| Geophysical motions can occur over a broad temporal scale,from short-duration highfrequency seismic movements to a very long period tectonic deformation.To create a terrific model and useful patterns of deformation relating to large earthquakes,collecting datasets and methods are crucial factors of the task.Although different methods of deformation monitoring techniques can provide deformation information in an efficient manner still there is some deficiency that limiting their use;thus leading us to use GPS as an effective tool to monitor deformation associated with massive earthquakes.Furthermore,observing and detecting deformation utilizing other conventional techniques normally provide poor information which cannot depend on it in future decision making.As such,our research has taken GNSS observations for estimating the deformation in the study area,which is New Zealand.New Zealand(NZ)is a micro-continent located between the Pacific and Australian plates,which are tectonically subjected to earthquakes,tsunami,volcano activities and crustal deformation.Indeed,both society and people might suffer in consequence of future earthquakes,and estimating the seismic risk related to these events for some future time period is crucial.Since the records of earthquakes have been kept in New Zealand i.e,about 1840,more than eight shallow earthquakes of magnitude 6.0 or greater have originated within the Kaikoura area.The reoccurrence of massive seismic events such as the 2016 Mw 7.8 Kaikoura earthquake that ruptured about 12 separate faults,triggered 2132 aftershocks within one week of the mainshock and induced several Mega Pascal(MPa)of Coulomb stress makes it a significant focal area.Besides,the growth of the Continuously Operating Reference Stations(CORS)in New Zealand has offered a useful technique for monitoring seismic activities.Notably,scholars tend to indicate less concern or overlook this significant research area.Existence of the significant focal area(New Zealand)along with the available data,even seismic or GNSS observations motivates us to characterize fault structures through analyzing crustal deformation by incorporating different innovative methods.Below are the main study outlines along with the results of this research:(1)Modeling of Coulomb Failure Stress Change(CFS)promoted by the Kaikoura earthquake,this special model is targeted to fill the gaps in the existing models by computing the effect of the CFS at different depths(0,5,10,15,20 and 25 km)instead of using a unique depth as in previous works.The study also seeks to establish the relationship between CFS and aftershocks.The CFS changes were calculated and compared at six depths of 0,5,10,15,20 and 25 km.As a result,the stress-change model is quite complicated.The primary factor that contributes to this model is caused by the setting of complicated active faults in the northern part of the South Island.At the depth 0 km,the average stress values are varied between 9.25 and-20.50 MPa and the direction of stresses is cluttered,mainly reflecting the inhomogeneity in the very shallow crust.Less information can be obtained,such as the positive stresses founded in three parts: at the southern section of Jordan thrust,Kaikoura region,and northeastern section Kekergen fault.The second layer(5 km),the stress change range values vary between 5.42 MPa and-13.40 MPa and the epicenter of the mainshock is situated at negative stress change area.However,the northeast area of the epicenter accommodates strong positive stress.This observation illustrates the propagation of the rupture towards the north,which concurs as a result of previous work.Also,the predominant stress change at this depth is a strong negative,which explains the inhomogeneity in the shallow crust.Generally,numerous positive stress zones are founded at depths 0 & 5 and10 & 15 km but the majority of positive stresses are observed at depths of 10 & 15 km relative to the shallow depths at 0 & 5 km.Positive stresses converge at fault ruptures,for example,Needles,Hundalee,Humps,Fidget and Jordan Thrust.In the northern and western areas,the combination of negative and positive stress change show the existence of several N-W faults near the main rupture area.In addition,the average stress ranges of the 10,15,20 and 25 km depths are(-58.00~+9.41),(-21.90~+6.02),(-9.65 ~+ 16.7)and(-11.9 ~+ 7.32)MPa,respectively.According to the above-mentioned figures,it may be useful to define the stresses at deeper depths to better understand the stress change structure.Many studies assumed that the productivity of aftershocks and subsequent main-shocks on nearby faults can easily be detected by monitoring the Coulomb stress change.In order to test this assumption,we adopted 4,261 aftershocks obtained from Geo Net between November 13,2016 and October 7,2017.Because the aftershocks locations have an uncertainty of different depths,the computation of the stresses considered different depths of 0,5,10,15,20 and 25 km.The result shows the spatial distribution of aftershocks is mainly clustered in the rupture areas along the SW-NE.Almost all aftershocks are located in regions with positive stresses such as Humps,Hundalee,Jordan Thrust and Needles.These aftershocks may be triggered by the Kaikoura earthquake.(2)The stability of our stress change models in terms of spatial distribution and amplitude was tested over both,friction coefficient and receiver fault parameters,and results show that accurate parameters are necessary for the computations.The experimental study findings justify the suitability of the chosen friction coefficient(?=0.4) and the receiver fault parameters(230°,70°,150°)to define good stress change estimates.Our study confirms that there is no remarkable change in spatial distributions of Coulomb stress when the value of the friction coefficient is altered.However,the amplitude of the stress may change.Also,it shows that different values of the receiver faults parameters can directly affect the spatial distribution and amplitude of the Coulomb stress models.Thus,considering the best values of the receiver parameters such as those published by USGS,Geo Net,GCMT and IRIS,is important.(3)The variation in gravity at the different depths is drawn to investigate the effect of gravity change induced by the Kaikoura earthquake against the depth element,which has previously been disregarded.The relationship between aftershocks and gravity change is also considered.The analysis of the gravity change can be helpful in future seismic studies.At various depths of 0,5,10,15,20 and 25 km,the general characteristics of gravity change models are different.But,we can clearly observe some common features between the 0 & 5 km models,10 & 15 km models and the 20 & 25 km models.In other words,at the shallower depths,the gravity change models are almost the same,while at the deeper depths close similarities can be observed over the gravity change models.Concisely,due to the first four gravity models,we can categorize the study region into three parts,in which the gravity changes happened significantly.The first part involves the northeast areas i.e.,Needles fault and London Hills fault.The second part comprises the faults of Kekeregnu,Fidget,Jordan Thrust and Hope as well as the Kaikoura area and upper segment of the Hundalee fault area.The third part consists of areas around Humps fault,the epicenter of the Kaikoura earthquake and North Canterbury.Finally,at20 km and 25 km depths models,we can observe the strong similarity between the two models except for the lower part of the Jordan Thrust fault area.Jordan Thrust lower part at 20 km depth was located in a negative gravity change area and changed to positive at 25 km.As mentioned before,many scholars confirmed that monitoring the stress changes can explain the linkage between mainshock and aftershocks on the surrounding faults.Whilst,there has been an increased awareness of monitoring the stress changes associated with large earthquakes,there is little research available to study the relationship between gravity changes and aftershocks distribution.Thus,our work also considered this relationship,which involves assumptions,mapping and analyzing.Using the study area is the same as stress changes and the same number of aftershocks(4266)previously mentioned as well as test the gravity changes at 0,5,10,15,20 and 25 km.The resulting maps relating to the gravity changes and aftershocks declared that almost all aftershocks locations are mainly clustered in the fault rupture regions along an SW-NE direction,where the gravity changes occur,particularly the positive changes.The regions with positive gravity changes are Kaikoura area,Humps,Hope,Jordan Thrust and Kekeregnu faults.That is,the clustering of aftershocks over positive regions can be interpreted by the reason mentioned before,the main rupture is propagated towards the northeast along several faults.Thus,the aftershocks and positive gravity changes may be found mostly in the north-eastern regions.(4)The study also used various statistical methods and verifications in order to conform to and complement the findings achieved by other methods.Statistical methods for hypothesis testing have been rarely employed in seismology.Various statistical methods and verifications such as Correlation,Regression and ANOVA were used in order to perform the task of data analysis,interpretation and best model selection.Such statistics were conducted to establish a statistical linkage between Coulomb failure stress and gravity change groups,which provided fruitful information within and between the groups.The output of those techniques are different and can be summarized as;descriptive analysis result revealed that the maximum Coulomb stress change is ~17 MPa and located at 20 km depth,whereas the minimum change(~59 MPa)observed at the depth of 10 km;this result proved that the total stress changes over the study area can be achieved using different depth levels.Likewise,the maximum gravity change-7.67 m Gal-was observed at 15 km and the minimum change value of 4.69 m Gal occurred at 0 km depth,worth mentioning the epicenter located at ~15 km depth,i.e.,the maximum gravity change happened near the epicenter and the change decreases away from the epicenter.The research is a scientific basis for the rational development and utilization of statistical analysis in this area,and it may be referenced by international scholars for similar research in other parts of the world.The work did not only provide high accuracy results but has also offered a complete framework which defines,observes,investigates and analyzes the massive Kaikoura earthquake.Moreover,the multidimensional analysis adopted in this contribution is helpful for making decisions and applications of stress and gravity change models in assessing seismic hazards. |
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