aftershocks (following a large event)

• seismic inactivity• strain accumulation• foreshocks• main event

effects of EQ’s

• shaking and ground rupture– immediate, damage to buildings, loss of life

• fires– from gas leaks, electrical lines, etc.

• disease– from dust with bacteria or contaminated drinking

water

effects of earthquakes• tsunamis

estimating risk

• US Geologic Survey (USGS) produces ‘seismic hazards maps’ for the US and other regions

• maps are based on the percentage likelihood that a certain size event will happen in a given timeframe

seismic hazard maps• shows a 2% chance that the color percent gravity

shaking will be exceeded in the next 50 years• public policy relationship: ability to properly place

emergency response facilities and specify building codes

forecasting methods

• we use ‘forecast’ instead of ‘predict’ because:– forecast includes the percentage chance that an

event will happen– predict indicates that the event will happen

• public policy relationship: how much of a risk does a possible event need to be before we take action?

forecasting methods: foreshocks

• foreshocks– M9.0 EQ in Japan occurred on a Friday, on that

Wednesday they had experienced a M7.2, and on that Thursday they experienced three M6.0+

– should we have forecasted a major event?

forecasting methods: radon gas• radon gas is naturally present in rocks• it is believe that before an EQ, rocks expand

and take in water• radon is removed from the rock as the water

leaves• the radon gas rises to the top of the

atmosphere, and can be seen as a temperature increase

forecasting methods: radon gas

forecasting methods: seismic gaps• based on the idea that the entire length of a

fault should experience similar amount of seismic activity

forecasting methods: past EQ’s

• develop by Dr. Alan Kafka of BC

• EQ’s are more likely to occur where EQ’s have occurred in the past

response to EQ hazards

• US National EQ Hazard Reduction Program– develop an understanding of the EQ source– determine EQ potential– predict effects of EQ’s– apply research results– what can we do with new information we gather?

adjustments to EQ activity• structural protection

– stricter building codes– flexible gas and electric lines

• EQ warning systems– ability of dangerous materials

to shut themselves off• land use planning

– putting important structures (schools, hospitals, government offices) away from active areas

– putting emergency response facilities close enough to be available to those who will most likely need them

Japan

• death toll will be over 10000• tsunami, flooding, fires• nuclear concern• building codes to allow tall buildings to sway

but not break• how much should the US help another

country?