The magnitude of an earthquake , as indicated on a seismic scale , is roughly proportional to the length of the rupture , while the ground motion in the region surrounding the fault is highly dependent upon the local soil conditions , somewhat upon the distance and relationship to the progression of the fault rupture , and ( as recently recognized in the 1989 Loma Prieta earthquake ) reflected energy from deep discontinuities in the Earth 's structure . The area affected by an earthquake is also dependent upon the density and uniformity of the soils surrounding the fault
1989 Loma Prieta earthquake
1989 Loma Prieta earthquake
Hayward Fault Zone