Description/Abstract Dense seismic arrays are a standard methodology in the oil exploration industry that is adding a new dimension to studies of seismic hazard and tectonics. We use ambient noise correlations of industry datasets recorded in the Los Angeles basin to derive near surface velocity variations that are important for characterizing the seismic hazard of the region. These same data can also be used to locate micro-seismicity, which can help understand the earthquake process itself. We are also building our own “dense array” – the Community Seismic Network- to make a microzonation map for the region and to monitor the state of health of buildings.