M 7.9 - off the coast, 280km SE of Kodiak, Alaska

[davenn]

M 7.9 - 280km SE of Kodiak, Alaska

Time: 2018-01-23 09:31:42 (UTC)
Location: 56.046°N 149.073°W
Depth: 25.0 km

from my seismo in Sydney Australia ... the opposite end of the Pacific Ocean

Location Map

cheers
Dave

 

[fresh_42]

Wow! Wow 7.9 and wow what a long distance to make such a measurement down under. This sounds heavy even for fire ring events. Do we expect a tsunami at the coast lines?

 

[Borg]

There are tsunami warnings out and water has been seen receeding from some shorelines. I haven't heard of anything beyond a 3 foot surge though. Hawaii initially had a warning but, it was cancelled.

 

[davenn]

There are tsunami warnings out and water has been seen receeding from some shorelines. I haven't heard of anything beyond a 3 foot surge though. Hawaii initially had a warning but, it was cancelled.

I haven't checked for tsunami warnings

ahhh here we go ...

WEAK51 PAAQ 231313
TSUAK1

BULLETIN
Public Tsunami Message Number 7
NWS National Tsunami Warning Center Palmer AK
413 AM AKST Tue Jan 23 2018

...THE TSUNAMI ADVISORY IS CANCELLED...

* The Tsunami Advisory is canceled for the coastal areas of
South Alaska and the Alaska PeninsulaOBSERVATIONS OF TSUNAMI ACTIVITY - UPDATED
------------------------------------------
* Observed max tsunami height is the highest recorded water level
above the tide level up to the time of this message.

TIME OBSERVED MAX
SITE OF MEASUREMENT TSUNAMI HEIGHT
---------------------------- ---------------- --------------
Kodiak Alaska 0329 PST Jan 23 0.6ft
Seward Alaska 0331 PST Jan 23 0.4ft
Old Harbor Alaska 0338 PST Jan 23 0.7ft
Sitka Alaska 0318 PST Jan 23 0.4ft
Yakutat Alaska 0335 PST Jan 23 0.5ft
Langara BC 0330 PST Jan 23 0.4ftRECOMMENDED ACTIONS - UPDATED
-----------------------------
* Do not re-occupy hazard zones until local emergency officials
indicate it is safe to do so.IMPACTS - UPDATED
-----------------
* A tsunami was generated by this event, but no longer
poses a threat.

* Some areas may continue to see small sea level changes.

* The determination to re-occupy hazard zones must be made
by local officials.

 

[davenn]

interestingly, this quake was located a decent way south of the plate boundary rather than on the interface between the overlying and subducting slab
This will be the main reason for the lack of a significant tsunami ... fortunately!

Interestingly it appears to be on a long linear feature ... possibly a fault, as indicated by the yellow arrows

Dave

 

[jim mcnamara]

Fault map of the area:

@davenn
So how does your proposed thrust fault show on this map? Fracture zone?

 

[fresh_42]

Could have been the Aleutian Thrust.

I wonder whether the Japanese volcanic eruption could have anything to do with it. I don't think so, but the simultaneity is weird.

 

[davenn]

Fault map of the area:

....So how does your proposed thrust fault show on this map? Fracture zone?

No it doesn't , but that isn't surprising, it's a relatively low resolution map ... just showing major features

so far I haven't been able to find anything with higher resolution than the Google Earth image I posted above

maybe it hasn't had a mapping program done in that region ?

Could have been the Aleutian Thrust.

not this time, as indicated by the location maps and comments above, it was somewhat south of the plate boundaryDave

 

[davenn]

Update after 4 days of aftershocks
and that linear feature is reasonably well defined by the events. The main shock is practically right on top of the feature.

There are a number of aftershocks located to the NW of the linear feature and they could indicate the dip angle of the fault that ruptured.

It has to be noted that the USGS have places a different strike angle for the ruptured fault...

DATA Process and Inversion
We used GSN broadband waveforms downloaded from the NEIC waveform server. We analyzed 50 teleseismic broadband P waveforms, 14 broadband SH waveforms, and 69 long period surface waves selected based on data quality and azimuthal distribution. Waveforms are first converted to displacement by removing the instrument response and are then used to constrain the slip history using a finite fault inverse algorithm (Ji et al., 2002). We begin modeling using a hypocenter matching or adjusted slightly from the initial NEIC solution (Lon. = -149.1 deg.; Lat. = 56.0 deg., Dep. = 25.0 km), and a fault plane defined using either the rapid W-Phase moment tensor (for near-real time solutions), or the gCMT moment tensor (for historic solutions).

Result
After comparing waveform fits based on the two planes of the input moment tensor, we find that the nodal plane (strike= 258.0 deg., dip= 69.0 deg.) fits the data better. The seismic moment release based upon this plane is 8.8e+27 dyne.cm (Mw = 7.9) using a 1D crustal model interpolated from CRUST2.0 (Bassin et al., 2000).

interesting results

Dave

 

[jim mcnamara]

Why is seismic moment in units of surface tension? dyne.cm is a unit for surface tension, right?
This is the definition of seismic moment from USGS:
https://earthquake.usgs.gov/learn/glossary/?term=seismic moment

I am clearly not getting something important here.

 

[Bystander]

I am clearly not getting something important here.

Force x displacement = energy.