Determination of Shear Wave Splitting Parameters in 2018 Lombok Earthquake Using Rotation Correlation Method: Preliminary Result
Annisa Trisnia SasmiAndri Dian NugrahaMuzli MuzliSri WidiyantoroZulfakriza ZulfakrizaShengji WeiDavid P. SaharaNanang T. PuspitoAwali PriyonoHaunan AfifPepen SupendiDaryono DaryonoArdianto ArdiantoDevy Kamil SyahbanaYayan M HusniBilly Sugiartono PrabowoKadek Hendrawan PalgunadiAchmad Fajar Narotama Sarjan
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Abstract Shear-wave splitting (SWS), or the propagation of two independent shear waves, can be used as an indicator of seismic anisotropy. In this study, we utilize this concept using aftershock data of the 2018 Lombok earthquake which had been acquired in period of August 4 – September 9, 2018. The goal of this research is to better understand the crack distribution related to the rupture zone of the 2018 Lombok earthquake. After applying instrument correction to the data, the waveform data were then windowed in each P and S arrival time. To determine the SWS parameters, we performed rotation in each horizontal seismogram components. The horizontal components were rotated from azimuth 0° to 180° with an increment of 1°. Cross-correlation coefficient (CCC) was determined for each rotation angle. The polarization direction and the SWS delay time were chosen from the parameters shown in the highest value of CCC.Keywords:
Seismogram
Shear wave splitting
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Three component seismograms of low magnitude earthquakes (M d < 3) recorded at the central station Tetitlan of the seismic network located in the Guerrero gap, Mexico show evidence of shear wave splitting. The data set contains several months of activity from the installation of the network in 1987 until late 1989. Even though the sampling rate is only 75 samples/sec and the signal to noise ratio is not very large, it was possible to estimate in most cases the polarization direction of the first shear wave as well as shear‐wave splitting time delays. Most polarizations show a direction which correlates with the main compressive stress in the area due to subduction of the Cocos Plate beneath the North American Plate. Two groups of hypocenters can be distinguished: a shallow group up to 25 Km in depth and a deeper group from 32 to 45 Km. As there is no difference in the polarization direction pattern of both groups and the time delays do not increase with depth, the shear wave splitting is interpreted in terms of an anisotropic layer above 25 km in depth. Filtering the seismograms to enhance the peak amplitude frequency band does not change the observed polarization directions but helps identify shear wave splitting.
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Shear wave splitting
S-wave
Shear waves
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Deformation characteristics, magnitude and time distributionof the aftershocks of the large earthquakes, occurred in the regionof Greece from 1926 till 1964 are investigated. An approximate relationbetween the number of aftershocks and the magnitude and focal depthof the main shock has been found. Also, an approximate relation has beenderived between the magnitude of the largest aftershock and the magnitudeand focal depth of the main sliok. The largest aftershock occurs withinfourteen days after the main shock. In many cases large " late aftershocks "occur one or more months after the main shock. One or more foreshocksof magnitude larger than 3.5 occurred in forty per cent of the cases. The probability for an earthquake to be preceded l>y a large foreshoek not muchsmaller than the main shock is 10%. It is shown that some properties ofthe Earth's material in the aftershock region can be derived by studying themagnitude distribution and deformation characteristics of the aftershocks.
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We present an analytical solution and numerical tests of the epidemic-type aftershock (ETAS) model for aftershocks, which describes foreshocks, aftershocks and mainshocks on the same footing. The occurrence rate of aftershocks triggered by a single mainshock decreases with the time from the mainshock according to the modified Omori law K/(t+c)^p with p=1+theta. A mainshock at time t=0 triggers aftershocks according to the local Omori law, that in turn trigger their own aftershocks and so on. The effective branching parameter n, defined as the mean aftershock number triggered per event, controls the transition between a sub-critical regime n<1 to a super-critical regime n>1. In the sub-critical regime, we recover and document the crossover from an Omori exponent 1-theta for t1 and theta>0, we find a novel transition from an Omori decay law with exponent 1-theta fot tt*. The case theta<0 yields an infinite n-value. In this case, we find another characteristic time tau controlling the crossover from an Omori law with exponent 1-theta for t
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Shear wave data from four nine component VSP's from the Iatan East Howard field, Mitchell County, Texas, have been analysed to determine the nature and extent of shear wave anisotropy. Oil production in this field is from Permian age Clearfork Dolomites. Cores indicate the presence of vertical fractures. Shear-wave splitting was observed on all VSP's and polarization of the leading split shear wave has been used to infer fracture orientation. The two anisotropic parameters, qS1 polarization and time delay between qS1 and qS2, were measured using two anisotropic estimation techniques. These measurements were confirmed by visual examination of seismograms and particle motions and then used to define an anisotropic model for the rockmass in the vicinity of the VSP. Synthetic seismograms were generated for the model, which gave a good match with observed seismograms and results.
Seismogram
Shear wave splitting
Shear waves
Seismic anisotropy
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Temporal features of the aftershock activity following some large shallow earthquakes of M≥7 in Japan have been studied quantitatively. The earthquakes concerned were accompanied by large aftershocks which triggered their own aftershock activity. The purpose of the present study is to seek any anomalous change in aftershock activity of the main shock before the occurrence of such large aftershocks. Aftershock activity shows an appreciable decrease from the level expected from the modified Omori formula before the occurrence of a large aftershock. The aftershock activity then recovers to the normal level or even increases beyond the normal level shortly before the occurrence of the large aftershock. The recovered activity generally occurs near the hypocenter of the forthcoming large aftershock. Such a feature has been recognized ha fourteen cases out of eighteen for which sufficient data are available. We have the possibility of predicting the occurrence of a large aftershock which might be as large and disastrous as the main shock, if we keep watch on the change of the aftershock activity immediately following the main shock. Moreover, a rough prediction of the place can be made by checking the hypocenter location of aftershocks occurring in the recovered stage.
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