Subduction Zone Earthquakes Research Articles

Duration Models for Subduction Earthquakes

ABSTRACT An empirical model for the significant duration of acceleration for subduction zone earthquakes is developed based on the normalized Arias intensity. The data set is a subset from the Next Generation Attenuation-Subduction database (Bozorgnia and Stewart, 2020). The functional form of the duration model uses an additive model for the source, path, and site terms with three lognormally distributed random effects for the source, path, and site. The remaining aleatory variability is modeled by a power-normal distribution. There are two parts of the duration model: a model for the D5–75 duration and a model for the D5–X/D5–75 ratio, which can be combined with the D5–75 model to estimate the DY–X for any interval. The D5–75 model includes regionalized coefficients for Japan, New Zealand, South America, and Taiwan. The large-magnitude (M >7) scaling of the D5–75 model is much weaker than the recent duration model by Bahrampouri et al. (2021). The residuals of the D5–75 model are negatively correlated with the lnPGA residuals with a correlation coefficient of −0.50 such that above-average peak ground acceleration values are associated with below-average duration values.

Evidence for Low Effective Stress Within the Crust of the Subducted Gorda Plate from the 2022 December Mw 6.4 Ferndale Earthquake Sequence

Abstract Stress levels on and adjacent to megathrust faults at seismogenic depths remain a key but difficult-to-constrain parameter for assessing seismic hazard in subduction zones. Although strong ground motions have been observed to be generated from distinct, high-stress regions on the down-dip end of the megathrust rupture areas in many great earthquakes, we lack direct constraints on the stress level in the lower seismogenic portion of the Cascadia megathrust. On 20 December 2022, an Mw 6.4 strike-slip earthquake occurred near Ferndale, California, in southern Cascadia and likely ruptured the Gorda slab crust in the lower seismogenic portion, providing an opportunity to assess the stress level in this region. Here, we relocate the Ferndale mainshock and the first two weeks of aftershocks using a high-resolution 3D velocity model and estimate rupture dimensions, directivity, and stress drop for several Mw 4–5 aftershocks and recent earthquakes. The aftershocks define a strike-slip fault in the slab crust striking east-northeast, consistent with the mainshock focal mechanism. The orientation of this fault is about 45° off the ideally oriented fault plane given the stress state in the slab. The aftershock zone is extensive and broad in the forward direction of the mainshock rupture but still constrained within the volume of high VP/VS in the slab crust. Our stress-drop estimates are generally lower for Mw 4–5 earthquakes located in the slab crust compared to those a few kilometers deeper in the slab mantle. Combined, our results support a relatively low effective stress level in the vicinity of the megathrust in the lower portion of the seismogenic zone in southern Cascadia, likely due to elevated fluid pressures. Consequently, the ground motion in the onshore region above this low-stress seismogenic portion in southern Cascadia may not be as intense as that observed during great earthquakes in other subduction zones.

Use of tsunami observations for resolving coseismic slip of recent and historic large submarine earthquakes

Tsunamis generated by seafloor displacements accompanying large submarine earthquakes provide sensitivity to absolute slip position and distribution for offshore faulting analogous to that of geodetic observations for landward faulting. Tsunami recordings at deep‐water and near‐shore ocean bottom pressure sensors and tide gauges, along with runup and inundation measurements, can now be reliably modeled using detailed bathymetric structures and robust numerical codes. As a result, tsunami observations now play an important role in quantifying coseismic slip distributions for large submarine earthquakes in subduction zones and other tectonic environments. Applications of joint modeling or inversion of seismic, geodetic and tsunami observations for recent major earthquakes are described, highlighting the specific contributions of the tsunami observations to source model resolution. Tsunami observations provide unique information on the up‐dip extent of earthquake coseismic slip on subduction zone megathrust faults and occurrence of near‐trench slip, which are usually not well constrained by seismic and land‐based geodetic signals. Tsunami signals also help to detect offshore slow slip that is not evident in seismic or land‐based geodetic data and to balance geophysical constraints on ruptures that extend from on‐shore to off‐shore. Tsunami runup measurements and stratigraphic deposits further provide unique constraints on large earthquake ruptures that occurred prior to modern geophysical instrumentation.

Frictional Strength and Frictional Instability of Glaucophane Gouges at Blueschist Temperatures Support Diverse Modes of Fault Slip From Slow Slip Events to Moderate‐Sized Earthquakes

AbstractFluid overpressure from the water released by subducted sediments and oceanic crust is an important mechanism for generating earthquakes via brittle failure and frictional instability. If unstable, such fault materials may also host diverse fault reactivation mechanisms from slow slip events to moderate‐sized earthquakes in cold subduction zones. We examine this hypothesis for glaucophane gouge ‐ a key index minerals for blueschist facies ‐ at lower confining stresses where behavior is poorly understood. We measure friction and stability at temperatures of 100°C–500°C and effective normal stresses of 50–200 MPa, to explore the controls of temperature, stresses and excess pore fluid pressures on fault friction. The frictional coefficient of glaucophane at representative temperatures and stresses is ∼0.70, insensitive to temperature but with a slight increase at lower effective stresses. Elevating temperature promotes a transition from velocity‐strengthening to weak velocity‐weakening behavior, indicating the destabilizing effect of high temperature downdip in subduction zones. Reducing effective normal stress or concomitantly elevating pore fluid pressure further strengthens the velocity‐weakening response and would be manifest as moderate‐sized earthquakes. Our results support the potential for enhanced unstable sliding of glaucophane gouges at lower effective stresses and blueschist facies temperatures ‐ with weak to moderate velocity‐weakening responses upon fluid pressurization vital for understanding the abundance of slow slip to moderate‐sized earthquakes apparent in cold subduction zones.

Assessing the Optimal Tsunami Inundation Modeling Strategy for Large Earthquakes in Subduction Zones

Assessing the Optimal Tsunami Inundation Modeling Strategy for Large Earthquakes in Subduction Zones

Transformational faulting in Mn2GeO4 from olivine to wadsleyite structure: Implications for physical mechanism of deep-focus earthquakes

High-pressure and temperature deformation experiments interfaced with acoustic emission (AE) monitoring have been conducted to study transformational faulting in Mn2GeO4 olivine, which transforms to the β phase, isostructural to wadsleyite. Metastable Mn2GeO4 olivine exhibits a marked embrittlement behavior at temperatures between 800 and 1100 K, emitting numerous AEs. At each temperature, brittle deformation is characterized by a two-stage process: (1) a “preparation” stage with numerous diffusedly located low-magnitude AEs and large b values (>2), and (2) a failure stage where larger-magnitude AEs form a planar distribution with b values about 1. Microstructure analysis reveals extensive kink band development in olivine grains in the recovered samples. Kink band boundaries (KBBs), with a typical thickness of ∼100 nm, are filled with a nanometric β-Mn2GeO4 “gouge”. A dense array of secondary shear localizations is often present within the kink bands, suggesting significant shear deformation therein. The combined observations suggest that faulting in metastable Mn2GeO4 olivine is a self-similar process, from grain-scale to the sample-scale. Both observed embrittlement behavior and the microstructure of metastable Mn2GeO4 olivine are essentially identical to those in Mg2GeO4 olivine we have reported previously, indicating that the physical mechanism of faulting in metastable olivine is insensitive to the specific crystallographic structure of the high-pressure phase. The low b values (about 1) observed in the faulting process in our experiments are similar to those of deep focus earthquakes in cold subduction zones. Our observed mechanism explains deep focus seismicity in cold metastable mantle wedges, provided that the self-similarity assumption holds to geological scales.

Subduction Zone Geometry Modulates the Megathrust Earthquake Cycle: Magnitude, Recurrence, and Variability

AbstractMegathrust geometric properties exhibit some of the strongest correlations with maximum earthquake magnitude in global surveys of large subduction zone earthquakes, but the mechanisms through which fault geometry influences subduction earthquake cycle dynamics remain unresolved. Here, we develop 39 models of sequences of earthquakes and aseismic slip (SEAS) on variably‐dipping planar and variably‐curved nonplanar megathrusts using the volumetric, high‐order accurate code tandem to account for fault curvature. We vary the dip, downdip curvature and width of the seismogenic zone to examine how slab geometry mechanically influences megathrust seismic cycles, including the size, variability, and interevent timing of earthquakes. Dip and curvature control characteristic slip styles primarily through their influence on seismogenic zone width: wider seismogenic zones allow shallowly‐dipping megathrusts to host larger earthquakes than steeply‐dipping ones. Under elevated pore pressure and less strongly velocity‐weakening friction, all modeled fault geometries host uniform periodic ruptures. In contrast, shallowly‐dipping and sharply‐curved megathrusts host multi‐period supercycles of slow‐to‐fast, small‐to‐large slip events under higher effective stresses and more strongly velocity‐weakening friction. We discuss how subduction zones' maximum earthquake magnitudes may be primarily controlled by the dip and dimensions of the seismogenic zone, while second‐order effects from structurally‐derived mechanical heterogeneity modulate the recurrence frequency and timing of these events. Our results suggest that enhanced co‐ and interseismic strength and stress variability along the megathrust, such as induced near areas of high or heterogeneous fault curvature, limits how frequently large ruptures occur and may explain curved faults' tendency to host more frequent, smaller earthquakes than flat faults.

Geoarchaeological Record of the AD 1700 Cascadia Subduction Zone Earthquake and Tsunami at the Salmon River Wet Site, Central Oregon Coast

Geoarchaeological Record of the AD 1700 Cascadia Subduction Zone Earthquake and Tsunami at the Salmon River Wet Site, Central Oregon Coast

The Impact of the Three-Dimensional Structure of a Subduction Zone on Time-dependent Crustal Deformation Measured by HR-GNSS

Accurately modeling time-dependent coseismic crustal deformation as observed on high-rate Global Navigation Satellite System (HR-GNSS) lends insight into earthquake source processes and improves local earthquake and tsunami early warning algorithms. Currently, time-dependent crustal deformation modeling relies most frequently on simplified 1D radially symmetric Earth models. However, for shallow subduction zone earthquakes, even low-frequency shaking is likely affected by the many strongly heterogeneous structures such as the subducting slab, mantle wedge, and the overlying crustal structure. We demonstrate that including 3D structure improves the estimation of key features of coseismic HR-GNSS time series, such as the peak ground displacement (PGD), the time to PGD (tPGD), static displacements (SD), and waveform cross-correlation values. We computed synthetic 1D and 3D, 0.25 Hz and 0.5 Hz waveforms at HR-GNSS stations for four M7.3+ earthquakes in Japan using MudPy and SW4, respectively. From these synthetics, we computed intensity-measure residuals between the synthetic and observed GNSS waveforms. Comparing 1D and 3D residuals, we observed that the 3D simulations show better fits to the PGD and SD in the observed waveforms than the 1D simulations for both 0.25 Hz and 0.5 Hz simulations. We find that the reduction in PGD residuals in the 3D simulations is a combined effect of both shallow and deep 3D structures; hence incorporating only the upper 30 km of 3D structure will still improve the fit to the observed PGD values. Our results demonstrate that 3D simulations significantly improve models of GNSS waveform characteristics and will not only help understand the underlying processes, but also improve local tsunami warning.

Low‐Frequency Earthquakes Downdip of Deep Slow Slip Beneath the North Island of New Zealand

AbstractWe report the first catalog of low‐frequency earthquakes in the Hikurangi subduction zone, located beneath the Kaimanawa Range of the North Island at 50 km depth, downdip of regularly recurring (every 4–5 years) deep M7 slow slip events. To systematically detect low‐frequency earthquakes within the regional continuous seismic data, we utilized a matched‐filter approach with template waveforms derived from previous observations of tectonic tremor. We built our catalog of 36 low‐frequency earthquake sources, that produced almost 21,000 events over more than a decade, with two matched‐filter search iterations. In each iteration, the detections were gathered into families and their coherent waveforms processed and stacked to extract high‐quality waveforms, allowing us to pick seismic phase arrivals to locate the low‐frequency earthquakes. We highlight three characteristic features to validate that our detected events are indeed low‐frequency earthquakes: the eponymous deficit of high frequencies in their seismic waveforms, the episodic swarms of activity that define their activity through time, and their location at the plate boundary with a double‐couple source mechanism and geometry consistent with the subduction interface. Considering the observed low‐frequency earthquakes' relationship to neighboring slow slip, we observe the event swarms to occur much more frequently than the M7 slow slip events located just updip. Similar to other deep low‐frequency earthquakes in other subduction zones, we suggest that this characteristic clustering in time is driven by more frequent, smaller slow slip events that are not clearly observable at the surface.

The importance of diatoms for understanding subduction zone earthquakes in Alaska

The importance of diatoms for understanding subduction zone earthquakes in Alaska

Testing Megathrust Rupture Models Using Tsunami Deposits

AbstractThe 26 January 1700 CE Cascadia subduction zone earthquake ruptured much of the plate boundary and generated a tsunami that deposited sand in coastal marshes from northern California to Vancouver Island. Although the depositional record of tsunami inundation is extensive in some of these marshes, few sites have been investigated in enough detail to map the inland extent of sand deposition and depict variability in tsunami deposit thickness and grain size. We collected 129 cores in marshes of the Salmon River estuary in Oregon and reanalyzed 114 core logs from a 1987–88 study that mapped the inland extent of circa 1700 CE sandy tsunami deposits. The ca. 1700 CE tsunami deposit in the Salmon River estuary is easily recognized in cores ≤1 m deep in which a buried marsh peat is overlain by a well sorted sand bed with a sharp lower contact that thins and fines inland. We use tsunami deposit data and models of sandy tsunami sediment transport (using Delft3D‐FLOW) to test 15 rupture models that could represent a ca. 1700 CE earthquake. At least 12–16 m of slip offshore of the Salmon River, which results in 0.8–1.0 m of coastal coseismic subsidence, is required to match the ca. 1700 CE sand deposit's inland extent, which is consistent with models of heterogeneous megathrust slip in ca. 1700 CE. Our methods of detailed tsunami deposit mapping, combined with sediment transport modeling, can be used to test models of megathrust ruptures and their tsunamis to potentially improve earthquake and tsunami hazard assessments.

Late Quaternary Surface Displacements on Accretionary Wedge Splay Faults in the Cascadia Subduction Zone: Implications for Megathrust Rupture

Because splay faults branch at a steep dip angle from the plate-boundary décollement in an accretionary wedge, their coseismic displacement can potentially result in larger tsunamis with distinct characteristics compared to megathrust-only fault ruptures, posing an enhanced hazard to coastal communities. Elsewhere, there is evidence of coseismic slip on splay faults during many of the largest subduction zone earthquakes, but our understanding of potentially active splay faults and their hazards at the Cascadia subduction zone remains limited. To identify the most recently active splay faults at Cascadia, we conduct stratigraphic and structural interpretations of near-surface deformation in the outer accretionary wedge for the ~400 km along-strike length of the landward vergence zone. We analyze recently acquired high-frequency sparker seismic data and crustal-scale multi-channel seismic data to examine the record of deformation in shallow slope basins and the upper ~1 km of the surrounding accreted sediments and to investigate linkages to deeper décollement structure. We present a new fault map for widest, most completely locked portion of Cascadia from 45 to 48°N latitude, which documents the distribution of faults that show clear evidence of recent late Quaternary activity. We find widespread evidence for active splay faulting up to 30 km landward of the deformation front, in what we define as the active domain, and diminished fault activity landward outside of this zone. The abundance of surface-deforming splay faults in the active outer wedge domain suggests Cascadia megathrust events may commonly host distributed shallow rupture on multiple splay faults located within 30 km of the deformation front.

Linking 3D Long‐Term Slow‐Slip Cycle Models With Rupture Dynamics: The Nucleation of the 2014 Mw 7.3 Guerrero, Mexico Earthquake

AbstractSlow slip events (SSEs) have been observed in spatial and temporal proximity to megathrust earthquakes in various subduction zones, including the 2014 Mw 7.3 Guerrero, Mexico earthquake which was preceded by a Mw 7.6 SSE. However, the underlying physics connecting SSEs to earthquakes remains elusive. Here, we link 3D slow‐slip cycle models with dynamic rupture simulations across the geometrically complex flat‐slab Cocos plate boundary. Our physics‐based models reproduce key regional geodetic and teleseismic fault slip observations on timescales from decades to seconds. We find that accelerating SSE fronts transiently increase shear stress at the down‐dip end of the seismogenic zone, modulated by the complex geometry beneath the Guerrero segment. The shear stresses cast by the migrating fronts of the 2014 Mw 7.6 SSE are significantly larger than those during the three previous episodic SSEs that occurred along the same portion of the megathrust. We show that the SSE transient stresses are large enough to nucleate earthquake dynamic rupture and affect rupture dynamics. However, additional frictional asperities in the seismogenic part of the megathrust are required to explain the observed complexities in the coseismic energy release and static surface displacements of the Guerrero earthquake. We conclude that it is crucial to jointly analyze the long‐ and short‐term interactions and complexities of SSEs and megathrust earthquakes across several (a)seismic cycles accounting for megathrust geometry. Our study has important implications for identifying earthquake precursors and understanding the link between transient and sudden megathrust faulting processes.

Earth's Free Surface Complicates Inference of Absolute Stress From Earthquake‐Induced Stress Rotations

AbstractThe stress redistribution from an earthquake can produce localized measurable rotations of the principal stress axes if the absolute level of differential stress in the crust is on the order of the earthquake stress drop. Two simple analytic solutions have been developed to estimate the differential stress from an observed stress rotation. However, each has assumptions that may not be accurate near Earth's free surface. I model synthetic earthquakes in an elastic half‐space, and show that the assumptions of the methods are accurate for strike‐slip earthquakes, and for deep dip‐slip earthquakes. However, they are incorrect for shallow dip‐slip earthquakes. I introduce a free surface correction for one of the methods for dip‐slip earthquakes. I revise an analysis of stress rotations due to great subduction zone earthquakes, including this correction. The results support the original conclusion of near complete stress drop for many shallow subduction zone earthquakes.

Nonergodic Ground-Motion Models for Subduction Zone and Crustal Earthquakes in Japan

ABSTRACT We investigate the nonergodic behavior of ground motions from subduction zone earthquakes and crustal earthquakes in Japan using the National Research Institute for Earth Science and Disaster Resilience strong-motion flatfile together with various reference ergodic ground-motion models (GMMs). For the nonergodic path effects, our nonergodic GMM has two path effects terms: a cell-specific linear-distance scaling, denoted as δP2PQ, that mimics the effects of a 3D Q structure, and a source- and site-specific term related to the effects of the 3D velocity structure, denoted δP2PV. The resulting model provides spatially varying nonergodic source, site, and path terms. The GMM smoothly interpolates and extrapolates the nonergodic terms in space so that the GMM can be applied to any combination of source and site locations in the region. In regions where data are sparse, the median nonergodic terms will approach zero but with large epistemic uncertainty. Over the period range of 0–10 s, the ranges of the standard deviations of the nonergodic source, path, and site terms are 0.2–0.65, 0.25–0.40, and 0.15–0.40 natural log units, respectively. The fully nonergodic models lead to a 40%–55% decrease in the aleatory standard deviation compared to the reference ergodic GMMs. This large reduction in the aleatory standard deviation combined with the change in the median given by the nonergodic terms can exert a significant impact on the computed seismic hazard for the Japan region.

Cyclic Resistance Models for Transitional Silts with Application to Subduction Zone Earthquakes

Cyclic Resistance Models for Transitional Silts with Application to Subduction Zone Earthquakes

Geodynamic processes in subduction zones and interpretation of grace measurement data

The paper discusses the results of studies of the process of preparing catastrophic earthquakes in the subduction zone in combination with the interpretation of measurement data from the GRACE space system (CS) regarding these zones. An analysis of digital maps of gravity anomalies in the form of the spatial distribution of the EWH parameter relative to the corresponding focal areas of the strongest subduction earthquakes that occurred over the past 20 years was performed. It is shown using examples of studying the strongest earthquakes in subduction zones that GRACE CS measurement data can serve as the basis for assessing the stress-strain state of the geoenvironment. At the same time these data make it possible to increase the accuracy and reliability of solving prognostic problems. In particular the productivity of the trigger and barycenter concepts has been confirmed in relation to the identified features of the strongest earthquakes.

In situ study of the reaction phase A plus high-P clinoenstatite to forsterite plus water at reduced water activity

Abstract. We examined the reaction phase A plus high-P clinoenstatite to forsterite plus water (Reaction R1) by means of in situ X-ray diffraction measurements with the large volume press at the synchrotron PETRA III, Hamburg. Contrary to the study of Lathe et al. (2022), in which all experiments on Reaction (R1) were performed at a water activity of 1, the reversed experiments presented in this study were performed at reduced water activity with mole fractions of about XH2O= XCO2=0.5. The intention of this investigation was to test the observation made by Perrillat et al. (2005), which was that dehydration reactions are kinetically faster at reduced than under water-saturated conditions. The position of Reaction (R1) at the reduced conditions was determined by reversal brackets at 9.1 and 9.5 GPa (630 and 700 ∘C), at 9.7 and 10.0 GPa (725 and 700 ∘C), at 9.8 and 10.2 GPa (675 and 750 ∘C), and at 10.5 GPa (675 and 740 ∘C). Additionally, we performed two offline experiments with brackets at 10.0 and 10.6 GPa (750 and 700 ∘C, respectively) that are in agreement with the results of the in situ experiments. We do not observe any “intermediate” precursor phase in our experiments. The equilibrium of Reaction (R1) is shifted by about 100 ∘C to lower temperature compared to the results under water-saturated conditions. Thus, at a water activity (aH2O) below 1 the phase A plus clinoenstatite dehydration reaction can only occur in extremely cold subduction slabs. The kinetics of Reaction (R1) dehydration at reduced water activity is slower than that determined previously by Lathe et al. (2022) under water-saturated conditions. Thus, the above-mentioned hypothesis of Perrillat et al. (2005) could not be confirmed. However, in both of our studies on Reaction (R1), the newly formed dehydration product forsterite was of nanometer size, which supports earlier experimental observations, which is that product phases of dehydration reactions are generally very fine-grained and might promote the concept that intermediate-depth earthquakes in subduction zones are initiated by mechanical instabilities from extremely fine-grained materials formed during dehydration reactions.

Characteristics of ring-shaped seismicity at depths up to 110 km prior to large and great earthquakes in subduction zones of the Pacific

We have been considering some seismicity characteristics at depths up to 110 km prior to 38 large and great earthquakes (Mw=7.0-9.0) in subduction zones of the Pacific. We revealed ring-shaped seismicity structures in three depth ranges: 0-33, 34-70 and 71-110 km. These structures are being formed during a few decades prior to large and great events with hypocenters at depths of 0-40 and 42-110 km. We call these events conditionally as shallow and deep ones correspondingly. The structures are characterized by threshold magnitude values: Mt1, Mt2 and Mt3 respectively. We analyzed differences of Mt1-Mt2, Mt2-Mt3 and Mt1-Mt3 values. It was established that parameters Mt2-Mt3 and Mt1-Mt3 are higher considerably for large shallow earthquakes in comparison to deep ones. Besides that, we found differences of mean Mt1-Mt2 values at the west and east of Pacific. We discuss the reasons of ring-shaped structures formation which most likely are connected with dehydration of the subducting plate material and deep-seated fluid migration. We estimate роssibilities of depths prognosis for preparing large earthquakes using characteristics of ring-shaped seismicity structures. The data obtained are essential for shaking intensity forecast and also for tsunami danger estimate prior to large and great events in subduction zones.