Large Effects of Moho Reflections (SmS) on Peak Ground Motion in Northwestern Taiwan

A total of 336 three-component strong-motion recordings from the Mw 6.35 Niu Dou earthquake of 25 June 1995 at a focal depth of 39.9 km in northern Taiwan are used to study the effects on strong ground motion due to Moho reflection of S waves. The residuals of both horizontal peak ground acceleration (PGA) and peak ground velocity (PGV) recorded from the earthquake are analyzed. The results confirm that many Class E soft soil stations in the Taipei Basin and the Ilan Plain had the expected large amplification of about 1.7 and 1.5 times, respectively, the predicted median PGA values. Surprisingly, a large group of Class C or D dense and stiff soil sites in Taoyuan (TCU007), Lungtan (TCU013), Guanshi (TCU021), Hsinchu (TCU095), and Miaoli (TCU047) areas in northwestern Taiwan had unusually large amplification of about 3.4–8.1 and 1.7–3.3 times the predicted median PGA and PGV values, respectively. They are interpreted in terms of focusing and interference between SmS waves reflected from the horizontal and inclined portions of an eastdipping Moho discontinuity in this area. This interpretation is supported by the close agreement between the expected amplitudes and arrival times of the largest shear waves with the observed data. Our results suggest that when a damaging earthquake occurs near an inclined Moho boundary, the reflected SmS waves can result in significantly amplified ground motions at distances beginning about 50 km. The exact distance range will depend on the thickness of the crust and the dip angle of the Moho boundary.