Earthquake hazard isn’t a physical thing we measure. It’s something mapmakers de

Earthquake hazard isn’t a physical thing we measure. It's something mapmakers de www.phwiki.com

Earthquake hazard isn’t a physical thing we measure. It’s something mapmakers de

Berra, Rich, Host has reference to this Academic Journal, PHwiki organized this Journal Earthquake hazard isn’t a physical thing we measure. It’s something mapmakers define in addition to then use computer programs to predict. To decide how much to believe a hazard map, we need to know what the mapmakers assumed, in addition to what the effects of those choices were. Definition of hazard (political, not scientific) Where in addition to when will earthquakes occur If they occur, then how large How large will ground motion be Strongly shaken areas MMI > VII as long as M 6 Assume that an earthquake of a certain size will strike in a certain time in addition to cause shaking within a certain area. Include earthquakes of different magnitudes, assume some areas more likely to have earthquakes, in addition to have stronger shaking close to the epicenter. Hazard at a given location is described by the maximum shaking due to earthquakes that is predicted to happen in a given period of time. Two methods of predicting hazard DHSA – deterministic seismic hazard assessment – chose the biggest earthquake to worry about, decide where & how big it will be, in addition to how much shaking it will cause. PSHA – probabilistic seismic hazard assessment – estimate combined hazard from many different earthquakes. Use the probabilities in addition to uncertainties of factors like the location in addition to times of earthquakes in addition to how much shaking will result from an earthquake of a given magnitude. DSHA makes society spend lots of money preparing as long as an event that is very unlikely to happen during a structure’s life. PSHA defines hazard via a mathematical event rather than real one, so results depend in complex ways on the probabilities in addition to uncertainties assumed. ”Simplicity is deeply veiled by user-hostile notation, antonymous jargon, in addition to proprietary software”(Hanks in addition to Cornell, 1994. As probabilistic models cover longer time windows they become about the same as deterministic ones, but emphasize extreme cases even more

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“Estimates of some specific PSHA studies are very surprising, particularly at small exceedance rates. High st in addition to ard deviations in ground motion prediction equations are a leading c in addition to idate to explain the surprising hazard predictions.” Anderson, 2010 SHORT RECORD OF SEISMICITY & HAZARD ESTIMATE Predicted hazard from historic seismicity is highly variable Likely overestimated near recent earthquakes, underestimated elsewhere More uni as long as m hazard seems more plausible – or opposite if time dependence considered Map changes after major earthquakes Africa-Eurasia convergence rate varies smoothly GSHAP NUVEL-1 Argus et al., 1989 SHORT RECORD OF SEISMICITY & HAZARD ESTIMATE Predicted hazard from historic seismicity is highly variable Likely overestimated near recent earthquakes, underestimated elsewhere More uni as long as m hazard seems more plausible – or opposite if time dependence considered Map changes after major earthquakes Africa-Eurasia convergence rate varies smoothly GSHAP NUVEL-1 Argus et al., 1989 2004 2003

Long record needed to see real hazard Swaf as long as d & Stein, 2007 “Our glacial loading model suggests that earthquakes may occur anywhere along the rifted margin which has been glaciated.” Stein et al., 1979 1985 2005 HIGH MODELED NMSZ HAZARD RESULTS FROM HIGH-END ASSUMPTIONS Future earthquakes will be like past ones in location & timing Redefined from maximum acceleration predicted at 10% probability in 50 yr to 2% in 50 yr (1/ 500 yr to 1/2500 yr) Arbitrary choice on policy grounds; no cost/benefit analysis Lack of data; chose high model Uncertainty in interpreting intensity data Doesn’t consider space-time variability Systematic Measurement – Large magnitude of 1811-12 in addition to thus future large earthquakes High ground motion in large events

Frankel et al., 1996 Algermissen et al., 1982 Hazard redefined from maximum acceleration predicted at 10% probability in 50 yr (1/ 500 yr ) to much higher 2% in 50 yr (1/2500 yr) New Madrid hazard higher than Cali as long as nia results largely from redefining hazard as largest shaking expected every 2500 yr: Not so as long as 500 yr 500 yr 2500 yr Searer & Freeman, 2002 500 yr 2500 yr Newman et al., 2001 PREDICTED HAZARD DEPENDS ON ASSUMED MAXIMUM MAGNITUDE OF LARGEST EVENTS AND ASSUMED GROUND MOTION MODEL Frankel/Toro: St Louis 1.8 Memphis 1.3

EFFECTS OF ASSUMED GROUND MOTION MODEL Effect as large as one magnitude unit Frankel model, developed as long as maps, predicts significantly greater shaking as long as M >7 Frankel M 7 similar to other models’ M 8 Frankel & Toro models averaged in 1996 maps; Atkinson & Boore not used Newman et al., 2001 ASSUMED HAZARD DEPENDS ON EARTHQUAKE PROBABILITY ASSUMPTION Constant since last event: time independent Small after last event, then grows: time dependent Time dependent lower until ~2/3 mean recurrence Results depend on model & parameters Hebden & Stein, 2008 Time dependent lower until ~2/3 mean recurrence Charleston & New Madrid early in their cycles so time dependent predicts lower hazard RELATIVE PREDICTED HAZARD DEPENDS ON POSITION IN EARTHQUAKE CYCLE Hebden & Stein, 2008

NEW MADRID 2% in 50 yr (1/2500 yr) Memphis: TD at present is 64% of TI Time dependent model as long as eastern US predicts lower New Madrid & Charleston hazard Effect larger than lowering Mmax in addition to thus ground motion model Including GPS makes effect much greater Mw 7.7 (NMSZ) Mw 7.3 (Charleston) Hebden & Stein, 2008 Assume from GPS data no M7 on the way Some hazard remains from earthquakes up to M ~ 6.7 Hazard ~ 1/10 that of USGS prediction USGS, 2500 yr, assumes M 7 coming GPS, 500 yr, assumes no M 7 coming Hard to assess possible hazard of M7 on other faults No evidence, but can’t exclude until we underst in addition to mechanics

CHARLESTON 2% in 50 yr (1/2500 yr) Hebden & Stein, 2008

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