 MINDANAO, PHILIPPINES EARTHQUAKE
August 17, 1976
EERI
Earthquake Engineering
Research Institute
Reconnaissance Report
The EERI Reconnaissance Team
James L. Stratta, Team Leader .Ted J.Canon
C. Martin Duke • Lawrence G. Solna
Contributing Authors
J. Eugene Haas • David J. Leeds
August 1977
Inside:
CONTENTS
1. INTRODUCTION
Fore shocks
Aftershocks
Acceleration
Macroseimic Reports
2. SITE CONDITIONS AND SOIL EFFECTS
3. DAMAGE TO LIFELINES
Transportation Communication Water
Energy
4. TSUNAMI DAMAGE
5. DAMAGE TO BUILDINGS rN COTABATO CITY Building Damage Observed,
Orientation of Earthquake Effects
6. DAMAGE IN ZAMBOANGA CITY
7. RESIDENTIAL DAMAGE'
8. BRIDGE DAMAGE Quirino Bridge
Tamontaka Bridge Comments
9. SUMMARY AND OBSERVATIONS
10. SOCIAL SCIENCE OBSERVATIONS ON THE RESPONSE OF COTABATO CITY
Patterns of Authority
Relief
Reconstruction Societal Values Conclusions
11. MINDANAO SEISMICITY
Selected Mindanao Earthquakes References
Pages
1. INTRODUCTION
A major earthquake occurred in Mindanao in the Philippine Islands on August 17, 1976. A map of the Philippines (see frontispiece) shows that the island of Mindanao lies at the southernmost tip of the island group, some 500 miles south of Manila.
Seismological reports of the main shock develop a refinement of the epicenter location with additional data. The most complete report to date is the USGS "Earthquake Data Report" (EDR 21-76, Apr 15, 1977, pp. 11-14), in which 231 observations are summarized, as follows:
UTC (GMT) time Aug 16 1976 16:11:07.30 ±0.07 sec
Local (120° EMT) time Aug 17 1976 00:11:07. 30 ± 0.07 sec
Latitude 6. 262°N ± 1.78 km
Longitude 124. 0230E ± 2.12 km
Depth of focus 33. 0 km (normal)
Magnitude
MB 6.4 (46 observations)
MSZ 7.9(8 observations)
MS-BRK 7.9
MS-PAS 7.7
The Philippine National Geophysical and Astronomical Service (PAGASA.) records show strong, remarkably recorded surface waves, indicating that the depth of focus was rather shallow. The depth, therefore, may be somewhat shallower than the "controlled" depth given in the EDR cited above. The PAGASA preliminary origin time is given as 16:11:44. 2, some 33 seconds later than the EDR time. This difference is attributed to the USGS interpretation of a foreshock preceding the main event. The location of the epicenter is offshore in the Moro Gulf approximately 100 km south of Cotabato City and about 12 km west of Cadiz Point. The entire Mindanao region is shown in Figure 1.
The PAGASA determination places the epicenter farther out into the gulf. The earthquake was felt as far north as the Visayas Islands and the southeastern tip of Luzon, and as far south as Borneo.
Fore shocks
Approximately a month before the major earthquake of August 17, 1976, two tremors were reported felt in Zamboanga City, near the epicentral area of the main shock. These preliminary tremors were not recorded at the Geophysical Observatory in Quezon City nor in any of its field stations. Analysis of seismic records for August prior to the earthquake showed that six tremors were recorded that had epicenters within the same area as the main shock. This brought to eight the total number of tremors which may be considered as foreshocks of the Moro Gulf
1
X
PREFACE
An Earthquake Engineering Research Institute reconnaissance team was appointed by President Henry J. Degenkolb on August 18, 1976 to assess the engineering aspects of the earthquake that occurred on the island of Mindanao in the Philippines on August 17, 1976. The EERI team consisted of James L. Stratta, team leader; Ted ,J, Canon, C. Martin Duke, and Lawrence G. Selna.* They were joined by two investigators from Canada, W. K, Tso and S. M. Uzumeri. These six departed San Francisco on August 21, A few days later EERI sent J. Eugene. Haas to examine the socio-economic aspects, and the United States Geological Survey (USGS) sent Robert E. Wallace to reconnoiter the scientific aspects of the earthquake. The original four team members arrived back at San Francisco on August 30.
This report, then, is the work of the original four investigators, augmented by notes on seismicity by David J. Leeds and social science observations by
J. Eugene Haas, as well as extracts from the preliminary report prepared by the Philippine National Geophysical and Astronomical Service (PAGASA) under the direction of Roman L. Kintanar, Administrator,
The present report is somewhat more comprehensive than the usual reconnaissance report. Because the amount and type of damage did not appear to warrant such detailed analyses as were made in some past major. earthquakes, the team concluded in the field that it could and should make an investigation and report of sufficient scope to document the earthquake in an engineering sense and to "calibrate" it relative to recent destructive United States and Latin America quakes. President Degenkolb was cabled from Manila to this effect on August 23, after the team had met with knowledgeable Philippine engineers.
Effective results could not be obtained by EERI teams investigating foreign earthquakes without the aid of local jurisdictions. The team wishes to express its appreciation to all who aided it, with special mention of the following:
* Affiliations
James L. Stratta, Simpson, Stratta & Associates, Menlo Park Ted J. Canon, H. J. Degenkolb & Associates, San Francisco C. Martin Duke, University of California, Los Angeles
Lawrence G. Selna, University of California, Los Angeles
J. Eugene Haas, Human Ecology Research Services, Inc. , Boulder David J. Leeds, David J. Leeds & Associates, Los Angeles Wai Keung Tso, McMaster University, Hamilton
S. M. Uzumeri, Toronto University, Toronto
Robert E. Wallace, U.S. Geological Survey, Menlo Park
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1) The Philippine Civil and Structural Engineering Associations, who greeted the team in Manila, briefed it on the damage, and arranged for Philippine Army assistance.
2) The Philippine Army, especially Colonel Castro, Commander of Region XII, who personally escorted the team on a helicopter flight over the tsunami areas and the areas damaged by shaking. Lt. Colonel Navera generously coordinated all aspects of the team's work and living accommodations. Colonel Gatmaitan, who was in charge of the Quirino Bridge repair, personally escorted the EERI team through the bridge problems.
3) The Cotabato City Civil Authorities - Mayor Juan J. Ty, City Attorney Ramon Della Fuente, and City Engineer Teodulo A. Ladub, who responded to requests for information with full cooperation and dispatch.
It would be impossible to name all of the Filipinos who aided in the study, but to each one the team extends its heartfelt thanks.
Financial support of the EERI team's travel costs was provided by the National Science Foundation.
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Item Details: Book Condition: Used, good condition Size: 8.5 x 11 - 106 pages Media: Soft cover
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