Seattle Fault Zone - marine seismic reflection surveys
ACTIVE TECTONICS OF THE SEATTLE FAULT AND CENTRAL PUGET SOUND, WASHINGTON — IMPLICATIONS FOR EARTHQUAKE HAZARDS
MARINE SEISMIC-REFLECTION SURVEYS
High-resolution seismic-reflection data
Our investigation relies mainly on a network of multichannel, high-resolution seismic-reflection data collected in 1995 and 1997 by the U.S. Geological Survey. Approximately 320 km of data were collected in the vicinity of the Seattle fault (Fig. 2A) at typical survey speeds of 5.5 to 9 km/hr. For the 1995 survey, the seismic source consisted of two 655 cm3 airguns fired at 12.5 m-intervals. Data were digitally recorded for 2 s using a 24-channel (6.25-m group interval; 150-m active length) streamer. Resulting common mid-point (CMP) stacked data are 6-fold and have a 3.125-m CMP spacing. For the 1997 survey, the seismic source consisted of a two-chambered, 1147 cm3 airgun fired at 20 m intervals, and data were digitally recorded for 2 s using a 24-channel (10-m group interval; 240-m active length) streamer. CMP stacked data are 6-fold and have a 5-m CMP spacing. Both 1995 and 1997 data were deconvolved and filtered before and after stack, then time-migrated using a smoothed velocity function. Data quality is typically good in the upper 1 s and degrades significantly between 1 and 2 s.
Industry conventional seismic-reflection data
Proprietary industry seismic-reflection data (3 to 5 s records) in the vicinity of the Seattle fault (Fig. 2B) have been displayed and described by Johnson and others (1994, 1996) and Pratt and others (1997). Most of these data were collected in the late 1960’s or early 1970’s when the area was considered a frontier for petroleum exploration. In this paper, we show a few east-west lines near the Seattle fault.
