Spasmodic tremor is an enigmatic type of seismicity. It is a type of seismic event that is "spastic" and "erratic" and can last anywhere from 10 minutes to over 1 hour. Unlike volcanic tremor which can go on and on for days and even months, carrying dominant frequencies between 1Hz-5Hz, spasmodic tremor can appear and disappear in a much shorter time frame and usually carries dominant frequencies that can reach to 20Hz. They can also occur in "swarms" which has been witnessed in Hawaii many times since January 23, 2019, which is when these spasmodic events made a drastic reappearance. 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. Therefore, the likely conclusion is that these events are signaling mass magma transport along the mantle plume conduit connected the mantle plume to the different magma systems. Directly below 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 can show 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 hypocenters:
1. Depths of spasmodic tremor can occur from about 20km to 60km in depth. However, most that have been reported to occur between 30km and 50km in depth.
2. Spasmodic tremor is NOT an earthquake or a sequence of earthquakes. Sometimes spasmodic tremor is made of multiple earthquakes occurring in rapid succession along with background tremor, and other times spasmodic tremor is more emergent (slowly builds) and is more tremor-like with a lack of clear earthquake patterns. One thing that is prevalent is the fact that each and every spasmodic tremor is unique.
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 detected since January 2019 to present day.
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 (example shown below).
5. It is hard to tell, but it seems that, at times, spasmodic tremor in Hawaii either becomes shallower 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 showing the strength of each event at such a depth.
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 and sometimes can reach a maximum dominant frequency of 20Hz, though that is rare.
8. Aseismic activity (a lack of seismic 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 spasmodic tremor within the conduit to the sub-surface reservoirs in the area?
So, we see spasmodic tremor in Hawaii is likely 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 at a constant rate. 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? Or is it truly showing the transport of magma itself?
1. Depths of spasmodic tremor can occur from about 20km to 60km in depth. However, most that have been reported to occur between 30km and 50km in depth.
2. Spasmodic tremor is NOT an earthquake or a sequence of earthquakes. Sometimes spasmodic tremor is made of multiple earthquakes occurring in rapid succession along with background tremor, and other times spasmodic tremor is more emergent (slowly builds) and is more tremor-like with a lack of clear earthquake patterns. One thing that is prevalent is the fact that each and every spasmodic tremor is unique.
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 detected since January 2019 to present day.
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 (example shown below).
5. It is hard to tell, but it seems that, at times, spasmodic tremor in Hawaii either becomes shallower 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 showing the strength of each event at such a depth.
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 and sometimes can reach a maximum dominant frequency of 20Hz, though that is rare.
8. Aseismic activity (a lack of seismic 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 spasmodic tremor within the conduit to the sub-surface reservoirs in the area?
So, we see spasmodic tremor in Hawaii is likely 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 at a constant rate. 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? Or is it truly showing the transport of magma itself?
Below we see reported spasmodic tremor for Hawaii starting on January 23, 2019. USGS used to report spasmodic tremor as "Other Event" but now only reports earthquakes that transpire as part of the tremor sequence. In the image below you can see the reported locations for spasmodic tremor. There are 81 reported events shown with many more spasmodic tremor events occurring from January 23, 2019 to present.
Again, spasmodic tremor events are no longer reported in this fashion.
Again, spasmodic tremor events are no longer reported in this fashion.
CLICK HERE to visit the USGS EQ map for reported "Other Event" events related to spasmodic tremor from Jan. 1, 2018 through Dec. 2, 2019.
Monitoring for Spasmodic Tremor
Want to learn how to monitor for volcanic spasmodic tremor in Hawaii? It is relatively easy. All you must do is learn how to access seismic data and review it in the program SWARM. If that sounds difficult, guess again! CLICK HERE to visit my easy 13-step process to learn how to access seismic data.
To start, you must know what spasmodic tremor looks like. Below is the largest spasmodic tremor from January 23, 2019 through September 9, 2019. It lasted approximately 38 minutes and was labeled as a M3.0 "Other Event" at 32.9km in depth with an epicenter off the coast of Pahala, Hawaii. Seismic audio of this event is included as well. Audio is compressed (sped up) and was retrieved using the IRIS Time Series tool. Headphones are suggested, but be wary of the volume just in case.
To start, you must know what spasmodic tremor looks like. Below is the largest spasmodic tremor from January 23, 2019 through September 9, 2019. It lasted approximately 38 minutes and was labeled as a M3.0 "Other Event" at 32.9km in depth with an epicenter off the coast of Pahala, Hawaii. Seismic audio of this event is included as well. Audio is compressed (sped up) and was retrieved using the IRIS Time Series tool. Headphones are suggested, but be wary of the volume just in case.
Below is the longest spasmodic tremor from January 23, 2019 through September 9, 2019. This one occurred on July 1, 2019 at 20:14UTC and lasted approximately 1 hour and 11 minutes. Seismic audio is included with this event as well.
There are two primary stations which are best for confirming spasmodic tremor has occurred: TRAD and KKUD, both of which are in the HV network. Although these stations are relatively far from the epicenter, which can be seen via the image below, spasmodic tremor occurs deep enough that it propagates through the mantle and the crust much more efficiently than if it were to occur within the crust. Therefore distant stations, within 40km or so, can still be used for accurate analysis.
As can be seen below, TRAD is about 57km (35mi) to the southwest of KKUD. Although spasmodic tremor can look like certain surface events, the great distance between these two stations allows you to confirm whether what you are viewing is spasmodic tremor or surface activity. Surface activity cannot travel 57km from TRAD to KKUD. Therefore, use these stations to confirm spasmodic tremor and the more stations you use, the more accurate the analysis.
Here are two examples of more spasmodic tremor which were detected on TRAD and KKUD. Even weak spasmodic tremor can slightly show on KKUD or HUAD.
Spasmodic Tremor Swarm
There will often be times when spasmodic tremor occurs multiple times in one day. Therefore, if spasmodic tremor has struck 3 or more times in a 24 hour period, it will be called a spasmodic swarm. Below are two examples of spasmodic swarming.
The first example is from May 15, 2019 where 9 spasmodic tremor events occurred. Can you spot all 9 events?
The second example is from June 1, 2019 where 5 spasmodic tremor events occurred. Can you spot all 5 events?
The first example is from May 15, 2019 where 9 spasmodic tremor events occurred. Can you spot all 9 events?
The second example is from June 1, 2019 where 5 spasmodic tremor events occurred. Can you spot all 5 events?