Magnitude scale and Nuclear Explosions and Magnitude and tectonic setting Flashcards
Why is the richter scale called the local scale?
Was designed for strike-slip earthquakes in southern california with a epicentral distance of <600km
What does the richter scale measure?
maximum s-wave amplitude
How is magnitude generated from richter scale?
epicentral distance and max s wave amplitude inputted into a nonogram
What is the crucial limitation of the richter scale?
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
What is magnitude moment?
derived from seismic energy released - seismic moment M0
M or Mw or Magnitude
How can M0 be accurately measured?
Modeling of the waveforms of very long period seismograms to estimate fault slip, fault area and earthquake source mechanism
what are aftershocks?
smaller shaking as the crust around the displaced fault plane adjusts
What is Baths law for aftershocks?
The difference in MW of the main quake and the largest aftershock is ~ constant and ~ 1.1-1.2
What is the problem with baths law?
Self fufilling as events larger then 1.1-1.2 will be discounted as aftershock
What is Omoris Law?
The frequency of aftershocks decreases roughly with the reciprocal of time after the main shock
When does the aftershock sequence end?
when rate is back to background rates
How do foreshocks follow omoris law?
In reverse increasing in frequency closer to rupture
What problem is there with using foreshocks for prediction?
Foreshocks do not always occur (~40% dont have foreshocks)
Hw is the siesmic moment calculated?
Rupture area x slip x rigidity
What can an underground nuclear test be equivalent to with earthquakes?
M5
why was the world wide seismic network setup?
set up in 1960s to detech nuclear explosions
How can we tell the difference between a nuclear explosion and an earthquake?
Radiation patterns are different
Nuclear bomb will be compression in all directions (P-wave)
Earthquake p wave compressional and extensional (double couple)
What will the relationship between body and surface waves be for earthquakes and nuclear?
Magnitude of bidy waves greater then surface for nuclear
What can be seen by the power of nuclear bombs tested using seismographs?
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
WHat might be done to try hide nuclear testing?
Put on active fault to induce eq to mask it but unsuccessful
What does a greater rupture area result in?
A larger EQ
What type of tectonic setting has the largest rupture area?
Subduction mega zones
What are EQs like at normal faults?
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
What re stike slip EQs like compared to normal faults?
Strike laterally larger extent follwing plate boundaries (but will often break into segments) so EQs are typically larger but limited depth only ~15km
Where will the epicentre for Eqs typically be on the fault?
At the edge of the rupture instead of centre
What is the reason subduction zone megathrusts are so large?
much greater down dip extent
What are the typical characteristics of subduction zone megathrusts?
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
What are the maximum magnitudes related to tectonic settings?
Normal - M7
Strike-slip- M8
Subduction zone megathrusts - 9.5
