L2 - Faulting in Large Earthquakes

Question 1

In the 1989 Loma Prieta earthquake, how were the aftershocks and hypocentre distributed? (1)

Answer 1

Dipping SW along the fault

Question 2

Before the 1989 Loma Prieta earthquake there were no small earthquakes in the rupture area but were surrounding it, how can this be explained? (2)
Where is this feature seen? (1)

Answer 2

Fault that ruptured was locked and no stress released
Surrounding areas not as locked so can move a bit
Close to the brittle-ductile transition = mostly deforming by ductile creep, only a few areas produce micro-seismicity

Question 3

In the 1989 Loma Prieta earthquake, why did it nucleate at the down-dip edge of the fault? (1)

Answer 3

Likely where the fault has been loaded by creep in the underlying material

Question 4

Why do aftershocks occur? (2)

Answer 4

To relieve stresses caused by ‘space problems’ in areas the fault is not perfectly planar
To release remaining stress on the fault plane

Question 5

What is an idea about why earthquakes don’t happen deep down?

Answer 5

Rocks are too hot so they creep instead (ductile behaviour over brittle)