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For #SeismogramSaturday #SeismoTwitter I thought it would be interesting to talk about a lesser known seismic phenomenon called “slow-rupture tsunami earthquakes, or simply “tsunami earthquakes”.
There’s been some great info about the M 7.5 Ecuador earthquake a couple days ago, and this topic will discuss frequency content of earthquakes. Notably why earthquakes with the same magnitude may be very different from one another.
Tsunami earthquakes differ from other M 9+ tsunamigenic earthquakes such as the 2004 Indian Ocean earthquake and the 2011 Japanese earthquake. Tsunami earthquakes have more moderate-size magnitudes but produce large tsunami for their size.
Tsunami earthquakes are typically reverse (thrust) faulting earthquakes (earthquake.usgs.gov/learn/animatio…) that occur in offshore regions.
The key characteristic that differentiates tsunami earthquakes from other earthquakes is the speed at which the rupture propagates. A fault will have a certain length, and the rupture speed is how fast the rupture travels from where it started.
Typical megathrust tsunamigenic earthquakes have rupture speeds ~2.5-3.5 km/s. Tsunami earthquakes have rupture speeds ~1 km/s, which means the rupture will have a very long duration.
Tsunami earthquakes are not common, but there are a few known cases since modern digital seismometers networks have been around. I’ll show an example from a tsunami earthquake in August 2012 off the coast of El Salvador.
I’ll compare this tsunami earthquake with a regular thrust faulting earthquake at almost the same location. In all plots the tsunami earthquake waveform is black, and the regular earthquake is red.
Both earthquakes had the same body-wave magnitude (mb), but different moment magnitudes (Mw). Mw is calculated at very low frequencies and represents the fault area and its displacement, while mb is measured at higher frequencies.
Another distinguishing feature of tsunami earthquakes is they have Mw much larger than other magnitude measurements, e.g. mb. In this case the tsunami earthquake was Mw 7.3 and mb 5.8, while the regular earthquake was Mw 6.2, mb 5.8.
This plot is 30 minutes of velocity data from the two earthquakes. It’s easy to see that the duration of the tsunami earthquake is much longer than the regular earthquake. After 30 minutes there is still noticeable signal from the tsunami earthquake. 
This plot zooms in more looking at the first 10 minutes of velocity data. Velocity data will show the speed of particle motion during the rupture. The two waveforms are also plotted together to give a better idea of the scale.
It’s easy to see that the regular earthquake has a much higher particle velocity than the tsunami earthquake. What’s also noticeable is a lack of high frequencies in the tsunami earthquake signal and the presence of a very strong low frequency signal.
Looking at the equivalent displacement waveforms shows the tsunami earthquake having much larger particle displacements than the regular earthquake. The large displacement is the cause of the tsunami as it displaces a large section of the seafloor.
A plot of the displacement spectra for the two earthquakes shows how the frequency content differs. The tsunami earthquake has a lot of energy at low frequencies and drops off considerably at high frequencies.
The regular earthquake has more high frequency energy, but far less low frequency energy. Also shown are the typical frequency bands where Mw and mb are measured.
It’s easy to see how the tsunami earthquake would have a much larger Mw than the regular earthquake, but the mb would be about the same or a bit larger for the regular earthquake.
The lack of high frequency energy in tsunami earthquakes is important. High frequency energy is what people will feel strongly. Therefore, a tsunami earthquake might be weakly felt or not felt at all, which means people may not realise a large tsunami may have been produced.
A big problem with these earthquakes is Mw is more difficult to calculate than other magnitudes. Therefore, it may not be available quickly, if it all.
This leads to severe underestimation of the true size of these earthquakes which can be disastrous if a large tsunami is produced. There may be little or no warning.
