Karen Felzer USGS Pasadena Summary Aftershock density decays with distance r from the mainshock surface as r n where n 13 25 and may vary for different mainshocks ID: 161759
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Slide1
A functional form for the spatial distribution of aftershocks
Karen Felzer
USGS PasadenaSlide2
Summary
Aftershock density decays with distance,
r
,
from the
mainshock
surface as
r
-n
where
n
=1.3 -- 2.5
and may vary for different
mainshocks
.
This decay holds out to distances of at least 50-100 km for
mainshocks
of all magnitudes.
The
azimuthal
distribution of aftershocks appears to vary according to receiver fault locations
(Powers,
2009) and
mainshock
propagation direction (
Kilb
et al.
2000). Slide3
1) Evidence from small
mainshocksSlide4
Advantages & disadvantages of using small
mainshocks
Mainshocks
can be treated as point sources at most distances – no worries about main shock fault plane location and complexity.
Many aftershock sequences are stacked to see the signal. The use of many sequences => results provide a good regional average.
The use of many sequences also drives up inclusion of background earthquakes => may make the decay appear too slow. Slide5
Small
mainshocks
and the background earthquake problem
Big
Mainshock
Observe aftershocks for
60
minutes after
mainshock
Observations include
60
minutes of background earthquakes
10 small main shocks
Observe aftershocks for
60
minutes after
mainshocks
Observations include
600
minutes of background earthquakesSlide6
8656 M 1—2 Northern California
mainshocks
from the NCSN catalog, not preceded by larger event for 3 days/200 km
Best fit aftershock decay for M 1—2 main shocks in Northern California from 1-10 km:
Density ~
r
-1.3Slide7
M ≥2 Aftershocks taken from the first 5 minutes after each
mainshock
From
Felzer and Brodsky
(2006)
Best fit aftershock decay for M 2—4 main shocks in Southern California from 1-100 km:
Density ~
r
-1.4Slide8
2) Evidence from big main shocksSlide9
Advantages and disadvantages of using
b
ig
main shocks
Main shocks can be inspected individually
,
decreasing interference from background seismicity
.
Results may be specific to a
partic
ular
location or event
.
Unknown
complexity of the main shock fault plane and incomplete catalogs may cause error.Slide10
Best fit aftershock decay for M ~ 5 Anza earthquakes, 4-40 km:
Density ~
r
-1.8
68 M≥0.5 aftershocks from 4-40 km
49 M≥0.5 aftershocks from 4-40 km
From
Felzer and
Kilb
(2009)Slide11
M 7.2 El Mayor-
Cucapah
earthquake:
Density ~
r
-2.0
Aftershocks to the north clearly concentrated on the Elsinore and San Jacinto fault zonesSlide12
Similar work by other authors
Marsan
and
Lengline
(2010)
M 3—6 main shocks, hard work to decrease background seismicity interference
Density ~
r
-1.7
--r
-2.1Slide13
Conclusions
Aftershock density decays with distance,
r
,
from the
mainshock
surface as
r
-n
where
n
~ 1.3 – 2
,
probably 1.8--2
??
This
decay is seen out to distances of 50—100 km for
mainshocks
as small as M 1.0.
The
azimuthal
distribution of aftershocks may be
influenced by existing faults.Slide14
More to come about big
mainshocks
in my next talk!
Hector Mine earthquake scarp