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Pent-up stress puts ‘the squeeze’ on Los Angeles Ken Hudnut U. S. Geological Survey Pasadena Office This presentation will probably involve audience discussion,

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Presentation on theme: "Pent-up stress puts ‘the squeeze’ on Los Angeles Ken Hudnut U. S. Geological Survey Pasadena Office This presentation will probably involve audience discussion,"— Presentation transcript:

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2 Pent-up stress puts ‘the squeeze’ on Los Angeles Ken Hudnut U. S. Geological Survey Pasadena Office This presentation will probably involve audience discussion, which will create action items. Use PowerPoint to keep track of these action items during your presentation In Slide Show, click on the right mouse button Select “Meeting Minder” Select the “Action Items” tab Type in action items as they come up Click OK to dismiss this box This will automatically create an Action Item slide at the end of your presentation with your points entered. USGS Public Lecture Caltech May 1, 2001

3 A very brief history of geodesy Geodesy literally means measurement of the Earth (we do geodesy to study strain build-up between earthquakes) Eratosthenes (a.k.a. ‘Beta’) measured circumference of the Earth very accurately – Knew that sun hit bottom of well at Syene – At same time (high noon), he measured sun’s angle at Alexandria – 500 miles away (stadia)  so C = 25,000 miles

4 More recently… Conventional geodesy: triangulation trilateration Space geodesy: VLBI, SLR and GPS

5 Elastic rebound theory Harry Reid – looking at geodetic data from the 1906 San Francisco earthquake – surmised that strain is accumulated and then released in an earthquake… over and over again… with similar strain accumulation and release – could it predict fault behavior? fault fence

6 Earthquake recurrence Reid’s elastic rebound theory was too good to be true - too simple Earthquakes do not behave this way, even in a simple system – Brick & bungee – B & K (1967) Time predictable, slip predictable, or non- predictable, i.e., chaotic? We are up against friction – notoriously unpredictable slip time strain event 1 event 2

7 Slightly more complex fault model – slider blocks Simple model of an earthquake fault, the Burridge-Knopoff slider block model (1967). A scaling distribution (fractal statistics) is seen for the slider block simulations, similar to form of Gutenberg-Richter distribution of naturally occuring relationship between number of earthquakes versus magnitude Log 10 {Area} Log 10 {Frequency}

8 Earthquake Terminology Strike-slip Faults Rupture surface Hypocenter Hypo- center Epicenter Fault plane Fault

9 Plate tectonic motions As the Farallon plate subducted, the San Andreas fault was born In the past 5 million years, this motion has been steady at about 5 cm/yr (that’s 5 meters per century or 50 km per million years!) Movie by T. Atwater, UCSB

10 Faults & Earthquakes San Andreas fault zone – North American and Pacific plate relative motions of 56 mm/yr in a right-lateral sense Eastern California shear zone – Accomodation of right-lateral motion inboard of Sierra Nevada block – Estimated rates of some 8-12 mm/yr (geological & space geodetic) – Easier to go through than the Big Bend? Big bend of San Andreas – Compression across Los Angeles basin Ventura basin

11 All of Earth’s tectonic plates are constantly moving, with respect to both the geocenter, and all of the other plates. Nothing is staying fixed, as the pieces of crust move across the surface of our sphere. Space geodesy allows us to measure absolute plate motions. We can readily estimate and then remove poles and rotation rates from our data, reducing the velocity field so that it is referred, say, to the ‘stable’ North American plate… … remember that not too long ago, baseline lengths were re-observed to obtain precise strain data. Space geodesy enabled precise reference frame realization, helping us to put together the big picture, allowing rigorous tests of plate tectonics and earthquake disloc- ation theory …

12 Southern California Integrated GPS Network Studying earthquakes using GPS technology State-of-the-art network for research and earthquake response – Software and hardware development – New networking technology – Automated processing systems developed Data also used widely for surveying, GIS mapping, engineering

13 A few acronyms Global Positioning System (GPS) Southern California Integrated GPS Network (SCIGN) Plate Boundary Observatory (PBO) Southern California Earthquake Center (SCEC)

14 GPS – Global Positioning System GPS is a U.S.-built constellation of navigation satellites Normally it is used for ‘coarse’ positioning – Handheld GPS units (~$100-$500) C/A code only 6 meter precision (with SA turned off) We ‘earthquake people’ do precise GPS – Top-notch GPS receivers ($11,000) P-code and phase on both L1 & L2 Differential phase - several millimeter precision (1000x better!) Wide range of uses for GPS data provided by SCIGN

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16 GPS signal correlation GPS phase differencing

17 ‘normal’ vs. ‘precise’ GPS 5/2/2000 ended SA normal GPS improved by about 10x

18 Continuous GPS & strainmeters Best tools ever devised for highly accurate, automated, constant monitoring of crustal strain for – long baselines – absolute ref. frame – displacement field – very high precision SCIGN & other PBO elements require sub-millimeter velocities on the plate boundary scale in order to answer the scientific questions

19 regional active faults You never know what you’re going to get… Fort Tejon 1857 Long Beach 1933 Arvin-Tehachapi 1952 San Fernando 1971 ECSZ sequence Northridge 1994

20 California relative plate motions for the past 20 million years Movie by T. Atwater, UCSB

21 Using GPS to study earthquakes Measure station positions every day to within a few millimeters Track stations through time (time series) Velocities derived from stations’ time series Strain derived from the velocity field Strain is proportional to seismic hazard and to seismic risk

22 Geodesy for risk estimation

23 Getting from GPS data to strain, and from strain to risk Daily positions show Station trajectory vs. time (a time series of position) In North-South, East-West and Up-Down components, we fit a line to the data This gives a vector for each station’s velocity time 1 time 2 t1t1 t2t2 North by 40 mm in 2 yrs. West by 70 mm in 2 yrs.

24 From station velocities to strain Velocities of all stations then are used to calculate strain (between each set of three sites) First case – all of the velocities are the same, so there is no strain occurring in the triangle between these three sites for this time interval

25 From station velocities to strain Velocities of all stations then are used to calculate strain (between each set of three sites) Second case – not all of the velocities are the same, so there is strain occurring in the triangle between these three sites for this time interval t1t1 t2t2

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27 SCEC Crustal Motion Map  Combined EDM, VLBI, survey- mode and continuous GPS rigorously  Released as a SCEC product  Set the bar very high for the SCIGN project

28 Types of Strain Shear strain – San Andreas fault has right- lateral slip (that results from shear strain between North American and Pacific Plates) Plane strain – Compression across the system of thrust faults beneath Los Angeles Dilatational strain Rotational strain t1t1 t2t2

29 From station velocities to strain Velocities of all GPS stations are then used to calculate strain (between each set of three sites) Maps can then be made to show each component of strain, (e.g., the shear strain both parallel to and perpendicular to the San Andreas fault) Strain values can then be contoured, or used in hazard calculations, to make maps that are more generally used, e.g., by engineers

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31 Probabilistic Seismic Hazard Analysis SCEC led using geodetic data as input to their seismic hazard Analysis (BSSA, 1995)

32 How to improve hazard information and reduce earthquake risk? [SoCal is home to 15 million people, and poses half of the national earthquake risk (FEMA #366)] Better geodetic and other data as input to geodynamic earthquake source models – SCIGN: state-of-the-art continuous GPS net – LARSE: deep crustal imaging of faults – TriNet: recording earthquake shaking Better methods of simulating shaking and estimating damage likely to be caused in scenario events – USGS & CDMG – official maps to have future revisions – SCEC ‘IT’, RELM – developing & testing new methods – HAZUS – convert hazard to risk – how to reduce it?

33 This presentation will probably involve audience discussion, which will create action items. Use PowerPoint to keep track of these action items during your presentation In Slide Show, click on the right mouse button Select “Meeting Minder” Select the “Action Items” tab Type in action items as they come up Click OK to dismiss this box This will automatically create an Action Item slide at the end of your presentation with your points entered. Operational Groups: Major Funding: Total $18 M SOPAC SCIGN is an integral part of SCEC

34 The major objectives of the SCIGN array are: * T o provide regional coverage for estimating earthquake potential throughout Southern California  T o identify active blind thrust faults and test models of compressional tectonics in the Los Angeles region  T o measure local variations in strain rate that might reveal the mechanical properties of earthquake faults  I n the event of an earthquake, to measure permanent crustal deformation not detectable by seismographs, as well as the response of major faults to the regional change in strain

35 Proper tools for the job… - air rotary rock drill & auger rig installations by contractor Had to build deeply anchored tripod ‘monuments’

36 Monumentation

37 SCIGN – a great GPS network: carefully planned - well reasoned Monuments –Each of 5 legs is drilled to 10 meters Lowermost 6 meters is anchored Upper 4 meters is isolated from soil –Stainless for longevity Innovative geodetic tools –SCIGN radomes & adaptors –Data acquisition software Redundant precise processing –GIPSY and GAMIT –Rigorous comparisons ongoing Accuracies are the highest ever achieved, exceeding even highly optimistic expectations for SCIGN movie by John Galetzka, USGS

38 SCIGN project installation status

39 Los Angeles deformation Two main models advanced for the observed 5 mm/yr contractional strain across Los Angeles: 1. Strain on NW-SE and SW- NE strike-slip faults (e.g., Walls et al.) 2. Strain on thrust fault system [including blind thrust faults] (e.g., Argus et al.)

40 Deep crustal studies - LARSE Fuis et al. (SCEC & USGS) set off many explosions to reflect energy from deep faults Tomographic imaging (like a CAT scan) Shows deep geometry of thrust faults beneath Los Angeles Images the faults, but doesn’t show if they are active or not Geodesy can now be used to estimate rate of slip on these faults

41 Deformation across LA SCEC and SCIGN GPS velocities - reduced so that San Gabriel Mtns. are held fixed (Argus et al., JPL) Contraction may be in a narrow belt, concent- rated across the San Gabriel Valley

42 Simple elastic models don’t fit the GPS data? So far, Argus et al. are finding that it’s going to take some fancy modeling to explain these GPS data Bawden et al. find land subsidence effects on data, and after correcting for that, they can fit the data Meanwhile, SCIGN is providing better data to help resolve these differences

43 and in the future… Soon we will build the SCIGN project’s 250 th station Early data have already been used for – Controversy over tectonic style in Los Angeles – SCEC v2 crustal motion map and Phase II hazards – Observation of static displacement field propagation and pushing the real-time GPS envelope (broad-band seismology using GPS?) Data so far are exceeding expectations, so we now expect to surpass original observational objectives (except where signals are masked by land subsidence) Leading the world in continuous GPS network technology and software development SCIGN project progress…

44 GPS & telemetry/networking Market for GPS boards is driven by Moore’s law (like PC’s) toward faster/better/cheaper, miniaturization, etc. Spread spectrum radio and satellite telemetry leading to high bandwidth IP field networking (e.g., TDMA) Allows higher sampling rates and more affordable real- time telemetry

45 Assess damage to infrastructure Were tilts or strains large enough to damage systems? (from regional measurements) Did damage occur to critical structures or systems? (from site-specific monitoring)

46 Structure monitoring Pacoima dam GPS monitoring since Sept with LA County GPS data can indicate damage to engineered structures such as overpasses and tall buildings

47 New methods: high-resolution topographic mapping and digital photography Laser scanning using an airborne platform requires high sampling-rate GPS data during flight to control aircraft position and attitude SCIGN stations were operated at 1 and 2 sample per second rates via the radio network

48 The Plate Boundary Observatory SCIGN as a prototype Plans for 875 additional continuous GPS stations, for earthquake research, throughout the Western U. S. A. (as well as in Canada and Mexico)

49 The San Andreas fault zone ‘focus array’ of PBO Geodetic networks for earthquake research, especially for observing transitional behavior between creeping and locked fault surfaces, along- strike and at depth, and aseismic fault loading processes (that cannot be observed with seismological instruments)

50 Conclusions SCIGN is the world-leading, state-of-the-art, continuously- operating GPS network We have innovated & devised the best-ever geodetic methods for earthquake research Having led in technology development, we now fully expect to lead in research results (especially for the Los Angeles urban region) as the network is now nearly fully operational – Original research on strain and fault motions – New and improved hazard and risk analyses Data from SCIGN are freely available to all, and are used extensively in surveying, engineering, and for GIS mapping (spatial referencing users may become main sponsors of network operations, removing some burden from agencies like NASA, USGS & NSF)

51 For more information: Kenneth W. Hudnut, Ph.D. Geophysicist USGS Pasadena Arthur C. Clarke's 2 nd Law: "The only way of discovering the limits of the possible is to venture a little way past them into the impossible."


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