a functional form for the spatial distribution of aftershocks karen felzer usgs pasadena
TRANSCRIPT
A functional form for the spatial distribution of aftershocks
Karen FelzerUSGS Pasadena
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).
1) Evidence from small mainshocks
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.
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 earthquakes
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-1.3
M ≥2 Aftershocks taken from the first 5 minutes after each mainshockFrom Felzer and Brodsky (2006)
Best fit aftershock decay for M 2—4 main shocks in Southern California from 1-100 km: Density ~ r-1.4
2) Evidence from big main shocks
Advantages and disadvantages of using big main shocks
• Main shocks can be inspected individually, decreasing interference from background seismicity.
• Results may be specific to a particular location or event.
• Unknown complexity of the main shock fault plane and incomplete catalogs may cause error.
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)
M 7.2 El Mayor-Cucapah earthquake: Density ~ r-2.0
Aftershocks to the north clearly concentrated on the Elsinore and San Jacinto fault zones
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-2.1
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.
More to come about big mainshocks in my next talk!
Hector Mine earthquake scarp