Hey guys! In this post I am going to show the usual seismic plots and audio associated with the spasmodic tremor events in Hawaii for July 4-5, 2019. I will also detail a reminder about what spasmodic tremor is and what it is likely caused by in Hawaii. As usual, please click the title of this post or "read more" to continue...
Mauna Loa is currently experiencing heightened volcanic unrest. Alert level is currently ADVISORY and the aviation color code is assigned to YELLOW. Please CLICK HERE to keep an eye on future updates issued by the HVO.
First off, click the USGS EQ map button above to see all events reported by USGS for this time period and location. Please keep in mind HVO at volcanoes.usgs.gov reports far more earthquakes than the USGS earthquake website does. Regardless, USGS does a great job at reporting most, if not all, earthquakes associated with spasmodic tremor. Also, another button resides above to see all active seismic stations on the Big Island of Hawaii.
Ever since I discovered my first spasmodic tremor event in late-Jan. 2019 for the Big Island of Hawaii, I have been utterly fascinated by them. I first incorrectly called them "DLP-HFEs", deep long period high frequency events. Then, thanks to a a professional seismologist, I was notified of a few publications about seismic events including spasmodic tremor. It first peaked my interest when USGS started posting "diamonds", reporting them as "Other Event", instead of the normal circles which represent earthquakes on their USGS EQ map. Sometimes USGS reports spasmodic tremor as "Other Event" and other times they simply report them as earthquakes. These events are for more complicated than simple earthquakes.
Spasmodic tremor is a type of seismic event that is "spastic" and "erratic". Unlike volcanic tremor which can go on and on for days and even months, carrying dominant frequencies between 1-5Hz, spasmodic tremor can appear and disappear in a much shorter time frame. They can also occur in "swarms", which has been witnessed in Hawaii many times since January 23, 2019. In the case of Hawaii's spasmodic tremor, these seismic events are occurring within the mantle plume which feeds magma into local magma reservoirs on the Big Island. Thus the likely conclusion is that these events are signaling mass magma transport. I have also asked a seismologist about these events and it seems that is a likely conclusion. Here is a cross section showing part of the Big Island of Hawaii from HVO:
The cross section above shows the depth of the crust and upper mantle beneath Kilauea volcano. It shows a maximum depth of 15 miles, or about 24 kilometers. This proves that these spasmodic tremor events are occurring deep within the mantle, of course likely occurring within the mantle plume itself. Why? Well, here are some characteristics of spasmodic tremor I have noted in my research from seismic analysis and reported depths:
1. Depths can occur from about 20km to 60km in depth. However, most I have seen reported occur between 30km and 45km in depth.
2. Spasmodic tremor is NOT an earthquake. Sometimes spasmodic tremor is made of multiple earthquakes occurring extremely fast, and other times spasmodic tremor is more emergent (slowly builds) and is more tremor-like with a lack of earthquake "spikes".
3. Spasmodic tremor in Hawaii can last anywhere from 10 minutes to 1 hour and 11 minutes! 1 hour and 11 minutes is the longest spasmodic tremor I have detected since January 2019.
4. Spasmodic tremor usually occurs at those depths stated above, but most epicenters are situated under Pahala, Hawaii or just off the coast of Pahala.
5. It is hard to tell, but it seems that, at times, spasmodic tremor in Hawaii either becomes more shallow or deeper with time. Could this be signaling both the ascent and descent of magma within the plume?
6. To the untrained eye, it seems like these events are indicative of surface activity. On the contrary, most stations around the entire island detect these events quite well, even though the power detected during these events is usually not greater than what a M3.0 would release. We also know they are occurring at the likely depths they are being reported at, seeing there is not much of a leeway of P wave arrivals from station to station.
7. Spasmodic tremor usually carries mid-range frequencies with dominant frequencies sometimes occurring between 10-15Hz depending on the event. It cannot be considered a low frequency event.
8. Aseismic activity is pronounced at depths shallower than ~19km directly after most spasmodic tremor. If this is magma recharge, which is possible, why then is there a lack of seismic activity connecting the spasmodic tremor events within the conduit to the sub-surface reservoirs in the area?
So, we see spasmodic tremor in Hawaii is probably associated with mass magma transport along the mantle plume conduit which feeds many sub-surface magma reservoirs for many of the volcanoes on the island. One thing that is not clear, is something associated with the fact that spasmodic tremor is not constant. Magma must be feeding these volcanoes at an almost constant rate, especially when seeing the swelling some of the areas on the Big Island are currently showing. So why is it that we don't see this occurring all the time every day? Is it possible spasmodic tremor is signaling a "widening" of the mantle plume conduit in response to a larger amount of magma? I don't know. But this is why I love this field of work!
Below I will show some plots detailing the spasmodic tremor events that occurred under the Big Island of Hawaii on July 4-5, 2019. They were pretty weak compared to recent events, and didn't last long compared to some other days I have seen. Regardless, I always like to report on them when I see more than one in a 48 hour period. As usual, I will show helicorder plots first. BE WARY: Helicorders below have 48 hours allotted with 60 minutes per line (seismogram), instead of the usual 24 hours with 30 minutes per line (seismogram). So that means events might appear smaller than usual.
Below I will show the usual seismogram, spectrogram, and spectra plots detailing the July 4-5, 2019 spasmodic tremor events from the four seismic stations labeled on the map in the beginning of this post. I will also include seismic audio from station PPLD which will allow you to hear all of the spasmodic tremor events. Regarding the seismic audio, it was obtained using the IRIS Time Series source. I suggest using headphones, but please be wary of loud volume just in case! All audio is always sped up to allow you to hear it better. Enjoy!
Event #1 occurred on July 4, 2019 at 22:14UTC. It lasted approximately 25 minutes.
Event #2 occurred on July 5, 2019 at 01:49UTC. It lasted approximately 26 minutes.
Event #3 occurred on July 5, 2019 at 22:38UTC. It lasted approximately 25 minutes.
This blog is specifically for activity that occurs in Hawaii. In light of 2018's eruptions on the Big Island, I felt it necessary to have a section entirely devoted to events in Hawaii. If there are too many posts, don't forget about the archives below!