Late Quaternary Volcanism Research Articles

A temporal dissection of late Quaternary volcanism and related hazards within the Rio Grande rift and along the Jemez lineament of New Mexico, USA

Abstract New 40Ar/39Ar ages, combined with selected ages from published studies, provide detailed insight into the late Quaternary (<500 ka) eruptive history and related hazards of the Rio Grande rift and Jemez lineament of New Mexico, USA. Most eruptions in the region during this time were within monogenetic volcanic fields, which largely produced cinder cones and mafic lava flows. 40Ar/39Ar ages of mafic groundmass determined using the high-sensitivity ARGUS VI multicollector mass spectrometer are significantly more precise, by as much as an order of magnitude, than prior 40Ar/39Ar dates. The high-precision data permit more rigorous interpretation of age spectra and isochrons, leading to a greater confidence in assigning eruption ages, and thus allowing more accurate and thorough calculations of eruptive rates and repose periods. For most fields, comprehensive dating identifies a greater number of late Quaternary eruptions than previously known and, for some fields, determines younger-than-previously established ages for the last eruptive events. Repose periods in the fields range from too short to measure with the 40Ar/39Ar method to a few hundred thousand years, which suggests that all 12 fields in the rift and lineament with late Quaternary activity should be considered dormant rather than extinct, with the possibility of future eruptions. Average recurrence intervals for these fields during the late Quaternary range from 16.5 k.y. to 170.8 k.y. Many fields display peak periods of activity where rates spike to a recurrence interval of 5 k.y. or less. At the scale of the entire rift and lineament, 75 late Quaternary eruptions were dated, yielding an average recurrence interval of 6.5 k.y., which is a minimum estimate considering the likelihood of undated eruptions (either not studied or buried). During the last 100 k.y., the volcanic record is better preserved, and the recurrence interval is 3.2 k.y., which indicates that the current hiatus of 3.9 ± 1.2 k.y. is typical for the region. Since ca. 36 ka, the average recurrence interval decreased to 2.3 k.y., which suggests a slight increase in recent activity. When ages are compared to vent locations, a previously unrecognized migrational pattern is observed in nearly all of the fields studied. Migration vectors vary from 1.0 cm/yr to 4.0 cm/yr, and always with an eastward component—similar to migration patterns at some other late Cenozoic fields throughout the American Southwest. Volcanic migration is attributed to a combination of mechanisms including asthenospheric convection along the margin of the Colorado Plateau, North American plate motion over partial mantle melt, and extensional tectonics. Developing similar high-precision chronologies for other Quaternary fields throughout southwestern North America will be necessary to better understand the volcanic hazards of the region.

Magnetic assessment of OSL and radiocarbon ages of sediments beneath a lava in Jeju Island, Korea: Implication of possible resetting of OSL signals and age constraint of the late Quaternary lava

Abstract Better chronological constraining of Late Pleistocene-Holocene volcanic eruptions is essential for figuring out spatio-temporal frequency distribution of ancient volcanism and assessing volcanic eruptions probability in a volcanic field. For this chronological frame work, radiocarbon (14C) and luminescence dating methods are being attempted alternatively, instead of K-Ar and 40Ar/39Ar techniques with lower resolutions and age limits of applicability. For the sediments capped by a lava flow in a Quaternary basaltic volcanic field of Jeju Island, Korea, we here introduce an interesting and probably frequently-encountered case of the discrepancy in age estimates between 14C and optically stimulated luminescence (OSL) chronologies. For samples with different positions in the sediments, the 14C ages of organic material extracts have a stratigraphically consistent but widely dispersed values between 2.25 ± 0.09 and 27.2 ± 0.5 calibrated (cal.) kyr BP, whereas the OSL ages of quartz extracts have a narrow range between 13.4 ± 1.3 and 16.1 ± 0.7 ka. Multiple magnetic parameters including Koenigsberger ratio, ratio of natural remanent magnetization (NRM) to saturation isothermal remanent magnetization intensities, and similarity in NRM direction compared with paleomagnetic direction for the upper lava comprehensively infer the paleo-heating of the sediments by the lava. Further, paleo-heating temperatures estimation by two independent magnetic methods, based on progressive thermal demagnetizations of NRM and repeated magnetic susceptibility versus temperature measurements with increasing peak temperatures, undoubtedly reveals that the sample positions where the OSL ages were yielded had paleo-temperatures between 200 and 250 °C and much higher than 550 °C, which are sufficiently high to reset their luminescence signals. Therefore, we interpret that a weighted mean OSL age of 14.8 ± 0.7 ka is the most probable age for the lava. Accordingly, the 14C ages older than ∼15.5 cal kyr BP are interpreted to be the deposition ages for their horizons, whereas the three exceptionally young 14C ages (∼2.3–∼7.7 cal kyr BP) for the upper sediments were contaminated by modern carbon. We suggest that magnetic applications, when combined with 14C and luminescence dating techniques, can help greatly improve the reliance of chronologies for late Quaternary volcanism of the world.

Geology and geochemistry of Late Quaternary volcanism in northern Harrat Rahat, Kingdom of Saudi Arabia: implications for eruption dynamics, regional stratigraphy and magma evolution

Abstract Harrat Rahat (<10 Ma) is one of the largest volcanic fields on western Arabia. In the north of the field, some of the youngest volcanic centres evolved through either point-like, complex or multiple aligned vents (i.e. along fissures), and have pyroclastic cones, lapilli fall deposits and/or lava flows associated with them. The products reflect dominantly Hawaiian eruptions, and only one centre experienced phreatomagmatism. Results from new 3 He surface-exposure dating provide constraints on stratigraphy of the youngest (<0.3 Ma) products. The rocks are compositionally alkali-basalt and hawaiite, with intra-plate basalt (prevalent mantle (PREMA)) affinity. Each eruption displays a distinct whole-rock composition in an overall linear trend. We suggest that the magma source for each centre is similar, and that composition of the products is different due to different degrees of fractionation. In a single eruption, the magma that reaches the surface first is the least evolved, with the most evolved magma erupting last. We also found that the most primitive magmas erupt less explosively. We think that the degree of magma evolution might correlate with ascent times, assuming that the more evolved magma spent more time en route. We suggest that magma ascent time is likely to be longer than that of other more primitive intra-plate basalts. Supplementary material: Whole-rock chemistry results, mineral chemistry results and fractional crystallization modeling data are available at https://doi.org/10.6084/m9.figshare.c.3488988

Trace element partitioning between clinopyroxene and trachy-phonolitic melts: A case study from the Campanian Ignimbrite (Campi Flegrei, Italy)

The partitioning of trace elements between crystals and melts provides an important petrogenetic tool for understanding magmatic processes. We present trace element partition coefficients measured between clinopyroxene phenocrysts and trachy-phonolitic magmas at the Campi Flegrei (Italy), whose late Quaternary volcanism has been characterized by two major caldera-forming events (Campanian Ignimbrite at ~39ka, and Neapolitan Yellow Tuff at ~15ka). Our data indicate that the increase of trivalent rare earth elements and yttrium into the crystal lattice M2 site is facilitated by the charge-balancing substitution of Si4+ with Al3+ on the tetrahedral site. Higher concentrations of tetravalent and pentavalent high field strength elements on the M1 site are also measured when the average charge on this site is increased by the substitution of divalent cations by Alvi. In contrast, due to these charge balance requirements, divalent transitional elements become less compatible within the crystal lattice. On the basis of the lattice strain theory, we document that the incorporation of rare earth elements and yttrium in clinopyroxene is influenced by both compositional and physical parameters. Data from this study allow to update existing partitioning equations for rare earth elements in order to construct a self-consistent model for trachy-phonolitic magmas based on the lattice strain theory. The application of this model to natural products from the Campanian Ignimbrite, the largest caldera-forming eruption at the Campi Flegrei, reveals that the complex rare earth element pattern recorded by the eruptive products can be successfully described by the stepwise fractional crystallization of clinopyroxene and feldspar where the clinopyroxene–melt partition coefficient changes progressively as a function of the physicochemical conditions of the system.

Late Quaternary deformation on the island on Pantelleria: New constraints for the recent tectonic evolution of the Sicily Channel Rift (southern Italy)

Abstract Structural observations carried out on the volcanic Island of Pantelleria show that the tectonic setting is dominated by NNE trending normal faults and by NW-striking right-lateral strike-slip faults with normal component of motion controlled by a ≈N 100°E oriented extension. This mode of deformation also controls the development of the eruptive fissures, dykes and eruptive centres along NNE–SSW belts that may thus represent the surface response to crustal cracking with associated magma intrusions. Magmatic intrusions are also responsible for the impressive vertical deformations that affect during the Late Quaternary the south-eastern segment of the island and producing a large dome within the Pantelleria caldera complex. The results of the structural analysis carried out on the Island of Pantelleria also improves the general knowledge on the Late Quaternary tectonics of the entire Sicily Channel. ESE–WNW directed extension, responsible for both the tectonic and volcano-tectonic features of the Pantelleria Island, also characterizes, at a greater scale, the entire channel as shown by available geodetic and seismological data. This mode of extension reactivates the older NW–SE trending fault segments bounding the tectonic troughs of the Channel as right-lateral strike-slip faults and produces new NNE trending pure extensional features (normal faulting and cracking) that preferentially develop at the tip of the major strike-slip fault zones. We thus relate the Late Quaternary volcanism of the Pelagian Block magmatism to dilatational strain on the NNE-striking extensional features that develop on the pre-existing stretched area and propagate throughout the entire continental crust linking the already up-welled mantle with the surface.

Late-Quaternary volcanism and transtensional tectonics in the Bay of Naples, Campanian continental margin, Italy

¶The Campanian continental margin is characterised by asymmetric half grabens and large-volume volcanic deposits. Vesuvius and Campi Flegrei are active volcanoes located along the coast of Naples Bay along one of these half grabens. The interpretation of an extensive set of seismic reflection data allowed to reconstruct the stratigraphy and structural pattern in Naples Bay and their relationships with volcanism. The stratigraphic succession is characterised by a complex architecture due to the presence of 157 Ka old submarine and subaerial vents, pyroclastic flow units associated to the Campanian Ignimbrite and the Neapolitan Yellow Tuff, and numerous tuff cones younger than 12 Ka. These volcanic units occur within marine sediments deposited during the late Quaternary sea level oscillations. The structural pattern is characterised by late Quaternary NE- trending normal faults, NW-trending transtensional faults, E–W trending left-lateral faults and WNW-ESE directed folds. The overall fault kinematics and the evaluated very high displacement rates are consistent with an E-trending left-lateral transtensional shear zone located between 41° and 40° N latitude. The relationship between volcanism and tectonics in the Bay of Naples is explained with block rotation associated with this shear zone. Finally, tectonic and stratigraphic data argue against the occurrence of a caldera in the Bay of Naples.

Fold uplift and synkinematic stratal architectures in a region of active transtensional tectonics and volcanism, eastern Tyrrhenian Sea

The detailed stratigraphic analysis of synkinematic stratal architectures permits the calculation of fold uplift rates and the reconstruction of fold kinematics. This approach was applied to active folds in the Bay of Pozzuoli, eastern Tyrrhenian Sea. This area features late Quaternary volcanism, shallow magmatic intrusions, and transtensional tectonics. Seismostratigraphic analysis of synkinematic strata is constrained by a physical correlation to an already established stratigraphic succession in the neighboring Campi Flegrei and by tephra layers in a gravity core located in Pozzuoli Bay. A stratigraphic resolution in thousands of years was established. Structural and stratigraphic analyses yield quantitative data on the timing of fold inception, kinematics, and the amount and rates of uplift. The folds extend over ∼2 km, and display a curved axial pattern and wedge-shaped packages of synkinematic strata. The rate of fold uplift ranges from 1 to 20 mm/yr. The structures feature limb rotation kinematics and decreasing uplift rates. The uplift rates and geometry of the Pozzuoli Bay folds suggest that they are of tectonic origin and are a product of detachment folding. Their limited longitudinal extent and arcuate axial trends are compatible with an east-trending left transtensional shear zone that was active along the eastern Tyrrhenian Sea during the late Quaternary. A relative sea-level curve was calculated for an anticline culminating in the city of Pozzuoli. This relative sea-level curve highlights the relationship between the deformation timing and the genesis of related abrupt changes in sedimentary facies and erosional surfaces.

Recent geodynamic characteristics in the Arabian–Eurasian and Indian–Eurasian collision region by active fault data

A part of the Alpine–Himalayan orogenic belt is studied, limited by 30–104°E and by 26–46°N to the west of 64°E and 26–56°N to the east of 64°E. A technique is proposed to calculate the field of the recent (Late Pleistocene and Holocene) deformation rate tensors in the upper crust (15 to 20 km) by active fault data. A hydrodynamic model of the medium is used for the calculation. Monotonous fault segments as long as 10–15 km are taken as the elementary cells of the medium. The cell square (the length multiplied by the depth) multiplied by the displacement vector magnitude, gives the geometric moment. Similar components of the local tensors of the geometric moments are summarized within the windows into which the region is divided. Finally, parameters of the axes of principal rates of deformation are calculated, and their directions and magnitudes are mapped. This tectonic deformation is compared with the seismotectonic one calculated by focal mechanisms of earthquakes. The compiled maps show: a concentration of high deformation rates in the plate boundary zones (especially in front of the southern plate syntaxes) and a smaller concentration in some microplate boundary zones; a predominance of the N–S-trending shortening and of strike-slip type of deformation. The rates of shortening and lengthening are usually almost equal to each other and differ essentially (the shortening rate is higher than the lengthening) only in specific sites. These are areas of intense recent detachment or late Quaternary volcanism. It seems that the double-axis lengthening (extension) is suitable for these processes.

Tephrochronology of the Toluca Basin, central Mexico

The major element glass geochemistry of 10 Late Pleistocene and Holocene tephras found in the Toluca Basin in the Trans-Mexican Volcanic Belt is reported for the first time, as a key step towards establishing a tephrochronology for the region. The context for this tephrochronology is provided by a review of Late Quaternary volcanism of the basin. New tephra samples were collected from reference profiles around the basin in order to establish major element glass geochemical data for tephras described in previous publications, but not previously analysed. In addition, cores were taken from the basin floor to extend the known spatial distribution of the tephra layers and establish correlations with the lacustrine record. Improved chronological control has been provided by seven new AMS and conventional radiocarbon dates on the major tephras. Geochemical data are presented for 10 tephras ranging in age from ca. 25,000 BP (Lower Toluca Pumice) to about 8500 BP (Tres Cruces Tephra). Based on geochemical data, it is possible to correlate between sites within the Toluca Basin and between the Toluca Basin and the Basin of Mexico. The basis for a tephrochronological framework has been established.

Why Does the Higashi-Izu Monogenetic Volcano Group Exist in the Izu Peninsula?: Relationships between Late Quaternary Volcanism and Tectonics in the Northern Tip of the Izu-Bonin Arc.

A synthesis of available geological, petrological, and geophysical data was made to construct a model for explaining the relationships between the late Quaternary volcanism (the Higashi-Izu monogenetic volcano group, HIMVG) and tectonics in and around the Izu Peninsula, Japan. The Izu Peninsula and adjacent areas can be divided into three tectonic blocks: the western Izu (W-Izu), eastern Izu (E-Izu), and Oshima-Zenisu blocks. The vents or intrusions of the HIMVG are formed as open cracks aligning with the nearly NW-SE azimuth of horizontal maximum compression of regional stress in the E-Izu block. This stress field is mainly controlled by the NW-SE compression caused by the strong mechanical coupling of the buoyant Izu-Bonin arc with the Japan arc. In spite of this compression, the upper crust of the E-Izu block is under weak lateral constraint on northeastward extension. Both the plate geometry involving an intra-plate fracture, the West Sagami Bay Fracture, and the existence of block rotations in the northern E-Izu block probably enable the northeastward upper crust extension in the E-Izu block, and make a monogenetic volcano field, where a new dike is formed in the upper crust at each eruptive or intrusive event.

Comment and Reply on "Geomorphic assessment of late Quaternary volcanism in the Yucca Mountain area, southern Nevada: Implications for the proposed high-level radioactive waste repository"

Comment and Reply on "Geomorphic assessment of late Quaternary volcanism in the Yucca Mountain area, southern Nevada: Implications for the proposed high-level radioactive waste repository"

Late Cenozoic volcanism, subduction, and extension in the Lassen Region of California, southern Cascade Range

Hundreds of short‐lived, small‐ to moderate‐volume, mostly mafic volcanoes occur throughout the Lassen region of NE California and surround five longer‐lived, large‐volume, intermediate to silicic volcanic centers younger than 3 Ma. Volcanic rocks older than 7 Ma are scarce in the Lassen region. We identify 537 volcanic vents younger than 7 Ma, and we classify these into five age intervals and five compositional categories based on SiO2 content. Maps of vents by age and composition illustrate regionally representative volcanic trends. By 2 Ma, the eastern limit of voicanism had contracted westward toward the late Quaternary arc. Late Quaternary volcanism is concentrated around and north of the silicic Lassen volcanic center. The belt of most recent volcanism (25–0 ka) has been active since at least 2 Ma. Most mafic volcanism is cakalkaline basalt and basaltic andésite. However, lesser volume of low‐potassium olivine tholeiite (LKOT), a geochemically distinctive basalt type found in the northern Basin and Range province, also has erupted throughout the Lassen segment of the Cascade arc since the Pliocene. Thus models of the mantle source and tectonic control of LKOT magmatism should be applicable both within and behind the subduction‐related arc. Normal faults and linear groups of vents are evidence of widespread crustal extension throughout most of the Lassen region. NNW alignments of these features indicate NNW orientation of maximum horizontal stress (ENE extension), which is similar to the stress regime in the adjacent northwestern Basin and Range and northern Sierra Nevada provinces. The large, long‐lived volcanic centers developed just west of a zone of closely spaced NNW trending normal faults. Within that zone of faulting, pervasive ENE extension has precluded growth of large, long‐lived crustal magma systems. We interpret the western limit of the zone of NNW trending normal faults as the western boundary of the Basin and Range province where it overlaps the Lassen segment of the Cascade arc. In our view, the Lassen volcanic region occurs above the subducting Gorda North plate but also lies within a broad zone of distributed extension that occurs in the North American lithosphere east and southeast of the present Cascadia subduction zone. An episode of ENE extension that began in the late Miocene in the northwestern Basin and Range province appears to have triggered widespread late Miocene to Quaternary mafic volcanism in the Lassen region. The scarcity of volcanic rocks older than 7 Ma suggests that a more compressive lithospheric stress regime prior to the late Miocene extensional episode may have suppressed volcanism, even though subduction probably was occurring beneath the Lassen region.

Geomorphic assessment of late Quaternary volcanism in the Yucca Mountain area, southern Nevada: Implications for the proposed high-level radioactive waste repository

Volcanic hazard studies for high-level radioactive waste isolation in the Yucca Mountain area, Nevada, require a detailed understanding of Quaternary volcanism to forecast rates of volcanic processes. Recent studies of the Quaternary Cima volcanic field in southern California have demonstrated that K-Ar dates of volcanic landforms are consistent with their geomorphic and pedologic properties. The systematic change of these properties with time may be used to provide age estimates of undated or questionably dated volcanic features. The reliability of radiometric age determinations of the youngest volcanic center, Lathrop Wells, near the proposed Yucca Mountain site in Nevada has been problematic. In this study, a comparison of morphometric, pedogenic, and stratigraphic data establishes that correlation of geomorphic and soil properties between the Cima volcanic field and the Yucca Mountain area is valid. Comparison of the Lathrop Wells cinder cone to a 15-20 ka cinder cone in California shows that their geomorphic-pedogenic properties are similar and implies that the two cones are of similar age. We conclude that previous determinations of ca. 0.27 Ma for the latest volcanic activity at Lathrop Wells, approximately 20 km from the proposed repository, may be in error by as much as an order of magnitude andmore » that the most recent volcanic activity is no older than 20ka.« less

Late Quaternary volcanism in New Zealand: Towards an integrated record using distal airfall tephras in lakes and bogs

AbstractStudies on distal airfall tephra layers preserved in lake sediments and peats in northern New Zealand have documented the stratigraphic, chronologic, and compositional relationships of 46 eruptives, aged c. 17000–700yr BP, which originated from six North Island volcanic centres: Taupo (9 tephras), Okataina (8), Maroa (1) (rhyolitic); Mayor Island (2) (peralkaline); Tongariro (11), Egmont (15) (andesitic). Sources were distinguished by mineralogy and composition, field relations, and 14C chronology. All known rhyolitic tephra‐producing eruptions from Taupo, Okataina, and Maroa volcanoes since c. 17000yr BP are represented, but only a small proportion of the known tephras erupted from Tongariro, Egmont, or Mayor Island volcanoes is recorded. The distal tephras from these latter volcanic centres may thus reflect atypically powerful (or oblique) eruptions, or dispersal by strong winds. An improved record of volcanism for the Tongariro, Egmont, and Mayor Island centres might be obtainable from suitable lakes or bogs more proximal to them.

The relationship between Quaternary volcanism in central Mexico and the seismicity and structure of subducted ocean lithosphere

Late Quaternary volcanism in central Mexico is related to the subduction of young ocean lithosphere at the Middle America. Trench. Along-arc variations in seismicity, volcano structure, and composition of volcanic products bear a remarkable correlation with the age and structural framework of the downgoing slab. Morphological and petrographic characteristics of major volcanoes within the Trans-Mexican Volcanic Belt (TMVB) serve to distinguish two calc-alkaline subprovinces: 1. A western arc, averaging 60 km in width, associated with aseismic subduction of the Rivera plate. The main cones of this region are dominated by two-pyroxene andesites, comprise volumes ⩽70 km 3 , and stand less than 3,000 m above sea level. 2. A broad central and eastern arc related to subduction of a gently inclined segment of the Cocos plate bounded by the Rivera transform and the Tehuantepec Ridge. Major volcanic edifices possess summit elevations in the range 4,000 to 6,000 m, have appropriately larger volumes (typically > 200 km 3 ) and are constructed with a high proportion of amphibole-bearing lavas. The boundary between these subprovinces is marked by a north-south–oriented structural depression, the Colima Graben, and it coincides with a 100-km offset in the “volcanic front.” Extensional tectonism in the Colima Graben, accompanied by mixed calc-alkaline and alkaline volcanism of potassic affinity, is likely related to a hinge-type transform fault which marks the Cocos-Rivera plate juncture in the downgoing slab. A third segment of ocean floor is presently interacting with continental lithosphere south of the Gulf of Tehuantepec, where Quaternary volcanism is weakly developed within a tectonically complex region that marks the diffuse Cocos-NOAM-Caribbean triple junction. The northern limit of this triple junction is defined by the seismically active Isthmus fault, which may be related to alkaline volcanic activity at San Andres Tuxtla. A tectonic reconstruction based on the evolution of oceanic crust reveals that the distribution of intermediate-depth earthquakes along the arc is directly dependent upon the age of the subducted slab. Ocean lithosphere younger than approximately 20 m.y. is subducted aseismically at convergence rates approaching 9 cm/yr. The length of the inclined seismic zone indicates that the time constant for thermal relaxation in the slab is approximately 4 m.y. The TMVB overlies the aseismic extension of this young ocean lithosphere. Several aspects of this study have a bearing on the segmented nature of converging margins in general: 1. The tectonic evolution of the ocean floor may determine the nature of segmentation at the site of subduction. 2. The complete record of volcanism in the TMVB over the past million years can be related to the present plate configuration. 3. Alkaline and calc-alkaline volcanism have developed contemporaneously at a converging plate margin. 4. Lineaments in volcanic arcs may reflect the structural complexity of the crust rather than segment boundaries in the subducted slab.

Precursors to ScS Phases and dipping interface in the upper mantle beneath southwestern Japan

Longitudinally polarized precursors to ScS phases observed in the Shikoku and Chugoku districts, southwestern Japan, are interpreted as ScSp arrivals, resulting from ScSto-P conversions at a dipping interface in the upper mantle. An ScSp phase recorded in the Tohoku district, northeastern Japan, also is examined. The location of the conversion interface, beneath the Shikoku district, determined from the ScSp observations agrees with the upper boundary of the descending Philippine Sea plate inferred from the seismicity pattern of subcrustal earthquakes. It has been proposed on the basis of no seismic activity in the upper mantle that the leading edge of the downgoing Philippine Sea plate has not reached the upper mantle beneath the Chugoku district. The ScSp observations, however, present a possibility of the existence of an aseismic continuation of the Philippine Sea plate in the upper mantle beneath the Chugoku district. An alternative interpretation of the conversion interface may be possible. The interface inferred in the present study may correspond to a boundary between the asthenosphere and an aseismic dead slab which had descended from the Nankai trough at the previous cycle of plate convergence. This ScS-to-P conversion interface may be closely related to the late Quaternary volcanism in the Chugoku district. Low-velocity zones are required in the vicinity of the inclined ScS-to-P conversion interfaces in the upper mantle beneath southwestern and northeastern Japan in order to explain the observed relative polarity between the ScSp and ScS phases. The first-order discontinuity of the velocity contrast of about 6%, which has been suggested between the lithosphere and overlying asthenosphere, cannot simultaneously explain all of the observed amplitudes, periods, and polarity of the ScSp phases. The simplest model which explains these observations is the low-velocity zone with the sharp upper and transitional lower boundaries. This low-velocity zone may be related to the melting or dehydration of the oceanic crust which has been subducted with the underlying lithospheric plate. The ScS-to-P conversion interface inferred beneath southwestern Japan is more transitional than that beneath northeastern Japan. This difference in nature of the conversion interfaces must correspond to the difference in geological histories of both the regions. A high dip angle of the Philippine Sea plate in the Kyushu district and a low dip angle for the Shikoku district suggest that the normal-faulting subcrustal earthquakes beneath the Bungo channel and Iyonada, the western part of the Sevo Inland Sea, can be explained in terms of tearing and hinge faulting at the junction of the two slabs with different dip angles.