Earthquake Size

Question 1

what is macroseismic intensity?

Answer 1

an intensity measure based off of people’s perceptions of the ground shaking event and damage reports

Question 2

what are some drawbacks of macroseismic scales?

Answer 2

very subjective to who is assessing the damage or interpreting the results

Question 3

what biases maximum intensity measures?

Answer 3

depth of the event
distribution of the population and public perception
construction practice
effect of local geology
This makes earthquake events from different regions rather uncomparable

Question 4

what is local magnitude?

Answer 4

commonly referred to as the Richter scale, used instrumental measurements of ground motion to determine magnitude, adjusted for depth and distance

Question 5

what was the original richter scale based off of?

Answer 5

the observation that the log of the amplitude waves decreased with epicentral distance

Question 6

what was a magnitude 0 event originally defined as?

Answer 6

an event that registered max ground motion of 1 micrometer at epicentral distance of 1 km, with a wood anderson instrument

Question 7

what are issues with the richter scale?

Answer 7

calibrated for southern california originally, requires adjustment for all other regions if it is to be used. also dependent on an instrument with a period of 0.8 seconds leading to saturation issues failing to capture large events

Question 8

whats a general form for a magnitude scale?

Answer 8

M = log (A/T) + f(depth, distance) + Cs + Cr
magnitude = log amplitude / period of instrument + adjustments for depth and distance, site and regional characteristics

Question 9

describe surface wave amplitude

Answer 9

based off of the amplitude of rayleigh waves, which have a dominant period of around 20seconds. used for shallow events, around 50km deep,

Question 10

describe body wave magnitude

Answer 10

based off of amplitude of first arrivals of P waves, used for deep earthquakes, dominant period of about 1 second.

Question 11

discuss magnitude saturation

Answer 11

the frequency / period that the earthquake spectrum is sampled at strongly controls the resolving power of the magniude type. Eg since Mb (body wave mag) is measured at 1 second period, it gets saturated lower as it misses the lower frequency content of the larger magnitude events

Question 12

why doesnt moment magnitude have a saturation problem?

Answer 12

because it is based off of semsic moment, which is based off of energy release, as opposed to measured amplitudes from an instrument.

Question 13

how do you measure Mo (scalar seismic moment)?

Answer 13

it is proportional to the far field earthquake spectrum at long periods.