Observed and Modeled Currents from the Tohoku-oki, Japan and other Recent Tsunamis in Northern California

摘要

We investigate the currents produced by recent tsunamis in Humboldt Bay and Crescent City, California. The region is susceptible to both near-field and far-field tsunamis and has a historic record of damaging events. Crescent City Harbor, located approximately 100 kms north of Humboldt Bay, suffered US $28 million in damages from strong currents produced by the 2006 Kuril Islands tsunami and an additional US $26 million from the 2011 Japan tsunami. In order to better evaluate these currents in northern California, we deployed a Nortek Aquadopp 600 kHz 2D acoustic Doppler current profiler (ADCP) with a 1-min sampling interval in Humboldt Bay, near the existing National Oceanic and Atmospheric Administration (NOAA) National Ocean Service (NOS) tide gauge station. The instrument recorded the tsunamis produced by the Mw 8.8 Chile earthquake on February 27, 2010 and the Mw 9.0 Japan earthquake on March 11, 2011. One other tsunami was recorded on the Humboldt Bay tide gauge during the period of ADCP operation, but was not visible on the ADCP, suggesting a threshold water level value of about 0.2 m to produce an observable ADCP record. The 2010 tsunami currents persisted in Humboldt Bay for approximately 30 h with peak amplitudes of about 0.35 m/s. The 2011 tsunami signal lasted for over 40 h with peak amplitude of 0.84 m/s. The strongest currents corresponded to the maximum change in water level approximately 67 min after the initial wave arrival. No damage was observed in Humboldt Bay for either event. In Crescent City, currents for the first three and onehalf hours of the 2011 Japan tsunami were estimated using security camera video footage from the Harbor Master, approximately 70 m away from the NOAA-NOS tide gauge station. The largest amplitude tide gauge water-level oscillations and most of the damage occurred within this time window. The currents reached a velocity of approximately 4.5 m/s and six cycles exceeded 3 m/s during this period. Measured current velocities both in Humboldt Bay and in Crescent City were compared to calculated velocities from the Method of Splitting Tsunamis (MOST) numerical model. The frequency and pattern of current amplification and decay at both locations are replicated by the MOST model for the first several hours after the tsunami onset. MOST generally underestimates 2011 peak current velocities by about 10-30 %, with a few peaks by as much as 50 %. At Humboldt Bay, MOST predicted attenuation of the signal after 4 h but the actual signal persisted at a nearly constant level for at least twice as long. The results from this project demonstrate that ADCPs can effectively record tsunami currents for small to moderate events and can be used to calibrate and validate models (i.e., MOST) in order to better understand hazardous tsunami conditions within harbors.