Upper mantle P structure on the ocean side of the Japan-Kurile Arc

Slowness measurements are made for the first and later arrivals of P waves from about seventy Kurile-Kamchatka earthquakes (13° < Δ < 30°) at the Wakayama Micro-Earthquake Observatory, Japan. The experimental error of dT/dd is not more than 3 per cent and the data points suffice to draw a dT/dΔ curve fairly uniquely. In the distance range 4° < Δ < 15° a travel-time curve, including a later arrival branch, is constructed for a specific Kurile event using the records along the Pacific coasts of the Japanese-Kurile islands. The results are inverted to obtain a velocity model for the upper 800 km of the mantle beneath the trench side of the Japan-Kurile Arc. The model includes a high-velocity lid extending down to 85 km depth. The low-velocity zone is of relatively high speed (8.1 km/s) and is terminated by a high-velocity gradient zone at depths 165-200 km, just below which the velocity is nearly constant with depth. The velocity increases very sharply near 400 km by about 6 per cent. An abrupt change in the slope of velocity occurs near 520 km. A major transition zone in the depth range 630-670 km consists of about 7 per cent increase in velocity and is sharp, especially near the base. A minor transition zone is tentatively suggested to exist around 740 km. Relative arrival times, crossover distances and qualitative amplitude behaviour calculated for this model are consistent with the observed data. Station residuals for the least-squares determination of dT/dΔ closely correlate with the local seismicity

Finite frequency whole mantle P wave tomography: Improvement of subducted slab images

We present a new whole mantle P wave tomographic model GAP_P4. We used two data groups; short-period data of more than 10 million picked-up onset times and long-period data of more than 20 thousand differential travel times measured by waveform cross-correlation. Finite frequency kernels were calculated at the corresponding frequency bands for both long- and short- period data. With respect to an earlier model GAP_P2, we find important improvements especially in the transition zone and uppermost lower mantle beneath the South China Sea and the southern Philippine Sea owing to broadband ocean bottom seismometers (BBOBS) deployed in the western Pacific Ocean where station coverage is poor. This new model is different from a model in which the full data set is interpreted with classical ray theory. BBOBS observations should be more useful to sharpen images of subducted slabs than expected from simple ray path coverage arguments

38. Focal Process of a Deep Focus Earthquake as Deduced from Long Period P and S Waves

The focal process of a deep shock which occurred at 580km depth in the Banda Sea region in 1964 is investigated by using long period P and S waves from WWSSN seismograms. The two nodal planes of this earthquake are given by dip direction=319°, dip angle=45°, and dip direction=180°, dip angle=53°, respectively. Assuming that a rectangle represents a far field pulse form, the S wave seismograms are synthesized for various pulse widths and epicentral distances. The effects on the wave form due to passage through the mantle, crust and instrument are removed. Trials are made to find the pulse width, Ts, of the original rectangular pulse which fits the observed S wave form. The variation of Ts with respect to the fault plane orientation is investigated. Ts is remarkably well correlated with the emergence angle measured from the slip vector on the nodal plane with dip direction=319°, dip angle=45°. This result, leads to the following conclusions: (1) The source is plane-like rather than spherical; a fault, model is more preferable to a phase transition model. (2) The nodal plane with dip direction=319° and dip angle=45° is the fault plane. (3) The fault surface is not circular but elongated. (4) The direction of elongation coincides, with the slip direction. (5) The fault width W should not exceed 15km. It may. probably be smaller than 10km. (6) The fault length L is estimated to be 40.km. (7) The process of the body wave radiation took place almost simultaneously, within about two seconds, over the entire fault surface; this model is significantly different from the ordinary propagating fault model. On the basis of this model, the seismic moment M0 is estimated to be 5.8×1026 dyne cm from the P wave amplitude and width. Assuming W=8km, the average dislocation, stress drop and elastic energy released when the fault surfaces become free, are. estimated to be 150 cm, 290 bars and 7.0×1022 ergs, respectively. The stress drop obtained here is one order of magnitude greater than that estimated for shallow earthquakes.|1964年バンダ海で起った深さ580kmの深発地震の震源過程を世界標準地震計観測網が記録した.長周期P波およびS波を用いて調べた.この地震の2つのP波節面はdip direction=319°dip angle=45°およびdip direction＝180°, dip angle＝53°で与えられる.S波の遠方での波形が矩形で近似できると仮定して種々のパルス巾,震央距離に対してP波理論記象を作った.その際,マントル,地殻および地震計を通過するときの波形への影響を考慮した.これらを実際の記録と比較して,もとの矩形波のパルス巾,Ts,を求めた.Tsの方位に関する著しい規則性より次の結論を得た.(1)震源は球状ではなく面状であり,従って相転移モデルより断層モデルの方が適当である.(2)dip direction=319°,dip angle=45°のP波節面が断層面である.(3)断層面は円ではなく,slip方向にのびた細長いものである.(4)断層の巾は15kmを越えず,恐らく10km以下と思われる.(5)地震は断層面全体にわたって殆ど同時に起ったと考えられる.この間約2秒である.普通の伝播性断層モデルでは結果を説明できない.(6)断層の長さは約40kmである.これらの結論はP波の実際の波形とも矛盾しない.P波理論記象と実際の記録との比較から地震モーメントは5.8×1026dyne cmと推定される.断層の巾を8kmと仮定すると,断層面上でのくい違いの大きさ,stress dropおよび断層面がfreeになったと仮定したとき解放される弾性エネルギーは,それぞれ150cm,290 barsおよび7.0×1022 ergsとなる

Tsunami earthquakes and subduction processes near deep-sea trenches

A tsunami earthquake is defined as a shock which generates extensive tsunamis but relatively weak seismic waves. A comparative study is made for the two recent tsunami earthquakes, and a subduction mechanism near a deep-sea trench is discussed. These two earthquakes occurred at extremely shallow depths far off the coasts of the Kurile Islands and of eastern Hokkaido on October 20, 1963, and on June 10, 1975, respectively. Both can be regarded as an aftershock of the preceding larger events. Their tsunami heights and seismic wave amplitudes are compared with those of the preceding events. The results show that the time constants involved in the tsunami earthquakes are relatively long but not long enough to explain the observed disproportionality between the tsunamis and the seismic waves. The process times are estimated to be less than 100 s. The spatio-temporal characteristics of the two events suggest that they represent a seaward and upward extension of the rupture associated with a great earthquake which did not break the free surface at the coseismic stage. The amplitude and phase spectra of long-period surface waves and the long-period P waveforms indicate that this extension of the rupture did not take place entirely along the lithospheric interface emerging as a trench axis. It rather branched upward from the interface in a complex way through the wedge portion at the leading edge of the continental lithosphere. This wedge portion consists in large part of thick deformable sediments. A large vertical deformation and hence extensive tsunamis result from such a branching process. A shallowest source depth, steepening of rupture surfaces, and a deformable nature of the source region all enhance generation of tsunamis. The wedge portion ruptured by a tsunami earthquake is usually characterized by a very low seismic activity which is presumably due to ductility of the sediments. We suggest that this portion fractures in a brittle way to generate a tsunami earthquake when it is loaded suddenly by the occurrence of a great earthquake and that otherwise it yields slowly. Upward branching of the rupture from the lithospheric interface produces permanent deformation of the free surface which is relative uplift landward and relative subsidence trenchward of the zone of surface break. This surface break zone geomorphologically corresponds to the lower continental slope between the deep-sea terrace and the trench. Such a mode of permanent deformation seems to be consistent with a rising feature of the outer ridge of the deep-sea terrace and a depressional feature of the trench. This consistency implies a causal relationship between great earthquake activities and geomorphological features near the trench

Seismological evidence for selectivity in slip planes under down.dip extension or compression

Un análisis reciente de temblores múltiples sugiere que hay una selectividad en los planos de falla en las zonas sísmicas bajo condiciones de extensión o compresión. Las soluciones de planos de falla no distinguen entre el plano de falla y el plano auxiliar. El análisis de un temblor múltiple permite resolver esta ambigüedad. Se presentan los resultados resumidos del análisis de más de veinte temblores múltiples profundos, en cuanto a la relación mutua de las pendientes entre los planos de falla y sus planos auxiliares. Los planos de falla tienden a tener pendientes más cercanas a la vertical que los auxiliares, sea que predomine la compresión o la extensión en la dirección de subducción. Las fuerzas exteriores que se ejercen desde arriba o desde abajo de la placa litosférica pueden causar extensión o compresión, pero no pueden determinar una selectividad en los planos de falla. La explicación más sencilla de tal selectividad es la idea de un hundimiento gravitacional de la litósfera. Esta explicación se basa en el hecho que todo desplazamiento de falla implica un trabajo en contra de la gravedad. Posiblemente pueda existir una selectividad similar para los temblores litosféricos someros debajo de las trincheras