Magnitude and Completeness Assessment of the ISC GEM

Magnitude and Completeness Assessment of the ISC -GEM Global Instrumental Earthquake Catalogue (1900 -2009) D. Di Giacomo, I. Bondár, E. R. Engdahl, D. A. Storchak, W. H. K. Lee, P. Bormann IASPEI 2013, Gothenburg

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. IASPEI 2013, Gothenburg 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 IASPEI 2013, Gothenburg

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 Major Sources of Phase Data: • Gutenberg Notepads (19041917) and BAAS (1913 -1917) • ISS Bulletins (1918 -1963) IASPEI 2013, Gothenburg DIGITALLY AVAILABLE, ISC database See Poster S 101 PS. 02 4

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; See Poster S 101 PS. 01 IASPEI 2013, Gothenburg 5

Mw from GCMT and literature search MW from GCMT is available from 1976 (plus some deep earthquakes between 1962 and 1975). For 967 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 6

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). IASPEI 2013, Gothenburg 7

Comparisons of MS with Abe and G&R catalogues Good fit with Abe’s catalogue (left), whereas with G&R catalogue (right) our recomputed MS confirm an average bias of ~0. 2 magnitude units, as already pointed out by Engdhal&Villaseñor (2002). IASPEI 2013, Gothenburg 8

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. IASPEI 2013, Gothenburg 9

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. 10

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 IASPEI 2013, Gothenburg the model). 11

MW proxy based on recomputed mb • The exponential model follows well the median values curve close to the saturation level of m IASPEI 2013, Gothenburg b. 12

Magnitude composition: Centennial vs ISC-GEM catalogue Centennial catalogue ISC-GEM catalogue IASPEI 2013, Gothenburg 13

Magnitude composition: timeline of direct/proxy Mw Direct values of Mw are prominent in the modern period. Considering only great or near-great earthquakes (Mw ≥ 7. 75), the entire catalogue includes 175 events with 86 direct Mw compiled from the bibliographic search, 52 from the GCMT, and for the remaining 37 a proxy Mw was computed. IASPEI 2013, Gothenburg 14

ISC-GEM catalogue: time-magnitude distribution Before 1918, only large earthquakes are included. Starting with 1918 the number of events in the catalogue starts to “take-off”. Black solid line is the cumulative number of earthquakes. 15

Frequency-Magnitude distributions • 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 shallow earthquakes the entire ISC-GEM catalogue may be used 16

Conclusions ü We recomputed MS and mb using uniform procedures, and new non-linear relationships are used to obtain MW proxies when direct computation of M 0 from GCMT or literature is not available. Thus, only four sources of Mw were used in the catalogue magnitude composition. ü The completeness estimation is 5. 6 for 1964 -2009 and 6. 4 between 1900 -1963. Further work is necessary to improve the completeness pre-1964. ü The ISC-GEM Global Instrumental Earthquake Catalogue represents the final product of one of the ten global components in the GEM program, and is available at the ISC website (www. isc. ac. uk). IASPEI 2013, Gothenburg 17

THANK YOU IASPEI 2013, Gothenburg 18

Appendix IASPEI 2013, Gothenburg 19

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), show EXP model works better than GOR 20