GEOL/METR 309: Investigating Land, Sea and Air Interactions Fall 2005, SFSU Pre-class Exercise #4: Introduction to Earthquakes Dr. Dave Dempsey Dr. Lisa White (Dept. of Geosciences)

This is the "preview" version of this exercise, suitable for printing and leisurely inspection before you submit your answers to the real thing, the interactive version. When you feel ready to submit your answers, go to Blackboard's "Pre-class Exercise" section, select the interactive version, and follow its instructions. Ther are three questions, worth 4 points altogether.

(Responses are due by 2:05 p.m. Tuesday, Sept. 27. Remember that if you leave the interactive version before submitting your answers, then Blackboard will forget whatever answers you've already entered and you'll have to reenter them when you return. You will be able to submit answers only once.)

Description: In this exercise, we explore some of the effects of earthquakes at different locations in the San Francisco Bay area.

Questions:

(1) Multiple Choice. Most of us have felt an earthquake. The severity of the shaking that we experience during an earthquake is called the intensity of the earthquake. The intensity is a function of (that is, depends on) multiple variables that we will begin to explore in the two questions following this one, but first we need to see how different levels of severity of shaking in an earthquake (that is, different intensities) are defined.

Click on the link below (from the web site of the Association of Bay Area Governments, or "ABAG") to go to a chart that shows the "Modified Mercalli Intensity Scale" and explains how each level of shaking severity is defined:

Based on the modified Mercalli intensity scale, what experiences could you expect to have if the severity of shaking was categorized as STRONG?

1. Felt outdoors; direction estimated. Sleepers wakened.
2. Felt by all. Many frightened and run outdoors.
3. Difficult to stand. Noticed by drivers of motor cars.
4. Steering of motor cars affected.

(2) Multiple Choice. The intensity of an earthquake (the severity of shaking) experienced at any location depend on:

1. the magnitude of the earthquake;
2. the distance of the location from the earthquake epicenter; and
3. the type of geologic material at the location.

The map below (also from the ABAG web site) shows the expected intensity of shaking, based only on the type of underlying geologic material (all else being equal).

Based on this figure (that is, based only on the type of geologic material at the earth's surface), where would you expect the highest intensity of earthquake shaking in the Bay Area to occur?

1. along the coast of the Pacific Ocean north of San Francisco
2. along the coast of the Pacific Ocean south of San Francisco
3. in the mountain ranges between the coast and San Francisco Bay
4. along the edges of San Francisco Bay

(3) Short Answer. The San Francisco Bay area has a number of earthquake faults, the best-known of which is the San Andreas fault. It is oriented from south-southeast to north-northwest and extends along much of the length of California. In the San Francisco Bay area it crosses the San Francisco Peninsula between San Francisco and San Jose, goes offshore at Daly City just south of San Francisco, passes in front of the Golden Gate, and comes back onshore near Point Reyes in Marin County. It separates Pt. Reyes from the rest of California and continues farther northward along the coast.

The map below shows the expected shaking intensity at various places in the SF Bay area if there was a large earthquake on the North Golden Gate Segment of the San Andreas fault (north of the Golden Gate Bridge). This map shows expected intensities of such an earthquake based on:

1. proximity (that is, nearness) to the fault; and
2. the geologic materials on the earth's surface.

1. Based on this map, where would "very strong", "violent", and "very violent" shaking intensities be expected?

2. Based on this map and on the factors that determine earthquake intensity at any particular location, why would high intensities be expected at these locations?