Sources of Long‐Range Anthropogenic Noise in Southern California and Implications for Tectonic Tremor Detection

Abstract

We study anthropogenic noise sources seen on seismic recordings along the central section of the San Jacinto fault near Anza, southern California. The strongest signals are caused by freight trains passing through the Coachella Valley north of Anza. Train-induced transients are observed at distances of up to 50 km from the railway, with durations of up to 20 min, and spectra that are peaked between 3 and 5 Hz. Additionally, truck traffic through the Coachella Valley generates a sustained hum with a similar spectral signature as the train transients but with lower amplitude. We also find that wind turbine activity in northern Baja California introduces a seasonal modulation of 1-to 5-Hz energy across the Anza network. We show that the observed train-generated transients can be used to constrain shallow attenuation structure at Anza. Using the results from this study as well as available borehole data, we further evaluate the performance of approaches that have been used to detect nonvolcanic tremor at Anza. We conclude that signals previously identified as spontaneous tremor (Hutchison and Ghosh, 2017) were probably generated by other nontectonic sources such as trains.Electronic Supplement: Seismograms during a transient, decomposition of the large amplitude signal into sequences of repeating patterns, detection statistics in the two stages of the tremor search procedure, velocity amplitude spectra of transients from two collocated borehole-surface station pairs, median weekday-night spectra for all stations in the Plate Boundary Observatory (PBO) Anza network, time variations of root mean square (rms) velocity for all stations in the PBO Anza network, cumulative population and kilometers of railroad on which the allowed speed for freight trains exceeds 50 mph, envelopes of vertical ground motions as a function of time, vertical ground motions recorded by the Anza borehole stations, and spectrograms of vertical ground motion and corresponding wind speeds.

Train Traffic as a Powerful Noise Source for Monitoring Active Faults With Seismic Interferometry

“…This relatively high value shows that these train‐radiated body waves can likely be detected at few tens of kilometers from the railways, showing promise for crustal applications of seismic interferometry. This is in good agreement with the study of Inbal et al () that report train tremor observations up to 50 km from the railways.…”

“…Figure shows the cumulative contribution of every individual seismic source attenuated using equation at each model location. The transition from yellow to no color corresponds to the expected level of train tremor detection using a seismometer located in a quiet area according to the observations of Inbal et al (). It roughly corresponds to a distance of 50 km from the railways for a single point source.…”

“…The closest source of cultural seismic noise near the San Jacinto Fault is the Coachella Valley (~40 km from the fault; Figure 1a). Following the study of Inbal et al (2018), we believe that vehicle traffic is the most energetic source of cultural noise in the valley. In order to evaluate the level of seismic emissions from train and car/truck traffic in the Coachella Valley and following the approach from the previous section 2, we compute the equivalent magnitude of the daily train traffic along a 10-km-long portion of freight railways in the Coachella Valley at Indio corresponding to the width of the stationary phase zone (~25 trains per day; USBTS, 2017).…”

Semiautomated Estimates of Directivity and Related Source Properties of Small to Moderate Southern California Earthquakes Using Second Seismic Moments

“…A bandpass filtering with corner frequencies at 0.5 and 20 Hz is then performed on the waveforms to suppress the noise associated with ocean tides, wind, and various other natural and anthropogenic noise sources (e.g. Hillers et al, ; Inbal et al, ; Johnson et al, ; Meng & Ben‐Zion, ). We primarily use the transverse component to get relatively clean SH wave data and the vertical component to get P wave data.…”

Seismic Velocity Change Patterns Along the San Jacinto Fault Zone Following the 2010 M7.2 El Mayor‐Cucapah and M5.4 Collins Valley Earthquakes

“…We can expect that the scattered or diffusive propagation regime coexists with a ballistic component (Hillers et al, ; Larose et al, ). This is likely more pronounced at the longer wavelengths due to longer scattering mean free paths and the proximity of the study area to the pelagic and coastal excitation region (Gerstoft & Tanimoto, ; Hillers et al, ), in contrast to the better wavefield mixing and the more distributed natural and anthropogenic noise sources at higher frequencies (Hillers & Ben‐Zion, ; Inbal et al, ). A stable anisotropic noise field still supports dv / v monitoring (Hadziioannou et al, ).…”