September 2014
Co- and Postseismic motion of a landslide; observations, modelling and analogy with tectonic faults.
Abstract
We document the first time‐series of a landslide reactivation by an earthquake, using continuous GPS measurements over the Maca landslide (Peru). Our survey shows a coseismic response of the landslide of about 2 cm, followed by a relaxation period of 5 weeks during which postseismic slip is three times greater than the coseismic displacement itself. Our results confirm the coseismic activation of landslides and provide the first observation of a postseismic displacement. These observations are consistent with a mechanical model where slip on the landslide basal interface is governed by rate and state friction, analogous to the mechanics of creeping tectonic faults, opening new perspectives to study the mechanics of landslides and active faults.
Foreshock activity related to enhanced aftershock production
Abstract
Foreshock activity sometimes precedes the occurrence of large earthquakes, but the nature of this seismicity is still debated, and whether it marks transient deformation and/or slip nucleation is still unclear. We here study at the world wide scale how foreshock occurrence affects the post‐seismic phase, and find a significant positive correlation between foreshock and aftershock activities: earthquakes preceded by accelerating seismicity rates produce 40 % more aftershocks on average, and the length of the aftershock zone after 20 days is 20 % larger. These observations cannot be reproduced by standard earthquake clustering models that predict the accelerating pattern of foreshock occurrence but not its impact on aftershock activity. This strongly suggests that slow deformation transients, possibly related to episodic creep, could initiate prior to the mainshock and extend past the co‐seismic phase, resulting in compound ruptures that include a very long period (up to tens of days) component.
Spatiotemporal analysis and interpretation of 1993–2013 ground deformation at Campi Flegrei, Italy, observed by advanced DInSAR
Abstract
Campi Flegrei is one of the most hazardous volcanic areas in the world because of its close proximity to the city of Naples. Here we apply the multidimensional small baseline subset (MSBAS) differential interferometric synthetic aperture radar (DInSAR) technique to obtain vertical and horizontal components of ground deformation for Campi Flegrei at high spatial and temporal resolutions that span, for the first time, 20 years. The area underwent continuous subsidence from 1993 through 1999. Moderate uplift began in 2010 and substantially increased through 2012, reaching approximately 13 cm by 2013. We model the observed deformation to determine source parameters for subsidence and uplift epochs. Both the inflation and deflation mechanisms involve large, extended sources in a layered hydrothermal system whose location is controlled by the caldera structure and stratigraphy. The temporal resolution of MSBAS approaches that of GPS daily time series, with superior precision and spatial resolution, making it an excellent alternative for volcano monitoring.
Removal of systematic seasonal atmospheric signal from interferometric synthetic aperture radar ground deformation time series
Abstract
Applying the Multidimensional Small Baseline Subset interferometric synthetic aperture radar algorithm to about 1500 Envisat and RADARSAT‐2 interferograms spanning 2003–2013, we computed time series of ground deformation over Naples Bay Area in Italy. Two active volcanoes, Vesuvius and Campi Flegrei, are located in this area in close proximity to the densely populated city of Naples. For the first time, and with remarkable clarity, we observed decade‐long elevation‐dependent seasonal oscillations of the vertical displacement component with a peak‐to‐peak amplitude of up to 3.0 cm, substantially larger than the long‐term deformation rate (<0.6 cm/yr). Analysis, utilizing surface weather and radiosonde data, linked observed oscillations with seasonal fluctuations of water vapor, air pressure, and temperature in the lower troposphere. The modeled correction is in a good agreement with observed results. The mean, absolute, and RMS differences are 0.014 cm, 0.073 cm, and 0.087 cm, respectively. Atmospherically corrected time series confirmed continuing subsidence at Vesuvius previously observed by geodetic techniques.
October 2013
Evidence of active mantle flow beneath South China
" The India-Eurasia collision is responsible for producing the Himalayan Mountains and Tibetan plateau and has been hypothesized to have significant far field influences, including driving the Baikal rift and the eastward extrusion of South China. However, quantification of lithospheric buoyancy forces and integrated effect of tractions acting at base of the lithosphere are unable to explain the observed surface motions within South China. We present 198 new SKS shear wave splitting observations beneath South China and invert these data along with published GPS data to solve for the subasthenospheric flow field beneath South China to assess the role of small-scale convection here. We find a 15–20 mm/yr southwestward-directed mantle flow toward the Burma slab. This flow is consistent with the mantle response of slab retreat over the past 25 Ma, and counter flow due to subduction of Burma/Sunda slabs demonstrating the importance of localized mantle convection on present-day plate motions. "
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" We numerically simulate slow slip events (SSEs) in the Shikoku region of Japan, incorporating the configuration of the subducting plate. We adopt a rate- and state-dependent friction law with cutoff velocities, assuming a frictional parameter distribution based on observed long-term SSEs and nonvolcanic tremors that reflects the slip of short-term SSEs. Our model reproduces recurrences of long- and short-term SSEs and segments of short-term SSEs. In our simulation, short-term SSEs' transition from episodic to continuous slip is reproduced. This feature is consistent with tremor activity recently reported in both Shikoku and Cascadia. In addition to the long-term SSEs in the Bungo Channel, our model also reproduces newly found long-term SSEs in central Shikoku and predicts that these SSEs recur during the interseismic period between megathrust earthquakes. Our model comprehensively reproduces various SSEs and their characteristics as reported in the Shikoku region. "
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" The Mojave Neovolcanic Province (MNVP), located in the Mojave block of southern California, comprises late Miocene to Quaternary small-volume basaltic centers. Geochemistry indicates an asthenospheric source for the MNVP beginning in the late Miocene, but no physical evidence of missing mantle lithosphere has been presented. We utilize receiver functions and ambient noise tomography to image the lithosphere beneath the Mojave block. Regionally, we find thin crust that thickens distal to sites of MNVP volcanism. Shear wave velocities between 40 and 75 km depth are consistent with the presence of mantle lithosphere in the southern Mojave block and very thin or missing mantle lithosphere to the north. With one exception, MNVP volcanoes lie along this sharp boundary. Our observations, together with the established geologic history and geochemistry of the MNVP, can be explained by small-scale edge-driven convection producing ongoing lithospheric removal within the Mojave block. Our results provide another example of lithospheric instability that occurs in response to rapid changes in mantle dynamics induced by major changes in tectonic plate geometry. "
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Spatial and temporal patterns of simulated slow slip events on the Cascadia megathrust
" In recent years, it has been discovered that sections of the subduction interface slip aseismically in slow slip events, during which stress is intermittently transferred to the section of the subduction zone that generates large or great earthquakes. Within the Cascadia subduction zone, the magnitude and frequency of SSEs and accompanying tectonic tremor exhibit complex patterns that vary systematically with depth. However, the loading mechanisms and interactions that precede great subduction earthquakes are poorly understood. Here we present results from physics-based simulations that reproduce the continuum of SSE characteristics reported for the Cascadia subduction zone. The simulations provide a basis for understanding the interactions that control both the observed complex patterns of SSEs and stress transfer to the seismogenic section that produce great earthquakes. "
Recovering coseismic point ground tilts from collocated high-rate GPS and accelerometers
" Rotational along with translational and strain measurements are essential for a complete description of the motion of a deformable body in a seismic event. We propose a new seismogeodetic approach where collocated high-rate GPS and accelerometer measurements are combined to estimate permanent and dynamic coseismic ground tilts at a point, whereas at present, only dynamic tilts are measured with either a dense seismic array or an expensive ring laser gyroscope. We estimate point tilts for a five-story structure on a shake table subjected to 13 earthquake strong motion records of increasing intensity. For the most intense record from the 2002 M7.9 Denali earthquake, we observe a peak-to-peak dynamic tilt of 0.12° and a permanent tilt of 0.16° for the structure's roof. Point tilts derived from networks of collocated GPS and accelerometers can be used to estimate the rotational component of the seismic wavefield for improved earthquake source characterization. "
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Modeling dynamic triggering of tectonic tremor using a brittle-ductile friction model
" We study the physics of dynamically triggered tectonic tremor by applying a brittle-ductile friction model in which we conceptualize the tremor source as a rigid block subject to driving and frictional forces. To simulate dynamic triggering of tremor, we apply a stress perturbation that mimics the surface waves of remote earthquakes. The tectonic and wave perturbation stresses define a phase space that demonstrates that both the timing and amplitude of the dynamic perturbations control the fundamental characteristics of triggered tremor. Tremor can be triggered instantaneously or with a delayed onset if the dynamic perturbation significantly alters the frictional state of the tremor source. "
" Scaling relations for seismic moment M0, rupture area S, average slip D, and asperity size Sa were obtained for large, great, and giant (Mw = 6.7–9.2) subduction-zone earthquakes. We compiled the source parameters for seven giant (Mw~9) earthquakes globally for which the heterogeneous slip distributions were estimated from tsunami and geodetic data. We defined Sa for subfaults exhibiting slip greater than 1.5 times D. Adding 25 slip models of 10 great earthquakes around Japan, we recalculated regression relations for 32 slip models: S = 1.34 × 10−10 M02/3, D = 1.66 × 10−7 M01/3, Sa = 2.81 × 10−11 M02/3, and Sa/S = 0.2, where S and Sa are in square kilometers, M0 is in newton meters, and D is in meters. These scaling relations are very similar to those obtained by Murotani et al. (2008) for large and great earthquakes. Thus, both scaling relations can be used for future tsunami hazard assessment associated with a giant earthquake. "
Characterization of nucleation during laboratory earthquakes
" We observe the nucleation phase of in-plane ruptures in the laboratory. We show that the nucleation is composed of two distinct phases, a quasi-static and an acceleration stage, followed by dynamic propagation. We propose an empirical model which describes the rupture length evolution: The quasi-static phase is described by an exponential growth while the acceleration phase is described by an inverse power law of time. The transition from quasi-static to accelerating rupture is related to the critical nucleation length, which scales inversely with normal stress in accordance with theoretical predictions, and to a critical surfacic power, which may be an intrinsic property of the interface. Finally, we discuss these results in the frame of previous studies and propose a scaling up to natural earthquake dimensions. "
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Shape evolution and finite deformation pattern in analog experiments of lithosphere necking
" Lithosphere necking evolution determines the 3-D architecture of crustal and upper mantle thinning and related basins, and the heat flow distribution in rifted regions. Despite a large number of studies, lithosphere necking evolution is still a matter of debate. We present the result from lithospheric-scale analog models designed for investigating the necking shape during extension and the vertical distribution of finite deformation in the mechanical lithosphere. In our experiments, lithosphere necking is asymmetric and, in particular, the 3-D distribution of thinning is cylindrical in the crust and very heterogeneous in the mantle. Overall, the evolution of rifting and necking progresses from delocalized to localized deformation. "
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" We derive a finite slip model for the 2013 Mw 8.3 Sea of Okhotsk Earthquake (Z = 610 km) by inverting calibrated teleseismic P waveforms. The inversion shows that the earthquake ruptured on a 10° dipping rectangular fault zone (140 km × 50 km) and evolved into a sequence of four large sub-events (E1–E4) with an average rupture speed of 4.0 km/s. The rupture process can be divided into two main stages. The first propagated south, rupturing sub-events E1, E2, and E4. The second stage (E3) originated near E2 with a delay of 12 s and ruptured northward, filling the slip gap between E1 and E2. This kinematic process produces an overall slip pattern similar to that observed in shallow swarms, except it occurs over a compressed time span of about 30 s and without many aftershocks, suggesting that sub-event triggering for deep events is significantly more efficient than for shallow events. "
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September 2013
" A recent global study has revealed that seismicity near the hypocenter prior to large earthquakes, which could be a proxy for preseismic moment rate, accelerates before interplate earthquakes, while it rarely does before intraplate earthquakes. Understanding the amplitude of preseismic deformation is important in assessing the possibility of its detection. For a class of rate-state friction laws without a characteristic speed-related parameter (e.g., aging law and slip law), a dimensional analysis has shown that if the moment rate increases more mildly than 1/tf where tf is the time-to-failure, then the amplitude of preseismic moment rate is smaller for a smaller quasistatic slip rate. Three-dimensional numerical simulations have revealed that the aging law yields 1/tf acceleration, while the slip law causes milder acceleration. If the latter is the case, faults with very low long-term slip rates (e.g., intraplate faults) may have very small preseismic moment rates. "
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" Active and passive seismic experiments in the Kii Peninsula, southwest Japan, revealed prominent structural features around the segment boundary of a megathrust earthquake associated with the subduction of the Philippine Sea Plate (PHS). A distinct reflection band in the uppermost part of the PHS shows significant lateral variation along its strike. The thicker reflection band, which corresponds to the deeper extension of the fault area of the 1944 Tonankai Earthquake, is interpreted to be a zone of high pore fluid pressure, causing a state of conditionally stable slip on the plate boundary by the reduction in effective normal stress. A thinner reflective band corresponds to the deepest part of the rupture area of the 1946 Nankai earthquake, where plate coupling is stronger due to less effect of fluids. This along-strike structural variation controls the differences in frictional properties and the lateral limit of rupturing along the plate boundary. "
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Permanently enhanced dynamic triggering probabilities as evidenced by two M ≥ 7.5 earthquakes
" The 2012 M7.7 Haida Gwaii earthquake radiated waves that likely dynamically triggered the 2013 M7.5 Craig earthquake, setting two precedents. First, the triggered earthquake is the largest dynamically triggered shear failure event documented to date. Second, the events highlight a connection between geologic structure, sedimentary troughs that act as waveguides, and triggering probability. The Haida Gwaii earthquake excited extraordinarily large waves within and beyond the Queen Charlotte Trough, which propagated well into mainland Alaska and likely triggering the Craig earthquake along the way. Previously, focusing and associated dynamic triggering have been attributed to unpredictable source effects. This case suggests that elevated dynamic triggering probabilities may exist along the many structures where sedimentary troughs overlie major faults, such as subduction zones’ accretionary prisms and transform faults’ axial valleys. Although data are sparse, I find no evidence of accelerating seismic activity in the vicinity of the Craig rupture between it and the Haida Gwaii earthquake. "
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Two-dimensional viscosity structure of the northeastern Japan islands arc-trench system
"Two-dimensional viscosity profiles were constructed for the northeastern Japan islands arc-trench system covering the source area of the 2011 Tohoku-Oki earthquake. From seismologically determined models of lithospheric structure, experimentally derived constitutive laws of various rocks, and densely measured geothermal gradient data, we have predicted the steady state effective viscosity across the subduction zone. The profile reveals strong lateral viscosity gradients both parallel and normal to the trench axis. The detailed viscosity structures presented here contribute to accurate evaluation of viscoelastic relaxation components when modeling geodetically measured postseismic deformation at high spatial and temporal resolution."
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Characteristics of a tsunamigenic megasplay fault in the Nankai Trough
"Slip on the shallow part of a megasplay fault that is an out-of-sequence thrust and branch of the main subduction plate boundary can cause devastating tsunamis after earthquakes. We analyzed the three-dimensional geometry, including dip amount and azimuths, roughness distributions, and thickness variations, of the shallow part of a megasplay fault in the Nankai Trough using a three-dimensional seismic data set. The fault is divided into three zones based on its geometry: thick, smooth, and simply convex in the east; complexly curved in the middle; and thin and kinked in the west. Results of scientific drilling indicate that the eastern region of the fault is most active, and local heterogeneities in fault geometry, including roughness and thickness, may control the slip on this part of the fault. The present findings can be used to evaluate the risk of future tsunamis arising from movement on shallow thrust faults at subduction margins."
Earthquake early warning for southern Iberia: A P wave threshold-based approach
" The south of the Iberian Peninsula is a region in which large, damaging earthquakes occur separated by long time intervals. An example was the great 1755 Lisbon earthquake (intensity Imax = X) which occurred SW of San Vicente Cape (SW Iberian Peninsula). Due to this risk of damaging earthquakes, the implementation of Earthquake Early Warning System (EEWS) technologies is of considerable interest. With the aim of investigating the feasibility of an EEWS in this region of the Iberian Peninsula, empirical scaling relationships have been derived between the early warning parameters and the earthquake size and/or its potential damaging effects for this region. An appropriate and suitable strategy is proposed for an EEWS in the SW Iberian Peninsula, which takes into account the limitations of the existing seismological networks. "
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"Upper mantle fabric, as detected by shear wave splitting measurements, records lithospheric strain history that may be related to past tectonic events. We present SKS (core-refracted shear wave) splitting measurements taken around the western Great Lakes, North America: the Archean Superior Province (SP) and surroundings. We analyze 40 sites in the USA and 5 in Canada and outline a region of strong anisotropy corresponding to a high-velocity zone in tomographic models, which we dub the Western Superior Mantle Anomaly (WSMA) and interpret to represent a preserved fabric from accretion of the SP. To the southwest of the WSMA, we locate a region of rotated fast direction corresponding to a linear low-velocity feature observed by tomography and which may represent a failed branch of the Mid-Continent Rift. Further south, within the Minnesota River Valley Terrane, the split times drop to near zero, suggesting isotropic lithosphere resulting from vertical (diapiric) rather than horizontal tectonic processes."
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August 2013
New ultrahigh-resolution picture of Earth's gravity field
We provide an unprecedented ultrahigh resolution picture of Earth's gravity over all continents and numerous islands within ±60° latitude. This is achieved through augmentation of new satellite and terrestrial gravity with topography data and use of massive parallel computation techniques, delivering local detail at ~200 m spatial resolution. As such, our work is the first-of-its-kind to model gravity at unprecedented fine scales yet with near-global coverage. The new picture of Earth's gravity encompasses a suite of gridded estimates of gravity accelerations, radial and horizontal field components, and quasi-geoid heights at over 3 billion points covering 80% of Earth's land masses. We identify new candidate locations of extreme gravity signals, suggesting that the Committee on Data for Science and Technology standard for peak-to-peak variations in free-fall gravity is too low by about 40%. The new models are beneficial for a wide range of scientific and engineering applications and freely available to the public.
Bias in estimates of lithosphere viscosity from interseismic deformation
The estimation of uniform viscosities representing the lower crust and uppermost mantle from postseismic or interseismic deformation (i.e., apparent viscosities) is inherently biased with respect to a depth dependence of the viscosities within each layer. Estimates are biased toward a more viscous lower crust or a less viscous lithospheric mantle, depending on the relative geometric mean viscosities of the two layers. When there is a low-viscosity shear zone beneath the fault, apparent viscosities are close to that of the shear zone immediately after the earthquake, although the apparent viscosities increase significantly during the later interseismic period. Inferences made from interseismic deformation that the lower crust is more viscous than the upper mantle may be entirely consistent with depth-dependent viscosity profiles that have a significant increase in viscosity from the lowermost crust to the uppermost mantle.
We provide a new link between tectonic tremor propagation, tremor amplitude, and tidal stresses by analyzing high-resolution tremor locations and amplitudes determined by multibeam backprojection of data from an array of subarrays. For two Cascadia episodic tremor and slip events, we observe repeating, high-amplitude rapid tremor reversals (RTRs) and tremor streaks. They tend to occur when tremor amplitudes are highest and occur almost exclusively during periods of thrust-encouraging, tidally induced shear stress on the fault. We speculate that thrust-encouraging shear stress from tidal loading forces trigger RTRs and streaks that energetically rerupture the weakened fault behind the slow slip front. The high rate and amplitude of tremor during RTRs and streaks stands in contrast to the hypothesis that activity at the leading edge of the slow slip zone is the most energetic and loudest. This implies that the spatiotemportal pattern of slow earthquake slip migration is even more intricate than previously reported.
Slab tears and intermediate-depth seismicity
Active tectonic regions where plate boundaries transition from subduction to strike slip can take several forms, such as triple junctions, acute, and obtuse corners. Well-documented slab tears that are associated with high rates of intermediate-depth seismicity are considered here: Gibraltar arc, the southern and northern ends of the Lesser Antilles arc, and the northern end of Tonga trench. Seismicity at each of these locations occurs, at times, in the form of swarms or clusters, and various authors have proposed that each marks an active locus of tear propagation. The swarms and clusters start at the top of the slab below the asthenospheric wedge and extend 30–60 km vertically downward within the slab. We propose that these swarms and clusters are generated by fluid-related embrittlement of mantle rocks. Focal mechanisms of these swarms generally fit the shear motion that is thought to be associated with the tearing process.
Scattered waves from low-frequency earthquakes and plate boundary structure in northern Cascadia
We use 3-D waveform modeling of LFEs (low-frequency earthquakes) to investigate their relation to plate boundary structure along a linear transect in northern Cascadia. To account for crustal velocity heterogeneity, a smoothed 3-D model of subduction zone structure is assembled that incorporates constraints from regional tomographic and plate boundary models. Scattered phases within LFE waveforms are identified based on synthetic predictions that incorporate thrust mechanisms aligned parallel to a dipping plate boundary atop a high-Vp/Vs low-velocity zone (LVZ). Scattering for near-vertical paths is dominated by S-to-P/S-to-S reflections/conversions from the LVZ. The modeling suggests that LFEs lie at or very close to the plate boundary and that the LVZ structure is laterally heterogeneous but broadly consistent with results from teleseismic analysis.
We analyze the mechanical properties needed to account for the large shallow slip during the 2011 Tohoku-Oki earthquake and the activation of landward normal faulting within the forearc. We show that the morphology and internal structure of the forearc follows closely the prediction of the critical Coulomb wedge in horizontal compression, implying a high internal pore pressure ratio (λ=0.7+0.14/−0.48) and a low effective basal friction. We then show that the activation of the normal fault requires a lower effective basal friction beneath the outer wedge than beneath the inner wedge (μouter≤0.015), possibly due to transient dynamic weakening associated to the seismic rupture. Forearc normal faults could be considered as evidence for very efficient dynamic weakening along the megathrust and typify megathrust with high tsunamigenic potential.
Teleseismic body wave analysis revealed that the 7 December 2012 off-Sanriku earthquake (MW 7.3) at the outer rise of Japan Trench consisted of two successive subevents. The first subevent with reverse-fault mechanism (Event 1, MW 7.1) at 56 km depth was followed by, approximately 20 s later, the second subevent with normal-fault mechanism (Event 2, MW 7.2) at 6 km depth. Finite-fault slip models show that the slip of Event 1 was concentrated around the initial rupture point with the maximum of 2.7 m and that Event 2 had two asperities with the maximum of 4.5 m at both sides of the initial rupture point. The static Coulomb Failure Function analyses suggested that Event 1 triggered Event 2 and that both subevents were promoted by the 2011 Tohoku earthquake (MW 9.1).
Stable, rapid rate of slip since inception of the San Jacinto fault, California
In California, where the San Jacinto fault (SJF) and San Andreas fault (SAF) accommodate the majority of the dextral shear deformation between the Pacific and North American plates, initiation of the SJF led to an apparent decline in the slip rate of the SAF. Previous studies suggest that since then, slip rate has covaried between these faults (possibly due to changes in fault strength, variation in topographic loading along a fault, or the development of new faults) and that presently the SJF is the dominant plate boundary structure. However, we dated displaced sedimentary deposits and landforms over three distinct time intervals since ~700 ka, and our results imply a constant slip rate of 12.1+3.4/−2.6 mm/yr. This rate is similar to the fault's lifetime rate and from rates derived from geodesy, suggesting that since the SJF initiated, its slip rate has remained relatively stable and does not exceed that of the SAF.
Interseismic coupling, stress evolution, and earthquake slip on the Sunda megathrust
The extent to which interseismic coupling controls the slip distribution of large megathrust earthquakes is unclear, with some authors proposing that it is the primary control and others suggesting that stress changes from previous earthquakes are of first-order importance. Here, we develop a detailed stress history of the Sunda megathrust, modified by coupling, and compare the correlation between slip and stress with that of slip versus coupling. We find that the slip distributions of recent earthquakes are more consistent with the stress field than with the coupling distributions but observe that in places, the stress pattern is strongly dependent on poorly constrained values of slip in historical earthquakes. We also find that of the 13 earthquakes in our study for which we have hypocentral locations, only two appear to have nucleated in areas of negative stress, and these locations correspond to large uncertainties in the slip distribution of pre-instrumental events.
We investigate the stick-slip behavior of a granular system confined and sheared by deformable solid blocks using three-dimensional discrete element method simulations. Our modeling results show that large slip events are preceded by a sequence of small slip events—microslips—whose occurrence accelerates exponentially before the large slip event onset. Microslips exhibit energy release several orders of magnitude smaller than the large slip events. The microslip event rate is proposed as a measure of slip activity in the granular gouge layer. A statistical analysis shows that microslip event rate correlates well with large slip event onset and that variations in it can be used to predict large slip events. The emergence of microslips and their duration are found to be controlled by the value of the slipping contact ratio and are therefore related to the jamming/unjamming transition of frictional granular packings.
Convergence of the frequency-magnitude distribution of global earthquakes: Maybe in 200 years
study the ability to estimate the tail of the frequency-magnitude distribution of global earthquakes. While power-law scaling for small earthquakes is accepted by support of data, the tail remains speculative. In a recent study, Bell et al. (2013) claim that the frequency-magnitude distribution of global earthquakes converges to a tapered Pareto distribution. I show that this finding results from data fitting errors, namely from the biased maximum likelihood estimation of the corner magnitude θin strongly undersampled models. In particular, the estimation of θdepends solely on the few largest events in the catalog. Taking this into account, I compare various state-of-the-art models for the global frequency-magnitude distribution. After discarding undersampled models, the remaining ones, including the unbounded Gutenberg-Richter distribution, perform all equally well and are, therefore, indistinguishable. Convergence to a specific distribution, if it ever takes place, requires about 200 years homogeneous recording of global seismicity, at least.
Influences of temperature-dependent thermal conductivity on surface heat flow near major faults
We studied the thermomechanical effects on surface heat flow near major faults from positive feedback between temperature-dependent thermal conductivity k(T) ∝ (1/T)b and frictional heating in a crust-lithosphere system using finite element simulations. Variable conductivity and frictional heating cause a drastic reduction in the thermal conductivity, and these changes can impact the heat flux. When b = 1, the temperature is 400 K higher around the fault than in the uniform conductivity case. This is caused by the reduction in thermal conductivity. In spite of the high temperature around the fault in the variable conductivity cases, the surface heat flux is 30% (for b = 0.5) to 50% (for b = 1) lower than in the uniform conductivity case. This thermal insulating effect may explain the lack of heat flow anomalies near major faults and is consistent with previous hypotheses about the nature of the shear strength associated with these faults.
Using core complex geometry to constrain fault strength
We present the first model results showing that some core complex detachment faults are strong and that their strength has to be in a narrow range to allow certain extensional structures to develop. The structures we simulate are kilometer-scale “rider blocks” that are particularly well observed on some oceanic core complexes as well as continental metamorphic core complexes. Previous numerical simulations of lithospheric extension produced the large-offset, core complex-forming, normal faults only when the faults were weaker than a given threshold. However, our new, high-resolution simulations indicate that rider blocks only result when the faults are stronger than a given level. A narrow range of fault weakening, relative to intact surrounding rock, allows for a consecutive series of rider blocks to emerge in a core complex-like geometry. Our results show that rider blocks develop when the dominant form of weakening is by reduction of fault cohesion while faults that weaken primarily by friction reduction do not form distinct rider blocks.
Magma dynamics feeding Yasur's explosive activity observed using thermal infrared remote sensing
A thermal infrared thermometer was used to record the passage of hot gases and fragments across a measurement area located at the exit of one of Yasur's active vents. Recording was completed over 2 h during September 2011. A total of 200 explosive events were recorded. We define two types of event: low-energy events with typical thermal energies of 14 kJ and high-energy events with typical thermal energies of 97 kJ. Around 180 low-energy events were recorded, which together released 457 kJ of radiant energy. In contrast, only 20 high-energy events were recorded, but they released 2042 kJ. We suggest that low-energy events originate from a relatively shallow, degassed magma reservoir and are associated with bursting of bubbles formed by bubble coalescence during ascent. Instead, high-energy events originate from a relatively deep, fresh magma reservoir and are associated with slugs formed by foam collapse.
Real-time detection and precise estimation of strong ground motion are crucial for rapid assessment and early warning of geohazards such as earthquakes, landslides, and volcanic activity. This challenging task can be accomplished by combining GPS and accelerometer measurements because of their complementary capabilities to resolve broadband ground motion signals. However, for implementing an operational monitoring network of such joint measurement systems, cost-effective techniques need to be developed and rigorously tested. We propose a new approach for joint processing of single-frequency GPS and MEMS (microelectromechanical systems) accelerometer data in real time. To demonstrate the performance of our method, we describe results from outdoor experiments under controlled conditions. For validation, we analyzed dual-frequency GPS data and images recorded by a video camera. The results of the different sensors agree very well, suggesting that real-time broadband information of ground motion can be provided by using single-frequency GPS and MEMS accelerometers.