ISCGEM Global Reference Earthquake Instrumental Catalogue 19002009 D

ISC-GEM Global Reference Earthquake Instrumental Catalogue (19002009) D. Di Giacomo, I. Bondár, E. R. Engdahl, D. A. Storchak, W. H. K. Lee, A. Villaseñor, J. Harris, P. Bormann ESC 2012, Moscow

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. ESC 2012, Moscow 2

ü Project in a nutshell Collecting, digitising and Cut-off magnitudes: ü 1900 -1917: MS≥ 7. 5 worldwide + processing data from a multitude smaller shallow events in stable of historical sources for continental areas earthquakes occurred up to 1970; ü 1918 -1959: MS≥ 6¼ ü 110 years of relocated ü 1960 -2009: MS≥ 5. 5 earthquake hypocenters; ü recomputed MS and mb values This Catalogue is unique for relocated events using because uniform procedures; it contains homogeneous ü MW values (with uncertainty) locations and magnitude based on: estimates with the estimates of 1. seismic moment from uncertainty for the entire GCMT (mainly 1976 -2009); period 1900 -2009 done using 2. seismic moments from the same tools and techniques literature search for to the extent possible. earthquakes up to 1979; 3 3. proxy values based on ESC 2012, Moscow

Phase and Amplitude Data Collection Period Body Wave Arrival Times Body/Surfac e Wave Amplitudes 19001917 19181959 19601970 19712009 ~10000 ~730, 000 Quality station bulletins DIGITALLY NOT AVAILABLE BEFORE THIS PROJECT DIGITALLY AVAILABLE, ISC database Major Sources of Phase Data: • Gutenberg Notepads (19041917) and BAAS (1913 -1917) • ISS Bulletins (1918 -1963) 4

Processing historical seismological bulletin 1906 San Francisco earthquake from station bulletin Göttingen, Germany 15, 257 individual seismic bulletins from 293 institutions over the period 1904 – 1970 were recovered from ISC storage The same report stored in digital format in the ISC database. Period and amplitude data finally available for magnitude recomputation. ESC 2012, Moscow 5

Amplitude Data from Quality Station Bulletins • ~300, 000 “brand new” amplitudes up to 1970 now available in the ISC database • Effort equivalent to ~70 personmonths Time Coverage: UPP, RIV, and LPZ nearly continuous, gaps for other stations Station timeline 6

Earthquake Location Procedure Location method: 1. Determine event depth using the EHB style of processing (Engdahl, van der Hilst and Buland, 1998): a) comprehensive analysis of near-event surface reflections off the earth surface inland ocean bottom or water surface in the oceans; b) Station patch corrections; 2. Use the new ISC location algorithm (Bondár and Storchak, 2011) with earthquake depths fixed to those from EHB analysis: a) more accurate epicentre locations due to correlated error structure taken into account (removes bias from uneven geometrical positioning of stations) b) independent depth confirmation using depth phase stacking; ESC 2012, Moscow 7

Earthquake Relocation results Before relocation… ESC 2012, Moscow 8

Earthquake Relocation results …. after relocation. ESC 2012, Moscow 9

Earthquake Relocation results After Before ESC 2012, Moscow 10

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) homogeneous magnitude calculations following the IASPEI standards; 4) network magnitudes based on several station measurements using alphatrimmed median (α = 20%) of the single station magnitudes (no network magnitude based on one station only). ESC 2012, Moscow 11

Mw from 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 M 0 were compiled from the literature. For the remaining relocated earthquakes, proxy MW values are obtained from the recomputed MS and mb using new empirical 12

MW proxy based on recomputed MS • Data population strongly dominated by earthquakes with magnitude below 6; Num=174 72 • 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. ESC 2012, Moscow 13

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 ESC 2012, Moscow the model). 14

MW proxy based on recomputed mb • The exponential model follows well the median values curve close to the saturation level of m ESC 2012, Moscow b. 15

Magnitude composition of the ISC-GEM catalogue Direct MW per year Proxy MW per year ESC 2012, Moscow 16

Magnitude composition: Centennial vs ISC-GEM catalogue Centennial catalogue ISC-GEM catalogue ESC 2012, Moscow 17

Magnitude distribution of the ISC-GEM catalogue ESC 2012, Moscow 18

Frequency-Magnitude distributions • Seismicity rates for large (M>7. 5 -7. 6) earthquakes better assessed considering a long time window (violet) Mc=6. 4 Mc=5. 6 ESC 2012, Moscow • For moderate earthquakes the modern period (red) is a better basis for magnitude-frequency studies, whereas for strong to major shallow earthquakes the entire ISC-GEM catalogue may be used 19

Conclusions ü We collected, digitised and processed an unprecedented amount of phase and amplitude data for earthquakes occurred before 1970; ü In the 110 years covered by the ISC-GEM catalogue, the relocation provided significant improvements especially in the first part of past century; ü We recomputed MS and mb using uniform procedures, and new nonlinear relationships are used to obtain MW proxies when direct computation of M 0 from GCMT or literature is not available; ü 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). ESC 2012, Moscow 20

THANK YOU ESC 2012, Moscow 21

Appendix ESC 2012, Moscow 22

A Brief Time Line in Seismology ESC 2012, Moscow 23

ISS bulletins (19181963) (predecessor of the ISC, phase data Converted into digital form by scanning only!) the bulletin pages and applying an optical character recognition (OCR) procedure (Engdahl and Villaseñor, 2002) Biggest source of earthquake data from 1918 to 1963. • 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). 24

Earthquake Relocation results After Before ESC 2012, Moscow 25

MW proxy based on recomputed MS • The relationship between MS and MW is • Data population strongly dominated by not linear; earthquakes with magnitude below 6; • Authors normally perform bi-linear • Median values for separated bins regression splitting the dataset at MS = (dashed black line) suggest that a non 6. 1; linear model could fit well the data over the • This separation, however, is arbitrary entire distribution. 26 ESC 2012, Moscow because slope change occurs in a

MW proxy based on recomputed The histogram MS 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. ESC 2012, Moscow 27

Magnitude composition of the ISC-GEM catalogue ESC 2012, Moscow 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 Proxyof. Mm. Wb. vs true MW (=10% of the original population not used for deriving the models), ESC 2012, Moscow show EXP model works better than GOR 29

Regional Frequency-Magnitude distributions ESC 2012, Moscow 30

Regional Frequency-Magnitude distributions (1) ESC 2012, Moscow 31

Regional Frequency-Magnitude distributions (2) ESC 2012, Moscow 32