As you've just seen, data suggest that the earthquake was the result of motion along a fault situated beneath the surface to the north of Northridge. The earthquake's depth (the focus) and the depths of aftershocks provide a way to see the three-dimensional shape of the fault.

Diagram 1 below shows a map view of epicenters and aftershocks for the Northridge earthquake. The diagram also shows the epicenter and aftershocks for a strong earthquake that was centered on nearby San Fernando.

Diagram 2 shows a cross section or side view along line A–A'. The side view clearly shows the depths of the epicenters and aftershocks along the line that trends from southwest to northeast.

  !   Examine the map view and the cross section. Interpret the diagrams carefully to visualize the 3-D shape of the fault planes outlined by the aftershocks.

The fault that moved in the Northridge earthquake did not rupture the surface, so it does not appear on the map. However, motion along any of the nearby faults might introduce further stress on the blind thrust fault, and the fault may eventually cause a break at the surface.

5. Which surface faults might be related to the fault responsible for the Northridge earthquake? Highlight these faults on your map. What evidence did you use to arrive at your conclusion?

6. At what depth did the Northridge earthquake occur?

7. At what depth range did most of the aftershocks occur?

8. Based on location of aftershocks compared to the Northridge epicenter, in what direction is the fault inclined?