environmental sciences research institute - University of Ulster

As it is impossible to know in advance the magnitude, or rupture length, of the next earthquake, we simulate 4
possibilities:
1. The rupture terminates at the northern edge of the 1833 event, total length 210 km.
2. The earthquake re-ruptures the area that failed in 1797, length 330 km.
3. The event re-ruptures the regions that slipped in 1797 and 1833, length 630 km.
4. In a worst case scenario, the rupture propagates south past Enggano Island for a total length of 840 km.
A further unknown is the slip distribution of the future event and hence for each rupture length we model about 25
different distributions. In total, then, we simulate about 100 possible earthquakes. For each simulated event, we calculate
the sea-floor displacement and resultant tsunami. An example of the seafloor displacement and tsunami resulting from
one simulated 630 km long earthquake is shown in Figure 2.
Figure 2: Seafloor displacement (left) and tsunami along the coast of Sumatra (right) for one modelled earthquake. Note that
the outer islands (e.g. Siberut) experience uplift in the event whereas the west coast of Sumatra drops by up to a meter. In this
scenario, the maximum wave height is about 5 m with Padang, a city of 840,000 people, experiencing a 3 m high tsunami. The
large variability in tsunami height along the coast line is largely controlled by local bathymetry.
Our 100 simulations led to some remarkable results. The first is that initial peak of the tsunami comes ashore on the
west coast of Sumatra approximately 33 minutes after the earthquake; regardless of the size of the earthquake. This is
illustrated in Figure 3 where we show two very different simulated events yet the time series of the tsunamis are very
similar.
Our second major result is that, in the near-field, there is a one-to-one correspondence between the coastal displacement
resulting from a particular earthquake and the maximum height of the tsunami triggered by it. This is shown in Figure
4 where we plot the wave height vs. coastal displacement at 3 locations. This result suggests that single measure of, for
example, the displacement of a seawall in an earthquake could be used to predict the height of the tsunami peak that
will arrive 33 minutes later.
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