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At least 307 records · Page 17Linked to original sources
USGS2012

Lithospheric loading by the 1896 Riku-u earthquake, northern Japan: Implications for plate flexure and asthenospheric rheology

Under favorable circumstances the time-dependent aseismic deformation resulting from the loading of the lithosphere by the stress drop of large dip slip earthquakes can be used to determine both the effective elastic plate thickness and the asthenospheric viscosity. The deformation has several similarities with the deflection of the lithosphere by surface loads and with movements due to postglacial rebound. Level changes obtained in the 80 years since the M = 7.5, 1896 Riku-u earthquake, an intraplate thrust event in northern Honshu, provide convincing evidence that asthenospheric readjustments are responsible for the observed movements. Leveling surveys crossing the zone of surface faulting have been repeated five times since 1900 and delineate a localized depression that has subsided at a continually decreasing rate. The depression is centered close to the 1896 faulting, and its shape and width, about 75 km, are matched by our model using a plate thickness of 30 km. The decaying subsidence rate constrains the viscosity of the uppermost asthenosphere to be 1×10 20 P. A linear viscous rheology matches the observed decay quite well, although measurements are sparse during the several decades following the earthquake.

Journal of Geophysical Research Solid Earth↗
USGS2013

Annual modulation of non-volcanic tremor in northern Cascadia

Two catalogs of episodic tremor events in northern Cascadia, one from 2006 to 2012 and the other from 1997 to 2011, reveal two systematic patterns of tremor occurrence in southern Vancouver Island: (1) most individual events tend to occur in the third quarter of the year; (2) the number of events in prolonged episodes (i.e., episodic tremor and slip events), which generally propagate to Vancouver Island from elsewhere along the Cascadia subduction zone, is inversely correlated with the amount of precipitation that occurred in the preceding 2 months. We rationalize these patterns as the product of hydrologic loading of the crust of southern Vancouver Island and the surrounding continental region, superimposed with annual variations from oceanic tidal loading. Loading of the Vancouver Island crust in the winter (when the land surface receives ample precipitation) and unloading in the summer tends to inhibit and enhance downdip shear stress, respectively. Quantitatively, for an annually variable surface load, the predicted stress perturbation depends on mantle viscoelastic rheology. A mechanical model of downdip shear stress on the transition zone beneath Vancouver Island—driven predominantly by the annual hydrologic cycle—is consistent with the 1997–2012 tremor observations, with peak-to-peak downdip shear stress of about 0.4 kPa. This seasonal dependence of tremor occurrence appears to be restricted to southern Vancouver Island because of its unique situation as an elongated narrow-width land mass surrounded by ocean, which permits seasonal perturbations in shear stress at depth.

Journal of Geophysical Research B: Solid Earth↗
USGS2020

Peak ground velocity spatial variability revealed by dense seismic array in southern California

Understanding and modeling variability of ground motion is essential for building accurate and precise ground motion prediction equations, which can net site‐specific characterization and reduced hazard levels. Here, we explore the spatial variability in peak ground velocity (PGV) at Sage Brush Flats along the San Jacinto Fault in southern California. We use data from a dense array (0.6 x 0.6 km 2 , 1,108 geophones, station spacings 10‐30 m) deployed in 2014 for ~1‐month. These data offer an opportunity to study small scale variability in this region. We examine 38 2≤M L ≤4.2 earthquakes within 200 km of the array. Fault strands and a small basin impact the ground motions, producing PGV variations up to 22% of the mean and a 40% reduction in P and S wave near‐surface velocities. We find along‐fault rupture directivity, source, and path effects can increase PGVs by 167%. Surface PGV measurements exceed the co‐located borehole station (depth 148 m) PGV by factors of 3‐10, confirming the impact on PGV from near surface fault structures, basins, topography, and amplifications from soft sediments. Consistently we find high PGVs within the basin structure. A pair of ~co‐located M L 2.6 events produce repeatable PGV values with similar spatial patterns. The average corner frequencies of these two events are 11‐16 Hz and viable measurements of stress drop can differ by 6.45MPa. Within this small array, the PGV values are variable implying spatial extrapolation of PGV to regions of known faults and basins, even across a small area, should be done with caution.

California↗
USGS2022

Toward next-generation lava flow forecasting: Development of a fast, physics-based lava propagation model

During effusive volcanic crises, the eruption and propagation of lava flows pose a significant hazard to nearby populations, homes, and infrastructure. Consequently, timely lava flow forecasts are a critical need for volcano observatory and emergency management operations. Previous lava flow modeling tools are typically either too slow to produce timely forecasts, or are fast, but lack critical aspects of lava physics or important forecasting outputs. In particular, the strong thermal stratification present in laminar, high-Prandtl number flows has generally been neglected. Bulk rheological changes have previously been computed from cell-averaged temperatures, assuming that the flow is thermally mixed. Here, we detail the development and initial testing of Lava2d, a new two-dimensional depth-averaged finite volume model of lava flow propagation over natural terrain which accounts for bulk rheological changes due to thermorheological stratification. We use a novel approach to energy conservation based on tracking cooling and solidifying at the flow base and at the moving flow surface, allowing for the estimation of more realistic vertical thermorheological profiles, while maintaining computational efficiency, producing very timely model runs. We validate our approach with three examples: comparison with theoretical propagation of crust-dominated lava flows, comparison with a large-scale molten basalt experiment from the Syracuse University Lava Project, and efficiency testing and comparison with the initial phase of the 1984 Mauna Loa lava flows. Our model is shown to produce rapid, realistic forecasts, making it a good candidate for operationalization in active volcanic regions such as in Hawai'i.

Journal of Geophysical Research - Solid Earth↗
USGS2011

A wavelength-dependent visible and infrared spectrophotometric function for the Moon based on ROLO data

The USGS's Robotic Lunar Observatory (ROLO) dedicated ground‐based lunar calibration project obtained photometric observations of the Moon over the spectral range attainable from Earth (0.347–2.39 μ m) and over solar phase angles of 1.55°–97°. From these observations, we derived empirical lunar surface solar phase functions for both the highlands and maria that can be used for a wide range of applications. The functions can be used to correct for the effects of viewing geometry to produce lunar mosaics, spectra, and quick‐look products for future lunar missions and ground‐based observations. Our methodology can be used for a wide range of objects for which multiply scattered radiation is not significant, including all but the very brightest asteroids and moons.

Journal of Geophysical Research E: Planets↗
USGS2020

Hydrologically induced deformation in Long Valley Caldera and adjacent Sierra Nevada

Vertical and horizontal components of GNSS displacements in the Long Valley Caldera and adjacent Sierra Nevada range show a clear correlation with hydrological trends at both multiyear and seasonal time scales. We observe a clear vertical and horizontal seasonal deformation pattern primarily attributable to the solid earth response to hydrological surface loading at large-to-regional (Sierra Nevada range) scales. Several GNSS sites, located at the eastern edge of the Sierra Nevada along the southwestern rim of Long Valley Caldera, also show significant horizontal deformation that cannot be explained by elastic deformation from surface loading. Due to the location of these sites and the strong correlation between their horizontal displacements and spring discharge, we hypothesize that this deformation reflects poroelastic processes related to snowmelt runoff water infiltrating into the Sierra Nevada slopes around Long Valley Caldera. Interestingly, this is also an area where water infiltrates to feed the local hydrothermal system, and where snowmelt-induced earthquake swarms have been recently detected.

California, Nevada↗
USGS2010

Seismic source mechanism of degassing bursts at Kilauea volcano, Hawaii: Results from waveform inversion in the 10–50 s band

The current (March 2008 to February 2009) summit eruptive activity at Kilauea Volcano is characterized by explosive degassing bursts accompanied by very long period (VLP) seismic signals. We model the source mechanisms of VLP signals in the 10–50 s band using data recorded for 15 bursts with a 10‐station broadband network deployed in the summit caldera. To determine the source centroid location and source mechanism, we minimize the residual error between data and synthetics calculated by the finite difference method for a point source embedded in a homogeneous medium that takes topography into account. The VLP signals associated with the bursts originate in a source region ∼1 km below the eastern perimeter of Halemaumau pit crater. The observed waveforms are well explained by the combination of a volumetric component and a vertical single force component. For the volumetric component, several source geometries are obtained which equally explain the observed waveforms. These geometries include (1) a pipe dipping 64° to the northeast; (2) two intersecting cracks including an east striking crack (dike) dipping 80° to the north, intersecting a north striking crack (another dike) dipping 65° to the east; (3) a pipe dipping 58° to the northeast, intersecting a crack dipping 48° to the west–southwest; and (4) a pipe dipping 57° to the northeast, intersecting a pipe dipping 58° to the west–southwest. Using the dual‐crack model as reference, the largest volume change obtained among the 15 bursts is ∼24,400 m 3 , and the maximum amplitude (peak to peak) of the force is ∼20 GN. Each burst is marked by a similar sequence of deflation and inflation, trailed by decaying oscillations of the volumetric source. The vertical force is initially upward, synchronous with source deflation, then downward, synchronous with source reinflation, followed by oscillations with polarity opposite to the volumetric oscillations. This combination of force and volume change is attributed to pressure and momentum changes induced during a fluid dynamic source mechanism involving the ascent, expansion, and burst of a large slug of gas within the upper ∼150 m of the magma conduit. As the slug expands upon approach to the surface and more liquid becomes wall supported by viscous shear forces, the pressure below the slug decreases, inducing conduit deflation and an upward force on the Earth. The final rapid slug expansion and burst stimulate VLP and LP oscillations of the conduit system, which slowly decay due to viscous dissipation and elastic radiation. Consideration of the fluid dynamic arguments leads us to prefer the dual‐crack VLP source model as it is the only candidate model capable of producing plausible values of length scales and pressure changes. The magnitudes of the vertical forces observed in the 15 bursts appear consistent with slug masses of 10 4 to 10 6 kg.

Hawaii↗
USGS2021

Airborne geophysical imaging of weak zones on Iliamna Volcano, Alaska: Implications for slope stability

Water‐saturated, hydrothermally altered rocks reduce the strength of volcanic edifices and increase the potential for sector collapses and far‐traveled mass flows of unconsolidated debris. Iliamna Volcano is an andesitic stratovolcano located on the western side of the Cook Inlet, ∼225 km southwest of Anchorage and is a source of repeated avalanches. The widespread snow and ice cover on Iliamna Volcano make surface alteration difficult to identify. However, intense hydrothermal alteration significantly reduces both the electrical resistivity and magnetization of volcanic rock and can therefore be identified with airborne geophysical measurements. We use airborne electromagnetic and magnetic data to map snow and ice thickness and identify underlying alteration zones at Iliamna Volcano, Alaska. Resistivities were calculated to an average depth of >300 m, and a 3‐D susceptibility model extends from the surface to the base of the volcano, about 3,000 m below the summit. Geophysical models image low resistivity (<30 ohm‐m) and low susceptibilities near the summit of Iliamna and below its older vent complex, with the low susceptibilities indicating alteration up to ∼800 m in thickness. Thin conductors (∼50–100 m thick) on the edifice slopes coincide with recorded locations of repeated debris avalanches over the past ∼60 years and are attributed to saturated zones at high elevation. Three‐dimensional slope stability models based upon the geophysically constrained alteration distribution suggest the edifice of Iliamna is unstable and could lead to collapse scars ∼400 m deep near the current and former vent complexes.

Alaska↗
USGS2012

Resonance of a fluid-driven crack: Radiation properties and implications for the source of long-period events and harmonic tremor

A dynamic source model is presented, in which a three-dimensional crack containing a viscous compressible fluid is excited into resonance by an impulsive pressure transient applied over a small area ΔS of the crack surface. The crack excitation depends critically on two dimensionless parameters called the crack stiffness, C = ( b /μ)( L / d ), and viscous damping loss, F = (12η L )/(ρ ƒ d 2 α), where b is the bulk modulus, η is the viscosity, ρ ƒ is the density of the fluid, μ is the rigidity, α is the compressional velocity of the solid, L is the crack length, and d is the crack thickness. The first parameter characterizes the ability of the crack to vibrate and shapes the spectral signature of the source, and the second quantifies the effect of fluid viscosity on the duration of resonance. Resonance is sustained by a very slow wave trapped in the fluid-filled crack. This guided wave, called the crack wave, is similar to the tube wave propagating in a fluid-filled borehole; it is inversely dispersive, showing a phase velocity that decreases with increasing wavelength, and its wave speed is always lower than the acoustic velocity of the fluid, decreasing rapidly as the crack stiffness increases. The source spectrum shows many sharp peaks characterizing the individual modes of vibration of the crack; the variation of spectral shape, both in the number and width of peaks, is surprisingly complex, reflecting the interference between the lateral and longitudinal modes of resonance, as well as nodes for these modes. The far-field spectrum is marked by narrow-band dominant and subdominant peaks that reflect the interaction of the various source modes. The frequency of the dominant spectral peak radiated by the source is independent of the radiation direction. The frequency, bandwidth, and spacing of the resonant peaks are strongly dependent on the crack stiffness, larger values of the stiffness factor shifting these peaks to lower frequencies and decreasing their bandwidth. The excitation of a particular mode depends on the position of the trigger and on the extent of the crack surface affected by the pressure transient. Fluid viscosity decreases the amplitudes of the main spectral peaks, smears out the finer structure of the spectrum, and greatly reduces the duration of the radiated signal. The energy loss by radiation is stronger for high frequencies, producing a seismic signature that is marked by a high-frequency content near the onset of the signal and dominated by a longer-period component of much longer duration in the signal coda. Such signature is in harmony with those displayed by long-period events observed on active volcanoes and in hydrofracture experiments. The very low velocity which is possible in a crack with high stiffness ( C ≥ 100) also provides an attractive explanation for very long period tremor, such as type 2 tremor at Aso volcano, Japan, without the requirement of an unrealistically large magma container. The standing wave pattern set up on the crack surface by the sustained resonance in the fluid is observable in the near field of the crack, suggesting that the location and extent of the source may be estimated from the mapping of the pattern of nodes and antinodes seen in its vicinity. According to the model, the long-period event and harmonic tremor share the same source but differ in the boundary conditions for fluid flow and in the triggering mechanism setting up the resonance of the source, the former being viewed as the impulse response of the tremor generating system and the latter representing the excitation due to more complex forcing functions.

Journal of Geophysical Research Solid Earth↗
USGS2005

Interferometric synthetic aperture radar study of Okmok volcano, Alaska, 1992-2003: Magma supply dynamics and postemplacement lava flow deformation

Okmok volcano, located in the central Aleutian arc, Alaska, is a dominantly basaltic complex topped with a 10-km-wide caldera that formed circa 2.05 ka. Okmok erupted several times during the 20th century, most recently in 1997; eruptions in 1945, 1958, and 1997 produced lava flows within the caldera. We used 80 interferometric synthetic aperture radar (InSAR) images (interferograms) to study transient deformation of the volcano before, during, and after the 1997 eruption. Point source models suggest that a magma reservoir at a depth of 3.2 km below sea level, located beneath the center of the caldera and about 5 km northeast of the 1997 vent, is responsible for observed volcano-wide deformation. The preeruption uplift rate decreased from about 10 cm yr −1 during 1992–1993 to 2 ∼ 3 cm yr −1 during 1993–1995 and then to about −1 ∼ −2 cm yr −1 during 1995–1996. The posteruption inflation rate generally decreased with time during 1997–2001, but increased significantly during 2001–2003. By the summer of 2003, 30 ∼ 60% of the magma volume lost from the reservoir in the 1997 eruption had been replenished. Interferograms for periods before the 1997 eruption indicate consistent subsidence of the surface of the 1958 lava flows, most likely due to thermal contraction. Interferograms for periods after the eruption suggest at least four distinct deformation processes: (1) volcano-wide inflation due to replenishment of the shallow magma reservoir, (2) subsidence of the 1997 lava flows, most likely due to thermal contraction, (3) deformation of the 1958 lava flows due to loading by the 1997 flows, and (4) continuing subsidence of 1958 lava flows buried beneath 1997 flows. Our results provide insights into the postemplacement behavior of lava flows and have cautionary implications for the interpretation of inflation patterns at active volcanoes.

Alaska↗
USGS2012

Hydrous carbonates on Mars?: Evidence from Mariner 6/7 infrared spectrometer and ground‐based telescopic spectra

Absorption features at 2.28 and 5.4 μm identified in Mariner 6/7 infrared spectrometer and terrestrial telescopic spectra are consistent with the spectra of hydrous magnesium carbonates such as hydromagnesite and artinite. Spectral characteristics of these hydrous carbonates are different from those of the anhydrous carbonates, as the former do not have the strong spectral features typically associated with anhydrous carbonates such as calcite and siderite. Theoretical mixing indicates that, depending on the type of hydrous carbonate, 10–20 wt % can be incorporated into the regolith without contradicting the spectral observations or the Viking x ray fluorescence chemical analysis. Hydrous carbonates form as weathering products of mafic minerals in the presence of H 2 O and CO 2 , even in the Antarctic. Their formation as evaporite minerals from either original magmas or hydrothermally altered rocks is consistent with the Martian environment, provided liquid water is or has been at least transiently present. On Earth, formation of hydrous Mg carbonates is associated with the production of amorphous iron oxides, which is consistent with both the environment and the inferred surface mineralogy of Mars. These minerals are about 60 wt % H 2 O, CO 3 , and OH; if they are abundant everywhere at the 10% level, then about 6% of the surface weight could be volatiles bound in this type of mineral. Although the stability of hydrous carbonates in a Martian environment is uncertain, there may be kinetic factors inhibiting the dehydration of these minerals, which may persist metastably in the current environment. Although the spectroscopic evidence for anhydrous carbonates is scant, the possible presence of hydrous carbonates provides an appealing mechanism for the existence of carbonates on Mars.

Journal of Geophysical Research↗
USGS2009

The 2006-2007 Kuril Islands great earthquake sequence

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

Journal of Geophysical Research B: Solid Earth↗
USGS2016

Volcano deformation source parameters estimated from InSAR: Sensitivities to uncertainties in seismic tomography

The eruption cycle of a volcano is controlled in part by the upward migration of magma. The characteristics of the magma flux produce a deformation signature at the Earth's surface. Inverse analyses use geodetic data to estimate strategic controlling parameters that describe the position and pressurization of a magma chamber at depth. The specific distribution of material properties controls how observed surface deformation translates to source parameter estimates. Seismic tomography models describe the spatial distributions of material properties that are necessary for accurate models of volcano deformation. This study investigates how uncertainties in seismic tomography models propagate into variations in the estimates of volcano deformation source parameters inverted from geodetic data. We conduct finite element model-based nonlinear inverse analyses of interferometric synthetic aperture radar (InSAR) data for Okmok volcano, Alaska, as an example. We then analyze the estimated parameters and their uncertainties to characterize the magma chamber. Analyses are performed separately for models simulating a pressurized chamber embedded in a homogeneous domain as well as for a domain having a heterogeneous distribution of material properties according to seismic tomography. The estimated depth of the source is sensitive to the distribution of material properties. The estimated depths for the homogeneous and heterogeneous domains are 2666 ± 42 and 3527 ± 56 m below mean sea level, respectively (99% confidence). A Monte Carlo analysis indicates that uncertainties of the seismic tomography cannot account for this discrepancy at the 99% confidence level. Accounting for the spatial distribution of elastic properties according to seismic tomography significantly improves the fit of the deformation model predictions and significantly influences estimates for parameters that describe the location of a pressurized magma chamber.

Journal of Geophysical Research B: Solid Earth↗
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↗
USGS2022

Microgravity change during the 2008-2018 Kı̄lauea summit eruption: Nearly a decade of subsurface mass accumulation

Results from nine microgravity campaigns from Kı̄lauea, Hawaiʻi, spanning most of the volcano's 2008–2018 summit eruption, indicate persistent mass accumulation at shallow levels. A weighted least squares approach is used to recover microgravity results from a network of benchmarks around Kı̄lauea's summit, eliminate instrumental drift, and restore suspected data tares. A total mass of 1.9 × 10 11 kg was determined from these microgravity campaigns to have accumulated below Kı̄lauea Caldera during 2009–2015 at an estimated depth of 1.3 km below sea level. Only a fraction of this mass is reflected in surface deformation, and this is consistent with previously reported discrepancies between subsurface mass accumulation and observed surface deformation. The discrepancy, amongst other independent evidence from gas emissions, seismicity, and continuous gravimetry, indicate densification of magma in the reservoirs below the volcano summit. This densification may have been driven by degassing through the summit vent. It is hypothesized that during the final years of the summit eruption, magma densification resulted in a buildup of pressure in the reservoirs that may have contributed to the lower East Rift Zone outbreak of 2018. The observed mass accumulation beneath Kı̄lauea could not have been detected through other techniques and illustrates the importance of microgravity measurements in volcano monitoring.

Hawai'i↗
USGS1998

Poroelastic rebound along the Landers 1992 earthquake surface rupture

Maps of surface displacement following the 1992 Landers, California, earthquake, generated by interferometric processing of ERS-1 synthetic aperture radar (SAR) images, reveal effects of various postseismic deformation processes along the 1992 surface rupture. The large-scale pattern of the postseismic displacement field includes large lobes, mostly visible on the west side of the fault, comparable in shape with the lobes observed in the coseismic displacement field. This pattern and the steep displacement gradient observed near the Emerson-Camp Rock fault cannot be simply explained by afterslip on deep sections of the 1992 rupture. Models show that horizontal slip occurring on a buried dislocation in a Poisson's material produces a characteristic quadripole pattern in the surface displacement field with several centimeters of vertical motion at distances of 10–20 km from the fault, yet this pattern is not observed in the postseismic interferograms. As previously proposed to explain local strain in the fault step overs [ Peltzer et al ., 1996b], we argue that poroelastic rebound caused by pore fluid flow may also occur over greater distances from the fault, compensating the vertical ground shift produced by fault afterslip. Such a rebound is explained by the gradual change of the crustal rocks' Poisson's ratio value from undrained (coseismic) to drained (postseismic) conditions as pore pressure gradients produced by the earthquake dissipate. Using the Poisson's ratio values of 0.27 and 0.31 for the drained and undrained crustal rocks, respectively, elastic dislocation models show that the combined contributions of afterslip on deep sections of the fault and poroelastic rebound can account for the range change observed in the SAR data and the horizontal displacement measured at Global Positioning System (GPS) sites along a 60-km-long transect across the Emerson fault [ Savage and Svarc , 1997]. Using a detailed surface slip distribution on the Homestead Valley, Kickapoo, and Johnson Valley faults, we modeled the poroelastic rebound in the Homestead Valley pull apart. A Poisson's ratio value of 0.35 for the undrained gouge rocks in the fault zone is required to account for the observed surface uplift in the 3.5 years following the earthquake. This large value implies a seismic velocity ratio V p /V s of 2.1, consistent with the observed low V s values of fault zone guided waves at shallow depth [ Li et al ., 1997]. The SAR data also reveal postseismic creep along shallow patches of the Eureka Peak and Burnt Mountain faults with a characteristic decay time of 0.8 years. Coseismic, dilatant hardening (locking process) followed by post-seismic, pore pressure controlled fault creep provide a plausible mechanism to account for the decay time of the observed slip rate along this section of the fault.

Journal of Geophysical Research B: Solid Earth↗
USGS2019

Coseismic slip and early afterslip of the M6.0 August 24, 2014 South Napa, California, earthquake

We employ strong motion seismograms and static offsets from the Global Positioning System, Interferometric Synthetic Aperture Radar, and other measurements in order to derive a coseismic slip and afterslip model of the M6.0 24 August 2014 South Napa earthquake. This earthquake ruptured an ∼13‐km‐long portion of the West Napa fault with predominantly right‐lateral strike slip. In the kinematic seismic slip inversions, we couple the coseismic slip and afterslip distributions by requiring both distributions to involve right‐lateral strike slip with positive amplitude, with the net static slip being the sum of the two. We consider several candidate fault geometries: a first involving two steeply east dipping fault planes that reach Earth's surface at the western surface trace (STW), where most surface rupture was observed, a second involving a steeply west dipping plane that also reaches Earth's surface at the STW, and a third involving a combination of two variably west dipping planes constrained to pass through the locus of postseismic seismicity located ∼1 km west of the STW. The data are best fit using the model of two east dipping fault planes, with coseismic slip up to ∼1.2 m on a dominant shallow asperity about 10 km north of the hypocenter and on deeper asperities on the southern part of the rupture. Afterslip up to 1 m is concentrated along the southern part of the rupture at depths 5 km, consistent with surface observations of afterslip. Seismic moments associated with coseismic slip and afterslip are 1.13×10 18 N m (Mw 6.00) and 3.64×10 17 N m, respectively.

California↗
USGS2016

Coseismic slip and early afterslip of the 2015 Illapel, Chile, earthquake: Implications for frictional heterogeneity and coastal uplift

Great subduction earthquakes are thought to rupture portions of the megathrust, where interseismic coupling is high and velocity-weakening frictional behavior is dominant, releasing elastic deformation accrued over a seismic cycle. Conversely, postseismic afterslip is assumed to occur primarily in regions of velocity-strengthening frictional characteristics that may correlate with lower interseismic coupling. However, it remains unclear if fixed frictional properties of the subduction interface, coseismic or aftershock-induced stress redistribution, or other factors control the spatial distribution of afterslip. Here we use interferometric synthetic aperture radar and Global Position System observations to map the distribution of coseismic slip of the 2015 M w 8.3 Illapel, Chile, earthquake and afterslip within the first 38 days following the earthquake. We find that afterslip overlaps the coseismic slip area and propagates along-strike into regions of both high and moderate interseismic coupling. The significance of these observations, however, is tempered by the limited resolution of geodetic inversions for both slip and coupling. Additional afterslip imaged deeper on the fault surface bounds a discrete region of deep coseismic slip, and both contribute to net uplift of the Chilean Coastal Cordillera. A simple partitioning of the subduction interface into regions of fixed frictional properties cannot reconcile our geodetic observations. Instead, stress heterogeneities, either preexisting or induced by the earthquake, likely provide the primary control on the afterslip distribution for this subduction zone earthquake. We also explore the occurrence of coseismic and postseismic coastal uplift in this sequence and its implications for recent hypotheses concerning the source of permanent coastal uplift along subduction zones.

Journal of Geophysical Research B: Solid Earth↗