JSON filter do not encode part of JSON

I'm using the json filter plugin and want to exclude parsing part of the message into JSON

filter {
        json {
            source => "message"

For example, below is an example JSON message. How can I keep the Event field as a string (as in tell json filter plugin to do not parse Event content, just keep as a string)

{"timestamp":"2016-08-12 14:10:52,385","level":"INFO","logger":"com.MyClass","thread":"Thread-48","Event":{"field1":"Value1","field2":"Value2"}}

or is there a way to convert Event back into a String and add as a new field