Longwave measurements for the coast of British Columbia and improvements to the tsunami warning capability

A few years ago the Canadian Hydrographic Service initiated a major upgrade to all tide gauges and tsunami stations on the coast of British Columbia ( B. C.). This program was undertaken to address shortcomings of the earlier digital systems and was driven by concerns about emergency response continuity in the year 2000. By 1999, thirteen tide gauge stations had been installed and were operational. Three of these stations (Tofino, Winter Harbour, and Langara) were selected for use as tsunami warning stations. Several years of continuous, high quality data have now been collected at these stations and used for analysis of long waves in the tsunami frequency band. Careful examination of these data revealed two weak tsunamis recorded by several B. C. stations: a distant tsunami of June 23, 2001 generated by the Peru Earthquake (M-w = 8.4), and a local tsunami of October 12, 2001 induced by the Queen Charlotte Earthquake (M-w = 6.3). Spectral characteristics of these two tsunamis are compared with the spectral characteristics of long waves generated by a strong storm ( October, 2000) and of ordinary background oscillations. The topographic admittance functions ( frequency responses) constructed for all stations showed that most of them ( in particular, Winter Harbour, Tofino, Bamfield, Port Hardy, and Victoria) have strong resonance at periods from 2.5 to 20 min, indicating that these locations are vulnerable to relatively high-frequency tsunamis. The Winter Harbour station also has two strong resonant peaks with periods of 30 and 47 min and with amplification factors of about 7. The estimated source functions show very clear differences between long waves associated with the seismic source ( typical periods 10-30 min) and those generated by a storm, which typically have shorter periods and strong energy pumping from high-frequencies due to non-linear interaction of wind waves.