Supplementary Tables 1-5, and Supplementary Text S1 The 2015 VLFE spectra as a sum of smaller events It is possible to model the 2015 M W 3.8 VLFE as the sum of about 10 M W 3.2 earthquakes occurring over the course of about 10 seconds, following the approach Gomberg et al. (2016) used to model VLFEs as sums of much smaller LFEs. To see this, imagine the VLFE is the sum of N subevents, all of which have moment M 0s and corner frequency f cs , and source spectra

The Ouachita system in northern Mexico can be subdivided into four unique tectonostratigraphic provinces: the foreland, the frontal zone, the interior zone, and the Coahuila terrane. Each province is defined by specific lithologic characteristics, structural styles, and regional Bouguer gravity anomalies. The Ouachita foreland is characterized by a carbonate dominated shelf which was disrupted during the late Paleozoic by several basement cored uplifts. The frontal zone is a northwest migrating foredeep and fold‐thrust belt; clastic sediments derived mainly from the fold‐thrust belt filled the foredeep and were subsequently deformed as the fold‐thrust belt migrated. The interior zone is the metamorphic core of the migrating fold‐thrust belt and is characterized by a distinctive positive Bouguer gravity anomaly. The Coahuila terrane is a composite terrane which includes a late Paleozoic volcanic arc and a piece of exotic continental crust which was probably afixed to North America during the final stages of the Ouachita orogeny and then left behind during the opening of the Gulf of Mexico.

No abstract

Ambient noise is useful for characterizing frequency-dependent noise levels and for assessing data quality for seismic stations. We use 4 years of ambient noise spectra from 16 stations in central Alaska to examine environmental and structural influences on seismic stations. The region contains a major river (Tanana River) that is ice covered for half the year and is underlain by a sedimentary basin (Nenana basin) that strongly influences the seismic wavefield. Nenana basin amplifies ambient seismic noise by 12-16 dB at 0.1-0.7 Hz and 17-30 dB at 0.7-3 Hz. A meteorological station and river gauge at Nenana provide environmental data for comparison with seismic stations. During the summer, the Tanana River produces noise levels elevated by 30-40 dB at frequencies near 10 Hz, as recorded by all five stations within 100 m of the main river channel. The Tanana River sediment is predominantly silt and sand in this region; therefore, we attribute the 10-Hz river signal to turbulence within the water and to unsteady shearing on the river bottom. The influence of wind is apparent on seismic noise at low (<0.05 Hz) frequencies, due to atmospheric-induced tilting, and at high (>2.0 Hz) frequencies, due to unsteady shearing and turbulence near the ground. Our empirical findings motivate future studies, such as how flow from air or water couples to the ground and how deep sedimentary basins influence the ambient noise wavefield. Our results have implications for seismic site selection, environmental monitoring, and detection and characterization of earthquakes.

Cook Inlet fore‐arc basin in south‐central Alaska is a large, deep (7.6 km) sedimentary basin with the Anchorage metropolitan region on its margins. From 2015 to 2017, a set of 28 broadband seismic stations was deployed in the region as part of the Southern Alaska Lithosphere and Mantle Observation Network (SALMON) project. The SALMON stations, which also cover the remote western portion of Cook Inlet basin and the back‐arc region, form the basis for our observational study of the seismic response of Cook Inlet basin. We quantify the influence of Cook Inlet basin on the seismic wavefield using three data sets: (1) ambient‐noise amplitudes of 18 basin stations relative to a nonbasin reference station, (2) earthquake ground‐motion metrics for 34 crustal and intraslab earthquakes, and (3) spectral ratios (SRs) between basin stations and nonbasin stations for the same earthquakes. For all analyses, we examine how quantities vary with the frequency content of the seismic signal and with the basin depth at each station. Seismic waves from earthquakes and from ambient noise are amplified within Cook Inlet basin. At low frequencies (0.1–0.5 Hz), ambient‐noise ratios and earthquake SRs are in a general agreement with power amplification of 6–14 dB, corresponding to amplitude amplification factors of 2.0–5.0. At high frequencies (0.5–4.0 Hz), the basin amplifies the earthquake wavefield by similar factors. Our results indicate stronger amplification for the deeper basin stations such as near Nikiski on the Kenai Peninsula and weaker amplification near the margins of the basin. Future work devoted to 3D wavefield simulations and treatment of source and propagation effects should improve the characterization of the frequency‐dependent response of Cook Inlet basin to recorded and scenario earthquakes in the region.