Investigation of a fault zone in urban environments by shallow reflected and reflected-converted waves

Shallow seismic surveys using reflected and reflected-converted waves were carried out in an area underlain by several presently unmapped faults in the northwestern district of Taipei metropolis. We used three-component geophones of 4.5 Hz natural frequency to record P, P-SV, and S-wave reflection data which were 50 m, 42 m and 40 m profiles, respectively. Field-recording parameters were planned according to the restricted space of the urban area. Our data processing demonstrates the advantages of employing filtering techniques in multifold sequences to attenuate complex urban noise. Three seismic profiles of reflected and reflected-converted waves recorded along the same survey line show different lateral and vertical resolutions. Reflection events on the northwest side of our S-wave profile differ in reflection character and frequency from those on the southeast side at the same time interval. Diffraction patterns in the raw S-wave data, together with variations in the velocity structure of S- and P-SV-wave data, suggest that the Chin-Shan fault may extend southwest to the Kuan-Tu plain of the Taipei Basin. The fault that we imaged could be an antithetic fault associated with the master Chin-Shan fault. The fault plane dips about 50°to 55°toward the southeast.