Sunday, February 19, 2017

Tree falls in the forest and triggers earthquake sensor

At 5am this morning I awoke to a heavy squall of weather hitting my house at Prairie Creek. It woke me up and the wind was very strong. Then immediately after I felt my whole house shake from an earthquake. The house shook pretty good but it was very short lived maybe five seconds. My house is surrounded by old growth redwoods on all sides so I immediately thought one fell nearby. I quickly checked my seismometer data from my garage seismometer and it triggered and recorded the earthquake. I then got in my car to go drive around the employee housing and campground to see if anyone was hurt. I couldn't find the tree so I knew it fell in the safety of the forest.

I Waited till sunrise to go hike and look for it and I didn't have to travel far! A monster 15 foot diameter old growth redwood! This tree had to have been over 300 feet tall!

I've got peak acceleration around 17.583 cm/sec/sec which is .17583 m/sec/sec / 9.8m/s/s = .0179g which puts it is about magnitude 4 on the modified Mercalli scale which is similar shaking to a 2.1 in moment magnitude. It is important to note that this is just a reading of ground shaking, not a measurement of energy released. It was only that intense because of the proximity of the event to the sensor. And since it was an above ground event it wouldn't have propagated too far so the shaking wouldn't be felt at greater distances.

The base of the tree.

Tree blocking the trail.

The waveform looks pretty good!

Here is a link to the seismogram:

***edit I should clarify as to why I have an earthquake sensor in my garage. I'm a geologist and work for the park during the summers at the visitors center. I host the small car battery size sensor as part of the USGS netquakes program. There is a small cloud of these sensors that help generate the shaking intensity maps that accompany a USGS earthquake report.

****Update 4-2-2017:
I had to edit the post a bit to remove the location of the tree. I also was contacted by who had some measurements of the tree from 2015. he said in his email:

"I took DBH and height measurements on that tree in September of 2015. It was notable for DBH, checking in at 18.4'. A ground-based survey indicated the tree was 265' tall. It was a spectacular tree. 18' is considered the cutoff point for notable dbh, so this was a very special tree in that regard. RIP Earthquake Tree."

I think Earthquake tree is a good name for this one. I was there today and the trail crew had already cleared the new trail. Please feel free to visit this monster tree on the Cathedral Trees trail but please do not walk off trail as it will damage the vulnerable plants on the forest floor.