need help on search with taxonomy term filter

i have a vocabulary like,
+ United States
++ State A
+++ City A1
+++ CIty A2
+++ City A3
...
+++City A30
++State B
+++City B1
+++CIty B2
+++City B3
...
+++City B50

The vocabulary is for a yellow page node type.

now if user select to search node under "United States" , the search result will only match the node posted to "United States", but what I want is when User select a parent term, the search result should contain matched nodes posted to all its child terms as well.

is it possible to do this and how to?