{"id":12016,"date":"2024-08-15T01:59:19","date_gmt":"2024-08-15T01:59:19","guid":{"rendered":"https:\/\/earthjay.com\/?p=12016"},"modified":"2024-08-16T02:21:15","modified_gmt":"2024-08-16T02:21:15","slug":"earthquake-report-m-7-1-japan","status":"publish","type":"post","link":"https:\/\/earthjay.com\/?p=12016","title":{"rendered":"Earthquake Report: M 7.1 Japan"},"content":{"rendered":"

A few days ago (as I write this) there was a magnitude M 7.1 earthquake offshore of Japan.<\/p>\n

https:\/\/earthquake.usgs.gov\/earthquakes\/eventpage\/us6000nith\/executive<\/a><\/p>\n

The plate tectonics of Japan is dominated by compressional forces. Convergent plate boundaries are where plates move towards each other.<\/p>\n

In Japan, these convergent plate boundaries are called subduction zones (one plate “subducts,” or dives, beneath another plate). <\/p>\n

The Japanese invest heavily in earthquake science because there has been a long history of subduction zone earthquakes.<\/p>\n

Earthquake History<\/font><\/H3><\/p>\n

In the 20th century, some are of note. The 1923 Great Kant\u014d Earthquake<\/a> was a magnitude M~8.0 earthquake that devastated Tokyo and the surrounding region. <\/p>\n

In 2011 there was the magnitude M 9.1 T\u014dhoku-oki subduction zone earthquake<\/a> in eastern Japan. This is where the Pacific plate subducts westward below the Okhotsk plate, forming the deep sea Japan trench.<\/p>\n

This 8 August 2024 M 7.1 earthquake was in southwest Japan, offshore of the Island Kyushu. The Pacific Ocean region offshore of Kyushu is called Hyuga-Nada.<\/p>\n

Here, the Philippine Sea oceanic plate subducts beneath the Eurasia plate to form the Nankai deep sea trench.<\/p>\n

In 1944 and 1946 there were two large megathrust earthquakes along the Nankai trench. The 1944 M 8.1 earthquake is called a Tokai earthquake. The 1946 M 8.1 earthquake is called a Tonankai earthquake. Earthquakes that rupture these segments are given the same names.<\/p>\n

These two earthquakes led to geologists subdividing this subduction zone into segments. Then geologists used prehistoric earthquake data (paleoseismic data) to try to understand if the subduction zone breaks along these segments in any patterns.<\/p>\n

Do earthquakes always only rupture one or the other of these two segments? Do both segments sometimes rupture at the same time? This is the same question people ask for most all subduction zones globally. <\/p>\n

The reason people ask this question is that the length of the earthquake is a primary factor that controls the magnitude of an earthquake. The width (length times width = area), the amount the fault slips, and properties about the elastic nature of the plates, are the other factors that control earthquake magnitude.<\/p>\n

So, if both segments rupture at the same time, the earthquake would be larger (like in 1707 and maybe in 1498).<\/p>\n

In 1707 there was the M 8.7 H\u014dei subduction zone earthquake that ruptured both of the 1944 and 1946 segments. Some have hypothesized that the 1707 earthquake slipped as far south as the 8 August 2024 M 7.1 earthquake. <\/p>\n

We also want to use paleoseismic data to get an idea about how often these types of earthquakes happen in a given area. This helps us design buildings so that they can “survive” future earthquakes.<\/p>\n

Earthquake Advisory<\/font><\/H3><\/p>\n

Before the 2011 earthquake, the largest magnitude earthquake along the Japan trench was in the mid M 8 range. So, the seismic hazard and tsunami hazard was based on this size of an earthquake.<\/p>\n

Two years before the 2011 earthquake, Japanese geologists found prehistoric evidence of a tsunami that was caused by a much larger sized earthquake. But, this information had not yet been integrated into the seismic and tsunami hazard analyses for east Japan.<\/p>\n

When the 2011 earthquake and tsunami happened, it was larger in size than expected. The mitigation that Japan had used was not sized properly and many people suffered greatly.<\/p>\n

Preceding the 2011 M 9.1 earthquake, there was a M 7.3 earthquake. Because of this “precursor” earthquake, Japan planned to place their society into an earthquake advisory when earthquakes of a sufficient size happens along the coast of Japan (Toda et al, 2024).<\/p>\n

This plan was particularly important for the region along the Nankai trench. The subduction zone earthquakes in Nankai are much closer to the coastline than for the 2011 earthquake (so tsunami arrive faster here). Placing the coast into an advisory is important particularly because of this shorter tsunami travel time.<\/p>\n

Following the 8 August 2024 earthquake, the Japanese government placed this part of the country into an earthquake advisory. The advisory is intended to last for a finite amount of time. However, we do know that sometimes earthquakes that are triggered are triggered following time periods longer than a week.<\/p>\n

In the social media section below there are some tweets that have links to stories about this advisory. I encourage everyone to read the Temblor article about this advisory<\/a>.<\/p>\n

Below is my interpretive poster for this earthquake<\/font><\/H2><\/p>\n
    \n
  • I plot the seismicity from the past month, with diameter representing magnitude (see legend). I include earthquake epicenters from 1924-2024 with magnitudes M \u2265 3.0 in one version. <\/li>\n
  • I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange), possibly in addition to some relevant historic earthquakes. <\/li>\n
  • A review of the basic base map variations and data that I use for the interpretive posters can be found on the Earthquake Reports page<\/a>. I have improved these posters over time and some of this background information applies to the older posters.<\/li>\n
  • Some basic fundamentals of earthquake geology and plate tectonics can be found on the Earthquake Plate Tectonic Fundamentals page<\/a>.<\/li>\n<\/ul>\n
      \n

      I include some inset figures. Some of the same figures are located in different places on the larger scale map below.<\/font><\/H3><\/p>\n
    • In the upper left corner is a map showing the plate tectonic boundaries (from the GEM).<\/li>\n
    • In the lower right corner is a map that shows the M 7.1 earthquake intensity using the modified Mercalli intensity scale<\/a>. Earthquake intensity is a measure of how strongly the Earth shakes during an earthquake, so gets smaller the further away one is from the earthquake epicenter<\/a>. The map colors represent a model of what the intensity may be. The USGS has a system called “Did You Feel It?” (DYFI) where people enter their observations from the earthquake and the USGS calculates what the intensity was for that person. The dots with yellow labels show what people actually felt in those different locations.<\/li>\n
    • In the upper left center is a plot that shows the same intensity (both modeled and reported) data as displayed on the map. Note how the intensity gets smaller with distance from the earthquake.<\/li>\n
    • To the right of this intensity plot is the USGS finite fault slip model. Colors represent the amount of fault slip that their model calculates.<\/li>\n
    • In the upper right corner are two maps showing the possibility of earthquake induced liquefaction for these two earthquakes. I discuss these phenomena in more detail later in the report.<\/li>\n
    • To the left of the ground failure maps is a low angle oblique cross section showing the plate configuration in three dimensions (from AGU Trembling Earth, Austin Elliot). I place a yellow star in the region of the M 7.1 earthquake. Note how we can see the Philippine Sea plate seismicity diving downwards beneath the Eurasia plate.<\/li>\n
    • In the lower right center are plots from three tide gage locations (shown on the map). These individual plots are included later in the report.<\/li>\n<\/ul>\n
        \n
      • Here is the map with a month’s seismicity plotted.<\/li>\n


        \n\"\"<\/a><\/p>\n<\/ul>\n

        Other Report Pages<\/font><\/strong><\/H2><\/p>\n
          \n
        • Temblor Article<\/a><\/li>\n
        • Earthquake Insights<\/a><\/li>\n<\/ul>\n

          Tsunami <\/font><\/h3>\n

          Tsunami can be caused by a variety of processes, including earthquakes, volcanic eruptions, landslides, and meteorological phenomena. <\/p>\n

          Earthquakes, eruptions, and landslides cause tsunami when these processes displace water in some way. <\/p>\n

          We may typically associate tsunami with subduction zone earthquakes because these earthquakes are the type that generate vertical land motion along the sea floor.<\/p>\n

            \n
          • Here is a great illustration of how a subduction zone earthquake can generate a tsunami (Atwater et al., 2005).<\/li>\n<\/ul>\n


            \n\"\"<\/a><\/p>\n

            When the fault slipped, it caused the seafloor to deform and move. This motion also displaced the overlying water column.<\/p>\n

            As the water column is elevated, it gains potential energy. As this uplifted water expends this energy by oscillating up and down, it radiates energy in the form of tsunami waves.<\/p>\n

            This M 7.1 earthquake generated a tsunami. The tsunami was observed on tide gages along the coast of Japan but was too small to generate a trans-Pacific tsunami.<\/p>\n

            This is an animated model of the Great East Japan tsunami of 2011. The warmer the colors, the larger the wave. The first surges reached the closest Japan coasts in about 25 minutes. The first surges reached Crescent City in 9.5 hours. <\/p>\n