Small Earthquake Can Also Generate a Massive Tsunami!

Some of the largest tsunamis in history were recorded following unexpectedly small earthquakes along convergent margins. The basic theory of tsunami genesis suggests that the tsunami occurs as the result of rapid seafloor deformation by rupture propagation. Thus theoretically, generation of tsunami along the trench, as well as the estimate of its magnitude, can be predicted quantitatively by analyzing the magnitude of fault slip that can be measured through seismic inversion analysis. However, historical record shows that large tsunamis are not necessarily associated with large earthquake. There has been a significant discrepancy between the magnitude of the earthquake (described as Mw or MS[1]/ML) and the magnitude of the following tsunami (MT[2]). This leads to a conclusion that there must be additional mechanism that is responsible for the generation of the tsunami. Several mechanisms were proposed for this phenomenon, one of which being the submarine landslide.

The mechanism of the submarine landslide has been described very well through the Tohoku earthquake 2011. Strasser (2013) proposed a causal link between earthquake slip to the trench and rotational slumping above a subducting horst structure. Bathymetric observation of the trench area has shown that there exist several subsidence and uplifts showing compensational interaction of about 60m long along the trench within a region of 2-3 km wide. He proposed that as the decollement fault moved and created a slip of 50m, the toe was put on the unconsolidated trench-fill sediment and then collapsed in a rotational slumping way. The rotational slumping mechanism is supported by undisturbed stratigraphic structures of the cores drilled in several sites throughout the tsunami-triggering area, indicating no significant horizontal displacement of sediment caused by geological event. The amount of sediments displaced during this landslide is equivalent to the additional contribution needed for the generation of such massive tsunami.

However, the slip of 60-80m is actually too large to have been caused by thrusting of an earthquake. Through the seismic inversion analysis, it was found that the thrust fault slipped about 20 meters in the hypocenter, which is much smaller than the slip around the toe region of the thrust which varies between 40-80m according to data from bathymetric surveys and ocean-bottom pressure. Then how is this possible? Kawamura (2012) suggested that this must have been due to additional fault slip by large normal faults that merge into the decollement. These normal faults formed rapidly as the results of the rapid elastic response, abrupt transition of basal frictional condition from static to dynamic, which reduces the frictional forces due to thermal pressurization of the region along the basal decollement. As the normal faults emerge, they create additional horizontal slip which makes displacement in the toe region much higher than it is in the hypocenter.

Mechanism above however requires a very crucial geological condition: the trench slope must be steep enough to create an unstable condition for submarine landslide to occur. The steep trench slope is one of the most important signature of the erosive active margin, which distinguished it from accretionary active margin. Erosive margin or non-accretionary margin often also associates with the existence of horst structure near trench on the oceanic floor. Some of the massive tsunamis associated with relatively small earthquake recorded in history coincidently happened along erosive active margins, such as Japan Trench (Meiji Sanriku 1896 & Tohoku 2011), Ryuku Trench (Meiwa 1771) and Costa Rica Margin (Nicoya slump). Therefore, a relatively small earthquake has actually high potential of causing a massive tsunami when it occurs along an erosive active margin.

Reference

Main journal:

Kiichiro Kawamura, Jan Sverre Laberg, Toshiya Kanamatsu, Potential tsunamigenic submarine landslides in active margins, Marine Geology, 2014, 356, 44.

Additional journal:

Kawamura, K., T. Sasaki, T. Kanamatsu, A. Sakaguchi, and Y. Ogawa (2012), Large submarine landslides in the Japan Trench: A new scenario for additional tsunami generation, Geophys. Res. Lett., 39, L05308, doi:10.1029/2011GL050661

Ide, S., Baltay, A., Beroza, G.C., 2011. Shallow dynamic overshoot and energetic deep rupture in the 2011 Mw 9.0 Tohoku-Oki earthquake. Science 332, 1425–1429.

[1] Ms = log (A/T) + 1.66 log Δ + 3.3 (A: amplitude, T: dominate period, measurement frequency: 20, empirical no direct connection to the physics of the earthquake)

[2] Mt = log Hr + log Re + 5.80, Hr: mean tsunami run-up height along a strech of coast, Re: the shortest distance to the epicenter of a tsunamigenic earthqauke