Magnitude scale and Nuclear Explosions and Magnitude and tectonic setting

Question 1

Why is the richter scale called the local scale?

Answer 1

Was designed for strike-slip earthquakes in southern california with a epicentral distance of <600km

Question 2

What does the richter scale measure?

Answer 2

maximum s-wave amplitude

Question 3

How is magnitude generated from richter scale?

Answer 3

epicentral distance and max s wave amplitude inputted into a nonogram

Question 4

What is the crucial limitation of the richter scale?

Answer 4

Only measures the single highest s-wave amplitude and was designed for local earthquakes in S California

Large earthquakes have a long duration, and thus much more energy for a given maximum amplitude.

So Richter scale underestimates the magnitude of these

Question 5

What is magnitude moment?

Answer 5

derived from seismic energy released - seismic moment M0

M or Mw or Magnitude

Question 6

How can M0 be accurately measured?

Answer 6

Modeling of the waveforms of very long period seismograms to estimate fault slip, fault area and earthquake source mechanism

Question 7

what are aftershocks?

Answer 7

smaller shaking as the crust around the displaced fault plane adjusts

Question 8

What is Baths law for aftershocks?

Answer 8

The difference in MW of the main quake and the largest aftershock is ~ constant and ~ 1.1-1.2

Question 9

What is the problem with baths law?

Answer 9

Self fufilling as events larger then 1.1-1.2 will be discounted as aftershock

Question 10

What is Omoris Law?

Answer 10

The frequency of aftershocks decreases roughly with the reciprocal of time after the main shock

Question 11

When does the aftershock sequence end?

Answer 11

when rate is back to background rates

Question 12

How do foreshocks follow omoris law?

Answer 12

In reverse increasing in frequency closer to rupture

Question 13

What problem is there with using foreshocks for prediction?

Answer 13

Foreshocks do not always occur (~40% dont have foreshocks)

Question 14

Hw is the siesmic moment calculated?

Answer 14

Rupture area x slip x rigidity

Question 15

What can an underground nuclear test be equivalent to with earthquakes?

Answer 15

M5

Question 16

why was the world wide seismic network setup?

Answer 16

set up in 1960s to detech nuclear explosions

Question 17

How can we tell the difference between a nuclear explosion and an earthquake?

Answer 17

Radiation patterns are different
Nuclear bomb will be compression in all directions (P-wave)
Earthquake p wave compressional and extensional (double couple)

Question 18

What will the relationship between body and surface waves be for earthquakes and nuclear?

Answer 18

Magnitude of bidy waves greater then surface for nuclear

Question 19

What can be seen by the power of nuclear bombs tested using seismographs?

Answer 19

Since limit of 15okt no one has breached that (keeping in line with regulations) some slighlty higher but this depends on how well the rocks radiate the waves

Question 20

WHat might be done to try hide nuclear testing?

Answer 20

Put on active fault to induce eq to mask it but unsuccessful

Question 21

What does a greater rupture area result in?

Answer 21

A larger EQ

Question 22

What type of tectonic setting has the largest rupture area?

Answer 22

Subduction mega zones

Question 23

What are EQs like at normal faults?

Answer 23

Associated with extension, i.e. thinning, bringing hot rocks near the surface.
Limited depth extent as steep faults and as high temperatures reached at shallow depth
Limited along strike extent
Shallow, moderate magnitude

Question 24

What re stike slip EQs like compared to normal faults?

Answer 24

Strike laterally larger extent follwing plate boundaries (but will often break into segments) so EQs are typically larger but limited depth only ~15km

Question 25

Where will the epicentre for Eqs typically be on the fault?

Answer 25

At the edge of the rupture instead of centre

Question 26

What is the reason subduction zone megathrusts are so large?

Answer 26

much greater down dip extent

Question 27

What are the typical characteristics of subduction zone megathrusts?

Answer 27

cool setting, reach down to >30 km depth, up to 1300km along strike, but up to 200 km down dip as much more gently dipping

Question 28

What are the maximum magnitudes related to tectonic settings?

Answer 28

Normal - M7
Strike-slip- M8
Subduction zone megathrusts - 9.5