Know how modern seismographs and an accelerographs measure and record earthquake motion.

Transcription

1 Lecture 3 - Engineering Measurement of Earthquakes Page 1 Learning Objectives Thursday, September 10, :10 AM LO3-1 Describe the engineering parameters used to characterize strong ground motion and common ways they are measured. 1. amplitude 2. frequency content 3. duration LO3-2 Know how modern seismographs and an accelerographs measure and record earthquake motion. LO3-3 Know how seismic networks and be used to locate earthquakes. Also, describe the scientific, engineering and emergency response uses of such networks. LO3-4 Define a response spectrum and know how to determine spectral values as a function of period for acceleration, velocity and dispalcement. Also, know how to calculate a displacement and velocity response spectrum from an acceleration response spectrum or vice versa. LO3-5 Know how to calculate rock and soil acceleration spectral values from common empirical attenuation relations. LO3-6 Know how to estimate earthquake magnitude, maximum and average fault displacement from geological maps. LO3-7 Know how to develop an earthquake intensity map from descriptions and accounts of strong ground motion.

2 Lecture 3 - Engineering Measurement of Earthquakes Page 2 Learning Objectives (cont.) Thursday, September 10, :10 AM LO3-8 Define the following: triggering error background noise baseline correction equation for simple harmonic motion circular frequency phase angle period of vibration peak ground acceleration equation of motion for SDOF system bracketed duration surface rupture length maximum fault displacement average fault displacement

3 Lecture 3 - Engineering Measurement of Earthquakes Page 3 Strong Motion Monday, September 14, :43 PM Earthquake Engineers are primarily interested in strong motion (i.e.. earthquake motion) of sufficient amplitude to affect people and their environment. Generally 0.05 g. or greater will begin to affect infrastructure. Ground motion parameters are: a. Amplitude b. Frequency content or period c. Duration A variety of parameters have been developed to characterize the amplitude, frequency and duration of strong ground motion. Measurement of Strong Motion Seismometers are very sensitive instruments that measure relatively weak motion. A recording from a seismometer is called a seismograph, which is a trace of the earthquake motion. Accelerometers measure strong ground motion from close, nearby earthquakes. A recording from an accelerometer is an accelerogram. Seismometer Pasted from < seismograph.jpg> Accelerometer Pasted from <

4 Lecture 3 - Engineering Measurement of Earthquakes Page 4 Seismometer as a Single Degree of Freedom System Monday, September 14, :03 PM Analog seismometers can be understood by a simple mass-spring-dashpot system where the base is connected to the ground surface. This is an analog type of seismometer because it is directly measuring the amplitude of the incoming waves.

5 Lecture 3 - Engineering Measurement of Earthquakes Page 5 Seismometers (cont.) Thursday, January 24, :41 PM The above cases show how the same physical system can act as both a displacement seismograph and an accelerograph. The system measures displacement at frequencies well above and accelerations at frequencies well below the natural frequency of the seismograph. Wood - Anderson Seismograph Used by Richter to develop the Richter Magnitude scale ML =log (max. amplitude trace) where: M L is the local or Richter magnitude determined from a Wood Anderson Seismograph at a distance of 100 km and max amplitude trace is the amplitude of the maximum P-wave trace in micrometers. (see Local or Richter Magnitude in Lecture 2.) Modern Seismometers Use electronic transducer (seismometer) senses motion and records digital electrical signal that is recorded for subsequent processing.

6 Lecture 3 - Engineering Measurement of Earthquakes Page 6 Accelerometers Monday, September 14, :59 PM Accelerometers for engineering use have an electronic transducer (accelerometer) that produces an output voltage proportional to acceleration. Types: (1) Servo (force balanced) uses a suspended mass to which a displacement transducer is attached. Signal is produced by the relative displacement between the housing and the suspended mass. The resisting force is measured electronically. This type has good accuracies for the range of frequencies of interest in earthquake engineering. (2) Piezoelectric uses a piezoelectric material (e.g., quartz, tourmaline, ferroelectric ceramic) to sense acceleration. The piezoelectric material acts as a spring in the SDOF system with negligible damping. When accelerated, inertial force strains the material which develops an electrical charge. The resulting voltage is proportional to the inertial force. This type is good for high frequency measurement because of the stiff piezoelectric material. (3) Geophones uses velocity transducers to measure velocity of waves. Used in geophysical surveys. (4) Seismoscope uses conical pendulum with metal stylus attached to a suspend mass which inscribes a record of ground motion on smoked glass plate, producing 2D record of movement. Pasted from < images/accelerometer.jpg> Pasted from Simple Servo Accelerometer Piezoelectric Accelerometer

7 Lecture 3 - Engineering Measurement of Earthquakes Page 7 Accelerometers (cont.) Monday, September 14, :08 PM Geophone From Wikipedia, the free encyclopedia The term geophone derives from the Greek word "geo" meaning "earth" and "phone" meaning "sound". A geophone is a device which converts ground movement (displacement) into voltage, which may be recorded at a recording station. The deviation of this measured voltage from the base line is called the seismic response and is analyzed for structure of the earth. Geosource Inc. MD-79 8Hz, 335Ω geophone Geophones have historically been passive analog devices and typically comprise a spring-mounted magnetic mass moving within a wire coil to generate an electrical signal. The frequency response of a geophone is that of a harmonic oscillator, fully determined by corner frequency (typically around 10 Hz) and damping (typically 0.707). The majority of geophones are used in reflection seismology to record the energy waves reflected by the subsurface geology. In this case the primary interest is in the vertical motion of the earth's surface. However, not all the waves are upwards travelling. A strong, horizontally transmitted wave known as ground-roll also generates vertical motion that can obliterate the weaker vertical signals. By using large areal arrays tuned to the wavelength of the ground-roll the dominant noise signals can be attenuated and the weaker data signals reinforced. Analog geophones are very sensitive devices which can respond to very distant tremors. These small signals can be drowned by larger signals from local sources. It is possible though to recover the small signals caused by large but distant events by correlating signals from several geophones deployed in an array. Signals which are registered only at one or few geophones can be attributed to small, local events and thus discarded. It can be assumed that small signals that register uniformly at all geophones in an array can be attributed to a distant and therefore significant event. The sensitivity of passive geophones is typically 30 Volts/(meter/second), so they are in general not a replacement for broadband seismometers.

8 Lecture 3 - Engineering Measurement of Earthquakes Page 8 Accelerometers (cont.) Monday, September 14, :21 PM Seismoscope Pasted from < 1c3c0c27b8.jpg> The Chinese philosopher Zhang Heng invented the earliest known seismoscope in 132 A.D. The instrument was said to resemble a wine jar of sixfoot diameter. On the outside of the vessel there were eight dragon heads, facing the eight principal directions of the compass. Below each of the dragon heads was a toad, with its mouth open toward the dragon. The mouth of each dragon held a ball. At the occurrence of an earthquake, one of the eight dragon-mouths would release a ball into the open mouth of the toad situated below. The direction of the shaking determined which of the dragons released its ball. The instrument is reported to have detected a four-hundred-mile distant earthquake which was not felt at the location of the seismoscope. Pasted from < Pasted from < en/popup_3.jpg> A seismoscope is an instrument that gives a qualitative measure of the oscillatory motion produced by an earthquake or other disturbance of the earth's surface. Unlike the seismograph, it lacks a device to calibrate the time. Several designs and variations exist, and many are easy to build with common materials. In this Demonstration, an oscillating cone filled with sand hangs by a string. The sand falls from a hole over a moving surface and draws the waveform that shows the general characteristics of the motion Pasted from < >

9 Lecture 3 - Engineering Measurement of Earthquakes Page 9 Processing of Strong Motion Records Monday, September 14, :50 PM Data Acquisition Early instruments - used pen, stylus, reflective mirror and recorded motion on paper or film attached to rotating drum Later generation recorded motions electronically in analog form or magnetic tape. Digital seismographs - Use analog transducers but convert the analog signal to digital form in the field. Can sample from 200 to 1000 samples per second in 12 to 16 bit resolution. Strong-Motion Processing Raw data from strong ground motion instruments must be processed to provide an accurate record of actual ground motion. Several sources of error: Background noise must be removed or suppressed ocean waves traffic construction activity atmospheric changes Instrument response Accelerographs have their own dynamic response characteristics Corrections are made numerically be modeling the instrument as a SDOF system and using the SDOF response to decouple the response of the instrument from the ground motion. However, this is only important for high frequencies which are usually above the range of engineering interest. Instrument shelters Triggering level Baseline correction - Any vibration that preceded triggering were not recorded, producing a baseline error in the record. Integration of an uncorrected acceleration time history will lead to a linear error in velocity and a quadratic error in displacement. An acceleration error as small as g at the beginning of a 30 second record could produce a erroneous permanent displacement of 441 cm. Baseline correction is done to correct for triggering error subtracting a best-fit parabola from the accelerogram before processing using high-pass filter

10 Lecture 3 - Engineering Measurement of Earthquakes Page 10 Strong Motion Networks Tuesday, September 15, :11 AM Sources of Strong Motion Records from Networks PEER Strong Motion Database LDEO/NCEER Earthquake Strong Motion Database National Geophysical Data Center

11 Lecture 3 - Engineering Measurement of Earthquakes Page 11 Strong Motion Network in Utah Thursday, September 10, :41 AM Pasted from <

12 Lecture 3 - Engineering Measurement of Earthquakes Page 12 Strong Motion Parameters Tuesday, September 15, :50 PM Relation between peak ground acceleration and modified Mercalli Intensity Scale

13 Lecture 3 - Engineering Measurement of Earthquakes Page 13 Peak Acceleration Tuesday, September 15, :54 PM

14 Lecture 3 - Engineering Measurement of Earthquakes Page 14 Peak Velocity Tuesday, September 15, :56 PM

15 Lecture 3 - Engineering Measurement of Earthquakes Page 15 Peak Displacement Tuesday, September 15, :53 PM

16 Lecture 3 - Engineering Measurement of Earthquakes Page 16 Frequency Content Tuesday, September 15, :00 PM

17 Lecture 3 - Engineering Measurement of Earthquakes Page 17 Frequency Content (cont.) Tuesday, September 15, :01 PM

18 Lecture 3 - Engineering Measurement of Earthquakes Page 18 Frequency Content - Tripartite Plot Tuesday, September 15, :02 PM

19 Lecture 3 - Engineering Measurement of Earthquakes Page 19 Frequency Content - Response Spectrum Tuesday, September 15, :03 PM

23 Lecture 3 - Engineering Measurement of Earthquakes Page 23 Duration Tuesday, September 15, :10 PM

24 Lecture 3 - Engineering Measurement of Earthquakes Page 24 Attenuation Relations Monday, September 14, :07 AM Attenuation relations describe how the strong motion attenuates (diminishes or decreases) as a function of earthquake magnitude (M) and distance from the earthquake source (R). The are given in terms of an elastic acceleration response spectrum of a single degree of freedom system with 5 percent damping.

25 Lecture 3 - Engineering Measurement of Earthquakes Page 25 Next Generation Attenuation Thursday, September 10, :15 AM

26 Lecture 3 - Engineering Measurement of Earthquakes Page 26 NGA Requirements Thursday, September 10, :19 AM

27 Lecture 3 - Engineering Measurement of Earthquakes Page 27 NGA Database Thursday, September 10, :43 AM

28 Lecture 3 - Engineering Measurement of Earthquakes Page 28 Abrahamson & Silva Attenuation Relation Thursday, September 10, :55 AM

29 Lecture 3 - Engineering Measurement of Earthquakes Page 29 Measurement of R Thursday, September 10, :17 AM

30 Lecture 3 - Engineering Measurement of Earthquakes Page 30 Parameters for A&S Attenuation Relation Thursday, September 10, :27 AM