This week we’re back to what many people probably regard as the basics of Earth science — earthquakes and volcanoes. There are a couple of items on new research and one that looks a little more closely at a particular current event. And there’s a sad scientific footnote to end with.
Papua New Guinea’s Earthquake Sequence Rumbles On
A couple of weeks ago I looked at a large earthquake — M7.5 — that had struck in Papua New Guinea’s Central Highlands. I don’t propose to go over its tectonic origins again (you can find them on the United States Geological Survey’s event page, listed below) but I do want to talk about its aftermath.
The M7.5 was, it’s now clear, just the beginning. Earthquakes often occur in sequence. The largest earthquake in a series is called the mainshock, while those that come before and after it are known as the foreshocks and aftershocks respectively. Large earthquakes often experience significant aftershocks, though they may not be preceded by foreshocks.
The Papua New Guinea earthquake is one of the latter types. The first recorded earthquake (on a map showing tremors of at least M4.5) was what now looks to be the mainshock, the M7.5 event of 25 February. In the subsequent two and a half weeks there have been 135 aftershocks of at least M4.5 and there will have been many, many more smaller ones which don’t appear on the USGS map.
The M6.7 was widely reported but the significant aftershocks — many of them large enough to be damaging in their own right — have largely passed under the media’s radar. But after the main event on 25 February there were shocks of M6.0 and M6.3 on 26 February; M6.1 on 28 February; M6.0 on 4 March and M6.7 on 6 March. The sequence is continuing.
An earthquake sequence like that releases a lot of energy. It’s killed at least 100 people so far, and almost certainly many more whose deaths, in remote areas, may never be recorded. In an area of high mountains, unstable slopes and poor access, the earthquakes trigger landslides which not only kill many people, but also prevent relief teams reaching damaged villages. It’s a reminder, once again, that earthquakes in themselves are problematic enough, but that the tremors, and the damage, can continue for long after the first rumbles have died away.
Thunderbolt and Lightning…Volcanically
The phenomenon of volcanic lightning is well documented. It’s caused when a charge builds up between the particles ejected from a volcano, and its spectacular displays are regularly photographed. But this week, news emerged of scientific research into a much more elusive occurrence — volcanic thunder.
Thunder and lightning are part of the same process and they go together, but up until now it’s been notoriously difficult to isolate the sound of thunder from the associated, often overwhelming, soundtrack of a major volcanic eruption. This week, however, a paper in Geophysical Research Letters reports, for the first time, on the observation of volcanic thunder at Alaska’s Bogoslof volcano.
The researchers recorded the volcano from a distance of about 60km and were able to analyse their recording to pick up a pattern of sounds which correlated with the occurrence of volcanic lightning — something which, as the study concludes: “offer[s] a new avenue for studying electrification processes in volcanic plumes”.
More Earthquakes: Mexico
Six months ago, a major earthquake in central Mexico killed around 300 people, caused a lot of damage and raised the spectre of the devastating Mexico City earthquake of 1986 (on whose anniversary, incidentally, it occurred).
Most earthquakes in Mexico occur offshore and, if large enough, are capable of causing damage well inland — as was the case in 1986. But the earthquake of 2017 was unusual in that it didn’t occur in this coastal subduction zone, but considerably further inland.
Because an earthquake is considered unusual doesn’t, of course, necessarily mean that it’s any more dangerous, but an unusual occurrence is by definition one that occurs less often than another and the chances are that it’s likely to be investigated in less detail and so less well understood.
The 2017 tremor showed that large and damaging earthquakes in Mexico don’t just occur off the coast, and raised the possibility that future earthquakes of this nature might strike closer to major settlements — perhaps most concerningly, nearer Mexico City.
This week, a paper published on the earthquake looked at the possibility of further damaging earthquakes inland and closer to. While concluding that such an event is unlikely, the research, which is part of an ongoing program, doesn’t rule it out — and the information it produces will be applicable to future understanding of earthquake hazard assessment and mitigation in the area.
Earthquakes and volcanoes are where I started off in Earth science, but the subject is so much wider than that. The Earth system is part of the solar system, the solar system part of a galaxy, the galaxy is part of the cosmos…
I woke up this morning to the news — and maybe you are waking up to it, too — of the death of the extraordinary physicist Steven Hawking. I’m not a physicist so a lot of what his achievements in that field are beyond my comprehension. But one of the key reasons for the articles I write is communication of science — and with the death of Professor Hawking, we have almost certainly lost one of the greatest scientific communicators of our time.