Few of our readers have missed the anomalous M6.2 earthquake that occured 20 km WNW of the Aniakchak Caldera. In this article we will go through the details of the earthquake and the possible effects of the main-shock and the series of after-shocks.
According to the Alaska Earthquake Center the waveform of the initial 6.2 was hard to calculate correctly and it initially looked more like several stacked earthquakes at the same location. This postponed the release of the earthquake data from both AEC and USGS with several minutes. In laymans terms, this earthquake started and almost halted before picking up speed again making it look like it had individual sets of P and S waves.
The initial earthquake followed the main stress-regimen of the subducting Aleutian arc which is common in the area. What was unusual was the depth of the earthquake. As you get further north from the faultline the depth of the earthquakes increases, near where the subduction starts the depth is normally around 20km and as you get further back they become progressively deeper until they reach 80 to 120 km depth around Aniakchak.
This earthquake was at 20 kilometers depth and this is the first earthquake at that depth recorded in the area.
All available evidence that we can see points towards that the large quake and the ensuing aftershocks are part of the accumulation and compression of material in the accretionary wedge. If there is an eruption, it will (in our opinion) be due to an existing magma chamber getting over-pressurized from the new increased compression stresses following the compaction driven quake. The moment tensor for the large quake clearly points to it being reverse mode faulting. Similar to the main subduction fault, but shallower, indicating that it’s likely stuff being piled up on each other.
Other systems like this are things like the entire Eastern coastal shelf of Japan, The Kamchatka Peninsula etc…
The main quake was probably along one of the area’s accretionary prism thrust faults. The M 6.2 quake had a dip angle of 49° for the NP1 plane and 42° for the NP2 plane and that fits our idea. Doesn’t prove it, but it fits. It also dovetails quite well with the observation that the M6.2 could be an aftershock of the 1964 Alaska quake since this is trending towards the area where that fault system petered out when it ruptured. Everything we are seeing now is related to the strata adjusting to the new stress regime.
Again, it’s just our opinion, but there will not be an eruption unless existing magma is re-mobilized from the new compression stress pushing a chamber over some critical pressure, sort of stepping on a mustard packet. And like mustard packets, it could be quite energetic if that happens. That could even be the same mechanism that originally made the caldera there. A massive pressure increase, really really fast that blew out the chamber of a previous volcano.
Now that I think about it… that could be what’s at play in New Zealand when those systems go nuts. The quick pressure rise and blow-out would help explain how Taupo was able to have almost no zonation in one of its eruptions. Everything went off at once when the lid let go. It would also help to explain how that area can have such rapid dike emplacements. I think I’ll call this the mustard packet scenario.
The tremor that is being seen on the local seismometer network in the area and that has been felt in Port Heiden by the village council is likely to be the magma chamber under the Aniakchack reacting to the new stress regimen.
Also, there is a marked resemblance to what was reported from Port Heiden prior to the 1931 eruption with written reports of earthquakes and volcanic tremor being felt there. For those who are not familiar with Aniakchaks eruptive history, this was a borderline VEI-4/5 eruption scoring in at 0.9 cubic kilometers of ejecta.
So even if this was not a volcanic earthquake swarm as such, the vicinity and relation to the general stress regime and the accretionary prism in combination with the tremor could over time affect the magma reservoir in the nearby Aniakchak Caldera. If it would lead to an eruption is an entirely different question and if that happens we will have reason to return to the largest volcano in Alaska.
CARL REHNBERG & GEOLURKING