Universidade de Brasília Instituto de Geociências Observatório Sismológico A GT5 Earthquake Identified in Central Brazil Lucas Vieira Barros (1) lucas@unb.br Marcelo Assumpção (2) marcelo@iag.usp.br Juraci Mario de Carvalho (3) juraci.carvalho@CTBTO.ORG (1) University of Brasília (2) University of São Paulo (3) PTS - CTBTO Vienna, June, 20, 2013. . 1
Universidade de Brasília Instituto de Geociências Observatório Sismológico Outline � Introduction � Brazilian Seismicity � Mara Rosa Earthquake • Aftershock Activity • Local and Regional Networks � Reference Events � Correlation between mainshock and reference events � Relative locations � Relative locations with hypoDD � Conclusions 2
Universidade de Brasília Instituto de Geociências Observatório Sismológico Introduction � Ground-truth (GT) events, located with a precision for instance of 5 km (GT5 event), and associated travel times to regional stations are important in the development of precise velocity models The low Brazilian seismicity and the low number of stations make it � difficult to detect events at regional distances that qualify as GT5 event, particularly in respect with the number of picks phases and azimuthal gaps 3
Universidade de Brasília Instituto de Geociências Observatório Sismológico Introduction � Recently PTS-CTBTO is encouraging seismologists from South and Central America to cooperate in identifying GT5 events to improve the 3-D regional velocity model � As a result we present MR earthquake (5.0 mb and MMI VI) detected only by regional stations located beyond 120 km, and in principle not suitable for GT5 event � However, using aftershocks well recorded by local and regional stations, as reference events it was possible to calculate station corrections and locate the mainshock accurately 4
Universidade de Brasília Instituto de Geociências Observatório Sismológico Brazilian seismicity (1922- 2013). M>=5,0 1. 22 earthquakes M>=5.0 2. 2 M >=6.0 Possible GT5 Events: 1. Event 22 (5.0 mb) on October 8, 2010; 2. Event 17 (5.2mb) and 18 (5.0 mb). in all cases the aftershock activities were studied with local seismic networks. 5
Universidade de Brasília Instituto de Geociências Observatório Sismológico Central Brazil earthquake - macroseismic survey Fallen plaster Fallen roof Craking
Universidade de Brasília Instituto de Geociências Observatório Sismológico Central Brazil earthquake Some waveforms at regional stations Min. dist = ~100 km Max. dist = ~2.000 km
Universidade de Brasília Instituto de Geociências Observatório Sismológico Ray path at regional stations Black line = small residuals Min. dist = ~100 km Blue line = negative residuals, high vel. Max. dist. = ~2.000 km Red line = positive res., low velocity 8
Universidade de Brasília Instituto de Geociências Observatório Sismológico Local seismic network Green stars are the mainshock epicenters: Local stations installed 10 days after USGS = 15km error; mainshock (green triangles) UnB1,2 (model 1,2) = 5-10km error Red circles = best located aftershocks . 9
Universidade de Brasília Instituto de Geociências Observatório Sismológico Reference events: 1. Feb.26.2011 M3.4 2. Mar.04.2011 M3.7 Data O. T. Lat. Long. Brazilian Reference Nº. Mag. (dd/mm/yyyy) (UT) (º) (º) Regional Stations Stations BDFB, CAN3, CPUP, IGA2, LAJE, IS09, CAN3, SSV2, 08/10/2010 5,0 JAN7, MAN1, NVXB, 01 SFA1, JAN7, Mainshock 20:16:54,8 -13,771 -49,160 PAL1, PRTB, PTGA, BDFB/BRA7 RCBR, SAML, SFA1, SSV2, TUCA BRA7, CAN3, FOR1, CAN3, SSV2, LAJE, MR03, MR07, 02 26/02/2011 3.4 SFA1, LAJE, 22:51:52,1 -13,757 -49,156 MR08, PAL1, PAL2, Aftershock 1 BDFB/BRA7 PEXB, SFA1, SSV2. BRA7, CAN3, LAJE, 04/03/2011 3.7 BRA7, CAN3, 03 MR07, MR08, PEXB, Aftershock 2 07:01:41,4 -13,755 -49,157 SSV2, SFA1 SFA1, SSV2. 10
Universidade de Brasília Instituto de Geociências Observatório Sismológico Correlations between mainshock and reference events � P and S waveforms of the aftershock and the main event were correlated to pick arrivals in a consistent way � Weak phases in the aftershocks were read using mainshock strong signal � Both signals were band pass filtered in the same band to compensate magnitude differences � Various band pass-filters were attempted � Cross correlation was used in order to identify common phases in both signals 11
Universidade de Brasília Instituto de Geociências Observatório Sismológico Correlations between mainshock and reference events P phase reference event 1 Station SSV2 Red main blue ref1 150 km distant 12
Universidade de Brasília Instituto de Geociências Observatório Sismológico Correlations between mainshock and reference events P phase reference event 1 Station BDFB Red main blue ref1 250 km distant 13
Universidade de Brasília Instituto de Geociências Observatório Sismológico Correlations between the three events in the same station BDFB Station ~250 km 14
Universidade de Brasília Instituto de Geociências Observatório Sismológico Correlations between the Main and 2 aftershocks at station SSV2, 150 km away 15
Universidade de Brasília Instituto de Geociências Observatório Sismológico Relative location The best half-space model (Vp= 6.0 km/s; Vp/Vs=1.70) was determined with the 18 best recorded local events. The aftershocks of 26-Feb-2011 and 04-Mar-2011 were recorded by only 3 and 2 local stations, respectively. P and S arrivals as well as back-azimuths were used to locate these event using “hypocenter” in Seisan. Travel times from the aftershocks to the regional stations were calculated with fixed hypocenter. . 16
Universidade de Brasília Instituto de Geociências Observatório Sismológico Regional model for localizations. Regional velocity model Local velocity model VELOCITY DEPTH Half space (km/sec.) (km) 6.00 0.0 6.00 0.00 6.60 12.0 6.60 12.0 6.80 25.0 7.20 35.0 8.30 38.0 Moho discontinuity Vp/Vs = 1.70 17
Universidade de Brasília Instituto de Geociências Observatório Sismológico Relative location – stations corrections Residuals of the 26-FEB-2011 at the regional stations DATE ORIGIN LAT S LONG W H 0226 2251 52.12 13.7572 49.1562 0.70km <= FIXED HYPOCENTER P and S corrections for Feb. P and S corrections for Mar. 26 event 04 event Station P-Res. S-Res. P-Res. S-Res. CAN3 -0.359 -0.320 -0.263 -0.098 SSV2 -0.448 +0.178 -0.187 -0.204 BRA7 +0.673 +4.718 +0.592 +4.775 BDFB +0.673 +4.718 +0.592 +4.775 SFA1 +0.122 +2.712 +1.301? +3.118 18
Universidade de Brasília Instituto de Geociências Observatório Sismológico Results: 62 events detect by six to eight stations . Yellow star denotes the best relative location of the main shock It is clear the dipping fault to the NW 19
Universidade de Brasília Instituto de Geociências Observatório Sismológico Results: 18 events detected by eight stations (red circles) Red star is the main shock epicenter, relative to the Feb. event Blue star is the main shock relative to the March event Yellow stars are the reference aftershocks located by local stations 20
Universidade de Brasília Instituto de Geociências Observatório Sismológico Relocation of 62 events with hypoDD – blue circles Reference events (yellow squares) also included in hypoDD Red star denotes mainshock relative to the Feb event Depths (km) hyp hDD Feb 0.70 1.3 Mar 1.80 1.2 21
Universidade de Brasília Instituto de Geociências Observatório Sismológico Composite focal mechanism solutions 1) 62 events detected by 6 to 8 stations 2) 18 events detected by eight stations Strike Dip Rake Strike Dip Rake 214 49 74 216 49 74 58 43 108 60 43 108 22
Universidade de Brasília Instituto de Geociências Observatório Sismológico CONCLUSIONS 1. The location of the main shock, using the reference event of 26- Feb, has a standard error of 0.87km (95% confidence = +- 2km), assuming no error in the aftershock location. The reference aftershock (26-Feb) had a standard error of 0.3 km when located with the 3 local stations. 2. Using the aftershock of 04-MAR-2011 as reference, the main shock had an std.error of 2.7 km (rms =0.07s). The correlations are not as good as the event of Feb 26. The absolute location of the aftershock of 04-Mar had a std.error of 0.2 km when located with only two stations (P+S, and two back-azimuths). 3. The difference of the two mainshock epicenters is only 0.7 km and 0.6 km in depth. The origin time differs by 0.01 s only! 23
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