Woody has prepared two early reports on the Hokkaido Earthquake on 6 September 2018.

TOKYO (Reuters) – A powerful earthquake paralyzed Japan’s northern island of Hokkaido on Thursday, killing at least seven people, triggering landslides and knocking out power to its 5.3 million residents.

The death toll from the 6.7-magnitude, pre-dawn quake was likely to rise as rescuers searched houses buried by landslides.

Source: Reuters

Preliminary Notes:

  • The PGAs at Oiwake were all greater than 0.90 g, with 4 of 6 directions greater than 1.0g, much greater than that experienced in all strong motion records during the 3/11/11 Great Eastern Japan Earthquake. Many, many landslides.
  • Much structural damage and will report about that in the next revision.
  • The calculated JMA intensities (called Shindo) were 6.73 and 6.40 on a scaleof 7.0 … extraordinarily high, and again, stronger than anyplace during 3.11.11.
  • The CAVs (cumulative absolute velocity) were all above 3.0 g/s; 8.32 g/s at Onagawa. EPRI considers a CAV greater than 0.16 should begin to show damage.
  • From the EPRI report: The CAV was determined to be the best damage indicating parameter and a CAV value of 0.16 g-sec was found to be a conservative characterization of the threshold between damaging earthquake motions and non-damaging earthquake motions for buildings of good design and construction as defined by the Modified Mercalli Scale.
  • It seems to me that the CAVs during 3.11.11 were an artifact of the very long shaking period of about 300 seconds; perhaps it is not the best damage indicating parameter.
  • The Tomari NPP suffered an LOSP; however, it was in cold shutdown and the EDGs cut in without a hitch.

Photgraphic Analysis Notes:

  • Of course, photos from the media cannot give us the true picture of damage from the earthquake engineering viewpoint.
  • However, from preliminary reports, the main damage was caused by land subsidence, liquefaction, landslides, mudslides, and other geological effects.
  • Resulting blackouts have hit as many as 2.95 million households.
  • The Nos 1-3 of the Tomari NPP in western Hokkaido had an LOSP event, butthe off-site power was restored by 13:00.
  • The LOSP event occurred when the fossil fuel plants in the prefecture shutdown automatically after the quake.
  • Although only one fossil plant was directly affected, the others shutdownbecause load balancing on the grid could not be maintained.
  • The blackout also affected around 40 hospitals as well as telephone servicesand television broadcasting in the prefecture.
  • A fire broke out at a Mitsubishi Steel Mfg. Co. plant in the city of Nemuro butwas later brought mostly under control.
  • There was also a fire at an oil refinery facility in Muroran, but it had almostbeen subdued by the afternoon.
  • New Chitose Airport was closed for the day after part of its terminal ceilingcollapsed, as well as due to the power outage.
  • Shinkansen and local train services were disrupted by the quake.
  • Some geologists believe that the earthquake was likely caused by a series ofslips on an inland active fault.
  • An active fault zone of more than 100 km in length runs north-south about 10kilometers west of the epicenter.
  • Although the Japan Meteorological Agency says it is unclear whether thezone had something to do with the quake, the geologists believe itsdisplacement could be the cause.
  • A seismologist, who examined strong motion time histories, found that therewere about three powerful shaking movements in a short period of time, which leads him to believe that consecutive fault slips may have caused a strong shaking for a long time and triggered mudslides, landslides, etc. (see below from my analysis).
  • The fault zone is massive, and its structure is complicated geomorphologists in Japan believe, but there is not enough data on the southern part of the zone to tell if it has caused large-scale quakes in the past.
  • Collecting data on the southern part, which is more than 54 km in length, is difficult as it extends into the sea and partly runs under the seafloor.