BotEC: The San Andreas Fault's Rate of Movement

Peter Kresan


Question: A diagram showing the San Andreas fault relative to California

The San Andrea is an active fault zone, marked by frequent earthquake activity. The crust southwest of this strike-slip fault (including Los Angeles) is sliding to the northwest relative to the other side.

It is possible to estimate the average annual rate of movement by recognizing rock of an earlier geologic age that have been cut and offset by the lateral movement along the fault. Below is a generalized geologic map of southern California, which shows Pliocene-Miocene age rocks offset along the San Andreas fault. The age of these rocks have been determined to be 25 million years. What is the average annual rate of movement in centimeters per year along the San Andreas fault?


Answer: 1.6 cm/year

References and Resources

This SERC page describes the use of Back of the Envelope Calculations

A View from the Back of the Envelope (more info) : This site has a good number of easy simulations and visualizations of back of the envelope calculations.

The Back of the Envelope : This page outlines one of the essays in the book "Programming Pearls" (ISBN 0-201-65788-0). The book is written for computer science faculty and students, but this portion speaks very well to back of the envelope calculations in general.