A puzzling case is presented by the occurrence of two large but dissimilar earthquakes at almost the same time and place. One must have acted as the trigger, but which one and how did it do so?
One earthquake can set off others. Most triggered earthquakes are aftershocks that result from adjustments on and near the plane of the fault that produced a larger mainshock. In this issue, Beavan et al.1 (page 959) and Lay et al.2 (page 964) take up the intriguing example of two earthquakes that overlapped in time and were adjoined in location, but differed drastically in mechanism. The authors reach opposing conclusions about which earthquake began first, and how one earthquake triggered the other.
The earthquakes in question occurred on 29 September 2009 in the southwest Pacific, near the Tonga trench. An associated tsunami claimed close to 200 lives across Samoa, American Samoa and Tonga1,2. The earthquakes ultimately resulted from the descent, or subduction, of the Pacific plate beneath the Australia plate (specifically its Tonga block), in an area where these two plates are moving towards one another at about 20 centimetres per year — faster than at any other plate convergence worldwide. Despite this, the Tonga trench had somehow failed to produce a single great earthquake of magnitude 8 or larger since 1917. That each of the September 2009 earthquakes attained magnitude 8 thus came as something of a surprise to geophysicists.
The main earthquake visible in seismic records was not a typical subduction-zone earthquake. Great earthquakes and associated tsunamis are caused by sudden slip occurring on the plate interface, releasing accumulated compressional strain between the two plates. A recent example of such a plate-boundary earthquake is the magnitude-8.8 earthquake that rattled central Chile and set off a tsunami on 27 February 2010. By contrast, the main Tonga event resulted from extensional faulting that occurred in an area known as the outer rise, where the descending plate begins to bend into the trench.
Still, it is not a geophysical surprise to find a great outer-rise earthquake. Several have been recorded during the past 100 years, and they are easily explained by downward pull by the descending plate. This pull force can be transmitted towards the outer rise if the two plates do not accumulate strain on the plate-boundary fault, and it can also increase suddenly if the plate boundary breaks in a great earthquake. A November 2006 plate-boundary earthquake of magnitude 8.3 along the Kuril trench set off an extensional outer-rise earthquake of magnitude 8.1 just two months later3, by causing the subducting plate to pull away from the outer rise. In a generic case, such triggering results from a change in static stress. Sudden displacement on a fault during an earthquake adds to the load on some neighbouring faults and subtracts from the load on others. These stress changes may hasten or retard earthquakes, respectively4.
In the Tonga case, both Beavan et al.1 and Lay et al.2 found that a plate-boundary earthquake was associated with the outer-rise earthquake (Fig. 1). The strongest evidence for this finding comes from satellite geodesy. By comparing pre- and post-earthquake measurements from northern Tonga, made by the Global Positioning System (GPS), Beavan et al.1 estimate that 35 cm of horizontal movement occurred in a direction opposite to that expected for an outer-rise earthquake. Continuous GPS measurements, such as those made during the 2010 Chilean earthquake, were not available, however; such measurements could have pinpointed which earthquake happened first.
Clues to the earthquakes' sequence can also be found in tsunami waveforms recorded on bottom-pressure (DART) sensors operated by the US National Oceanic and Atmospheric Administration (NOAA). These waveforms are sensitive to the parent earthquake because plate-boundary earthquakes and outer-rise earthquakes produce opposite sea-surface displacement above the earthquake fault. NOAA modellers implicitly assumed a plate-boundary earthquake model in their successful real-time data assimilation to forecast the far-field tsunami5. Beavan et al. likewise show that the tsunami waveforms recorded at the DART stations are better explained by a plate-interface earthquake, and they obtained the best match by postulating the occurrence of a slow plate-boundary earthquake before the outer-rise earthquake. They point out that this sequence can be explained by static stress change, as in the Kuril example.
When two earthquakes occur nearly simultaneously, the signal from the later event may be buried in the seismic waves from the first. Lay et al.2 carried out non-routine, detailed and comprehensive analyses of the available seismic data, and succeeded in detecting signals from earthquakes after the outer-rise earthquake. Their model indicates that the main outer-rise earthquake triggered the rupture of the plate boundary by shaking it. Such dynamic triggering is plausible: it has been documented on faults hundreds of kilometres from the initiating earthquake6.
But it is still difficult to tell whether the plate-interface earthquake really happened later. If that event was generated slowly in comparison to seismic-wave periods, it would not have been detected in ordinary seismic records1. Analysis of ultra-long-period seismograms2 can indicate the existence of such slow earthquakes, but it is difficult to achieve an accurate estimate of timing from such ultra-long-period records. Lay et al.2 locate the plate-boundary earthquake (as a pair of subevents) close to the trench. It has been shown that the shallower on the plate interface and closer to the trench axis slip occurs, the slower it is7. Therefore, the plate-boundary earthquake rupture might have been slow.
Taken together, the two papers1,2 leave uncertainty as to which of the two earthquakes happened first. And, until we learn which of them was the cause and which the effect, it will be difficult to know whether the trigger was the release of static stress on an extensional fault, or of dynamic stress on a compressional one.
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Pure and Applied Geophysics (2015)
Pure and Applied Geophysics (2014)