A functional form for the spatial distribution of aftershocks Karen Felzer USGS Pasadena

Summary Aftershock density decays with distance, r, from the mainshock surface as r -n where n= and may vary for different mainshocks. This decay holds out to distances of at least 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 mainshock From Felzer and Brodsky (2006) Best fit aftershock decay for M 2—4 main shocks in Southern California from 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 M≥0.5 aftershocks from 4-40 km 49 M≥0.5 aftershocks from 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 r -2.1

Conclusions Aftershock density decays with distance, r, from the mainshock surface as r -n where n ~ 1.3 – 2, probably ?? 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