Low-frequency Earthquakes Research Articles

Massive formation of lawsonite in subducted sediments from the Schistes Lustrés (W. Alps): Implications for mass transfer and decarbonation in cold subduction zones

This study investigates the reactions allowing crystallization of large amounts of lawsonite (CaAl2Si2O7(OH)2•[H2O]) found in calcschists metamorphosed in a subduction zone setting. Previous studies of lawsonite-forming reactions in metasediments have highlighted its importance for the large-scale budget of CO2, as the calcium required to form lawsonite is thought to originate from decarbonation reactions. Yet, thermodynamic modelling as well as isotopic measurements have indicated that 80 to 90% of the carbon is retained in sediments, and there is no evidence of major decarbonation in the field. As lawsonite contains abundant H2O and has a large stability field, understanding its crystallization is also important to assess fluid migration and mass transfer in a critical part of the subduction system where slow earthquakes are nucleating (such as low frequency earthquakes or episodic tremor and slip). The upper units of the blueschist facies metasediments of the Schistes Lustrés complex (Western Alps), buried to depths of ~30–40 km, have been selected as an ideal case study as they host up to 40 vol% of lawsonite. Lawsonite is found crystallized over several generations, in veins and in reactions fronts as well as in the rock matrix. Three types of lawsonite were identified. The most abundant type of lawsonite is associated with quartz and ankerite. This assemblage formed from phyllosilicates and calcite in a continuous reaction: chlorite + calcite + kaolinite = lawsonite + ankerite + quartz + H2O. According to thermodynamic modelling, this reaction is restricted to a narrow pressure-temperature domain and initiates around 180 °C and 0.4 GPa. Lawsonite is also observed and predicted to grow from Fe-Mg-carpholite at higher metamorphic conditions. None of these reactions allow efficient net export of carbon, as one carbonate replaces another, and most observations are consistent with closed-system behaviour at outcrop-scale, in agreement with geochemical studies. At sample-scale and below, crystallization of lawsonite is linked to homogenization of carbonate and pelitic domains in geologically fast reactions. Dissolution of calcite produces reactive fluids prone to react with pelitic domains and crystallize lawsonite and another carbonate. Although metamorphic veins are ubiquitous to the upper units of the Schistes Lustrés Complex, most of them result from local reactions and do no indicate large-scale mass transfer.

Near‐Field Effects of Earthquake Rupture Velocity Into Tsunami Runup Heights

AbstractTsunamis have often been treated as an isolated phenomena from the earthquake itself. In tsunami modeling, seafloor deformation is generated from an earthquake. That deformation is copied into the sea surface, and then, the tsunami is propagated over the ocean. On the other hand, rupture velocities from earthquakes are in the span of 1.5–2.5 km/s; therefore, it is safe to approximate the earthquake rupture propagation as an instantaneous phenomena relative to the tsunami propagation. However, this is not necessarily true for all earthquakes. Several types of large slow earthquakes or nonregular earthquakes, such as low frequency earthquakes and very low frequency earthquakes, and tsunami earthquakes have been detected and observed in certain zones around the world. A key question is: Do giant thrust tsunamigenic earthquakes produce slow rupture (0.1–0.5 km/s) velocities? In this study, we model heterogeneous earthquakes sources using very slow rupture velocities (0.1– 2.5 km/s) with the aim of understanding how this parameter affects the tsunami propagation and runup. We compute the amplification due to a very slow moment release in megathrust earthquakes. Our research shows that rupture velocity plays a key role on runup amplification, and the classic instantaneous case might not work as expected for every case.

Stress Sensitivity of Instantaneous Dynamic Triggering of Shallow Slow Slip Events

AbstractDynamic triggering of large, detectable slow slip events (SSEs) is rarely observed, even though regional earthquakes often trigger tectonic tremors and very low frequency earthquakes. In this study, we investigate stress sensitivity of dynamic triggering of shallow SSEs in the Nankai Trough offshore of Kii Peninsula, Japan. We first identify additional shallow SSEs that have not been reported in previous studies and obtain a 15‐year‐long catalog of SSEs, some of which are triggered by passing seismic waves originating from large regional earthquakes. We then quantify dynamic and static stress perturbations on the plate interface induced by 19 candidate regional earthquakes using numerical simulations of seismic wave propagation. We find that SSE propensity to dynamic triggering depends mainly on the maximum Coulomb stress change and that relatively large dynamic stresses (>10–20 kPa) are needed to trigger a shallow SSE in the Nankai Trough. Regional earthquakes that can induce such large amplitude of dynamic stresses on the plate interface are relatively rare, which might explain the scarcity of dynamic triggering of large, detectable SSEs along the Nankai as well as other subduction zones. In addition, our analysis suggests that intraslab earthquakes can efficiently trigger SSEs in subduction zones via less‐attenuated, slab‐guided waves. Moreover, our results support the idea that an accretionary wedge in subduction zones promotes the dynamic triggering of shallow SSEs.

Spatial relationship between shallow very low frequency earthquakes and the subducted Kyushu-Palau Ridge in the Hyuga-nada region of the Nankai subduction zone

SUMMARYShallow very low frequency earthquakes (sVLFEs) have occurred recurrently at the shallow plate interface of the Hyuga-nada region of the western Nankai subduction zone, Japan. Although the locations of sVLFE epicentres have been determined using land-based seismic records with moderate accuracy, it is necessary to determine their locations more precisely to explore the relationship between sVLFEs and other shallow slow earthquakes and examine the structural factors that may control sVLFE activity. Here, we identified sVLFE epicentres using seismic records obtained from temporarily deployed ocean bottom seismometers (OBSs) in the Hyuga-nada region. Seismic observations involved the deployment of 5–13 OBSs for approximately 1 yr, with deployments conducted three times during 2014–2016 each time with changing OBS numbers and array distribution. As a result, one sVLFE episode, containing successive Rayleigh wave pulses with slow velocities due to marine sediments, could be detected at a frequency band of 0.1–0.15 Hz per observation, resulting in a total of three episodes. Rayleigh wave amplitudes of ordinary earthquakes in the continuous records were suppressed using earthquake catalogues. We estimated the dispersion curve for the Rayleigh wave group velocity for each array, which represented the averaged group velocity within the array, using coda interferometry, and applied an envelope correlation method (ECM) using the group velocities to continuous records. These processing provided sVLFE epicentres with horizontal distance errors of <5 km. Our results showed that sVLFEs depths, which were inferred from the contour line of the top of the Phillipine Sea Plate, had increased from <10 km to 10–15 km in the region of the subducted Kyusyu-Palau Ridge (KPR). It was also apparent that migration of sVLFE epicentres occurred in 2015 from a depth of 15 km to shallower depths along the northern margin of the subducted KPR. These results identified the subducted KPR as a structural factor controlling the excitation conditions of sVLFE activities.

Self-similarity of low-frequency earthquakes

Low-frequency earthquakes are a particular class of slow earthquakes that provide a unique source of information on the physical processes along a subduction zone during the preparation of large earthquakes. Despite increasing detection of these events in recent years, their source mechanisms are still poorly characterised, and the relation between their magnitude and size remains controversial. Here, we present the source characterisation of more than 10,000 low-frequency earthquakes that occurred during tremor sequences in 2012–2016 along the Nankai subduction zone in western Shikoku, Japan. We show that the scaling of seismic moment versus corner frequency for these events is compatible with an inverse of the cube law, as widely observed for regular earthquakes. Their radiation, however, appears depleted in high-frequency content when compared to regular earthquakes. The displacement spectrum decays beyond the corner frequency with an omega-cube power law. Our result is consistent with shear rupture as the source mechanism for low-frequency earthquakes, and suggests a self-similar rupture process and constant stress drop. When investigating the dependence of the stress drop value on the rupture speed, we found that low-frequency earthquakes might propagate at lower rupture velocity than regular earthquakes, releasing smaller stress drop.

Frictional Strengths of Subduction Thrust Rocks in the Region of Shallow Slow Earthquakes

AbstractEarthquakes are expected to nucleate within velocity‐weakening materials; however, at the updip limit of the subduction seismogenic zone, the principal lithologies exhibit velocity‐strengthening behavior. At an exhumed analogue for present‐day subduction at Nankai (the Mugi Mélange, Japan) two examples of paleoseismic features occur within altered basalts, suggesting that they may be velocity‐weakening. We shear altered basalt and shale matrix from the mélange in the triaxial saw cut configuration at in situ conditions of deformation (Pc = 120 MPa, T = 150 °C) for two pore fluid factors (λ = 0.36, 0.7). The shale matrix exhibits a coefficient of friction between 0.4 and 0.5, and velocity‐strengthening behavior. Altered basalt exhibits Byerlee friction and velocity‐weakening behavior. In most experiments deformation was partitioned into Riedel shears, which have a lower percentage of clasts, clast size distributions with higher fractal dimensions, and shape factors indicating rounder clasts compared to the matrix between Riedel shears. We hypothesize that earthquakes preferentially nucleate within altered basalt at the updip limit of the seismogenic zone. More complex forms of deformation (e.g., shallow very low frequency earthquakes) may occur by mixing velocity‐weakening altered basalt into the velocity‐strengthening shale matrix. In subduction zones where the matrix is composed of shale, this complex behavior is limited to shallow depths (T < ~250 °C), above the updip transition to velocity‐weakening behavior for the matrix. Altered basalt is a ubiquitous subducting material, and future studies on its behavior through a range of subduction zone conditions are required for a full understanding of the mechanical behavior of subduction zones.

Inter‐episodic Tremor and Slip Event Episodes of Quasi‐spatiotemporally Discrete Tremor and Very Low Frequency Earthquakes in Cascadia Suggestive of a Connective Underlying, Heterogeneous Process

AbstractExploring relationships between different types of slow earthquakes is essential for determining their source properties and controlling parameters. Studies show that very low frequency earthquakes (VLFEs) and tremor are discrete events that necessitate a further comprehensive study with respect to each other. Spatiotemporal analyses of VLFE and tremor catalogs from three interepisodic tremor and slip (inter‐ETS) events in Cascadia show that seemingly uncorrelated events have more nuanced quasi‐spatiotemporal relationships. Some of these events are either only related spatially, where tremor and VLFEs occur in the same location at different times, or related temporally, occurring at a distance but during the same time period. Including quasi‐related events, nearly all VLFEs and tremor that do not exhibit immediate spatiotemporal relationships become correlated. I interpret this to mean they are related by a connective process that either is itself heterogenous or has heterogenous expressions based on other physical characteristics.

Stress state along the western Nankai Trough subduction zone inferred from b-values, long-term slow-slip events, and low-frequency earthquakes

The b-value of the Gutenberg–Richter law represents the ratio of earthquake magnitude to frequency of occurrence and is inversely proportional to differential stress. Repeating long-term slow-slip events (SSEs) and low-frequency earthquakes (LFEs) occur at subducting plate interfaces and have stress-dependent characteristics near the interface. In this study, a comprehensive regional b-value distribution is produced for the western Nankai Trough region, which highlights the relationship between b-values, SSEs, and LFEs. b-values vary along the strike direction of the subducting plate and are significantly lower left( {b sim 0.6} right) in central Shikoku district than elsewhere, where LFEs frequently occur. However, b-values in the source regions of other LFEs are moderate to high. These findings imply that b-values in the focal region are controlled by more than the LFE source process; indeed, if this source process were solely responsible, then high b-values would be expected. Meanwhile, the V_{P} /V_{S} and QP around the plate interface in central Shikoku estimated from seismic velocity and attenuation structure are smaller and larger than those in other regions with LFEs, respectively. SSEs with the migration toward central Shikoku also occurred during the analysis period, suggesting significant accumulation of shear stresses in the focal region, which reduced the b-values. These findings suggest that the spatial distributions of b-values are influenced by complicated stress and shear strength perturbations caused by SSEs and LFEs. On the other hand, the b-values in the region that underwent the greatest slip during the 1946 Nankai earthquake are not necessarily low, although the area covered by the b-value distribution is small owing to the lack of events on the updip side. Whereas the asperity areas of huge earthquakes are characterized by low b-values, the b-value distribution in the Nankai megathrust area is more complicated. It is considered that slow earthquakes, including SSEs and LFEs, are related to megathrust earthquakes via stress transfer from slow earthquakes to adjacent megathrust source regions. A unified analysis of b-values in the source regions of slow and megathrust earthquakes may be required to make precise estimates of the seismic hazard produced by a megathrust event.

Comprehensive Detection of Very Low Frequency Earthquakes Off the Hokkaido and Tohoku Pacific Coasts, Northeastern Japan

AbstractVery low frequency earthquakes (VLFEs), classified as one type of slow earthquake, occur near seismogenic zones and can be a proxy for interplate slow slip. We investigated VLFE activity off the Hokkaido and Tohoku Pacific coasts, where the Pacific plate subducts under the North American plate. Comprehensive detection of VLFEs is performed for the period from January 2003 to July 2018 by a matched‐filter technique utilizing synthetic waveforms calculated based on a three‐dimensional velocity structure model. The spatiotemporal consistency between the afterslip of the 2003 Tokachi‐Oki earthquake (Mw 8.0) and VLFE distribution off the Hokkaido coast suggests that VLFE activity can reflect interplate slow slip. Inside a large coseismic slip area of the 2011 Tohoku earthquake (Mw 9.0), the VLFE activity was low thereafter, whereas outside the area, VLFE activity increased after the 2011 Tohoku earthquake. These characteristics are consistent with aseismic slip distributions estimated from repeating earthquakes. VLFEs were sometimes activated by afterslips of interplate earthquakes with Mw of 6–7, such as the 2008 Fukushima‐Oki earthquake and aftershocks of the 2011 Tohoku earthquake. Even in the coseismic slip areas of very large earthquakes, we found the occurrence of episodic bursts every 3 months to a year during an interseismic period between very large earthquakes. Such VLFE bursts may suggest small slips during the interseismic period.

Study on the mechanical properties and rheological model of an anchored rock mass under creep-fatigue loading

The stress environment of deep rock masses is complex. Under the action of earthquakes or blasting, the strength and stability of anchored rock masses in fracture zones or faults are affected. To explore the variation in anchored rock masses under creep-fatigue loading, shear creep comparative testing of anchored marble specimens with or without fatigue loading is performed. Considering the damage variable of rock under fatigue loading, a rheological model is established to characterize the whole shear creep process of anchored rock masses under creep-fatigue loading. The results show that (1) the overall deformation of marble under creep-fatigue loading is larger than that under only shear creep loading, and the average deformation is increased by 18.3%. (2) By comparing the creep curves with and without fatigue loading, the two curves basically coincide when the first level stress is applied, and the two curves are stable with the increase in stress level. The results show that the strain difference among the specimens increases gradually in the steady-state stage and reaches the maximum at the fourth level. (3) The shear creep is described by considering the creep mechanical properties of anchored rock masses under fatigue loading. The accuracy of this creep-fatigue model is verified by laboratory tests, and the applicability of the model is illustrated by the fitting parameter R2. The proposed model provides a theoretical basis for the study of anchored rock masses under low-frequency earthquakes or blasting and new methods for the stability and reinforcement of rock masses.

Migration of shallow and deep slow earthquakes toward the locked segment of the Nankai megathrust

The Nankai megathrust is located offshore Shikoku and Kyushu, Japan and is characterized by various kinds of slow earthquakes whose relative motions across the plate boundary faults are slower than regular earthquakes. In the area, the interplate locking is stronger in the northern area (offshore Shikoku) than in the southern area (offshore Kyushu) and Mw ∼8 earthquakes (Nankai earthquakes) have occurred repeatedly in the northern area. In this paper, the spatio-temporal distributions of slow earthquakes (very low frequency earthquakes, tremors and slow-slip events) are examined based on the analyses of repeating earthquakes and slow earthquakes with special focus on the interaction between different activities. A comprehensive analysis of the seismic and geodetic data from 2003 to 2016 indicates complementary distribution of various types of slow earthquakes down to 35–50 km depth outside the Nankai main locking area. We also found interactions between different kinds of activities. The interactions between the repeating earthquakes and slow earthquakes suggest that the area of the repeating earthquakes activity can be divided into deeper (depth ≥ 20 km) and shallower (depth < 20 km) areas. The analyses of deep repeating earthquakes and the inland Global Navigation Satellite System (GNSS) data suggests slow northward migrations of long-term slow slip events (SSEs) in 20–50 km (offshore Kyushu) and 20–35 km (under Shikoku) depths along the plate boundary. These migrations occurred during a period of 2–3 years that includes the 2003 and 2010 large slow-slip events in the Bungo channel located in between Kyushu and Shikoku. The analysis has also shown interaction between shallow repeating earthquakes and shallow very low frequency earthquakes which indicates faster northward migrations of short-term SSEs from the shallow plate boundary offshore Kyushu to the deeper area under Shikoku over the duration of a month during the 2010 long-term slow-slip episode. The deep slow migration and the shallow to deep fast migration of SSEs in a ∼300 km area towards and around the source area of the recurrent Nankai earthquake (Mw 8.0–8.6) indicates the occurrence of a widespread non-steady stress build-up process around the source area of the Nankai megathrust earthquake.

Characteristics of Shallow Low‐Frequency Earthquakes off the Kii Peninsula, Japan, in 2004 Revealed by Ocean Bottom Seismometers

AbstractThe characteristics of shallow low‐frequency earthquakes (LFEs) are related to stress changes on the shallow plate boundary, which are important for understanding the megathrust earthquake cycle. The 5 September 2004 off the Kii Peninsula earthquakes (MJMA = 7.1, 7.4) occurred near the Nankai trough subduction zone, off southwest Japan. Ocean bottom seismometer observations from 22 September to 30 November 2004 detected many shallow LFEs among the ordinary aftershocks. During the observation period, the frequency of shallow LFEs steadily decayed with the exception of episodic activities, which were very sensitive to stress changes caused by tides and the 23 October 2004 Niigata earthquake (MJMA = 6.8). We also confirmed correlations between shallow LFEs and shallow very low frequency earthquakes, which suggest that these slow events represent the same slip phenomenon. These findings will contribute to clarifying the impact of LFEs on megathrust earthquakes.

Migrations and Clusters of Shallow Very Low Frequency Earthquakes in the Regions Surrounding Shear Stress Accumulation Peaks Along the Nankai Trough

AbstractThe spatiotemporal characteristics of shallow slow earthquake activity are linked to the tectonic environments of shallow plate boundaries. In this work, the spatiotemporal variations of shallow very low frequency earthquake (SVLFE) activity along the Nankai Trough were investigated using a cross‐correlation analysis. The SVLFEs migrated or spread eastward along the strike direction of the trench during large SVLFE episodes. Migrations and clusters of SVLFEs suggest the occurrence of shallow slow slip events. The observed lateral variations in SVLFE activity patterns reflect the heterogeneous distributions of effective strengths at the shallow plate boundary along the Nankai Trough. Migrations and clusters of SVLFEs tended to be concentrated in the regions surrounding the stress accumulation peaks on the Philippine Sea Plate boundary. The stress accumulated in the transitional regions between high‐strength and low‐strength zones can be released by shallow slow earthquakes.

Daily measurement of slow slip from low-frequency earthquakes is consistent with ordinary earthquake scaling.

Slow slip transients on faults can last from seconds to months and stitch together the earthquake cycle. However, no single geophysical instrument is able to observe the full range of slow slip because of bandwidth limitations. Here, we connect seismic and geodetic data from the Mexican subduction zone to explore an instrumental blind spot. We establish a calibration of the daily median amplitude of the seismically recorded low-frequency earthquakes to the daily geodetically recorded moment rate of previously established slow slip events. This calibration allows us to use the precise evolution of low-frequency earthquake activity to quantitatively measure the moment of smaller, subdaily slip events that are unresolvable by geodesy alone. The resulting inferred slow slip moments scale with duration and inter-event time like ordinary earthquakes. These new quantifications help connect slow and fast events in a broad spectrum of transient slip and suggest that slow slip events behave much like ordinary earthquakes.

Low-frequency earthquakes beneath Tullu Moye volcano, Ethiopia, reveal fluid pulses from shallow magma chamber

The active magmatic processes beneath volcanoes in continental rifts is poorly understood. For example, until recently in the East African rift (EAR), the majority of the young volcanoes were thought to be inactive. More recent studies have shown that numerous volcanoes in the EAR are seismically active and deforming rapidly. However, an unambiguous sign of actively degassing magma hosted in shallow magma bodies has eluded most investigators. Here we present detailed analysis of the first low-frequency (LF) earthquake swarms to be observed in the Main Ethiopian Rift. The earthquakes locate to beneath Tullu Moye volcano and are directly related to the presence of a shallow magma body with a high fluid content. Using spectral modelling we show that the LF earthquakes appear to have low stress-drops (1–50 kPa) which we interpret in terms of low rupture velocities and high pore-fluid pressure. Careful relocation of the LF earthquakes place them approximately 4 km below the surface within one of two possible clusters. However, analysis of the correlation between earthquake waveforms show that each swarm contains a range of earthquake families and as such a diversity of earthquake source mechanisms. To explain these observations, we propose the seismicity is induced by H2O/CO2 fluid pulses from the shallow magma body into a highly fractured region. Fluid pulses cause high pore fluid pressures, which also cause the low rupture velocities.

Focal Mechanisms of Deep Low‐Frequency Earthquakes Beneath Zao Volcano, Northeast Japan, and Relationship to the 2011 Tohoku Earthquake

AbstractSmall, deep low‐frequency earthquakes (LFEs) with dominant frequencies of 2–8 Hz occur at depths of 20–40 km and are thought to be related to movement of magma or crustal fluid, but their physical source processes remain largely unknown. Therefore, we determine the focal mechanisms of LFEs beneath Zao volcano, Northeast Japan, using the S/P amplitude ratio. Most focal mechanisms are classified into five groups: three types of double couples, a compensated linear vector dipole group, and a single force group. Double couples in 2007–2012 are consistent with those expected for the regional stress field, but no such events have been observed since 2013. This transition in focal mechanisms was simultaneous with a rapid increase in LFE activity beneath Zao. Our results suggest that LFEs beneath Zao were controlled mainly by the local stress field, but the stress field changed about two years after the 2011 Tohoku earthquake.

Two‐Dimensional Thermal Modeling of the Philippine Sea Plate Subduction in Central Japan: Implications for Gap of Low‐Frequency Earthquakes and Tectonic Tremors

AbstractWe performed 2‐D high‐resolution thermomechanical numerical simulations associated with subduction of the Philippine Sea (PHS) plate in central Japan, where a well‐defined gap of low‐frequency earthquakes (LFEs) and tectonic tremors (TTs) is observed. We investigate the complex relationships among megathrust earthquakes, LFEs, TTs, and interplate temperature distributions and dehydrations caused by phase transformations of hydrous mid‐ocean ridge basalt (MORB) of the oceanic crust and ultramafic rock of the slab mantle. Our models predict a temperature range of 480–600 °C corresponding to both active LFEs and TTs regions and their gap region located on the upper surface of the PHS plate. A moderate amount of fluids (&lt;0.8 wt%) is released due to phase transformations of the hydrous MORB near the thermally estimated seismogenic zone on the upper surface of the PHS plate. Hydrous MORB was found to transform from greenschist to epidote amphibole, releasing a small amount of fluids (&gt;0.3 wt%) near the belt‐like form regions of LFEs and TTs. We found that the total amount of dehydrated water near the active region of LFEs and TTs is larger than that near the gap of LFEs and TTs. Our modeling results show that this difference stems from the presence of the phase transformation from epidote amphibole to eclogite in the former, and it may play a significant role for the occurrence of LFEs and TTs. The difference in the amount of released fluid between the active and gap regions of LFEs and TTs may be related to their generation mechanism.

Classes of low-frequency earthquakes based on inter-time distribution reveal a precursor event for the 2011 Great Tohoku Earthquake

Recently, slow earthquakes (slow EQ) have received much attention relative to understanding the mechanisms underlying large earthquakes and to detecting their precursors. Low-frequency earthquakes (LFE) are a specific type of slow EQ. In the present paper, we reveal the relevance of LFEs to the 11 March 2011 Great Tohoku Earthquake (Tohoku-oki EQ) by means of cluster analysis. We classified LFEs in northern Japan in a data-driven manner, based on inter-time, the time interval between neighboring LFEs occurring within 10 km. We found that there are four classes of LFE that are characterized by median inter-times of 24 seconds, 27 minutes, 2.0 days, and 35 days, respectively. Remarkably, in examining the relevance of these classes to the Tohoku-oki EQ, we found that activity in the shortest inter-time class (median 24 seconds) diminished significantly at least three months before the Tohoku-oki EQ, and became completely quiescent 30 days before the event (p-value = 0.00014). Further statistical analysis implies that this class, together with a similar class of volcanic tremor, may have served as a precursor of the Tohoku-oki EQ. We discuss a generative model for these classes of LFE, in which the shortest inter-time class is characterized by a generalized gamma distribution with the product of shape parameters vκ = 1:54 in the domain of inter-time close to zero. We give a possible geodetic interpretation for the relevance of LFE to the Tohoku-oki EQ.

Identifying significant earthquake intensity measures for evaluating seismic damage and fragility of nuclear power plant structures

Abstract Seismic design practices and seismic response analyses of civil structures and nuclear power plants (NPPs) have conventionally used the peak ground acceleration (PGA) or spectral acceleration (Sa) as an intensity measure (IM) of an earthquake. However, there are many other earthquake IMs that were proposed by various researchers. The aim of this study is to investigate the correlation between seismic responses of NPP components and 23 earthquake IMs and identify the best IMs for correlating with damage of NPP structures. Particularly, low- and high-frequency ground motion records are separately accounted in correlation analyses. An advanced power reactor NPP in Korea, APR1400, is selected for numerical analyses where containment and auxiliary buildings are modeled using SAP2000. Floor displacements and accelerations are monitored for the non- and base-isolated NPP structures while shear deformations of the base isolator are additionally monitored for the base-isolated NPP. A series of Pearson's correlation coefficients are calculated to recognize the correlation between each of the 23 earthquake IMs and responses of NPP structures. The numerical results demonstrate that there is a significant difference in the correlation between earthquake IMs and seismic responses of non-isolated NPP structures considering low- and high-frequency ground motion groups. Meanwhile, a trivial discrepancy of the correlation is observed in the case of the base-isolated NPP subjected to the two groups of ground motions. Moreover, a selection of PGA or Sa for seismic response analyses of NPP structures in the high-frequency seismic regions may not be the best option. Additionally, a set of fragility curves are thereafter developed for the base-isolated NPP based on the shear deformation of lead rubber bearing (LRB) with respect to the strongly correlated IMs. The results reveal that the probability of damage to the structure is higher for low-frequency earthquakes compared with that of high-frequency ground motions.

Erratum: Bayesian approach for detecting dynamically triggered very low-frequency earthquakes in the Nankai subduction zone and application to the 2016 Mw 5.9 off-Kii Peninsula earthquake, Japan

Erratum: Bayesian approach for detecting dynamically triggered very low-frequency earthquakes in the Nankai subduction zone and application to the 2016 Mw 5.9 off-Kii Peninsula earthquake, Japan