The problem you have described is a complex one and requires an expert's review.To begin with, read an American Lifeline Alliance document on seismic analysis of buried pipes. Doing google search will also take you to a document on the same topic published by Prof S.K.Jain of Indian Institute of Technology Kanpur.
The differential settlement can be due to soil liquefaction and can have added complications. Talk to your company's geotechnical group also.
Regards
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anindya