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At least 883 records · Page 49Linked to original sources

USGS2019

Evaluation of ground‐motion models for U.S. Geological Survey seismic hazard models: 2018 Anchorage, Alaska, Mw 7.1 subduction zone earthquake sequence

Instrumental ground‐motion recordings from the 2018 Anchorage, Alaska ( ⁠ M w "> M w 7.1), earthquake sequence provide an independent data set allowing us to evaluate the predictive power of ground‐motion models (GMMs) for intraslab earthquakes associated with the Alaska subduction zone. In this study, we evaluate 15 candidate GMMs using instrumental ground‐motion observations of peak ground acceleration and 5% damped pseudospectral acceleration (0.02–10 s) to inform logic‐tree weights for the update of the U.S. Geological Survey seismic hazard model for Alaska. GMMs are evaluated using two methods. The first is a total residual visualization approach that compares the probability density function, mean, and standard deviations σ "> σ of the observed and predicted ground motion. The second GMM evaluation method we use is the common total residual probabilistic scoring method (log likelihood [LLH]). The LLH method provides a single score that can be used to weight GMMs in the Alaska seismic hazard model logic trees. To test logic branches in previous seismic hazard models, we evaluate GMM performance as a function of depth and we demonstrate that some GMMs show improved performance for earthquakes with focal depths greater than 50 km. Ten of the initial 15 candidate GMMs fit the observed ground motions and meet established criteria for inclusion in the next update of the Alaska seismic hazard model.

Alaska↗

USGS1999

Seismic hazard map of the western hemisphere

Vulnerability to natural disasters increases with urbanization and development of associated support systems (reservoirs, power plants, etc.). Catastrophic earthquakes account for 60% of worldwide casualties associated with natural disasters. Economic damage from earthquakes is increasing, even in technologically advanced countries with some level of seismic zonation, as shown by the 1989 Loma Prieta, CA ($6 billion), 1994 Northridge, CA ($ 25 billion), and 1995 Kobe, Japan (> $ 100 billion) earthquakes. The growth of megacities in seismically active regions around the world often includes the construction of seismically unsafe buildings and infrastructures, due to an insufficient knowledge of existing seismic hazard. Minimization of the loss of life, property damage, and social and economic disruption due to earthquakes depends on reliable estimates of seismic hazard. National, state, and local governments, decision makers, engineers, planners, emergency response organizations, builders, universities, and the general public require seismic hazard estimates for land use planning, improved building design and construction (including adoption of building construction codes), emergency response preparedness plans, economic forecasts, housing and employment decisions, and many more types of risk mitigation. The seismic hazard map of the Americas is the concatenation of various national and regional maps, involving a suite of approaches. The combined maps and documentation provide a useful global seismic hazard framework and serve as a resource for any national or regional agency for further detailed studies applicable to their needs. This seismic hazard map depicts Peak Ground Acceleration (PGA) with a 10% chance of exceedance in 50 years for the western hemisphere. PGA, a short-period ground motion parameter that is proportional to force, is the most commonly mapped ground motion parameter because current building codes that include seismic provisions specify the horizontal force a building should be able to withstand during an earthquake. This seismic hazard map of the Americas depicts the likely level of short-period ground motion from earthquakes in a fifty-year window. Short-period ground motions effect short-period structures (e.g., one-to-two story buildings). The largest seismic hazard values in the western hemisphere generally occur in areas that have been, or are likely to be, the sites of the largest plate boundary earthquakes. Although the largest earthquakes ever recorded are the 1960 Chile and 1964 Alaska subduction zone earthquakes, the largest seismic hazard (PGA) value in the Americas is in Southern California (U.S.), along the San Andreas fault.

Annals of Geophysics↗

USGS1999

Analysis of earthquake recordings obtained from the Seafloor Earthquake Measurement System (SEMS) instruments deployed off the coast of southern California

For more than 20 years, a program has been underway to obtain records of earthquake shaking on the seafloor at sites offshore of southern California, near oil platforms. The primary goal of the program is to obtain data that can help determine if ground motions at offshore sites are significantly different than those at onshore sites; if so, caution may be necessary in using onshore motions as the basis for the seismic design of oil platforms. We analyze data from eight earthquakes recorded at six offshore sites; these are the most important data recorded on these stations to date. Seven of the earthquakes were recorded at only one offshore station; the eighth event was recorded at two sites. The earthquakes range in magnitude from 4.7 to 6.1. Because of the scarcity of multiple recordings from any one event, most of the analysis is based on the ratio of spectra from vertical and horizontal components of motion. The results clearly show that the offshore motions have very low vertical motions compared to those from an average onshore site, particularly at short periods. Theoretical calculations find that the water layer has little effect on the horizontal components of motion but that it produces a strong spectral null on the vertical component at the resonant frequency of P waves in the water layer. The vertical-to-horizontal ratios for a few selected onshore sites underlain by relatively low shear-wave velocities are similar to the ratios from offshore sites for frequencies less than about one-half the water layer P -wave resonant frequency, suggesting that the shear-wave velocities beneath a site are more important than the water layer in determining the character of the ground motions at lower frequencies.

California↗

USGS2025

Near-surface material and topography generate anomalous high-frequency ground motion amplification in Chugiak, Alaska

An ∼3 km long nodal array oriented approximately east–west was deployed in Chugiak, Alaska, by the U.S. Geological Survey during 2021. The array intersects with the permanent NetQuakes station NP.ARTY, where peak ground acceleration (PGA) value of 1.98 g was recorded during the 2018 M w 7.1 Anchorage, Alaska, earthquake, in sharp contrast to the PGA of ∼0.3 g at a site just 4 km to the west. Seismic data for M w 1.8–4.3 aftershocks from the M w 7.1 event recorded by the nodal array confirm the anomalously large ground motions obtained at NP.ARTY as well as similar amplifications at nodes within ∼1 km to the east. Here, we performed 0–10 Hz 3D finite‐difference simulations, including high‐resolution surface topography, to explore the cause of the unexpectedly large amplification. As expected, the simulations computed with a regional 3D tomography velocity model severely underpredict the 0–10 Hz acceleration records at almost all sites. Adding a near‐surface low‐velocity taper to 300 m depth amplifies the accelerations by up to a factor of 5 and enables a reasonable match between the nodal data and simulations at sites to the west of NP.ARTY. However, this model still underpredicts the spectral energy in the area covered by glacial sediments by up to an order of magnitude. The addition of a till layer using a depth‐dependent shear‐wave velocity ( ⁠⁠ V s ) profile along with a homogeneous, 8 m thick low‐velocity layer with V s = 250 m/s representing the kame terraces improves the fit to data to within a factor of 2 at nodes located on top of the glacial sediments. Our study shows that the anomalously large high‐frequency amplification recorded at and near NP.ARTY can be explained by a combination of topographic effects and near‐surface low‐velocity material with amplification effects on the high‐frequency ground motion by up to about 40% and an order of magnitude, respectively.

Alaska↗

USGS2023

Comparison of co-recorded analog and digital systems for characterization of responses and uncertainties

One of the most prominent challenges related to legacy seismic data is determining how these data can be appropriately used in modern research applications. The wide variety of instrumentation used in the analog era, the format of recording on paper wrapped around a helicorder drum, and limited metadata information introduces ambiguities that are not typical of modern digital data. Therefore, techniques must be developed to help characterize uncertainties in legacy data. This article presents an analysis that compares corecorded signals from two instruments—a Trillium Compact or Press‐Ewing (PE) seismometer for sensing ground motion and two recording systems: a modern Q330 digitizer or heated‐stylus system. Analyses of the recordings in both time and frequency domains indicate time uncertainty on the order of one second, identify a flat response in a 10–60 s band for the PE and drum recorder, and highlight how specific features of scans and paper seismograms (e.g., repeated portions of scans and line thickness) can cause timing jumps or reduced trace amplitude.

Seismological Research Letters↗

USGS2018

A flatfile of ground motion intensity measurements from induced earthquakes in Oklahoma and Kansas

We have produced a uniformly processed database of orientation-independent (RotD50, RotD100) ground motion intensity measurements containing peak horizontal ground motions (accelerations and velocities) and 5-percent-damped pseudospectral accelerations (0.1–10 s) from more than 3,800 M ≥ 3 earthquakes in Oklahoma and Kansas that occurred between January 2009 and December 2016. Ground motion time series were collected from regional, national, and temporary seismic arrays out to 500 km. We relocated the majority of the earthquake hypocenters using a multiple-event relocation algorithm to produce a set of near-uniformly processed hypocentral locations. Ground motion processing followed standard methods, with the primary objective of reducing the effects of noise on the measurements. Regional wave-propagation features and the high seismicity rate required careful selection of signal windows to ensure that we captured the entire ground motion record and that contaminating signals from extraneous earthquakes did not contribute to the database. Processing was carried out with an automated scheme and resulted in a database comprising more than 174,000 records ( https://dx.doi.org/10.5066/F73B5X8N ). We anticipate that these results will be useful for improved understanding of earthquake ground motions and for seismic hazard applications.

Kansas, Oklahoma↗

USGS1993

The role of acoustic emission in the study of rock fracture

The development of faults and shear fracture systems over a broad range of temperature and pressure and for a variety of rock types involves the growth and interaction of microcracks. Acoustic emission (AE), which is produced by rapid microcrack growth, is a ubiquitous phenomenon associated with brittle fracture and has provided a wealth of information regarding the failure process in rock. This paper reviews the successes and limitations of AE studies as applied to the fracture process in rock with emphasis on our ability to predict rock failure. Application of laboratory AE studies to larger scale problems related to the understanding of earthquake processes is also discussed. In this context, laboratory studies can be divided into the following categories. 1) Simple counting of the number of AE events prior to sample failure shows a correlation between AE rate and inelastic strain rate. Additional sorting of events by amplitude has shown that AE events obey the power law frequency-magnitude relation observed for earthquakes. These cumulative event count techniques are being used in conjunction with damage mechanics models to determine how damage accumulates during loading and to predict failure. 2) A second area of research involves the location of hypocenters of AE source events. This technique requires precise arrival time data of AE signals recorded over an array of sensors that are essentially a miniature seismic net. Analysis of the spatial and temporal variation of event hypocenters has improved our understanding of the progression of microcrack growth and clustering leading to rock failure. Recently, fracture nucleation and growth have been studied under conditions of quasi-static fault propagation by controlling stress to maintain constant AE rate. 3) A third area of study involves the analysis of full waveform data as recorded at receiver sites. One aspect of this research has been to determine fault plane solutions of AE source events from first motion data. These studies show that in addition to pure tensile and double couple events, a significant number of more complex event types occur in the period leading to fault nucleation. 4) P and S wave velocities (including spatial variations) and attenuation have been obtained by artificially generating acoustic pulses which are modified during passage through the sample. ?? 1993.

International Journal of Rock Mechanics and Mining↗

USGS2019

Combining dynamic rupture simulations with ground motion data to characterize seismic hazard from Mw 3-5.8 earthquakes in Oklahoma and Kansas

Many seismically active areas suffer from a lack of near‐source ground‐motion recordings, making ground‐motion prediction difficult at distances within ∼ 40    km "> ∼ 40 km ∼40 km from an earthquake. We aim to aid the development of near‐source ground‐motion prediction equations (GMPEs) by generating synthetic ground‐motion data via simulation. Building on previous work using point‐source moment tensor sources to simulate small ( ⁠ M w "> M w Mw 3–4) earthquakes for a target region encompassing north central Oklahoma and south central Kansas, we perform dynamic rupture simulations of earthquakes up to M w "> M w Mw 5.8. We introduce complexity into the rupture process by adding stochastically generated heterogeneity to initial stress conditions. Our simulated ground‐motion data are added to a catalog of recorded ground‐motion data to construct a composite recorded‐simulated ground‐motion catalog that we use to develop a GMPE for the target region. This procedure can be generalized and used for GMPE development in other regions with near‐source ground‐motion data scarcity, which could directly benefit critical applications such as the National Seismic Hazard Maps produced by the U.S. Geological Survey (USGS).

Kansas, Oklahoma↗

USGS2012

Comparison of Vibroseis and explosive source methods for deep crustal seismic reflection profiling in the Basin and Range province

Direct comparison of low-fold, high-energy explosive and high-fold, lower-energy Vibroseis methods for acquiring deep crustal seismic reflection data in the Basin and Range Province suggests that the high-fold common midpoint (CMP) method there does not provide the best possible image of lower crustal structure. During the recent acquisition of a Vibroseis profile in the Basin and Range Province we fired single deep shot holes to obtain a coincident single-fold explosive section. Within the upper crust (upper 3 s) the explosive source and Vibroseis records are nearly equivalent. For record times below 3 s, however, comparison of the explosive source gathers and the coincident final 60-fold Vibroseis section demonstrates that low-fold explosive profiling provides a higher-quality image of the midcrust to lower crust (3–10 s). The higher record quality of the explosive sources results primarily from the larger seismic energy levels produced by the explosives, making them less sensitive to common noise sources. Whereas deeper than 4–5 s the Vibroseis energy levels on individual source efforts fall to that of ambient noise levels, the explosions provide signal-generated energy exceeding ambient noise levels down to 18–19 s. Although individual reflections can be correlated on explosion and Vibroseis shot gathers, reflection events on the 60-fold Vibroseis stack do not correlate to those on the single-fold explosion profile, suggesting that the high-fold CMP method in our study did not maintain the integrity of the weak lower crustal reflected arrivals. Reasons why the high-fold CMP method apparently failed include complex, even time-varying, statics, nonhyperbolic moveout at long offsets, and the difficulty in resolving stacking velocities with data having low signal-to-noise ratios. Reflections on the explosion section are longer and imply a greater degree of layering than one would infer from the lower-energy Vibroseis section.

Journal of Geophysical Research↗

USGS2009

The 2006-2007 Kuril Islands great earthquake sequence

[1] The southwestern half of a ∼500 km long seismic gap in the central Kuril Island arc subduction zone experienced two great earthquakes with extensive preshock and aftershock sequences in late 2006 to early 2007. The nature of seismic coupling in the gap had been uncertain due to the limited historical record of prior large events and the presence of distinctive upper plate, trench and outer rise structures relative to adjacent regions along the arc that have experienced repeated great interplate earthquakes in the last few centuries. The intraplate region seaward of the seismic gap had several shallow compressional events during the preceding decades (notably an M S 7.2 event on 16 March 1963), leading to speculation that the interplate fault was seismically coupled. This issue was partly resolved by failure of the shallow portion of the interplate megathrust in an M W = 8.3 thrust event on 15 November 2006. This event ruptured ∼250 km along the seismic gap, just northeast of the great 1963 Kuril Island (M w = 8.5) earthquake rupture zone. Within minutes of the thrust event, intense earthquake activity commenced beneath the outer wall of the trench seaward of the interplate rupture, with the larger events having normal-faulting mechanisms. An unusual double band of interplate and intraplate aftershocks developed. On 13 January 2007, an M W = 8.1 extensional earthquake ruptured within the Pacific plate beneath the seaward edge of the Kuril trench. This event is the third largest normal-faulting earthquake seaward of a subduction zone on record, and its rupture zone extended to at least 33 km depth and paralleled most of the length of the 2006 rupture. The 13 January 2007 event produced stronger shaking in Japan than the larger thrust event, as a consequence of higher short-period energy radiation from the source. The great event aftershock sequences were dominated by the expected faulting geometries; thrust faulting for the 2006 rupture zone, and normal faulting for the 2007 rupture zone. A large intraplate compressional event occurred on 15 January 2009 (M w = 7.4) near 45 km depth, below the rupture zone of the 2007 event and in the vicinity of the 16 March 1963 compressional event. The fault geometry, rupture process and slip distributions of the two great events are estimated using very broadband teleseismic body and surface wave observations. The occurrence of the thrust event in the shallowest portion of the interplate fault in a region with a paucity of large thrust events at greater depths suggests that the event removed most of the slip deficit on this portion of the interplate fault. This great earthquake doublet demonstrates the heightened seismic hazard posed by induced intraplate faulting following large interplate thrust events. Future seismic failure of the remainder of the seismic gap appears viable, with the northeastern region that has also experienced compressional activity seaward of the megathrust warranting particular attention.

Journal of Geophysical Research B: Solid Earth↗

USGS2003

2001 Bhuj, India, earthquake engineering seismoscope recordings and Eastern North America ground-motion attenuation relations

Engineering seismoscope data collected at distances less than 300 km for the M 7.7 Bhuj, India, mainshock are compatible with ground-motion attenuation in eastern North America (ENA). The mainshock ground-motion data have been corrected to a common geological site condition using the factors of Joyner and Boore (2000) and a classification scheme of Quaternary or Tertiary sediments or rock. We then compare these data to ENA ground-motion attenuation relations. Despite uncertainties in recording method, geological site corrections, common tectonic setting, and the amount of regional seismic attenuation, the corrected Bhuj dataset agrees with the collective predictions by ENA ground-motion attenuation relations within a factor of 2. This level of agreement is within the dataset uncertainties and the normal variance for recorded earthquake ground motions.

Bulletin of the Seismological Society of America↗

USGS2016

Seismic envelope-based detection and location of ground-coupled airwaves from volcanoes in Alaska

Volcanic explosions and other infrasonic sources frequently produce acoustic waves that are recorded by seismometers. Here we explore multiple techniques to detect, locate, and characterize ground‐coupled airwaves (GCA) on volcano seismic networks in Alaska. GCA waveforms are typically incoherent between stations, thus we use envelope‐based techniques in our analyses. For distant sources and planar waves, we use f ‐ k beamforming to estimate back azimuth and trace velocity parameters. For spherical waves originating within the network, we use two related time difference of arrival (TDOA) methods to detect and localize the source. We investigate a modified envelope function to enhance the signal‐to‐noise ratio and emphasize both high energies and energy contrasts within a spectrogram. We apply these methods to recent eruptions from Cleveland, Veniaminof, and Pavlof Volcanoes, Alaska. Array processing of GCA from Cleveland Volcano on 4 May 2013 produces robust detection and wave characterization. Our modified envelopes substantially improve the short‐term average/long‐term average ratios, enhancing explosion detection. We detect GCA within both the Veniaminof and Pavlof networks from the 2007 and 2013–2014 activity, indicating repeated volcanic explosions. Event clustering and forward modeling suggests that high‐resolution localization is possible for GCA on typical volcano seismic networks. These results indicate that GCA can be used to help detect, locate, characterize, and monitor volcanic eruptions, particularly in difficult‐to‐monitor regions. We have implemented these GCA detection algorithms into our operational volcano‐monitoring algorithms at the Alaska Volcano Observatory.

Alaska↗

USGS2010

The MW 7.0 Haiti Earthquake of January 12, 2010: USGS/EERI Advance Reconnaissance Team Report

Executive Summary A field reconnaissance in Haiti by a five-member team with expertise in seismology and earthquake engineering has revealed a number of factors that led to catastrophic losses of life and property during the January 12, 2010, Mw 7.0 earthquake. The field study was conducted from January 26 to February 3, 2010, and included investigations in Port-au-Prince and the heavily damaged communities to the west, including Leogane, Grand Goave, Petite Goave, and Oliver. Seismology Despite recent seismic quiescence, Haiti has suffered similar devastating earthquakes in the historical past (1701, 1751, 1770 and 1860). Despite this knowledge of historical seismicity, Haiti had no seismograph stations during the main earthquake, so it is impossible to estimate accurately the intensity of ground motions. Nonetheless, the wide range of buildings damaged by the January 12, 2010, earthquake suggests that the ground motions contained seismic energy over a wide range of frequencies. Another earthquake of similar magnitude could strike at any time on the eastern end of the Enriquillo Fault, directly to the south of Port-au-Prince. Reconstruction must take this hazard into account. The four portable seismographs installed by the team recorded a series of small aftershocks. As expected, the ground motions recorded at a hard-rock site contained a greater proportion of high frequencies than the motions recorded at a soil site. Two of the stations continue to monitor seismic activity. A thorough field investigation of the mapped Enriquillo Fault south of the city of Leogane failed to find any evidence of surface faulting. This led the team to conclude that the earthquake was unlikely to have produced any surface rupture in the study area. Geotechnical Aspects Soil liquefaction, landslides and rockslides in cut slopes, and road embankment failures contributed to extensive damage in Port-au-Prince and elsewhere. A lack of detailed knowledge of the physical conditions of the soils (for example, lithology, stiffness, density, and thickness) made it difficult for us to quantitatively assess the role of ground-motion amplification in the widespread damage. Buildings The Haitian Ministry of Statistics and Informatics reported that one-story buildings represent 73 percent of the building inventory. Most ordinary, one-story houses have roofs made of sheet metal (82 percent), whereas most multistory houses and apartments have roofs made of concrete (71 percent). Walls made of concrete/block/stone predominate both in ordinary houses and apartments. It appears that the widespread damage to residences and commercial and government buildings was attributable to a great extent to the lack of earthquake-resistant design. In many cases, the structural types, member dimensions, and detailing practices were inadequate to resist strong ground motions. These vulnerabilities may have been exacerbated by poor construction practices. Reinforced concrete frames with concrete block masonry infill appeared to perform particularly poorly. Structures with light (timber or sheet metal) roofs performed better compared to structures with concrete roofs and slabs. The seismic performance of some buildings was adequate, and some of the damaged buildings appeared to have had low deformation demands. These observations suggest that structures designed and constructed with adequate stiffness and reinforcing details would have resisted the earthquake without being damaged severely. A damage survey of 107 buildings in downtown Port-au-Prince indicated that 28 percent had collapsed and another 33 percent were damaged enough to require repairs. A similar survey of 52 buildings in Leogane found that 62 percent had collapsed and another 31 percent required repairs. Bridges There was no evidence of bridge collapses attributable to the earthquake. Most bridges in Port-au-Prince are simple box culverts consisting of box girders 2.0 to 2.

Open-File Report↗

USGS2008

Observation and prediction of dynamic ground strains, tilts, and torsions caused by the Mw 6.0 2004 Parkfield, California, earthquake and aftershocks, derived from UPSAR array observations

The 28 September 2004 Parkfield, California, earthquake (Mw 6.0) and four aftershocks (Mw 4.7-5.1) were recorded on 12 accelerograph stations of the U.S. Geological Survey Parkfield seismic array (UPSAR), an array of three-component accelerographs occupying an area of about 1 km2 located 8.8 km from the San Andreas fault. Peak horizontal acceleration and velocity at UPSAR during the mainshock were 0.45g and 27 cm/sec, respectively. We determined both time-varying and peak values of ground dilatations, shear strains, torsions, tilts, torsion rates, and tilt rates by applying a time-dependent geodetic analysis to the observed array displacement time series. Array-derived dilatations agree fairly well with point measurements made on high sample rate recordings of the Parkfield-area dilatometers (Johnston et al., 2006). Torsion Fourier amplitude spectra agree well with ground velocity spectra, as expected for propagating plane waves. A simple predictive relation, using the predicted peak velocity from the Boore-Atkinson ground-motion prediction relation (Boore and Atkinson, 2007) scaled by a phase velocity of 1 km/sec, predicts observed peak Parkfield and Chi-Chi rotations (Huang, 2003) well. However, rotation rates measured during Mw 5 Ito, Japan, events observed on a gyro sensor (Takeo, 1998) are factors of 5-60 greater than those predicted by our predictive relation. This discrepancy might be caused by a scale dependence in rotation, with rotations measured over a short baseline exceeding those measured over long baselines. An alternative hypothesis is that events having significant non-double-couple mechanisms, like the Ito events, radiate much stronger rotations than double-couple events. If this is true, then rotational observations might provide an important source of new information for monitoring seismicity in volcanic areas.

Bulletin of the Seismological Society of America↗

USGS2004

Seismic velocity models for the Denali fault zone along the Richardson Highway, Alaska

Crustal-scale seismic-velocity models across the Denali fault zone along the Richardson Highway show a 50-km-thick crust, a near vertical fault trace, and a 5-km-wide damage zone associated with the fault near Trans-Alaska Pipeline Pump Station 10, which provided the closest strong ground motion recordings of the 2002 Denali fault earthquake. We compare models, derived from seismic reflection and refraction surveys acquired in 1986 and 1987, to laboratory measurements of seismic velocities for typical metamorphic rocks exposed along the profiles. Our model for the 1986 seismic reflection profile indicates a 5-km-wide low-velocity zone in the upper 1 km of the Denali fault zone, which we interpret as fault gouge. Deeper refractions from our 1987 line image a 40-km wide, 5-km-deep low-velocity zone along the Denali fault and nearby associated fault strands, which we attribute to a composite damage zone along several strands of the Denali fault zone and to the obliquity of the seismic line to the fault zone. Our velocity model and other geophysical data indicate a nearly vertical Denali fault zone to a depth of 30 km. After-shocks of the 2002 Denali fault earthquake and our velocity model provide evidence for a flower structure along the fault zone consisting of faults dipping toward and truncated by the Denali fault. Wide-angle reflections indicate that the crustal thickness beneath the Denali fault is transitional between the 60-km-thick crust beneath the Alaska Range to the south, and the extended, 30-km-thick crust of the Yukon-Tanana terrane to the north.

Alaska↗

USGS2021

Seismic wave propagation and basin amplification in the Wasatch Front, Utah

Ground‐motion analysis of more than 3000 records from 59 earthquakes, including records from the March 2020 M w "> M w 5.7 Magna earthquake sequence, was carried out to investigate site response and basin amplification in the Wasatch Front, Utah. We compare ground motions with the Bayless and Abrahamson (2019 ; hereafter, BA18) ground‐motion model (GMM) for Fourier amplitude spectra, which was developed on crustal earthquake records from California and other tectonically active regions. The Wasatch Front records show a significantly different near‐source rate of distance attenuation than the BA18 model, which we attribute to differences in (apparent) geometric attenuation. Near‐source residuals show a period dependence of this effect, with greater attenuation at shorter periods ( ⁠ T < 0.5    s "> T < 0.5 s ) and a correlation between period and the distance over which the discrepancy manifests ( ⁠ ∼ 20 – 50    km "> ∼ 20 – 50 km ⁠ ). We adjusted the recorded ground motions for these regional path effects and solved for station site terms using linear mixed‐effects regressions, with groupings for events and stations. We analyzed basin amplification by comparing the site terms with the basin geometry and basin depths from two seismic‐velocity models for the region. Sites over the deeper parts of the sedimentary basins are amplified by factors of 3–10, relative to sites with thin sedimentary cover, with greater amplification at longer periods ( ⁠ T ≳ 1    s "> T ≳ 1 s ). Average ground‐motion variability increases with period, and long‐period variability exhibits a slight increase at the basin edges. These results indicate regional seismic wave propagation effects requiring further study, and potentially a regionalized GMM, as well as highlight basin amplification complexities that may be incorporated into seismic hazard assessments.

Utah↗

USGS1994

Lake-level history of Lake Michigan for the past 12,000 years: the record from deep lacustrine sediments

Collection and analysis of an extensive set of seismic-reflection profiles and cores from southern Lake Michigan have provided new data that document the history of the lake basin for the past 12,000 years. Analyses of the seismic data, together with radiocarbon dating, magnetic, sedimentologic, isotopic, and paleontologic studies of core samples, have allowed us to reconstruct lake-level changes during this recent part of the lake's history. The post-glacial history of lake-level changes in the Lake Michigan basin begins about 11.2 ka with the fall from the high Calumet level, caused by the retreat of the Two Rivers glacier, which had blocked the northern outlet of the lake. This lake-level fall was temporarily reversed by a major influx of water from glacial Lake Agassiz (about 10.6 ka), during which deposition of the distinctive gray Wilmette Bed of the Lake Michigan Formation interrupted deposition of red glaciolacustrine sediment. Lake level then continued to fall, culminating in the opening of the North Bay outlet at about 10.3 ka. During the resulting Chippewa low phase, lake level was about 80 m lower than it is today in the southern basin of Lake Michigan. The rise of the early Holocene lake level, controlled primarily by isostatic rebound of the North Bay outlet, resulted in a prominent, planar, transgressive unconformity that eroded most of the shoreline features below present lake level. Superimposed on this overall rise in lake level, a second influx of water from Lake Agassiz temporarily raised lake levels an unknown amount about 9.1 ka. At about 7 ka, lake level may have fallen below the level of the outlet because of sharply drier climate. Sometime between 6 and 5 ka, the character of the lake changed dramatically, probably due mostly to climatic causes, becoming highly undersaturated with respect to calcium carbonate and returning primary control of lake level to the isostatically rising North Bay outlet. Post-Nipissing (about 5 ka) lake level has fallen about 6 m due to erosion of the Port Huron outlet, a trend around which occurred relatively small (± ∼2 m), short-term fluctuations controlled mainly by climatic changes. These cyclic fluctuations are reflected in the sed-imentological and sediment-magnetic properties of the sediments.

Indiana, Illinois, Michigan, Wisconsin↗

USGS2000

Seismic anisotropy of the shallow crust at the Juan de Fuca Ridge

Microearthquake data recorded on four ocean bottom seismometers are used to study shear-wave splitting on the Endeavour Segment of the Juan de Fuca Ridge. The covariance matrix decomposition method is used to determine the sensor orientation from explosive shot data and to estimate the anisotropy parameters for 238 earthquake records. At three of the four sites, the results show a remarkably consistent fast direction parallel to the ridge axis. The time delays between the fast and the slow waves range from 40 to 200 ms, with an average of 90 ms. They are not clearly related to earthquake range, focal depth or source-receiver azimuth. The splitting of the shear waves is interpreted as an effect of structural anisotropy due to the presence of ridge-parallel cracks in the shallow crust. If we assume that anisotropy is concentrated in the upper 1-2 km, the splitting times require a high crack density of ~0.1.

Geophysical Research Letters↗