Review of Catalogs and Rate Determination in UCERF2
and Plans for UCERF3
Andy Michael
Historical Earthquake Catalog: 1850 – 1932
Primary compilation: Toppozada and Branum (2003)
1769-1999, M≥5.5, magnitude based on area with MMI V, VI, VIII,
relies on many other sources for magnitude of individual events.
417 events from 1850-1931
Location and magnitude solved independently.
Bakun (1999, 2000, 2006) independently analyzed 84 events
location and magnitude jointly based on fitting intensity at
points to a ground motion prediction relationship.
Magnitudes generally agree well. In the Bay Area:
15 are given the same magnitudes
17 differ by 0.1
11 differ by 0.2
3 differ by more (0.4, 0.5, and 0.6) – sparse data
Instrumental Catalog: 1932 – 2006
CGS compilation of the Southern and Northern California Catalogs
and for Nevada also use NEIC and Nevada Seismological Lab.
Mostly ML for M≥4, some Md in northern California
Mw for larger recent events taken from Global CMT catalog
From 1972 on used ANSS merged catalog
Some problems with updated SCSN catalog not in ANSS and so
the updated SCSN version of events was used. This appears to
have been fixed.
Magnitude Error and Rounding
Toppozada and Branum: ±0.3 units when data is sparse
Bakun: individually estimated errors, similar when data is sparse.
Bakun’s uncertainties could be underestimates because the
magnitude uncertainty does not take into account location
uncertainty.
When magnitudes agree and Bakun’s errors are smaller then
used Toppozada’s magnitude and Bakun’s uncertainty.
When error is unknown ±0.333 is used but true errors could be
higher.
If errors are higher than estimated then rates could be
overestimated.
Magnitude Error and Rounding
SCSN: new uncertainties caluclated using a bootstrap of the
amplitudes used to calculate the magnitudes.
NCSN: uncertainties for most events listed in catalogs.
Global CMT: 0.09 uncertainty from Kagan et al. (2006) which is
consistent with 0.08 estimated by Bakun (1999).
Rounding:
Historic events: 0.1
SCSN: 0.01.
NCSN: uncertainties for most events listed in catalogs.
Global CMT: calculated to 0.01 from moment.
1910-1932: 0.1 or 0.5 depending on event (estimated from
histograms of magnitudes)
logN=a−bM
The a and b values found here are calculated using methods
employed by the 1996 and 2002 National Hazard Maps, with
several revisions.
These revisions include:
making corrections for magnitude error and rounding before
calculating a values,
using only modern instrumental data to calculate b value, and
using a new comprehensive and spatially variable assessment of
the magnitude completeness threshold as a function of time.
We also calculate the seismicity rate in several different ways to
account for the fact that the seismicity rate may change with time
(for example, the higher seismicity rates in the San Francisco Bay
Area before 1927 than after), and
perform simulations to evaluate the accuracy with which the
seismicity rate averaged over the last 156 years represents the
true long term seismicity rate.
Finally, the National Hazard Maps have traditionally only used the
historical earthquake solutions of Toppozada, most recently
compiled in Toppozada et al. (2002).
We do our calculations both with the Toppozada solutions and
with 84 of the Toppozada solutions substituted with historical
earthquake solutions of Bakun (Bakun 1999; Bakun 2000; Bakun
2006).
We find that this substitution creates an insignificant increase in
the statewide seismicity rate of 0.6%, although it may produce
larger differences on a regional level.
Completeness
Schorlemmer et al. (2006) bases completeness thresholds in
Southern California on observations of how frequently individual
seismic stations detect earthquakes of different magnitudes and
distances. For each location determine completeness by proximity
to stations that record earthquakes with some quality.
Extended to
historical
sources.
Plans for UCERF3:
Further consider uncertainties and biases in intensity assignments
and magnitudes of historic earthquakes.
Could these change the seismicity rate in the historic catalog?
Could this change the empirical model?
Could this improve the fit between observed and modeled rates?
What can be done without redoing the entire historic catalog?
Declustering:
Traditionally use Gardner and Knopoff (1974)
Produces change in b-value from 1 to 0.8
ETAS models use the same magnitude frequency distribution for
mainshocks and aftershocks.
Consider other declustering methods
How will this affect rates?
How do we do this while being consistent with national maps?
More Plans for UCERF3:
Changes in instrumental magnitudes.
Characterize off-fault seismicity focal mechanisms and Mmax
New assignments of historic events to faults (?)
New approaches to smoothed seismicity rates
Magnitude-Frequency Distributions:
Characteristic versus Gutenberg-Richter