GEAR1 forecast: Distribution of largest earthquakes and number test,

Yan Y. Kagan, & David D. Jackson

Published June 1, 2016, SCEC Contribution #6228

We have obtained new results in the statistical analysis of
global earthquake catalogs with special attention to the
largest earthquakes, and we examined the statistical behavior
of earthquake rate variations.
These results can serve as an input for updating our recent
earthquake forecast, known as the "Global Earthquake
Activity Rate 1" model (GEAR1), which is based on past
earthquakes and geodetic strain rates.
The GEAR1 forecast is expressed as the rate density of all
earthquakes above magnitude 5.8 within 70 km of sea level
everywhere on earth at 0.1 by 0.1 degree resolution, and it is
currently being tested by the Collaboratory for Study of
Earthquake Predictability.
The seismic component of the present model is based on a
smoothed version of the Global Centroid Moment Tensor (GCMT)
catalog from 1977 through 2013.
The tectonic component is based on the Global Strain Rate Map,
a "General Earthquake Model" (GEM) product.
The forecast was optimized to fit the GCMT data from 2005
through 2012, but it also fit well the earthquake locations
from 1918 to 1976 reported in the International Seismological
Centre-Global Earthquake Model (ISC-GEM) global catalog of
instrumental and pre-instrumental magnitude determinations.
We have improved the recent forecast by optimizing the
treatment of larger magnitudes and including a longer duration
(1918-2011) ISC-GEM catalog of large earthquakes to estimate
smoothed seismicity.
We revised our estimates of upper magnitude limits, described
as corner magnitudes, based on the massive earthquakes since
2004 and the seismic moment conservation principle.
The new corner magnitude estimates are somewhat larger than
but consistent with our previous estimates.
For major subduction zones we find the best estimates of
corner magnitude to be in the range 8.9 to 9.6 and consistent
with a uniform average of 9.35.
Statistical estimates tend to grow with time as larger
earthquakes occur.
However, by using the moment conservation principle that
equates the seismic moment rate with the tectonic moment rate
inferred from geodesy and geology, we obtain a consistent
estimate of the corner moment largely independent of seismic
history.
These evaluations confirm the above-mentioned corner magnitude
value.
The new estimates of corner magnitudes are important both for
the forecast part based on seismicity as well as the part
based on geodetic strain rates.
We examine rate variations as expressed by annual earthquake
numbers.
Earthquakes larger than magnitude 6.5 obey the Poisson
distribution.
For smaller events the negative-binomial distribution fits
much better because it allows for earthquake clustering.

Key Words
Probability distributions; Seismicity and tectonics; Statistical seismology; Dynamics: seismotectonics; Subduction zones processes

Citation
Kagan, Y. Y., & Jackson, D. D. (2016). GEAR1 forecast: Distribution of largest earthquakes and number test,. Geophysical Journal International, 206(1), 630-643.