A GT5 Earthquake Identified in Central Brazil Lucas Vieira Barros (1) - - PowerPoint PPT Presentation

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A GT5 Earthquake Identified in Central Brazil Lucas Vieira Barros (1) - - PowerPoint PPT Presentation

May 24, 2023 135 likes •426 views

Universidade de Braslia Instituto de Geocincias Observatrio Sismolgico A GT5 Earthquake Identified in Central Brazil Lucas Vieira Barros (1) lucas@unb.br Marcelo Assumpo (2) marcelo@iag.usp.br Juraci Mario de Carvalho (3)

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Observatório Sismológico Instituto de Geociências Universidade de Brasília 1

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. .

A GT5 Earthquake Identified in Central Brazil

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Observatório Sismológico Instituto de Geociências Universidade de Brasília

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

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Observatório Sismológico Instituto de Geociências Universidade de Brasília

Introduction

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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

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Observatório Sismológico Instituto de Geociências Universidade de Brasília

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

  • nly 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

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Observatório Sismológico Instituto de Geociências Universidade de Brasília

Brazilian seismicity (1922- 2013). M>=5,0

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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.

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Observatório Sismológico Instituto de Geociências Universidade de Brasília

Central Brazil earthquake - macroseismic survey

Fallen plaster Craking Fallen roof

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Observatório Sismológico Instituto de Geociências Universidade de Brasília

Central Brazil earthquake Some waveforms at regional stations

  • Min. dist = ~100

km

  • Max. dist = ~2.000

km

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Observatório Sismológico Instituto de Geociências Universidade de Brasília 8

Ray path at regional stations

  • Min. dist = ~100 km
  • Max. dist. = ~2.000 km

Black line = small residuals Blue line = negative residuals, high vel. Red line = positive res., low velocity

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Observatório Sismológico Instituto de Geociências Universidade de Brasília 9

Local stations installed 10 days after mainshock (green triangles) Green stars are the mainshock epicenters: USGS = 15km error; UnB1,2 (model 1,2) = 5-10km error Red circles = best located aftershocks.

Local seismic network

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Observatório Sismológico Instituto de Geociências Universidade de Brasília 10

Nº. Data (dd/mm/yyyy)

  • O. T.

(UT) Lat. (º) Long. (º) Mag. Brazilian Regional Stations Reference Stations 01 08/10/2010 Mainshock 20:16:54,8

  • 13,771
  • 49,160

5,0 BDFB, CAN3, CPUP, IGA2, LAJE, IS09, JAN7, MAN1, NVXB, PAL1, PRTB, PTGA, RCBR, SAML, SFA1, SSV2, TUCA CAN3, SSV2, SFA1, JAN7, BDFB/BRA7 02 26/02/2011 Aftershock 1 22:51:52,1

  • 13,757
  • 49,156

3.4 BRA7, CAN3, FOR1, LAJE, MR03, MR07, MR08, PAL1, PAL2, PEXB, SFA1, SSV2. CAN3, SSV2, SFA1, LAJE, BDFB/BRA7 03 04/03/2011 Aftershock 2 07:01:41,4

  • 13,755
  • 49,157

3.7 BRA7, CAN3, LAJE, MR07, MR08, PEXB, SFA1, SSV2. BRA7, CAN3, SSV2, SFA1

Reference events: 1. Feb.26.2011 M3.4

  • 2. Mar.04.2011 M3.7
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Observatório Sismológico Instituto de Geociências Universidade de Brasília

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

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Observatório Sismológico Instituto de Geociências Universidade de Brasília

Correlations between mainshock and reference events

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P phase reference event 1 Red main blue ref1 Station SSV2 150 km distant

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Observatório Sismológico Instituto de Geociências Universidade de Brasília

Correlations between mainshock and reference events

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P phase reference event 1 Red main blue ref1 Station BDFB 250 km distant

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Observatório Sismológico Instituto de Geociências Universidade de Brasília 14

Correlations between the three events in the same station BDFB Station ~250 km

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Observatório Sismológico Instituto de Geociências Universidade de Brasília

Correlations between the Main and 2 aftershocks at station SSV2, 150 km away

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Observatório Sismológico Instituto de Geociências Universidade de Brasília

Relative location

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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

  • nly 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. .

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Observatório Sismológico Instituto de Geociências Universidade de Brasília

Regional model for localizations.

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VELOCITY (km/sec.) DEPTH (km) Half space 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

Regional velocity model Local velocity model

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Observatório Sismológico Instituto de Geociências Universidade de Brasília

Relative location – stations corrections

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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. 26 event P and S corrections for Mar. 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

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Observatório Sismológico Instituto de Geociências Universidade de Brasília 19

Results: 62 events detect by six to eight stations . Yellow star denotes the best relative location

  • f

the main shock It is clear the dipping fault to the NW

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Observatório Sismológico Instituto de Geociências Universidade de Brasília 20

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

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Observatório Sismológico Instituto de Geociências Universidade de Brasília 21

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

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Observatório Sismológico Instituto de Geociências Universidade de Brasília 22

Strike Dip Rake 214 49 74 58 43 108 Strike Dip Rake 216 49 74 60 43 108

Composite focal mechanism solutions

1) 62 events detected by 6 to 8 stations 2) 18 events detected by eight stations

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Observatório Sismológico Instituto de Geociências Universidade de Brasília 23

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

  • nly 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!

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Observatório Sismológico Instituto de Geociências Universidade de Brasília 24

  • 4. The main event is in the middle of the aftershock zone, in an area

with little aftershock activity. The rupture of the main event is probably less than that defined by the late aftershocks.

  • 5. Using arrival times picked by cross-correlation of a reference event

is a good method to obtain reliable location and depth of a main event, well within the 5 km confidence of a GT5 event.

  • 6. The new Brazilian GT5 event:

Date O.T Lat Long Depth No Gap RMS ERH ERZ 2010 1008 201655 -13.7713 -49.1602 1.36 6 256 0.010 0.87 0.74 Feb 2010 1008 201655 -13.7655 -49.1587 2.0 6 244 0.069 2.68 2.33 Mar Conclusion

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Observatório Sismológico Instituto de Geociências Universidade de Brasília 25

Thank you very much!

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Observatório Sismológico Instituto de Geociências Universidade de Brasília

Correlations between mainshock and reference event2

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P phase reference event 2 Red main blue ref2 Station SSV2 and CAN3 120 km and 150 distant

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Observatório Sismológico Instituto de Geociências Universidade de Brasília 27

SSV2 Station ~ 150 km BDFB Station ~250 km