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Results for “Journal of Physics of the Earth”

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USGS2014

The value of geophysical methods in ground‐water studies

Two meanings are unfortunately given to the term geophysics. In the broad sense, based on the etymology of the term, geophysics means the physics of the Earth. This is its significance in the names “Geophysical Laboratory of the Carnegie Institution of Washington,” “International Union of Geodesy and Geophysics,” and “American Geophysical Union.” In this sense the sciences of geology and hydrology are largely geophysics, although in part they are geochemistry or biology. Geologists and hydraulic engineers who are elected to membership in the American Geophysical Union are, however, often surprised to find that they are regarded as “geophysicists.” I hope that when the Union establisnes a journal, its name will include the expression “Earth physics” or “Physics of the Earth,” in order that there will be no misunderstanding as to its scope.

Eos, Transactions, American Geophysical Union↗

USGS1996

Slip history of the 1995 Kobe, Japan, earthquake determined from strong motion, teleseismic, and geodetic data

Near-source ground motions, teleseismic body waveforms, and geodetic displacements produced by the 1995 Kobe, Japan, earthquake have been used to determine the spatial and temporal dislocation pattern on the faulting surfaces. A linear, least-squares approach was used to invert the data sets both independently and in unison in order to investigate the resolving power of each data set and to determine a model most consistent with all the available data. A two-fault model was used, with a single rupture plane representing faulting beneath Kobe and a second plane representing slip underneath Awaji Island. The total seismic moment is estimated to be 2.4×10 19 Nm (M W 6.9), with rupture partitioned such that about 40% of the slip was relatively deep (5-20 km) and northeast of the epicenter toward Kobe, and about 60% was toward the southwest and shallower (mostly 0-10 km) beneath Awaji Island. Analysis of the slip model indicates that the ground motions recorded within the severely damaged region of Kobe originated from the region of relatively low slip (about 1 m) deep beneath Kobe and not from the shallow, higher slip regions (about 3 m) beneath Awaji Island. Although the slip was relatively low beneath Kobe, the combined effects of source rupture directivity, a short slip duration, and site amplification conspired to generate very damaging ground motions within the city.

Journal of Physics of the Earth↗

USGS1969

History of the geomagnetic field

Direct measurements of the direction and strength of the earth's magnetic field have provided a knowledge of the field's form and behavior during the last few hundreds of years. For older times, however, it has been necessary to measure the magnetism of certain rocks to learn what the geomagnetic field was like. For example, when a lava flow solidifies (at temperatures near 1000°C) and cools through the Curie point of the magnetic minerals contained in it (around 500°C) it acquires a remanent magnetism that is (1) very weak, (2) very stablel, (3) paralle to the direction of the ambient geomagnetic field, and (4) proportional in intensity to the ambient field. Separating, by various analytical means, this magnetization from other ``unwanted'' magnetizations has allowed paleomagnetists to study the historical and prehistorical behavior of the earth's field. It has been learned, for example, that the strength of the field was almost twice its present value 2000 years ago and that it has often completely reversed its polarity. Paleo‐magnetists have also confirmed that most oceans are, geologically speaking, relatively new features, and that the continents have markedly changed their positions over the surface of the earth.

Journal of Applied Physics↗

USGS2014

Maximum magnitude earthquakes induced by fluid injection

Analysis of numerous case histories of earthquake sequences induced by fluid injection at depth reveals that the maximum magnitude appears to be limited according to the total volume of fluid injected. Similarly, the maximum seismic moment seems to have an upper bound proportional to the total volume of injected fluid. Activities involving fluid injection include (1) hydraulic fracturing of shale formations or coal seams to extract gas and oil, (2) disposal of wastewater from these gas and oil activities by injection into deep aquifers, and (3) the development of enhanced geothermal systems by injecting water into hot, low-permeability rock. Of these three operations, wastewater disposal is observed to be associated with the largest earthquakes, with maximum magnitudes sometimes exceeding 5. To estimate the maximum earthquake that could be induced by a given fluid injection project, the rock mass is assumed to be fully saturated, brittle, to respond to injection with a sequence of earthquakes localized to the region weakened by the pore pressure increase of the injection operation and to have a Gutenberg-Richter magnitude distribution with a b value of 1. If these assumptions correctly describe the circumstances of the largest earthquake, then the maximum seismic moment is limited to the volume of injected liquid times the modulus of rigidity. Observations from the available case histories of earthquakes induced by fluid injection are consistent with this bound on seismic moment. In view of the uncertainties in this analysis, however, this should not be regarded as an absolute physical limit.

Journal of Geophysical Research B: Solid Earth↗

USGS2006

Cometary impact and amino acid survival - Chemical kinetics and thermochemistry

The Arrhenius parameters for the initiating reactions in butane thermolysis and the formation of soot, reliable to at least 3000 K, have been applied to the question of the survival of amino acids in cometary impacts on early Earth. The pressure/temperature/time course employed here was that developed in hydrocode simulations for kilometer-sized comets (Pierazzo and Chyba, 1999), with attention to the track below 3000 K where it is shown that potential stabilizing effects of high pressure become unimportant kinetically. The question of survival can then be considered without the need for assignment of activation volumes and the related uncertainties in their application to extreme conditions. The exercise shows that the characteristic times for soot formation in the interval fall well below the cooling periods for impacts ranging from fully vertical down to about 9?? above horizontal. Decarboxylation, which emerges as more rapid than soot formation below 2000-3000 K, continues further down to extremely narrow impact angles, and accordingly cometa??ry delivery of amino acids to early Earth is highly unlikely. ?? 2006 American Chemical Society.

Journal of Physical Chemistry A↗

USGS2011

Referees and the publications crisis

Routine refereeing by peers of manuscripts submitted to scholarly journals is a logical and effective device that can obviate ‘crankiness, irrelevance and gross incompetence’ [ Ziman , 1970 a ] on the one hand and minimize editorial arbitrariness on the other. As reviewed by Zuckerman and Merton [1971], the device has historical roots that extend back to the beginning of the first scholarly journals, such as Transactions of the Royal Society, and Journal des Scavans . The growth of science since World War II and the pressure of burgeoning manuscript submission appear to have been important factors in making automatic peer review nearly universal in American and British Commonwealth natural science journals and in international journals influenced by Anglo‐North Americans. A 1962 survey cited by Zuckerman and Merton [1971] found that 71% of 156 physical and biological science journals in 13 countries made use of referees.

Eos, Earth and Space Science News↗

USGS2015

Book review: Rare earth elements—A new approach to the nexus of supply, demand and use: exemplified along the use of neodymium in permanent magnets

This book is part of the “Springer Theses” published by Springer, a book series designed to highlight and share outstanding Ph.D. research. As explained by Springer (on the second page), this series “brings together a selection of the very best Ph.D. theses from around the world and across the physical sciences.” I admire Springer for this endeavor because a large number of exceptional theses are never published, and thus are difficult to obtain, while most of those that are published are presented only in highly condensed form as journal articles. Review info: Rare Earth Elements—A New Approach to the Nexus of Supply, Demand and Use: Exemplified Along the Use of Neodymium in Permanent Magnets . By Volker Zerf. 2013. ISBN 978-3-642-35457-1. 157 pp.

Economic Geology↗

USGS2019

Earthquake stress drop and Arias Intensity

We determine earthquake stress drops directly from the Arias intensity database of NGA-West2. Arias intensity (Arias, 1970) is an engineering measure proportional to the integral of the absolute value of acceleration squared, over the significant duration of the signal. As such, it is closely related to root-mean-square acceleration, and can readily be connected to earthquake stress drop (Hanks and McGuire, 1981). Arias intensity records out to 100 km yield stable stress drops for moderate-to-large magnitude earthquakes, M6.5+; for smaller events ~M4.5 – 6.5, only closer-in records yield stable results. For the 116 events considered, stress drops are about 35% larger for Class 1 mainshocks than for traditional on-fault Class 2 aftershocks, and smaller for those aftershocks close to the main fault plane. Aftershock stress drops show large variability, however, implying that on average they re-rupture weakened patches, but can also rupture intact rock or high-stress asperities. We observe an increase of stress drop with earthquake depth similar to that of other studies but do not find any significant faulting mechanism dependence. The variability of the Arias intensity-based stress drop is lower than that of eGf-based stress drops from Baltay et al. (2010, 2011), and nearly on par with variability seen in ground-motion prediction equations. The Arias intensity stress drop is a novel and promising method to estimate stress drop without the need for path and site corrections, and yields further insight into the connection between source physics and ground-motion.

Journal of Geophysical Research B: Solid Earth↗

USGS2012

Tidal triggering of low frequency earthquakes near Parkfield, California: Implications for fault mechanics within the brittle-ductile transition

Studies of nonvolcanic tremor (NVT) have established the significant impact of small stress perturbations on NVT generation. Here we analyze the influence of the solid earth and ocean tides on a catalog of ∼550,000 low frequency earthquakes (LFEs) distributed along a 150 km section of the San Andreas Fault centered at Parkfield. LFE families are identified in the NVT data on the basis of waveform similarity and are thought to represent small, effectively co-located earthquakes occurring on brittle asperities on an otherwise aseismic fault at depths of 16 to 30 km. We calculate the sensitivity of each of these 88 LFE families to the tidally induced right-lateral shear stress (RLSS), fault-normal stress (FNS), and their time derivatives and use the hypocentral locations of each family to map the spatial variability of this sensitivity. LFE occurrence is most strongly modulated by fluctuations in shear stress, with the majority of families demonstrating a correlation with RLSS at the 99% confidence level or above. Producing the observed LFE rate modulation in response to shear stress perturbations requires low effective stress in the LFE source region. There are substantial lateral and vertical variations in tidal shear stress sensitivity, which we interpret to reflect spatial variation in source region properties, such as friction and pore fluid pressure. Additionally, we find that highly episodic, shallow LFE families are generally less correlated with tidal stresses than their deeper, continuously active counterparts. The majority of families have weaker or insignificant correlation with positive (tensile) FNS. Two groups of families demonstrate a stronger correlation with fault-normal tension to the north and with compression to the south of Parkfield. The families that correlate with fault-normal clamping coincide with a releasing right bend in the surface fault trace and the LFE locations, suggesting that the San Andreas remains localized and contiguous down to near the base of the crust. The deep families that have high sensitivity to both shear and tensile normal stress perturbations may be indicative of an increase in effective fault contact area with depth. Synthesizing our observations with those of other LFE-hosting localities will help to develop a comprehensive understanding of transient fault slip below the “seismogenic zone” by providing constraints on parameters in physical models of slow slip and LFEs.

California↗

USGS1998

CRUST 5.1: A global crustal model at 5° x 5°

We present a new global model for the Earth's crust based on seismic refraction data published in the period 1948-1995 and a detailed compilation of ice and sediment thickness. An extensive compilation of seismic refraction measurements has been used to determine the crustal structure on continents and their margins. Oceanic crust is modeled with both a standard model for normal oceanic crust, and variants for nonstandard regions, such as oceanic plateaus. Our model (CRUST 5.1) consists of 2592 5° × 5° tiles in which the crust and uppermost mantle are described by eight layers: (1) ice, (2) water, (3) soft sediments, (4) hard sediments, (5) crystalline upper, (6) middle, (7) lower crust, and (8) uppermost mantle. Topography and bathymetry are adopted from a Mandard database (ETOPO-5). Compressional wave velocity in each layer is based on field measurements, and shear wave velocity and density are estimated using recently published empirical V p -V s and V p -density relationships. The crustal model differs from previous models in that (1) the thickness and seismic/density structure of sedimentary basins is accounted for more completely, (2) the velocity structure of unmeasured regions is estimated using statistical averages that are based on a significantly larger database of crustal structure, (3) the compressional wave, shear wave, and density structure have been explicitly specified using newly available constraints from field and laboratory studies. Thus - this global crustal model is based on substantially more data than previous models and differs from them in many important respects. A new map of the thickness of the Earth's crust is presented, and we illustrate the application of this model by using it to provide the crustal correction for surface wave phase velocity maps. Love waves at 40 s are dominantly sensitive to crustal structure, and there is a very close correspondence between observed phase velocities at this period and those predicted by CRUST 5.1. We find that the application of crustal corrections to long-period (167 s) Rayleigh waves significantly increases the variance in the phase velocity maps and strengthens the upper mantle , velocity anomalies beneath stable continental regions. A simple calculation of crustal isostacy indicates significant lateral variations in upper mantle density. The model CRUST 5.1 provides a complete description of the physical properties of the Earth's crust at a scale of 5° × 5° and can be used for a wide range of seismological and nonseismological problems.

Journal of Geophysical Research B: Solid Earth↗

USGS2021

Role of future reef growth on morphological response of coral reef islands to sea-level rise

Coral reefs are widely recognised for providing a natural breakwater effect that modulates erosion and flooding hazards on low‐lying sedimentary reef islands. Increased water depth across reef platforms due sea‐level rise (SLR) can compromise this breakwater effect and enhance island exposure to these hazards, but reef accretion in response to SLR may positively contribute to island resilience. Morphodynamic studies suggest that reef islands can adjust to SLR by maintaining freeboard (island crest elevation above still water level) through overwash deposition and island accretion, but the impact of different future reef accretion trajectories on the morphological response of islands remain unknown. Here we show, using a process‐based morphodynamic model, that, although reef growth significantly affects wave transformation processes and island morphology, it does not lead to decreased coastal flooding and island inundation. According to the model, reef islands evolve during SLR by attuning their elevation to the maximum wave runup and islands fronted by a growing reef platform attain lower elevations than those without reef growth, but have similar overwash regimes. The mean overwash discharge Q over across the island crest plays a key role in the ability of islands to keep up with SLR and maintain freeboard, with a Q over value of O (10 l m ‐1 s ‐1 ) separating island construction from destruction. Islands, therefore, can grow vertically to keep up with SLR via flooding and overwash if specific forcing and sediment supply conditions are met, offering hope for uninhabited and sparely populated islands. However, this physical island response will negatively impact infrastructure and assets on developed islands.

Journal of Geophysical Research--Earth Surface↗

USGS2013

Foreshocks during the nucleation of stick-slip instability

We report on laboratory experiments which investigate interactions between aseismic slip, stress changes, and seismicity on a critically stressed fault during the nucleation of stick-slip instability. We monitor quasi-static and dynamic changes in local shear stress and fault slip with arrays of gages deployed along a simulated strike-slip fault (2 m long and 0.4 m deep) in a saw cut sample of Sierra White granite. With 14 piezoelectric sensors, we simultaneously monitor seismic signals produced during the nucleation phase and subsequent dynamic rupture. We observe localized aseismic fault slip in an approximately meter-sized zone in the center of the fault, while the ends of the fault remain locked. Clusters of high-frequency foreshocks (M w ~ −6.5 to −5.0) can occur in this slowly slipping zone 5–50 ms prior to the initiation of dynamic rupture; their occurrence appears to be dependent on the rate at which local shear stress is applied to the fault. The meter-sized nucleation zone is generally consistent with theoretical estimates, but source radii of the foreshocks (2 to 70 mm) are 1 to 2 orders of magnitude smaller than the theoretical minimum length scale over which earthquake nucleation can occur. We propose that frictional stability and the transition between seismic and aseismic slip are modulated by local stressing rate and that fault sections, which would typically slip aseismically, may radiate seismic waves if they are rapidly stressed. Fault behavior of this type may provide physical insight into the mechanics of foreshocks, tremor, repeating earthquake sequences, and a minimum earthquake source dimension.

Journal of Geophysical Research B: Solid Earth↗

USGS2018

Seismicity induced by wastewater injection in Washington County, Ohio: Influence of preexisting structure, regional stress regime, and well operations

Recent seismicity in Washington County, Ohio, has been suggested to be induced by wastewater disposal operations despite injection ~2 km above the Precambrian basement. We investigated the relationships between disposal well locations and operational histories, spatiotemporal patterns of seismicity enhanced by waveform correlation, and mapped subsurface structures. We also analyzed proxies for in situ stress conditions to evaluate existing principal stress orientations and magnitudes. Double‐difference relocations of small ( M L ≤ 2.1) seismicity revealed linear clusters defining a NE‐SW orientation that parallels the trend of small‐amplitude folds in Upper Paleozoic rocks within 1–2 km of the injection well. Hypocenter depths of ~3.1 to 4.3 km indicate seismicity occurred along faults below the Silurian injection interval in lower Paleozoic strata and Precambrian crystalline basement. Similar folds/faults imaged in nearby seismic reflection lines were associated with basement‐involved fault systems that cut the injection interval targeted by the injection well, providing permeability pathways for fluid pressure increases that initiated slip. Interpreted faults were also optimally oriented in the regional stress field for reactivation due to injection‐related decreases in effective stress. Well completion records suggest that during the recorded seismicity, the Long Run‐1 well was injecting disposal fluids close to or above the fracture pressure of the targeted Silurian injection interval. A comparison of observed seismicity rates and monthly injection records from the disposal well also indicate a close correspondence between earthquake frequency and injection rate. Taken together, these results provide a physical framework for understanding why wastewater disposal operations induced the Washington County seismic sequence.

Ohio↗

USGS2012

The new IASPEI standards for determining magnitudes from digital data and their relation to classical magnitudes

Why there is a need for measurement standards of magnitudes: In October 2005, the Commission on Seismic Observation and Interpretation of the International Association of Seismology and Physics of the Earth´s Interior (IASPEI) adopted the summary recommendations made by the IASPEI Working Group on Magnitudes on new measurement standards for widely used local, regional and teleseismic magnitude scales (IASPEI, 2005). These recommendations have recently been refined and detailed (IASPEI, 2013) and a final scientific report, to be published in a reputable international journal, is currently under preparation.

Book chapter↗

USGS2015

Near-surface versus fault zone damage following the 1999 Chi-Chi earthquake: Observation and simulation of repeating earthquakes

We observe crustal damage and its subsequent recovery caused by the 1999  M 7.6 Chi-Chi earthquake in central Taiwan. Analysis of repeating earthquakes in Hualien region, ~70 km east of the Chi-Chi earthquake, shows a remarkable change in wave propagation beginning in the year 2000, revealing damage within the fault zone and distributed across the near surface. We use moving window cross correlation to identify a dramatic decrease in the waveform similarity and delays in the S wave coda. The maximum delay is up to 59 ms, corresponding to a 7.6% velocity decrease averaged over the wave propagation path. The waveform changes on either side of the fault are distinct. They occur in different parts of the waveforms, affect different frequencies, and the size of the velocity reductions is different. Using a finite difference method, we simulate the effect of postseismic changes in the wavefield by introducing S wave velocity anomaly in the fault zone and near the surface. The models that best fit the observations point to pervasive damage in the near surface and deep, along-fault damage at the time of the Chi-Chi earthquake. The footwall stations show the combined effect of near-surface and the fault zone damage, where the velocity reduction (2–7%) is twofold to threefold greater than the fault zone damage observed in the hanging wall stations. The physical models obtained here allow us to monitor the temporal evolution and recovering process of the Chi-Chi fault zone damage.

Journal of Geophysical Research B: Solid Earth↗

USGS2007

Sorted bed forms as self-organized patterns: 2. complex forcing scenarios

We employ a numerical model to study the development of sorted bed forms under a variety of hydrodynamic and sedimentary conditions. Results indicate that increased variability in wave height decreases the growth rate of the features and can potentially give rise to complicated, a priori unpredictable, behavior. This happens because the system responds to a change in wave characteristics by attempting to self-organize into a patterned seabed of different geometry and spacing. The new wavelength might not have enough time to emerge before a new change in wave characteristics occurs, leading to less regular seabed configurations. The new seabed configuration is also highly dependent on the preexisting morphology, which further limits the possibility of predicting future behavior. For the same reasons, variability in the mean current magnitude and direction slows down the growth of features and causes patterns to develop that differ from classical sorted bed forms. Spatial variability in grain size distribution and different types of net sediment aggradation/degradation can also result in the development of sorted bed forms characterized by a less regular shape. Numerical simulations qualitatively agree with observed geometry (spacing and height) of sorted bed forms. Also in agreement with observations is that at shallower depths, sorted bed forms are more likely to be affected by changes in the forcing conditions, which might also explain why, in shallow waters, sorted bed forms are described as ephemeral features. Finally, simulations indicate that the different sorted bed form shapes and patterns observed in the field might not necessarily be related to diverse physical mechanisms. Instead, variations in sorted bed form characteristics may result from variations in local hydrodynamic and/or sedimentary conditions.

Journal of Geophysical Research F: Earth Surface↗

USGS1999

Analysis of stress-induced oval fractures in a borehole at Deep Sea Drilling Project Site 504, eastern equatorial Pacific

Deep Sea Drilling Project (DSDP) Hole 504B is located in the eastern equatorial Pacific Ocean and extends to a total depth of 2111 m beneath the seafloor (mbsf). Several acoustic televiewer logs have been obtained in this well during successive stages of drilling, and the resulting digital images have revealed numerous oval-shaped fractures seemingly etched into the borehole wall. A theoretical examination of these stress-induced features identifies a unique and ephemeral set of stress distributions and magnitudes that are necessary for their production. Consequently, the ovals provide a basis for quantifying the magnitudes and orientations of the maximum and minimum horizontal principal stresses, S H and S h at this site. Vertical, truncated breakouts and horizontal tensile fractures define the spatial boundaries of the ovals. Explicit criteria for their occurrence are combined with estimates for various physical properties of the rock to yield a range of possible values for the horizontal principal stresses. The conspicuous oval geometry is completed by a curved fracture that joins the vertical and horizontal components. Its degree of curvature is delineated by the modified Griffith failure criterion and is directly related to the principal stress difference ( S H − S h ). Matching a series of type curves corresponding to specific values for ( S H − S h ) with the actual undistorted well bore images allows the magnitude of the stress difference to be further constrained. With a value for ( S H − S h ) of 45 ± 5 MPa the individual magnitudes of S H and S h are determined more precisely. Final estimates for the horizontal principal stresses in DSDP Hole 504B at a depth of 1200 mbsf are 141 MPa ≤ S H ≤ 149 MPa and 91 MPa ≤ S h ≤ 109 MPa. Stress magnitudes derived from this approach rely heavily upon the values of a variety of physical properties, and complementary laboratory measurements performed on relevant rock samples provide critical information. Uncertainties in estimating these properties translate into less precise predictions of principal stresses.

Journal of Geophysical Research B: Solid Earth↗

USGS2003

Importance of a stochastic distribution of floods and erosion thresholds in the bedrock river incision problem

Fluvial erosion of bedrock occurs during occasional flood events when boundary shear stress exceeds a critical threshold to initiate incision. Therefore efforts to model the evolution of topography over long timescales should include an erosion threshold and should be driven by a stochastic distribution of erosive events. However, most bedrock incision models ignore the threshold as a second‐order detail. In addition, climate is poorly represented in most landscape evolution models, so the quantitative relationship between erosion rate and measurable climatic variables has been elusive. Here we show that the presence of an erosion threshold, when combined with a well‐constrained, probabilistic model of storm and flood occurrence, has first‐order implications for the dynamics of river incision in tectonically active areas. First, we make a direct calculation of the critical shear stress required to pluck bedrock blocks for a field site in New York. Second, we apply a recently proposed stochastic, threshold, bedrock incision model to a series of streams in California, with known tectonic and climatic forcing. Previous work in the area has identified a weak relationship between channel gradient or relief and rock uplift rate that is not easily explained by simpler detachment‐limited models. The results with the stochastic threshold model show that even low erosion thresholds, which are exceeded in steep channels during high‐frequency flood events, fundamentally affect the predicted relationship between gradient and uplift rate in steady state rivers, in a manner consistent with the observed topography. This correspondence between theory and data is, however, nonunique; models in which a thin alluvial cover may act to inhibit channel incision in the low uplift rate zone also provide plausible explanations for the observed topography. Third, we explore the broader implications of the stochastic threshold model to the development of fluvial topography in active tectonic settings. We suggest that continued field applications of geomorphic models, including physically meaningful thresholds and stochastic climate distributions, are required to advance our knowledge of interactions among surficial, climatic, and crustal processes.

California↗