A field trip to observe features of Lake Bonneville, mountain glaciation, and Great Salt Lake near Salt Lake City, Utah, USA

ABSTRACT On this field trip we visit three sites in the Salt Lake Valley, Utah, USA, where we examine the geomorphology of the Bonneville shoreline, the history of glaciation in the Wasatch Range, and shorezone geomorphology of Great Salt Lake. Stop 1 is at Steep Mountain bench, adjacent to Point of the Mountain in the Traverse Mountains, where the Bonneville shoreline is well developed and we can examine geomorphic evidence for the behavior of Lake Bonneville at its highest levels. At Stop 2 at the mouths of Little Cottonwood and Bells Canyons in the Wasatch Range, we examine geochronologic and geomorphic evidence for the interaction of mountain glaciers with Lake Bonneville. At the Great Salt Lake at Stop 3, we can examine modern processes and evidence of the Holocene history of the lake, and appreciate how Lake Bonneville and Great Salt Lake are two end members of a long-lived lacustrine system in one of the tectonically generated basins of the Great Basin.

The role of coarse aerosol particles as a sink of HNO₃ in wintertime pollution events in the Salt Lake Valley

Wintertime ammonium nitrate (NH4NO3) pollution events burden urban mountain basins around the globe. In the Salt Lake Valley of Utah in the United States, such pollution events are often driven by the formation of persistent cold-air pools (PCAPs) that trap emissions near the surface for several consecutive days. As a result, secondary pollutants including fine particulate matter less than 2.5 µm in diameter (PM2.5), largely in the form of NH4NO3, build up during these events and lead to severe haze. As part of an extensive measurement campaign to understand the chemical processes underlying PM2.5 formation, the 2017 Utah Winter Fine Particulate Study, water-soluble trace gases and PM2.5 constituents were continuously monitored using the ambient ion monitoring ion chromatograph (AIM-IC) system at the University of Utah campus. Gas-phase NH3, HNO3, HCl, and SO2 along with particulate NH4+, Na+, K+, Mg2+, Ca2+, NO3-, Cl−, and SO42- were measured from 21 January to 21 February 2017. During the two PCAP events captured, the fine particulate matter was dominated by secondary NH4NO3. The comparison of total nitrate (HNO3 + PM2.5 NO3-) and total NHx (NH3 + PM2.5 NH4+) showed NHx was in excess during both pollution events. However, chemical composition analysis of the snowpack during the first PCAP event revealed that the total concentration of deposited NO3- was nearly 3 times greater than that of deposited NH4+. Daily snow composition measurements showed a strong correlation between NO3- and Ca2+ in the snowpack. The presence of non-volatile salts (Na+, Ca2+, and Mg2+), which are frequently associated with coarse-mode dust, was also detected in PM2.5 by the AIM-IC during the two PCAP events, accounting for roughly 5 % of total mass loading. The presence of a significant particle mass and surface area in the coarse mode during the first PCAP event was indicated by size-resolved particle measurements from an aerodynamic particle sizer. Taken together, these observations imply that atmospheric measurements of the gas-phase and fine-mode particle nitrate may not represent the total burden of nitrate in the atmosphere, implying a potentially significant role for uptake by coarse-mode dust. Using the NO3- : NH4+ ratio observed in the snowpack to estimate the proportion of atmospheric nitrate present in the coarse mode, we estimate that the amount of secondary NH4NO3 could double in the absence of the coarse-mode sink. The underestimation of total nitrate indicates an incomplete account of the total oxidant production during PCAP events. The ability of coarse particles to permanently remove HNO3 and influence PM2.5 formation is discussed using information about particle composition and size distribution.

The 18 March 2020 M 5.7 Magna, Utah, Earthquake: Strong-Motion Data and Implications for Seismic Hazard in the Salt Lake Valley

The 2020 oblique normal-faulting M 5.7 Magna mainshock has provided the best dataset of recorded strong ground motions for an earthquake within the Wasatch Front region, Utah, and the larger Basin and Range Province. We performed a preliminary evaluation of the strong motion and broadband data from this earthquake and compared the data with the Next Generation Attenuation - West2 Project (NGA-West2) ground-motion models (GMMs). The highest horizontal peak ground acceleration (PGA) recorded was 0.43g (geometric mean of the two horizontal components) at a station located above the rupture plane at a rupture distance of 8 km. Eleven stations recorded PGAs >0.20g. Most of these stations are located on the deep sedimentary deposits within the Salt Lake Valley, and all are at rupture distances <20 km. The data compare favorably with the NGA-West2 GMMs, although the expected variability was observed. PGAs exceed the GMM predictions at the closest distances for the source model that we used. The area of the strongest ground shaking encompassed the town of Magna, where some of the heaviest damage occurred. A significant implication of the 2020 Magna earthquake for seismic hazards in the Salt Lake Valley arises from the possibility that this earthquake occurred on the Salt Lake City segment of the Wasatch fault. If so, then the dip of this fault segment must decrease with depth to ≤30°–35°, as proposed by Pang et al. (2020)—at least along the northern part of the segment where the earthquake occurred. Because of the lack of information about the subsurface geometry of the Wasatch fault zone, modeling of this fault zone in seismic hazard analyses has assumed a moderate dip of 50°±15°. Assuming a more shallowly dipping fault results in higher estimates of ground shaking in future large earthquakes on this fault. Alternative interpretations of the Magna earthquake are that it occurred (1) on an auxiliary fault within the Wasatch fault zone or (2) on a listric section of the northern Salt Lake City segment that is not representative of the geometry of the whole fault segment.

Monitoring the 2020 Magna, Utah, Earthquake Sequence with Nodal Seismometers and Machine Learning

Immediately following the 18 March 2020 Mww 5.7 Magna, Utah, earthquake, work began on installing a network of three-component, 5 Hz geophones throughout the Salt Lake Valley. After six days, 180 geophones had been sited within 35 km of the epicenter. Each geophone recorded 250 samples per second data onsite for ∼40 days. Here, we integrate the geophone data with data from the permanent regional seismic network operated by the University of Utah Seismograph Stations (UUSS). We use machine learning (ML) methods to create a new catalog of arrival time picks, earthquake locations, and P-wave polarities for 18 March 2020–30 April 2020. We train two deep-learning U-Net models to detect P waves and S waves, assigning arrival times to maximal posterior probabilities, followed by a two-step association process that combines deep learning with a grid-based interferometric approach. Our automated workflow results in 142,000 P picks, 188,000 S picks, and over 5000 earthquake locations. We recovered 95% of the events in the UUSS authoritative catalog and more than doubled the total number of events (5000 vs. 2300). The P and S arrival times generated by our ML models have near-zero biases and standard deviations of 0.05 s and 0.09 s, respectively, relative to corresponding analyst times picked at backbone stations. We also use a deep-learning architecture to automatically determine 70,000 P-wave first motions, which agree with 93% of 5876 hand-picked up or down first motions from both the backbone and nodal stations. Overall, the use of ML led to large increases in the number of arrival times, especially S times, that will be useful for future tomographic studies, as well as the discovery of thousands more earthquakes than exist in the UUSS catalog.

Temperature Inversion and Ultrafine Particulate/Near Ultrafine Particulate Matter Concentrations in the Salt Lake Valley

Ultrafine particulate (UFP) matter exposures are associated with negative health outcomes. UFPs (<100nm) and near UFP (NUFP) matter (4.5nm - 250nm) are trapped by the bowl-like geography of the Salt Lake Valley causing winter inversions (i.e., trapped particulate matter (PM)). Enmont PUFP C100 and Grimm 1.109 particle counters were used to define NUFP concentrations during inversion (n=5) and non-inversion (n=5) days at 7 sites. NUFP concentrations served as a proxy for the UFP fraction. NUFP concentrations were log-transformed and multivariable mixed effects linear regression models determined if NUFP concentration differed between inversion and non-inversion or by length of inversion. Difference in fraction NUFP was also analyzed. The mean NUFP concentration was 1.49-fold higher during inversions (95% CI 1.11–2.02), whereas the fraction declined by 0.22 (95% CI -0.31– -0.13). Increased NUFP concentrations during inversions may lead to increased adverse health outcomes. These findings have serious implications for inversion-prone regions.

Backprojection Imaging of the 2020 Mw 5.5 Magna, Utah, Earthquake Using a Local Dense Strong-Motion Network

We present the application of a backprojection method for imaging the detailed rupture of the 2020 Mw 5.5 Magna, Utah, earthquake. This is the first time that this method is applied to an earthquake smaller than Mw 6 in a local scale using a dense strong-motion network. The 2020 Magna earthquake occurred in a very well instrumented area, the Salt Lake valley, with tens of strong-motion seismic stations. We use envelopes of high-frequency S waves recorded on the transverse component at 45 seismic stations that are located at distances up to 100 km from the epicenter. The nearest station is ∼4.5 km. Backprojection resolves the epicentral location of the mainshock with an absolute error of less than 1 km, whereas the depth resolution is within the centroid depth range of multiple moment tensor solutions. Spatial distribution of the imaged subevents shows an up-dip unilateral west-northwest–east-southeast rupture with a length of ∼10 km, consistent with the distribution of early aftershocks. The average rupture speed is between 2.9 and 3.2 km/s for the first 2 and 3 s, respectively. The shallow dip (∼35°) of the Wasatch fault at depth, which failed during the Magna earthquake, combined with the up-dip unilateral rupture, indicates that ground-motion scenarios for future larger earthquakes in the Salt Lake Valley should be re-evaluated. This study underlines the need for instrumenting metropolitan areas of high seismic risk and adopting backprojection techniques in the near-real-time network products immediately after a strong earthquake.

Responding to the 2020 Magna, Utah, Earthquake Sequence during the COVID-19 Pandemic Shutdown

Two days after the University of Utah Seismograph Stations (UUSS) staff were required to leave campus and work remotely, an Mw 5.7 earthquake struck the Salt Lake Valley near the town of Magna, Utah. This event was the largest instrumentally recorded earthquake in the Salt Lake Valley and the largest earthquake ever felt by most residents. The timing of this event—at the start of a lockdown in response to the COVID-19 pandemic—made the UUSS response to this earthquake an extra challenge. Other factors such as a toxic plume caused by the ground shaking, inclement weather, and a mountain lion also impacted the work. The response tested the continuity of operations plan that had been in place since 2007, response protocols, and communications with partners and the public. Overall, the UUSS earthquake response was successful: A valuable and arguably unprecedented dataset of strong ground motions from normal faulting was generated, magnitudes and locations of thousands of earthquakes were shared in a timely fashion, unfounded rumors and general questions were promptly responded to via traditional and social media, and initial scientific results were submitted for publication.