Deep Earthquake Zone Research Articles

Anomalously low aftershock productivity of the 2019 MW 8.0 energetic intermediate-depth faulting beneath Peru

The 26 May 2019 Peru (MW 8.0) earthquake struck within the nearly-horizontal underthrust Nazca plate at depths from ∼110 to 150 km below the upper Amazon, near a steep bend in the plate where it plunges down to a deep earthquake zone. Little prior seismicity occurred in this region, but large intraslab events with similar normal-faulting mechanisms have occurred to the west. The event is situated in a similar slab position to the 2017 Puebla-Morelos, Mexico earthquake, but the remote location resulted in limited loss of life and damage. Back-projection imaging and finite-fault inversion based on teleseismic data suggest a brittle and energetic rupture process with unilateral expansion northward over a 170-km-long zone at a rupture speed, Vr ∼3 km/s, with three normal-faulting patches of up to ∼4.5 m slip. Despite the mainshock size, it produced only three M4.0+ aftershocks within 300 km (one nearby); the aftershock productivity of the 2019 Peru earthquake is very low even among all large intermediate-depth earthquakes, but similar to that for other large Peruvian intraslab events. Comparison of aftershock productivity of the Peru event with that of global large earthquakes in various tectonic settings suggests that the low aftershock productivity can largely be attributed to regionally homogeneous faulting systems and relatively uniform stress state in the flat Peru slab.

Influence of Tidal Forces on the Triggering of Seismic Events

Tidal stresses are generated in any three-dimensional body influenced by an external inhomogeneous gravity field of rotating planets or moons. In this paper, as a special case, stresses caused within the solid Earth by the body tides are discussed from viewpoint of their influence on seismic activity. The earthquake triggering effects of the Moon and Sun are usually investigated by statistical comparison of tidal variations and temporal distribution of earthquake activity, or with the use of mathematical or experimental modelling of physical processes in earthquake prone structures. In this study, the magnitude of the lunisolar stress tensor in terms of its components along the latitude of the spherical surface of the Earth as well as inside the Earth (up to the core-mantle boundary) were calculated for the PREM (Dziewonski and Anderson in Phys Earth Planet Inter 25(4):297–356, 1981). Results of calculations prove that stress increases as a function of depth reaching a value around some kPa at the depth of 900–1500 km, well below the zone of deep earthquakes. At the depth of the overwhelming part of seismic energy accumulation (around 50 km) the stresses of lunisolar origin are only (0.0–1.0).103 Pa. Despite the fact that these values are much smaller than the earthquake stress drops (1–30 MPa) (Kanamori in Annu Rev Earth Planet Sci 22:207–237, 1994) this does not exclude the possibility of an impact of tidal forces on outbreak of seismic events. Since the tidal potential and its derivatives are coordinate dependent and the zonal, tesseral and sectorial tides have different distributions from the surface down to the CMB, the lunisolar stress cannot influence the break-out of every seismological event in the same degree. The influencing lunisolar effect of the solid earth tides on earthquake occurrences is connected first of all with stress components acting parallel to the surface of the Earth. The influence of load tides is limited to the loaded area and its immediate vicinity.

A recent deep earthquake doublet in light of long-term evolution of Nazca subduction

Earthquake faulting at ~600 km depth remains puzzling. Here we present a new kinematic interpretation of two Mw7.6 earthquakes of November 24, 2015. In contrast to teleseismic analysis of this doublet, we use regional seismic data providing robust two-point source models, further validated by regional back-projection and rupture-stop analysis. The doublet represents segmented rupture of a ∼30-year gap in a narrow, deep fault zone, fully consistent with the stress field derived from neighbouring 1976–2015 earthquakes. Seismic observations are interpreted using a geodynamic model of regional subduction, incorporating realistic rheology and major phase transitions, yielding a model slab that is nearly vertical in the deep-earthquake zone but stagnant below 660 km, consistent with tomographic imaging. Geodynamically modelled stresses match the seismically inferred stress field, where the steeply down-dip orientation of compressive stress axes at ∼600 km arises from combined viscous and buoyant forces resisting slab penetration into the lower mantle and deformation associated with slab buckling and stagnation. Observed fault-rupture geometry, demonstrated likelihood of seismic triggering, and high model temperatures in young subducted lithosphere, together favour nanometric crystallisation (and associated grain-boundary sliding) attending high-pressure dehydration as a likely seismogenic mechanism, unless a segment of much older lithosphere is present at depth.

Arc Magmatic Tempos: Gathering the Evidence

Arc Magmatic Tempos: Gathering the Evidence

Three-dimensional structure of so-called double seismic zone of deep earthquakes in Northeast Japan.(<Special issue>Crustal structure and geotectonic history, Northeast Japan Arc)

Three-dimensional structure of so-called double seismic zone of deep earthquakes in Northeast Japan.(<Special issue>Crustal structure and geotectonic history, Northeast Japan Arc)

The Application of Finite Element Analysis to Body Wave Propagation Problems

Summary The finite element method is shown to be a powerful tool for the numerical modelling of seismic body wave propagation problems. Applications extend to both problems on a scale of interest to engineers and also to large-scale seismological problems. Solutions are sought in the time domain. Efficient programs have been written to accomplish this. The scope of numerical solutions has been greatly enhanced by the use of a previously reported scheme for exactly cancelling reflections at the boundaries of the model. The finite difference results of Boore and the analytical results of Trifunac for the amplification due to a mountain and an alluvial valley respectively are compared with new finite element results. The new results agree well, although there are some difficulties with resonance in the alluvial valley problem. Boore's SH results have been extended to vertical P and SV incidence. A deep earthquake zone has been modelled realistically in two dimensions and earthquakes simulated at depth. It is suggested that the variation in observed amplitude across the top of the zone, due to refraction away from the slab, may be used to provide an estimate of the thickness of the slab from long-period observations of local earthquakes.

Deep magma transport at Kilauea volcano, Hawaii

The shallow part of Kilauea's magma system is conceptually well-understood. Long-period and short-period (brittle-failure) earthquake swarms outline a near-vertical magma transport path beneath Kilauea's summit to 20 km depth. A gravity high centered above the magma transport path demonstrates that Kilauea's shallow magma system, established early in the volcano's history, has remained fixed in place. Low seismicity at 4–7 km outlines a storage region from which magma is supplied for eruptions and intrusions. Brittle-failure earthquake swarms shallower than 5 km beneath the rift zones accompany dike emplacement. Sparse earthquakes extend to a decollement at 10–12 km along which the south flank of Kilauea is sliding seaward. This zone below 5 km can sustain aseismic magma transport, consistent with recent tomographic studies. Long-period earthquake clusters deeper than 40 km occur parallel to and offshore of Kilauea's south coast, defining the deepest seismic response to magma transport from the Hawaiian hot spot. A path connecting the shallow and deep long-period earthquakes is defined by mainshock–aftershock locations of brittle-failure earthquakes unique to Kilauea whose hypocenters are deeper than 25 km with magnitudes from 4.4 to 5.2. Separation of deep and shallow long-period clusters occurs as the shallow plumbing moves with the volcanic edifice, while the deep plumbing is centered over the hotspot. Recent GPS data agrees with the volcano-propagation vector from Kauai to Maui, suggesting that Pacific plate motion, azimuth 293.5° and rate of 7.4 cm/yr, has been constant over Kilauea's lifetime. However, volcano propagation on the island of Hawaii, azimuth 325°, rate 13 cm/yr, requires southwesterly migration of the locus of melting within the broad hotspot. Deep, long-period earthquakes lie west of the extrapolated position of Kilauea backward in time along a plate-motion vector, requiring southwesterly migration of Kilauea's magma source. Assumed ages of 0.4 my for Kilauea and 0.8 my for Mauna Loa are consistent with this model. Younger ages would apply if Kilauea began its growth south of the locus of maximum melting, as is true for Loihi seamount. We conclude that Kilauea is fed from below the eastern end of the zone of deep long-period earthquakes. Magma transport is vertical below 30 km, then sub-horizontal, following the oceanic mantle boundary separating plagioclase- and spinel-peridotite, then near-vertical beneath Kilauea's summit. The migration of the melting region within the hotspot and Kilauea's sampling of different sources within the melting region can explain (1) the long-term geochemical separation of Kilauea from neighboring volcanoes Mauna Loa and Loihi, and (2) the short-term changes in trace-element and isotope signatures within Kilauea.

Variations in P wave speeds and outboard earthquakes: Evidence for a petrologic anomaly in the mantle transition zone

We investigate the nature of laterally varying P wave speeds (Vp) and their relationship to a group of unusual deep earthquakes beneath the backarc of Tonga. This subhorizontal swath of “outboard” earthquakes occurs within the transition zone of the mantle but is several hundred kilometers away from the inclined Wadati‐Benioff zone. Surprisingly, the seismogenic region is not associated with fast Vp. It follows that the effect of cold temperature, as implied by the presence of large earthquakes, must have been counteracted by other factors such as compositional or mineralogical effects. Our results are based on a sequence of seismic profiles in a fan‐shot geometry. For a given azimuthal sector, we seek the simplest model of radially varying Vp that explains observed broadband waveforms and travel times. The lateral extent of anomalous Vp is then constrained by comparing the results from a wide range of azimuths. This approach fully utilizes information from triplicate P arrivals that are sensitive to seismic wave speeds at the depths of interest. Our results identify prominent anomalies of fast Vp (∼3% over average Earth models) both in the transition zone and in the lower mantle beneath the Tonga backarc. However, the region of fast Vp in the transition zone is offset by ∼300 km to the south of that in the lower mantle, and the region of offset coincides with the outboard earthquakes. A straightforward interpretation is that a large flux of subducted Pacific lithosphere lingers in the transition zone before descending into the lower mantle. Within this broad remnant of subducted lithosphere, the outboard earthquakes occur in a region where impounding of subducted material is most significant, a condition favorable for the accumulation of met astable olivine or volatiles. Such materials reduce Vp and are candidates for triggering earthquakes, thus accounting for the absence of fast Vp in the source zone of deep earthquakes.

Imaging of subducted lithosphere beneath South America

Tomographic images are produced for the deep structure of the Andean subduction zone beneath western South America. The data used in the imaging are the delay times of P, pP and pwP phases from relocated teleseismic earthquakes in the region. Regionally, structural features larger than about 150 km are resolved by the data. Presentations of layer anomaly maps and cross sections reveal: (1) The Nazca slab is probably continuous laterally and at depth over most regions; (2) The offset between the north and south deep earthquake zones, containing the 1994 deep Bolivia main shock and its aftershocks, can be modelled by a northwest striking and steeply northeast dipping slab structure; and (3) The Nazca slab clearly penetrates the lower mantle beneath central South America, but is partly deflected in the southern deep zone.

Kiyoo Wadati and early research on deep focus earthquakes: Introduction to Special Section on Deep and Intermediate Focus Earthquakes

This special section of the Journal of Geophysical Research is dedicated to Kiyoo Wadati. This paper evaluates his contribution to our knowledge of geophysics and, especially, deep earthquake phenomena. In at least three areas, Wadati wrote several papers before 1936 which profoundly influenced modern geophysics. First, his work provided the first convincing evidence that deep earthquakes existed. Second, he was a leader in the construction of travel time tables and the determination of mantle velocity structure. Finally, he published the first accurate description of the inclined planar zone of deep earthquakes which extends from trenches beneath volcanic island arcs. Wadati's work strongly influenced research of many other scientists both before and after World War II, particularly, Jeffreys, Gutenberg, and Benioff. Several of the questions raised by the research of Wadati and others before 1940 are still unanswered today.

九州-琉球列島における稍深発地震とテクトニクス

The minute investigation of the spatial distribution of intermediate-depth earthquakes reveals that the deep earthquake zone beneath the Kyushu-Ryukyu region is dislocated transversely and changes its strike under the Tokara Channel.Left-lateral dislocations of the isobaths of intermediate-depth earthquakes amount to 70-80km and 30-40km at depths of 80km and 150km, respectively. The isobath of the depth of 200km, however, is scarcely dislocated. The change in strike of the deep earthquake zone comes up to about 30° there.The discontinuity of the zone appears to be related closely to the surface geology. The Tokara Channel represents the left-lateral geological fault offsetting by 80-100km the tectonic belts in the pre-Miocene basement rocks running parallel to the island arc. In this region, there are a few left-lateral faults including the Tokara Channel, which traverse the Ryukyu Islands and divide the pre-Miocene tectonic belts into segments of 200 to 400km in strike length. But the other transverse discontinuities of the deep earthquake zone corresponding to the surface geological structure can not be found from the present analyses.

A regional study of mantle velocity variations beneath eastern Australia and the southwestern Pacific using short-period recordings of P, S, PcP, ScP and ScS waves produced by Tongan deep earthquakes

Deep earthquakes located in the Tonga-Kermadec region produce exceptionally clear and sharp short-period P, S, PcP, ScP, and ScS phases which are recorded at many stations at distances of less than 60°. The data used in this study are produced by short-period stations located in oceanic-type regions (Fiji and New Caledonia), a mobile continental region (eastern Australia) and a shield region (central Australia). Differential travel-time residuals of the above phases at these stations are investigated to determine the contribution to the differential residuals from: (1) the upper part of the mantle (S-P residuals); (2) the core-to-station portion of the mantle (ScS-ScP residuals); and (3) the hypocenter-to core portion of the mantle (ScP-PcP residuals). The use of differential travel-time residuals considerably reduces near-station effects and effects due to inaccurate determination of the source parameters, and hence the results can be interpreted as due to variations along the propagation paths. The results show that (S-P) residuals from phases traveling along event-to-station paths are about 7 s smaller at the shield station than at the oceanic stations. This correlation with surface tectonic environments is equally strong for the (ScS-ScP) residuals, with the shield/oceanic station difference being about 4 s. Moreover, the data suggest that this correlation between differential residuals and surface tectonic environments is caused by variations in shear velocity within the upper part of the mantle. However, the data cannot uniquely resolve the required depth of these variations within the mantle. For example, if the shear velocity variations extend to a depth of 400 km beneath the recording stations, then the average shear velocity difference between shield- and oceanic-type environments is about 4%. However, if the variations extend only to a depth of 200 km, this difference is more than 8%. (ScP-PcP) and (ScS-PcS) residuals vary from about +1 to about +4 s at the different stations, apparently because of compressional velocity variations in the mantle along the Pc path. If the variation in compressional velocity within the mantle below a depth of about 600 km is about 10% and occurs near the source region, these results suggest that, in the vicinity of deep earthquake zones, variations in compressional velocity extend to a depth of about 1000 km. However, these results can equally be explained by a 1% variation in compressional velocity, evenly distributed along the entire Pc path. An estimate of Q determined from the observed predominant frequency of ScS waves, as recorded at the shield station, suggests that the average 〈 Q s〉 of the mantle beneath about 600 km is about 1050 at frequencies of about 1 Hz.

Fault-plane solutions and stress regime in Italy and adjacent regions

The paper is a sequel to one published earlier (Cagnetti et al., 1976) in which 81 earthquakes that occurred in Italy and adjacent regions were summarized and discussed. Here the characteristics of the positions of the principal stress axes of these solutions, of five separate seismic regions within the investigated area, have been studied. The stress model for each region has been determined, and by a comparison of these stress fields it appears that the whole region can be divided into two parts, with different stress regimes. The first one includes the Alps, the Dalmatian coast, Albania, western Greece and the southeastern Ionic margin of the Tyrrhenian arc and is characterized by a compressional regime; the direction of compression is generally perpendicular to the orographic axis of the mountain chains and to the seismic lines, and is originated and controlled by the relative movement between the plates in contact in the Mediterranean area. The latter, characterized by tensional stress regime, includes the central-northern Apennines, the Tyrrhenian basin and some narrow areas along the Dalmatian coast. This area is in a state of tension in a roughly east-west direction, and in the deep earthquake zone is affected by nearly vertical compressional stresses. These results are compared with the seismotectonics of the area and other geophysical data.

THE FORMATION OF INTERMEDIATE AND DEEP EARTHQUAKE ZONE IN RELATION TO THE GEOLOGIC DEVELOPMENT OF EAST ASIA SINCE MESOZOIC

The intermediate and deep earthquake zone comes into existence under the condition that the continental side upheaves and the oceanic side subsides, judging from the geologic development of East Asia including the Japanese Islands since the Mesozoic age.The authors try an experiment by F.E.M. to examine the internal condition accompanied by the vertical movement in the deeper part of the earth and explain the geologic development of East Asia including the Japanese Islands and the earthquake zone.

Subcrustal earthquakes beneath New Zealand; Locations determined with a laterally inhomogeneous velocity model

Abstract Inconsistencies have become apparent in locations allocated to subcrustal earthquakes beneath the main seismic region of New Zealand. To reduce these, a simple model has been set up in which velocities are increased along ray paths presumed to be entirely in the zone of deep earthquakes. The results are most consistent when the velocities of both P and S waves are increased by 11%, compared with the standard Jeffreys-Bullen model. The effect of this velocity change is to reduce the dip of the zone of subcrustal foci determined in previous studies, and to make the foci more closely planar. The newly defined plane strikes N 45° E and dips at 50° to the north-west.

Identification of fault planes associated with deep earthquakes

Application of the method of Joint Hypocenter Determination (JHD) to carefully determined arrival time data from a small network of sensitive and well‐distributed local and teleseismic stations provides very precise locations of deep earthquakes. The precision of locations attained by this method—the relative locations are good to about 5 km—is limited mainly by the accuracy and precision of the measurements of arrival times. Application of the technique to a small and very active part of the deep earthquake zone of the Tonga island arc (depths near 600 km) reveals probable fault planes associated with the occurrence of deep earthquakes. The spatial distribution of precisely relocated hypocenters is closely related to the orientation of the focal mechanism solutions of two deep earthquakes. Hypocenters cluster in a plane parallel to one of the nodal planes for each of the two solutions examined. The planar features are yearly vertical and have linear dimensions of about 40 km. In one case the shocks defining the fault plane also tended to cluster in time near the occurrence of the largest event. These and other data suggest the existence of several distinct fault planes in the area studied. Our study is being extended to examine the structure of the entire Tonga deep seismic zone.

On the P-wave velocity and plate-tectonics implications for the Tyrrhenian deep-earthquake zone

P-wave velocities in the Tyrrhenian mantle have been determined for the 230–480 km depth range. Analysis of P-wave travel times for a set of Tyrrhenian deep earthquakes gives a velocity-distribution law which shows different behaviours in the 230–300 km and 300–480 km depth intervals. For the first interval the velocity gradient is 0.64 · 10 −2 sec −1 and for the second one it is 0.59 · 10 −2 sec −1. At a depth of 300 km the velocity decreases rapidly from 8.75 to 8.43 km/sec. The results have been analyzed in the framework of a Tyrrhenian structural model characterized by a lithospheric slab dipping 55–60° in the WNW direction. It is also pointed out that the analysis of some geodynamic features of the slabs of Pacific island arcs carried out by Oliver et al. (1973) and Sleep (1973) can be applied to the Tyrrhenian mantle geodynamic features.

A comparison of the anelasticity structure beneath western South America and Japan

Similar spectral ratio techniques were used for Q-determinations in two regions. In South America, Q P in the (somewhat seismically active) wedge bounded by the seismic plane, the trench and volcanoes, is 1000–2000. Beneath Japan for the corresponding region (which is a seismic) Q P is 400–500; this value is identical to that in the asthenosphere below the trench, i.e., there is no evidence for extra low Q-values beneath the volcanoes. In South America, the high Q P-values (up to 3000) extend to 350 km, but there do not seem to be any paths from deep earthquakes (600 km) with Q P greater than 1000, i.e., there must be a Q-inversion below 350 km. In Japan, however, the high Q P-values in the Benioff zone (≈3000) persist down to the deep earthquakes. Above the deep earthquake zone in Japan and Fiji, very low Q P-values (100) were found, in contrast to a minimum value of 350 in South America. The tectonic implications of these structures are that lithospheric plates have varying thicknesses; continental plates are several times thicker than oceanic plates.

深発地震域内の地震波速度

By the application of the refraction law to the readings of P and S times at Morioka (MOR), Miyako (MIY) and Ofunato (OFU), in northern Honshu, for deep earthquakes that occurred in Hokkaido and the Sea of Okhotsk, the seismic wave velocities in the "deep earthquake zone" (high-V, high-Q zone) are estimated.The seismic waves of relatively shallow earthquakes occurring in this zone travel from their origins to the above stations over the deep earthquake zone alone. However, waves from origins deeper than a certain depth refract on their way in the "low-V, low-Q region" that exists just beneath the deep earthquake zone (Fig.7). This low-V, low-Q region is also characterized by high-a, as was suggested by Utsu (1969) from the analysis of the values of Vp/Vs. The observed values of Vp/Vs may suggest that the waves passed through the low-V, low-Q region or not.For the present data, the values of Vp/Vs for earthquakes deeper than about 300 km are somewhat larger than those for earthquakes shallower than that depth (Fig.8). This suggests that the waves from earthquakes deeper than about 300 km may have travelled on the way in the low-V, low-Q region beneath the deep earthquake zone. Then, only those data that are shallower than about 300 km are available for estimating the velocity.In order to strip off the upp e rmost layer of 50 km thickness, the travel times of P and 5, and values of dT/dA for an origin of 50 km depth are obtained from earthquakes of about 50 km depth which occurred off the east coast of northeastern Honshu (Fig.9). On reference to the observed values of dT/di, the data on the surface of the earth are converted to those at the surface of a depth of 50 km.In the calculation of the velocity, a layered structure is assumed and the velocity in a layer of depths from 50 km to 100 km, v1, is given properly. In the first place, the average velocities for layers from 100 km to the depths of the earthquakes are estimated from the converted A and T for respective shocks of depths of 114 km and 132 km. Next, by applying Bullen's relation, v=A exp (-kh), to the value of v1 and estimated velocities, v, the value of k for the layer from 50 km to 130 km is estimated, and the velocity at a depth of 115 km is adopted as the average one in the layer from 100 km to 130 km.By the same procedure, average velocities for layers of depths from 130 km to 160 km and from 160 km to 220 km are estimated from earthquakes occurring in these layers. Finally, Bullen's relation is applied once again to all values of v.Velocities with depth in the deep earthquake zone, larger by about 2-6% than those by Jeffreys-Bullen or Honda, and Sagisaka and Takehana, were obtained and shown in Fig.12, where the values given of v1 are 8.05 km/sec and 4.63 km/sec for P and S waves, respectively.

The upper mantle beneath Japan inferred from P traveltime anomalies by means of three dimensional ray tracing. I.

The nuclear explosion Cannikin in the Central Aleutian islands detonated in 1971 was observed at about 150 seismometrical stations in Japan. These data were used with the purpose of investigation of lateral inhomogeneity in the upper mantle associated with the Japan Island arc by means of a three-dimensional-ray-tracing method recently developed. The upper mantle was divided into three zones in terms of velocity contrast based on the work by Utsu; 1) the low velocity zone, uppermost 275km in the mantle, on the continental side of the inclined deep and intermediate earthquake zone, 2) the high velocity inclined seismic zone of 100-km thick reaching to a 350-km depth, 3) the low velocity zone, uppermost 350km in the mantle, on the Pacific side of the inclined seismic zone. The best-fit velocity contrasts in each zone to a standard earth model were -4, +3, and -1% for the above mentioned zones 1), 2), and 3), respectively. This model is in good agreement with the observed P-wave traveltime anomalies in north-east Japan, however, inconsistent in detail with the observation in south-west Japan, although it explains a general trend of the observed traveltime anomalies in whole Japan. Suitability of the present model is discussed comparing with tho observation.