Transcript ppt

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.
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Project in a nutshell
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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.
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.
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Phase and Amplitude Data Collection
Period 1900-1917
Body Wave
Arrival Times
Body/Surface
Wave
Amplitudes
~10000
1918-1959 1960-1970 1971-2009
~730,000
Quality station bulletins
DIGITALLY NOT AVAILABLE
BEFORE THIS PROJECT
Major Sources of Phase Data:
• Gutenberg Notepads (1904-1917) and
BAAS (1913-1917)
• ISS Bulletins (1918-1963)
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DIGITALLY
AVAILABLE,
ISC database
See Poster
S101PS.02
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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 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;
See Poster S101PS.01
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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 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…
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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 alpha-trimmed median (α =
20%) of the single station magnitudes (no network magnitude based on one station only).
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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).
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Mw proxy based on recomputed MS
• Data population strongly
dominated by earthquakes with
magnitude below 6;
Num=17472
• 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.
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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.
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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).
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MW proxy based on recomputed mb
• The exponential model follows well the median
values curve close to the saturation level of mb.
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Magnitude composition: Centennial vs ISC-GEM catalogue
Centennial catalogue
ISC-GEM catalogue
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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.
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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.
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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 magnitudefrequency studies,
whereas for strong to
major shallow
earthquakes the entire
ISC-GEM catalogue may
be used
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Conclusions
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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. Thus, only four sources of Mw were used in
the catalogue magnitude composition.
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The completeness estimation is 5.6 for 1964-2009 and 6.4 between 19001963. Further work is necessary to improve the completeness pre-1964.
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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).
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THANK YOU
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Appendix
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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.
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