CyberShake Background Seismicity

Goal: determine the importance of background seismicity in different parts of CA to either validate it's exclusion in CyberShake or motivate an inclusion strategy.

UCERF2

To illustrate what events UCERF2 includes in background seismicity, below are plots of the magnitude-frequency distribution for the UCERF2 ERF in both linear and log scale. Blue is the full UCERF2 distribution, and black is the on-fault ERF used in CyberShake. All UCERF2 finite-fault ruptures are included in CyberShake, regardless of magnitude, though the GR distribution applied to type B faults is only for M>=6.5, and floating rupture sources start at M6.5, which is why there are fewer CyberShake events for M<6.5.

Calculation parameters:

• GMPE: NGA-West2 GMPE (average of 4 models)
• ERF: UCERF2, with and without background seismicity
• Site Effects: Vs30 is from Wills (2015), no basin depth terms included
• IMT: 2 second SA.

Model	10% in 50yr	2% in 50yr	1% in 50yr
Excluding Background
Including Background
Ratio Exclude/Include

UCERF3

Calculation parameters:

• GMPE: NGA-West2 GMPE (average of 4 models)
• ERF: UCERF3, with the following configurations
  • Supra-seismogenic ruptures only (only supra-seismogenic ruptures have finite fault surfaces in UCERF3, all others treated as gridded)
  • Supra and Sub-seismogenic ruptures only (this adds gridded ruptures that correspond to sub-seimogenic ruptures on known faults)
  • Supra, Sub-seismogenic, and off fault ruptures (complete model)
• Site Effects: Vs30 is from Wills (2015), no basin depth terms included
• IMT: 2 second SA.

Model	10% in 50yr	2% in 50yr	1% in 50yr
Supra-Seismogenic On-Fault Only
Sub+Supra-Seismogenic On-Fault Only
Complete Model (Supra+Sub+Off)
Ratio Supra-Only/Complete
Ratio Supra+Sub/Complete