Overview of lecture Acquisition of Data Measures of ground motion Processing of Data Properties of Data from Observations –Data distribution in M-D space –Path dependence of ground motion –Magnitude dependence of ground motion –Site dependence of ground motion

The first known instrument for earthquakes measurement is the Chang seismoscope built in China in 132 B.C. Balls were held in the dragons’ mouths by lever devices connected to an internal pendulum. The direction of the epicenter was reputed to be indicated by the first ball released. MEASURING EARTHQUAKES

Jargon seismoscope – an instrument that documents the occurrence of ground motion (but does not record it over time) seismometer – an instrument that senses ground motion and converts the motion into some form of signal accelerometer – a seismometer that records acceleration, also known as strong ground motion geophone – another name for a seismometer, commonly used in active source seismology

More Jargon seismograph – a system of instruments that detects and records ground motion as a function of time seismogram – the actual record of ground motion produce by a seismograph seismometry – the design and development of seismic recording systems data logger – device that converts analog to digital signal and stores the signal

How Seismometers Work Fundamental Idea: To record ground motion a seismometer must be decoupled from the ground. If the seismometer moves with the ground then no motion will be recorded.

Principles of seismographs

Strong-Motion Accelerographs Analog

Magnification curves Not shown: broadband (0.02—DC sec)

The nature of the seismogram and the waves shown depends directly on the type of seismograph

It is easier to make a stable, small short-period oscillator than a long-period oscillator. Note that modern strong-motion sensors use force-balance accelerometers with resonant frequencies near 50 Hz, where the quantity being measured is the current in a coil required to keep the mass centered. This current is proportional to the force on the mass. “Broadband” seismometers (velocity sensors, using electronics to extend the frequency to low values) are starting to be used in engineering seismology: the boundary between traditional strong-motion and weak- motion seismology is becoming blurred.

Digital strong-motion recording Broadband: nominally flat response from dc to at least 40 Hz –But noise/ baseline problems can limit low-frequency information –High-frequency limit generally not a problem because these frequencies are generally filtered out of the motion by natural processes (exception: very hard rock sites) High dynamic range (ADC 16 bits or higher) Pre-event data usually available

Trifunac & Todorovska (2001)

Many networks of instruments, both traditional “strong-motion” and, more recently, very broad-band, high dynamic- range sensors and dataloggers

digital instruments installed after the Kobe earthquake of 1995 free field stations with an average spacing of 25 km velocity profile of each station up to 20 m by downhole measurement data are transmitted to the Control Center and released on Internet in 3-4 hours after the event INSTRUMENTATION Kyoshin Net: Japanese strong motion network

Reminder: Play Chuettsu and Tottori movies

A number of web sites provide data from instrument networks But no single web site containing data from all over the world. An effort is still need to add broad-band data into the more traditional data sets.

WEB SITES COSMOS Consortium of Organizations for Strong - Motion Observation Systems

Measures of ground-motion intensity for engineering purposes PGA, PGV Response spectra (elastic, inelastic) Others (avg. spectra over freq., power spectra, Fourier amplitude spectra) Time series

Peak ground acceleration (pga) easy to measure because the response of most instruments is proportional to ground acceleration liked by many engineers because it can be related to the force on a short-period building convenient single number to enable rough evaluation of importance of records BUT it is not a measure of the force on most buildings and it is controlled by the high frequency content in the ground motion (i.e., it is not associated with a narrow range of frequencies); records can show isolated short-duration, high-amplitude spikes with little engineering significance

Peak ground velocity (pgv) Many think it is better correlated with damage than other measures It is sensitive to longer periods than pga (making it potentially more predictable using deterministic models) BUT it requires digital processing (no longer an important issue)

Peak ground displacement (pgd) The best parameter for displacement-based design? BUT highly sensitive to the low-cut (high-pass) filter that needs to be applied to most records (in which case the derived pgd might not represent the true pgd, unlike pga, for which the Earth imposes a natural limit to the frequency content). For this reason I recommend against the use of pgd.

Elastic response spectra (many structures can be idealized as SDOF oscillators)

At short periods, oscillator response proportional to base acceleration

At long periods, oscillator response proportional to base displacement

convert displacement spectrum into acceleration spectrum (multiply by (2π/T) 2 ) Acceleration spectrum usually used in engineering

At short and very long periods, damping not significant

PGA generally a poor measure of ground-motion intensity. All of these time series have the same PGA:

But the response spectra (and consequences for structures) are quite different (lin-lin and log-log plots to emphasize different periods of motion):

Data Processing Data processing = removing long-period noise Processing at high frequencies of much less concern

Baseline problems are common Even for digitally recorded records There can be many reasons for the shifts, and as a result it is not possible to design a single correction scheme to remove the long-period noise without affecting the long-period signal.

Many possible causes Mechanical: –Hysteresis (mechanical/ electrical) –“Popcorn” noise –Other Ground deformation –Tilt near earthquakes –Differential settlement –Other Analog-Digital Conversion (ADC)

Seismologists may want residual displacements. Schemes have been tailored that claim to produce these. Although OK in some cases of large signal to noise, in general I am pessimistic about being to remove long- period noise and retain long-period signal.

A possible correction scheme Modification of one proposed by Iwan et al. (1985) Guarantees that velocity will have a value around 0.0 in the later part of the record (a physical constraint) Choice of critical parameters is arbitrary unless they can be associated with a physical mechanism (as for the specific instrument studied by Iwan et al.)

Although the results look physically plausible, the residual displacements can be sensitive to t1, t2

But response spectra at periods of engineering interest can be insensitive to the baseline correction, which says that the “noise” is very long period If abandon desire to recover residual displacements, then many methods are available for removing long- period noise, in addition to baseline correction: filtering, polynomial fits, combinations of above.

in this case Although the results look physically plausible, the residual displacements can be sensitive to t1, t2 Quadratic fit to velocity gives best fit to GPS residual displacement in this case

In spite of large differences in waveforms, the response spectra at periods of engineering interest are similar. Two general conclusions to be made here: –Filtering alone is often all that is needed –Response spectra at periods of engineering interest are often insensitive to filter cutoff periods for modern digital records

More examples, comparing displacements and SD from accelerograms and “high-rate” (1 sps) gps

From Wang et al.

From Guoquan Wang

Still have many analog records, for which choosing the filter corner can be very important if want long-period response spectra (e.g., project in Italy to determine regression equation for T=10 s SD)

Choosing Filter Corners Choosing filter corners often guided by –Shape of Fourier acceleration spectrum (look for f^2 slope) –Appearance of displacement waveforms (do they “look reasonable”?)

Choosing Filter Corners near- and intermediate-field contributions to ground displacement can fool our ideas of what is “normal” or “reasonable”

Note very different shape for EW, NS components, and peculiar shape for NS waveform until fc=0.16 Data from C. Di’Alessandro

But data from a nearby station (2 km) shows that the “peculiar” features are real, and suggest that a filter somewhere between 0.04 and 0.08 is appropriate

To convince you that differences are independent of acausal filter transients:

In spite of the resemblance of the displacement traces, the response spectra are less similar than I would have expected, demonstrating that the spatial variation in ground motions can be large

Characteristics Characteristics of Data Magnitude-Distance depends on region Change of amplitude with distance for fixed magnitude Change of amplitude with magnitude after removing distance dependence Site dependence Scatter

Observed data adequate for regression except close to large ‘quakes (the recently developed NGA database contains such records, primarily from Taiwan and Turkey) Observed data not adequate for regression, use simulated data

Path effects Wave types –Body (P, S) –Surface (Love, Rayleigh) Amplitude changes due to wave propagation –Geometrical spreading (1/r in uniform media, more rapid decay for velocity increasing with depth) –Critical angle reflections –Waveguide effects Amplitude changes due to intrinsic attenuation (conversion to heat) and scattering attenuation

How does the motion depend on distance? Generally, it will decrease (attenuate) with distance But wave propagation in a layered earth predicts more complicated behavior (e.g., increase at some distances due to critical angle reflections (“Moho-bounce”) Equations assume average over various crustal structures

More distant data show limitations of function fit to closer data (but ground motions at greater distances are of little engineering interest) From V. Graizer

Scaling with magnitude

Chi-Chi data are low at short periods (note also scatter, distance dependence) Illustrating distance and magnitude dependence

And so are Denali and Kocaeli (or is Northridge high?)

Magnitude scaling Note that each symbol represents the average of the strong-motion recordings for a single earthquake, correcting for the distance dependence. (note strong correlation of events breaking to the surface with magnitude)

Magnitude scaling quadratic fit (used for empirical ground-motion prediction equations) Note decrease of motion with increase of M for T = 0.1 and 0.3 sec

Site Effects, Basin Waves,Spatial Variability, Azimuthal Dependence

People have known for a long time that motions on soil are greater than on rock e.g., Daniel Drake (1815) on the New Madrid sequence: –"The convulsion was greater along the Mississippi, as well as along the Ohio, than in the uplands. The strata in both valleys are loose. The more tenacious layers of clay and loam spread over the adjoining hills … suffered but little derangement."

Steidl

2002 M 7.9 Denali Fault

Site Classes are based on the average shear- wave velocity in the upper 30 m (discussed later).

pulses 1 & 2: subevent 1 pulse 3: subevent 2 pulse 4: subevent 3

Basin Waves

"It is an easy matter to select two stations within 1,000 feet of each other where the average range of horizontal motion at the one station shall be five times, and even ten times, greater than it is at the other” John Milne, (1898, Seismology) Spatial Variability

Comparing the 1966 and 2004 Aftershocks Both Earthquakes Ruptured the Same Segment

But with Some Important Differences

Most Extensively Observed Earthquake to Date in the Near- Fault Region

Potential Contributing Factors to the Observed Ground Motion Site conditions Rupture propagation Stopping phases Prestress (“Asperities”) Fault geometry

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Azimuth-dependent amplification of weak and strong ground motions within a fault zone strong ground motions within a fault zone (Nocera Umbra, central Italy) (Nocera Umbra, central Italy) G. Cultrera, A. Rovelli, G. Mele, R. Azzara, A. Caserta, and F. Marra (2003)

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Magnitude scaling Expected scaling for simplest self- similar model (to be discussed later)