Geophysics

We investigate the local seismotectonic of the Molucca Sea area using moment tensor calculations for the earthquakes that occurred in July 2019 at a depth of 10–55 km. The mainshock of Mw 6.8 occurred on July 7, followed by aftershocks until July 18, with magnitudes ranging from Mw 4.6 to Mw 5.8. Moment tensor solutions are calculated by applying Isolated Asperities (ISOLA) software using the full waveform data recorded at regional seismic stations. The analyzed frequency bands used in this study are 0.01–0.03 Hz and 0.04–0.05 Hz for the event with Mw ≥ 5 and Mw 

Seismic method can provide detailed information on the subsurface structure to determine the prospect / potential of hydrocarbons. There are few previous studies have used KINGDOM software in processing and interpreting seismic data to determine hydrocarbon potential. Therefore, in this study we provide an overview of how the KINGDOM software works which can be used as an option in processing and interpreting seismic data. This research area is the area of East Java basin. The data used are secondary data. The seismic data used is the final PSTM - STACK - SUKOWATI 3D. Extension is used in the form of SGY File. Selected area in interpretation is inline within range 6100 to 6500 (dimensionless) and crossline within range 12700 to 13000 (dimensionless), with the increment about 10. KINGDOM 6.7.1 software is used for seismic data processing. Interpretation of seismic data is done to obtain information from seismic data, to provide a conclusion that can be accounted by the analysis of al...

Spectra ratios of direct and coda waves between large and small earthquake represent the differences of source time function, and have been used to evaluate the corner frequencies and magnitudes of tectonic earthquakes. We applied the spectral ratio method to explosion earthquakes observed at Sakurajima volcano, and found the spectral ratio changes with the lapse time. We inferred that the seismic waves of explosion earthquakes are generated mainly by initial rapid pressure release in the conduit but also by successive oscillations of the magma system during continuous ash emission. This observation is contrary to the results of tectonic earthquakes in many previous papers which show almost same spectral ratio between the direct and coda waves. In the present study, we further apply the spectral ratio method to the volcano tectonic earthquakes (VTs) observed at Sakurajima to examine whether or not the characteristics of explosion earthquakes represent the source process and are not ...

Volcanic eruptions generally excite seismic and acoustic waves during their activities. Investigation of relationship between seismic and acoustic waves excited by volcanic eruptions is necessary to understand their mechanism. In this preliminary study, we examine temporal changes of the radiations of acoustic and seismic waves during the Vulcanian eruptions at Sakurajima volcano, giving attention into the energy distributions in time between the acoustic and seismic waves. The results show the characteristics of ratios of acoustic to seismic wave radiation, which can be described as follows: the ratios rapidly decrease for 10 s from the onset, and keep the minimum values for about 10-15 s with gradual increases. Since then, the seismic waves are dominant because the acoustic waves are not well recorded beyond the noise level. These results suggest that the volcanic fluid motions are strong in the shallow parts in the beginning, and the active zone gradually becomes deep.

We perform investigation of crustal anisotropy in the Sumatran forearc through shear wave splitting analysis. We employ high quality splitting measurements from around 59 local events with depth less than 25 km recorded at a seismic station in Nias Island. The result shows an average splitting time of 0.19 ± 0.01 s and the fast directions aligned parallel to the strike of the Sumatran Trench or perpendicular to the absolute plate motion. In comparison with the direction of regional strain rates and fast directions obtained from the previous mantle anisotropy studies, the fast orientations of our result are nearly perpendicular to the results of the previous studies. This suggests that anisotropy in the shallow depth around the island is either due to structural induced anisotropy associated with the regional tectonic activity or the fossil anisotropic feature created from the past tectonic events.

By applying detrended fluctuation analysis (DFA), we investigated the short-term earthquake precursor related to the 2018 Banten earthquake. It took place on January 23, 2018. The magnitude of the earthquake is 6.1. DFA is a method to determine the data scaling behavior of time series data. The advantage of using DFA is that we can calculate the scaling behavior of the analyzed data from its trend without understanding more detail of its origin and shape. Our result confirms that the DFA value decreased about two weeks before the earthquake. We also compared the result to the previous studies, which applied spectral density ratio (SDR) to investigate the short-term earthquake precursor related to the M6.1 earthquake. The result shows that the DFA value simultaneously decreased when the spectral density ratio (SDR) associated with the earthquake increased. Furthermore, the result also reveals that the characteristics of DFA value related to the 2018 Banten earthquake are the same as those of the previous study related to the Pelabuhan Ratu earthquake that occurred on September 2, 2009.

West Java, Indonesia is part of Java Island, which is the most populous Island in Indonesia. The region is characterized with earthquakes from subducting plate of Indo-Australian beneath Eurasian plate. The region is also characterized by the presence of Cimandiri fault zone encompassing cities along its trend. In this preliminary study, we estimated crust and upper mantle structure beneath CISI station of GE network. Joint inversion of observed receiver function from teleseismic events and surface wave dispersion from a regional earthquake was carried out. Surface wave dispersion used in this study because it is more sensitive to the shear velocity model, while receiver function is more sensitive to the medium discontinuity. A total number of receiver functions used in the inversion is about 74 receiver functions. Inversion results suggest that sediment thickness varies about 1-2 km thick. Moho depth is estimated to be about 36 km depth. A notable intra crustal low velocity zone is observed from the shear velocity zone at depth of 24-28 km depth. Shear velocity reduction is ~16% compared to the upper and lower layer. By assuming the mechanism to be partial melt, relationship model suggests that 6-7% melt is present beneath the station. Further studies such as heat flow analysis, rock parameters are necessary to constrain the possible mechanism for the seismic velocity decrease.

Peatlands have a unique hydrological system so that water management based on the peatland water system is a must. Peatland management by building drainage will cause changes in the structure of the peat, causing a high risk of forest fires, especially during the dry season. In this paper, we propose a model of tropical peatland drainage based on the 2D Darcy equation. With the initial and boundary conditions that we provide, the solution is solved using the expansion of the eigen-function. The simulation results show that the dynamics of the peat drainage water table is strongly influenced by rainfall rather than water input from the canal. There is a time lag between the rainfall and the increasing of the water table with the hydraulic coefficient determine the length of the time lag.

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.

Observations of seismic waves from earthquakes at depths between 100 and 200 km beneath Japan show that the initial portion of the S-wave arrival has greater duration than can be accounted for by earthquake source-time duration. The observed long duration of S waves has been explained as being caused by multiple forward scattering around the ray path between source and receiver. Array observations of Lg waveforms have also shown that multiple forward scattering along the path between the source and receiver are important influences on the character of Lg waveforms and that the scattering cannot be explained only by vertical variations in velocity. Multiple forward scattering in 3D has been modeled using the Markov approximation for the parabolic-wave equation, which allows the calculation of seismogram envelopes in statistically characterized random media. To test the range of validity of the Markov approximation-derived solutions, we made 2D numerical calculations of wavefields in random media using approximations to the parabolic wave equation, which only models forward scattering, and by finite-difference solution of the scalar-wave equation, which gives complete wavefields. Media of background velocity 4 km/sec are characterized using a Gaussian autocorrelation function with a 5 km correlation distance and 5% rms fractional fluctuation. To compare with envelopes obtained from the Markov approximation, we calculated wavefields for source-receiver distances ranging from 50 to 200 km for several statistically identical realizations of random media. We obtain ensemble average envelopes by averaging envelopes from the realizations. We find a good agreement between ensembleaverage envelopes obtained from the numerical solution of the parabolic-wave equation and envelopes obtained from the Markov approximation. The later portion of the ensemble-average envelopes calculated using finite difference have larger amplitudes than those from the Markov approximation, which is probably due to the late-arriving energy that has been scattered at wide angles from the global propagation direction between the source and the receiver. We observe that the variations among the numerically calculated envelopes of individual realizations of random media are well fit by a Rayleigh distribution, which describes the distribution of envelope amplitude when signals of one frequency but random phase are summed. Our results show that the Markov approximation provides reliable information about envelope shapes for forward-scattered wavefields but that the influence of wide-angle scattering and backscattering have some influences on envelope shapes and should be considered when analyzing data using random media models.

Broadening of seismogram envelopes around direct S waves of deep earthquakes that occurred in a lithospheric slab reveals random inhomogeneities in the upper mantle and crust. Regional differences in the character of envelope broadening were studied in relation to the volcanic front in the central part of Japan. A total of 58 earthquakes that occurred along the subducting Pacific plate ranging from 80 to 500 km in depth were observed at 73 stations of the Kanto-Tokai seismic observation network and used in this analysis. Time lags of the maximum peak arrival and the halfmaximum arrival were measured from the onset of the direct S wave on root-meansquared traces of horizontal component seismograms in frequency bands of 1, 2, 4, and 8 Hz. The envelope broadening is weak at stations located along the coastline of the Pacific Ocean, however, the dependence of time lags on the hypocentral distance and frequency becomes strong at stations near the volcanic front. Moreover, the time lags become very long with increasing hypocentral distances and frequencies west of the volcanic front. This broadening of the seismogram envelope can be interpreted as the effect of the diffraction and multiple forward scattering on the seismic wave propagation through random inhomogeneities. The parabolic approximation is applied to simulate the envelope broadening when the correlation distance of velocity inhomogeneities is longer than the seismic wavelength. The numerical simulation indicates that the dependence of the envelope broadening on the travel distance and frequency depends greatly on the type of the autocorrelation function for the randomness. By comparing the observed frequency dependence of the envelope broadening with the result of the numerical simulation, the type of the autocorrelation function was estimated to be close to Gaussian at stations east of the volcanic front and close to exponential west of the volcanic front. The regional difference in the randomness on both sides of the volcanic front might reflect the difference in the shortwavelength component inhomogeneities. Paper number 94JB02644. 0148-0227/95/94JB-02644505.00 northern part of the area in accordance with the bending of the trench axis. The upper mantle above the descending plate on the back arc side of the VF is characterized as an aseismic region called a mantle wedge. Seismological studies have been done to clarify the inhomogeneous structure beneath the Kanto-Tokai are_a_ lshida and Haxemi [ 1988] and !shido [!992] resolved the three-dimensional structure of the seismic velocity and indicated that the velocity below the crust on the back arc side of the VF is lower than that on the forearc side. Sekiguchi [1991] investigated the three-dimensional Q structure in the central part of Honshu and showed that a high Q zone exists coincident with the subducting plate and that a low Q zone in the uppermost mantle extends westward from the VF. Results of these studies indicate that the mantle wedge west of the VF is characterized as a low-velocity, low Q zone. Moreover, the distribution of the heat flow is quite different on both sides of the VF [Yuhara, 1973]. Thus the melt fraction, thermal structure, and other physical conditions in the upper mantle might differ considerably between the forearc side and the back arc side of the VF. 2103 2104 OBARA AND SATO: S ENVELOPE BROADENING IN KANTO-TOKAI, JAPAN

1] We observe tidal triggering of earthquakes by measuring the correlation between the Earth tide and earthquake occurrence. We used the times, locations, and focal mechanisms of the 9350 globally distributed earthquakes with magnitude 5.5 or larger from the Harvard centroid moment tensor catalog. The tidal stress was theoretically computed by using the Preliminary Reference Earth Model and a recent ocean tide model, NAO.99b. We considered the shear stress on the fault plane and the trace of stress tensor, J1. Defining the tidal phase angle at the occurrence time for each earthquake, we statistically tested the phase selectivity using the Schuster's method. For all the earthquakes, no significant correlation is found between the Earth tide and earthquake occurrence both for the shear stress and for J1. By classifying the data set according to fault types, however, we find a high correlation with the shear stress for reverse fault type. The correlation is particularly clear for shallow and smaller earthquakes of this type. Significant correlation with J1 also appears for larger earthquakes of reverse fault type and for shallow and larger ones of normal fault type. We find no correlation for strike-slip type. For all the cases of high correlation, earthquakes tend to occur when the tidal stress accelerates the fault slip, indicating that high correlation is not coincidental but is physically justified. This result strongly suggests that a small stress change due to the Earth tide encourages earthquake occurrence when the stress in the future focal area is near a critical condition.

From the analysis of the tide gauge records and precise leveling data, we found a postseismic deformation following the 1978 Miyagi±Oki earthquake M 7:4; Japan, on June 12, 1978. The postseismic deformation in the vicinity of the focal region lasted at least four years. By assuming that the deformation was caused by an afterslip on the subducting plate interface, we estimated the area and amount of the afterslip by the inversion analysis. The result of the inversion analysis indicates that, in about two years following the mainshock, the afterslip was restricted to the mainshock fault and its vicinity on the plate interface. In the following years, the afterslip ceased on the mainshock fault, and it migrated towards the down-dip extension of the mainshock fault. The total amount of aseismic moment release was 80±140% of that of the coseismic moment release. Similar afterslip has been reported for the 1994 Sanriku±Haruka±Oki earthquake M 7:7 that occurred about 280 km north of the 1978 Miyagi±Oki earthquake. The occurrence of the large afterslip on the mainshock fault and its migration towards the deeper portion of the plate boundary may be a common feature for the interplate earthquakes occurring in the northeast Japan along the Japan Trench. q