Slab Earthquakes Research Articles

Seismic structure of subducted oceanic crust near the slow-earthquake source region in the southern Ryukyu arc

Seismic tomography and receiver function analysis were carried out to investigate the relation between the slab structure in the southern Ryukyu arc region and the occurrence of slow-slip events that repeat biannually. For calculation of the receiver function, 212 teleseismic earthquakes with magnitudes larger than 6.0 were selected, and teleseismic waveforms were observed using two short-period seismometers and one broadband seismometer. Assuming that each later phase in a receiver function was a wave converted from P to S at depth, the time-domain receiver function was transformed into depth domain along each ray path using a reference velocity model. P- and S-wave arrival times, selected manually by the Japan Meteorological Agency (JMA), were used for the analysis of seismic tomography. In all, 6,750 earthquakes from January 2002 to March 2014 were used. The results showed that a cluster of slab earthquakes is distributed about 10 km below the plate interface. This suggests that the slab earthquakes occur in the lower part of the oceanic crust near the oceanic Moho within the slab. Moreover, the fault of the slow-slip events corresponds with the plate interface. The results also showed that a low Vp and high Vp/Vs area is distributed within the subducted oceanic crust underlying the fault associated with the slow-slip events. These structures are similar to those in the Tokai district, which suggests that the thermal condition of the southern Ryukyu arc is similar to the case of a hot-slab area where slow-slip events occur.

Along-strike variation in subducting plate seismicity and mantle wedge attenuation related to fluid release beneath the North Island, New Zealand

We analyze seismicity in the subducted slab of the Hikurangi subduction zone, along a 600-km length beneath the North Island, New Zealand. The volcanic character changes along strike with extremely productive rhyolitic volcanism in the central Taupo Volcanic Zone, moderate andesitic volcanism in the northern and south-central zones, and subduction without volcanism in the southernmost Hikurangi zone. We have relocated slab earthquakes with 3-D velocity models. The relocated seismicity shows more detail of the varied distribution and abundance of slab seismicity, which below 50-km depth may be related to embrittlement from high fluid pressure. The depth of the buoyant subducted Hikurangi Plateau ranges from 40 to 140km depth.At depths where the subducting slab interacts with the mantle wedge, along-arc variation in slab seismicity is a fundamental characteristic, with patches of abundant seismicity separated by low seismicity zones. The largest most numerous patch, at 150–220km depth, underlies a pronounced low Qp zone in the mantle wedge, which is associated with the rhyolite-dominant Taupo caldera. Extensive melt in the region of low Qp requires high H2O flux from the underlying slab. The abundant Taupo seismicity suggests a correlation between melt production and regions of earthquake fracture permeability following embrittlement which promote migration of dehydration fluid. The hydration history of the incoming slab may be a key factor in producing variations in dehydration and intraslab fluid migration. Broader, extensive outer-rise yielding and hydration may have occurred near the approach of the re-entrant Hikurangi Plateau, forming the slab section that currently has high seismicity. Slab seismicity deepens from 240 to 330km along-arc as the subduction rate increases from <20 to >40mm/yr. The southwestern slab seismicity is bounded by an unusually narrow zone with 110-km depth extent. This is inferred to be a dehydration front related to heating at a slab edge that is located 70km further southwest.

Sub-slab seismic anisotropy and mantle flow beneath the Caribbean and Scotia subduction zones: Effects of slab morphology and kinematics

Subduction systems are vitally important to plate tectonics and mantle convection, but questions remain about many aspects of subduction dynamics, particularly the nature of sub-slab mantle flow. Observations of seismic anisotropy can shed light on the pattern of mantle flow in subduction systems, but major uncertainties remain regarding the interpretation of sub-slab anisotropy. Here, we present measurements of shear wave splitting due to anisotropy in the sub-slab mantle beneath the Caribbean and Scotia subduction zones. These subduction systems are morphologically similar, with high arc curvature and short arc length, but there are differences between them in terms of their kinematic and tectonic settings. We apply the source-side shear wave splitting technique to direct teleseismic S phases originating from slab earthquakes. We restrict our analysis to seismic stations at which we have examined the receiver-side splitting in detail to ensure accurate corrections for anisotropy in the upper mantle beneath the station. We observe a well-defined pattern of trench-parallel fast directions (ϕ) in the northern half of the Caribbean subduction zone, with a transition to dominantly trench-perpendicular ϕ at the southern end. There is more scatter in the measurements for Scotia, but we observe generally trench-parallel ϕ in the northern portion of the subduction system, with a mix of trench-parallel, -perpendicular, and -oblique fast directions to the south. Our preferred interpretation of these splitting patterns is that they reflect 3-D return flow of the sub-slab mantle due to trench rollback. In both systems, we infer that sub-slab flow is being driven from south to north. Beneath Scotia, this is likely driven by differential migration of the Scotia trench along strike. Beneath the Caribbean, we hypothesize that mantle flow around the southern edge of the slab is inhibited by the presence of the South American continental keel, enabling trench-perpendicular stretching in the sub-slab mantle and forcing mantle flow to escape to the north. The similarities and differences we observe between the two systems yield insight into the relative contributions of slab morphology and plate kinematics in controlling mantle flow beneath subducting slabs.

Effect of tectonic setting on the fit and performance of a long-range earthquake forecasting model

The Every Earthquake a Precursor According to Scale (EEPAS) long-range earthquake forecasting model has been shown to be informative in several seismically active regions, including New Zealand, California and Japan. In previous applications of the model, the tectonic setting of earthquakes has been ignored. Here we distinguish crustal, plate interface, and slab earthquakes and apply the model to earthquakes with magnitude M≥4 in the Japan region from 1926 onwards. The target magnitude range is M≥ 6; the fitting period is 1966-1995; and the testing period is 1996-2005. In forecasting major slab earthquakes, it is optimal to use only slab and interface events as precursors. In forecasting major interface events, it is optimal to use only interface events as precursors. In forecasting major crustal events, it is optimal to use only crustal events as precursors. For the smoothed-seismicity component of the EEPAS model, it is optimal to use slab and interface events for earthquakes in the slab, interface events only for earthquakes on the interface, and crustal and interface events for crustal earthquakes. The optimal model parameters indicate that the precursor areas for slab earthquakes are relatively small compared to those for earthquakes in other tectonic categories, and that the precursor times and precursory earthquake magnitudes for crustal earthquakes are relatively large. The optimal models fit the learning data sets better than the raw EEPAS model, with an average information gain per earthquake of about 0.4. The average information gain is similar in the testing period, although it is higher for crustal earthquakes and lower for slab and interface earthquakes than in the learning period. These results show that earthquake interactions are stronger between earthquakes of similar tectonic types and that distinguishing tectonic types improves forecasts by enhancing the depth resolution where tectonic categories of earthquakes are vertically separated. However, when depth resolution is ignored, the model formed by aggregating the optimal forecasts for each tectonic category performs no better than the raw EEPAS model.

Continental and oceanic crustal structure of the Pampean flat slab region, western Argentina, using receiver function analysis: new high-resolution results

SUMMARY The Pampean flat slab of central Chile and Argentina (30°–32°S) has strongly influenced Cenozoic tectonics in western Argentina, which contains both the thick-skinned, basement-cored uplifts of the Sierras Pampeanas and the thin-skinned Andean Precordillera fold and thrust belt. In this region of South America, the Nazca Plate is subducting nearly horizontally beneath the South American Plate at ∼100 km depth. To gain a better understanding of the deeper structure of this region, including the transition from flat to ‘normal’ subduction to the south, three IRIS-PASSCAL arrays of broad-band seismic stations have been deployed in central Argentina. Using the dense SIEMBRA array, combined with the broader CHARGE and ESP arrays, the flat slab is imaged for the first time in 3-D detail using receiver function (RF) analysis. A distinct pair of RF arrivals consisting of a negative pulse that marks the top of the oceanic crust, followed by a positive pulse, which indicates the base of the oceanic crust, can be used to map the slab's structure. Depths to Moho and oceanic crustal thicknesses estimated from RF results provide new, more detailed regional maps. An improved depth to continental Moho map shows depths of more than 70 km in the main Cordillera and ∼50 km in the western Sierras Pampeanas, that shallow to ∼35 km in the eastern Sierras Pampeanas. Depth to Moho contours roughly follow terrane boundaries. Offshore, the hotspot seamount chain of the Juan Fernandez Ridge (JFR) is thought to create overthickened oceanic crust, providing a mechanism for flat slab subduction. By comparing synthetic RFs, based on various structures, to the observed RF signal we determine that the thickness of the oceanic crust at the top of the slab averages at least ∼13–19 km, supporting the idea of a moderately overthickened crust to provide the additional buoyancy for the slab to remain flat. The overthickened region is broader than the area directly aligned with the path of the JFR, however, and indicates, along with the slab earthquake locations, that the flat slab area is wider than the JFR volcanic chain observed in the offshore bathymetry. Further, RFs indicate that the subducted oceanic crust in the region directly along the path of the subducted ridge is broken by trench-parallel faults. One explanation for these faults is that they are older structures within the oceanic crust that were created when the slab subducted. Alternatively, it is possible that faults formed recently from tectonic underplating caused by increased interplate coupling in the flat slab region.

Shear wave anisotropy in the crust, mantle wedge, and subducting Pacific slab under northeast Japan

To study the anisotropic structure beneath northeast (NE) Japan, we made 4366 shear wave splitting measurements using high-quality seismograms of many earthquakes occurring in the crust and the subducting Pacific slab. Our results provide important new information on the S wave anisotropy in the upper crust, lower crust, mantle wedge, and subducting Pacific slab. In the upper crust, the anisotropy is mainly caused by the stress-aligned fluid-saturated microcracks. The measured delay times (DTs) increase to 0.10 s at 10–11 km depth; the fast velocity directions (FVDs) are parallel to either the tectonic stress or the strike of active faults. The maximum DTs for the low-frequency earthquakes near the Moho are 0.15–0.17 s, suggesting strong anisotropy at the base of the crust or in the uppermost mantle. The measurements for the intermediate-depth earthquakes in the Pacific slab show dominant E-W (trench-normal) FVDs in the back-arc area and N-S (trench-parallel) FVDs in the fore-arc area. The trench-normal FVDs in the back-arc area are caused by the corner flow in the mantle wedge as a result of the subduction of the Pacific plate. The maximum DTs for the slab earthquakes reach 0.30–0.32 s at 100 km depth, but only half of the total DTs are produced in the mantle wedge. The small DTs in the mantle wedge may result from an isotropic or weak anisotropic zone in the middle of the mantle wedge. In the fore arc, the dominant trench-parallel FVDs for the slab earthquakes are consistent with those for the upper crust earthquakes, and ∼80% of the total DTs can be accounted for by the anisotropy in the crust. In the subducting Pacific slab, the trench-parallel FVDs may reflect either the original fossil anisotropy in the Pacific plate when the plate was produced in the mid-ocean ridge or the preferred orientations of the crystals and cracks in the upper part of the subducting slab.

Vrancea slab earthquakes triggered by static stress transfer

Abstract. The purpose of this paper is to study the interaction of the Vrancea seismic activity (Romania) in space as result of Coulomb, static stress transfer during M=7+ events. In this area, three large events occurred in 1977, 1986 and 1990 at mid-lower, lithospheric depths and with similar focal mechanisms. Assuming elastic rheology for the deforming rocks it is suggested that frictional sliding on pre-existing fault produced the 1986 M=7.1 event (depth 131 km), that was possibly triggered by the 1977 M=7.4 event (depth 94 km). We calculated a static stress transfer of 0.52–0.78 bar to the hypocentre of the 1986 event. On the contrary, the occurrence of the 1990 event is uncertain: it is located inside the relaxed (shadow) zone of the combined 1977 and 1986 static stress field considering an azimuth for maximum compression of N307° E. It follows that, the 1990 earthquake most likely represents an unbroken patch (asperity) of the 1977 rupture plane that failed due to loading. However, if a different compression azimuth is assumed (N323° E) then the 1990 event was also possibly triggered by static stress transfer of the 1977 and 1986 events (combined). Our modeling is a first-order approximation of the kind of earthquake interaction we might expect at intermediate lithospheric depths (80–90 to 130–140 km). It is also suggested that static stress transfer may explain the clustering of Vrancea earthquakes in space by the rupturing of two (possibly three) NW-dipping major zones of weakness (faults) which accommodate the extension (vertical elongation) of the slab.

Side-effect of using response spectral amplification ratios for soft soil sites—Earthquake source-type dependent amplification ratios

Our previous studies show that site effects (amplification of rock motions), source and path effects are coupled when response spectra are used to characterize the amplification ratios for a soil site modelled as nonlinear or elastic. The coupling is referred to as a “side effect” of using response spectral amplification ratios. In the present study we use a suite of rock site records, well distributed with respect to magnitude and source distance, from crustal, subduction interface and slab earthquakes to evaluate the response spectral amplification ratio for soft soil sites. We compare these side-effects for ground motions generated by three types of earthquakes, and we find that, at periods much shorter or much longer than the natural period of a soil site modelled as elastic, the average amplification ratios with respect to rock site ground motions from three types of earthquakes are moderately different and are very similar for other spectral periods. These differences are not statistically significant because of the moderately large scatter of the amplification ratios. However, the extent of magnitude- and source-distance-dependence of amplification ratios differs significantly. After the effects of magnitude and source distance on the amplification ratios are accounted for, the differences in amplification ratios between crustal and subduction earthquake records are very large in some particular combinations of source distance and magnitude range. These findings may have potential impact in establishing design spectra for soft soil sites using strong motion attenuation models or numerical modelling.

RELATIONSHIPS AMONG DIFFERENT MAGNITUDES FOR INLAND EARTHQUAKES CLASSIFIED BY FAULT TYPE AND THEIR APPLICATION TO STRONG MOTION PREDICTION

We examined how the relationships systematically change among the moment magnitude MW, the Japan Meteorological Agency magnitude MJ, and the short-period magnitude m for inland earthquakes classified by fault type. The examination was extended to the relationships among the magnitudes for inter-plate earthquakes and to those for slab earthquakes. It was found out that MJ was about 0.5 larger than MW in inland earthquakes caused by strike-slip faults, MJ was about 0.2 larger than MW in inland earthquakes caused by dip-slip faults, m was about 0.2 smaller than MW in inland earthquakes caused by strike-slip faults, and m was about 0.1 larger than MW in inland earthquakes caused by dip-slip faults. We applied these relationships to the procedure for evaluating fault parameters, and showed two examples of strong motions predicted for strike-slip and dip-slip faults 40 km long.

Shear wave splitting from local events beneath the Ryukyu arc: Trench-parallel anisotropy in the mantle wedge

We present shear wave splitting measurements from local slab earthquakes at eight seismic stations of the Japanese F-net array located in the Ryukyu arc. We obtained high-quality splitting measurements for 70 event-station pairs and found that the majority of the measured fast directions were parallel to the strike of the trench and perpendicular to the convergence direction. Splitting times for individual measurements ranged from 0.25 to 2 s; most values were between 0.75 and 1.25 s. Both the fast directions and the split times were similar to results for teleseismic S( K) KS and S wave splitting at the same stations, which suggests that the anisotropy is located in the mantle wedge above the slab. We considered several mantle deformation scenarios that would result in predominantly trench-parallel fast directions, and concluded that for the Ryukyu subduction system the most likely explanation for the observations is corner flow in the mantle wedge combined with B-type olivine fabric. In this model, the flow direction in the wedge is perpendicular to the trench, but the fast axes of olivine crystals tend to align perpendicular to the flow direction, resulting in trench-parallel shear wave splitting.

A tectonic interpretation of NW-SE strike-slip faulting during the 2004 off the Kii peninsula earthquakes, Japan: Probable tear of the Philippine Sea plate

Abstract The 2004 off the Kii peninsula earthquakes (M w 7.5 for the main shock) occurred within the subducting Philippine Sea (PHS) plate near its boundary, the Nankai trough, southwest Japan. The rupture mode of the foreshock-main shock-aftershock sequence was complicated, a combination of ENE-WSW striking (almost trough parallel) reverse faulting beneath the trough and NW-SE trending (almost trough normal) strike-slip faulting mostly on the landward side of the former. In this paper, we discuss the tectonic meaning of this NW-SE running strike-slip fault. We examined hypocenter distribution and focal mechanisms of slab earthquakes from October 1997 through September 2004 and confirmed a NW-SE striking tear of the PHS slab beneath the middle part of the Kii Peninsula pointed out by Miyoshi and Ishibashi (2004). According to the Earthquake Research Committee (2004) there is a NW-SE trending structural discontinuity in the PHS crust to the southeast of the main shock epicenter. Putting all features together, we interpret that there is a NW-SE striking fracture within the PHS plate continuously from the Nankai trough region to the slab beneath the Kii Peninsula, and that a partial rupture of this fracture occurred during the off the Kii peninsula earthquakes together with trough-parallel reverse faulting. It should be noted that two disastrous M 7-class slab earthquakes seem to have occurred along this tear beneath the peninsula in 1899 and 1952.

Analysis of the 2001 Geiyo, Japan, earthquake using high‐density strong ground motion data: Detailed rupture process of a slab earthquake in a medium with a large velocity contrast

Detailed rupture process of the 2001 Geiyo, Japan, earthquake (Mw = 6.8), which is a slab earthquake in the Philippine Sea slab, is investigated from the inversion of strong motion waveforms. In the inversion, a realistic curved fault model is assumed, and the one‐dimensional underground structure models considering the deep slab structure and the shallow surface layers are used. The estimated seismic moment and the maximum slip are 2.1 × 1019 N m and 2.4 m, respectively. The total rupture duration is ∼10 s. The obtained rupture process is very complex. The rupture changed gradually from a normal fault type into a more strike‐slip type during its propagation. The slip distribution is heterogeneous, and the rupture area extends over both of the slab oceanic crust and the slab oceanic mantle. In the medium of the source area, there exists a large velocity contrast at the slab oceanic crust‐mantle boundary. The largest slip is seen in the low‐velocity (low‐rigidity) oceanic crust. It is reasonable from the viewpoint of rupture dynamics that large slip occurs in the low‐rigidity layer, which is easier to deform. Thus the obtained slip model suggests that the rupture process of the 2001 Geiyo earthquake was strongly influenced by the heterogeneity of the background medium.

震源分布からみた伊勢湾から四国西部にかけてのフィリピン海スラブの形状

We inferred minutely the geometry of the upper surface of the seismic Philippine Sea (PHS) slab beneath the region from Ise Bay to western Shikoku, southwest Japan, based on hypocentral distribution. Despite the importance of the slab geometry for the subduction zone seismotectonics as well as for the assessment and forecasting of destructive interplate and slab earthquakes, the shape of the PHS slab has been obscure and controversial. We selected well-determined hypocenters during the period from Oct. 1997 through Dec. 2002 from the integrated hypocenter database prepared by the Japan Meteorological Agency. At first, we composed depth data set of the slab upper surface from vertical cross sections of hypocentral distribution along longitude and latitude of every 0.1 degree, and drew first-trial isodepth contours of the upper surface of the PHS slab assuming that the upper boundary of slab earthquake distribution roughly coincides with the slab upper surface. Then, in many areas we made vertical cross sections of hypocenter distribution in many directions including inferred dip-direction and strike of the slab, and revised isodepth contours. The main results are as follows: (1) In the region to the east of Lake Biwa, northwestward gently-dipping PHS slab reaches around the depth of 35km, in which disastrous slab earthquakes presumably took place. (2) To the northeast it continues to the slab beneath the central Tokai district, but to the southwest it is obscure whether it continues to the slab beneath the eastern Kii Peninsula or it is separated from the latter. (3) The slab beneath the middle part of the Kii Peninsula may be torn into two parts, with the southwestern part underlying the northeastern part. (4) There are double seismic zones beneath the southern Kii Peninsula. (5) In the west part of the Kii Peninsula no slab earthquake occurs in deeper part, whereas just in this area a remarkable shallow crustal swarm earthquake activity exists. (6) Beneath Shikoku the slab is being subducted with gentle dip angle and forms a broad ridge. The slab looks continuous from Shikoku to Kyusyu. Recently the PHS slab geometry has been investigated by seismic refraction/reflection profilings and receiver function analyses. Those results are shallower than our result in eastern Shikoku suggesting that slab earthquakes in this region occur in the oceanic mantle. We relocated slab earthquakes in this region using a velocity structure model consistent with the result of seismic profiling and found that the shallower main activity lays in the slab mantle of the assumed model with deeper remarkable events beneath it. In order to clarify the true slab geometry it is essentially important to know in which part of the slab, oceanic crust or mantle, slab earthquakes are actually taking place in each region.

Estimating Slab Earthquake Response Spectra from a 3D Q Model

The problem of estimating response spectra for a heterogeneous lithosphere can be addressed by directly computing attenuation from physical models. In a subduction zone, slab earthquakes will have different attenuation through the mantle wedge than the slab. This article is primarily concerned with very high loss paths through the attenuating mantle underlying the volcanic zone of the North Island, New Zealand, where low Q requires modification of the “standard” New Zealand engineering response spectrum model. A lack of strong-motion data prevents a standard regression analysis for paths from deep slab earthquakes through the highly attenuating mantle zone. Instead, modifications have been derived using a 3D frequency-independent Q model that has been developed for the North Island subduction zone from 2-20 Hz local earthquake t * data. By calibrating the t * crustal results to the standard New Zealand model, amplitudes can be compared between the standard model and the 3D Q model. Additional path-averaged attenuation rate coefficients, CQ , for each source and station pair are determined. This results in simple expressions for CQ as a function of centroid depth, for modifying the standard model. This modification reduces the model spectra by a factor of approximately 2-4 for mantle wedge paths below the volcanic region. This reduction is similar to the observed variation in response spectra, at periods below 0.4 sec, for a 160-km-deep M w 6.0 earthquake. For shallow earthquakes propagating through the shallow volcanic region, the new model gives results that are similar to a volcanic-path attenuation term derived by regression analysis. Manuscript received 24 February 2003.

Slab Earthquakes: To Dehydrate or to Transform

Slab Earthquakes: To Dehydrate or to Transform

既存のスペクトルインバージョン結果と震源インバージョン結果から推定されるアスペリティの実効応力と断層タイプおよび深さとの経験的関係

Empirical relations of the effective stress on the asperity to the fault type and the depth were examined for more accurate prediction of strong ground motions from a future earthquake. The existing spectrum inversion results showed that the level of the acceleration source spectrum in a short period range, called short period level in this paper, varied several hundred percent as a function of the fault type (inland earthquake, subduction earthquake, and slab earthquake) and 10 percent as a function of the focal depth. The effective stress on the asperity, inferred from the short period level, had the same feature as the short period level. The existing variable-slip rupture models showed that the effective stress on the subfaults in the asperity varied several hundred percent as a function of the fault type (strike slip fault with a surface rupture, reverse fault with a buried rupture, and oblique slip fault with a buried rupture) and 10 to 30 percent as a function of the depth of the subfault.

Mode of stress release within a subducting slab of lithosphere: implication of source mechanism of deep and intermediate-depth earthquakes

We inverted the long-period P-wave seismograms of 28 deep and intermediate-depth earthquakes including 16 Tonga events to their sources. The results are incorporated with our previous results for 18 deep and intermediate-depth earthquakes to obtain a general picture of slab earthquakes. A large event with seismic moment Mo > 5 × 10 27 dyn cm tends to be a multiplet that consists of several subevents. A small event with Mo < 5 × 10 26 dyn cm remains a singlet. Stress drop Δσ of the largest subevent is 800 bar on average and significantly higher than Δσ(∼ 200 bar) of the largest subevent for interplate thrust earthquakes. Stress drop tends to scatter for smaller earthquakes beyond the resolution limit of our analysis. Deep and intermediate-depth shocks in the Tonga region possess higher Δσ, indicating a significant regionality in the mode of stress release within a subducting slab. Fault planes have, in general, preferred orientation according to the focal depth, indicating alignment of weak planes within the slab. The slab is therefore not simply elongated or shortened under down-dip extension or compression but rather rotates by bulk simple shear. We suggest that this bulk shear is a dominant mechanism for the deformation of a subducting plate, including its bending and unbending near deep-sea trenches.