SIO15-SS1 2020: Topic 7 Earthquake Seismology
P and S Waves are body waves Image: S. Marshak “Earth, Portrait of a Planet” SIO15-SS1 2020: Topic 7 Earthquake Seismology
SIO15-SS1 2020: Topic 7 Earthquake Seismology
R 1. P first, typically smallest signal 2. S, typically larger (felt by humans) 3. Love 4. Rayleigh, longest/largest wavetrain, most damaging short video 7 on : p wave used in early warning systems seismic waves warning time: up to 1 minute how to locate an earthquake SIO15-SS1 2020: Topic 7 Earthquake Seismology
Fig. 5.18 there is already a fault why don’t blocks move all the time? Friction Stress no EQ Stress Friction EQ! asperities: protrusions that increase friction along a fault EQ: when stresses high enough to overcome friction fault breccia/fault gouge: broken-off, ground up asperities SIO15-SS1 2020: Topic 7 Earthquake Seismology
fault locked very little creep fault locked very little creep 1. stress builds up; asperities; no motion 2. stress overcomes friction -> EQ + aftershocks 3. slip along fault; stress drop Fig. 5.19 • start from 1) SIO15-SS1 2020: Topic 7 Earthquake Seismology
• some history of local seismicity in Seattle/Tacoma area • no recorded history of megathrust EQ • 1700 orphan tsunami in Japan has no corresponding local EQ -> M W =9.0 in Cascadia • recurrence time ~ 300 years • complex aseismic slip and episodic tremors (GPS and seismometers) Fig. 5.21 SIO15-SS1 2020: Topic 7 Earthquake Seismology
• > 3.5 Mio per year • small events often • large events rare SIO15-SS1 2020: Topic 7 Earthquake Seismology
Fig. 5.13 Get recurrence time from # of EQ per year E.g. 3 Mio EQ/year -> 1 EQ every 10.5s 0.3 EQ/year -> 1 EQ every 3.3 years SIO15-SS1 2020: Topic 7 Earthquake Seismology
Fig. 5.15 � EQs not regular � recurrence time has a certain probability < 100% source: Abbott “Natural Disasters” SIO15-SS1 2020: Topic 7 Earthquake Seismology
Fig. 5.16 � recurrence time varies along same fault source: Abbott “Natural Disasters” Probability of EQ before 2032 • 30% chance for a mag 7 quake before 2032, for most places along SAF prognosis for before 2004 • 90% chance for a mag 6 at Parkfield seismic gap/recurrence SIO15-SS1 2020: Topic 7 Earthquake Seismology
Fig. 5.17 use seismic gap method � EQ recurrence time � Parkfield EQs remarkably regular in > 100 years � next predicted: 1993 28 September 2004, 10:15 PSDT * #7 late or #8 early??? SIO15-SS1 2020: Topic 7 Earthquake Seismology
Fig. 5.17 - Mw= 6.5 December 22, 2003 San Simeon EQ increased stress at Parkfield - made #8 more likely??? - but why did #7 not happen? - EQs can trigger/influence other EQs on nearby faults (1992 Landers/Big Bear) - very large EQs may trigger volcanism far away (Rayleigh waves!) (2002 Denali EQ/Yellowstone; 2004 Sumatra/Alaska volcanoes) SIO15-SS1 2020: Topic 7 Earthquake Seismology
Fig. 6.43 InSAR: Interferometric Synthetic Aperture Radar • overlay satellite altimetry map before and after EQ • interferogram -> fringes show amount of slip along fault Sep 17, 1999 Izmit, Turkey Earthquake SIO15-SS1 2020: Topic 7 Earthquake Seismology Image: NASA/JPL; wikipedia
• example: southern SAF • last major EQ: 250 years ago, while north and middle SAF ruptured 1906, 1989 and 1857 • typical co-seismic slip for big EQs: 7-10m relaxed loaded • use stack of INSAR images to get strain rate • extrapolate to 250yrs SIO15-SS1 2020: Topic 7 Earthquake Seismology
Fig. 6.44 site amplification takes into account local geology probability of strong shaking takes into account local geology, tectonics, seismicity Fig. 6.45 SIO15-SS1 2020: Topic 7 Earthquake Seismology
Fig. 6.46 http:movie SCEC: Southern California Earthquake Center SIO15-SS1 2020: Topic 7 Earthquake Seismology
USGS Did-you-feel-it map Mw=7.2 El Mayor Easter (4 Apr) 2010 * understand shaking characteristics of EQs * improve local forecasts for strength of shaking SIO15-SS1 2020: Topic 7 Earthquake Seismology
SIO15-SS1 2020: Topic 7 Earthquake Seismology
Fig. 6.42 What Government and People can do - don’t build, build to code, retrofit (e.g. use wood, not bricks) - construct lifelines to EQ building codes - problematic in less developed countries - grading according to code (landslides!) - secure life lines/make disaster plans Got water? ...flashlight canned food batteries battery-op radio SIO15-SS1 2020: Topic 7 Earthquake Seismology
SIO15-SS1 2020: Topic 7 Earthquake Seismology
We just don’t know! ……yet….. forecasting weather is much easier! SIO15-SS1 2020: Topic 7 Earthquake Seismology
SIO15-SS1 2020: Topic 7 Earthquake Seismology
So, when is it coming? SIO15-SS1 2020: Topic 7 Earthquake Seismology
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