Geography 140

Introduction to Physical Geography

Lab 9: Isoseismal Mapping

Answers

The convexities and concavities may reflect the different soils and rock types the quake waves passed through. Seismic waves travel fastest in uniform, dense, coherent material (many kinds of bedrock), so their amplitude is correspondingly lower. This produces less shaking on such materials. On a map like this, the isoseism would bend closer to the epicenter on well-consolidated bedrock as a result. That is, there would be less shaking at a given distance from the focus of the quake than you'd expect.

Conversely, seismic waves are slowed down when they encounter unconsolidated, non-uniform, and less dense materials, such as the alluvium or river deposits that accumulate on the valley floors. When waves slow down, they compensate by developing greater amplitude, which produces more violent shaking as subjectively experienced by the people in the area. So, the isoseisms bend outward from the epicenter in areas dominated by such loose materials. This is because you would experience more violent shaking at a given distance from the focus than you would expect otherwise.

The fault plane that failed forms a 45° angle with the ground, dipping north. The focus of the quake was about 10 km below the ground. Applying the geometry of a right triangle, we see that the fault must intersect the surface about 10 km to the south of the epicenter. So, the area 10 km south of the epicenter was receiving the concentration of seismic energy associated with the fault plane discontinuity. Hence, the shaking there, where the fault plane intersected the ground, is about as intense as that right over the focus (the epicenter). This gives the VII intensity area a shape distributed roughly from the epicenter to the strike of the responsible fault.

abridged and modified from a lab on the Southern California Earthquake Center web site

first placed on the web: 11/24/00
last revised: 07/05/07
Dr. Christine M. Rodrigue