Actually, earthquake magnitude can be projected to negative numbers. It’s well defined but it stops describing earthquakes. For instance, a -3 magnitude earthquake is the energy released by a cat knocking your cell phone off of a nightstand. (see page 290 of this book). Pretty sure the others are also logarithmic scales which are well-defined for any negative number. It just so happens that those negative numbers don’t describe anything we care to describe with those scales.
Actually, earthquake magnitude can be projected to negative numbers. It’s well defined but it stops describing earthquakes. For instance, a -3 magnitude earthquake is the energy released by a cat knocking your cell phone off of a nightstand. (see page 290 of this book). Pretty sure the others are also logarithmic scales which are well-defined for any negative number. It just so happens that those negative numbers don’t describe anything we care to describe with those scales.
That negative number comes from taking a logarithm of a number less than one. Not from a negative scalar.