Geology Reports⌕ Search

SEARCH · Geology Reports

Results for “The Earth Observer”

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

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

At least 1,135 records · Page 63Linked to original sources
USGS2024

Snow avalanches are a primary climate-linked driver of mountain ungulate populations

Snow is a major, climate-sensitive feature of the Earth’s surface and catalyst of fundamentally important ecosystem processes. Understanding how snow influences sentinel species in rapidly changing mountain ecosystems is particularly critical. Whereas effects of snow on food availability, energy expenditure, and predation are well documented, we report how avalanches exert major impacts on an ecologically significant mountain ungulate - the coastal Alaskan mountain goat ( Oreamnos americanus ). Using long-term GPS data and field observations across four populations (421 individuals over 17 years), we show that avalanches caused 23−65% of all mortality, depending on area. Deaths varied seasonally and were directly linked to spatial movement patterns and avalanche terrain use. Population-level avalanche mortality, 61% of which comprised reproductively important prime-aged individuals, averaged 8% annually and exceeded 22% when avalanche conditions were severe. Our findings reveal a widespread but previously undescribed pathway by which snow can elicit major population-level impacts and shape demographic characteristics of slow-growing populations of mountain-adapted animals.

Alaska↗
USGS2017

Emplacement of Holocene silicic lava flows and domes at Newberry, South Sister, and Medicine Lake volcanoes, California and Oregon

This field guide for the International Association of Volcanology and Chemistry of the Earth’s Interior (IAVCEI) Scientific Assembly 2017 focuses on Holocene glassy silicic lava flows and domes on three volcanoes in the Cascade Range in Oregon and California: Newberry, South Sister, and Medicine Lake volcanoes. Although obsidian-rich lava flows have been of interest to geologists, archaeologists, pumice miners, and rock hounds for more than a century, many of their emplacement characteristics had not been scientifically observed until two very recent eruptions in Chile. Even with the new observations, several eruptive processes discussed in this field trip guide can only be inferred from their final products. This makes for lively debates at outcrops, just as there have been in the literature for the past 30 years. Of the three volcanoes discussed in this field guide, one (South Sister) lies along the main axis defined by major peaks of the Cascade Range, whereas the other two lie in extensional tectonic settings east of the axis. These two tectonic environments influence volcano morphology and the magmatic and volcanic processes that form silicic lava flows and domes. The geomorphic and textural features of glass-rich extrusions provide many clues about their emplacement and the magma bodies that fed them. The scope of this field guide does not include a full geologic history or comprehensive explanation of hazards associated with a particular volcano or volcanic field. The geochemistry, petrology, tectonics, and eruption history of Newberry, South Sister, and Medicine Lake volcanic centers have been extensively studied and are discussed on other field excursions. Instead, we seek to explore the structural, textural, and geochemical evolution of well-preserved individual lava flows—the goal is to understand the geologic processes, rather than the development, of a specific volcano.

California, Oregon↗
USGS2020

A newly emerging thermal area in Yellowstone

Yellowstone is a large restless caldera that contains many dynamic thermal areas that are the surface expression of the deeper magmatic system. In 2018, using a Landsat 8 nighttime thermal infrared image, we discovered the emergence of a new thermal area located near Tern Lake on the northeast margin of the Sour Creek dome. A high-spatial-resolution airborne visible image from August 2017 revealed a large (~33,000 m2) area of recently fallen trees, mostly devoid of vegetation, with bright soil, similar to other nearby thermal areas. Field observations in August 2019 confirmed that this was a steam-heated acid-sulfate thermal area, with an arc-shaped zone of hydrothermally altered soil and heated ground, with surface temperatures of 60-80 °C, several steaming fumaroles, and boiling temperatures (93 °C) just beneath the surface. Fallen trees in contact with warm ground were being carbonized, yet there were some cooler areas with new trees growing. Observations of stressed or dying vegetation from archived satellite and airborne remote sensing data going back to 1994 indicated that this thermal area started emerging around 2000. It increased in size slowly until around 2005, when the radiative heat output started measurably increasing. From 2005 to 2012 it grew more rapidly; and from 2012 through 2019 the growth rate slowed and the heat output stabilized. We predict that this stabilizing trend will continue in the coming years. The initial formation of this new thermal area was not clearly linked to any distinct seismic or geodetic events, although the period of rapid growth partly coincided with a period of rapid local uplift, possibly suggesting a causative relationship. The identification of this emerging thermal area illustrates the importance of satellite thermal infrared imaging combined with high-spatial-resolution remote sensing data and field observations for mapping, measuring, and monitoring Yellowstone's thermal areas. It is also an example of the dynamics we expect to observe within large caldera systems like Yellowstone, where changes in the size and distribution of thermal areas are normal and do not indicate an impending eruption nor any significant changes in the broader magmatic system.

Wyoming↗
USGS2022

Lava fountain jet noise during the 2018 eruption of fissure 8 of Kīlauea volcano

Real-time monitoring is crucial to assess hazards and mitigate risks of sustained volcanic eruptions that last hours to months or more. Sustained eruptions have been shown to produce a low frequency (infrasonic) form of jet noise. We analyze the lava fountaining at fissure 8 during the 2018 Lower East Rift Zone eruption of Kīlauea volcano, Hawaii, and connect changes in fountain properties with recorded infrasound signals from an array about 500 m from the fountain using jet noise scaling laws and visual imagery. Video footage from the eruption reveals a change in lava fountain dynamics from a tall, distinct fountain at the beginning of June to a low fountain with a turbulent, out-pouring lava pond surrounded by a tephra cone by mid-June. During mid-June, the sound pressure level reaches a maximum, and peak frequency drops. We develop a model that uses jet noise scaling relationships to estimate changes in volcanic jet diameter and jet velocity from infrasound sound pressure levels and peak frequencies. The results of this model indicate a decrease in velocity in mid-June which coincides with the decrease in fountain height. Furthermore, the model results suggest an increase in jet diameter, which can be explained by the larger width of the fountain that resembles a turbulent lava pond compared to the distinct fountain at the beginning of June. The agreement between the infrasound-derived and visually observed changes in fountain dynamics suggests that jet noise scaling relationships can be used to monitor lava fountain dynamics using infrasound recordings.

Hawaii↗
USGS2011

C-band radar observes water level change in swamp forests

C-band radar pulses backscatter from the upper canopy of swamp forests, and consequently interferometric synthetic aperture radar (InSAR) analysis of C-band imagery has not been exploited to study water level changes in swamp forests. This article explores C-band ERS-1 (European Remote Sensing Satellite) and ERS-2 InSAR data over swamp forests composed of moderately dense trees with a medium-low canopy closure in southeastern Louisiana to measure water level changes beneath tree cover. Wetlands cover more than 4% of the Earth's land surface and interact with hydrologic, biogeochemical, and sediment transport processes that are fundamental in understanding ecological and climatic changes [ Alsdorf et al , 2003; Prigent et al ., 2001 ; Melack and Forsberg , 2000; Dunne et al ., 1998]. Measurement of water level changes in wetlands, and consequently of changes in water storage capacity, provides a required input for hydrologic models, and is required to comprehensively assess flood hazards [e.g., Coe , 1998].

Eos, Transactions, American Geophysical Union↗
USGS2013

Seismicity of the Earth 1900-2012 Philippine Sea plate and vicinity

The complex tectonics surrounding the Philippine Islands are dominated by the interactions of the Pacific, Sunda, and Eurasia plates with the Philippine Sea plate (PSP). The latter is unique because it is almost exclusively surrounded by zones of plate convergence. At its eastern and southeastern edges, the Pacific plate is subducted beneath the PSP at the Izu-Bonin, Mariana, and Yap trenches. Here, the subduction zone exhibits high rates of seismic activity to depths of over 600 km, though no great earthquakes (M>8.0) have been observed, likely because of weak coupling along the plate interface. In the northeast, the PSP subducts beneath Japan and the eastern margin of the Eurasia plate at the Nankai and Ryukyu trenches, extending westward to Taiwan. The Nankai portion of this subduction zone has hosted some of the largest earthquakes along the margins of the PSP, including a pair of Mw8.1 megathrust events in 1944 and 1946. Along its western margin, the convergence of the PSP and the Sunda plate is responsible for a broad and active plate boundary system extending along both sides of the Philippine Islands chain. The region is characterized by opposite-facing subduction systems on the east and west sides of the islands, and the archipelago is cut by a major transform structure: the Philippine Fault. Subduction of the Philippine Sea plate occurs at the eastern margin of the islands along the Philippine Trench and its northern extension, the East Luzon Trough. On the west side of Luzon, the Sunda Plate subducts eastward along a series of trenches, including the Manila Trench in the north, the smaller Negros Trench in the central Philippines, and the Sulu and Cotabato trenches in the south. Twentieth and early twentyfirst century seismic activity along the boundaries of the Philippine Sea plate has produced seven great (M>8.0) earthquakes and 250 large (M>7) events. Among the most destructive events were the 1923 Kanto, the 1948 Fukui, and the 1995 Kobe, Japan, earthquakes; the 1935 and the 1999 Chi-Chi, Taiwan, earthquakes; and the 1976 M7.6 Moro Gulf and 1990 M7.6 Luzon, Philippines, earthquakes.

Open-File Report↗
USGS2026

Constraining the permeability and outer-rise hydration at the Central America margin

Subduction zones are important drivers of Earth's geochemical evolution; the presence of water within the subduction system has substantial implications for geohazards, including the style of seismicity and the volume of magma generated at arc volcanoes. We utilize estimates of pore fluid distributions in the outer-rise region of the Central American margin imaged using controlled-source electro-magnetic surveys and novel two-dimensional forward models simulating poro-elastic deformation of slab bending to constrain the background permeability structure of the Cocos Plate and within outer-rise fault damage zones. The model results suggest that a relatively low background permeability structure is required to match the observed porosity at depths greater than 3 km (less than ~10 −21 m 2 . Similarly, increasing the permeability within the damage zones of outer-rise faults by more than a factor of 10 also over-predicts observed porosity distributions at depths greater than 3 km. However, the simulations consistently underpredict pore fluid distributions between depths of 1–3 km, which may reflect nonlinear elastic or anelastic deformation processes not included in the simulations or inherent uncertainty in the conversion of resistivity measurements to porosity distributions. The best-fitting depth-permeability relationship is relatively low, which has important implications for the timing of pore fluid release down-dip of the trench, as it restricts shallow fluid release, likely leading to higher pore fluid pressures, more widespread hydraulic fracturing, and deeper release of pore fluid along the plate interface.

Journal of Geophysical Research, Solid Earth↗
USGS2024

An accuracy assessment of the surface reflectance product from the EMIT imaging spectrometer

The Earth surface Mineral dust source InvesTigation (EMIT) is an imaging spectrometer launched to the International Space Station in July 2022 to measure the mineral composition of Earth’s dust-producing regions. We present a systematic accuracy assessment of the EMIT surface reflectance product in two parts. First, we characterize the surface reflectance product’s overall performance using multiple independent vicarious calibration field experiments with hand-held and automated field spectrometers. We find that the EMIT surface reflectance product has a standard error of ±1.0 % in absolute reflectance units for temporally coincident observations. Discrepancies rise to ±2.7 % for spectra acquired at different dates and times of day, which we attribute mainly to changes in solar geometry. Second, we develop an error budget that explains the differences between EMIT and in-situ field spectrometer data. We find that uncertainties in spatial footprints, field spectroscopy, and the EMIT-reported measurement were sufficient to explain discrepancies in most cases. Our approach did not detect any systematic calibration or reflectance errors in the timespan considered. Together, these findings demonstrate that a space-based imaging spectrometer can acquire high-quality spectra across a wide range of observational and atmospheric conditions.

Nevada↗
USGS2019

Variability in synthetic earthquake ground motions caused by source variability and errors in wave propagation models

Numerical simulations of earthquake ground motions are used both to anticipate the effects of hypothetical earthquakes by forward simulation and to infer the behaviour of the real earthquake source ruptures by the inversion of recorded ground motions. In either application it is necessary to assume some Earth structure that is necessarily inaccurate and to use a computational method that is also inaccurate for simulating the wavefield Green's functions. We refer to these two sources of error as ‘propagation inaccuracies’, which might be considered to be epistemic. We show that the variance of the Fourier spectrum of the synthetic earthquake seismograms caused by propagation inaccuracies is related to the spatial covariance on the rupture surface of errors in the computed Green's functions, which we estimate for the case of the 2009 L'Aquila, Italy, earthquake by comparing erroneous computed Green's functions with observed L'Aquila aftershock seismograms (empirical Green's functions). We further show that the variance of the synthetic seismograms caused by the rupture variability (aleatory uncertainty) is related to the spatial covariance on the rupture surface of aleatory variations in the rupture model, and we investigate the effect of correlated variations in Green's function errors and variations in rupture models. Thus, we completely characterize the variability of synthetic earthquake seismograms induced by errors in propagation and variability in the rupture behaviour. We calculate the spectra of the variance of the ground motions of the L'Aquila main shock caused by propagation inaccuracies for two specific broad-band stations, the AQU and the FIAM stations. These variances are distressingly large, being comparable or in some cases exceeding the data amplitudes, suggesting that the best-fitting L'Aquila rupture model significantly overfits the data and might be seriously in error. If these computed variances are typical, the accuracy of many other rupture models for past earthquakes may need to be reconsidered. The results of this work might be useful in seismic hazard estimation because the variability of the computed ground motion, caused both by propagation inaccuracies and variations in the rupture model, can be computed directly, not requiring laborious consideration of multiple Earth structures.

Geophysical Journal International↗
USGS2003

Slip history and dynamic implications of the 1999 Chi-Chi, Taiwan, earthquake

[1] We investigate the rupture process of the 1999 Chi‐Chi, Taiwan, earthquake using extensive near‐source observations, including three‐component velocity waveforms at 36 strong motion stations and 119 GPS measurements. A three‐plane fault geometry derived from our previous inversion using only static data [ Ji et al. , 2001 ] is applied. The slip amplitude, rake angle, rupture initiation time, and risetime function are inverted simultaneously with a recently developed finite fault inverse method that combines a wavelet transform approach with a simulated annealing algorithm [ Ji et al. , 2002b ]. The inversion results are validated by the forward prediction of an independent data set, the teleseismic P and SH ground velocities, with notable agreement. The results show that the total seismic moment release of this earthquake is 2.7 × 10 20 N m and that most of the slip occurred in a triangular‐shaped asperity involving two fault segments, which is consistent with our previous static inversion. The rupture front propagates with an average rupture velocity of ∼2.0 km s −1 , and the average slip duration (risetime) is 7.2 s. Several interesting observations related to the temporal evolution of the Chi‐Chi earthquake are also investigated, including (1) the strong effect of the sinuous fault plane of the Chelungpu fault on spatial and temporal variations in slip history, (2) the intersection of fault 1 and fault 2 not being a strong impediment to the rupture propagation, and (3) the observation that the peak slip velocity near the surface is, in general, higher than on the deeper portion of the fault plane, as predicted by dynamic modeling.

Journal of Geophysical Research B: Solid Earth↗
USGS2009

Inducing in situ, nonlinear soil response applying an active source

[1] It is well known that soil sites have a profound effect on ground motion during large earthquakes. The complex structure of soil deposits and the highly nonlinear constitutive behavior of soils largely control nonlinear site response at soil sites. Measurements of nonlinear soil response under natural conditions are critical to advancing our understanding of soil behavior during earthquakes. Many factors limit the use of earthquake observations to estimate nonlinear site response such that quantitative characterization of nonlinear behavior relies almost exclusively on laboratory experiments and modeling of wave propagation. Here we introduce a new method for in situ characterization of the nonlinear behavior of a natural soil formation using measurements obtained immediately adjacent to a large vibrator source. To our knowledge, we are the first group to propose and test such an approach. Employing a large, surface vibrator as a source, we measure the nonlinear behavior of the soil by incrementally increasing the source amplitude over a range of frequencies and monitoring changes in the output spectra. We apply a homodyne algorithm for measuring spectral amplitudes, which provides robust signal-to-noise ratios at the frequencies of interest. Spectral ratios are computed between the receivers and the source as well as receiver pairs located in an array adjacent to the source, providing the means to separate source and near-source nonlinearity from pervasive nonlinearity in the soil column. We find clear evidence of nonlinearity in significant decreases in the frequency of peak spectral ratios, corresponding to material softening with amplitude, observed across the array as the source amplitude is increased. The observed peak shifts are consistent with laboratory measurements of soil nonlinearity. Our results provide constraints for future numerical modeling studies of strong ground motion during earthquakes.

Journal of Geophysical Research B: Solid Earth↗
USGS2012

Absence of strain accumulation in the Shumagin seismic gap, Alaska, 1980-1987

Measurements of the deformation of a trilateration network in the Shumagin seismic gap in the interval 1980–1987 failed to detect any significant strain accumulation (observed extension rate in the direction of plate convergence 0.00±0.03 μstrain/yr). Dislocation models of the subduction process and measurements at a comparable network at a known seismic subduction zone (Nankai Trough, Japan) suggest that a rate of the order of −0.2 μstrain/yr should have been observed if the main thrust zone beneath the Shumagin Islands were locked. The simplest explanation of the observed absence of strain accumulation in the Shumagin seismic gap is that the main thrust zone beneath the Shumagin Islands is not presently locked. Other possible explanations depend upon very particular circumstances.

Journal of Geophysical Research Solid Earth↗
USGS2019

Relaxation response of critically stressed macroscale surficial rock sheets

Rock environments both underground and on Earth’s surface show indications of energetic macroscale fracture. In tunnels and excavations, these manifest as rockbursts—energetic explosions of rock that can damage engineering projects, and may pose ongoing financial and safety risk as rock stresses adjust during post-failure relaxation. In natural settings at the surface, evidence for rockbursts exist in the form of tent-like structures of ruptured exfoliation sheets, but few direct observations of such events exist, precluding the analysis of how natural rock formations may evolve after rupture. Here we investigate the post-failure evolution of a granitic rock dome following rapid fracture events (i.e., surficial rockbursts) that occurred in California, USA during 2014–2016. Building upon previous work that showed a thermal stress origin for the observed fracturing, we investigate the return to background stress conditions (i.e., stress relaxation) observed in both short- (week, month) and long-term (multi-year) rock deformation trends. Acoustic emissions, deformation, and environmental monitoring data indicate that partially detached rock sheets forming the surface of the dome undergo fracture aperture closing during cooling periods, concurrent with reduction of rock stress by the source of forcing (i.e., thermal stress). However, with sufficient critical and/or subcritical fracture, our observations also show that rock sheets can become decoupled from the source of stress, resulting in a long-term return to background stress conditions. Our results provide insight into the cyclic and likely ephemeral nature of rock fracture in surficial rock domes, as well as in underground rockburst environments.

Rock Mechanics and Rock Engineering↗
USGS1981

Mars and Earth: Comparison of cold-climate features

On Earth, glacial and periglacial features are common in areas of cold climate. On Mars, the temperature of the present-day surface is appropriate for permafrost, and the presence of water is suspected from data relating to the outgassing of the planet, from remote-sensing measurements over the polar caps and elsewhere on the Martian surface, and from recognition of fluvial morphological features such as channels. These observations and the possibility that ice could be in equilibrium with the atmosphere in the high latitudes north and south of ±40° latitude suggest that glacial and periglacial features should exist on the planet. Morphological studies based mainly on Viking pictures indicate many features that can be attributed to the action of ice. Among these features are extensive talus aprons; debris avalanches; flows that resemble glaciers or rock glaciers; ridges that look like moraines; various types of patterned ground, scalloped scarps, and chaotically collapsed terrain that could be attributed to thermokarst processes; and landforms that may reflect the interaction of volcanism and ice.

Icarus↗
USGS1992

Rare earth, major, and trace element composition of Monterey and DSDP chert and associated host sediment: Assessing the influence of chemical fractionation during diagenesis

Chert and associated host sediments from Monterey Formation and Deep Sea Drilling Project (DSDP) sequences were analyzed in order to assess chemical behavior during diagenesis of biogenic sediments. The primary compositional contrast between chert and host sediment is a greater absolute SiO 2 concentration in chert, often with final SiO 2 ≥ 98 wt%. This contrast in SiO 2 (and Si Al "> SiAl ) potentially reflects precursor sediment heterogeneity, diagenetic chemical fractionation, or both. SiO 2 concentrations and Si Al "> SiAl ratios in chert are far greater than in modern siliceous oozes, however and often exceed values in acid-cleaned diatom tests. Compositional contrasts between chert and host sediment are also orders-of-magnitude greater than between multiple samples of the host sediment. Calculations based on the initial composition of adjacent host, observed porosity reductions from host to chert and a postulated influx of pure SiO 2 , construct a chert composition which is essentially identical to observed SiO 2 values in chert. Thus, precursor heterogeneity does not seem to be the dominant factor influencing the current chert composition for the key elements of interest. In order to assess the extent of chemical fractionation during diagenesis, we approximate the precursor composition by analyzing host sediments adjacent to the chert. The SiO 2 concentration contrast seems caused by biogenic SiO 2 dissolution and transport from the local adjacent host sediment and subsequent SiO 2 reprecipitation in the chert. Along with SiO 2 , other elements are often added (with respect to Al) to Monterey and DSDP chert during silicification, although absolute concentrations decrease. The two Monterey quartz chert nodules investigated, in contrast to the opal-CT and quartz chert lenses, formed primarily by extreme removal of carbonate and phosphate, thereby increasing relative SiO 2 concentrations. DSDP chert formed by both carbonate/phosphate dissolution and SiO 2 addition from the host. Manganese is fractionated during chert formation, resulting in MnO Al 2 O 3 "> MnOAl2O3 ratios that no longer record the depositional signal of the precursor sediment. REE data indicate only subtle diagenetic fractionation across the rare earth series. Ce Ce ∗ "> CeCe* values do not change significantly during diagenesis of either Monterey or DSDP chert. Eu Eu ∗ "> EuEu* decreases slightly during formation of DSDP chert. La n Yb n "> LanYbn is affected only minimally as well. During formation of one Monterey opal-CT chert lens, REE Al "> REEAl ratios show subtle distribution changes at Gd and to a lesser extent near Nd and Ho. REE compositional contrasts between diagenetic states of siliceous sediment and chert are of a vastly smaller scale than has been noted between different depositional environments of marine sediment, indicating that the paleoenvironmental REE signature is not obscured by diagenetic overprinting.

Geochimica et Cosmochimica Acta↗
USGS2015

Self-similar rupture implied by scaling properties of volcanic earthquakes occurring during the 2004-2008 eruption of Mount St. Helens, Washington

We analyze a group of 6073 low-frequency earthquakes recorded during a week-long temporary deployment of broadband seismometers at distances of less than 3 km from the crater at Mount St. Helens in September of 2006. We estimate the seismic moment ( M 0 ) and spectral corner frequency ( f 0 ) using a spectral ratio approach for events with a high signal-to-noise (SNR) ratio that have a cross-correlation coefficient of 0.8 or greater with at least five other events. A cluster analysis of cross-correlation values indicates that the group of 421 events meeting the SNR and cross-correlation criteria forms eight event families that exhibit largely self-similar scaling. We estimate the M 0 and f 0 values of the 421 events and calculate their static stress drop and scaled energy ( E R / M 0 ) values. The estimated values suggest self-similar scaling within families, as well as between five of eight families (i.e., and constant). We speculate that differences in scaled energy values for the two families with variable scaling may result from a lack of resolution in the velocity model. The observation of self-similar scaling is the first of its kind for such a large group of low-frequency volcanic tectonic events occurring during a single active dome extrusion eruption.

Journal of Geophysical Research B: Solid Earth↗
USGS2020

Thresholds for post-wildfire debris flows: Insights from the Pinal Fire, Arizona, USA

Wildfire significantly alters the hydrologic properties of a burned area, leading to increases in overland flow, erosion, and the potential for runoff-generated debris flows. The initiation of debris flows in recently burned areas is well-characterized by rainfall intensity-duration (ID) thresholds. However, there is currently a paucity of data quantifying the rainfall intensities required to trigger post-wildfire debris flows, which limits our understanding of how and why rainfall ID thresholds vary in different climatic and geologic settings. In this study, we monitored debris-flow activity following the Pinal Fire in central Arizona, which differs from both a climatic and hydrogeomorphic perspective from other regions in the western U.S. where ID thresholds for post-wildfire debris flows are well-established, namely the Transverse Ranges of southern CA. Since the peak rainfall intensity within a rainstorm may exceed the rainfall intensity required to trigger a debris flow, the development of robust rainfall ID thresholds requires knowledge of the timing of debris flows within rainstorms. Existing post-wildfire debris-flow studies in Arizona only constrain the peak rainfall intensity within debris-flow-producing storms, which may far exceed the intensity that actually triggered the observed debris flow. In this study, we used pressure transducers within 5 burned drainage basins to constrain the timing of debris flows within rainstorms. Rainfall ID thresholds derived here from triggering rainfall intensities are, on average, 22 mm/h lower than ID thresholds derived under the assumption that the triggering intensity is equal to the maximum rainfall intensity recorded during a rainstorm. We then use a hydrologic model to demonstrate that the magnitude of the 15-minute rainfall ID threshold at the Pinal Fire site is associated with the rainfall intensity required to exceed a recently proposed dimensionless discharge threshold for debris-flow initiation. Model results further suggest that previously observed differences in regional ID thresholds between Arizona and the San Gabriel Mountains of southern CA may be attributed, in large part, to differences in the hydraulic properties of burned soils.

Arizona↗
USGS2013

Runoff-generated debris flows: observations and modeling of surge initiation, magnitude, and frequency

Runoff during intense rainstorms plays a major role in generating debris flows in many alpine areas and burned steeplands. Yet compared to debris flow initiation from shallow landslides, the mechanics by which runoff generates a debris flow are less understood. To better understand debris flow initiation by surface water runoff, we monitored flow stage and rainfall associated with debris flows in the headwaters of two small catchments: a bedrock-dominated alpine basin in central Colorado (0.06 km 2 ) and a recently burned area in southern California (0.01 km 2 ). We also obtained video footage of debris flow initiation and flow dynamics from three cameras at the Colorado site. Stage observations at both sites display distinct patterns in debris flow surge characteristics relative to rainfall intensity (I). We observe small, quasiperiodic surges at low I; large, quasiperiodic surges at intermediate I; and a single large surge followed by small-amplitude fluctuations about a more steady high flow at high I. Video observations of surge formation lead us to the hypothesis that these flow patterns are controlled by upstream variations in channel slope, in which low-gradient sections act as “sediment capacitors,” temporarily storing incoming bed load transported by water flow and periodically releasing the accumulated sediment as a debris flow surge. To explore this hypothesis, we develop a simple one-dimensional morphodynamic model of a sediment capacitor that consists of a system of coupled equations for water flow, bed load transport, slope stability, and mass flow. This model reproduces the essential patterns in surge magnitude and frequency with rainfall intensity observed at the two field sites and provides a new framework for predicting the runoff threshold for debris flow initiation in a burned or alpine setting.

California;Colorado↗