Double Seismic Zone Research Articles

Ruptures of deep-focus earthquakes in the north-western Pacific and their implications on seismogenesis

SUMMARY The high resolution of broad-band seismic data is highly effective in studying ruptures associated with deep-focus earthquakes. In this study, we report results from an analysis of P and SH waveforms from eight of the largest deep-focus earthquakes that occurred between 1987 and 1992 along the north-western Pacific. In this region, broad-band data recorded at close-in distances, directly above the source zone, offer an unusual source-receiver geometry that complements observations at teleseismic distances. Among the eight earthquakes studied, six showed clear indications of directivity. The best constraint on the extent of rupture came from the steeply dipping Izu-Bonin Wadati-Benioff zone, where subhorizontal ruptures typically originate in the interior of subducted slab and propagate toward the top at high angles. In particular, rupture of the 1988 September 7 event consists of two en echelon line sources. At a depth of approximately 480 km, the overall rupture length of this earthquake constrains the minimum thickness of the seismogenic zone to be 27 ± 12 km. At this depth, our estimated thickness of the seismogenic zone seems remarkably close to the maximum thickness (˜35 km) predicted by the metastable olivine wedge model of seismogenesis. This model interprets deep-focus earthquakes as a result of transformational faulting within a region bounded approximately by the 700°C isotherm. The extent of rupture during the largest events in this region seems to be only slightly larger than the 20-25 km spacing between the two layers of a double seismic zone, found at depths between 300 and 400 km in the same general area. Beneath the Island of Sakhalin, a rupture speed greater than that of the shear wave is not required for the large, deep-focus earthquake (˜615 km) of 1990 May 12. Almost all complexity in observed waveforms for this event can be explained by a northward propagating, subhorizontal rupture composed of only two subevents.

Seismic structure of the northeastern Japan convergent margin: A synthesis

Many studies recently made on the basis of seismic observations have revealed a detailed structure of the crust and the upper mantle beneath the northeastern Japan arc and its relationship to seismic and volcanic activity. Spatial distributions of the depths to the Conrad and the Moho discontinuities, estimated from shallow earthquake data and seismic explosion data, show that both discontinuities are deep in the middle of the land area and shallow toward the coastlines of the Japan Sea and the Pacific Ocean. The Pn velocity has a lateral variation; it is as low as ∼7.5 km/s beneath the land area, while that beneath the Japan Sea and the Pacific Ocean is 8.0–8.2 km/s. It changes abruptly at the transition zones, which are located along the coastlines. Precise structure and location of the subducted Pacific plate beneath the land area is inferred from converted or reflected seismic waves at the top or bottom of the plate. The Pacific plate is composed of a thin (∼5 km) low‐velocity upper layer and a thick high‐velocity lower layer, its total thickness being 80–90 km. The upper plane seismicity of the double seismic zone is confined to the thin low‐velocity upper layer, which probably corresponds to the subducted former oceanic crust. The lower plane seismicity lies at the middle of the high‐velocity lower layer, and the lower half of the plate below it is incapable of generating earthquakes. The shallower portion of the upper surface of the plate beneath the Pacific Ocean, along which major seismicity with low‐angle thrust faultings is actually occurring, is also located by seismic observations on land and in the sea. The Pacific plate subducts at an extremely low angle of ∼5° for the first ∼25‐km depth, and then the dip steepens rather abruptly to ∼30°. Normal‐fault type events at the top of the plate have not been detected in the portion where the downward‐bending is the largest, but have been ditected near the trench axis, where it is rather small. Tomographic inversions for seismic velocity structure clearly delineate the inclined high‐velocity Pacific plate with a thickness of 80–90 km and low‐velocity zones in the crust and the mantle wedge beneath active volcanoes. Seismic attenuation tomography also shows similar zones of low‐Q value beneath active volcanoes, although its spatial resolution is much lower. The low‐velocity zones with 2–6% velocity lows are continuously distributed from the upper crust just beneath active volcanoes to a depth of 100–150 km in the mantle wedge, and are approximately parallel to the dip of the underlying Pacific plate. These low‐velocity zones probably reflect the pathway of magma ascent from a depth in the mantle wedge to the Earth's surface, corresponding to a portion of the subduction‐induced secondary mantle wedge flow.

A shallow double seismic zone beneath the central New Hebrides (Vanuatu): evidence for fragmentation and accretion of the descending plate?

A shallow double seismic zone (SDSZ) has been found in the descending Australian plate beneath the central part of the New Hebrides island arc, directly above a large gap in intermediate depth seismicity and between two seismic boundaries. Ambient seismicity occurs mostly in the upper part of the SDSZ, while earthquakes in the lower part occur in clusters (swarms or aftershocks of large earthquakes). The distance between the upper and lower levels of the SDSZ is 50–70 km, and they are joined at 80 km depth by a near‐horizontal band of seismicity. Thrust‐faulting mechanisms predominate for earthquakes in the upper level of the SDSZ. Those in the lower level, however, appear to be normal faulting, despite their being aftershocks of large thrust events. We suggest that with the absence of a pull from the detached lithosphere the upper part of the Australian plate in the region of the SDSZ is resistant to subduction, and thus the downward displacements caused by large earthquakes in the adjoining regions result in a localized rebound. The location of the aftershocks within the plate suggests that a new plate boundary is forming, which will eventually replace that outlined by the residual seismicity in the upper level. Thus the leading edge is decoupling, and the boundary will eventually shift back to the lower level of the SDSZ.

A double‐planed seismic zone in Kamchatka from local and teleseismic data

The fine structure of a double‐planed deep seismic zone is studied over a wide area of the Kamchatka peninsula. This prominent feature of deep seismic zone configuration is ascertained through the analysis of microearthquake hypocenters from the local seismic network of the Institute of Volcanology of Kamchatka and 22 focal mechanism solutions from the formal inversion of long‐period P and SH waves for events with mb≥5.5. Additionally, 11 focal mechanism solutions estimated from the first motion of P‐waves and 12 centroid moment tensor solutions of Harvard University are used. The maximum depth of the double seismic zone is 170–180 km. The two planes of seismicity are separated by 40 km at a depth of 50 km, and by 10–15 km at 180 km depth. The focal mechanism solutions of shallow earthquakes show an abrupt change from the thrust events to down‐dip compressional events at approximately 60 km depth at the upper boundary of the descending slab. Within the descending slab, the earthquakes with down‐dip tensional axis form the lower plane of the double‐planed deep seismic zone. Several earthquakes with down‐dip tensional axis are discovered in a narrow area of the upper seismic zone at the depth of about 50 km. The double seismic zone is revealed clearly in the area between ∼52°N to ∼54°N and probably extends up to ∼56°N.

The double seismic zone in Kuril‐Kamchatka: The tale of two overlapping single zones

The classic double seismic zone in the central portion of the Kuril‐Kamchatka arc seems to turn into a regular Wadati‐Benioff zone with downdip compression and extension in the northeastern and southwestern ends of the arc, respectively. To delineate this lateral changeover, we studied 33 large‐ to moderate‐sized (mb≥5.1) earthquakes that occurred at depths shallower than 200 km between 1963 and 1991. These events took place in two regions where the double seismic zone terminates and single seismic zones emerge. Precise earthquake source parameters, including focal depths and mechanisms, are determined from a careful analysis of a mixture of analog and digital P and SH waveforms recorded at teleseismic distances. Our results indicate that the double seismic zone is confined between approximately 46° and 53°N. Northeast of 53°N, intermediate‐depth earthquakes (∼70–200 km) show only downdip compressional focal mechanisms. Furthermore, the Wadati‐Benioff zone there seems to be a continuation of the upper, compressional layer of the double seismic zone. Thus the changeover from a double to a single seismic zone in this case is simply disappearing of the lower, extensional sheet of the double seismic zone. Similarly, southwest of 46°N, intermediate‐depth earthquakes show only downdip extensional mechanisms. The single seismic zone is unlikely to consist of two layers of extensional seismicity. In fact, nowhere along the entire arc did we observe two distinct seismogenic layers with a single state of strain. In other words, these extensional events did not occur in the same seismogenic structure as the upper (compressional) layer of the double seismic zone. However, we cannot ascertain whether the single seismic zone in extension is simply a southwestern extension of the lower (extensional) layer of the double seismic zone. Lateral variation from a single zone under extension in the southwest, to a double seismic zone, and finally to a single zone under compression cannot be accounted for by conventional models of a double seismic zone. On the other hand, the addition of a component of stress transmitted from depths greater than 250 km can explain the observed lateral variation in the state of strain at intermediate depths because the slab between depths of 250–450 km is under extension in the southwestern end of the arc, whereas the rest of the arc is under compression. Vanishing of the lower (extensional) layer of the double seismic zone toward the northeastern end of the arc is interpreted as a result of compression transmitted from below overshadowing extension caused by effects such as unbending. A similar reasoning applies to the southwestern termination of the double seismic zone.

Double Seismic Zone for Deep Earthquakes in the Izu-Bonin Subduction Zone

A double seismic zone for deep earthquakes was found in the Izu-Bonin region. An analysis of SP-converted phases confirms that the deep seismic zone consists of two layers separated by approximately 20 kilometers. Numerical modeling of the thermal structure implies that the hypocenters are located along isotherms of 500 degrees to 550 degrees C, which is consistent with the hypothesis that deep earthquakes result from the phase transition of metastable olivine to a high-pressure phase in the subducting slab.

An explanation for the double seismic layers north of the Mendocino Triple Junction

We propose that the gently eastward dipping double planed seismic zone observed at 15–25 km depths in the southern Cascadia subduction zone, just north of the Mendocino triple junction, is a direct consequence of the thermally controlled rheology. As the oceanic lithosphere subducts to a depth of about 15 km, the temperature regime causes a brittle‐plastic transition to occur within the oceanic crust. Thus, a ductile layer forms in the lower oceanic crust, sandwiched between the brittle upper crust and brittle upper mantle. The very high strain rates near the triple junction caused by the northward push of the Pacific plate on the Gorda plate increase the seismicity and thus accentuate the double seismic zone in this region. This model explains the focal mechanisms observed in the seismic zone and their spatial change. The double seismic layers clearly define the position of the subducting Gorda plate, previously uncertain in the Cape Mendocino region.

An Inverted Double Seismic Zone in Chile: Evidence of Phase Transformation in the Subducted Slab

Data from two microseismic field experiments in northern Chile revealed an elongated cluster of earthquakes in the subducted Nazca plate at a depth of about 100 kilometers in which down-dip tensional events were consistently shallower than a family of compressional earthquakes. This double seismic zone shows a distribution of stresses of opposite polarity relative to that observed in other double seismic zones in the world. The distribution of stresses in northern Chile supports the notion that at depths of between 90 to 150 kilometers, the basalt to eclogite transformation of the subducting oceanic crust induces tensional deformation in the upper part of the subducted slab and compressional deformation in the underlying mantle.

The 1993 Kushiro‐Oki, Japan, Earthquake: A high stress‐drop event in a subducting slab

A spatial slip distribution of the 1993 Kushiro‐Oki earthquake (Mw; 7.6) is undoubtedly constrained using near‐field data and the detailed aftershock distribution. This earthquake has peculiar features in that the slip on the fault plane is restricted within an extremely small area (about 40 km × 20 km) between the upper and lower planes of the double seismic zone in the subducting slab beneath Hokkaido. The total seismic moment, averaged slip, and stress‐drop in this area are 3.3 × 1020 Nm, 5.5 m, and 4.2 × 107 Pa, respectively. This earthquake indicates that the region of the slab between the planes of the double seismic zone beneath Hokkaido is strong enough to sustain a stress of about 4 × 107 Pa. The slip motion of this event is consistent with retrograde slab migration along the Southern Kuril trench, and may represent a process by which retrograde motion of subducting slabs could occur.

Evidence for transformational faulting from a deep double seismic zone in Tonga

DOUBLE seismic zones, planes or earthquakes parallel to the dip of a subducting slab and separated by 20–40 km, provide important clues about the earthquake generating mechanisms and strain distribution inside subducting slabs. Double seismic zones have been found at intermediate depths (70–200 km) in many subduction zones1–6 but have not been previously reported in deep slabs. Here, by relocating earthquakes with a hypocentroidal decomposition technique7 and visualizing the earthquake positions and uncertainties in three dimensions, we identify a double seismic zone at depths of 350–460 km in the Tonga subduction zone. Source parameters of the earthquakes determined by waveform analysis suggest different stress orientations for the two zones, with in-plane compression in the lower zone and in-plane tension in the upper zone. The double zone may be due to transformational faulting, as olivine along the edges of a metastable olivine wedge becomes warmer and transforms to spinel8–11.

Seismicity of the Gorda Plate, structure of the continental margin, and an eastward jump of the Mendocino Triple Junction

Analysis of 9 years of data from the Humboldt Bay seismic network sheds new light on the structure and evolution of the Gorda plate and Mendocino triple junction. Significant findings include the buttressing effect of the Pacific plate which demonstrates that there is no underthrusting of the Gorda plate along the Mendocino fault, and the pattern of left‐lateral northeast‐trending faults, which demonstrates how the Gorda plate accommodates N‐S shortening without such underthrusting. Focal mechanisms are consistent with northward compression of the Gorda plate by the Pacific plate until the Gorda plate passes the triple junction, beyond which the N‐S compressive stress is effectively removed, and the focal mechanisms show a change from strike slip to normal faulting with downslab tension. Another significant feature is the shallow Benioff zone (10–40 km) which shows a double seismogenic layer. Unlike most other double seismic layers in deep subduction zones, this one appears to be due to underplating, with a new subduction zone being developed east of the former one. A corner of continental margin material has been partially subducted. We attribute this double seismic zone to reactivation of both the old and new subduction boundaries under N‐S compression produced by Pacific‐Gorda plate interaction at the triple junction. Because the geometry of this triple junction indicates that it is unstable and because there is evidence that as part of its evolution the San Andreas fault has migrated eastward, we have constructed a model that accounts for both the eastward migration of the San Andreas fault and the doubling of the seismic zone in the Gorda plate. This can be done by assuming that at the time of the last eastward jump of the fault, the overriding continental margin north of the triple junction was broken and underthrust producing a new subduction zone that is collinear with the San Andreas fault. Other scenarios for the evolution of the Mendocino triple junction do not require that the subduction zone migrate eastward. However, the position and dip of the double zone indicate that an eastward jump of 100 km some 5 my ago could have taken place. This is consistent with other evidence for an eastward jump of the San Andreas fault.

3-D Spatial Variation ofb-Values of Magnitude-Frequency Distribution Beneath the Kanto District, Japan

A method is developed for estimation and interpolation of b-values in space. A 3-D spline function is considered for the logarithm of the b-value at each location in the space. Since many parameters for the spline coefficients are required to obtain a sensible estimate of the spatial variation of b-values, we consider the penalized log-likelihood with the standard roughness penalties for the spline function. Further the error bands of the b-value estimation at each location can be calculated. Using the current method, the spatial distribution of b-values beneath the Kanto District down to the depth of 100 km is determined based on hypocentral data of microearthquakes from the Kanto-Tokai Observational Network of the National Research Center for Disaster Prevention. The stability of the estimated pattern is checked by comparing with the results using alternative cut-off magnitudes. This is further ensured by comparison with the result obtained by an alternative model using equally divided blocks. On the whole, the vertical change in b-value is greater than the horizontal one. It is high in the crust of the Eurasian plate, especially above the upper boundary of the subducting Pacific plate and in the northwest part, or the volcanic area, in the Kanto District. A steep decrease of the b-values is seen to take place in perpendicular direction to the subducting Pacific plate boundary. Also, a similar change is seen in the boundary between the Eurasian and Phillippine Sea plates, especially beneath the southern part of the Kanto Plain. The b-value is low in the upper boundary of the subducting plates, but high in the lower plane of the double seismic zone in the Pacific plate. It appears that, even within a narrow area of aftershocks, the b-value can change significantly. It is also found that the variation of the b-value estimate is in good agreement with the structure of seismic wave fractional velocity perturbations. The regions of high and low b-values correspond, respectively, to the lower and higher parts of the P-wave velocity. The similar relationship is seen with the spatial structure of the seismic wave attenuation.

Earthquake focal mechanisms and seismotectonic deformation in the Kamchatka-Commander region

The system of stresses in the lithosphere (0–59 km) and upper asthenosphere (60–150 km) was analysed on the basis of focal-mechanism data for events of M > 5.0 in 1964–1982. The zoning was made considering types of faulting and predominant stress systems. Four uniformly-stressed zones have been identified within the lithosphere and two zones within the upper asthenosphere. The deep distribution of types of faulting and stresses shows the presence of a double seismic focal zone under Kamchatka. Schemes of seismotectonic-deformation (STD) rate-tensor components were dran separately for the lithospheric and asthenospheric layers. To this end, data on individual and averaged focal-mechanism solutions and regional seismic moment-energy class relationships were used. The average STD rate for the lithosphere is 10 −9 yr −1, and for the upper asthenosphere 10 −10 yr −1. Vertical and horizontal components of STD are of the same order.

Regional extent of the S wave reflector beneath the Kanto District, Japan

An S wave reflector was found near the upper plane of the double seismic zone associated with the subducting Pacific plate beneath the southern Kanto district of Japan. Here the areal extent of the reflector is confirmed for a region ranging from 34.5°N to 36.7°N, from the analysis of the travel times of the reflected S waves on 8‐Hz band seismograms. A bend of the reflector is found at about 36°N. It is coincident with the bend of the upper boundary of the subducting Pacific plate obtained from the seismicity analysis. In the middle of the Kanto district the reflector extends to a depth of 120 km, whereas in the northern Kanto district it extends just beneath the volcanic front. The reflector could not be verified for depths shallower than 60 km depth because the reflected S wave cannot be identified at the eastern most stations of the network.

Existence of an S wave reflector near the upper plane of the double seismic zone beneath the Southern Kanto District, Japan

The amplitude of coda from local shallow earthquakes in the southern Kanto district of Japan does not decay smoothly with time. A clear phase appears in the S coda at lapse time from 15 to 45 s measured from the S wave arrival, and it is predominant for frequencies around 8 Hz. This phase seems to be an S wave reflected from a discontinuity inclined westward at upper mantle depths since it is predominantly on the horizontal components and propagated westward with a high apparent velocity. Applying an inversion method to the travel times of the phases read from 8‐Hz band seismograms, we locate the S wave reflector near the upper plane of the double seismic zone associated with the subducting Pacific plate. The S wave reflector is resolved east of the volcanic front at a depth of 80–100 km. If we put Qs−l = 10−3, the reflection coefficient is estimated to be around 0.5; however, the scatter is large. The S wave impedance contrast at the interface seems to be very large. It may suggest the existence of a zone containing many liquid bodies.

Plastic instabilities: Implications for the origin of intermediate and deep focus earthquakes

Adiabatic or catastrophic plastic shear has been reported in metals, polymers, and metallic glasses. The phenomenon is associated with rapid stress drops and audible pings or clicks as the material deforms in a plastic manner. The driving force for the plastic instability is the stored elastic strain energy of the loading system, and in many respects the behavior is reminiscent of the shear stress response arising from stick slip events during unstable frictional sliding, although the precise mechanism is different. It is shown here that adiabatic plastic shear is capable of explaining the detailed distribution of intermediate and deep focus earthquakes within subduction zones, the earthquake events being the result of instabilities in material undergoing plastic flow. It is argued that for a particular strain rate there exists a critical temperature, TC, which is depth dependent; for temperatures below TC the material is strain rate softening and, for a soft enough loading system, may undergo catastrophic plastic shear. For temperatures above TC the material is strain rate hardening and is always stable during plastic shear. The cutoff depth for deep focus earthquakes then corresponds to the transition from strain rate softening to strain rate hardening material, and for commonly accepted geothermal gradients within the slab corresponds to approximately 800 km. The precise distribution of earthquakes within the slab is a function of the subtle interplay between the geothermal gradient and the TC gradient. In particular, a decrease in seismic activity is to be expected below about 300 km in the slab with total stress drops decreasing from a maximum of 700 MPa above 300 km to a maximum of ≈ 50 MPa below 300 km. The differences in foci distribution between subduction zones such as Tonga, New Hebrides, and Peru result from minor differences in the geothermal gradients within the slabs. The model predicts the development of triple seismic zones high in the slab, double seismic zones down to approximately 300 km, and single seismic zones down to ≈ 800 km. Such a distribution is to be expected of relatively young, cool slabs; as the slab heats up, the seismic activity retreats up the slab. The paper only proposes a deformation mechanism for earthquake generation, it does not address the stress field within the slab but only the distribution of strength. Thus the distribution of focal plane mechanisms is not considered, only the locations where earthquakes due to plastic instabilities are possible. The absence of earthquakes does not necessarily mean that the slab does not exist, it only means that the slab is too hot to undergo plastic instability. This means that aseismic subduction is a distinct possibility in many regions of high geothermal gradient within the slab (i.e. > circa 3°C km−1).

Three‐dimensional fine velocity structure and hypocentral distribution of earthquakes beneath the Kanto‐Tokai District, Japan

We determine the three‐dimensional P wave velocity structure beneath the Kanto‐Tokai district, central Japan, and simultaneously relocated by inverting travel time data for 91 events that were observed at 61 stations operated by the National Research Center for Disaster Prevention. The variance of the travel time data is reduced by 75%, as compared to the variance for the routinely used laterally homogeneous model. In general, our velocity model is consistent with tectonic models of the Kanto‐Tokai district. Our velocity model is also in good agreement with seismic explosion data and Bouguer gravity anomalies. The uppermost layer (from 0 to 16 km depth) is characterized by lateral velocity differences of more than 20%. We find low‐velocity zones beneath Tokyo Bay, the middle of the Boso Peninsula, and Omae‐zaki, where there are thick sedimentary layers. A marked 30‐km‐thick high‐velocity zone extends to the northeast and the west of the Izu Peninsula. This zone dips gradually from the surface to a depth of 70 km in the eastern region and to a depth of 40 km in the western region and is interpreted as the Philippine Sea (PHS) plate, which is being subducted at the Sagami and the Suruga troughs. The data unquestionably require a high‐velocity zone beneath Mount Tsukuba that cannot be explained in terms of present tectonic models. We use our three‐dimensional velocity model to relocate about 23,000 earthquakes that took place from 1983 through 1985. The accuracy of the hypocentral parameters improved considerably. As a result, the double seismic zone within the subducting Pacific plate is more clearly delineated than in previous studies. The inclined seismic zone for the subducting PHS plate coincides fairly well with the high‐velocity zone of our three‐dimensional velocity model.

Slab pull and the seismotectonics of subducting lithosphere

This synthesis links many seismic and tectonic processes at subduction zones, including great subduction earthquakes, to the sinking of subducted plate. Earthquake data and tectonic modeling for subduction zones indicate that the slab pull force is much larger than the ridge push force. Interactions between the forces that drive and resist plate motions cause spatially and temporally localized stresses that lead to characteristic earthquake activity, providing details on how subduction occurs. Compression is localized across a locked interface thrust zone, because both the ridge push and the slab pull forces are resisted there. The slab pull force increases with increasing plate age; thus because the slab pull force tends to bend subducted plate downward and decrease the force acting normal to the interface thrust zone, the characteristic maximum earthquake at a given interface thrust zone is inversely related to the age of the subducted plate. The 1960 Chile earthquake (Mw 9.5), the largest earthquake to occur in historic times, began its rupture at an interface bounding oceanic plate <30 m.y. old. However, this rupture initiation was associated with the locally oldest subducting lithosphere (weakest coupling); the rupture propagated southward along an interface bounding progressively younger oceanic lithosphere, terminating near the subducting Chile Rise. Prior to a great subduction earthquake, the sinking subducted slab will cause increased tension at depths of 50–200 km, with greatest tension near the shallow zone resisting plate subduction. Plate sinking not only leads to compressional stresses at a locked interface thrust zone but may load compressional stresses at plate depths of 260–350 km, provided that the shallow sinking occurs faster than the relaxation time of the deeper mantle. This explains K. Mogi's observations of M ≥ 7 thrust earthquakes at depths of 260–350 km, immediately downdip and within 3 years prior to five great, shallow earthquakes of northern Japan. The slab pull model explains the lower layer of double seismic zones as due to tension from the deeper, sinking plate and the upper layer as due to localized in‐plate compression, as plate motion is resisted by the bounding mantle. Just downdip of the interface thrust zone, there occurs an aseismic 20°–50° dip increase of subducted plate. This slab bend reflects the summed slab pull force of deeper plate and probably is at the crustal basalt to eclogite phase change. Resistance to subduction provided by a continually developing slab bend may be an important factor in the size of slab pull force delivered to an interface thrust zone.

Transition from double to single Wadati‐Benioff seismic zone in the Shumagin Islands, Alaska

A discontinuity exists in the moderate‐depth seismicity located by the Shumagin seismic network in the eastern Aleutian subduction zone. The boundary is nearly perpendicular to the arc and has a double planed Wadati‐Benioff zone to the west and a single planed zone to the east. The transition does not appear to be an artifact of mislocations, as shown by three‐dimensional ray trace modeling and examination of phases of representative earthquakes. Current theory on the presence or absence of double seismic zones suggests a change in stress across the boundary. If so, the east and west regions may either have different aseismic to seismic slip ratios, or be in different stages of the earthquake cycle.

Downdip tensional earthquakes beneath the Tonga Arc: A double seismic zone?

Characteristics of seismicity at intermediate depth beneath the Tonga arc are studied. Special attention is paid to newly identified downdip tensional events because the dominance of downdip compression has been known in this area. We reexamine the relatively shallow (<70 km) downdip tensional events, reported in the Harvard centroid moment tensor solutions, by modeling bodywave waveforms of World‐Wide Standard Seismograph Network seismograms and conclude that they are in fact intraplate downdip tensional events. Furthermore, these downdip tensional events are consistently located below and seaward of downdip compressional events and appear to constitute a double seismic zone. The result of a relative relocation technique applied to these earthquakes also supports this observation, which was originally based on routinely determined epicenters. The subducting slab beneath the Tonga arc has been considered as a prototype of a compressionally loaded slab. The existence of a double seismic zone in Tonga, therefore, would suggest that double seismic zones, located at intermediate depth in subducting slabs, are more common than previously thought.