Karen Felzer USGS Pasadena Preliminary Work Subject to change The ETAS simulations assign aftershock density with distance r from the mainshock as Aftershock density 1r dmin ID: 658551
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Slide1
Evaluation of simulation results: Aftershocks in space
Karen Felzer
USGS PasadenaSlide2
Preliminary Work!!
Subject to change!Slide3
The ETAS simulations assign aftershock density with distance,
r
, from the mainshock as:
Aftershock density = 1/(r+dmin)-distDecay
dmin
(km)
0.3
0.50.5distDecay1.72.02.5
Trial parameter combinations:
Which simulation parameters recreate real data the best?Slide4
Trial Cases
Landers earthquake
Northridge earthquakeSlide5
Before we start: Where’s the fault?
The parameter
r must be measured to the
mainshock fault plane, but where exactly is the real rupture surface?Slide6
Faults can be very complex
Detail of El Mayor-
Cucapah
rupture, (Rymer et al. in preparation, courtesy of Katherine Kendrick)Slide7
Quick and dirty aftershock-based fault-tracing approach
Place aftershocks into 5x5x5 km bins
Sort the bins by aftershock densityCalculate
Nbin = min((Fault area)/10,(Total number of populated bins)/4)Place fault points at the median aftershock location in the Nbin most populated binsThe distance
r
is measured from aftershock hypocenters to the nearest fault point.
Slide8
Fault points for the Landers earthquake
M 3+ aftershocksSlide9
Fault points for a simulated Landers earthquake with
minDist
= 0.3 km and
distDecay = 1.7M 2.5+ aftershocksSlide10
Fault points for the Northridge earthquake
M 2.0+ aftershocksSlide11
Fault points for a simulated Northridge earthquake with
minDist
= 0.3 km and
distDecay = 1.7M 2.5+ aftershocksSlide12
Data and simulation comparison: Landers earthquake
Aftershock density decays more quickly in the near field, and more slowly in the far field, than any of the simulations
Suggests we need dmin
<0.3 km, distDecay<1.7
Real dataSlide13
Data and simulation comparison: Northridge earthquake
Aftershock density decay rate
is similar to the
distDecay
=2.0
and
distDecay
=2.5 simulations in the near field
Far field decay is closest to the
distDecay
=2.5 simulation.
Different results than for Landers!!
Real dataSlide14
How similar is the aftershock decay for different real earthquakes?
I compare aftershock density vs. distance for Landers, Hector Mine, Northridge, and Joshua Tree.
Each
earthquake is different, but Hector Mine and Joshua Tree are closer to Northridge-type than Landers-type behavior:
supports
distDecay
=2.0-2.5.
Big Bear aftershock may be an issue?Slide15
Future work
Refine method for finding fault points.
Do aftershock measurements for many more large to moderate
mainshocks.Do measurements with relocated aftershocks.Test sensitivity of results to minimum magnitude and duration of aftershocks used.Look for systematic variation of aftershock behavior with mainshock characteristics.Decide whether sequence-specific parameters will be necessary.