ISC-GEM Global Reference Earthquake Instrumental Catalogue (1900-2009)

1. ISC-GEM Global Reference Earthquake Instrumental Catalogue (1900-2009) D. Di Giacomo, I. Bondár, E.R. Engdahl, D.A. Storchak, W.H.K. Lee, A. Villaseñor, J. Harris, P. Bormann 43rd Nordic Seismology Seminar, 24-26 October 2012, Tallinn

2. Motivation Seismic hazard studies need accurate knowledge of the spatial distribution of seismicity and the magnitude-frequency relation. Existing catalogues for past century, however, are compilations of different sources covering different time periods, and therefore contain inhomogeneous locations and magnitudes. There is the need for an improved global instrumental catalogue for large earthquakes spanning the entire 100+ years period of instrumental seismology. 43rd Nordic Seismology Seminar, 24-26 October 2012, Tallinn

3. Project in a nutshell • Cut-off magnitudes: • 1900-1917: MS≥7.5 worldwide + smaller shallow events in stable continental areas • 1918-1959: MS≥6¼ • 1960-2009: MS≥5.5 This Catalogue is unique because it contains homogeneous locations and magnitude estimates with the estimates of uncertainty for the entire period 1900-2009 done using the same tools and techniques to the extent possible. • Collecting, digitising and processing data from a multitude of historical sources for earthquakes occurred up to 1970; • 110 years of relocated earthquake hypocenters; • recomputed MS and mb values for relocated events using uniform procedures; • MW values (with uncertainty) based on: • seismic moment from GCMT (mainly 1976-2009); • seismic moments from the literature search for earthquakes up to 1979; • proxy values based on recomputed MS and mb in other cases using appropriate empirical relationships. 3 43rd Nordic Seismology Seminar, 24-26 October 2012, Tallinn

4. Phase and Amplitude Data Collection Period Body Wave Arrival Times Body/Surface Wave Amplitudes 1900-1917 1918-1959 1960-1970 1971-2009 ~10,000 ~1,000,000 Quality station bulletins DIGITALLY AVAILABLE, ISC database DIGITALLY NOT AVAILABLE BEFORE THIS PROJECT Major Sources of Phase Data: • Gutenberg Notepads (1904-1917) and BAAS (1913-1917) • ISS Bulletins (1918-1963) 4 43rd Nordic Seismology Seminar, 24-26 October 2012, Tallinn

5. 1906 San Francisco earthquake report from Göttingen (Germany) station bulletin. The same report stored in digital format in the ISC database. Period and amplitude data finally available for magnitude recomputation. Processing historical seismological bulletin ~15,000 individual seismic bulletins from 290 institutions over the period 1904 – 1970 were recovered from ISC storage 43rd Nordic Seismology Seminar, 24-26 October 2012, Tallinn

6. Amplitude Data from Quality Station Bulletins Station timeline • ~110,000 “brand new” amplitudes up to 1970 now available in the ISC database • Effort equivalent to ~70 person-months Time Coverage: UPP, RIV, and LPZ nearly continuous, gaps for other stations 6

7. Earthquake Location Procedure • Location method: • Determine event depth using the EHB style of processing (Engdahl, van der Hilst and Buland, 1998): • comprehensive analysis of near-event surface reflections off the earth surface inland and ocean bottom or water surface in the oceans; • Station patch corrections; • Use the new ISC location algorithm (Bondár and Storchak, 2011) with earthquake depths fixed to those from EHB analysis: • more accurate epicentre locations due to correlated error structure taken into account (removes bias from uneven geometrical positioning of stations) • independent depth confirmation using depth phase stacking; 43rd Nordic Seismology Seminar, 24-26 October 2012, Tallinn

8. Earthquake Relocation results Before relocation… 8 43rd Nordic Seismology Seminar, 24-26 October 2012, Tallinn

9. Earthquake Relocation results ….after relocation. 9 43rd Nordic Seismology Seminar, 24-26 October 2012, Tallinn

10. Earthquake Relocation results After Before 10 43rd Nordic Seismology Seminar, 24-26 October 2012, Tallinn

11. MS and mb recomputation The recomputed MS and mb benefit from:1) amplitude data added up to 1970;2) station magnitudes consistent with newly computed hypocentre solutions; 3) network magnitudes based on several station measurements using alpha-trimmed median (α = 20%) of the single station magnitudes (no network magnitude based on one station only). 11 43rd Nordic Seismology Seminar, 24-26 October 2012, Tallinn

12. Mwfrom GCMT and literature search MW from GCMT is available from 1976 (plus some deep earthquakes between 1962 and 1975). For 970 relocated earthquakes direct measurements of M0 were compiled from the literature. For the remaining relocated earthquakes, proxy MW values are obtained from the recomputed MS and mb using new empirical relationships… 12

13. MW proxy based on recomputed MS Num=17472 • Data population strongly dominated by earthquakes with magnitude below 6; • The relationship between MS and MW is not linear over the entire distribution; • Median values for separated bins (dashed black line) suggest that a non-linear model could fit well the data. • Histogram equalization scheme: bins of varying width so that each bin contains the same number of data points. • For each bin a randomly chosen 10% of the data is assigned to the validation dataset, while the 90% to the training dataset used to obtain the regression model. 13

14. MW proxy based on recomputed MS We applied a non-linear regression using an exponential model of the form My = exp(a+b*Mx)+c (EXP, purple). • The exponential model follows well the median values curve over the entire population. • Proxy MW vs true MW(=10% of the original population not used for deriving the model). 14 43rd Nordic Seismology Seminar, 24-26 October 2012, Tallinn

15. MW proxy based on recomputed mb • The exponential model follows well the median values curve close to the saturation level of mb. 15 43rd Nordic Seismology Seminar, 24-26 October 2012, Tallinn

16. Magnitude composition of the ISC-GEM catalogue Direct MW per year Proxy MW per year 16 43rd Nordic Seismology Seminar, 24-26 October 2012, Tallinn

17. Magnitude composition: Centennial vs ISC-GEM catalogue Centennial catalogue ISC-GEM catalogue 17 43rd Nordic Seismology Seminar, 24-26 October 2012, Tallinn

18. Magnitude distribution of the ISC-GEM catalogue 18 43rd Nordic Seismology Seminar, 24-26 October 2012, Tallinn

19. Frequency-Magnitude distributions Mc=6.4 Mc=5.6 • Seismicity rates for large (M>7.5-7.6) earthquakes better assessed considering a long time window (violet) • For moderate earthquakes the modern period (red) is a better basis for magnitude-frequency studies, whereas for strong to major shallowearthquakes the entire ISC-GEM catalogue may be used 19 43rd Nordic Seismology Seminar, 24-26 October 2012, Tallinn

20. Conclusions • We collected, digitised and processed an unprecedented amount of phase and amplitude data for earthquakes occurred before 1970; • In the 110 years covered by the ISC-GEM catalogue, the relocation provided significant improvements especially in the first part of past century; • We recomputed MS and mb using uniform procedures, and new non-linear relationships are used to obtain MW proxies when direct computation of M0 from GCMT or literature is not available; • The ISC-GEM Global Instrumental Earthquake Catalogue represents the final product of one of the ten global components in the GEM program, and will be available to researchers at the ISC website (www.isc.ac.uk). 43rd Nordic Seismology Seminar, 24-26 October 2012, Tallinn

21. THANK YOU 43rd Nordic Seismology Seminar, 24-26 October 2012, Tallinn

22. Appendix 22 43rd Nordic Seismology Seminar, 24-26 October 2012, Tallinn

23. ISS bulletins (1918-1963)(predecessor of the ISC, phase data only!) • Over 1.1 million phases (~1000 seismic stations between 1918 and 1963) from ISS have been used in the relocation process; over 730,000 have been inserted into the ISC database during this project for earthquakes occurred between 1918 and 1959. • Over 5000 phases (from ~160 seismic stations) have been added before 1918 (mostly from BAAS and G&R notepads). Converted into digital form by scanning the bulletin pages and applying an optical character recognition (OCR) procedure (Engdahl and Villaseñor, 2002) Biggest source of earthquake data from 1918 to 1963. 23 23

24. Earthquake Relocation results After Before 24

25. MW proxy based on recomputed MS • The relationship between MS and MW is not linear; • Authors normally perform bi-linear regression splitting the dataset at MS = 6.1; • This separation, however, is arbitrary because slope change occurs in a transition zone between MS ~6 and ~6.7. • Data population strongly dominated by earthquakes with magnitude below 6; • Median values for separated bins (dashed black line) suggest that a non-linear model could fit well the data over the entire distribution. 25 43rd Nordic Seismology Seminar, 24-26 October 2012, Tallinn

26. MW proxy based on recomputed MS The histogram equalization defines magnitude bins varying width so that each bin contains the same number of data points. For each bin a randomly chosen 10% of the data is assigned to the validation dataset, while the 90% to the training dataset used to obtain the regression model. 26 43rd Nordic Seismology Seminar, 24-26 October 2012, Tallinn

27. Magnitude composition of the ISC-GEM catalogue 27 43rd Nordic Seismology Seminar, 24-26 October 2012, Tallinn

28. MW proxy based on recomputed mb We applied both the GOR (green) and a non-linear regression using an exponential model of the form My = exp(a+b*Mx)+c (EXP, purple). • The exponential model follows well the median values curve close to the saturation level of mb. • Proxy MW vs true MW(=10% of the original population not used for deriving the models), show how EXP model works better than GOR models, especially for MW < 6. 28

29. Regional Frequency-Magnitude distributions 29 43rd Nordic Seismology Seminar, 24-26 October 2012, Tallinn

30. Regional Frequency-Magnitude distributions (1) 30 43rd Nordic Seismology Seminar, 24-26 October 2012, Tallinn

31. Regional Frequency-Magnitude distributions (2) 31 43rd Nordic Seismology Seminar, 24-26 October 2012, Tallinn