The Pacific Tsunami Warning Center response to the Mw8.1 Samoan earthquake of September 29, 2009

Over 90% of tsunami-related casualties occur within a few hundred km of the causative event, usually an earthquake. The Mw 8.1 (GCMT) Samoan earthquake and tsunami of September 29, 2009, represents a best-case scenario for response and self-evacuation by a population near the epicenter of a tsunamigenic earthquake. The Samoan population felt over 60 seconds of strong ground shaking and saw the first tsunami wave motion as a recession rather than an onshore wave. Their observations coupled with effective public awareness saved many lives. Such phenomena do not precede all dangerous tsunamis, however, and Samoans may not receive these natural warnings for future local tsunamis. For example, a “tsunami earthquake” (Kanamori, 1972) can generate a destructive tsunami with little or no strong ground motion (cf. Nicaragua 1992 and Java 2006). Furthermore, if the Samoan earthquake had ruptured as a thrust mechanism more typical for the nearby subduction zone, then the first observed tsunami wave would have likely caused inundation, and thus the ocean would not have warned the population. The Pacific Tsunami Warning Center (PTWC) mitigates such hazards by monitoring earthquakes in real time and using semi-automated analysis to rapidly characterize seismic sources for their tsunami-generating potential in order to warn coastlines of any tsunami threats. As part of its mission PTWC also uses a dense local seismic network in order to produce local warnings for the State of Hawaii within 3 minutes of earthquake origin time. In this presentation we detail the analysis and response performed by the PTWC for the Samoan event. We highlight how the current sparse deployment of seismometers in the southwest Pacific Ocean resulted in PTWC issuing a warning 16 minutes after the earthquake's origin time, as compared to what can be done using a denser seismic network. Therefore, we advocate for a denser network of seismometers in the region that will allow the PTWC to halve the time needed to issue tsunami warnings after future earthquakes in the region that may not be as well suited for local response and self-evacuation as this recent event. Currently, there are new and developing seismic networks in Tonga, Fiji and Samoa. These data will be needed to reduce the time lapse between the earthquake and the tsunami warning.