Back-of-the-Envelope Calculations: Energy Released in an Earthquake
A magnitude 8.5 earthquake (such as the 1964 Good Friday earthquake in Alaska) releases about 1x1018 joules of energy. The atomic bomb exploded over Hiroshima released about 1.5x1013 joules of energy. How many Hiroshima bombs would one have to explode simultaneously to equal the amount of energy released in a single magnitude 8.5 earthquake?
67,000 (!).If you want to divide 1x1018 by 1.5x1013, just subtract the exponents (18-13) to get 105, and divide 1 by 1.5 to get .67, giving you .67x105, or 6.7x104 (67,000).
References and Resources
This SERC page describes the use 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.