Measuring Possible Tsunami Currents from the April 1, 2014 Mw 8.2 Chile Earthquake in Crescent City, California
Monday, 15 December 2014
Crescent City, California has a long history of damaging tsunamis. Thirty-nine tsunamis have been recorded since 1933, including five that caused damage. Crescent City’s harbor and small boat basin are particularly vulnerable to strong currents. Humboldt State University has installed Acoustic Doppler Profilers (ADPs) in order to directly measure water pressure fluctuations and currents caused by tsunamis. An instrument in Humboldt Bay, ~100 km south of Crescent City, recorded tsunamis generated by the 2010 Mw 8.7 Chile and 2011 Mw 9.0 Japan earthquakes and demonstrated the usefulness of ADPs in measuring tsunami currents. In 2013, an ADP was deployed in Crescent City’s harbor adjacent to the NOAA tide gauge. On April 1, 2014, a Mw 8.2 earthquake occurred in northern Chile, producing a modest Pacific-wide tsunami and a 16 cm peak amplitude on the Crescent City tide gauge. We analyze the ADP data before and during the expected arrival of the April 2 tsunami to see if a tsunami signal is present. Tidal currents are generally small (5 cm/s or less). For two months before the tsunami, intermittent, high-frequency variability is present in velocity and pressure at periods on the order of 20, 9 and 5 min, which compare favorably to modal periods predicted using some simplified models of open-ended basins. For several hours after the tsunami arrival on April 2, spectral power levels in velocity and pressure around the 20 min period are notably enhanced. These results suggest that: (1) the observed periods of enhanced variability represent the first three modes (n=0, 1 and 2) of free oscillations in the harbor, (2) the dominant period of (non-tidal) oscillations observed during the April 2, 2014 tsunami (~20 min) and during previous tsunamis (e.g., the water level record for the March 11, 2011 tsunami; also ~20 min) represents harbor resonance corresponding to the lowest order mode, and (3) this event is very near the ADP limit of detectability with peak tsunami currents of 5-10 cm/s and higher frequency variability and instrument noise root-mean-squared amplitude of 4-5 cm/s.