Strong motion earthquake accelerograms, digitized and plotted data, Volume I - uncorrected accelerograms; Part D - accelerograms ID056 through ID070

Abstract

The first set of twenty uncorrected digitized earthquake accelerograms was published in July, 1969, as Volume I, Part A, of the present series. That volume also contained introductory material and background information describing the methods used, accuracies, etc., which should be referred to by all users of the data. The San Fernando Earthquake provided records of sufficient importance that the original plan was interrupted after two parts, and Part C contained the first of the San Fernando Earthquake accelerograms. This issue continues the San Fernando accelerograms and contains fifteen records consisting of three records each from four buildings, two ground level records at the Hollywood Storage Building and the record from the Old Ridge Route at Castaic. Some comments on these records follow. ID056. Castaic, Old Ridge Route. At 0.97 seconds after triggering the drive mechanism malfunctioned briefly and the film was stationary for a short length of time with the recording light on. The actual time is indicated by asterisks in the computer printout and by arrows in the computer plot. The lost portion of trace can be estimated to be one to two seconds long for the following reasons. The gap between the time marks is normally 0.5 seconds, but the particular gap spanning the missing part measures only 0.3 seconds. Hence at least 0.2 seconds is missing. In addition, at least one of the time marks occurred during the pause, so that the possible lengths of the pause are 0.7, 1.2, 1.7, 2.2, ... seconds. The maximum amplitudes of the missing traces can be ascertained from the thickness of the recorded vertical line for two of the three traces and these are less than the peak amplitudes occurring during the following one or two seconds. The changes in density along the recorded vertical line indicate that not a great many of the large oscillations of the trace took place during the pause and that a duration of 1.2 seconds is realistic. Also the records from the Lake Hughes Array Stations 4 and 12, within 26 km of Castaic, and a similar distance from the epicenter, 25-29km, indicate that the strong motion portion in this vicinity was only a few seconds in duration. These complete records show a marked similarity to that portion of the Castaic record outside the lost part, further confirming that not more than one or two seconds was lost. As another means of estimating the time lost, a laboratory test was made on the Castaic accelerograph in which the recording paper was held stationary for various time intervals with the recording light on, The intensities of the resulting traces were compatible with the above estimates. ID057, 58, Hollywood Storage Building, Basement and P. E, Lot. The total record length for the main shock is included. ID059, 60, 61, 1900 Avenue of the Stars, Subbasement, 9th and 21st Floors. The instrument on the 9th floor several times responded with a very high frequency, 80 - 100 cps, lightly damped oscillation, probably initiated by an impact loading on the structure near the instrument. Such impacts arise from dislodged material or equipment, or safety devices operating in elevator mechanisms. These were not digitized as part of the project. The instrument on the 21st floor operated for the same 58 - 60 seconds as the other two, but the first 11 seconds were not recorded. ID062, 63, 64. 1640 South Marengo Street, lst, 4th and 8th Floors. The 1st and 8th floor records were digitized to 54 seconds, by which time the first aftershocks had diminished. The 4th floor record was digitized to 98 seconds, for the total length of the record. ID065, 66, 67. 3710 Wilshire Blvd, Basement 5th and 10th Floors. The instrument on the 10th floor faces a different direction from the other two instruments and the component directions are therefore different. ID068, 69, 70. 7080 Hollywood Blvd., Basement 6th and 12th Floors. The total record length for the main shock is included. As in previous reports the printout and plots contain asterisks and arrows, respectively, indicating where the record was shifted in the digitizer. Two further reports (see References) have been issued at the California Institute of Technology describing the San Fernando Earthquake. "The Engineering Features of the San Fernando Earthquake, February 9, 1971" contains preliminary studies of some of the more important and interesting engineering features of the earthquake. The various chapters were prepared by staff and students working in Earthquake Engineering within the Division of Engineering and Applied Science. "Strong Motion Instrumental Data on the San Fernando Earthquake of February 9, 1971", a joint report with the Seismological Field Survey of the National Oceanic and Atmospheric Administration, U. S. Department of Commerce, presents an overall picture of the instrumentation results, updating and bringing together a number of preliminary reports issued by both the Earthquake Engineering Research Laboratory and the Seismological Field Survey. An account is given there of the performance of the Southern California strong motion networks, including the seismoscope results. Both of these reports are available from the National Information Service for Earthquake Engineering at the California Institute of Technology. We should like once again to express our gratitude to personnel from the Jet Propulsion Laboratory who earlier assisted so capably on the digitizing, and to our regular digitizers and assistants, students and otherwise, for their conscientious work. We would like to acknowledge the general support of the Earthquake Research Affiliates, the generous grants from the National Science Foundation and the cooperation of the Seismological Field Survey of the National Oceanic and Atmospheric Administration.

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