MyShake is a growing smartphone-based network for seismological research applications. We study how dense array analysis of the seismic wavefield recorded by smartphones may enhance microearthquake monitoring in urban environments. In such areas, the microearthquake signal-to-noise ratio on smartphones is not well constrained. We address this issue by compiling a seismic noise model for the Los Angeles (LA) metropolitan area using over 500,000 seismograms recorded by stationary phones running MyShake. We confirm that smartphone noise level is reduced during nighttime, and identify strong noise sources such as major traffic highways, the LA airport, and the Long Beach seaport. The noise analysis shows that stationary smartphones are sensitive to human-induced ground motions, and therefore smartphone-derived seismograms may be used to infer the elastic properties of the shallow subsurface. We employ array backprojection analysis on synthetic data to estimate what fraction of LA's smartphone user population is required to install MyShake to enable the location of events whose induced ground motions are below the smartphone noise level. We find that having 0.5% of LA's population download the MyShake app would be sufficient to accurately locate M > 1 events recorded during nighttime by stationary phones located at epicentral distances < 5 km. Currently, the MyShake user coverage in LA is approaching a value that will allow us to locate events whose magnitude is near the regional catalog's magnitude of completeness.