We discuss several outstanding aspects of seismograms recorded during >4 weeks by a spatially dense Nodal array, straddling the damage zone of the San Jacinto fault in southern California, and some example results. The waveforms contain numerous spikes and bursts of high-frequency waves (up to the recorded 200 Hz) produced in part by minute failure events in the shallow crust. The high spatial density of the array facilitates the detection of 120 small local earthquakes in a single day, most of which not detected by the surrounding ANZA and regional southern California networks. Beamforming results identify likely ongoing cultural noise sources dominant in the frequency range 1-10 Hz and likely ongoing earthquake sources dominant in the frequency range 20-40 Hz. Matched-field processing and back-projection of seismograms provide alternate event location. The median noise levels during the experiment at different stations, waves generated by Betsy gunshots, and wavefields from nearby earthquakes point consistently to several structural units across the fault. Seismic trapping structure and local sedimentary basin produce localized motion amplification and stronger attenuation than adjacent regions. Cross correlations of high-frequency noise recorded at closely spaced stations provide a structural image of the subsurface material across the fault zone. The high spatial density and broad frequency range of the data can be used for additional high resolution studies of structure and source properties in the shallow crust.

P-wave travel-time residuals from USArray helped improve the scale and consistency with which the mantle beneath North America is resolved. Beginning in 2008, we published a series of P-wave velocity models based on a global ray theoretical inversion of USArray and global catalog data. Here, we present the final model update, MITP_2016MAY, which includes the complete set of travel-time residuals from USArray Transportable Array (TA) in the contiguous United States. In this model, the area of high resolution extends to the eastern margin of the continent, allowing us to better estimate the location and extent of slow features in Central Virginia and New England. An increasing number of data from the TA in Alaska also allows us to recover the structure of subducting Pacific plate and Yakutat terrane. In addition to highlighting new features in the final model, we visualize and discuss the improvements to the model due to the addition of USArray data through time. Electronic Supplement: MATLAB MITP_2016MAY model and plotting scripts, figures of checkerboard tests, and animations of model evolution.

We develop a simple methodology for reliable automated estimation of the low-frequency asymptote in seismic body wave spectra of small to moderate local earthquakes. The procedure corrects individual P-and S-wave spectra for propagation and site effects and estimates the seismic potency from a stacked spectrum. The method is applied to >11 000 earthquakes with local magnitudes 0 < M L < 4 that occurred in the Southern California plate-boundary region around the San Jacinto fault zone during 2013. Moment magnitude M w values, derived from the spectra and the scaling relation of Hanks & Kanamori, follow a Gutenberg-Richter distribution with a larger b-value (1.22) from that associated with the M L values (0.93) for the same earthquakes. The completeness magnitude for the M w values is 1.6 while for M L it is 1.0. The quantity (M w − M L) linearly increases in the analysed magnitude range as M L decreases. An average earthquake with M L = 0 in the study area has an M w of about 0.9. The developed methodology and results have important implications for earthquake source studies and statistical seismology.