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

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

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.


Project in a nutshell 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:

1. seismic moment from GCMT (mainly 1976-2009);

2. seismic moments from the literature search for earthquakes up to 1979;

3. proxy values based on recomputed MS and mb in other cases using appropriate empirical relationships.

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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.

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.

Cut-off magnitudes:

1900-1917: MS≥7.5 worldwide + smaller shallow events in stable continental areas1918-1959: MS≥6¼

1960-2009: MS≥5.5

Cut-off magnitudes:

1900-1917: MS≥7.5 worldwide + smaller shallow events in stable continental areas1918-1959: MS≥6¼

1960-2009: MS≥5.5


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Major Sources of Phase Data:

• Gutenberg Notepads (1904-1917) and BAAS (1913-1917)

• ISS Bulletins (1918-1963)

Phase and Amplitude Data Collection

~1,000,000~10,000

1960-19701918-19591900-1917

Body/Surface Wave

Amplitudes

Body Wave Arrival Times

Period

DIGITALLY AVAILABLE, ISC database

Quality station bulletins

1971-2009


DIGITALLY NOT AVAILABLE BEFORE THIS PROJECT


Processing historical seismological bulletin

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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.

~15,000 individual seismic bulletins from 290 institutions over the period 1904 – 1970 were recovered from ISC storage

Amplitude Data from Quality Station Bulletins

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• ~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

Station timeline

Earthquake Location Procedure

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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 and 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;


8Before relocation…

Earthquake Relocation results



9….after relocation.



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Before After


MS and mb recomputation

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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).


Mw from GCMT and literature search

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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…

MW proxy based on recomputed MS

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• 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.

Num=17472

• 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.


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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).


MW proxy based on recomputed mb

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• The exponential model follows well the median values curve close to the saturation level of mb.


Magnitude composition of the ISC-GEM catalogue

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Direct MW per year

Proxy MW per year


Magnitude composition: Centennial vs ISC-GEM catalogue

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Centennial catalogue ISC-GEM catalogue


Magnitude distribution of the ISC-GEM catalogue


Frequency-Magnitude distributions

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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 shallow earthquakes the entire ISC-GEM catalogue may be used


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).


http://www.isc.ac.uk/

THANK YOU


Appendix


ISS bulletins (1918-1963)(predecessor of the ISC, phase data only!)

2323

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.

• 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).


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AfterBefore


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• 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.



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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.


Magnitude composition of the ISC-GEM catalogue


MW proxy based on recomputed mb

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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.

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Regional Frequency-Magnitude distributions


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Regional Frequency-Magnitude distributions (1)


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Regional Frequency-Magnitude distributions (2)


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

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