Update README.md

Author: meihuisu

README.md

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 [![ivlsu-ucvm-ci Actions Status](https://github.com/SCECcode/ivlsu/workflows/ivlsu-ucvm-ci/badge.svg
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 SSIP Imperial Valley model provides P-wave velocities for the shallow crust in
 the southern Salton Trough, from the southern Salton Sea to the USA-Mexico border.
 The model combines travel-time observations from explosions recorded during the 
 Salton Seismic Imaging Project (SSIP) and the IV1979 active source experiment 
 with local earthquake recordings. The model extends from the surface to 8-km depth.
 
+Persaud P., (2016) Integrated 3-D Seismotectonic-Velocity Model of the Salton Trough Region based on Seismicity and Explosive Shots from the Salton Seismic Imaging Project Final Technical Report to U.S. Geological Survey NEHRP, Award G15AP00062
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 ## Installation
 
 This package is intended to be installed as part of the UCVM framework,