How did they know that there was a "desert tsunami" in a Death Valley cave after a 7.6 magnitude earthquake near the southwest of Mexico?

Question

Phys.org's story from September 23, 2022 entitled Mexico earthquake triggers 'desert tsunami' 1,500 miles away in Death Valley cave includes the following:

About five minutes after the 7.6 magnitude earthquake hit near Mexico's southwest coast Monday, typically calm water deep in a Death Valley National Park cave started sloshing against the surrounding limestone rock.

The reverberations from the earthquake more than 1,500 miles away created what experts have called a "desert tsunami," which on Monday made waves erupt up to 4 feet high in the cave known as Devils Hole, a pool of water about 10 feet wide, 70 feet long and more than 500 feet deep, in Amargosa Valley, Nevada.

The water in the partially filled cave has become an "unusual indicator of seismic activity" across the world, with earthquakes across the globe—as far as Japan, Indonesia and Chile—causing the water to splash up Devils Hole, according to the National Park Service website.

and

"It depends on the depth, magnitude and location around the world," Wilson said. He said typically earthquakes along the Pacific's "Ring of Fire" that reach at or above a magnitude 7 will register in Devils Hole.

Question: How did they know that there was a "desert tsunami" in a Death Valley cave after a 7.6 magnitude earthquake near the southwest of Mexico?

I'm wondering if there are video cameras inside, or water height sensors that record wave height, or was there just a puddle of water outside the opening in the morning?

Since the article begins "About five minutes after the 7.6 magnitude earthquake hit..." it suggests there were some type of live data collected, although that could simply be an estimate somehow?

Answer 1

The news section of the Death Valley National Park website has the story: Mexican earthquake sloshes Devils Hole. There is footage of the waves at the end of the page. Here is a screenshot from the video:

NPS photo by Ambre Chaudoin, public domain

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Edit: At first I thought that this was the answer to how did they know that there was a "desert tsunami" (a seiche) in a Death Valley cave after a 7.6 magnitude earthquake: they had visual information from the video camera. However, as @uhoh noted in a comment, they also know that another earthquake did not trigger a seiche, and this one occurred at 1 am, so they could not know that through visual output only. Indeed, water level in the cave has been monitored for a while. From Matthew Weingarten PhD dissertation:

Since August 1989, the National Park Service has monitored the water level in Devils Hole with multiple instruments. Continuous water level was recorded by a float-pulley device and an accompanying strip chart recorder (Fig. 2.2). The strip chart recorder graphically records real-time water level relative to a fixed elevation. The majority of the water level was recorded on 0.0005 m of strip chart per 30 minutes. The resolution of the float-pulley device is 0.003 m and the maximum possible recorded water level fluctuation recorded is 0.3 m (16). Two pressure transducers and an electronic data logger were also added to record water levels to the nearest 0.0003 m on 15-minute intervals. The strip chart recorder was removed in May 2010. In December 2012 a new pressure transducer was installed and a data logger reprogrammed to record water level at 1-second (1 Hz) intervals when an offset of 0.003 m was detected. The updated monitoring protocol records higher frequency, digital water level data.

The monitoring setup is shown on figure 2.2, while some examples of water level response to earthquakes is shown on figure 2.3 (not included here because of copyright). Alternatively, there is also a publication on the same topic (Weingarten & Ge, 2014), but the photo of the setup was not included.

Bottom-line is: they know that some earthquakes do trigger seiches at Devils Hole, and that some don't, because the water level of the cave is monitored in near real-time.

Answer 2

As to the how did they know, and how did they register: The observations are a byproduct of the conservation efforts at the cave. The Devils Hole pupfish are an endangered species that are observed by biologists.

From Wikipedia's Devils Hole pupfish; Threats:

In addition to the indirect threat of groundwater depletion, human actions can impact the pupfish in other ways as well. A 2004 flash flood swept scientific monitoring equipment into Devils Hole, causing the deaths of an estimated eighty pupfish.[40] In April 2016, three men broke into the Devils Hole protected area, destroying scientific equipment and wading onto the shallow shelf of Devils Hole, smashing pupfish eggs and larvae, as well as vomiting into the water.

The effects of seismic activity on the water of the cave affects these pupfish, and is a regular known phenomenon.

• Waves Crash Against Rocks In U.S. National Park's Devils Hole After Earthquake Shook Mexico
• Mexican earthquake triggers 4-foot waves in Death Valley National Park — 1,500 miles away

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Nearly the entire natural range of the species is visible in this photo. The equipment is used to monitor the water level.

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