Lecture 11 06 Earthquakes Chapter 8 bolt for midterm Richter Scale Only small and moderate earthquakes 6 Has a fatal flaw Low frequency wave energy propagates more effectively than high frequency Also body waves includes shear waves attenuates die off in amplitude with distance than surface waves Surface waves less frequency Body waves short period long frequency Smaller the rupture richer in high frequency Small earthquakes tend to produce high frequency violin P Big earthquakes produce a lot of low frequency Stand up Bass P M2 saturates at around 6 5 M2 is bad for measuring distance earthquakes M3 surface wave magnitude scale long period 20 40s surface waves better for measuring big earthquakes distant earthquakes Moment earthquake Mw w work Based on seismic moment torque Can measure all earthquakes of all sizes on a single scale Better measure of the energy released during earthquake Seismic moment increases by 32xfor every Mw unit E g Mw 6 2 vs Mw 8 2 32x32 1000 Mw 6 2 vs Mw 9 2 32x32x32 32000 Magnitude scales are open Mo UAd treat U as a constant Seismic moment shear modulus or rigidity area of fault that slipped displacement amount of slipped Mo increases by 32x Area that slipped 10x 3 2x10 320 Displacement 3 2x 1906 San Francisco earthquake Mw 7 9 400km long 15km surface to base of seismogenic zone vertical fault plane 400km x 15 km 6000km2 avg displacement 5m 1994 Northridge Mw 6 7 rupture was 20km x 30km 600km2 Mw 5 0 2km2 Mw 8 2 releases 2x as much energy as an Mw 8 0 SAF 5o 20o 50 150Km 15Km Mw 7 9 1906 SF earthquake Mw 8 9 32x 9 1 32x2 64 9 3 64x2 128 Mw 9 5 1960 Chile earthquake Ms 1999 Ismit Eq Turkey USGS was saying 7 8 The govt was saying MD 6 7 Mw says it was 7 4 7 5 MD Where D duration Based on how long the ground shakes near the Eq So earthquake depends on which magnitude scale one s using Media always wants to portray the earthquake as bad as they can most damaging 1995 Kobe MW 6 9 But MJ 7 2 Where J stands for Japanese