Scientists baffled by seismic waves

Recently seismic waves traversed thousands of kilometres across the globe, ringing earthquake sensors from Zambia to New Zealand – but the activity is baffling scientists because there was no earthquake.

The seismic source was traced to an area near the shores of Mayotte, a French island sandwiched between Africa and the northern tip of Madagascar.

The waves made noise for more than 20 minutes, but it seems no human felt them.

But an earthquake enthusiast based in Wellington did – @matarikipax posted the curious activity on Twitter which sparked a global ‘‘wave hunt’’ to find the source.

Based on the scientific sleuthing done so far, the tremors seem to be related to a seismic swarm that’s gripped Mayotte since last May, according to Maya WeiHaas at National Geographic.

Hundreds of quakes have rattled the small nation during that time, most radiating from about 50 kilometres offshore, just east of the odd ringing.

Most were minor trembles, but the largest clocked in at magnitude 5.8 on May 15, the mightiest in the island’s recorded history.

Yet the frequency of these shakes has declined in recent months and no traditional quakes rumbled there when the mystery waves began on November 11.

Mayotte was formed from volcanic activity, but there has been no eruption in more than 4000 years. Instead, the French Geological Survey analysis suggests the new activity could point to magmatic movement offshore – miles from the coast and under thousands of feet of water.

Though this is good news for the 235,000 people who live on the archipelago in the Indian Ocean, it’s an area that hasn’t been studied in detail by geologists.

In New Zealand, GeoNet seismologist John Ristau said he saw his contemporaries discussing the unusual activity on Twitter when it first happened.

‘‘It is a very strange signal and it can be seen pretty much everywhere around the world. The signal is clearly not like a regular earthquake, it’s more like a burst of energy.’’

Ristau points out he has not investigated what the source of the signals is, but said the constant rumble was similar to a volcanic tremor which was a constant lowfrequency tremor. An earthquake had a wide range of frequencies and periods that it produced energy at, he said.

‘‘But this signal is like a large truck idling outside your office with a steady rumble of the engine.’’

Because the signal was unusual, it was difficult to know its origin, and that was baffling those monitoring the seismic waves.

In a normal earthquake, the buildup to a jolt can happen in seconds and emit two types of energy. The first is called primary (P) wave energy, which rarely causes damage. Measuring this data allows an estimate of the location and magnitude of the quake.

That’s followed by the secondary (S) wave – these travel about 2km a second slower than P-waves through rock – which is the strong shaking that usually causes most of the damage.

Ristau produced a copy of the seismic activity felt from Mayotte and compared it with a magnitude-6.3 North Atlantic earthquake across three stations in New Zealand – PYZ (at the southern end of Fiordland), WEL (Wellington), and OUZ (Northland).

"You can see that the Mayotte signal and the M6.3 earthquake in the North Atlantic are easily visible. They also look much different from each other. The red lines mark where each signal begins.

‘‘You can see that the amplitude of the signal varies over time; however, the frequency, or period, of the signal is virtually uniform for the entire time.

‘‘This implies a source that is producing a signal at one consistent frequency, but the strength varies.’’

. . . the tremors seem to be related to a seismic swarm that’s gripped Mayotte since last May, according to Maya Wei-Haas.