Database

[{"_id":ObjectId("622afbb16c5bba83829e8033"),"type":"Feature","properties":{"mag":0.6,"place":"6km NW of The Geysers, California","time":NumberLong("1370255770900"),"updated":NumberLong("1370256367681"),"tz":-420,"url":"http://earthquake.usgs.gov/earthquakes/eventpage/nc72001595","detail":"http://earthquake.usgs.gov/earthquakes/feed/v1.0/detail/nc72001595.geojson","cdi":null,"mmi":null,"alert":null,"status":"AUTOMATIC","tsunami":null,"sig":6,"net":"nc","code":"72001595","ids":",nc72001595,","sources":",nc,","types":",general-link,geoserve,nearby-cities,origin,phase-data,scitech-link,","nst":null,"dmin":0.01796631,"rms":0.05,"gap":205.2,"magType":"Md","type":"earthquake","iso_date":ISODate("2013-06-03T10:36:10.900Z"),"types_as_array":["general-link","geoserve","nearby-cities","origin","phase-data","scitech-link"]},"geometry":{"type":"Point","coordinates":[-122.8173,38.8115,9.4]},"id":"nc72001595"}]

Query

db.collection.update({},[{$set:{"depth":{"$arrayElemAt":["$geometry.coordinates",2]}}}])

Result