Explanation of the slow surprise in the middle of the sandwich (earthquake)

Newswise – The August 12, 2021 South Sandwich Island earthquake had a surprise hidden in its complex rupture sequence: a slow, shallow magnitude 8.16 sub-event that was “invisible” to researchers at first glance.

At the annual meeting of the American Seismological Society, Zhe Jia, a graduate student in geophysics at CalTech, described how he and colleagues discovered the sub-event by looking at seismic data of long duration, waves of up to 500 seconds in length.

Jia and colleagues note that the results demonstrate that subduction-zone earthquakes can display a wide range of rupture events, and that further research is needed to understand and issue tsunami and aftershock warnings for these events.

Researchers were intrigued by some of the unusual features of the South Sandwich Island event. Initially, the earthquake was reported as an event measuring 7.5 on the Richter scale with a depth of about 47 kilometers. But it did create a massive 400-kilometre aftershock and tsunami across three oceans – both predicted for a larger, shallower earthquake.

“At first, we thought the indexed size didn’t match the 400-kilometre aftershock region, and something special had to happen,” Jia explained. “Later, we saw size 8+ reported by different catalogs, but they had huge differences in mechanism, depth and location. It must have been a really complex event. That’s when we started trying to figure out the complexity of the rupture.”

The team initially analyzed short to medium seismic data, in keeping with the usual process of examining complex earthquakes. This analysis showed four sub-events with a total size of 7.9, located at the beginning and end of the event.

These sub-events “do not appear to be sufficient to generate the observed global tsunamis, and the middle segment was missing,” Jia said. “We are beginning to believe that the long rupture length could indicate a slow nature, making it invisible in short to medium periods.”

After examining seismic data for up to 500 seconds, Jia and colleagues identified a “hidden” shallow 8.16-magnitude event that was sandwiched between other ruptures. They calculated that the slow event contributed more than 70% of the earthquake’s total seismic moment, tearing a 200-kilometre portion of the plate’s frontage and lasting about 200 seconds – an unusually long time for an event of this size.

Jia said it was an “open question” whether the initial regular sub-events somehow led to the slow event. He said that the slip at deep depth may have transferred dynamic pressure to a shallower part of the subduction zone interface, but that other launches would also be needed. For example, slipping at a deeper depth may generate high temperature in the fault, causing any fluids to expand or evaporate and reduce frictional resistance.

“Another possible reason is the different geological conditions of the subduction zone,” Jia said. Less hard rocks in the subduction zone’s upper plate, at shallow depths, “have been shown to produce tsunami-like properties. Therefore, a change from higher to lower upper plate hardness along the impact may result in a secondary slow occurrence.”

Jia also noted that it is also unclear whether other tsunami earthquakes may contain a “trapped” slow rupture component, or whether the South Sandwich Island event is a unique case. “More historical events could have been ‘buried’ like in recent seismic records, but more importantly, our global observations of large earthquakes were only about 50 years old, which is relatively short compared to the frequency of large earthquakes at intervals of hundreds or thousands of years.”

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