Explanation of Differences in Hazard: UCERF3.2 Vs NSHM 2008 Peter Powers

Explanation of differences in hazard: UCERF3.2 vs NSHM 2008 Peter Powers – 4/1/2013

The accompanying figures and text summarize the significant changes in hazard (probabilistic ground motions) implied by the UCERF3 earthquake rupture forecast, relative to the the 2008 NSHM. The purpose of this document is to explain all those areas where the 2% in 50yr. PGA ground motions have changed more than 10%. Bear in mind that the figures and explanatory text are not complete with respect to identifying every change, however, the largest have been addressed and the many numerous small changes (i.e. those spanning a few pixels in the accompanying statewide maps) also fall into the broad categories identified herein. The higher resolution local maps that follow offer insight as to the causes of smaller changes.

Gridded source contributions Many changes (mostly increases) in probabilistic ground motion are due to the use of the "UCERF3 smoothed seismicity" model for background sources. Although this background source model only receives 50% weight (a model based on UCERF2 smoothed seismicity receives the other half), it uses a tighter, adaptive smoothing kernel, which commonly gives rise to spikes in ground motion over small areas. The locations of the most signiﬁcant grid source contributions to hazard are circled in Fig. 1A. Note that the California "deep" sources were not included in the NSHM hazard calculation because they are treated as subduction slab events and use a different set of GMPEs, however, they are present in the UCERF3 smoothed seismicity model. This gives rise to the broad area of increased ground motion in northwest California (circle in Figure 1B). These sources are slated for removal in UCERF3.3.

Fault source contributions The remaining changes to probabilistic ground motion arise from (1) slip or moment rate changes on existing NSHM 2008 faults, (2) geometric changes to existing faults, (3) the introduction of new faults, and (4) fault participation in multi-fault ruptures. Although the rates of large multi-fault events are generally quite low, they use signiﬁcant moment and do not contribute as much to hazard at 2% in 50yr. PGA, resulting in a general decrease in ground motion along the major California fault zones. Figures 2A and 2B highlight the signiﬁcant increases and decreases, respectively, in ground motion implied by UCERF3.

Table 1: Ground motion increases (See Fig. 2A) ID Fault Notes 1 Big Lagoon–Bald Mountain Faults Extended north ~60 km and has 8x moment rate for ABM deformation model. 2 South Klamath Lake East & West Faults New faults. 3 Goose Lake, Fitzhugh Creek, Jess New faults. Valley, and Davis Creek Faults 4 Likely Fault Moment rate doubled. 5 Pittville Fault New fault. 6 Honey Lake Fault Fault extended to west. 7 Almanor, Walker Spring, Keddie Ridge, 7 new faults. Skinner Flat, Mohawk Valley, Dog Valley, Polaris, and Incline Village Faults 8 West Tahoe, Carson Range (Genoa), 2 new faults and southern extension of West Tahoe and Antelope Valley Faults fault. 9 Maacama Fault 20% increase in moment rate. 10 West Napa Fault 3.5x increase in moment rate, mostly due to ABM deformation model. 11 Great Valley 03, 05, &06, and Los New faults. Medanos–Roe Island Faults 12 Greenville South & Ortigliata Faults Ortigliata: geometric and moment rate changes; Greenville South extended further south. 13 Silver Creek, Monte Vista–Shannon, and New or extension of existing (Monte Vista– Sargent Faults Shannon) faults. Zayante–Vergeles Fault Moment rate increase (x9 increase for reverse fault representation) 14 Oceanic–West Hausna New fault with high moment rate. 15 Los Alamaos, Santa Ynez River, Santa New faults. Ynez West, and Hosgri Extension Faults 16 Lost Hills Fault New fault. 17 Fish Slough, Independence, and Owens Fault geometry change (Fish Slough) and moment Valley Faults rate increases on all three faults. 18 Sierra Nevada (N. extension)and White New faults comingled with gridded seismicity Wolf Extension Faults, and Scodie and sources. Lake Isabella Seismicity Lineaments 19 15+ new faults Many new faults added in the easter Mojave desert; some were present in UCERF2 with 0 zero slip rate; others are entirely new (e.g. Cleghorn Pass and Lake Faults) 20 San Clemente, San Diego Trough, Santa Multiple new offshore faults. Cruz Catalina Ridge, Oceanside, and San Pedro Basin Faults 21 Elmore Ranch Fault Moment rate increase. 21 Cerro Prieto New fault.

Table 2: Ground motion decreases (See Fig. 2B) ID Fault Notes 1 Gillem–Big Crack & Cedar Mtn.– Lower slip rates. Mahogany Mtn. Faults 2 Surprise Valley Fault Moment rate decrease. 3 Hat Creek–McArthur–Mayﬁeld Fault Moment rate decrease. 4 Little Salmon (onshore) Fault Moment rate decrease. 5 Maacama High slip rate fault shortened at northern end. 6 Bartlett Springs Fault Trace geometry changes. 7 Great Valley 03, 04 Faults Moment rate decrease. 8 Great Valley 08 (Orestimba) 60% decrease in moment rate. 9 Mono Lake Fault Moment rate decrease. 10 Death Valley North, South, and Black 40% decrease in moment rate due to participation Mtn. Frontal Faults in multi-fault ruptures. 11 San Gregorio South Fault geometry change. 12 Hosgri Fault Down dip width increase and participation in multi- fault ruptures contributes to lower hazard although total moment rate has changed little. 13 San Cayetano, Anacapa–Dume, Oak New faults and moment rate increases on existing. Ridge Offshore, and Channel Islands Deep Ramp Faults 14 Cucamonga Fault Methodological difference; Cucamonga participates in multi fault ruptures rather than by itself in high- rate 'characteristic' events. Down dip width also increased. 15 North Frontal and Cleghorn Faults Moment rate decreases.

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Log10(Ratio) Local Changes To examine local changes in hazard, map resolutions were increased to 0.02 degrees / pixel (as opposed to 0.1 degree / pixel in the statewide comparisons) and the scale was changed such that in subsequent ﬁgures yellow spans ±4% changes in ground motion (as opposed ±10% in the statewide comparisons). As with the statewide comparisons, most changes in ground motion arise from changes to existing faults (slip or moment rate, or geometry) or the introduction of new faults. A number of such changes are identiﬁed in Figure 3; the higher resolution map and scale reveal how sensitive hazard is to slight changes in geometry (e.g. a once vertical fault that now has a slight dip picks up a hanging-wall effect from 2 of the 3 GMPE's used to calculate probabilistic ground motions). Another contribution to changes in hazard that stands out at the local scale is the presence of segment boundaries in the 2008 NSHM fault model, across which slip-rate changes are common.

The comparisons here are also for 2% in 50 PGA ground motions but please note that local ground motion and ratio maps are available online for 5Hz and 1Hz spectral accelerations and for 10% in 50yr return periods; 40% in 50yr return period is also available for PGA.

Table 3: San Francisco Bay area ground motion changes (see Fig. 3) 1 Oakland: Hazard decrease due to the Hayward fault as a whole participating in multi-fault ruptures and the presence of the Hayward N. – Hayward S. segment (slip-rate change) boundary in the 2008 NSHM (UCERF2). 2 Numerous new fault and updated fault geometries Monte Vista - Shannon extension Hayward S. Extension addition Silver Creek addition 3 New fault: Greenville S. 4 Numerous Contra Costa Faults connecting Calaveras N. and West Napa 5 Increased hazard between Calaveras N. and Hayward South due to change in fault geometry. Both dip towards each other, whereas they were both vertical in UCERF2, thereby increasing hanging-wall effects. 6 Green Valley relocated slightly to west but also now dipping to the west thereby signiﬁcantly reducing hazard on what is now the footwall (east) side of the fault trace.

Table 4: Los Angeles area ground motion changes (see Fig. 4) 1 Conversion from independent overlapping faults to 'stepovers' on both the San Jacinto and Elsinore faults. 2 (Note: there are two markers in ﬁgure) Decreased hazard due to presence of segment boundaries in 2008 NSHM (UCERF2). 3 Multiple new faults considered that had 0mm/yr slip rates in NSHM. Although rates are generally low (~1mm/yr) (e.g. Compton, San Pedro escarpment), these signiﬁcantly increase 2% in 50 ground motions. 4 Anacapa-Dume; 7-% reduction in moment 5 Cucamonga methodological change; now participates with other faults. 6 San Cayetano; 10% reduction in moment.

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