Deep Seismic Zone Research Articles

Alkali mapping of the japanese quaternary volcanic rocks

Of ca. 200 Quaternary volcanic centers in the Japanese islands, ca. 120 are characterized by K 2O and Na 2O concentrations of their ejecta normalized to SiO 2 = 60% K 2O values give a geographically well-defined trend of regularly increasing away from the volcanic front. Na 2O values show no geographical trend. Relationships between the SiO 2-normalized K 2O and the depth of the deep seismic zone (K- h relation) are different for different arcs. The Southwest Japan arc shows distinctly higher K 2O and less regular geographic trend than the others probably due to crustal contamination. In the normal segments of the Northeastern Japan belt, K 2O increases ca. 2.5 to 3.0% per 100 km depth of the seismic zone. Groups of volcanic centers near the junction between the Northeast Honshu and Izu-Mariana arcs give characteristically lower K 2O while Rishiri volcano far behind the arc junction between the Kurile and Honshu arcs also gives a low K 2O value.

Triple seismic zone and the regional variation of seismicity along the Northern Honshu Arc

The regional variation of seismicity along the northern Honshu arc, Japan, is studied using accurate focal depths and focal mechanism types. We use focal depths determined from pP‐P time intervals reported in the ISC bulletins. For submarine earthquakes, depths are corrected by considering the pP phase reported in the bulletins as the pwP phase (the reflection from the ocean surface). Out of more than 600 well‐located earthquakes selected from the ISC bulletins, we determine the types of the focal mechanisms of 184 events using P wave first motion data. Based on historical seismicity of great and large earthquakes, we divide the zone of thrust type earthquakes at the plate interface into two regions: the shallow thrust zone (0–40km), where great earthquakes (MS∼8.0) occur, and the deep thrust zone (40–60km), where large earthquakes (MS∼7.4) occur. The activity of great or large earthquakes shows a variation along the arc; in some regions, both the shallow and deep thrust zones are active, and in other regions, only one of the thrust zones is active. The seismicity of recent moderate size earthquakes (mb>4) combined with the focal mechanism type shows a variation along the arc, which reflects the variation of the activity of great or large earthquakes. Where large earthquakes do not occur in the deep thrust zone, neither thrust type nor down‐dip compression/tension type events occur in and beneath the deep thrust zone. Where large earthquakes do occur in the deep thrust zone, we find a number of thrust type earthquakes. Further, in the latter case, in some regions, the down‐dip compression and tension type events of the double seismic zones extend seaward just beneath the deep thrust zone and form a triple‐planed structure of seismicity (the triple seismic zone). This study confirms the hypothesis of previous workers (Seno and Pongsawat, 1981) on the causal relation between the strong seismic coupling of two converging plates at the deep thrust zone and extension of the double seismic zone; i.e., the presence or absence of activity within the slab beneath the deep seismic zone occurs when the deep thrust zone has a strong or weak coupling, respectively. Here, the weak coupling could be interpreted as either aseismic slip or as low stress buildup since the last large event occurred at the deep thrust zone. Triple seismic zones are found offshore of Miyagi prefecture, where the deep thrust zone has been broken recently in 1978, and offshore of Fukushima prefecture. We expect a future large earthquake at the deep thrust zone offshore of Fukushima prefecture because the presence of the triple seismic zone suggests stress has been accumulating and 40 years have passed since the deep thrust zone was ruptured in 1938.

Reconnaissance microearthquake survey of Sulawesi, Indonesia

Several hundred earthquakes were recorded during the operation of a two‐week five‐station portable seismograph network on Sulawesi, Indonesia. Forty one of these events were locatable and half of these occurred beneath the eastern Gorontalo Basin in a north dipping zone which extends from the Batui Thrust on the East Arm of Sulawesi to about 100 km depth beneath the Gorontalo Basin. The Batui Thrust is the site of thrusting of the Banggai Islands continental fragment beneath the East Arm ophiolite and oceanic crust of the Gorontalo Basin. The observation of a zone of earthquakes dipping to the north from this thrust zone suggests that the leading edge of the Banggai Island block was subducted to at least 100 km depth. The eastern Gorontalo Basin earthquake zone may connect with a deep seismic zone beneath the Celebes Basin. Beneath the western Gorontalo Basin, a very narrow zone of earthquakes dips to the south, probably within lithosphere of the Celebes Basin subducted at the North Sulawesi Trench. Three shallow earthquakes occurred near Lake Matano in central Sulawesi, possibly on the Matano Fault, and their composite focal plane solution suggests east‐west extension. The occurrence of only one earthquake on the Palu Fault and the lack of short S‐P times at a station operated on the fault indicate that this feature was very quiet during the survey period.

Seismicity and seismic gap in the Lesser Antilles arc and earthquake hazard in Guadeloupe

Summary. A seismic study of the Lesser Antilles arc has been carried out, first for the period 1950–1978, for which we can use local seismic networks to draw maps of instrumental seismicity, then for the period 1530–1950, for which we have catalogues of felt earthquakes. The striking feature of the spatial distribution of foci is the cluster of epicentres in the northern half of the arc; all large earthquakes (M > 7.5) are located north of 14° latitude. Seismicity cross-sections through the arc show a variable dipping subduction zone along the arc; the deep seismic zone is steeper in the centre of the arc than on the extremity. The time-space diagram for historical seismicity, and the evidence of a seismic gap at the east of Guadeloupe lead us to consider the northern half arc as a likely site for a large earthquake in the near future. The seismic slip rate calculated from all major earthquakes since 1530 is of much greater value than that obtained from recent plate tectonic models, suggesting that the recurrence rate of earthquakes is more than many hundreds of years with a possible aseismic creep.

Deep root of andesitic volcanoes: New evidence of magma generation at depth in the Benioff zone

The association of andesitic volcanoes with island arcs and continental margins is well known and the close spatial coincidence of these volcanoes with inclined deep seismic zones suggests a direct relation of cause and effect between earthquakes at depth (100–300 km) beneath the recent volcanoes and their activities. Since 1962, the author has drawn attention to the temporal correlation between earthquakes at depth beneath volcanoes and their eruptions, as observed in the New Hebrides and other island arc regions. To test these time correlations, attempts have been made to predict volcanic eruptions in the New Hebrides and in New Zealand. From 1970 until 1977 the author has supplied predictions to be tested against the records of volcanic activity collected in the New Hebrides by the Geological Survey. A significant number of successful eruption predictions, 65%, notably for Ambrym, Lopevi and Karua volcanoes, has emerged. Activity at these volcanoes is actually correlated to specific intermediate-depth earthquakes within nests which are located around aseismic gaps seated straight beneath these volcanoes. In 1977 and 1978, investigation carried out in New Zealand, particularly on White Island volcano, shows a similar correlation between occurrence of subcrustal earthquakes and volcanic activities. Crater floor level changes on White Island are also correlated with seismic activity at depth beneath the volcano. Successful tests of prediction of recent eruptions at White Island, using a simple relation with the depth of earthquakes, get confirmation of a deep root of andesitic volcanoes which is seated in the Benioff zone and where seismic activity is related to magma generation.

北海道の微小地震活動

The seismicity in and near Hokkaido, Japan during the period from 1976 to 1980 has been studied by means of the telemeter observation system having nine high-gain seismograph stations.The highest activity of shallow microearthquakes (0 to 60km depth) is in Pacific coast from the coast of Urakawa to east off the Shimokita peninsula. In northern part of this area great earthquake with M7 or greater has not occurred for 50 years. In southern part of this area the largest aftershock (M7.5) of the 1968 Tokachi-Oki earthquake (M7.9) occurred. Seismicities in the aftershock areas of the 1968 Tokachi-Oki earthquake and the 1973 Nemuro-Oki earthquake (M7.4) are inactive, while the number of microearthquakes in the aftershock area of the 1952 Tokachi-Oki earthquake (M8.1) has an increasing tendency since 1979. Teshikaga-Akan area of eastern Hokkaido, in which nine shallow earthquakes with magnitudes 5 or 6 occurred during the period from 1938 to 1967 shows low seismic activity during the period of observation.The accurate location of microearthquakes in deep seismic zone shows two dipping zones 25-30km apart beneath the area from the southwest to the middle of Hokkaido. Beneath southern Hokkaido (the northern side of the Northeast Japan arc), the seismicity of the upper zone is more active than that of the lower zone, while beneath central Hokkaido (the western side of the Kurile arc), the seismicity of the lower zone is more active than that of the upper zone.

Phase changes and the frequency‐magnitude distribution in the upper plane of the deep seismic zone beneath Tohoku, Japan

The frequency‐magnitude distribution (b value) in the upper plane of the deep seismic zone beneath Tohoku, Japan, shows significant variation with depth which we attribute to dewatering of the oceanic crust during the subduction process. High b value from the surface to 40 km in the upper seismic plane and overlying mantle wedge is thought to be caused by the advection of free water from the oceanic crust. The fact that the b value drops to <0.7 in the upper seismic plane from 40‐ to 70‐km depth reflects the low pore pressures left in the subducting oceanic crust after the advection of this free water. From 70 to 90 km an increase in b to >1.0 is attributed to the passing of the hydrous oceanic crust through a dehydration front. The resulting increase in pore pressure lowers the effective stress and increases the b value in the upper plane. Beneath the volcanic line, b drops back to <0.7, implying that the newly released water advects immediately into the asthenosphere, lowering the mantle melting temperature and producing the calc‐alkaline volcanism on Tohoku. At depths greater than that beneath the volcanic line, the b value again rises, indicating a water source at 100–125 km which might be the further dehydration, of talc, chlorite, and amphibole in the oceanic crust.

Comments on 'Lengths of intermediate and deep seismic zones and temperatures in downgoing slabs of lithosphere' by Peter Molnar, David Freedman and John S. F. Shih

Molnar, Freedman & Shih (1979) recently studied the lengths of intermediate and deep seismic zones and the dependence on convergence rate, lithospheric thickness and age of the lithosphere. In their paper reference is made to Vlaar & Wortel (1976), in which the role of lithospheric age in the subduction process is investigated. I would like to comment on a few points in their paper which are related to our work:

Double-planed deep seismic zone and anomalous structure in the upper mantle beneath northeastern Honshu (Japan)

Seismicity located by using the most recent data obtained from the high-gain seismograph network of Tohoku University shows that the deep seismic zone beneath northeastern Honshu, Japan, is composed of two thin planes which are parallel to each other and are 30–40 km apart. Focal mechanisms derived from the earthquakes in the upper plane are reverse-faulting, or, some of them, down-dip compression. As a contrast, those in the lower plane are down-dip extension. The location of the upper boundary of the descending lithospheric slab, inferred from the arrival-time difference between ScS and ScSp waves and from the travel-time anomaly of intermediate-depth earthquakes observed at the small-scale seismic array, coincides exactly with the upper plane of the double-planed deep seismic zone. Anelasticity (1/ Q) structure of the upper mantle consists of three distinct zones: a high- Q ( Qs− 1500) inclined lithospheric slab, an intennediate- Q ( Qs−350) land-side mantle between the Pacific coast and the volcanic front, and a low- Q ( Qs − 100) land-side mantle between the volcanic front and the coast of the Japan Sea. The evidence obtained here provides valuable information as to the definition of the type of mechanism producing the plate motion beneath island arcs.

A detailed cross-section of the deep seismic zone beneath northeastern Honshu, Japan

A cross-section of earthquakes located in northeastern Japan is presented by using pPdepths reported by the International Seismological Centre. Travel-time corrections for the water layer were used to recompute pP-depths of earthquakes located below the sea regions. Seven new focal-mechanism solutions, based on teleseismic and Japanese data, were determined for this region. The reconstructed cross-section shows a double seismic zone at intermediate depths of 80–150 km. Earthquakes located within the upper seismic plane are characterized by down-dip compression while those in the lower plane, located about 35 km below the other seismic plane, are characterized by down-dip extension. These observations suggest that, at these depths, stresses attributable to a simple “unbending” of a plate may contribute to the generation of earthquakes in addition to stresses generated by the gravitational sinking of the lithosphere. A detailed cross-section of shallow earthquakes in the same area between the trench and eastern coast of northeastern Honshu is presented along with focal-mechanism solutions. This cross-section delineates more clearly the seismic zones characterized by normal and low-angle thrust faulting.

Lengths of intermediate and deep seismic zones and temperatures in downgoing slabs of lithosphere

Summary. If intermediate and deep earthquakes occur in the coldest portions of the downgoing slabs of lithosphere, then different lengths of seismic zones represent different temperatures in the slabs. As the slab descends through the aesthenosphere, it warms primarily by conduction of heat through its upper and lower surfaces. Isotherms are advected downwards to distances approximately proportional both to the rate of subduction and to the square of the thickness of the lithosphere. Consequently, the lengths of seismic zones should be approximately proportional to the product of the rates and the squares of these thicknesses. As these thicknesses are approximately proportional to the square root of the age of the lithosphere, the lengths ought therefore to be approximately proportional to the product of the convergence rates times the ages. Although there is considerable scatter, observed lengths are approximately proportional to such products, and are not simply related to the rate, the age or the thickness alone. The data crudely fit the relationship: length = rate × age/10. Using this relationship, we infer that the Philippine sea and Pacific plates move slowly, if at all, with respect to one another and that the Farallon plate may have been too young to be subducted to a great distance beneath western North America in the Palaeogene. Calculations of temperatures at the depths of the deepest events suggest that these cut-off temperatures increase from about 600 ± 100°C at 200 km to 830 ± 50°C at 650 km depth, but the cut-off potential temperature is approximately constant. Assuming that the strength is a thermally activated parameter, and using the activation energy for olivine, a crude estimate of activation volume is obtained from the dependence of the cut-off temperature on depth.

A detailed cross-section of the deep seismic zone beneath northeastern Honshu, Japan. : Yoshii, Toshikatsu, 1979. Tectonophysics, 55(3/4): 349–360

A detailed cross-section of the deep seismic zone beneath northeastern Honshu, Japan. : Yoshii, Toshikatsu, 1979. Tectonophysics, 55(3/4): 349–360

Double-planed deep seismic zone and upper-mantle structure in the Northeastern Japan Arc

Summary. The ScSp wave converted from the ScS wave at the boundary between the descending lithospheric slab and the mantle above it was clearly observed from a nearby deep earthquake with magnitude 7.7 at some stations of the seismic network of Tohoku University which covers the Tohoku District, the northeastern part of Honshu, Japan. By applying the threedimensional seismic-ray tracing method, the location of this boundary was determined from the difference in arrival time between the ScS and ScSp waves. The result shows that the upper boundary of the descending slab lies exactly on the upper plane of the double-planed deep seismic zone found in the Northeastern Japan Arc. There is an additional evidence that the boundary is located on the upper plane of the double-planed deep seismic zone. The hypocentre distribution of intermediate-depth earthquakes located by the small-scale seismic-array observation is extremely different from that obtained by the relatively largescale seismic network. The discrepancy in the distribution of hypocentres of the same earthquake independently located is well explained by the inclined lithospheric slab model derived from the difference in arrival time between the ScS and ScSp waves. The earthquakes with reverse faulting or with down-dip compressional stresses occur at the upper boundary of the descending slab. Within the descending slab, the earthquakes with down-dip extensional stresses also occur in a very narrow zone from 30 to 40 km below the dipping boundary in the depth range from 50 to about 200 km, and these shocks form the lower plane of the double-planed deep seismic zone.

Double-planed structure of the deep seismic zone in the northeastern Japan arc

A double-planed structure of deep seismic zone has been found over a wide area more then 300 km × 200 km in the Tohoku District, the northeastern part of Honshu, Japan. This prominent feature of the configuration of the deep seismic zone has been ascertained through a precise determination of the microearthquake hypocenters by using the data from the seismic network of Tohoku University. The two planes are nearly parallel to each other, the distance between the two planes being from 30 to 40 km.Composite focal mechanism solutions are derived from the superposition of the distribution of first motions of P waves, and the different fault types are obtained for the two groups of earthquakes; the earthquakes which occurred in the upper plane are characterized by reverse faulting, some of them by down-dip compressional stresses, and those in the lower plane by down-dip extensional stresses. The evidence obtained here provides valuable information for the definition of the type of mechanism producing the plate motion beneath the island arc.

Deep seismic zone as an upper mantle reflector of body waves

THE deep seismic zone associated with the Izu–Bonin arc dips at about 40° towards the south-west beneath the Dodaira Observatory, Japan (Fig. 1). At this observatory an unexplained phase with apparent velocity of 16.5 km s−1 was observed 10–20 s after the first arrival for earthquakes in the Ryukyu Islands. No corresponding phase has been found for earthquakes of different azimuths. This unexpected phase can be explained by post-critical reflection of P waves at the deep seismic zone after strong focusing by the 400-km transition zone: this zone of reflection is apparently sharp, of the order of 10 km in thickness. Typical ray diagrams are shown in the inset of Fig. 1. The ray path involves both the deep seismic zone and the 400 km transition zone.

Regionality of the upper mantle around Northeastern Japan as derived from explosion seismic observations and its seismological implications

Regionality of the upper mantle around Northeastern Japan was derived through the analysis of data obtained by large explosions at sea. Beneath Northeastern Japan, a low Pn velocity of 7.5 km/sec was determined with a good accuracy, while beneath the ocean the Pn velocity is larger than or close to 8.0 km/sec. Change of Pn velocity from 7.5–7.7 to 8.1 km/sec takes place within a narrow zone near the coast. This zone is almost coincident with the Aseismic Front. The high seismicity seems to be related to a high Pn velocity also in the uppermost part of the mantle as in the deep seismic zone. Observed travel times are smaller by about 5 sec than the standard travel times. The accuracy of hypocenter determination was improved to a large extent by taking the regionality of the structure thus obtained into account.

SEISMIC ACTIVITY IN THE NORTHEASTERN JAPAN ARC

The most recent data obtained from the high-gain seismograph network of Tohoku University are used to make an investigation on seismic activity and tectonics in the northeastern Japan arc. Seismic activity is closely related to the tectonic development of island arcs, and detailed analyses of the spatial distribution of microearthquakes in the northeastern Japan arc have led to conclusive evidence that almost all the shallow microearthquakes in the land area occur only in the layer with a P-wave velocity of 5.9 km/sec which was determined from explosion seismic observations. The configuration of the deep seismic zone in this region is discussed in relation to the plate subduction beneath the arc. The accurate determination of the hypocenters and focal mechanisms of microearthquakes has made it possible to investigate the subduction process beneath the arc.

Petrology and geochemistry of basaltic rocks of the Lau Basin

The Lau Basin is a marginal sea, located between the Tonga and Lau Ridges, in the southwestern Pacific. The basin is on the “inner” or concave side of the Tonga Trench-Arc system and is situated above the deep seismic zone dipping westward from the Tonga Trench. The Tonga Trench-Arc system is undoubtedly located above a zone of crustal shortening as evidenced by the deep seismicity and vulcanism. However, the geological and geophysical data give strong support to the contention that the Lau Basin has formed by crustal dilation. Rocks dredged from ridges and seamounts in the basin are sub-alkaline basalt. The average major element composition of least altered samples is: SiO 2 48.8%, TiO 2 1.2%, K 2O 0.18%, P 2O 5 0.08%, H 2O+ 0.30%, Fe III/Fe II = 0.26,CaO/Al 2O 3 = 0.77. The data for Lau Basin basalt (LBB) show close similarity to data of typical oceanic ridge basalt (ORB). Trace element abundances (ppm): Ni 160, Cr 390, Sr 100, Ba < 31, Rb < 1 also resemble ORB values. K/Rb in a least altered and unfractionated sample is 860, Ba/Sr is 0.1, Ba/Rb is 8. Strontium isotope data show the only marked variance from ORB chemistry with LBB values ranging from 87Sr/ 86Sr=0.7020 to 0.7051. The low Sr abundances in the samples suggest the possibility of crustal Sr contamination to explain the radiogenic Sr enrichment. An alternate possibility is that the mantle source rocks were enriched in 87Sr. Variation within dredge hauls and between dredge sites may be explained by low-pressure fractional crystallization of magmas separated from the mantle at about 50 km depth. The basin probably began to open in middle to late Miocene time either by the disruption of a single andesitic island arc by splitting along its axis or by dilation of the area between two closely spaced concentric arcs. Mantle counterflow in the asthenosphere above the downgoing oceanic lithosphere slab is the probable driving force for dilation and has provided a continuous supply of parent material for the basalt of the basin floor.

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.

The segmented nature of the Chilean seismic zone

The better located earthquakes from the Earthquake Data File of NOAA during the period from 1967 to December 1971 define a seismic zone dipping eastward below Chile with a thickness of 20–70 km. Vertical sections were used to examine the morphology of the zone. The orientation of the vertical sections and the limits of the area projected onto the sections were chosen to produce plots of the seismic zone showing the least thickness and steepest dip. These conditions should be characteristic of sections which are perpendicular to the seismic zone. Seven segments that differ in strike and dip were found. The segments range in dip from 20° to 30° and have lengths of 300–850 km. There are offsets of the seismic zone between adjacent segments of up to 50 km and changes in strike of up to 30°. There are similar offsets in the historically active volcanic chain. The breaks in the deep seismic zone correspond to abrupt changes in topography and tectonic structures. The margins of great earthquake aftershock zones correspond well with the segment boundaries proposed here.