  We study the Newton polygon jumping locus of a Mumford family in char $p$.
Our main result says that, under a mild assumption on $p$, the jumping locus
consists of only supersingular points and its cardinality is equal to
$(p^r-1)(g-1)$, where $r$ is the degree of the defining field of the base curve
of a Mumford family in char $p$ and $g$ is the genus of the curve. The
underlying technique is the $p$-adic Hodge theory.
