Answer
$I=10^4=10,000$ cm
Work Step by Step
$M=8$
$S=10^{-4}$cm
Putting in the formula
$M=\log\frac{I}{S}$
and solve for $I$:
$8=\log \frac{I}{10^{-4}}$
$8=\log( 10^4I )$
$8=\log( 10^4I )$
$10^4I=10^8$
$I=\frac{10^8}{10^4}$
$I=10^{8-4}=10^4=10,000$