The Weertmans and the San Andreas Fault: How a dislocation solution became a key tool in earthquake science
Seth Stein & James Neely Earth & Planetary Sciences, Northwestern
1 SAN FRANCISCO EARTHQUAKE April 18, 1906 3000 deaths 28,000 buildings destroyed (most by fire) $10B damage “The whole street was undulating as if the waves of the ocean were coming toward me.” “I saw the whole city enveloped in a pile of dust caused by falling buildings.” “Inside of twelve hours half the heart of the city was
gone” 2 Average 4m of motion West side moved north
Motion along hundreds of km of San Andreas Fault
Questions arose: What is the fault? Why does the ground move? Will another quake happen 3 and when? USGS 1960’s discovery of Plate Tectonics: Earth's outer shell made up of ~15 major rigid plates ~ 100 km thick that move relative to each other at speeds of a few cm/yr
Deformation occurs at their boundaries, giving rise to earthquakes, mountain building, volcanism, and other spectacular phenomena.
4 San Andreas Fault is boundary between Pacific and North America plates It’s a strike-slip fault!
North America Pacific Plate Plate
This Dynamic Earth: Scott Cronk (YouTube) The Story of Plate Tectonics, USGS
5 ELASTIC REBOUND OR SEISMIC CYCLE MODEL
Materials at distance on opposite sides of the fault move relative to each other, but friction on the fault "locks" it and prevents slip
Eventually strain accumulated is more than the rocks on the fault can withstand, and the fault slips in an earthquake
Earthquake releases strain 6 Long term slip rate and earthquake recurrence on the San Andreas
Wallace Creek is offset Robert Wallace by 130 m NU Geology BA 1938 Offset developed over 3700 years
130 m / 3700 yr = 35 mm/ yr
Large earthquakes with San ~4 m slip should often on average ~ 115 years Andreas apart Fault
1906 + 115 = 2021 7 San Andreas Fault – Over thousands of Wallace Creek, California years have had 35 mm/yr motion between Pacific and North America
How fast is the motion today, which is storing strain that will be released in the next big earthquake?
Crucial parameter for estimating earthquake hazard 8 Weertmans’ solution:
Can provide insight into size and frequency of earthquakes 9 Locking depth is depth to which rock is strong enough to accumulate elastic D strain that will be released in future earthquakes, and Jiang and Lapusta (2017) thus affects their magnitude
D Depends on rock properties, temperature Brittle and pressure, pore Ductile pressure, and strain rate
10 Savage & Burford is heavily cited and their method applied worldwide
Geologists don’t have copies of Elementary Dislocation Theory, but we have GPS 11 Pacific North plate America plate
ARCTANGENT
FAR FIELD SLIP RATE ~ 35 mm/yr
GPS and stream offset agree!
12 Z.-K. Shen Fix Rate, Change Locking Depth
San Andreas Fault Carrizo Plain GPS Data Schmalze et al. 2006
13 Change Rate, Fix Locking Depth
San Andreas Fault Carrizo Plain GPS Data Schmalze et al. 2006
14 15 San Andreas Fault Carrizo Plain GPS Data Schmalze et al. 2006
16 Slip rate and locking depth from GPS profiles across the southern Dead Sea Transform
Arctangent
Sinai microplate Arabian plate
Israel Jordan
Site of many historical earthquakes since Biblical times, more expected Eurasian plate Arctangent
Anatolian plate
18 Site of 1999 Mw 7.6 Izmit earthquake, another expected soon Site of 1975 Mw 7.5 Guatemala earthquake 19 Another arctangent!
20 Most earthquakes at plate boundaries, where motion is fast Some from slow motion inside plates
New Madrid seismic zone in central U.S. M 7 earthquakes in 1811-12 Small earthquakes continue Big ones might happen again
What’s happening PACIFIC today? 21 New Madrid GPS surveys
Significant motion expected as strain builds up for next earthquake
Best fitting arctangent gives rate 0.2 ︎ ± 2.4 mm/year.
No significant motion! 22 Hans & Julia Weertman‘s 1964 solution has become geophysicists’ standard tool for inferring slip rates on strike-slip faults worldwide and making crucial inferences about plate motions and earthquake recurrence
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Vƀš˃ These are used to infer mantle viscosity from post-glacial rebound
GPS
Canada rises & US sinks Hinge line agrees with lake level data
25 Sella et al., 2007 BRITTLE
DUCTILE
Brace & Kohlstedt, 1980
And to infer the strength of rocks due to increasing temperature and pressure and thus where earthquakes do and don’t occur at depth 26 The adage “Nice guys finish last” didn’t apply to Hans and Julia Weertman – nice people and superb scientists who finished first.
We in earth science were lucky to have had them as friends.
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