M7.1 Earthquake, Banda Sea, Indonesia + aftershocks

[Astronuc]

M 7.1 - Banda Sea - https://earthquake.usgs.gov/earthquakes/eventpage/us7000l9h4/executive

• 2023-11-08 04:53:51 (UTC)
• 6.456°S 129.513°E
• 10.0 km depth

Fairly shallow

Aftershocks in the 4-6 M range, but one at M 6.7 - Banda Sea

• 2023-11-08 13:02:06 (UTC)
• 6.146°S 129.914°E
• 10.0 km depth

https://earthquake.usgs.gov/earthquakes/eventpage/us7000l9ku/executive

 

[berkeman]

Ouch. Any tsunami warnings?

 

[Astronuc]

Ouch. Any tsunami warnings?

According to various news sources, "No tsunami warning was issued for the quake -- initially reported as a magnitude 6.9 -- which hit at 11:53 am local time. The 7.1-magnitude earthquake struck Indonesia's Banda Sea on Wednesday, the United States Geological Survey said, with no immediate reports of damage or casualties."

The M7.1 was preceded by a Mag 6.7 quake (considered a fore-shock).

M 6.7 - Banda Sea​

https://earthquake.usgs.gov/earthquakes/eventpage/us7000l9h2/executive

• 2023-11-08 04:52:52 (UTC)
• 6.382°S 129.779°E
• 10.0 km depth

The quake was not expected to produce tsunamis along the nearby coastlines.

From the USGS seismic summary

The November 8, 2023, M 7.1 earthquake in the Banda Sea occurred as a result of strike-slip faulting at shallow depth. This event was preceded approximately 1 minute earlier by an M 6.7 earthquake, referred to as a foreshock, with a similar mechanism and location. These events occurred in the Banda Sea in a region of complex tectonics, broadly defined by the boundary between the Australia and Sunda Plates. These events occurred on or near the plate boundary between two smaller tectonic plates, the Timor and Banda Sea plates. Focal mechanism solutions for both events indicate that rupture occurred on either a left-lateral northeast-striking fault or a right-lateral southwest-striking fault. At the location of the M 7.1 earthquake, the Australia plate moves towards the north-northeast with respect to Sunda at a velocity of about 73 mm/yr. Motion between the two plates is dominantly convergent, but many styles of faulting are present in the region.

Maybe because it occurred on a striking fault, a tsunami was not expected, unlike for a subduction movement.

 

[DaveE]

According to various news sources, "No tsunami warning was issued for the quake -- initially reported as a magnitude 6.9 -- which hit at 11:53 am local time. The 7.1-magnitude earthquake struck Indonesia's Banda Sea on Wednesday, the United States Geological Survey said, with no immediate reports of damage or casualties."

The M7.1 was preceded by a Mag 6.7 quake (considered a fore-shock).

M 6.7 - Banda Sea​

https://earthquake.usgs.gov/earthquakes/eventpage/us7000l9h2/executive

• 2023-11-08 04:52:52 (UTC)
• 6.382°S 129.779°E
• 10.0 km depth

The quake was not expected to produce tsunamis along the nearby coastlines.

From the USGS seismic summary

Maybe because it occurred on a striking fault, a tsunami was not expected, unlike for a subduction movement.

Help! What does this phrase mean: "rupture occurred on either a left-lateral northeast-striking fault or a right-lateral southwest-striking fault"?

For strike-slip faults, isn't a 'northeast-striking fault' the same as a 'southwest-striking fault'? Is it about the direction of the applied stress vector?

For context:

 

[Astronuc]

Help! What does this phrase mean: "rupture occurred on either a left-lateral northeast-striking fault or a right-lateral southwest-striking fault"?

For strike-slip faults, isn't a 'northeast-striking fault' the same as a 'southwest-striking fault'? Is it about the direction of the applied stress vector?

I was wondering that myself. I expect it depends on which mass if moving in whatever direction.

 

[davenn]

Help! What does this phrase mean: "rupture occurred on either a left-lateral northeast-striking fault or a right-lateral southwest-striking fault"?

For strike-slip faults, isn't a 'northeast-striking fault' the same as a 'southwest-striking fault'?

Yes it is, other than the direction of slip. Which could be either but is dependent on the focal mechism derived from first motions. I wonder if there was a typo there ?
I think ( but cant confirm) they really meant to say NE striking and SE striking as per the beachball.
They also seem to be using opposite colouring to everyone else. everone else uses the dark colour ( blue in this case) as the quadrant being compressed (P = Pressure) but they are using the light colour ( white) as the P = Pressure quadrant.

I was wondering that myself. I expect it depends on which mass if moving in whatever direction.

Seems I need to do a thread on focal mechanisms "beachballs".

here is the beachball for that event .....

This focal mech. is as almost as pure strike slip as can be obtained.
there are two possible fault plane solutions for this event.
If we use the dark colour as the compressional area and the white as the tensional areas,
we would have .....

2 possible solutions (fault planes)
Solution 1 (in red arrows) A left lateral strike-slip along a fault trending NE to SW.
That is Q1 and Q4 are moving towards the SW relative to Q2 and Q3.
Left Lateral means, if you stand on one side of the fault and look across the fault,
which direction did the ground move on the opposite side of the fault.
Standing in Q3 or Q2 quadrants, the other side (Q1 and Q4) moved to the left = Left Lateral.
Note, it doesnt matter what side you stand on, you see the same left lateral motion.

Solution 2 ( in black arrows) would be a fault trending NW to SE.
So if you stand in Q1 or Q2 (Quadrants), the other side (Q3 and Q4) has moved towards the NW.
That is towards the right - therefore a right lateral fault.
Once again, it doesnt matter which side of this fault you stand on, looking across the fault shows
movement to the right.

Clear as mud ? hopefully not too bad

Looking at the focal mechanism, we do not know which of the 2 fault planes is the correct one.
Choosing the correct one comes from knowledge of the tectonics region where the event occurred.

I will continue this in another following post. looking at how the P and T quadrants are defined
I need to save this much and head home from work

cheers
Dave

 

[DaveE]

Seems I need to do a thread on focal mechanisms "beachballs".

Yes, please!