Question

The San Andreas is long but is a vertical fault, so only ~25 km of down-dip width is cold enough to break in an earthquake (i.e. below 25 km depth, the rocks are too warm to fail in a brittle earthquake and instead deform in other ways). Imagine a 500 km length of the fault breaks, slipping 10 m (this huge earthquake is unlikely but not impossible). What magnitude would this earthquake be?

Area is width multiplied by length... and don't forget to convert values from km to m

Answer 1

Ans -

Given the dynamics of fault movement , we can calculate the moment magnitude of the earthquake using the formula

where   = moment magnitude of the earthquake and = seismic moment .

where ,   = rigidity modulus of the rock at the source depth , S = area of the fault , D = slip

In the question above , we are given that S = 25km * 500 km , D = 10 m but no value for the rigidity modulus on the San Andreas fault is given , therefore we can quantify the magnitude of the earthquake .

If somehow is known , calculate in the units of dyne cm . Then the substitution of the values in the formula of gives the moment magnitude of the earthquake.