On 29 July 2021, an Mw 8.2 megathrust earthquake struck the Alaska Peninsula. Quantifying the coseismic slip and the afterslip that followed this earthquake provides us the opportunity to clarify the megathrust slip budget and the earthquake hazard potential there. However, the estimated coseismic slip distribution inversion result is strongly affected by assumptions made in the inversion. The spatial pattern of stress-driven afterslip is mainly controlled by the coseismic slip distribution, so that it can provide new information about the coseismic slip distribution and is useful to assess the assumptions made in the coseismic inversion. The orientation and relative magnitudes of postseismic displacements at sites on the Alaska Peninsula require that the afterslip be concentrated ~130km from the trench. As a result, coseismic slip models including slip at that distance or less to shore, predict postseismic deformation that systematically misfits the observations. A narrower coseismic rupture plane with an abrupt downward termination of slip provides a much better fit to the observed postseismic signal than models where the slip tapers gently with depth. We considered multiple different viscoelastic relaxation models and find that these conclusions about the coseismic model are required regardless of the viscoelastic relaxation models used. The contribution of viscoelastic relaxation to the observed signal is not negligible, and the early postseismic observations are best reproduced with a model that features a 50 km thick elastic lithosphere for the overriding plate, and an elastic cold nose to the mantle wedge.

Early Postseismic Deformation of the 29 July 2021 Mw8.2 Chignik Earthquake Provides New Constraints on the Downdip Coseismic SlipZ. Zhuo1, J.T. Freymueller1, Z. Xiao2, J. Elliott1, and R. Grapenthin31Michigan State University2Kunming University of Science and Technology3University of Alaska FairbanksCorresponding author: Zechao Zhuo([email protected])Key Points:The spatial pattern of afterslip provides new information about the coseismic slip distribution of the 2021 Mw8.2 Chignik earthquake.Displacements due to viscoelastic depend strongly on the viscosity model, but sensitive to the details of the coseismic slip.The maximum depth of the Chignik coseismic rupture constrained by the stress-driven afterslip is about 35km based on the lab2.0 geometry.AbstractOn 29 July 2021, an Mw 8.2 megathrust earthquake struck the Alaska Peninsula. Quantifying the coseismic slip and the afterslip that followed this earthquake provides us the opportunity to clarify the megathrust slip budget and the earthquake hazard potential there. However, the estimated coseismic slip distribution inversion result is strongly affected by assumptions made in the inversion. The spatial pattern of stress-driven afterslip is mainly controlled by the coseismic slip distribution, so that it can provide new information about the coseismic slip distribution and is useful to assess the assumptions made in the coseismic inversion. The orientation and relative magnitudes of postseismic displacements at sites on the Alaska Peninsula require that the afterslip be concentrated ~130km from the trench. As a result, coseismic slip models including slip at that distance or less to shore, predict postseismic deformation that systematically misfits the observations. A narrower coseismic rupture plane with an abrupt downward termination of slip provides a much better fit to the observed postseismic signal than models where the slip tapers gently with depth. We considered multiple different viscoelastic relaxation models and find that these conclusions about the coseismic model are required regardless of the viscoelastic relaxation models used. The contribution of viscoelastic relaxation to the observed signal is not negligible, and the early postseismic observations are best reproduced with a model that features a 50 km thick elastic lithosphere for the overriding plate, and an elastic cold nose to the mantle wedge.

Okmok volcano located on the northeastern part of the Umnak Island is one of the most active volcanoes in the Aleutian Arc. It was initially built as a large shield volcano, but was strongly destroyed by two caldera-forming eruptions 12,000 and 2,040 years ago. The post-caldera eruptions occur mostly along the inner perimeter of the caldera from a series of distinct cones. Here, we perform seismic tomography to explore the deep sources of magmatic activity beneath Okmok. We use the local earthquake data of the Alaska Volcano Observatory (AVO) in the time period from 2003 to 2017 to build a model with the 3D distributions of the P and S wave velocities and Vp/Vs ratio. At depths of more than 10 km, we observe a vertically aligned anomaly of high Vp/Vs ratio representing a steady conduit likely responsible for the volcano evolution since its origin. Above this conduit, we reveal a large anomaly of high Vp/Vs ratio representing the main magma reservoir providing the material for all recent eruptions in the caldera. It appears to be connected with another large shallow reservoir located below the Cone A that was the source of most of Okmok’s historical eruptions. The most recent eruption in 2008 took place right above the deep conduit. To reach the surface, the magma for this eruption passed through the shallow ductile zone, where it was saturated by silicic components. This interpretation is consistent with the petrology studies and modeling of ground deformations.

Since the inception and realization of the Global Positioning System (GPS) in the 1970-1980s, the Global Satellite Navigation System (GNSS) has become a ubiquous tool in civil, business, and scientific life. Major breakthroughs in our understanding of dynamic Earth processes were only achievable through this precise positioning technology. While positioning is the chief objective of the system, the nature of its design requires satellite signals to traverse the ionosphere and the troposphere, and results in signal reflections off the ground. In addition to crustal dynamics, this enables the study of the atmosphere and local environmental sensing, impacting fields far beyond solid earth research, including space physics, atmospheric science, glaciology, hydrology, and natural hazards. In this paper I review some of the history of this technology and its impact on the Earth sciences. Using the example of GPS, I introduce how satellite positioning systems work and how we can infer precise positions from the signals broadcast by the satellites. For this, I give an overview on reference systems, different observation models, the predominant precise positioning strategies and how the various error terms can be corrected. Once a solid understanding of precise positioning is developed , I present some of the complications that arise in high-rate (1 or more sample per second observations) sub-daily and real-time kinematic positioning, which is of great utility in the characterization and monitoring of many natural hazards. GNSS enables observations beyond precise positioning. I provide background and observation models for instantaneous velocity estimations, useful in real-time applications particularly where precise orbits and intial positions are not available, and GNSS reflectometry, which allows to perform local environmental sensing around GNSS monuments, including the inference of snow depth or tidal heights. Throughout the paper, each method is illustrated by a number of applications either from the literature or novel work. The focus is on some highlights from the last 1 2 Ronni Grapenthin decade of geodetic work, with a clear slant towards examples from solid Earth and hydrologic research.

In April 2022 a seismic swarm near Mt. Edgecumbe in southeast Alaska suggests renewed activity at this dormant volcano located in a transform fault setting. Oral Tlingit history describes low-level basaltic eruptions $\approx$800 years ago. Thin rhyolitic tephras deposited 5-4 ka. We analyze synthetic aperture radar data from 2014-2022 and resolve rapid inflation up to 8.7\,cm/yr beginning in August 2018. Bayesian modeling suggests a gently westward dipping sill opened 0.65\,m between 7.6\,km to 5.3\,km depth, centered about 2-3\,km east of Mt. Edgecumbe. Reanalyzed seismicity, recorded 25\,km away, shows increased activity since July 2019. We hypothesize mafic magma ascent through ductile material, accumulating below a silicic seal or in a silicic reservoir, and triggering seismicity in the overburden. Cloud-native open data and workflows enabled discovery and analysis of this rapid inflation within days after going unnoticed for $>$3 years.