SEARCH · USGS Science

Results for “Letter Report”

Search indexed USGS publications on groundwater, aquifers, geologic maps, mineral resources and earthquakes. Explore source records by subject and place.

SourceResearch connectionFrom yearThrough year

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 397 records · Page 22Linked to original sources

USGS1983

40Ar/39Ar and U-Th-Pb dating of separated clasts from the Abee E4 chondrite

Determinations of 40 Ar/ 39 Ar and U-Th-Pb are reported for three clasts from the Abee (E4) enstatite chondrite, which has been the object of extensive consortium investigations. The clasts give 40 Ar/ 39 Ar plateau ages and/or maximum ages of 4.5 Gy, whereas two of the clasts give average ages of 4.4 Gy. Within the range of 4.4–4.5 Gy these data do not resolve any possible age differences among the three clasts. 206 Pb measured in these clasts is only ∼1.5–2.5% radiogenic, which leads to relatively large uncertainties in the Pb isochron age and in the 207 Pb/ 206 Pb model ages. The Pb data indicate that the initial 207 Pb/ 206 Pb was no more than 0.08±0.07% higher than this ratio in Can˜on Diablo troilite. The U-Th-Pb data are consistent with the interpretation that initial formation of these clasts occurred 4.58 Gy ago and that the clasts have since remained closed systems, but are contaminated with terrestrial Pb. The 40 Ar/ 39 Ar ages could be gas retention ages after clast formation or impact degassing ages. The thermal history of Abee deduced from Ar data appears consistent with that deduced from magnetic data, and suggests that various Abee components experienced separate histories until brecciation no later than 4.4 Gy ago, and experienced no appreciable subsequent heating.

Earth and Planetary Science Letters↗

USGS2021

A review of timing accuracy across the Global Seismographic Network

The accuracy of timing across a seismic network is important for locating earthquakes as well as studies that use phase‐arrival information (e.g., tomography). The Global Seismographic Network (GSN) was designed with the goal of having reported timing be better than 10 ms. In this work, we provide a brief overview of how timing is kept across the GSN and discuss how clock‐quality metrics are embedded in Standard for Exchange of Earthquake Data records. Specifically, blockette 1001 contains the timing‐quality field, which can be used to identify time periods when poor clock quality could compromise timing accuracy. To verify the timing across the GSN, we compare cross‐correlation lags between collocated sensors from 1 January 2000 to 1 January 2020. We find that the mean error is less than 10 ms, with much of the difference likely coming from the method or uncertainty in the phase response of the instruments. This indicates that timing across the GSN is potentially better than 10 ms. We conclude that unless clock quality is compromised (as indicated in blockette 1001), GSN data’s timing accuracy should be suitable for most current seismological applications that require 10 ms accuracy. To assist users, the GSN network operators have implemented a “gsn_timing” metric available via the Incorporated Research Institutions for Seismology Data Management Center that helps users identify data with substandard timing accuracy (the 10 ms design goal of the GSN).

Seismological Research Letters↗

USGS2024

Reply to “comment on ‘an evaluation of the timing accuracy of global and regional seismic stations and networks’ by Yang et al. (2021)” by Xin Zhang and Lianxing Wen

The finding of Yang et al. (2021 ; hereafter, YSR21) casts fundamental doubts on all the previous studies on the temporal changes of the inner core (IC) surface that are based on absolute arrival times, including Wen (2006 ; hereafter, WEN06) and its follow‐up studies. The reports of small time shifts of the IC‐reflected phase ( PKiKP ) likely misinterpreted station‐related issues (clock errors and instrument changes) as evidence for rapid localized deformation at the IC surface. The comment by Zhang and Wen (2024 ; hereafter, ZW24) aims at defending the results and conclusions of WEN06 by questioning a tiny portion of the results (2 out of over 400 doublets and 3 out of over 400 problematic stations) in YSR21. In response, we acknowledge an accidental error in table S1 of YSR21, which is corrected in an erratum ( Yang et al. , 2023 ), although this does not affect the results or interpretations of YSR21. However, we strongly disagree with the criticisms in ZW24 due to many problems therein, especially the issue of irreproducibility due to its inconsistent procedures with YSR21. While the problems are related to the use of absolute arrivals in the previous studies by Wen’s group, key problems in this new comment are related to questionable data selection and inconsistent processing. We further caution the metadata updates, particularly when involving small temporal change signals. We focus on the main points in ZW24 and demonstrate that its arguments are problematic in many aspects and do not diminish the credibility of YSR21 or other related works by Yang and Song in support of the IC differential rotation.

Seismological Research Letters↗

USGS2018

Unraveling the dynamics of magmatic CO2 degassing at Mammoth Mountain, California

The accumulation of magmatic CO 2 beneath low-permeability barriers may lead to the formation of CO 2 -rich gas reservoirs within volcanic systems. Such accumulation is often evidenced by high surface CO 2 emissions that fluctuate over time. The temporal variability in surface degassing is believed in part to reflect a complex interplay between deep magmatic degassing and the permeability of degassing pathways. A better understanding of the dynamics of CO 2 degassing is required to improve monitoring and hazards mitigation in these systems. Owing to the availability of long-term records of CO 2 emissions rates and seismicity, Mammoth Mountain in California constitutes an ideal site towards such predictive understanding. Mammoth Mountain is characterized by intense soil CO 2 degassing (up to ∼1000 t d −1 ) and tree kill areas that resulted from leakage of CO 2 from a CO 2 -rich gas reservoir located in the upper ∼4 km. The release of CO 2 -rich fluids from deeper basaltic intrusions towards the reservoir induces seismicity and potentially reactivates faults connecting the reservoir to the surface. While this conceptual model is well-accepted, there is still a debate whether temporally variable surface CO 2 fluxes directly reflect degassing of intrusions or variations in fault permeability. Here, we report the first large-scale numerical model of fluid and heat transport for Mammoth Mountain. We discuss processes (i) leading to the initial formation of the CO 2 -rich gas reservoir prior to the occurrence of high surface CO 2 degassing rates and (ii) controlling current CO 2 degassing at the surface. Although the modeling settings are site-specific, the key mechanisms discussed in this study are likely at play at other volcanic systems hosting CO 2 -rich gas reservoirs. In particular, our model results illustrate the role of convection in stripping a CO 2 -rich gas phase from a rising hydrothermal fluid and leading to an accumulation of a large mass of CO 2 (∼10 7 –10 8 t) in a shallow gas reservoir. Moreover, we show that both, short-lived (months to years) and long-lived (hundreds of years) events of magmatic fluid injection can lead to critical pressures within the reservoir and potentially trigger fault reactivation. Our sensitivity analysis suggests that observed temporal fluctuations in surface degassing are only indirectly controlled by variations in magmatic degassing and are mainly the result of temporally variable fault permeability. Finally, we suggest that long-term CO 2 emission monitoring, seismic tomography and coupled thermal–hydraulic–mechanical modeling are important for CO 2 -related hazard mitigation.

California↗

USGS2023

Deep root activity overprints weathering of petrogenic organic carbon in shale

The oxidation of organic carbon in sedimentary bedrock (petrogenic OC, OC p e t r o "> petro ) is increasingly recognized as a potential source of CO 2 to the atmosphere. Recent studies provide evidence for the mobilization and oxidation of OC p e t r o "> petro in sedimentary bedrock during rock weathering. However, the mechanisms and rates remain uncertain, particularly where overlying soils and vegetation drive contemporaneous oxidation of recently fixed organic carbon. Here, we quantify OC p e t r o "> petro weathering across a 16 m shale depth profile in a steep, rapidly eroding forested hillslope in the Northern California Coast Ranges. We report solid and gas phase radiocarbon and stable isotope analyses of samples extracted from specialized in-situ samplers, and a supporting laboratory incubation experiment of the shale regolith. OC p e t r o "> petro is removed from the weathered bedrock at a rate of approximately 0.12 gC/m 3 yr, which is orders of magnitude lower than the rate of OC p e t r o "> petro oxidation we achieved in the laboratory with crushed samples (557.1 gC/m 3 /yr). This disparity occurs despite high O 2 ( g ) "> 2(g) content across the depth profile, indicating that physical accessibility of OC p e t r o "> petro can regulate oxidative weathering. There is no direct radiocarbon evidence of OC p e t r o "> petro oxidation in CO 2 ( g ) "> 2(g) across the upper 13 m of the weathering profile during both wet and dry seasons. Instead, vadose zone CO 2 ( g ) "> 2(g) production at the site is dominated by respiration of recently fixed carbon associated with deep rooting. OC p e t r o "> petro is clearly mobilized across the vadose zone during weathering in this rapidly eroding, oxygen-rich, biologically dynamic hillslope, but at rates far below what can be measured given the contribution of root-derived CO 2 ( g ) "> 2(g) .

California↗

USGS2018

Spatiotemporal analysis of the Foreshock-Mainshock-Aftershock sequence of the 6 July 2017 M5.8 Lincoln, Montana, earthquake

A MW 5.8 earthquake occurred on 6 July 2017 at 12.2 km depth, 11 km southeast of Lincoln in west central Montana. No major damage or injuries were reported; however, the widely felt mainshock generated a prolific aftershock sequence with more than 1200 located events through the end of 2017. The Lincoln event is the latest in a series of moderate-to-large earthquakes that have affected western Montana. We characterize the spatiotemporal evolution of the sequence using matched filter detection and multiple-event relocation techniques. Moment tensor solutions and aftershock locations indicate faulting occurred on a 9-km-long NNE-striking, near-vertical, strike-slip fault antithetic to the Lewis and Clark Line, the main through-going fault system. Seismicity primarily occurs between 6 and 16 km depth, consistent with seismicity in the Intermountain Seismic Belt. We estimate a fault rupture area of ~64 km2 and ~30 cm of average fault displacement. We identified four foreshocks in the three days prior to, and 3005 aftershocks in the three weeks following the mainshock. The supplemented catalog frequency-magnitude distribution has a b-value of 0.79 and a minimum magnitude of completeness of 0.7. The overall decay rate is consistent with a modified Omori decay law p-value of 0.76 and c-value of 0.32. This event demonstrates that unmapped faults antithetic to major geologic structures play a role in accommodating regional strain in Western Montana and can host significant earthquakes

Montana↗

USGS2011

Rapid cooling rates at an active mid-ocean ridge from zircon thermochronology

Oceanic spreading ridges are Earth's most productive crust generating environment, but mechanisms and rates of crustal accretion and heat loss are debated. Existing observations on cooling rates are ambiguous regarding the prevalence of conductive vs. convective cooling of lower oceanic crust. Here, we report the discovery and dating of zircon in mid-ocean ridge dacite lavas that constrain magmatic differentiation and cooling rates at an active spreading center. Dacitic lavas erupted on the southern Cleft segment of the Juan de Fuca ridge, an intermediate-rate spreading center, near the intersection with the Blanco transform fault. Their U–Th zircon crystallization ages (29.3 − 4.6 + 4.8 ka; 1σ standard error s.e.) overlap with the (U–Th)/He zircon eruption age (32.7 ± 1.6 ka) within uncertainty. Based on similar 238 U− 230 Th disequilibria between southern Cleft dacite glass separates and young mid-ocean ridge basalt (MORB) erupted nearby, differentiation must have occurred rapidly, within ~ 10–20 ka at most. Ti-in-zircon thermometry indicates crystallization at 850–900 °C and pressures > 70–150 MPa are calculated from H 2 O solubility models. These time-temperature constraints translate into a magma cooling rate of ~ 2 × 10 − 2 °C/a. This rate is at least one order-of-magnitude faster than those calculated for zircon-bearing plutonic rocks from slow spreading ridges. Such short intervals for differentiation and cooling can only be resolved through uranium-series ( 238 U– 230 Th) decay in young lavas, and are best explained by dissipating heat convectively at high crustal permeability.

Juan de Fuca ridge and the Blanco transform fault ↗

USGS2017

Tree mortality across biomes is promoted by drought intensity, lower wood density and higher specific leaf area

Drought events are increasing globally, and reports of consequent forest mortality are widespread. However, due to a lack of a quantitative global synthesis, it is still not clear whether drought-induced mortality rates differ among global biomes and whether functional traits influence the risk of drought-induced mortality. To address these uncertainties, we performed a global meta-analysis of 58 studies of drought-induced forest mortality. Mortality rates were modelled as a function of drought, temperature, biomes, phylogenetic and functional groups and functional traits. We identified a consistent global-scale response, where mortality increased with drought severity [log mortality (trees trees −1 year −1 ) increased 0.46 (95% CI = 0.2–0.7) with one SPEI unit drought intensity]. We found no significant differences in the magnitude of the response depending on forest biomes or between angiosperms and gymnosperms or evergreen and deciduous tree species. Functional traits explained some of the variation in drought responses between species (i.e. increased from 30 to 37% when wood density and specific leaf area were included). Tree species with denser wood and lower specific leaf area showed lower mortality responses. Our results illustrate the value of functional traits for understanding patterns of drought-induced tree mortality and suggest that mortality could become increasingly widespread in the future.

Ecology Letters↗

USGS1997

Some comparisons between recent ground-motion relations

We provide an overview of new ground-motion relations for eastern North America (ENA) developed over the last five years. The empirical-stochastic relations of Atkinson and Boore (1995) are compared to relations developed by the Electric Power Research Institute (EPRI, 1993; also Toro et al. , 1994), Frankel et al. (1996), and the consensus ENA ground-motion values as reported by SSHAC (1996). The main difference between our relations and those of EPRI or Frankel is in the low-frequency amplitudes ( f < 2 Hz, all magnitudes). We predict lower amplitudes (by more than a factor of two) at 1 Hz, largely due to our use of an empirical source model rather than a single-corner-frequency Brune source model; the use of the empirical source model is motivated by the desire to match the ENA ground-motion database as closely as possible. We also compare our new ENA relations to empirical relations for California. The comparison is complicated by the need to adjust the ENA hard-rock motions to obtain equivalent motions for typical California soil conditions. Two alternative methods of making this correction lead to somewhat different conclusions. One possible conclusion is that our ENA relations predict similar low-frequency amplitudes to those predicted by Boore et al. (1993, 1994) and Abrahamson and Silva (1996) for California, but our predicted ENA amplitudes are much higher (factor > 2) than California values at high frequencies. The alternative soil correction leads to the conclusion that our ENA relations are moderately lower (factor<2) than the California relations at low frequencies, and moderately higher at high frequencies. Both of these conclusions imply that ground-motion relations or time series for earthquakes in one region cannot be simply modified for use in engineering analyses in another region.

Seismological Research Letters↗

USGS2003

Comment on “Isotopic fractionation between Fe(III) and Fe(II) in aqueous solutions” by Clark Johnson et al., [Earth Planet. Sci. Lett. 195 (2002) 141–153]

In a recent contribution [1] , Johnson et al. reported the equilibrium isotope fractionation factor between dissolved Fe(II) and Fe(III) in aqueous solutions at pH=2.5 and 5.5. They suggest that because the iron isotope fractionation observed in their experiments spans virtually the entire range observed in sedimentary rocks, Fe(II)–Fe(III) aqueous speciation may play a major role in determining iron isotope variations in nature where Fe(II) and Fe(III) can become physically separated. They discounted earlier conclusions by us and others [2] ; [3] that significant equilibrium fractionation between specific coexisting Fe(II)- or Fe(III)-aqueous complexes (e.g., between aqueous Fe(II)(OH) x(aq) and Fe(II) (aq) ion) is capable of producing iron isotope contrasts that can be preserved in nature. This is an important contribution not only because the authors recognize the importance of abiotic equilibrium iron isotope fractionation in nature in contrast to previous assertions [4] , but also because it will help to focus discussion on the development and evaluation of experimental approaches that can reveal abiotic fractionation mechanisms. However, in this Comment we propose that the experiments presented in this paper cannot be interpreted as straightforwardly as Johnson et al. contend. In particular, we show that in one of their critical experiments attainment of either isotope mass balance or equilibrium was not demonstrated, and thus the results of that experiment cannot be used to calculate an Fe(II)–Fe(III) equilibrium fractionation factor.

Earth and Planetary Science Letters↗

USGS2015

The 2014 Mw6.1 South Napa Earthquake: A unilateral rupture with shallow asperity and rapid afterslip

The M w 6.1 South Napa earthquake occurred near Napa, California on August 24, 2014 (UTC), and was the largest inland earthquake in Northern California since the 1989 M w 6.9 Loma Prieta earthquake. The first report of the earthquake from the Northern California Earthquake Data Center (NCEDC) indicates a hypocentral depth of 11.0km with longitude and latitude of (122.3105°W, 38.217°N). Surface rupture was documented by field observations and Lidar imaging (Brooks et al. 2014; Hudnut et al. 2014; Brocher et al., 2015), with about 12 km of continuous rupture starting near the epicenter and extending to the northwest. The southern part of the rupture is relatively straight, but the strike changes by about 15° at the northern end over a 6-km segment. The peak dextral offset was observed near the Buhman residence with right-.‐lateral motion of 46 cm, near the location where the strike of fault begins to rotate clock-.‐wise (Hudnut et al., 2014). The earthquake was well recorded by the strong motion network operated by the NCEDC, the California Geological Survey and the U.S. Geological Survey (USGS). There are about 12 sites within an epicentral distance of 15km, with relatively good azimuthal coverage (Fig.1). The largest peak-ground-velocity (PGV) of nearly 100 cm/s was observed on station 1765, which is the closest station to the rupture and lies about 3 km east of the northern segment (Fig. 1). The ground deformation associated with the earthquake was also well recorded by the high-resolution COSMO-SkyMed satellite and Sentinel-1A satellite, providing independent static observations.

California↗

USGS2015

The Mw6.0 24 August 2014 South Napa earthquake

The M w 6.0 South Napa earthquake, which occurred at 10:20 UTC 24 August 2014 was the largest earthquake to strike the greater San Francisco Bay area since the M w 6.9 1989 Loma Prieta earthquake. The rupture from this right‐lateral earthquake propagated mostly unilaterally to the north and up‐dip, directing the strongest shaking toward the city of Napa, where peak ground accelerations (PGAs) between 45% g and 61% g were recorded and modified Mercalli intensities (MMIs) of VII–VIII were reported. Tectonic surface rupture with dextral slip of up to 46 cm was observed on a 12.5 km long segment, some of which was along a previously mapped strand of the West Napa fault system, although the rupture extended to the north of the mapped Quaternary strand. Modeling of seismic and geodetic data suggests an average coseismic slip of 50 cm, with a maximum slip of about 1 m at depths of 10–11 km. We observed up to 35 cm of afterslip along the surface trace in the week following the mainshock, primarily along the southern half of the surface rupture that experienced relatively little coseismic offset. Relocation of the sparse aftershock sequence suggests en echelon southwest‐ and northeast‐dipping fault planes, reflective of the complex fault geometry in this region. The Napa basin and historic and late Holocene alluvial flood deposits in downtown Napa amplified the ground motions there. Few ground failures were mapped, reflecting the dry season (as well as a persistent drought that had lowered the groundwater table) and the short duration of strong shaking in the epicentral area.

California↗

USGS2022

Trace elements in olivine fingerprint the source of 2018 magmas and shed light on explosive-effusive eruption cycles at Kīlauea Volcano

Understanding magma genesis and the evolution of intensive parameters (temperature, pressure, composition, degree of melting) in the mantle source of highly active volcanic systems is crucial for interpreting magma supply changes over time and recognizing cyclic behavior to anticipate future volcanic behavior. Major and trace elements in olivine are commonly used to study variations in mantle lithologies and melting conditions (e.g., temperature, pressure, oxygen fugacity) affecting the mantle over time. Here, we track the temporal evolution of primary melts through the most recent cycle of explosive and effusive eruptions at Kīlauea (Hawai‘i), which spans the last ∼500 years. We report major and trace elements in olivine from the last explosive period (∼1500 – early 1820’s Keanakāko‘i Tephra) and the most recent decade of the current effusive period (2018 LERZ, 2015–2018 Pu‘u‘ō‘ō, 2008–2018 lava lake and 2020 eruption in Halema‘uma‘u). Scandium concentrations in olivine allow characterizing changes in mantle source between 1500 and 2018, and suggest that the recent (2015–2018) magma feeding the Pu‘u‘ō‘ō cone did not significantly interact with the magma that erupted in the LERZ in 2018. The evolution of olivine and melt compositions over the past 500 years is not easily reconcilable with variations in mantle potential temperature, pressure of mantle melt pooling and storage, or oxygen fugacity. Instead, Sc, Mn, and Co concentrations and Ni/Mg ratio in high forsterite (Fo >87) olivine advocate for an increase in the proportion of clinopyroxene in the mantle source associated with a slightly higher degree of partial melting from 1500 to 2018. Changes in primitive melt compositions and degrees of mantle melting may well modulate magma supply to the crust and formation-replenishment of steady or ephemeral summit reservoirs, and thereby control transitions between explosive and effusive periods at Kīlauea. Analyzing trace elements in olivine at Kīlauea and elsewhere could therefore provide important clues on subtle changes occurring at the mantle level that might herald changes in volcanic behavior.

Hawaii↗

USGS2020

The community code verification exercise for simulating sequences of earthquakes and aseismic slip (SEAS)

Numerical simulations of sequences of earthquakes and aseismic slip (SEAS) have made great progress over past decades to address important questions in earthquake physics. However, significant challenges in SEAS modeling remain in resolving multiscale interactions between earthquake nucleation, dynamic rupture, and aseismic slip, and understanding physical factors controlling observables such as seismicity and ground deformation. The increasing complexity of SEAS modeling calls for extensive efforts to verify codes and advance these simulations with rigor, reproducibility, and broadened impact. In 2018, we initiated a community code-verification exercise for SEAS simulations, supported by the Southern California Earthquake Center. Here, we report the findings from our first two benchmark problems (BP1 and BP2), designed to verify different computational methods in solving a mathematically well-defined, basic faulting problem. We consider a 2D antiplane problem, with a 1D planar vertical strike-slip fault obeying rate-and-state friction, embedded in a 2D homogeneous, linear elastic halfspace. Sequences of quasi-dynamic earthquakes with periodic occurrences (BP1) or bimodal sizes (BP2) and their interactions with aseismic slip are simulated. The comparison of results from 11 groups using different numerical methods show excellent agreements in long-term and coseismic fault behavior. In BP1, we found that truncated domain boundaries influence interseismic stressing, earthquake recurrence, and coseismic rupture, and that model agreement is only achieved with sufficiently large domain sizes. In BP2, we found that complexity of fault behavior depends on how well physical length scales related to spontaneous nucleation and rupture propagation are resolved. Poor numerical resolution can result in artificial complexity, impacting simulation results that are of potential interest for characterizing seismic hazard such as earthquake size distributions, moment release, and recurrence times. These results inform the development of more advanced SEAS models, contributing to our further understanding of earthquake system dynamics.

Seismological Research Letters↗

USGS2019

Chlorinated byproducts of neonicotinoids and their metabolites: An unrecognized human exposure potential?

We recently reported the initial discovery of neonicotinoid pesticides in drinking water and their potential for transformation through chlorination and alkaline hydrolysis during water treatment. The objectives of this research were: (1) to determine if neonicotinoid metabolites are relevant to drinking water exposure and (2) to identify the products formed from chlorination of neonicotinoids and their metabolites. Desnitro-imidacloprid and imidacloprid-urea, two known metabolites of imidacloprid, are documented for the first time in drinking water. Desnitro-imidacloprid was present above the lower level of detection (0.03 ng/L) in 67% of samples (six of nine) from drinking water systems but detectable in all samples (up to 0.6 ng/L). Although concentrations of desnitro-imidacloprid were lower than concentrations of the parent neonicotinoids, desnitro-imidacloprid exhibits significantly greater mammalian toxicity than imidacloprid. Using LC-HR-ToF-MS/MS analysis of results from laboratory experiments, we propose structures for novel transformation products resulting from the chlorination of clothianidin, imidacloprid, desnitro-imidacloprid, imidacloprid-urea, and hydrolysis products of thiamethoxam. Formation of chlorinated neonicotinoid byproducts occurs at time scales relevant to water treatment and/or distribution for the imidacloprid metabolites ( t 1/2 values from 2.4 min to 1.0 h) and thiamethoxam hydrolysis products (4.8 h). Neonicotinoid metabolites in finished drinking water and potential formation of novel disinfection byproducts during treatment and/or distribution are relevant to evaluating the exposure and potential impacts of neonicotinoids on human health.

Environmental Science & Technology Letters↗

USGS2015

Quantifying 10 years of improved earthquake-monitoring performance in the Caribbean region

Over 75 tsunamis have been documented in the Caribbean and adjacent regions during the past 500 years. Since 1500, at least 4484 people are reported to have perished in these killer waves. Hundreds of thousands are currently threatened along the Caribbean coastlines. Were a great tsunamigenic earthquake to occur in the Caribbean region today, the effects would potentially be catastrophic due to an increasingly vulnerable region that has seen significant population increases in the past 40–50 years and currently hosts an estimated 500,000 daily beach visitors from North America and Europe, a majority of whom are not likely aware of tsunami and earthquake hazards. Following the magnitude 9.1 Sumatra–Andaman Islands earthquake of 26 December 2004, the United Nations Educational, Scientific and Cultural Organization (UNESCO) Intergovernmental Coordination Group (ICG) for the Tsunami and other Coastal Hazards Early Warning System for the Caribbean and Adjacent Regions (CARIBE‐EWS) was established and developed minimum performance standards for the detection and analysis of earthquakes. In this study, we model earthquake‐magnitude detection threshold and P‐wave detection time and demonstrate that the requirements established by the UNESCO ICG CARIBE‐EWS are met with 100% of the network operating. We demonstrate that earthquake‐monitoring performance in the Caribbean Sea region has improved significantly in the past decade as the number of real‐time seismic stations available to the National Oceanic and Atmospheric Administration tsunami warning centers have increased. We also identify weaknesses in the current international network and provide guidance for selecting the optimal distribution of seismic stations contributed from existing real‐time broadband national networks in the region.

Seismological Research Letters↗

USGS2011

More than a century of bathymetric observations and present-day shallow sediment characterization in Belfast Bay, Maine, USA: Implications for pockmark field longevity

Mechanisms and timescales responsible for pockmark formation and maintenance remain uncertain, especially in areas lacking extensive thermogenic fluid deposits (e.g., previously glaciated estuaries). This study characterizes seafloor activity in the Belfast Bay, Maine nearshore pockmark field using (1) three swath bathymetry datasets collected between 1999 and 2008, complemented by analyses of shallow box-core samples for radionuclide activity and undrained shear strength, and (2) historical bathymetric data (report and smooth sheets from 1872, 1947, 1948). In addition, because repeat swath bathymetry surveys are an emerging data source, we present a selected literature review of recent studies using such datasets for seafloor change analysis. This study is the first to apply the method to a pockmark field, and characterizes macro-scale (>5 m) evolution of tens of square kilometers of highly irregular seafloor. Presence/absence analysis yielded no change in pockmark frequency or distribution over a 9-year period (1999–2008). In that time pockmarks did not detectably enlarge, truncate, elongate, or combine. Historical data indicate that pockmark chains already existed in the 19th century. Despite the lack of macroscopic changes in the field, near-bed undrained shear-strength values of less than 7 kPa and scattered downcore 137 Cs signatures indicate a highly disturbed setting. Integrating these findings with independent geophysical and geochemical observations made in the pockmark field, it can be concluded that (1) large-scale sediment resuspension and dispersion related to pockmark formation and failure do not occur frequently within this field, and (2) pockmarks can persevere in a dynamic estuarine setting that exhibits minimal modern fluid venting. Although pockmarks are conventionally thought to be long-lived features maintained by a combination of fluid venting and minimal sediment accumulation, this suggests that other mechanisms may be equally active in maintaining such irregular seafloor morphology. One such mechanism could be upwelling within pockmarks induced by near-bed currents.

Maine↗

USGS2023

Trends in tree cover change over three decades related to interannual climate variability and wildfire in California

The U.S. State of California has experienced frequent drought events, hotter temperatures and other disruptions to the climate system whose effects on ecosystems have been widely reported in recent decades. Studies primarily confined to specific vegetation communities or species, individual drought incidents, or analysis over a relatively short intervals, has limited our understanding of the broad-scale effects on tree cover and the spatiotemporal variability of effects across broader regions. We focused analysis on multi-annual land cover and land surface change to assess patterns and trends in tree cover loss in tree-dominated Californian ecoregions from 1986 to 2019. The top three years of total tree cover loss for the state were 2018 (1901 km 2 ), 2015 (1556 km 2 ), and 2008 (1549 km 2 ). Overall, annual tree cover loss had upward trends. Tree cover loss rapidly surged later in the study period and was apparently driven by climate stress and wildfires. Underlying geographic variability was apparent in both non-fire and fire-related tree cover loss that sharply increased during hotter multi-year droughts. The increasingly hotter and drier climate conditions were associated with significant increases in fire-induced mortality. Our findings indicate that a possible effect of future hotter and drier climate would lead to further tree cover loss, thereby endangering California's ecosystem goods and services. Geographic variability in tree cover trends indicates that ecoregion-specific mitigation and adaptation strategies would be useful to conserve the region's forest resources. Such strategies may benefit from consideration of historical disturbances, ecoregion's sensitivity to disturbance types, as well as potential ecoregion-specific climate-vegetation-fire feedbacks.

California↗