The Newport–Inglewood fault (NIF) is a slowly deforming fault cutting through a thin continental crust with a normal geothermal; yet it hosts some of the deepest earthquakes in southern California. The nucleation of deep earthquakes in such a continental setting is not well understood. Moreover, the deep seismogenic zone implies that the maximum NIF earthquake magnitude may be larger than expected. Here, we quantify the resolution of the Long Beach (LB) and the Extended Long Beach (ELB) dense arrays used to study deep NIF seismicity. Previous study of the regional catalog and of downward‐continued LB array data found NIF seismicity extending into the upper mantle beneath LB. Later studies, which analyzed the ELB raw data, found little evidence for such deep events. To resolve this inconsistency, we quantify the array’s microearthquake detectability and resolution power via analysis of pre‐ and postdownward migrated LB seismograms and benchmark tests. Downward migration focuses energy onto the source region and deamplifies the surface noise, thus significantly improving detectability and resolution. The detectability is also improved with the increase in the array aperture‐to‐source‐depth ratio. The LB array maximum aperture is only 20% larger than the ELB aperture, yet its resolution for deep (>20 km) events is improved by about a factor of two, suggesting that small changes to the array geometry may yield significant improvement to the resolution power. Assuming a constant aperture, we find the LB array maintain resolution with 1% of its sensors used for backprojection. However, the high‐sensor density is essential for improving the signal‐to‐noise ratio. Analysis of the regional and array‐derived NIF catalogs together with newly acquired Moho depths beneath the NIF suggests that mantle seismicity beneath LB may be a long‐lived feature of this fault.

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