Solving Sudoku Earthquake puzzles

Sudoku Earthquake starts with a regular 9x9 Sudoku grid with a “fault line” running from one side of the puzzle to another. At the bottom of the puzzle the digits that will appear on that fault line are shown. A fault is rarely nine digits long so the digits on the fault may be repeated or a digit may not even appear at all.

When solving the Earthquake puzzle you’ll usually be moving back and forth between the puzzle above and the fault below, so an important thing to remember when solving Earthquake puzzles is to cross off a digit each time you place one in the grid.

The main thing to look for on the fault is either a digit that’s not there (or that has had all of a digit crossed off) or multiples of the same digit.

Once a digit is gone, then you now know that that digit cannot appear in any of the blank spots in the fault line. Let’s say that you are working on a puzzle where the digit “7” does not appear along the fault. Look at the rows, columns and boxes that are along the fault line. In any of them without a “7”, if there’s a single empty space remaining, it must be a “7”.

A similar tactic is used when you have multiple copies of a digit. Let’s say the fault line has three “2”s left in it.  If there are exactly two boxes left which don’t have a “2” filled in to them intersecting the fault line, you know that you’ll be putting a “2” into one of those spaces.

The last check is for a single digit remaining in the fault line. Sometimes you can simply look at all the spaces and eliminate those where it can’t go, leaving you with hopefully only one remaining space for it.

Often these tactics aren’t very helpful when just starting a puzzle, but often partway through the puzzle you’ll be able to fill in all the digits along the fault line.