Abstract

The 16 July 1990 Luzon Earthquake Ground Rupture
Raymundo S. Punongbayan*, Rolly E. Rimando*, Jessie A. Daligdig*
Glenda M. Besana*, Arturo S. Daag*, Takashi Nakata**, and
Hiroyuki Tsutsumi**
*Philippine Institute of Volcanology and Seismology
** Hiroshima University, Japan

ABSTRACT

The 16 July 1990 earthquake (Ms = 7.8) produced a 125 km-long ground rupture that stretches from Dingalan, Aurora to Kayapa, Nueva Vlzcaya as a result of strike-slip movements along the NW segment of the Philippine Fault Zone and its splay, the Digdig Fault. The earthquake epicenter was placed at 15º 42' N and 121º 7' E near the town of Rizal, Nueva Ecija. The surface rupture essentially followed the pre-earthquake active fault trace along previously identified fault-related geologic and geomorphic features such as mole tracks, sag ponds, offset streams and fluvial terraces, shutter and pressure ridges, scarplets, and similar features, with only slight deviations in certain places. Secondary shears are present as localized features along portions of the main rupture trace. Ground rupture, which had a general trend of N 40-60º W, was predominantly left-lateral with measured vertical and horizontal displacements varying from 0.1-2.5 m and 0.2-6.2 m, respectively. Movement is concentrated along the main rupture although right-stepping en echelon faults and trace discontinuities interrupt the narrow fault trace. The spatial distribution of these en echelon faults suggests that these are surface expressions of fault bends and are more pronounced to the SE of the epicenter. Asymmetry of fault length with respect to the epicenter, rupture arrest and displacement distribution can be explained in terms of these rupture propagation barriers. Variation of horizontal and vertical displacement values with distance shows a wavy pattern with some observed scissor-like reversals in the vertical displacement along some segments of the fault. Rupture length and measured maximum horizontal (6.2 m) and vertical (2.5 m) displacements are within the range of values observed worldwide, for earthquakes of this magnitude.

Damage to buildings, infrastructures, and properties amounted to at least P 10B, a part of which was caused by ground rupturing. Structures directly straddling the ground rupture were totally damaged as a result of large lateral shifting and substantial vertical displacement. However, some houses within 1-2 m on either side of the ground rupture survived owing to their light-weight construction while those built of reinforced concrete within this zone suffered partial damage. Damages beyond 2m depended mainly on the structural integrity of the building and effects of local topography and ground conditions. These observations underscore the advantage of using lightweight materials for construction purposes as well as the need to observe sound construction and design of buildings particularly in areas close to the ground rupture and in places that may be affected by future movements along active faults.

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