RELOCATION OF EARTHQUAKES IN MYANMAR BY MJHD METHOD: AFTERSHOCKS OF LARGE EARTHQUAKES AND SEISMICITY ALONG THE SAGAING FAULT

Transcription

1 RELOCATION OF EARTHQUAKES IN MYANMAR BY MJHD METHOD: AFTERSHOCKS OF LARGE EARTHQUAKES AND SEISMICITY ALONG THE SAGAING FAULT PHYO Maug Maug MEE0859 Supervsor: Buchro SHIBAZAKI Nobuo HURUKAWA ABSTRACT To uderstad the sesmcty Myamar, detaled aalyss of hypoceters ad fault plaes determato are mportat. Frst, we relocated seve earthquakes of whch magtudes are M w > 6.0, together wth ther aftershocks usg modfed ot hypoceter determato (MJHD) method ad used P-wave arrval tmes from Iteratoal Sesmologcal Cetre (ISC). We relocated aftershocks of oe day, oe week, or oe moth based o the aftershocks occurreces. Target area s + degree from both the lattude ad logtude from Global CMT for each large earthquake. After the relocatos of the large earthquakes ad ther aftershocks, we could detfy the fault plae of three earthquakes but some earthquakes are dffcult to detfy the fault plae due to aftershocks occurreces. Furthermore, we relocated earthquakes cludg the hstorcal earthquakes to vestgate the sesmcty alog the Sagag fault. We used P-wave arrval tmes from ISC (96 to March, 2007) ad those for seve hstorcal earthquakes from Iteratoal Sesmologcal Summary (ISS). Oce the hypoceters relocated precsely, the earthquakes occurrece ad sesmc gap alog the Sagag fault would become clear. Some relocated epceters are closer to the fault tha those before relocato. We could determe the hypoceters of hstorcal earthquakes ad recet earthquakes o the fve sub-regos. It meas our results are cosstet wth the geologcal settg of ths rego ad are useful for sesmc hazard assessmet ths rego. Keywords: Hypoceter relocato, Sagag fault, Hstorcal earthquakes. INTRODUCTION Myamar s located at a very actve tectoc area, whch cludes the subducto zoe ad the actve Sagag fault. From sesmologcal pot of vew, Myamar s oe of sesmcally actve coutres of the world. Shallow ad termedate earthquakes occur alog the Burma subducto zoe ad shallow strog earthquakes occur alog the Sagag fault (Fgure ). The Sagag fault s a actve rght lateral strke slp fault crossg from orth to south through Myamar. Most of hstorcal earthquakes occurred alog the fault. Fftee large earthquakes (of M > 7.0) ht the coutry from 92 to 2003 (Fgure 2). The frequecy of the occurrece of large earthquakes s aroud oce 2 to 6 years. Accordg to the sesmcty map, the orthwester part of Myamar s oe of strog earthquake sources of the world. The earthquakes are spreadg all over the coutry except the souther coastal regos, whch dcates oretato of actve faults. I the preset study, we relocate aftershocks of seve large earthquakes from 988 to 2003 to determe the fault plae by MJHD method. I order to kow the precse sesmcty alog the Sagag fault, we relocated earthquakes alog the Sagag fault cludg evets from 96 to March, 2007 ad hstorcal earthquakes. We use the data from Iteratoal Sesmologcal Cetre (ISC) ad Iteratoal Sesmologcal Summary (ISS). Departmet of Meteorology ad Hydrology (DMH), Myamar. **Iteratoal Isttute of Sesmology ad Earthquake Egeerg, Buldg Research Isttute, Japa.

2 Fgure. The epceter dstrbuto map of Myamar s used USGS data from 973 to 2009, March 3. The dfferet sze ad color of symbols dcates the sze ad depth of earthquakes. Fgure 2. The epceter dstrbuto map of hstorcal earthquakes Myamar occurred from 92 to 2003 ad data from Departmet of Meteorology ad Hydrology (DMH). 2. METHOD I ths study, we used the Modfed Jot Hypoceter Determato (MJHD) method developed by Hurukawa ad Imoto (990, 992) for the determato of hypoceters ad fault plaes Myamar. The equato show as follows; t t t ( O C) = ( t To ) T = dλ + dϕ + dz + dto + ds () λ ϕ z where t s arrval tme of th evet at th stato, T s the calculated travel tme of the th evet at the th stato, ds s the stato correcto at the th stato, To s the org tme, O s the observed travel tme, C s the calculated travel tme, (O-C) s the travel tme resdual of th evet at th stato, dλ, dϕ, dz ad dto are correcto of tral hypoceter of th evet. Due to the heterogeeous earth s structure, whe the stato coverage s ot good, the JHD solutos become ustable ad urelable because of the trade off betwee stato correctos ad focal depths of earthquakes. For ths reaso, Hurukawa ad Imoto (990, 992) developed usg the costrats below. S = 0 S cosθ = 0 S sθ = 0 S = 0 (2) D where S s the stato correcto at th stato, D s the dstace betwee the th stato ad the ceter of the rego, θ s the azmuth of the th stato from the ceter of the rego ad s the umber of statos. 2

3 3. DATA 3. Aftershock relocato ad ts fault plae Foreshocks, a mashock ad ts aftershocks are relocated ths study by usg P-wave arrval tmes from Iteratoal Sesmologcal Cetre (ISC) order to determe the fault plae. The, the correspodg fault plae s determed based o the obtaed aftershock dstrbuto. We relocated seve large earthquakes from 988 to 2003 wth M w > 6.0 wth data take from Global CMT catalog. 3.2 Hstorcal earthquakes relocato We relocate seve hstorcal earthquakes usg data from Iteratoal Sesmologcal Summery (ISS). These are the Jauary 9 th, 929 (M=7.0), August 8 th, 929 (M=7.0), May 5 th, 930 (M=7.3), December 3 rd, 930 (M=7.3), Jauary 27 th, 93 (M=7.3), September 2 th, 946 (M=7.3) ad July 6 th, 956 (M=7.0) earthquakes. I addto, we relocated the earthquakes to kow the precse sesmcty ad to determe hypoceter o the fve sub-regos alog the Sagag fault usg Iteratoal Sesmologcal Ceter (ISC) from 96 to March, Aftershock relocato ad ts fault plae 4. RESULTS It s dffcult to determe the fault plae of the mashock usg oly ISC hypoceter dstrbuto due to suffcet accuracy. Therefore, we aalyzed the earthquakes to determe the fault plae usg MJHD method. I order to determe the fault plae of the August 6 th, 988 earthquake, we relocated the aftershocks from August 6 th, 988 to August 2 st, 988. We caot determe the fault plae of the mashock usg Iteratoal Sesmologcal Cetre (ISC) hypoceter dstrbuto map Fgure 3. Fgure 3. Hypoceter dstrbuto of the August 6 th, 988 earthquake ad ts aftershocks by ISC. The sze of symbols represets the magtude of the evets. The color ad shape of the symbols deote the depth rage of the evets. Fgure 4. MJHD hypoceter of the August 6 th, 988 earthquake ad ts aftershocks. Symbols are same as Fgure 3. The A-B ad C-D les are two odal plaes show by sold les cross sectos. The momet tesor soluto s take from Global CMT catalog. 3

4 We detfed 236 statos worldwde ad phase data wth travel tme resduals (O-C) > 2.0 secods were excluded. The mashock ad ts aftershocks are relocated to detfy the fault plae usg MJHD method Fgure 4. The hypoceter dstrbuto of ths evet suggests the odal plae show cross secto C-D could be the fault plae related to that evet as show Fgure 4, because the mashock ad all aftershocks were located alog ths odal plae. The strke ad dp of the fault plae are 48 ad 54 determed by Global CMT. Smlarly, we could determe the fault plae of the November 6 th, 988 earthquake wth M w 7.0 occurred the Cha-Myamar border rego as show Fgure 5. The largest aftershock followed wth M w 6.9 wth 2 mutes after the mashock ad some e aftershocks followed o the same day wth M w > 4.5. From the MJHD hypoceters dstrbuto map, we ca clearly fd the fault plae of the earthquake after relocato from two odal plaes. The odal plae show the cross secto A-B s the fault plae because aftershocks cocetrated alog ths plae. The strke ad dp agle of ths fault plae are 333 ad 78 as determed by Global CMT. Ad also, we could determe the fault plae of the September 2 st, 2003 earthquake as show Fgure 6. The odal plae, whch s show cross secto C-D could be the fault plae of the evet. Aftershocks are cocetrated alog ths odal plae ad all earthquakes are aroud 20 to 50 km depths. The strke ad dp of the fault plae are 00 ad 83 determed by Global CMT. Fgure 5. MJHD hypoceters of the November 6 th, 988 Earthquake ad ts aftershocks. Symbols are same as Fgure 4. Fgure 6. MJHD hypoceters of the September 2 st, 2003 Earthquake ad ts aftershocks. Symbols are same as Fgure 4. But, t s dffcult of determe the fault plae of the Jauary 5 th, 99 earthquake due to the lack of aftershocks. However, we thk that the odal plae show the cross secto A-B mght be the fault plae because aftershocks occurred ear the odal plae. We could determe the accurate hypoceters but we could ot determe the fault plae by usg MJHD method aloe. For the Aprl 23 rd, 992, July th, 995 ad Jue 7 th, 2000 earthquakes, we could ot determe the fault plae due to lack of aftershocks eve though earthquakes occurred the shallow depth. 4.2 Hstorcal earthquakes relocato I order to uderstad the precse sesmcty alog the Sagag fault, we relocate hypoceters of hstorcal large earthquakes from 929 to 956 usg data from Iteratoal Sesmologcal Summary (ISS). Combg hstorcal earthquakes ad recet earthquakes from 96 to March, 2007 usg the data from Iteratoal Sesmologcal Cetre (ISC), we relocated them smultaeously order to mprove accuracy of hstorcal earthquakes locato by MJHD method (Hurukawa et al. 2008). 4

5 Fgure 7 (a) shows locato of the fve sub-regos ad locatos of seve hstorcal earthquakes by ISS data ad Fgure 7 (b) shows the relocato results by MJHD method. Oly P-wave frst arrvals are used ths hypoceters relocato because the readg accuracy of S-wave arrval tmes s worse tha P-wave arrvals. (a) (b) Fgure 7. Comparso betwee before ad after relocatos o the fve sub-regos. (a) ISS locatos ad (b) MJHD locatos. The rectagles wth blue color (A-E) dcate the fve sub-regos for relocato of earthquakes alog the Sagag fault ad red crcles dcate the epceters of hstorcal earthquakes. I the rego (A), we relocated the 223 earthquakes from 96 to March, 2007 ad two hstorcal recorded earthquakes: the Jauary 9 th, 929 (M 7.0) ad Jauary 27 th, 93 (M 7.3). We selected earthquakes that had occurred the rectagle betwee N N lattude ad E E logtude. Phase data wth travel tme resduals (O-C) > 2.0 secods were excluded. After the relocato by MJHD method, we ca see clearly that the earthquakes cocetrate alog the actve Sagag fault ad depths of the shallow earthquakes are aroud 0-40 km. Two hstorcal earthquakes are moved closer to the Sagag fault wth focal depths of aroud 0-20 km as show Fgure 8, whle some of earthquakes are occurred at km depth. The dfferece of locato for the Jauary 9 th,929 (M 7.0) S.F 93 EQ 929 EQ Fgure 8. Relocated hypoceters by the MJHD method rego A. Symbols are same as Fgure 3. S.F s the Sagag fault. earthquake s 0.47 N lattude ad.0 E logtude ad depth s 20.4 km. We could determe the hypoceter of hstorcal earthquakes wth accuracy because the errors of the hypoceters are ot so large. Epcetral dstrbuto ad E-W ad N-S cross secto alog A-B ad C-D les are show Fgure 8 ad cross bar the crcles represet the stadard errors of the hypoceters. 5

6 Oe hstorcal earthquake (M=7.3) occurred o September 2 th, 946 the rego (B). The deeper earthquakes occurred outsde of the Sagag fault. Accordg to the relocato result, the locato of the 946 hstorcal earthquake moved to the orth ad depth s stll shallow. We relocated the July 6 th, 956 hstorcal earthquake the rego (C). We thk that the hypoceter would be mslocated because we relocate ths earthquake usg few statos data ad statos are far from the epceter. We relocate the earthquakes from 96 to March, 2007 ad the August 8 th, 929 hstorcal earthquake the rego (D). The earthquake cluster occurred betwee the Sagag fault ad other small fault from epceter dstrbuto map. About the hstorcal earthquake, we thk that the hypoceter would be mslocated because we used few statos data ad the statos are far from the epceter. I the rego (E), we relocated 6 earthquakes durg from 96 to March, 2007 cludg the two hstorcal earthquakes that occurred o the May 5 th, 930 ad December 3 rd, 930. After relocato, we could determe the focal depth of the hstorcal earthquakes to be aroud 5-40 km. 5. DISCUSSION AND CONCLUSION We relocated the seve earthquakes from 988 to 2003 wth M w > 6.0 ad ther aftershocks to determe the fault plae usg Modfed Jot Hypoceter Determato (MJHD) method developed by Hurukawa ad Imoto (992). We obtaed hgh accuracy hypoceters of the earthquakes ad our results are more relable tha hypoceters of Iteratoal Sesmologcal Cetre (ISC) catalog. We could determe the fault plae of the August 6 th, 988 earthquake, November 6 th, 988 earthquake ad September 2 st, 2003 earthquake. However, we could ot determe the fault plae of some earthquakes whch are the Jauary st, 99 earthquake, Aprl 23 rd, 992 earthquake, July th, 995 ad Jue 7 th, 2000 earthquake. These results dcate that we caot determe the fault plae of small earthquakes. Moreover, we otced the deep ad shallow earthquakes have aftershocks wth dfferet behavor. Large shallow earthquakes are lkely to be followed by aftershocks more tha deep earthquake. Ths MJHD method s very useful for determg the accurate hypoceter of the earthquake. I addto, we relocated the earthquakes alog the Sagag fault usg ISC data from 96 to March 2007 ad ISS data for hstorcal earthquakes from 929 to 956. We used the oly P-wave arrval tmes ad determed the hypoceters of earthquakes durg 47 years ad the hstorcal earthquakes usg the MJHD method. We determed the depths of the Jauary 9 th, 929 (M 7.0) ad Jauary 27 th, 93 (M 7.3) hstorcal earthquakes are 5 to 30 km rego (A). The 946 hstorcal earthquake epceter s o the Sagag fault before relocato. After the relocato, the earthquake moves to the orth although depth s stll shallow. We could determe the hypoceters of the December 3 rd, 930 (M=7.3) earthquake that moved to the orth ad May 5 th, 930 (M=7.3) earthquake that moved to the west. The depths of earthquakes are aroud 0-50 km. Before relocato, these two earthquakes are located o the same place accordg to the ISS data. We thk that the July 6 th, 956 ad August 8 th, 929 earthquakes were mslocated due to the poor azmuth coverage ad low qualty of data from ISS. Therefore, we cocluded that the azmuth coverage, earby statos ad readg data are mportat order to relocate hypoceter usg MJHD method. We have to be careful for the put parameters for relocatos ad the data to get accurate ad relable results. Fally, we expect that the results of ths study wll be useful for the future sesmologcal study Myamar. AKNOWLEDGEMENT I am scerely grateful to Mr. Dmtry A. Storchak, Drector ad Mr. James Harrs, System ad Database Admstrator from ISC for ther cotuous assstace o mapulatg computer data. REFERENCES Hurukawa, N. ad Imoto, M., 990, J. Sesm. Soc. Japa, Ser. 2, 43, Hurukawa, N. ad Imoto, M., 992, Geophys. J. It., 09, Hurukawa, N., 995, Geophy. Res. Letters, Vol. 22, No. 23, pp Hurukawa, N. M. Popa, ad M. Radula, 2008, Earth Plaets Space, 60,