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At least 217 records · Page 12Linked to original sources

USGS2017

Using structural damage statistics to derive macroseismic intensity within the Kathmandu valley for the 2015 M7.8 Gorkha, Nepal earthquake

We make and analyze structural damage observations from within the Kathmandu valley following the 2015 M7.8 Gorkha, Nepal earthquake to derive macroseismic intensities at several locations including some located near ground motion recording sites. The macroseismic intensity estimates supplement the limited strong ground motion data in order to characterize the damage statistics. This augmentation allows for direct comparisons between ground motion amplitudes and structural damage characteristics and ultimately produces a more constrained ground shaking hazard map for the Gorkha earthquake. For systematic assessments, we focused on damage to three specific building categories: (a) low/mid-rise reinforced concrete frames with infill brick walls, (b) unreinforced brick masonry bearing walls with reinforced concrete slabs, and (c) unreinforced brick masonry bearing walls with partial timber framing. Evaluating dozens of photos of each construction type, assigning each building in the study sample to a European Macroseismic Scale (EMS)-98 Vulnerability Class based upon its structural characteristics, and then individually assigning an EMS-98 Damage Grade to each building allows a statistically derived estimate of macroseismic intensity for each of nine study areas in and around the Kathmandu valley. This analysis concludes that EMS-98 macroseismic intensities for the study areas from the Gorkha mainshock typically were in the VII–IX range. The intensity assignment process described is more rigorous than the informal approach of assigning intensities based upon anecdotal media or first-person accounts of felt-reports, shaking, and their interpretation of damage. Detailed EMS-98 macroseismic assessments in urban areas are critical for quantifying relations between shaking and damage as well as for calibrating loss estimates. We show that the macroseismic assignments made herein result in fatality estimates consistent with the overall and district-wide reported values.

Kathmandu Valley↗

USGS2017

Source modeling of the 2015 Mw 7.8 Nepal (Gorkha) earthquake sequence: Implications for geodynamics and earthquake hazards

The Gorkha earthquake on April 25th, 2015 was a long anticipated, low-angle thrust-faulting event on the shallow décollement between the India and Eurasia plates. We present a detailed multiple-event hypocenter relocation analysis of the Mw 7.8 Gorkha Nepal earthquake sequence, constrained by local seismic stations, and a geodetic rupture model based on InSAR and GPS data. We integrate these observations to place the Gorkha earthquake sequence into a seismotectonic context and evaluate potential earthquake hazard. Major results from this study include (1) a comprehensive catalog of calibrated hypocenters for the Gorkha earthquake sequence; (2) the Gorkha earthquake ruptured a ~ 150 × 60 km patch of the Main Himalayan Thrust (MHT), the décollement defining the plate boundary at depth, over an area surrounding but predominantly north of the capital city of Kathmandu (3) the distribution of aftershock seismicity surrounds the mainshock maximum slip patch; (4) aftershocks occur at or below the mainshock rupture plane with depths generally increasing to the north beneath the higher Himalaya, possibly outlining a 10–15 km thick subduction channel between the overriding Eurasian and subducting Indian plates; (5) the largest Mw 7.3 aftershock and the highest concentration of aftershocks occurred to the southeast the mainshock rupture, on a segment of the MHT décollement that was positively stressed towards failure; (6) the near surface portion of the MHT south of Kathmandu shows no aftershocks or slip during the mainshock. Results from this study characterize the details of the Gorkha earthquake sequence and provide constraints on where earthquake hazard remains high, and thus where future, damaging earthquakes may occur in this densely populated region. Up-dip segments of the MHT should be considered to be high hazard for future damaging earthquakes.

Gorkha↗

USGS2017

Characterizing the Kathmandu Valley sediment response through strong motion recordings of the 2015 Gorkha earthquake sequence

We analyze strong motion records and high-rate GPS measurements of the M 7.8 Gorkha mainshock, M 7.3 Dolakha, and two moderate aftershock events recorded at four stations on the Kathmandu basin sediments, and one on rock-outcrop. Recordings on soil from all four events show systematic amplification relative to the rock site at multiple frequencies in the 0.1–2.5 Hz frequency range, and de-amplification of higher frequencies ( >2.5–10 Hz). The soil-to-rock amplification ratios for the M 7.8 and M 7.3 events have lower amplitude and frequency peaks relative to the ratios of the two moderate events, effects that could be suggestive of nonlinear site response. Further, comparisons to ground motion prediction equations show that 1) both soil and rock mainshock recordings were severely depleted of high frequencies, and 2) the depletion at high frequencies is not present in the aftershocks. These observations indicate that the high frequency deamplification is additionally related to characteristics of the source that are not captured by simplified ground motion prediction equations, and allude to seismic hazard analysis models being revised – possibly by treating isolated high frequency radiation sources separately from long period components to capture large magnitude near-source events such as the 2015 Gorkha mainshock.

Kathmandu Valley↗

USGS2017

Application and evaluation of a rapid response earthquake-triggered landslide model to the 25 April 2015 Mw 7.8 Gorkha earthquake, Nepal

The 25 April 2015 M w 7.8 Gorkha earthquake produced strong ground motions across an approximately 250 km by 100 km swath in central Nepal. To assist disaster response activities, we modified an existing earthquake-triggered landslide model based on a Newmark sliding block analysis to estimate the extent and intensity of landsliding and landslide dam hazard. Landslide hazard maps were produced using Shuttle Radar Topography Mission (SRTM) digital topography, peak ground acceleration (PGA) information from the U.S. Geological Survey (USGS) ShakeMap program, and assumptions about the regional rock strength based on end-member values from previous studies. The instrumental record of seismicity in Nepal is poor, so PGA estimates were based on empirical Ground Motion Prediction Equations (GMPEs) constrained by teleseismic data and felt reports. We demonstrate a non-linear dependence of modeled landsliding on aggregate rock strength, where the number of landslides decreases exponentially with increasing rock strength. Model estimates are less sensitive to PGA at steep slopes (> 60°) compared to moderate slopes (30–60°). We compare forward model results to an inventory of landslides triggered by the Gorkha earthquake. We show that moderate rock strength inputs over estimate landsliding in regions beyond the main slip patch, which may in part be related to poorly constrained PGA estimates for this event at far distances from the source area. Directly above the main slip patch, however, the moderate strength model accurately estimates the total number of landslides within the resolution of the model (landslides ≥ 0.0162 km 2 ; observed n = 2214, modeled n = 2987), but the pattern of landsliding differs from observations. This discrepancy is likely due to the unaccounted for effects of variable material strength and local topographic amplification of strong ground motion, as well as other simplifying assumptions about source characteristics and their relationship to landsliding.

Tectonophysics↗

USGS2016

Introduction to the special issue on the 25 April 2015 Mw 7.8 Gorkha(Nepal) earthquake

On April 25, 2015, a moment magnitude (M w ) 7.8 earthquake struck central Nepal, breaking a section of the broader Himalayan Front that had been largely quiescent in moderate-to-large earthquakes for much of the modern seismological era. Ground shaking associated with the event resulted in a broad distribution of triggered avalanches and landslides. The ensuing aftershock sequence was punctuated by a Mw 7.3 event 17 days after the mainshock. The combined effects of these earthquakes and secondary hazards have led to the Gorkha earthquake becoming the worst natural disaster in Nepal since the 1934 Nepal-Bihar earthquake, causing close to 9000 deaths and severely injuring over 21,000 people ( OCHA, 2015 ). Despite the devastating effects of this earthquake, the convergent margin that hosted it is thought to be capable of much larger ruptures—perhaps as large as Mw 9 ( Feldl and Bilham, 2006 ). The 2015 Gorkha rupture lies just to the west of the 1934 M 8.0–8.4 event ( Sapkota et al., 2013; Bollinger et al., 2014 ). Unlike the 1934 event, which has been documented in paleoseismic trenches along the Himalayan Front (e.g., Sapkota et al., 2013 ), and other large ruptures along the arc (e.g., Lavé et al., 2005; Kumar et al., 2006 ), the 2015 event did not rupture to the surface (e.g., Galetzka et al., 2015 ). As a result, some researchers have suggested that the Gorkha earthquake was not as large, or as damaging, as might have been expected based on our (albeit limited) understanding of historic earthquakes, seismic hazard and risk (e.g., Bilham, 2015; Hough, 2015 ). Important questions surrounding the earthquake and its regional setting thus arise. What were the detailed characteristics of the rupture and the aftershock sequence, and what is the relationship between mainshock slip and subsequent seismicity? Why did this event not rupture to the surface? Was damage less than should have been expected; and if so, why? What role did path effects, such as basin amplification, play? Do details of the earthquake sequence allow us to better understand regional seismotectonics, and in turn, future risk? Discussion of these and other issues has been ongoing since the earthquake; a large body of literature already exists that characterizes details of the earthquake sequence and its effects. This special issue attempts to gather a wide variety of detailed studies that wholly characterize this event to a degree that has not yet been possible. The studies herein provide an improved understanding of the Gorkha earthquake, its impact on the region, and its place in the broader seismotectonic history of the Himalayan Front.

Gorkha↗

USGS2017

Rapid exhumation of Cretaceous arc-rocks along the Blue Mountains restraining bend of the Enriquillo-Plantain Garden fault, Jamaica, using thermochronometry from multiple closure systems

The effect of rapid erosion on kinematic partitioning along transpressional plate margins is not well understood, particularly in highly erosive climates. The Blue Mountains restraining bend (BMRB) of eastern Jamaica, bound to the south by the left-lateral Enriquillo-Plantain Garden fault (EPGF), offers an opportunity to test the effects of highly erosive climatic conditions on a 30-km-wide restraining bend system. No previous thermochronometric data exists in Jamaica to describe the spatial or temporal pattern of rock uplift and how oblique (> 20°) plate motion is partitioned into vertical strain. To define the exhumation history, we measured apatite (n = 10) and zircon (n = 6) (U-Th)/He ages, 40Ar/39Ar (n = 2; amphibole and K-spar) ages, and U/Pb zircon (n = 2) crystallization ages. Late Cretaceous U/Pb and 40Ar/39Ar ages (74–68 Ma) indicate rapid cooling following shallow emplacement of plutons during north-south subduction along the Great Caribbean Arc. Early to middle Miocene zircon helium ages (19–14 Ma) along a vertical transect suggest exhumation and island emergence at ~ 0.2 mm/yr. Older zircon ages 10–15 km to the north (44–35 Ma) imply less rock uplift. Apatite helium ages are young (6–1 Ma) across the entire orogen, suggesting rapid exhumation of the BMRB since the late Miocene. These constraints are consistent with previous reports of restraining bend formation and early emergence of eastern Jamaica. An age-elevation relationship from a vertical transect implies an exhumation rate of 0.8 mm/yr, while calculated closure depths and thermal modeling suggests exhumation as rapid as 2 mm/yr. The rapid rock uplift rates in Jamaica are comparable to the most intense transpressive zones worldwide, despite the relatively slow (5–7 mm/yr) strike-slip rate. We hypothesize highly erosive conditions in Jamaica enable a higher fraction of plate motion to be accommodated by vertical deformation. Thus, strike-slip restraining bends may evolve differently depending on erosivity and local climate.

Tectonophysics↗

USGS2018

The 2015 Gorkha (Nepal) Earthquake sequence: I. Source modeling and deterministic 3D ground shaking

To better quantify the relatively long period (< 0.3 Hz) shaking experienced during the 2015 Gorkha (Nepal) earthquake sequence, we study the finite rupture processes and the associated 3D ground motion of the Mw7.8 mainshock and the Mw7.2 aftershock . The 3D synthetics are then used in the broadband ground shaking in Kathmandu with a hybrid approach, summarized in a companion paper (Chen and Wei, 2017, submitted together). We determined the coseismic rupture process of the mainshock by joint inversion of InSAR/SAR, GPS (static and high-rate), strong motion and teleseismic waveforms. Our inversion for the mainshock indicates unilateral rupture towards the ESE, with an average rupture speed of 3.0 km/s and a total duration of ~ 60 s. Additionally, we find that the beginning part of the rupture (5–18 s) has about 40% longer rise time than the rest of the rupture, as well as slower rupture velocity. Our model shows two strong asperities occurring ~ 24 s and ~ 36 s after the origin and located ~ 30 km to the northwest and northeast of the Kathmandu valley, respectively. In contrast, the Mw7.2 aftershock is more compact both in time and space, as revealed by joint inversion of teleseismic body waves and InSAR data. The different rupture features between the mainshock and the aftershock could be related to difference in fault zone structure. The mainshock and aftershock ground motions in the Kathmandu valley, recorded by both strong motion and high-rate GPS stations, exhibited strong amplification around 0.2 Hz. A simplified 3D basin model, calibrated by an Mw5.2 aftershock, can match the observed waveforms reasonably well at 0.3 Hz and lower frequency. The 3D simulations indicate that the basin structure trapped the wavefield and produced an extensive ground vibration. Our study suggests that the combination of rupture characteristics and propagational complexity are required to understand the ground shaking produced by hazardous earthquakes such as the Gorkha event.

Tectonophysics↗

USGS2018

The influence of tectonic environment on dynamic earthquake triggering: A review and case study on Alaskan volcanoes

The phenomenon of dynamic earthquake triggering, when seismic waves from an earthquake trigger seismicity at distant sites, has been recognized for over 25 years, yet knowledge of the global distribution of dynamic triggering remains far from complete. Because occurrences of dynamic triggering provide in-situ information of the stress-state of a responding site, a more complete global picture of susceptible crustal environments becomes an important component of seismic hazard evaluations. Here we 1) review evidence for tectonic regime dependence of dynamic triggering susceptibility in the shallow brittle crust, and 2) explore triggering susceptibility at Alaskan volcanoes. We search for significant increases in seismicity rates at 19 Alaskan volcanic areas between 2006 and 2013 within 3 days following regional and teleseismic earthquakes of magnitude 7 and greater. We find evidence for 12 triggered responses at 9 volcanoes, but no evidence for triggered responses following hundreds of other earthquakes. The most impressive response was that of Pavlof Volcano to the 2011 M W 9.0 Tohoku-Oki earthquake. Our results suggest that triggered responses are difficult to predict, as they do not depend on background seismicity rates or amplitude of incident seismic waves from distant earthquakes. Taken together, observational and theoretical evidence suggests that dynamic triggering occurs in all tectonic environments. So far however, it appears to be more common in extensional and transtensional environments and regions with high pore-fluid pressure than in compressional and transpressional environments. Although some volcanic and geothermal areas may be primed for dynamic earthquake triggering, volcanoes in Alaska are relatively insensitive. The weak and variable responses of Alaskan volcanoes suggest that triggering susceptibility depends on time varying crustal conditions, such as the state of stress and distribution of pore pressure. Results also indirectly suggest that eruption triggering by distant earthquakes is unlikely in Alaska over the time scale of days.

Alaska↗

USGS2018

A proposed rupture scenario for the 1925 Mw 6.5 Santa Barbara, California, earthquake

The 29 June 1925 Santa Barbara earthquake is among the largest 20th century earthquakes in southern California. The earthquake also predated the installation of strong motion and local monitoring instruments in southern California; some instrumental data are, however, available from long-period instruments at regional and teleseismic distances. The current catalog moment magnitude is M W 6.8. Initial intensity magnitudes (M I ) estimated from original Coast and Geodetic Survey intensity assignments were lower (M I 6.3). In this study we assign modified Mercalli intensity values at 239 locations, including 144 specific locations within the city of Santa Barbara for which detailed damage information is available. Comparing the reinterpreted intensities with Did You Feel it? intensities for recent events in California, we estimate M W = 6.5, with a plausible range of 6.3–6.6. We further consider reported instrumental amplitudes to estimate an instrumental moment magnitude of M W = 6.6 ± 0.5. Our preferred final estimate is M W 6.5. Based on available constraints including aftershock locations inferred from data recorded on portable instruments, we propose that the earthquake nucleated east of the city of Santa Barbara, closer to the coast than previously estimated, and ruptured unilaterally ~30 km to the west, possibly along the south-dipping Mesa-Rincon Creek, and the More Ranch fault systems. Contrary to suggestions made in earlier studies (e.g. Willis, 1925a), relatively high intensities ~50 km west of Santa Barbara can then be explained by directivity rather than involvement of the Santa Ynez fault. Finally, we discuss the possibility that the earthquake was triggered by the larger M W = 6.6 Clarkston, Montana earthquake the previous day or induced by oil production in the Summerland oil field.

California↗

USGS2019

Crustal structure of the northern Harrat Rahat volcanic field (Saudi Arabia) from gravity and aeromagnetic data

New gravity data reveal a prominent negative anomaly along the main vent axis of the northern Harrat Rahat volcanic field in Saudi Arabia. The low continues north of the volcanic field onto exposures of Proterozoic rocks, indicating that the low is caused not only by the volcanic field (and possibly also underlying Cenozoic sediments), but also the underlying Proterozoic basement. An inversion of the gravity field guided by analysis of aeromagnetic data indicates (1) a broad depression of the basement surface that is deeper along the main vent axis in the eastern part of the harrat and in the southwest part of the volcanic field and (2) less dense basement beneath the vent axis. Low densities within the basement most likely arise from lithologic variations in the basement, predating Cenozoic volcanism, although our analysis does not rule out small volumes of partial melt and higher temperatures or extensive fracturing at depth.

Tectonophysics↗

USGS2020

Along-strike segmentation in the northern Caribbean plate boundary zone (Hispaniola sector): Tectonic implications

The North American (NOAM) plate converges with the Caribbean (CARIB) plate at a rate of 20.0 ± 0.4 mm/yr. towards 254 ± 1°. Plate convergence is highly oblique (20–10°), resulting in a complex crustal boundary with along-strike segmentation, strain partitioning and microplate tectonics. We study the oblique convergence of the NOAM and CARIB plates between southeastern Cuba to northern Puerto Rico using new swath multibeam bathymetry data and 2D multi-channel seismic profiles. The combined interpretation of marine geophysical data with the seismicity and geodetic data from public databases allow us to perform a regional scale analysis of the shallower structure, the seismotectonics and the slab geometry along the plate boundary. Due to differential rollback between the NOAM oceanic crust north of Puerto Rico and the relative thicker Bahamas Carbonate Province crust north of Hispaniola a slab tear is created at 68.5°W. The northern margin of Puerto Rico records the oblique high-dip subduction and rollback of the NOAM plate below the island arc. Those processes have resulted in a forearc transpressive tectonics (without strain partitioning), controlled by the Septentrional-Oriente Fault Zone (SOFZ) and the Bunce Fault Zone (BFZ). Meanwhile, in the northern margin of Hispaniola, the collision of the Bahamas Carbonate Province results in high plate coupling with strain partitioning: SOFZ and Northern Hispaniola Deformed Belt (NHDB). In the northern Haitian margin, compression is still relevant since seismicity is mostly associated with the deformation front, whereas strike slip earthquakes are hardly anecdotal. Although in Hispaniola intermediate-depth seismicity should disappear, diffuse intermediate-depth hypocenter remains evidencing the presence of remnant NOAM subducted slab below central and western Hispaniola. Results of this study improve our understanding of the active tectonics in the NE Caribbean that it is the base for future assessment studies on seismic and tsunamigenic hazard.

Caribbean Plate↗

USGS2021

Rapid strain release on the Bear River fault zone, Utah–Wyoming—The impact of preexisting structure on the rupture behavior of a new normal fault

Earthquake clustering (grouping in space and time) is a widely observed mode of strain release in the upper crust, although this behavior on individual faults is a departure from classic elastic rebound theory. In this study, we consider factors responsible for a cluster of earthquakes on the Bear River fault zone (BRF), a recently activated, 44-km-long normal fault on the eastern margin of Basin and Range extension in the Rocky Mountains. The entire surface-rupturing history of the BRF, as gleaned from paleoseismic and geomorphic observations, began only 4500 years ago and consists of at least three large events. Rupture of the BRF is spatially complex and is clearly conditioned by preexisting structure. In particular, where the south end of the fault intersects older thrust faults and upturned strata along the south-dipping flank of the Precambrian basement-cored Uinta arch, the main trace ends abruptly in a set of orthogonal splays that accommodate down-dropping of a large hanging-wall graben against the arch. We hypothesize that the geomechanically strong Uinta arch crustal block impeded the development of the BRF and, over time, enabled a significant accumulation of elastic strain energy, eventually giving rise to a pulse of strain release in the mid- to late Holocene. We surmise that variations in fault strength, both in space and time, is a cause of earthquake clustering on the BRF and on other faults that are structurally and tectonically immature. The first two earthquakes on the BRF occurred during the same period of time as a regional cluster of earthquakes in the Middle Rocky Mountains, suggesting that isolated faults in this slowly extending region interact through widespread changes in stress conditions.

Idaho, Utah, Wyoming↗

USGS2022

Seismic evidence for magmatic underplating along the Kodiak-Bowie Seamount Chain, Gulf of Alaska

Oceanic crust formed at mid-ocean ridges may be later modified by off-ridge magmatism forming seamounts, guyots, and islands. We investigate processes associated with seamount formation in the Gulf of Alaska Seamount Province using two coincident seismic reflection/wide-angle profiles. A north-south profile crosses the Kodiak-Bowie Seamount Chain and Aja fracture zone (FZ), and an orthogonal east-west profile is located about 90 km south of the seamount chain over Pacific plate oceanic crust. Structure along the profile away from the seamount chain is consistent with typical oceanic crust. Crust in our study region is thinnest (about 5.6 km) at the Aja FZ. Unlike observations from active transform faults, no low-velocity anomaly is observed at the Aja FZ suggesting that the crustal velocities have recovered to normal values through crack closure and crack healing. Higher lower crustal velocities (∼7.3 and > 7.5 km/s) and thicker crust (∼8.5 and ∼7.0 km) are observed near the Pratt and Durgin Seamounts and at the intersection of the Kodiak-Bowie Seamount Chain linear trend, respectively. These observations are attributed to magmatic underplating associated with seamount province magmatism. Lithospheric thickness variations across the Aja FZ may form a barrier or impediment to magmatic flow. The thickest crust (8.5 km) along our two profiles is located on the younger side of the FZ, and we suggest that the majority of magmatism jumped south of the Aja FZ when thinner lithosphere was encountered by the Bowie hot spot. The crustal structure near the Kodiak-Bowie Seamount Chain is most similar to that of other seamounts and guyots that formed on similarly young lithosphere (8–12 Ma). Our results suggest that lithospheric thickness at the time of hot spot interaction has a large control on magmatic underplating at seamounts and seamount provinces.

Alaska↗

USGS2023

Unzipping supercontinent Pangea: Geologic, potential field data, and buried structures, and a case for sequential Atlantic opening

Amalgamation of Pangea culminated with zippered N-to-S closing of the Theic ocean during the Alleghanian orogeny. Transpressional-rotational collision produced widespread dextral faulting throughout the eastern Appalachian hinterland, and thrust faulting in the western hinterland and foreland. The partially buried southern Appalachian Eastern Piedmont fault system is a product of late Paleozoic transpressional dextral faulting. Eastern Piedmont fault system faults are cut by two 200–190 Ma Central Atlantic Magmatic Province (CAMP) diabase dike sets (∼NW and N S), which postdate initial rifting, producing the Late Triassic-Early Jurassic basins. Sinistral reactivation of suitably oriented Paleozoic faults (e.g., Towaliga) that offset Jurassic dikes implies faulting is coeval with CAMP dike emplacement, or occurred very soon after 200–190 Ma dike emplacement. A large NE-striking sinistral fault (Estill fault) offsets magnetic and gravity highs in the Brunswick (Charleston) terrane in South Carolina and Georgia. This fault is at least 185 km long, with ca. 75–80 km maximum of sinistral separation. The Estill fault geometry agrees with the stress field associated with Central Atlantic Magmatic Province dike emplacement. We conclude that movement along the Estill and other sinistral faults represents Early Jurassic displacement that is well documented in eastern North America prior to Atlantic opening. The early Mesozoic post-failed rifting (basin formation) preceded dike emplacement and pre-spreading sinistral faulting. We recognize four stages in the Late Triassic–Jurassic tectonics and kinematics of eastern North America based on field, crosscutting relationships, and available geophysical data: (1) initial W-to-E failed transtensional rifting with formation of the S-to-N-younging Triassic-Jurassic basins; (2) CAMP diabase dikes intruded 200–190 Ma; (3) sinistral movement of large blocks of crust in the North Atlantic region including southeastern North America; and (4) spreading and opening of the Atlantic Ocean 195–170 Ma. These stages mark the reversal of Alleghanian rotational dextral transpressional (zippered) collision forming Pangea supercontinent and ‘unzipping’ the supercontinent prior to Atlantic opening.

Alabama, Georgia↗

USGS2023

Regional crustal structure of Indonesia from receiver functions

Characterizing the crustal structure of Indonesia is important to gain a better understanding of its geodynamic evolution and improve seismic hazard assessments in the area. However, a unified crustal model of the entire Indonesian region and its surroundings is lacking. We present new maps of crustal thickness and bulk V p /V s ratio in Indonesia and the surrounding area that are obtained using P-wave receiver functions at 36 seismic stations from several permanent regional networks. The measured crustal thickness varies from ∼24 km to ∼38 km. The thickest crust, ∼38 km, is beneath Flores Island, southern Maluku, and neighboring northernmost Australia, whereas the thinnest crust, ∼24 km, is found under eastern Malaysia. Thus, crustal thickness varies by ∼14 km (from ∼24 km to ∼38 km) despite the small changes in elevation at the measurement points. The V p /V s ratios are 1.79 ± "> ± 0.11, with high values (>1.85) found along the Banda-Sunda arc-trench system. We attribute these high values to: (1) the presence of mafic island arc and oceanic crust and (2) partial melting within this volcanic region, which causes a larger decrease in S-wave velocities compared with P-wave velocities. The comparison of the seismic properties of Indonesian island arc crust, particularly the V p /V s ratio, with laboratory measurements and the petrology of the exhumed Talkeetna island arc, Alaska, allows us to infer the crustal composition of Indonesian island arc crust.

Tectonophysics↗

USGS2024

Fairweather transform boundary Oligocene to present orogenesis: Fairweather Range vertical extrusion and rotation of the Yakutat microplate at ca. 3 Ma

Oblique-slip along transform fault boundaries is often partitioned between a strike-slip system and thrust faults that accommodate contraction. However, topography along the Yakutat-North American transform (Fairweather fault), is asymmetric with low-terrain above active thrusts on the western, Yakutat side of the transform and high topography on the continental side with peaks >4500 m (Mount Fairweather: 4671 m) to the west of the Border Ranges fault, limited recorded earthquakes >M4, and no apparent reverse faults to generate the highest terrain. In this study we compile, for the first time, published U-Pb zircon, 40 Ar/ 39 Ar and K-Ar (hornblende, muscovite, and biotite) and U-Th/He and fission-track (zircon and apatite) bedrock ages (109) from 75 samples to investigate the exhumation history of the Fairweather Range region, complemented by a published detrital sample (ZFT and AFT) and 13 new 40 Ar/ 39 Ar (hornblende, biotite, and K-feldspar) ages on 9 bedrock samples from both sides of the Fairweather fault. Additionally, we examined published seismicity and geodetic data of the Fairweather region and assessed if plate paleo-vectors correlate with the cooling history of the Fairweather Range. Cooling age, seismic, and block-motion patterns indicate the Fairweather Range has been vertically extruded between the Fairweather and the Border Ranges faults as a coherent block since ca. 25 Ma. The pre-6 Ma Pacific plate motion (N30°W) aligns with the N33°W strike of the Fairweather Fault whereas a hypothetical pre-6 Ma Yakutat microplate paleo-vector of (N39°W) does not: indicating a post-6 Ma timing for Yakutat microplate counter-clockwise rotation (9°). We infer that rotation and impingement of the Yakutat microplate along the Fairweather fault at ca. 3 Ma led to the development of the Fairweather restraining bend and increased cooling rates. The resultant thickened Fairweather welt and the ∼30 km thick southeast end of the Yakutat microplate compounded double-indenter tectonics into Alaska's southeast convergent corner

Tectonophysics↗

USGS2024

A scaling relationship for the width of secondary deformation around strike-slip faults

Simple mechanical arguments suggest that slip along interlocked, rough faults, damages surrounding rocks. The same arguments require that the scale of secondary damage is proportional to the size of geometric irregularities along the main fault. This relationship could apply at all scales, but has, so far, been difficult to observe at the 10s to 100 s of km scales of large, natural faults, often because large-scale deformation is distributed across wide, complex plate-boundary fault systems, like the San Andreas Fault. The geometry and geology of another large-scale plate-boundary strike slip fault—the Queen Charlotte Fault (QCF)—is, in contrast, especially simple. Here, we show that observations of secondary deformation are well-aligned with predictions of stress variations caused by geometric irregularities along the QCF, suggesting a geometric relationship between primary fault geometry and secondary deformation. The analytic stress solution reveals that the highest stresses and highest likelihood of failure are confined to a zone of influence (ZOI) with a width quantified by ZOI = λ / 2 π "> ZOI=λ/2π , where λ is the wavelength of geometric variations along the main fault. This simple model is consistent with ∼100-km-scale observations along the QCF and can theoretically be used to predict the width of secondary deformation at all scales.

Queen Charlotte Fault↗

USGS2024

Geophysical characterization of an alkaline‑carbonatite complex using gravity and magnetic methods at Magnet Cove, Arkansas, USA

The Magnet Cove alkaline‑carbonatite complex (MCC), located in the Ouachita Mountains of south-central Arkansas in the United States, hosts an extensive variety of rare rock types and critical mineral resources with physical properties (density and magnetization) that contrast significantly with the sedimentary rocks into which they have intruded. Newly acquired ground-based gravity and magnetic data were used to develop two-dimensional and three-dimensional geophysical models of the Cretaceous-aged Magnet Cove intrusive complex. The models reveal that the MCC: (1) widens out at middle crustal depths to as much 22 km across, and may reach a depth of 20 km; (2) has a total volume (exposed and subsurface) that may be over 800 km 3 ; (3) is likely connected at depth to other intrusions in the Arkansas alkaline province; and (4) has a geometry that is aligned with pre-existing structures such as the Reelfoot rift and the Ouachita orogenic belt, some of which were likely structurally controlled by the Precambrian crystalline basement and the continent-ocean transition zone buried beneath the Ouachita orogen. For the first time, the magnetic models of the MCC account for the presence of strong remanent magnetization. This results in a geophysical workflow necessary to accurately interpret magnetic anomalies over the much larger Arkansas alkaline province, its geologic and structural framework, and critical mineral potential.

Arkansas↗