May 25, 2004CSMIP Processing, Shakal et al1 CSMIP Strong Motion Data Processing Anthony Shakal, Moh Huang and Vladimir Graizer California Strong Motion Instrumentation Program California Geological Survey (was CDMG) Sacramento, California

May 25, 2004CSMIP Processing, Shakal et al2 CSMIP Processing Development CSMIP began joint processing project with USGS in late ‘70s (film scanning by Towill Co.- software devel. and processing at Lawrence Berkeley Lab) In early ‘80s standalone processing at CSMIP –Scanning system installed patterned after that developed at Univ. Southern Calif. by Trifunac & Lee –Processing software of Caltech Bluebook project (Hudson et al) as modified by Trifunac & Lee Software upgraded for production, with noise level improvement at CSMIP

May 25, 2004CSMIP Processing, Shakal et al3 Uniform Processing – Guiding filter period selection based on Signal and Noise Spectrum (Trifunac, 1977) Digitized accelerogram as sum of desired acceleration and background noise

May 25, 2004CSMIP Processing, Shakal et al4 Signal spectrum moves up and to the right with increasing magnitude Noise spectrum controlled by –digitization (by film scanner, or by A-to-D converter) –sensor properties Initial filter corner estimate - above & left of junction, an SNR 2 or 3 Final corner guided by time domain output of suite of runs having filter near this period

May 25, 2004CSMIP Processing, Shakal et al5 Current CSMIP procedure is to use one filter corner for all components Pro: Multi-dimensional aspects can be studied by end user –Particle motion –Torsional response in structures Con: Period controlled by the lowest signal/highest noise channel (often vertical, or lowest n building)

May 25, 2004CSMIP Processing, Shakal et al6 Steps in processing (analog) 1.Baseline correction – minimal (remove mean; perhaps remove slope) 2.Instrument correction 3.High-frequency filtering (25 Hz Ormsby classically) 4.Initial integration & long period filtering 5.Maximum-bandwidth response spectra 6.Time-history suite for long-period filter selection 7.Final product preparation

May 25, 2004CSMIP Processing, Shakal et al7 Example – Whittier analog record Digitize at 200 points/cm Digitize two fixed traces for reference-trace subtraction to remove film shift (earthquake) and film drift (canister) problems Use 2 pulse/sec time trace to correct film-speed change errors Digitize fiducial marks placed on film to control multiple-panel concatenation

May 25, 2004CSMIP Processing, Shakal et al8 Example – Digitized (Vol. 1) Digitized, time-corrected, 200 pts/sec, scaled by sensitivity Match-test to film image, check for offsets, drifts, panel-junction effects

May 25, 2004CSMIP Processing, Shakal et al9 Response Spectrum Response spectrum (Nigam-Jennings) of Vol.1 data ‘Wide-open’ bandwidth Compare the long period decay of signal spectrum with long-period noise Initial corner estimate

May 25, 2004CSMIP Processing, Shakal et al10 Suite of time-histories Range of corners from 12 second to 2.5 sec period Period chosen was 3.5 second

May 25, 2004CSMIP Processing, Shakal et al11 Final Accel, Veloc, Displ

May 25, 2004CSMIP Processing, Shakal et al12 Final Spectrum Filter corners given on plot Plotted only out to corner selected

May 25, 2004CSMIP Processing, Shakal et al13 Usable Data Bandwidth 3 dB (half-power) points (whether Ormsby, Butterworth or other filter) define UDB for user User assumed to be knowledgeable, but not necessarily in data processing

May 25, 2004CSMIP Processing, Shakal et al14 Digital Records Frequency domain processing Noise level controlled by A-to-D converter’s effective number of bits (last bits often noise) In general, more dynamic range (72, 96 dB, or more vs ~50-60 dB) Sensor noise/drift more critical – the next focus in getting the most from recorded data

May 25, 2004CSMIP Processing, Shakal et al15 Automatically Processed Record Record processed automatically at the time of the earthquake (2 am Sunday morning, May 9, M4.4 off Santa Barbara).

May 25, 2004CSMIP Processing, Shakal et al16 CISN Internet Quick Report – tied to automatically generated ShakeMap

May 25, 2004CSMIP Processing, Shakal et al17 Summary (1) CSMIP processing evolved from the Caltech Bluebook project as extended at USC Nearly 1000 records digitized/processed (1000s of traces) General approach goal is to release as much signal as possible, with as little noise accompanying the signal as practical Using one filter corner per record means that results may be: –more conservative than another policy would yield; but –less difficult to use, for most users

May 25, 2004CSMIP Processing, Shakal et al18 Summary (2) No acausal filters routinely used; processing of special problem or offset records done by hand, on case-by-case basis, or restricted band pass provided Automatic processing done by straightforward processing, with diagnostic checks/set-asides if potential problems (DC shifts, electronic noise events, etc) Automated processing and Internet Quick Report providing rapid release for response and post-earthquake engineering evaluations.