Two papers by researchers from ASE and the EOS Geodesy group, published recently in the same issue of Nature Geoscience, shine new light on the geological processes leading up to the most destructive earthquakes, in particular the ones located far from shore that can generate devastating tsunamis. In view of the new findings, earthquake and tsunami risk assessment need to be revised for coastlines near subduction zones in Asia and worldwide.
The biggest and most destructive tsunami waves are caused by megathrust earthquakes, which occur in subduction zones, where one tectonic plate slips in under another. The converging process is ongoing also between earthquakes. If the converging plates are slipping against each other continuously, stress is released without producing the shaking we associate with earthquakes, and earthquake risk is low.
However, if a portion of the fault gets stuck and stops slipping (while the plates keep converging), stress builds up over time that will eventually need to be released through “catch up slip”, possibly very rapidly and therefore generating an earthquake. Measuring the rate at which the plates are slipping (or not slipping) can reveal if such a slip deficit is building up. Lindsey et al. found a way to predict the slip rate in an inaccessible part of subduction zones, far offshore, where big tsunamis may be generated if this slip happens quickly, causing an earthquake.
In a separate study, Mallick et al. used 200-year-old sea level records from coral growth to discover a “slow slip event”: a slip that happened over the course of 32 years, too slow to generate seismic signals or tsunamis, but faster than regular in-between earthquake slip. This period of slow slip preceded the devastating 1861 Simeulue Island earthquake and tsunami, and is the longest slow slip event ever identified.
The Indo-Australian and Asian tectonic plates converging at the Sunda megathrust. The map shows GPS stations providing information on the Sunda megathrust and the Sumatran fault. Source: https://earthobservatory.sg/facilities/sumatran-gps-array-sugar
New geodetic method for detecting stress in far offshore part of fault where tsunamis are generated
The slip rate of faults can be measured through observations that track how the Earth’s surface moves over time, for example by using highly precise GPS sensors installed on land. However, it is still hard to detect what is going on at faults that are far from land, below kilometers of water, where traditional GPS instruments cannot operate. Knowledge of the behaviour of this part of the fault relies largely on computer models.
Previously it had been assumed that the parts of subduction zones located further out to sea slip independently from the parts closer to the coast. However, Lindsey et al. showed that previous models have failed to take into account the fact that if the part of the fault near the coast gets stuck between earthquakes, the part far offshore can’t move either, even if it has no frictional locking. It is in a “stress shadow”. Consequently, a slip debt builds up, and like any debt it has to be paid off eventually; in this case payday is earthquake day.
Taking this effect into account, the team developed a technique that uses the same land-based data but results in a vast improvement in the ability to “see” the fault slip and detect any slip debt/stress shadow in the areas that are farthest from shore, allowing researchers to reassess the hazard presented by the offshore parts of subduction zones most prone to tsunami generation.
When the team applied the new method to real data from before the 2011 Tohoku earthquake, they found that in contrast to models at the time, their model predicts high risk. "If these areas can slip seismically, the global tsunami hazard could be higher than currently recognised" says Eric Lindsey, lead author of the study and currently an Assistant Professor of the University of New Mexico, who conducted the study during his time as a Research Fellow at EOS.Microatoll from the Latiung village site on southeastern Simeulue, 7 km south of Labuhan Bajau village. Photo credit: Imam Suprihanto (left). Microatoll from the Labuhan Bajau village site on southeastern Simeulue (right). The radial slab that was extracted from the microatoll was 2.6 m long and was used to reconstruct changes in relative sea level and land level. Photo credit: Aron Meltzner.
Coral records revealed world’s longest earthquake off Simeulue Island, Indonesia
A few years ago, researchers from ASE/EOS discovered that the southeastern part of Simeulue Island off the west coast of Sumatra began sinking unexpectedly during the decades leading up to the 1861 Simeulue earthquake and tsunami. In a recent study, led by ASE PhD student Rishav Mallick, they suggest that the cause was a long and slow “earthquake” that lasted for 32 years – the longest such event ever recorded!
This super long “earthquake” didn’t cause any “quaking” because the slip was too slow, but it did release built-up stress. However, the long silent earthquake was followed by a damaging earthquake and tsunami, indicating that though these so-called slow-slip events may not cause strong shaking, they can still lead to earthquakes by affecting nearby faults. The paper suggests that we may currently be missing these drawn-out earthquakes in local and global risk assessments.
The gradual process of accelerating slip rate that began around 1830 and continued for more than 30 years may have relieved tectonic stress on the shallow part of the megathrust but transferred that stress to a neighboring deeper segment, which then failed in a massive earthquake and tsunami in 1861. The authors highlight a potential ongoing drawn-out slow-slip event at Enggano Island, Indonesia, which, if correct, could increase the hazard to nearby communities in Indonesia.
The study site off Simeulue Island is located in the part of the subduction zone further offshore, which is typically “quieter” - it does not produce as many earthquakes. Though that might still be the situation in most cases, Mallick and co-authors knew from Lindsey et al.’s study that this zone was potentially more prone to earthquakes than previously believed, and their study shows that sometimes this zone may slip in other ways. If similar behaviour is observed leading up to earthquakes elsewhere, this process might eventually be recognized as an earthquake precursor.
Data from modern geodetic measuring instruments only go back a couple decades, but in this case nature provided a long-term prehistoric sea-level record in ancient coral microatolls). Growing both sideways and upwards, the disc-shaped coral microatolls are natural recorders of changes in sea level and land elevation, through their visible growth patterns. “Normally these coral microatolls record changes in sea level due to climate change, but if tectonic stresses push the coastline up or down, then the corals will “see” this too” says Asst Prof Aron Meltzner, second author of the study.
“It is interesting just how much we were able to discover from just a handful of ideally located coral sites. Thanks to the long timespans of the ancient corals, we were able to probe and find answers to secrets of the past. The method that we adopted in this paper will also be useful for future studies of other subduction zones - places that are prone to earthquakes, tsunamis, and volcanic eruptions. Our study can therefore contribute to better risk assessments in the future” says Rishav Mallick, lead author of the study.
Read more about these two studies on the EOS Blog
Written by Anna Lagerstroem