At 8:18 P.M. of 17 August 1983, an earthquake with a magnitude of 5.3 (Ml) on the Richter Scale and an intensity of VII on the Rossi-Forel Scale hit the province of Ilocos Norte. The tremor was perceptible over a distance of 400 kilometers from the epicenter. This was the most sever earthquake in North-western Luzon in 52 years and probably the second largest earthquake event to hit Laoag city and it's immediate vicinity in historical times. This earthquake has caused death of 16 people and injuries of forty seven persons (PDE).
Date of Event August 17, 1983
Origin Time 8:17 pm (12:17 GMT)
Epicenter 18.231 N Latitude 120.860 E Longitude or approximately 30 aerial kilometers east-northeast of Laoag City.
Magnitude 6.5 Ms ( 5.3 Ml on the Richter Scale)
Depth approximately 42 km from the surface.
Intensity Report:
Intensity VII Laoag City, Pasuquin- Ilocos Norte
Intensity VI Vigan-Ilocos Norte
Intensity V Aparri-Cagayan, Santa-locos Sur
Intensity IV Tuguegarao-Cagayan, Baguio City
Intensity III Dagupan City, Callao-Cagayan, Manila
Note: Intensity scale used in these observation was the Rossi-Forel Earthquake Intensity Scale.
Historical Background:
Since 1862 upto 1981, (excepting the years 1941 to 1949) fifty-six earthquakes have affected Laoag City. Of these, the strongest was recorded on 19 March, 1931. This earthquake reportedly had an intensity of VII - IX. Prior to the 17 August earthquake, two tremors were recorded on the eleventh and the thirteenth of August 1983. These were believed to be foreshocks of the intensity VII earthquake (Macalincag, T. G., personal communication). The first had an intensity of V and the succeeding one an intensity of II in the Rossi-Forrel Scale.
Summary of Damages:
Damages on buildings:
A number of reinforced concrete buildings either totally crumbled or sustained major structural damage beyond rehabilitation. The failure in most of the damaged buildings can be attributed to shear and compressional waves, thereby producing horizontal and vertical stresses. The most heavily damaged structures in Laoag City are those situated near the Laoag River flood plain and along reclaimed stream channels. These buildings were condemned by the City Engineer's Office. Nearly all the damaged buildings in the area were of reinforced concrete frame. Most of the external walls and internal partitions were of concrete hollow blocks. There are however, some buildings with wood partitions.
List of buildings that totally collapsed, suffered severe structural damage, sustained considerable damage or considerable non-structural damage:
Laoag City The Laoag Cathedral
Sinking Bell Tower
Yabes Juan Department Store Building
Denson Building
Golden Hardware Building
Laeno Building
Avenue Lumber and Hardware
Philippine Veterans Bank Building
Teresita Building
Philippine National Bank Building
Shirley Building
Far East Building
Sunrise Building
Isabel Building
SE Asia building
Julian Building
Pichay's Building
Castro Building
Mique Residence
Gerardo Building
E. Ang Residence
San Nicolas Kaunlaran Building
San Nicolas Church and Bell Tower
Bacarra Bacarra Church and Bell Tower
Sarrat Sta. Monica Church and Bell Tower
Sarrat Municipal Hall
Marcos Guesthouse
Vintar Vintar Church
San Nicolas Academy
Vintar Municipal Hall
Burgos Bojeador Lighthouse
Minor Effects:
Landslides: Several earthquake induced landslides were observed in places where the slopes along road cuts were steep to very steep. This condition had been aggravated by prolonged rainy days, absence of vegetation to hold the soil, moderately weathered and indurated rocks. Areas affected by landslides were the Sarong Valley in Vintar and Patapat Mountains in Pagudpud, both in Ilocos Norte.
Sandboils or Sandblows: Several sandboils were reportedly observed in Barangay Zamboanga, Laoag City; Barangay Puyupuyan, Pasuquin; and Barangay Calayab, Paoay. The diameter of their craters vary from a few centimeters to 2.5 meters. Sandblows or sandboils are the spouting of hydrated sand caused by moderate to severe earthquakes. This connate water that has been entrapped in the interstices of sediments at the time of deposition may have come from either South China Sea or Laoag River.
Differential Settlement: Majority of the bridges in Ilocos Norte had experienced differtial settlement of approach and or abutments. Some of the buildings were also observed to have differential settlement in addition to being out of plumb. Step fractures due to collapse of foundation were observed at Marcos Guesthouse in Sarrat. Magnitude of differential settlement measured range from a few centimeters to approximately 30 centimeters.
Shear Fractures: A tilted road pavement along J. P. Rizal Street, Laoag City was observed after the main tremor. Gaping tension fractures along Vintar-Bacarra Road and along asphalt pavement on the southern approach of Bacarra Bridge were also observed. Gaping Step tension fracture along Vintar Poblacion-Tamdagan road was found. Numerous irregular cracks and small fissures were discovered along seashores, river banks and alluvial fans.
References:
Santiago, N.G and Rillon, E.A ( December 1983): Assessment on the effects of the August 17, 1983 Earthquake in Laoag City: Bureau of Mines and Geo-sciences.
Valenzuela, R.G. and Garcia, L.C. (10 October 1983) Laoag Earthquake of 17 August 1983 SummaryReport: PAGASA.
