Evaluation of simulation results: Aftershocks in space

Karen FelzerUSGS Pasadena

Preliminary Work!!

Subject to change!

The ETAS simulations assign aftershock density with distance, r, from the mainshock as:

Aftershock density = 1/(r+dmin)-distDecay

dmin (km) 0.3 0.5 0.5

distDecay 1.7 2.0 2.5

Trial parameter combinations:

Which simulation parameters recreate real data the best?

Trial Cases

Landers earthquake Northridge earthquake

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?

Faults can be very complex

Detail of El Mayor-Cucapah rupture,

(Rymer et al. in preparation, courtesy of Katherine Kendrick)

Quick and dirty aftershock-based fault-tracing approach

• Place aftershocks into 5x5x5 km bins• Sort the bins by aftershock density• Calculate Nbin = min((Fault area)/10,(Total

number of populated bins)/4)• Place fault points at the median aftershock

location in the Nbin most populated bins• The distance r is measured from aftershock

hypocenters to the nearest fault point.

Fault points for the Landers earthquake

M 3+ aftershocks

Fault points for a simulated Landers earthquake with minDist = 0.3 km and distDecay = 1.7

M 2.5+ aftershocks

Fault points for the Northridge earthquake

M 2.0+ aftershocks

Fault points for a simulated Northridge earthquake with minDist = 0.3 km and distDecay = 1.7

M 2.5+ aftershocks

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 data

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 data

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?

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.