Millihertz Ground Motion at the Seafloor Excited by Large Regional Earthquakes

Date: 4/24/2019

Time: 04:15 PM

Room: Grand Crescent

The effects of sedimentary basins on strong ground motion have been recognized through the amplification and excitation of long-duration coda. Most studies evaluating these effects using seismograms have been land-based. Ocean bottom seismometers have recently been deployed in several subduction zones, that commonly use high-sensitivity and/or broadband sensors in the ocean bottom networks, which makes it difficult to measure large or strong ground motion without saturating the recorded seismograms. Herein, we analyze the ground motion retrieved from ocean bottom pressure gauges (OBPGs) as strong-motion seismometers, and discuss the effects of shallow submarine structures on the amplification and excitation of coda waves, particularly the Hikurangi Trough accretionary wedges.

The dynamic pressure changes (caused by strong motion) on the OBPGs can be converted to ground motion acceleration, based on the ground motion frequency and the water depth of a given OBPG. The water depth can be expressed in terms of the fundamental acoustic resonant frequency, which depends on the velocity of the ocean acoustic wave and the water depth above the OBPG. When the ground motion frequency is sufficiently low compared to the acoustic resonant frequency, the vertical acceleration of the ground motion at the seafloor can be estimated from the dynamic pressure change observed on the OBPG.

We investigated the ocean bottom seismograms of both M7.8 Kaikoura and M7.1 Te Araroa earthquakes from ocean bottom pressure recorders. Our results show that anomalous long-period waves, dominating in the range of 0.005–0.05 Hz, occurred near the coastline. However, the long-period waves have not been observed at onshore stations. This suggests that infragravity, or ocean surface gravity waves, might have been generated by the ground motion at shallow water depths, near the coastline.

Presenting Author: Yoshihiro Ito

Authors