The Earthquake Cycle Chapter :: n/a A German seismogram of the 1906 SF EQ Image courtesy of San Francisco Public Library
Stages of the Earthquake Cycle The Earthquake cycle is split into several distinct phases / stages based on the deformation observed: Interseismic The time between large earthquakes Preseismic The time just before an earthquake when anomalous things happen Coseismic The time during an earthquake Postseismic The time after a large earthquake when anomalous deformation occurs. The preseismic phase has proven elusive and inconsistent It may not even exist! The other three phases are commonly observed Postseismic involves complex math! We will only briefly discuss this stage
The Revolution :: Elastic Rebound After the M w 7.9 1906 SF EQ, H.F. Reid proposed that Earthquakes represent rapid release of strain/stress built up over a long period of time (hundreds of years) Called elastic rebound theory Confirmed by Geodetic measurements of surface motion (triangulation) Geologic measurements of offset 450 km long rupture (360 km on land) Average slip 4.5 m Reid postulated: Pacific Ocean floor must be spreading, pushing the west side of the SAF to the NW. He recommended a monitoring program Not adopted until 60 yrs later
Reid s Evidence for Elastic Rebound
Farallon Lighthouse Duxbury Point, Bolinas Beach
Before the 1906 Earthquake Farallon Lighthouse Duxbury Point, Bolinas Beach Locations far from the fault were moving fast Locations near the fault were moving slow Same was true on other side of the fault, but motions were in the opposite direction
During the 1906 Earthquake Farallon Lighthouse Duxbury Point, Bolinas Beach Locations near the fault were displaced very far Locations far from the fault were displaced very little Same was true on other side of the fault, but motions were in the opposite direction
Reid s Hypothesis :: Elastic Rebound Theory Although plate tectonics theory was ~50+ years from being developed, Reid s hypothesis is consistent with plate tectonics Elastic rebound is also consistent with geologic observations! Interseismic Coseismic Long-Term Block Offset + = Elastic Strain is localized near fault Elastic strain is released After the EQ, elastic strain has been released
The Earthquake Cycle: Graphical Form Reid proposed: Interseismic strain accumulates slowly and is eventually released in an EQ The coseismic strain release = total accumulated interseismic strain The net result: Block offsets over geologic timescales He made the prediction that the next EQ would happen when the same amount of interseismic strain had accumulated Called a time-predictable model Turned out to be unreliable Interseismic Coseismic Long-Term / Geologic
What is Happening During the EQ Cycle? Interseismic Deep, steady, & slow aseismic slip (i.e. creep) Coseismic Rapid shallow slip
Conventional Interseismic Model x y Semi-infinite vertical dislocation embedded in an elastic earth. Semi-infinite height Infinite length u x = displacement of ground around the fault x = distance from fault b = fault slip rate D = locking depth b x This is an analytical model based on mathematics developed by the engineering community
Displacement Displacement = u - u o Final position initial position Measured anywhere in a medium Applies to the motion of a single particle A vector quantity (has a magnitude and direction) Difficult to measure in the geologic record Don t know initial position, only know final position final position = u Initial position = u 0
Slip = u + + u - Slip A.k.a: Offset / Displacement Discontinuity / Burgers Vector Displacements are discontinuous across a fault This is why geophysicists refer to faults as discontinuities or dislocations Slip is the sum of the displacements on both sides of a fault A vector quantity (recall that the slip vector has a rake ) Applies to the relative motion across a fault So it is only measured across faults! Slip Offset Feature Fault So, slip measures the distance along a fault surface between two points that used to be connected
Slip vs. Displacement slip = 1 u + = 1 u - = 0 Can t determine displacements unless you know the original position slip = 1 u + = 0.5 In geology, you almost never know the original position In geophysics, you sometimes know the original position (GPS) u - = 0.5 It is the sum of the displacements on both sides of a fault (i.e. the slip) that matters when considering earthquakes
Coseismic Rupture Dimensions Coseismic ruptures commonly Are longer than they are deep Can be approximated by a rectangle If surface ruptures Can be measured by geologists If no surface rupture Let s Trench! Rupture can be mapped by aftershocks Rupture can be estimated by surface deformation models Can also be determined by analyzing seismic wave patterns slip Fault Trace Offset road from the Mw7.1 1999 Hector Mine EQ
The 1966 Parkfield EQ brittle-ductile transition
Controls on Rupture Dimensions Recall the two main layers of the Earth: Lithosphere: Brittle Rocks Asthenosphere: Ductile Rocks Earthquakes only occur in the lithosphere Heat flow / geothermal gradient controls the level of the brittle ductile transition Hot rocks: ductile Cold rocks: brittle Subduction zones have greatest potential rupture width (depth) Mid ocean ridges have smallest potential rupture width Pressure (Kbar) Brittle-Ductile Transition 0 200 400 600 800 0 4 8 12 Temperature ( o C) 0 20 40 Depth (km)
The 3 largest earthquakes recorded: Largest EQ s: Subduction M W 9.2 1964 Good Friday EQ, Anchorage, Alaska M W 9.1-9.3 2004 Great Sumatra EQ 20 m maximum slip!! 1200 km long rupture! M W 9.5 1960 Chile EQ
Slip on an earthquake fault START Surface of the earth Depth Into the earth 100 km (60 miles) Distance along the fault plane
Slip on an earthquake fault Second 2.0
Slip on an earthquake fault Second 4.0
Slip on an earthquake fault Second 6.0
Slip on an earthquake fault Second 8.0
Slip on an earthquake fault Second 10.0
Slip on an earthquake fault Second 12.0
Slip on an earthquake fault Second 14.0
Slip on an earthquake fault Second 16.0
Slip on an earthquake fault Second 18.0
Slip on an earthquake fault Second 20.0
Slip on an earthquake fault Second 22.0
Slip on an earthquake fault Second 24.0
Rupture on a Fault Total Slip in the M7.3 Landers Earthquake
Quantifying Earthquake Size There are two basic ways to quantify the size of an earthquake. Intensity Measures the amount of shaking at a given location Depends on location i.e. a given earthquake will have lots of different intensities Magnitude Measures the amount of energy released at the source Does not depend on location A given earthquake will just have one magnitude (on each scale) Haiti Photo Courtesy: UN Photo/Logan Abassi United Nations Development Programme
Intensity Measured on the Modified Mercalli Scale (1931) Twelve categories Denoted by Roman numerals Plotted as isoseismals: zones of same intensity Intensity in general decreases away from epicenter, but local geology can completely control intensity in some cases (only measured by instruments)
Severity of Shaking Depends On: Magnitude of the earthquake Distance from hypocenter The nature of the substrate at location Stiff bedrock shakes less Soft rock shakes a lot Sedimentary basins can amplify waves E.g. 1985 Mexico city M W 8.0 > 350 km away The frequency of the seismic waves High frequency waves do most damage but do not travel very far (i.e. they attenuate) Car stereo analogy (bass) In general Long ruptures generate long wavelengths (low frequencies) Short ruptures generate short wavelengths (high frequencies) Mexico city
Buildings - Mexico City, 1985 Thousands of buildings destroyed Prompted Mexico to develop building codes After Before [TerraShake Animations]
Magnitude Magnitude = A measure of the amount of energy released at the source of the EQ. Richter Scale: A type of magnitude measurement coined by Charles Richter in 1935. M L = log 10 (max amplitude of S-waves in units of 10-6 m) Used a logarithmic scale to make the wide range of measurements easy to deal with A change of one in Richter magnitude = 10x the ground motion and 30x the energy. Also called the local magnitude Based on measurements of S-wave amplitudes at 100 km from epicenter Can be effectively corrected for seismometers at different distances Photos of Charles Richter (1900-1985) courtesy of USGS
How seismogram readings are made into M L Can have negative magnitude No mathematical upper limit on magnitude i.e. 10 is not max The Richter Nomogram
Richter s ups and Downs Richter scale advantages: First quantitative measure of energy release Can be computed minutes after an EQ Good for nearby, shallow, and moderate EQ s The Richter scale shortcomings: At epicentral distances > 600 km, surface waves have greater amplitude than S-waves M L underestimates distant events Instead, we use M S, surface wave magnitude, which is based on the amplitude of surface waves (R-waves) Underestimates deep earthquakes (S-waves attenuate faster than P-waves) Instead we use mb, body wave magnitude, for deep events. Uses the maximum amplitude of either body wave. All of these underestimate very large EQ s We now use Moment Magnitude, M W = 2/3 log 10 M 0-10.7 M 0 is the Seismic Moment Background: seismogram from M W 9.2 1964 Alaska EQ, courtesy USGS
Seismic Moment Seismic moment, M 0, is mathematically based on the torque exerted by the shear stress couple (i.e. the deformation on both sides of a fault) M 0 = μad μ = shear modulus A = fault rupture area d = average slip during earthquake μ does not greatly vary for different rock types at depth Typically ~ 30 GPa So, A, and d are what matter But what controls A and d?
Seismic Moment and the Sizes of Ruptures Small EQ s have small rupture areas and small average slip Slip is much smaller than rupture length Due to finite fault width (brittle-ductile transition), small earthquakes follow different scaling Where would subduction EQ s plot below?
Bigger (longer) Faults Make Bigger Earthquakes 1000 Kilometers 100 10 1 5.5 6 6.5 7 7.5 Magnitude 8
Bigger Earthquakes Last Longer 100 Seconds 10 1 5.5 6 6.5 7 7.5 8 Magnitude
Earthquake Prediction (?) Currently scientists can t make short term predictions of earthquakes e.g. there will be an earthquake next Tuesday at 8:07 AM. We can make some long term predictions There will very likely be a large earthquake on the San Andreas fault in the next hundred years. In the next hundred years it is unlikely that there will be a large earthquake in central Canada Seismic Hazard Is there a seismic source? Seismic Risk What sort of risk does this source pose to civilization? E.g. no people no risk Seismic Hazard Assessments are based on: Locations of faults Slip rates of faults Recurrence intervals (time between events) Local geology effects (liquefaction / basin fill) Seismic gaps
Building Codes In response to the 1971 M6.6 Sylmar EQ, the state of California passed new laws prohibiting the building of public buildings within ¼ mile of an active fault zone (private houses within 50 feet) Called Alquist-Priolo Earthquake Fault Zones Since short term earthquake predictions may be impossible, building codes are the main way to save lives in future earthquake events Building codes (zoning laws) are based on seismic hazard assessments Insurance companies also are very interested in seismic hazard maps
Seismic Gaps & The North Anatolian Fault, Turkey Seismic Gaps: Areas where the fault has not moved in a long time These regions may be the next to go Stress Triggering: When an earthquake happens, the motion changes the stress on nearby faults, possibly making them more or less likely to fail. The North Anatolian Fault is an excellent example of both of these phenomenon (Ross Stein animations)