How compression dictates new contacts
M. Wyart, 2005
Compression increases { } term, weakens restoring forces, reduces ws
1/2
Lowest modes go unstable
New contacts z are needed beyond zc
Dw2 ~ - pressure ~  -(f - fc)
Extra contacts stabilize modes up to frequency
w* ~ (z - zc)  
Þ (z - zc)2 > (f - fc)
Agrees with simulation