M 7.5 Indonesia (2018-09-28)

[davenn]

Main Question or Discussion Point

huge on my recorder

http://www.sydneystormcity.com/seismograms.htm

78km N of Palu, Indonesia
2018-09-28 10:02:43 (UTC)
10.0 km

M 7.5 - 78km N of Palu, Indonesia

Time: 2018-09-28 10:02:43 (UTC)
Location: 0.178°S 119.840°E
Depth: 10.0 km

Dave

 

[davenn]

from my recorder just before it started over writing the signal …

There were M4.8, 4.7 and 6.1 foreshocks prior to the main M7.5 main shock
since then there have been M5.8, 5.7, 5.8, 5.4, 5.1, 5.1 aftershocks

Map of the area …

Dave

 

[fresh_42]

Oops, that sounds serious. You already trained me to automatically first ask: Depth? Thank you, Mr. Pavlov, Sir!

 

[Bystander]

Borneo?

 

[Astronuc]

Region is Sulawesi Tengah (Central Sulawesi). Borneo or Kalimantan is west across Makassar Strait.

Looking at USGS site, all preceding and subsequent quakes are shallow (10 km), except for M 6.1 - 55km NNW of Palu, Indonesia.

 

[StevieTNZ]

https://www.nzherald.co.nz/world/news/article.cfm?c_id=2&objectid=12133935

Only became aware of it now..... didn't see anything this morning on the online news.

 

[davenn]

https://www.nzherald.co.nz/world/news/article.cfm?c_id=2&objectid=12133935

Only became aware of it now..... didn't see anything this morning on the online news.

that will teach you for not following me

Borneo?

As Astronuc said, no.

That is Borneo that you can partly see in the upper left of the map

And this event has produced a 2 metre tsunami that have caused further damage.




Dave

 

[Astronuc]

I read this morning that there were tsunami waves of up to 3 meters in Palu and up to 6 m in some places. Perhaps the initial waves were 2 m and subsequent runup was up to 6 m?

Death toll is at least 400.

The 7.5 mag earthquake was inland. So what is this earthquake that caused the tsunami, since it was undersea?
https://earthquake.usgs.gov/earthquakes/eventpage/us1000h3t2#executive
M 5.2 - 106km NNW of Palu, Indonesia
2018-09-28 14:26:00 UTC
Location 0.022°N 119.576°E, Depth 10.0 km

Or was the 7.5 mag earthquake large enough to cause seafloor displacement?

The 7.5 mag and related earthquakes occurred in the Palu-Koro fault zone.
http://temblor.net/earthquake-insights/the-palu-koro-fault-ruptures-in-a-m7-5-quake-in-sulawesi-indonesia-triggering-a-tsunami-and-likely-more-shocks-7797/

Today’s M 7.5 earthquake has changed the stresses within the crust surrounding the earthquake. The amount of this stress change is moderate, especially when compared with the amount of stress that is typically released during an earthquake.

This change in stress is called a change in “static coulomb stress” and a paper that discusses the fundamental factors controlling these stress increases is from Lin and Stein (2004). There is software available to the public from the USGS to perform these calculations. This software is called “Coulomb 3” and is available online here. An introduction to this software and the physics behind the calculations can be found in Stein (2003)

Seismic and Tsunmai Hazard Potential in Sulawesi Island, Indonesia
https://ir.lib.hiroshima-u.ac.jp/files/public/3/31339/20141016181157824412/JIDC_17-1_17.pdf

 

[davenn]

I read this morning that there were tsunami waves of up to 3 meters in Palu and up to 6 m in some places. Perhaps the initial waves were 2 m and subsequent runup was up to 6 m?

I tend to agree, specially when I watched the video(s) in the links I posted. It appeared that there was a smaller initial wave,
that can be seen washing through the street and building by the seashore and the larger second wave in the background.
It was then followed by that much larger wave that hit with a lot of force completely covering those buildings on the shoreline
and being a considerable way up that large street sign on the right side of the video which is much higher than the buildings.

It is very hard to estimate heights of waves when videoed from an angle looking down on them

The 7.5 mag earthquake was inland. So what is this earthquake that caused the tsunami, since it was undersea?
https://earthquake.usgs.gov/earthquakes/eventpage/us1000h3t2#executive
M 5.2 - 106km NNW of Palu, Indonesia
2018-09-28 14:26:00 UTC
Location 0.022°N 119.576°E, Depth 10.0 km

I strongly doubt a 5.2 could produce a tsunami, specially one of that size. I have never seen such reported in the past.

Tho they show the location of the 7.5 as being on land, remember there is a reasonable error circle around that location
and the fault that it occurred on is offshore, so seafloor displacement is likely.

Tho the fault is noted as being primarily strike-slip, it obviously does have an oblique slip component ... see the "beachball" moment tensor solution

That solution allows for a vertical offset of the seafloor.

The 7.5 mag and related earthquakes occurred in the Palu-Koro fault zone.
http://temblor.net/earthquake-insig...gering-a-tsunami-and-likely-more-shocks-7797/

Excellent article …. it amazes me how fast they can produce indepth info for events.
And it was good to see the offshore fault location

(cropped to fit here)

Dave

 

[StevieTNZ]

Just 34 minutes after Indonesia was hit by another major earthquake, officials called off a tsunami warning.

-- https://www.stuff.co.nz/world/asia/107491729/questions-raised-about-indonesian-tsunami-alerts-as-death-toll-expected-to-reach-thousands

Over 800 people are confirmed dead and hundreds are missing after Indonesian quake and tsunami.

-- https://www.stuff.co.nz/world/asia/107474716/earthquake-jolts-central-sulawesi-in-indonesia

 

[davenn]

OK with a bit of searching, I have discovered where that video of the tsunami was taken from. It was from right at the head of that long narrow bay.
This leads to the possibility of wave amplification as it travels up the bay.

some severely reduced in size pic's from road level view.

1) aerial view of the immediate area

2) the carpark spiral ramp from where the video was done . The Mosque can be seen in behind the ramp. It was severely damaged in the quake shaking.
To the left side of the image is a red mast/pole, just past the street lamp pole ( the unfinished mast is where there is now a large black sign with red writing
on it ( obviously not finished when Google went through here with their street cameras)
The videographer was standing with the other people at that narrower ( in height) level, level with the word "SIMPLY"

3) looking further to the left ( west)

4) and further again to the left ….

go to the location in google earth and go into street view for a larger sized image ....
Lat -0.883346 Long 119.843607

The first thing that becomes very apparent is the changes of height from beach level to the upper road/bottom of car ramp level.
I found one spot where there was a clear view of the sea between buildings and there would appear to be approx. one to two metres
between sea level and those shoreline building floor level. The building floor level is just a little below road level

Now note from the video, the wave came right over the tops of those shoreline buildings and to around half way up that that large signage
pole (red on GE, painted white in the video). This infers a wave height of at least 4 metres and possibly more at this location.


Dave

 

[Nik_2213]

At least 800 confirmed dead in city, sadly thrice more expected, but no signs of life in big town further along coast.

Looks bad...
:-((

 

[davenn]

At least 800 confirmed dead in city, sadly thrice more expected, but no signs of life in big town further along coast.

Looks bad...
:-((

yeah, very bad and sad

 

[StevieTNZ]

What has happened is very very sad.

Early warning system that might've prevented deaths in Indonesian tsunami stalled in testing phase for years.

-- https://www.stuff.co.nz/world/asia/107497821/indonesia-tsunami-stalled-warning-system-might-have-saved-lives