South Pacific, Mexico (Yet Again) and Offshore California: Earthquakes 21-27 September 2017

After the traumas of the past couple of weeks in Mexico, the digest for this week, 21-27 September 2017, feels like a catalogue of calm. Yes, there have been plenty of earthquakes, and yes, they’ve included some that are significant in magnitude, but none of them has brought any serious damage, at least as far as I’ve seen reported.

That said, in any other week I’d have been commenting on the high levels of seismic activity appearing on the United States Geological Survey’s real time earthquake map, which showed almost 1900 earthquakes this week.

The map, which includes (broadly speaking) earthquakes of all magnitudes in the SU and its territories and those of at least magnitude 4 elsewhere, included four tremors in excess of M6.0, along with 32 of at least M5.0 and 143 of at least M4.0.

The number was slightly inflated by aftershocks from Mexico’s M8.1 earthquake earlier in the month, with 31 tremors of ≥M4.0 in the area. Other than that, most of the larger earthquakes occurred at or near the active seismic margins of the western Pacific — including three of the four largest registering this week.

The Week’s Biggest Earthquake: M6.4, South of Tonga

There were two earthquakes this week registering at M6.4, but I’ve chosen the one that occurred south of the Tongan islands on the grounds that this is a part of the world this digest visits a little less often than the Vanuatu region, which is where the other occurred.

Tectonically, the setting of this earthquake is so straightforward it merits the description ‘classic’. It’s a subduction zone, with the Pacific plate moving eastwards against the Australian plate and descending beneath it along a stretch of ocean that extends from Samoa to the South Island of New Zealand. This straight(ish) and uncomplicated margin is characterised by a pattern of earthquakes in the over-riding plate increasing equally with distance and depth from the margin.

Bearing this in mind, it was surprising to see that this week’s earthquake doesn’t fit that pattern. Its epicentre (the point in the Earth’s crust directly above the point at which faulting occurred) is around 200km from the trench and the depth is 98km, implying that the crust ruptured within the over-riding plate other than at or near the margin.

This is borne out by additional data from the USGS, which suggest normal faulting (resulting from extension) rather than the compressional motion we’d expect at the plate interface itself. The implication is that the earthquake was caused by deformation within the crust rather than movement along the subduction zone.

M6.1 Earthquake in Mexico

It was always unlikely that Mexico wasn’t going to feature among the notable earthquakes, just two weeks after a tremor of M8.1 and a week from one of M7.1. The close proximity in time of the two has raised questions about whether they were connected, while the distance between them suggests that the answer is that they probably aren’t. But where does the third earthquake fit in to this sequence?

The tremor occurred between the two, slightly closer to the first, larger ‘quake than the second. The over-riding tectonic setting is the same — subduction of the Cocos plate beneath Central America — but the depth is much shallower at just 9km, although the motion, as with the others, is extensional.

Last week we considered — and largely dismissed — a connection between the M8.1 and the M7.1, but in this case there does seem to be a connection between the larger tremor and the most recent one. On a map reproduced on the Temblor blog, it appears at the edge of the cluster of aftershocks, and the conclusion reached in that analysis was that it was, in fact, an aftershock.

US Earthquakes: Offshore California

California is most notorious for the San Andreas fault zone, but actually the state’s larger and more regular earthquakes occur off its northern coast. The San Andreas zone terminates near Ferndale, at a so-called triple junction, where the smaller Juan de Fuca plate runs up against North America. San Andreas forms the southern leg of the the three: the northern leg is the Cascadia subduction zone and, to the west, the Mendocino Fracture Zone is a strike-slip margin.

It’s the latter that’s the source of this week’s earthquake which, at M5.7, would be a very significant and possibly very damaging event were it to occur on land. At over 200km off shore, however, and with a strike-slip motion and relatively low magnitude in terms of generating a tsunami, it passed largely unnoticed.

Last Thoughts: Thankful for a Quiet Week

It’s so much easier to deal with numbers. An earthquake of Mx.x, at a depth of y km, a distance of z from a preceding event… the nth in a series of however many going back to such and such a date.

The past couple of weeks have been a reminder, if we needed it, that earthquakes are a part of the Earth system in which humans play an integral part. Nature is not malicious, but it can cause unbelievable harm and damage.

Frankly, this week it’s a relief not to have to discuss earthquakes in terms of numbers dead and injured.