Strong Motion Data: Structures Adam Pascale Chief Technology Officer, Seismology Research Centre a division of ESS Earth Sciences Treasurer, Australian Earthquake Engineering Society
Why monitor buildings? Before a major earthquake Determine natural frequency of structure Monitor response to minor earthquakes During a major earthquake Alarm generation for occupants Record dynamic response of building After a major earthquake Look for changes in natural frequency Compare actual performance to design
Natural Period Taller buildings = longer periods Greater than 10 storeys, >1 sec period Smaller buildings = higher frequency 1 to 10 storeys resonate at 2 to 30Hz If a building is shaken at its natural frequency, the shaking is amplified Low-rise buildings at greater risk
Natural Period Video
Seismograms Terms: Seismometer: sensor that measures ground velocity Accelerometer: sensor that measures ground acceleration Seismograph: a data logger that takes input from any seismic sensor Accelerograph: a seismograph with an acceleration sensor connected Seismogram/Accelerogram: recording from seismograph/accelerograph Usually triaxial North-South, East-West, Up-Down
Positioning Instruments DPWH recommendations Basement: record input ground motion Tall buildings: also middle and top Locate near columns Synchronised recording (usually GPS) More sensors give engineers a better idea of how building responds to earthquake Basement instrument position not critical as it is not measuring building response
Placement: Corner vs Centre Linear motion Similar amplitudes Rotational motion Different amplitudes Corner mounted sensor will detect rotational motion more clearly
Amplification Structures on sediment or saturated sand, surface waves can be amplified Large nearby earthquakes typically have most energy in 2-20Hz range If natural frequency of structure matches earthquake frequency, further amplification is possible
Measuring Natural Frequency Need to detect very low amplitude motion Usually recorded with a seismometer Modern high-sensitivity, high-resolution accelerometers can detect natural period Natural noise provides excitation source (weather, population activity) MEMS accelerometers (used in cellphones) and geophones not sensitive enough
Comparing sensors Feedback coil accelerometer Noise level <1µg MEMS accelerometer Noise floor ~300µg Seismometer 1Hz noise <0.05µm/s Geophone 1Hz noise ~10µm/s Magnitude 1.8 at 115km range
Accelerometer Technologies Force Feedback Coil type Technology proven for many decades Sensitivity has steadily improved Micro-Electro-Mechanical Systems (MEMS) type Developed from airbag sensor technology Cheap, but sensitivity worse than oldest coil tech Optical Laser Interferometry type New technology, extremely sensitive Twice the cost of coil accelerometer Performance rivals broadband seismometers Quarter the cost of a broadband seismometer
Recorded naturally in Velocity Acceleration converted to Velocity Comparison Prism vs STS2 New accelerometer tech as sensitive as expensive broadband seismometer Magnitude 6.1 earthquake at a range of 2000km
Myanmar M6.8, 24 August 2016 Even coil type accelerometers can detect distant earthquakes Recorded in JGC office building in Alabang at a range of 2900km!
Frequency Analysis
P-wave frequency 6.5Hz
S-wave frequency 2Hz
Surface wave 2.5Hz
Pre-event (natural freq.) 0.4Hz
Log scaling for clarity Longer background sample would produce more low frequency detail
Spectrogram View
Convert to Displacement
Vector Sum
Earthquake Magnitude Many magnitude scales Richter is most famous, limited Formula unreliable above ML 6 Unreliable for quakes >600km away Moment magnitude for large quakes Richter magnitude easily estimated
Magnitude calculated from Distance and Displacement
P-to-S time gives Distance, software calculates Displacement & Magnitude Rule of thumb: S minus P in seconds multiplied by 8 gives you distance in km.
Data Management Seismic data much more useful when telemetered in real time Centralised data allows concentration on data analysis, not data collection Remote management of stations over web (browser or software)
Centralised Data Storage All data rapidly accessible for analysis by centralised engineering resources Reduce survey time after a major earthquake Help to prioritise emergency services Data backup (on instrument and on server) Easy data sharing
The SRC Observatory Online Data & Network Management
Summary Building performance is measured by recording continuous waveform data from before, during & after earthquakes Low rise buildings more likely to be affected Good accelerographs can be very sensitive Even basic waveform analysis tools can reveal much about building response Centralised data collection and analysis is critical for effective emergency response
Thank You Upcoming joint conference in Melbourne, Australia Friday to Sunday 25-27 November 2016: Australian Earthquake Engineering Society Conference 11 th Asian Seismological Commission General Assembly http://asc2016.asia