Seismic Coupling Research Articles

Roughness Characteristics of Oceanic Seafloor Prior to Subduction in Relation to the Seismogenic Potential of Subduction Zones

AbstractWe have developed a new approach to characterize the seafloor roughness seaward of the trenches, as a proxy for estimating the roughness of the subduction interface. We consider that abrupt elevation changes over given wavelengths play a larger role in the seismogenic behavior of the subduction interface than the amplitude of bathymetric variations alone. The new database, SubRough, provides roughness parameters at selected spatial wavelengths. Here we mainly discuss the spatial distribution of short‐ (12–20 km) and long‐wavelength (80–100 km) roughness, RSW and RLW, respectively, along 250‐km‐wide strips of seafloor seaward of the trenches. Compared with global trend, seamounts show distinct roughness signature of much larger amplitudes at both wavelengths, whereas aseismic ridges only differ from the global trend at long wavelengths. Fracture zones cannot be distinguished from the global trend, which suggests that their potential effect on rupture dynamics is not the consequence of their roughness, at least not at these wavelengths. Based on RLW amplitude, segments along subduction zones can be defined from rough to smooth. Subduction zones like the Solomons or the Ryukyus appear dominantly rough, whereas others like the Andes or Cascadia are dominantly smooth. The relative contribution of smooth versus rough areas in terms of respective lateral extents probably plays a role in multipatch rupture and thus in the final earthquake magnitude. We observe a clear correlation between high seismic coupling and relatively low roughness and conversely between low seismic coupling and relatively high seafloor roughness.

A “Geodetic Gap” in the Calabrian Arc: Evidence for a Locked Subduction Megathrust?

AbstractSubduction of old Ionian seafloor beneath the Calabrian Arc (southern Italy) is the geological process with the greatest mass flux in the central Mediterranean, yet its seismogenic behavior is largely obscured. No unambiguous evidence of subduction‐related earthquakes exists in historical times, and local GPS velocities indicate very low strain rates. Nevertheless, the region hosted some of the deadliest normal‐faulting earthquakes of the entire Mediterranean basin. We show that the low strain rates recorded in southern Calabria can be reconciled with the regional vigorous seismic moment release by assuming high interseismic coupling but low seismic coupling of the subduction interface. The alternative scenario of steadily creeping subduction cannot be ruled out but requires the historical seismicity record to be dismissed as unrepresentative. We refer to the peculiar spatial pattern of short‐term strain rates in southern Calabria as a “geodetic gap” resulting from destructive interference between upper‐plate extension and temporary compression due to locking along the subduction interface. Seismic hazard modelers must understand that within such gaps, the long‐term seismic hazard is greater than that suggested by the low geodetic strain rates.

Aseismic Transform Fault Slip at the Mendocino Triple Junction From Characteristically Repeating Earthquakes

AbstractThe Mendocino Triple Junction (MTJ), at the northern terminus of the San Andreas Fault system, is an actively deforming plate boundary region with poorly constrained estimates of seismic coupling on most offshore fault surfaces. Characteristically repeating earthquakes provide spatial and temporal descriptions of aseismic creep at the MTJ, including on the oceanic transform Mendocino Fault Zone (MFZ) as it subducts beneath North America. Using a dataset of earthquakes from 2008 to 2017, we find that the easternmost segment of the MFZ displays creep during this period at about 65% of the long‐term slip rate. We also find creep at slower rates on the shallower strike‐slip interface between the Pacific plate and the North American accretionary wedge, as well as on a fault that accommodates Gorda subplate internal deformation. After a nearby M5.7 earthquake in 2015, we observe a possible decrease in aseismic slip on the near‐shore MFZ that lasts from 2015 to at least early 2017.

Role of Equatorial Anomaly in Earthquake time precursive features: A few strong events over West Pacific zone

The earthquake (EQ) time coupling processes between equator-low-mid latitude ionosphere are complex due to inherent dynamical status of each latitudinal zone and qualified geomagnetic roles working in the system. In an attempt to identify such process, the paper presents temporal and latitudinal variations of ionization density (foF2) covering 45°N to 35°S, during a number of earthquake events (M > 5.5). The approaches adopted for extraction of features by the earthquake induced preparatory processes are discussed in the paper through identification of parameters like the ‘EQ time modification in density gradient’ defined by δ = (foF2 max − foF2 min)∕τmm, where τmm – time span (in days) between EQ modified density maximum and minimum, and the Earthquake time Equatorial Anomaly, i.e. EEA, one of the most significant phenomenon which develops even during night time irrespective of epicenter position. Based on the observations, the paper presents the seismic time coupling dynamics through anomaly like manifestations between equator, low and mid latitude ionosphere bringing in the global Total Electron Content (TEC) features as supporting indices.

Goce derived geoid changes before the Pisagua 2014 earthquake

The analysis of space – time surface deformation during earthquakes reveals the variable state of stress that occurs at deep crustal levels, and this information can be used to better understand the seismic cycle. Understanding the possible mechanisms that produce earthquake precursors is a key issue for earthquake prediction. In the last years, modern geodesy can map the degree of seismic coupling during the interseismic period, as well as the coseismic and postseismic slip for great earthquakes along subduction zones. Earthquakes usually occur due to mass transfer and consequent gravity variations, where these changes have been monitored for intraplate earthquakes by means of terrestrial gravity measurements. When stresses and correspondent rupture areas are large, affecting hundreds of thousands of square kilometres (as occurs in some segments along plate interface zones), satellite gravimetry data become relevant. This is due to the higher spatial resolution of this type of data when compared to terrestrial data, and also due to their homogeneous precision and availability across the whole Earth. Satellite gravity missions as GOCE can map the Earth gravity field with unprecedented precision and resolution. We mapped geoid changes from two GOCE satellite models obtained by the direct approach, which combines data from other gravity missions as GRACE and LAGEOS regarding their best characteristics. The results show that the geoid height diminished from a year to five months before the main seismic event in the region where maximum slip occurred after the Pisagua Mw = 8.2 great megathrust earthquake. This diminution is interpreted as accelerated inland-directed interseismic mass transfer before the earthquake, coinciding with the intermediate degree of seismic coupling reported in the region. We highlight the advantage of satellite data for modelling surficial deformation related to pre-seismic displacements. This deformation, combined to geodetical and seismological data, could be useful for delimiting and monitoring areas of higher seismic hazard potential.

Stability of faults with heterogeneous friction properties and effective normal stress

Abundant geological, seismological and experimental evidence of the heterogeneous structure of natural faults motivates the theoretical and computational study of the mechanical behavior of heterogeneous frictional fault interfaces. Fault zones are composed of a mixture of materials with contrasting strength, which may affect the spatial variability of seismic coupling, the location of high-frequency radiation and the diversity of slip behavior observed in natural faults. To develop a quantitative understanding of the effect of strength heterogeneity on the mechanical behavior of faults, here we investigate a fault model with spatially variable frictional properties and pore pressure. Conceptually, this model may correspond to two rough surfaces in contact along discrete asperities, the space in between being filled by compressed gouge. The asperities have different permeability than the gouge matrix and may be hydraulically sealed, resulting in different pore pressure. We consider faults governed by rate-and-state friction, with mixtures of velocity-weakening and velocity-strengthening materials and contrasts of effective normal stress. We systematically study the diversity of slip behaviors generated by this model through multi-cycle simulations and linear stability analysis. The fault can be either stable without spontaneous slip transients, or unstable with spontaneous rupture. When the fault is unstable, slip can rupture either part or the entire fault. In some cases the fault alternates between these behaviors throughout multiple cycles. We determine how the fault behavior is controlled by the proportion of velocity-weakening and velocity-strengthening materials, their relative strength and other frictional properties. We also develop, through heuristic approximations, closed-form equations to predict the stability of slip on heterogeneous faults. Our study shows that a fault model with heterogeneous materials and pore pressure contrasts is a viable framework to reproduce the full spectrum of fault behaviors observed in natural faults: from fast earthquakes, to slow transients, to stable sliding. In particular, this model constitutes a building block for models of episodic tremor and slow slip events.

Source of high tsunamis along the southernmost Ryukyu trench inferred from tsunami stratigraphy

Four paleotsunamis deposits are exposed in a trench on the coastal lowland north of the southern Ryukyu subduction zone trench. Radiocarbon ages on coral and bivalve shells show that the four deposits record tsunamis date from the last 2000yrs., including a historical tsunami with a maximum run-up of 30m in 1771, for an average recurrence interval of approximately 600yrs. Ground fissures in a soil beneath the 1771 tsunami deposit may have been generated by stronger shaking than recorded by historical documents. The repeated occurrence of the paleotsunami deposits supports a tectonic source model on the plate boundary rather than a nontectonic source model, such as submarine landslides. Assuming a thrust model at the subduction zone, the seismic coupling ratio may be as low as 20%.

Postdoc at NCPA: Explosive growth and entertainment

My postdoctorate from August 1988 to December 1991 at the University of Mississippi and NCPA favorably influenced my career. Thinking back on the cast of characters and the dramatic science being pursued there brings a smile. My main mentors were Dr. James Sabatier, Dr. Richard Raspet, and Dr. Henry Bass. I was involved with theory and experiments for acoustic to seismic coupling; land mine detection; nondestructive measurements of soil properties; and fundamental analysis of Celcor extruded ceramic catalysts as materials for thermoacoustic stacks. We often worked in real-world settings—cotton and soybean fields, and at “Audi Acres” where we used massive low frequency speaker elevated with construction scaffolding and a winch as sound sources. I worked hours with Mike the machinist constructing acoustical resonators and thermoacoustic heat engines. The vibrant social and political scene was something to behold—including early morning jogging in the Faulkner woods, regular basketball games at lunch; visiting politicians and program managers, dancing on tables at the Gin; Friday nights at Dr. Bass’ house; visitors from acoustic institutions around the world; Starnes catfish restaurant.

Crustal Structure beneath the Kashmir Basin Adjoining the Western Himalayan Syntaxis

We present a crustal shear‐wave velocity model for the intermontane Kashmir valley derived from eight broadband seismic stations located on hard‐rock sites surrounding and within the valley. Receiver functions at these sites were calculated using the iterative time‐domain deconvolution method of Ligorria and Ammon (1999) jointly inverted with fundamental‐mode Rayleigh‐wave group velocity dispersion data to estimate the underlying shear‐wave velocity structure. The inverted Moho depths were further constrained within ±2 km by forward modeling and conform to those independently estimated using H‐κ (crustal thickness vs. VP/VS⁠) stacks. The Moho descends steeply (⁠>15° NE⁠) beneath the Pir Panjal range from a depth of ∼40 km south of the Main Central thrust to ∼58 km on the southwestern flank of the valley and undergoes an ∼4 km upwarp beneath the valley, thinning the overlying crust. Further northeast (NE) of the valley, the Moho dips gently (⁠∼3°⁠) beneath the Zanskar range. A persistent low‐velocity zone at a depth of ∼12–16 km is interpreted as a sheared zone associated with the decollement separating Himalayan rocks from the Indian plate. Relocated seismicity from the International Seismological Centre (ISC, 2013) catalog (1964–2013), together with a small number of local earthquakes recorded by our network, is apparently confined south of the NE edge of the valley, suggesting that the transition of the Indian plate from locked decollement to aseismic creep lies near here. This result is consistent with the geodetic findings that a broad zone of partial seismic coupling exists beneath the Kashmir valley.

Mapping subduction interface coupling using magnetotellurics: Hikurangi margin, New Zealand

AbstractThe observation of slow‐slip, seismic tremor, and low‐frequency earthquakes at subduction margins has provided new insight into the mechanisms by which stress accumulates between large subduction (megathrust) earthquakes. However, the relationship between the physical properties of the subduction interface and the nature of the controls on interplate seismic coupling is not fully understood. Using magnetotelluric data, we show in situ that an electrically resistive patch on the Hikurangi subduction interface corresponds with an area of increased coupling inferred from geodetic data. This resistive patch must reflect a decrease in the fluid or sediment content of the interface shear zone. Together, the magnetotelluric and geodetic data suggest that the frictional coupling of this part on the Hikurangi margin may be controlled by the interface fluid and sediment content: the resistive patch marking a fluid‐ and sediment‐starved area with an increased density of small, seismogenic‐asperities, and therefore a greater likelihood of subduction earthquake nucleation.

Mapping fluids to subduction megathrust locking and slip behavior

AbstractIn subduction zones, high fluid content and pore pressure are thought to promote aseismic creep, whereas well‐drained conditions are thought to promote locking and failure in earthquakes. However, observations directly linking fluid content and seismic coupling remain elusive. Heise et al. (2017, https://doi.org/10.1002/2017GL074641) use a magnetotelluric survey to image the electrical resistivity structure of the northern Hikurangi subduction thrust to ~30 km depth, as an indicator of interconnected fluid content. The authors document a clear correlation between high resistivity and a distinct geodetically locked patch and between conductive areas and weak coupling. Their study, together with other recent geophysical investigations, provides new evidence for the role of fluids in governing subduction thrust locking.

Objective estimation of spatially variable parameters of epidemic type aftershock sequence model: Application to California

AbstractThe Epidemic Type Aftershock Sequence (ETAS) model is widely employed to model the spatiotemporal distribution of earthquakes, generally using spatially invariant parameters. We propose an efficient method for the estimation of spatially varying parameters, using the expectation maximization (EM) algorithm and spatial Voronoi tessellation ensembles. We use the Bayesian information criterion (BIC) to rank inverted models given their likelihood and complexity and select the best models to finally compute an ensemble model at any location. Using a synthetic catalog, we also check that the proposed method correctly inverts the known parameters. We apply the proposed method to earthquakes included in the Advanced National Seismic System catalog that occurred within the time period 1981–2015 in a spatial polygon around California. The results indicate significant spatial variation of the ETAS parameters. We find that the efficiency of earthquakes to trigger future ones (quantified by the branching ratio) positively correlates with surface heat flow. In contrast, the rate of earthquakes triggered by far‐field tectonic loading or background seismicity rate shows no such correlation, suggesting the relevance of triggering possibly through fluid‐induced activation. Furthermore, the branching ratio and background seismicity rate are found to be uncorrelated with hypocentral depths, indicating that the seismic coupling remains invariant of hypocentral depths in the study region. Additionally, triggering seems to be mostly dominated by small earthquakes. Consequently, the static stress change studies should not only focus on the Coulomb stress changes caused by specific moderate to large earthquakes but also account for the secondary static stress changes caused by smaller earthquakes.

Change in seismicity along the Japan trench, 1990–2011, and its relationship with seismic coupling

AbstractWe investigate the temporal evolution of the background seismicity rate related to the subduction of the Pacific plate in northeast Japan, at latitudes 34° to 42°, for the 1 January 1990 to 9 March 2011 period. Two declustering methods are used to identify robust features. We find that the dominant behavior is a lowering down of activity, especially in the northern half of our studied area, where changes appear related to the cycle of M7.5+ earthquakes, in particular the 1968 Tokachi and the 1994 Sanriku earthquakes. Acceleration of background seismicity is observed offshore the Kanto region and could mark a long‐term decoupling of the Pacific and the Philippine Sea plates over a 100 km long segment along the Sagimi Trough, which cannot be simply explained by the earthquake cycle model. Our analysis further suggests that changes in background seismicity are plausibly related to changes in seismic coupling and thus further strengthens the recent observation that seismic coupling does vary at the timescales of tens of years, for the Japanese subduction zone.

Trench dynamics: Effects of dynamically migrating trench on subducting slab morphology and characteristics of subduction zones systems

Understanding the mechanisms of trench migration (retreat or advance) is crucial to characterizing the driving forces of Earth’s tectonics plates, the origins of subducting slab morphologies in the deep mantle, and identifying the characteristics of subduction zones systems, which are among the fundamental issues of solid Earth science. A series of numerical simulations of mantle convection, focusing on plate subduction in a three-dimensional (3-D) regional spherical shell coordinate system, was performed to examine subduction zone characteristics, including geodynamic relationships among trench migration, back-arc stress, and slab morphology. The results show that a subducting slab tends to deflect around the base of the mantle transition zone and form a sub-horizontal slab because its front edge (its ‘toe’) is subject to resistance from the highly viscous lower mantle. As the sub-horizontal slab starts to penetrate into the lower mantle from its ‘heel,’ the toe of the slab is drawn into the lower mantle. The results for models with dynamically migrating trenches suggest that trench retreat is the dynamically self-consistent phenomenon in trench migration. The reason for this is that the strong lateral mantle flow that is generated as a sequence of events leading from corner flow at the subduction initiation to return flow of the formation of a sub-horizontal slab in the shallower part of mantle wedge produces the retreat of the subducting slab. In fact, a ‘mantle suction force,’ which is generated in the mantle wedge to fill space left by the retreating subducting plate, is enhanced by the subsequent trench retreat. Even when upwelling flow with significant positive buoyancy originates just above a mantle phase boundary at a depth of 410km (as inferred from independent seismic tomographic, geodynamic, geochemical, and mineral physics), reaches the base of the overriding plate, and the overriding plate is slightly thinned, lithospheric stress tends to be compressed above the upwelling flow. The reason for this is that the strong lateral mantle flow originating from the upwelling flow generates resistance drag force at the base of the overriding plates. This situation may apply to a case of East Asia, under which the typical morphology of sub-horizontal slabs can be seen by seismic tomography. The strong lateral velocity observed in the shallower mantle wedge in the present numerical simulation may account for both the compressional subduction tectonics and back arc compression in the Japan–Kuril–Kamchatka, Aleutian, and South Chile trenches, as well as for weak plate-slab coupling, strong seismic coupling, and the possibility of great earthquakes along these trenches.

Slow and fast ruptures on a laboratory fault controlled by loading characteristics

AbstractRecent geodetic observations indicate that a single fault may slip slowly and silently or fast and seismically in different circumstances. We report laboratory experiments that demonstrate how the mode of faulting can alternate between fast “stick‐slip” events (70 to ~500 mm/s sliding rates) and slow silent events (~0.001 to 30 mm/s) as a result of loading conditions rather than friction properties or stiffness of the loading machine. The 760 mm long granite sample is close to the critical nucleation length scale for unstable slip, so small variations in nucleation properties result in measurable differences in slip events. Slow events occur when instability cannot fully nucleate before reaching the sample ends. Dynamic events occur after long healing times or abrupt increases in loading rate which suggests that these factors shrink the spatial and temporal extents of the nucleation zone. Arrays of slip, strain, and ground motion sensors installed on the sample allow us to quantify seismic coupling and study details of premonitory slip and afterslip. We find that seismic coupling decreases when slip rates fall below about 70 mm/s. The slow slip events we observe are primarily aseismic (less than 1% of the seismic coupling of faster events) and produce swarms of very small M −6 to M −8 events. These mechanical and seismic interactions suggest that faults with transitional behavior—where creep, small earthquakes, and tremor are often observed—could become seismically coupled if loaded rapidly, either by a slow slip front or dynamic rupture of an earthquake that nucleated elsewhere.

Seismicity characterization of the Maravatío-Acambay and Actopan regions, central Mexico

We studied the seismic activity in the Maravatío-Acambay and Actopan regions in Central Mexico. These regions are of great importance due to the occurrence of shallow crustal normal-faulting earthquakes that caused widespread destruction near their epicenter and as far away as Mexico City. That was the case of the 19 November 1912 Acambay (Mw 6.9). We determined statistical seismicity characteristics such as the Båth's law (the size of largest aftershock with respect to that of the mainshock), the b-value and p-value. For the Maravatío aftershock sequence, we obtained a b-value of 0.88 and p-value of 0.68. Based on reported seismicity, we obtained a b-value of 1.12 for the Actopan region. We estimated the size of the largest aftershock of the Acambay event in the range of 4.7 <Mw < 5.9. By using the fragment-asperity model, we also performed the analysis of the magnitude distribution. The analysis showed similar values for the non-extensivity parameter (q-value): q = 1.6465 and q = 1.6555 for the Maravatío aftershock sequence and for the seismicity in the Actopan region, respectively. The similitude in the q-values could be associated with the similarities in the tectonic environment. In this model, the constant of proportionality (a) between released energy and the size of fragments showed that more energy is released for the Maravatío aftershock sequence than for the regular seismicity rate in the Acambay region. Finally, we analyzed the relation of the seismicity and the tectonic environment by quantifying the seismic coupling and the thickness of the seismogenic layer. The estimated seismogenic layer for the Maravatío-Acambay and Actopan regions are 14.6 km and 20.8 km, respectively. The seismic coupling coefficient at the Venta the Bravo fault in the Maravatío-Acambay region and Actopan region are 0.21 and 0.46, respectively. Our estimation of the seismic coupling coefficients shows that the regions can be classified as low-to-intermediate-coupling zones.

Seismic performance of new-type box steel bridge piers with embedded energy-dissipating shell plates under tri-directional seismic coupling action

Accurate numerical models are necessary to evaluate the seismic performance and load-bearing mechanism of new-type box steel bridge piers with embedded energy-dissipating shell plates under tri-directional seismic coupling action. Numerical simulations of seismic performance under six types of tri-directional seismic coupling action was conducted. The effects of this stress on the seismic performance of the new-type steel bridge piers was evaluated through analysis of damage mode, hysteresis curve, skeleton curve, stiffness and strength degradation characteristic, and energy-dissipating capacity. This study also compared the numerical analysis with experimental results in order to validate the accuracy of the proposed finite element model. Based on this model, the range of relevant parameters expanded and 88 numerical specimens were analysed for seismic performance, producing further information about the influence of thickness and curvature of the embedded shell plate, spacing of transverse stiffening ribs of the shell, axial compression ratio, and slenderness ratio. Results showed that tri-directional seismic coupling action significantly affects the specimen’s deformation capacity; the embedded shell plate effectively improves the piers’ load-carrying and deformation capacity; and the thickness of the embedded shell plate, width-to-thickness ratio of the wall plate, axial compression ratio, and slenderness ratio significantly affect the seismic performance of the new-type steel bridge piers. To promote the ease of seismic design of new-type box steel bridge piers, this study used theoretical analysis and numerical simulation to calculate equations for the minimum height of the energy-dissipating zone of the bottom embedded shell plate. Finally, formulas were also established to calculate the relevant stability bearing capacity and displacement ductility factor of the new-type steel bridge piers under tri-directional seismic coupling action in order to improve their seismic design.

From the seismic cycle to long-term deformation: linking seismic coupling and Quaternary coastal geomorphology along the Andean megathrust

Measurement of interseismic strain along subduction zones reveals the location of both locked asperities, which might rupture during Megathrust earthquakes, and creeping zones, which tend to arrest such seismic ruptures. The heterogeneous pattern of interseismic coupling presumably relates to spatial variations of frictional properties along the subduction interface and may also show up in the forearc morphology. To investigate this hypothesis, we compiled information on the extent of earthquake ruptures for the last 500 years and uplift rates derived from dated marine terraces along the South American coastline from central Peru to Southern Chile. We additionally calculated a new interseismic coupling model for that same area based on a compilation of GPS data. We show that the coastline geometry, characterized by the distance between the coast and the trench, the latitudinal variations of long-term uplift rates and the spatial pattern of interseismic coupling are correlated. Zones of faster and long-term permanent coastal uplift, evidenced by uplifted marine terraces, coincide with peninsulas, and also with areas of creep on the megathrust where slip is mostly aseismic and tend to arrest seismic ruptures. We conclude that spatial variations of frictional properties along the megathrust dictate the tectono-geomorphological evolution of the coastal zone and the extent of seismic ruptures along strike.

Assessing the seismic coupling of shallow continental faults and its impact on seismic hazard estimates: a case-study from Italy

Assessing the seismic coupling of shallow continental faults and its impact on seismic hazard estimates: a case-study from Italy

Dynamic Characteristics Analysis of a Seismic Vibrator-Ground Coupling System

A mathematical model to describe the seismic vibrator and ground coupling is proposed based on dynamic analyses of the vibrator baseplate structure and properties of soil. This mathematical model is solved using Newmark’s method. It produces reasonable results and the trend of amplitude envelope follows the experimental data. With this model, characterization of the vibrator-ground coupling is performed. Modal analysis study shows that the motions between the vibrator reaction mass and baseplate experience a combination of a first-order vibration mode and a second-order vibration mode. The corresponding frequencies of two vibration modes are located at 2.77 Hz and 488.7 Hz, respectively. As the frequency goes up, the motion behavior of the reaction mass and baseplate is dominated by the second-order vibration mode. It is realized that the second-order vibration mode is responsible for the phase difference between the input sweep signal and the output force signal. Furthermore, the impact of the coupling system parameters on the vibrator output force is also investigated. It is observed that the vibrator output force decreases as the sweep frequency increases. The weight ratio of the reaction mass and the baseplate has an impact on the vibrator output force.