Earthquake Report: M 7.4 Earthquake in Taiwan

Just as my workday was wrapping up, I got my notification of an earthquake in Taiwan and, shortly thereafter, a call from my boss. There was a possibility of a tsunami, so I stay tuned in the case that I needed to support my work at the California Geological Survey.

However, based on the magnitude and location, it did not seem that there would be a tsunami impacting California. BUT, the magnitude could be wrong. So, we needed to wait and see.

Initially, there were two M 7.5 earthquakes. But, since they were close in time, I suspected that there was really just one. Turns out, that was correct (and the magnitude settled at M 7.4):

Then a second large earthquake hit, a M 6.5 near the M 7.4. The 6.5 was shallower.

In the past few years, there have been a number of significant earthquakes in this region of eastern Taiwan. Notably, events in March and September of 2022.

Today’s M 7.4 earthquake has a reverse earthquake mechanism, the result of compression.

In this part of Taiwan, there are reverse faults that could be responsible for the M 7.4.

The Longitudinal Valley fault (LVF) and the Central Range fault (CRF) are both reverse faults that dip in opposite directions.

The LVF dips to the east and the CRF dips to the west. Both faults daylight (intersect the Earth’s surface) along the main valley along eastern Taiwan (the Longitudinal Valley).

This earthquake sequence seems to be related to the LVF, though the M 7.4 is too deep. Therefore, if the fault is east dipping, the fault tip (where it daylights) must be far to the west of the Longitudinal Valley. Let’s see what people come up with.

Alternatively, perhaps this M 7.4 is related to the Offshore Eastern Taiwan thrust belts (e.g., the Takangkou thrust) from Huang and Wang (2022). See their map below.

Another possibility is that the M 7.4 was on the Milun fault. See the Huang and Wang (2022) cross-section below.

There was a small tsunami observed offshore of Taiwan at Ishigakijima, Japan.

Here is the plot:

And, here are some observations from the National Tsunami Warning Center in Palmer, Alaska:

Here are the data plotted and interpreted.

Below is my interpretive poster for this earthquake

  • I plot the seismicity from the past month, with diameter representing magnitude (see legend). I include earthquake epicenters from 1922-2022 with magnitudes M ≥ 3.0 in one version.
  • I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange), possibly in addition to some relevant historic earthquakes.
  • 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. I have improved these posters over time and some of this background information applies to the older posters.
  • Some basic fundamentals of earthquake geology and plate tectonics can be found on the Earthquake Plate Tectonic Fundamentals page.

    I include some inset figures. Some of the same figures are located in different places on the larger scale map below.

  • In the upper left corner is a map that shows the plates, their boundaries, and a century of seismicity.
  • In the upper right are two maps that show models of how there may have been landslides or liquefaction because of the earthquake shaking and impacts. Read more about landslides and liquefaction here. I include both the USGS epicenter and the Central Weather Bureau Seismological Center epicenter (which is probably more accurate). However, these ground failure models are based on the USGS epicenter/location.
  • To the left of those two maps is a low angle oblique view of the tectonic plates and how they are oriented relative to each other.
  • Below that figure, in the center, is a map from Chen at al. (2020) that shows the earthquake fault mapping along eastern Taiwan. I place a yellow star in the location of the M 7.4 epicenter (the location of the earthquake on the ground surface).
  • In the lower right corner is a map that shows the ground shaking from the earthquake, with color representing intensity using the Modified Mercalli Intensity (MMI) scale. The closer to the earthquake, the stronger the ground shaking. The colors on the map represent the USGS model of ground shaking. The colored circles represent reports from people who posted information on the USGS Did You Feel It? part of the website for this earthquake. There are things that affect the strength of ground shaking other than distance, which is why the reported intensities are different from the modeled intensities.
  • To the left of the intensity map is a map that shows the regional tectonics (Shyu et al., 2005), with a yellow star in the location of the M 7.5.
  • To the left of this seismicity map is a plot that shows how the shaking intensity models and reports relate to each other. The horizontal axis is distance from the earthquake and the vertical axis is shaking intensity (using the MMI scale, just like in the map to the right: these are the same datasets).
  • In the upper left-center is a figure that shows a tsunami recorded at Ishigakijima, Japan.
  • Here is the map with 3 month’s seismicity plotted.

  • Here is an updated map with aftershocks and my tide gage interpretive plot.

Shaking Intensity

  • Here is a figure that shows a more detailed comparison between the modeled intensity and the reported intensity. Both data use the same color scale, the Modified Mercalli Intensity Scale (MMI). More about this can be found here. The colors and contours on the map are results from the USGS modeled intensity. The DYFI data are plotted as colored dots (color = MMI, diameter = number of reports).
  • In the upper panel is the USGS Did You Feel It reports map, showing reports as colored dots using the MMI color scale. Underlain on this map are colored areas showing the USGS modeled estimate for shaking intensity (MMI scale).
  • In the lower panel is a plot showing MMI intensity (vertical axis) relative to distance from the earthquake (horizontal axis). The models are represented by the green and orange lines. The DYFI data are plotted as light blue dots. The mean and median (different types of “average”) are plotted as orange and purple dots. Note how well the reports fit the green line (the model that represents how MMI works based on quakes in California).
  • Below the lower plot is the USGS MMI Intensity scale, which lists the level of damage for each level of intensity, along with approximate measures of how strongly the ground shakes at these intensities, showing levels in acceleration (Peak Ground Acceleration, PGA) and velocity (Peak Ground Velocity, PGV).
  • The first figure is for the M 7.4 earthquake and the second figure is for the M 6.4 earthquake. Note the large difference in shaking intensity between these two figures.
  • For the M 7.4, why do you think that the Did You Feel It? observations are so much higher in intensity than the models “predict?”

    Supportive Figures

  • I could not help myself. I am so excited to have this website back up and running, like a fully operational space station, that I include below some additional figures that help us understand the tectonic setting.
  • Here is the low angle oblique view of the plate configuration in Taiwan.

  • Here is the map from Chen at al. (2020) that shows the fault mapping in this area of eastern Taiwan.

  • Geologic map of the Coastal Range on shaded relief (after Wang and Chen, 1993). The Longitudinal Valley Fault (LVF) can be subdivided into the Linding and Juisui locked Fault and the Chihshang and Lichi creeping Fault. Vertical cross-sections of VS perturbation tomography along the AeA0 and BeB0 profiles denote the Central Range, the Coastal Range, and the LVF. EU: Eurasian Plate; PH: Philippine Sea Plate.

  • Here is an oblique view of the plate configuration in this region. This is from Chang (2001).

  • Here is a great interpretation showing how the Island of Taiwan is being uplifted and exhumed. This is from Lin (2002).

  • Needless to say, this is an excellent map showing the complicated faulting of this region. This is from Theunissen et al. (2012).

  • Here is another tectonic interpretation map from here.
  • Here is a great general overview of the tectonics of the region from Shyu et al. (2005). I include their figure caption below the image as a blockquote.

  • A neotectonic snapshot of Taiwan and adjacent regions. (a) Taiwan is currently experiencing a double suturing. In the south the Luzon volcanic arc is colliding with the Hengchun forearc ridge, which is, in turn, colliding with the Eurasian continental margin. In the north both sutures are unstitching. Their disengagement is forming both the Okinawa Trough and the forearc basins of the Ryukyu arc. Thus, in the course of passing through the island, the roles of the volcanic arc and forearc ridge flip along with the flipping of the polarity of subduction. The three gray strips represent the three lithospheric pieces of Taiwan’s tandem suturing and disarticulation: the Eurasian continental margin, the continental sliver, and the Luzon arc. Black arrows indicate the suturing and disarticulation. This concept is discussed in detail by Shyu et al. [2005]. Current velocity vector of the Philippine Sea plate relative to the Eurasian plate is adapted from Yu et al. [1997, 1999]. Current velocity vector of the Ryukyu arc is adapted from Lallemand and Liu [1998]. Black dashed lines are the northern and western limits of the Wadati-Benioff zone of the two subducting systems, taken from the seismicity database of the Central Weather Bureau, Taiwan. DF, deformation front; LCS, Lishan-Chaochou suture; LVS, Longitudinal Valley suture; WF, Western Foothills; CeR, Central Range; CoR, Coastal Range; HP, Hengchun Peninsula. (b) Major tectonic elements around Taiwan. Active structures identified in this study are shown in red. Major inactive faults that form the boundaries of tectonic elements are shown in black: 1, Chiuchih fault; 2, Lishan fault; 3, Laonung fault; 4, Chukou fault. Selected GPS vectors relative to the stable Eurasian continental shelf are adapted from Yu et al. [1997]. A,Western Foothills; B, Hsueshan Range; C, Central Range and Hengchun Peninsula; D, Coastal Range; E, westernmost Ryukyu arc; F, Yaeyama forearc ridge; G, northernmost Luzon arc; H, western Taiwan coastal plains; I, Lanyang Plain; J, Pingtung Plain; K, Longitudinal Valley; L, submarine Hengchun Ridge; M, Ryukyu forearc basins.

  • This figure from Shyu et al. (2005) shows their interpretation of the different tectonic domains in Taiwan. This is a complicated region that includes collision zones in different orientations as the Okinawa Trough, Ryukyu Trench, and Manila Trench (all subduction zones) each intersect beneath and adjacent to Taiwan. I include their figure caption below the image as a blockquote.

  • Map of major active faults and folds of Taiwan (in red) showing that the two sutures are producing separate western and eastern neotectonic belts. Each collision belt matures and then decays progressively from south to north. This occurs in discrete steps, manifested as seven distinct neotectonic domains along the western belt and four along the eastern. A distinctive assemblage of active structures defines each domain. For example, two principal structures dominate the Taichung Domain. Rupture in 1999 of one of these, the Chelungpu fault, caused the disastrous Chi-Chi earthquake. The Lishan fault (dashed black line) is the suture between forearc ridge and continental margin. Thick light green and pink lines are boundaries of domains.

  • This map from Shyu et al. (2005) shows the earthquake slip regions for proposed earthquake scenarios. I include their figure caption below the image as a blockquote.

  • Proposed major sources for future large earthquakes in and around Taiwan. Thick red lines are proposed future ruptures, and the white patches are rupture planes projected to the surface. Here we have selected only a few representative scenarios from Table 1. Earthquake magnitude of each scenario is predicted value from our calculation.

  • This map from here shows the basement geology of Taiwan. Note the accretionary belts, including the forearc basin. This is a compilation from Teng et al. (2001) and Hsiao et al. (1998) as presented in Ustaszewski et al. (2012).

  • Here is a map from Huang and Wang (2022) that shows the different fault systems in this region.

  • General tectonic map of Taiwan showing major active tectonic elements near Eastern Taiwan. a Eastern Taiwan is experiencing tectonic collisions between the volcanic arc (Coastal Range/Luzon arc) and the basement-involved accretionary wedge (Central Range) at the western edge of the Philippine Sea Plate. More than half of the active convergence between the Philippine Sea Plate and the Eurasian Plate is absorbed by the Longitudinal Valley Suture system (LVS) and its associated structure. In the north, both the coastal range and the Longitudinal Valley Suture intersects to the Ryukyu Trench, with the Okinawa Trough and Ryukyu Forearc Ridge formed above the Ryukyu subduction zone. The opening and tectonic rotation near the LVS-Ryukyu trench junction enhance the N–S convergence along the Ryukyu Trench. b Active tectonic map along the Longitudinal Valley shows three major structures from the base of the Central Range to the offshore eastern coast. These structures on the surface include the Central Range Fault, The Longitudinal Valley Fault, and the Chimei Canyon Thrust as the deformation front of the Offshore Eastern Taiwan Thrust Belt (Active fault map is modified from Shyu et al. (2005a, b), Shyu et al. (2020), velocity of PSP is from Yu et al. (1997); the velocity of Ryukyu Arc is from Lallemand and Liu (1998))

  • Here is the seismicity used by Huang and Wang (2022) to investigate the subsurface structures along the Longitudinal Valley.
  • These authors used advanced methods to relocate earthquakes (using a 3-D velocity model, since seismic wave velocity can affect the location calculations). Once relocated, they could identify faults in the Earth.
  • The profiles displayed on this map show the locations for the cross-sections in the next figure.

  • Distribution of background seismicity (grey-colored dots) and extracted earthquake clusters (colored dots) from January 1991 to August 2021. Locations for the profiles shown in Figs. 6, 7 and 8 are indicated as black lines here

  • Here is a group of cross-sections from Huang and Wang (2022) that shows the different fault systems in this region.
  • The view is from outer space, looking to the northwest.

  • A schematic structural model of the northern Longitudinal Valley showing the truncation relationship between the active Central Range Fault system and the active Longitudinal Valley Fault system. Black lines are the simplified seismogenic fault of the Central Range Fault system, with blue sub-vertical splay faults developed within the Central Range blocks. The Red lines are faults within the Longitudinal Valley Fault system, including the Longitudinal Valley Fault (LVF) and the Chimei Fault. Gray lines represent the location and approximate geometry of the Takangkou-Height Thrust from Hsieh et al. (2020), while the Green lines show the Milun fault (M.L.F.) and its associated faults offshore Hualien


    Geodesy is the study of the movement of the Earth.

    Geodesists use measurements between the locations of different places at different times to calculate how those locations move relative to each other.

    One way to do this is to use GPS (or, GNSS) observations for these locations. As these locations more relative to each other, we can quantify this.

    These motions are quantified by relative direction and by the rate (e.g., the speed, like meters per second, or miles per hour).

    • This map shows the relative motion of GPS/GNSS locations.
    • The map on the left shows horizontal motion and the map on the right shows vertical motion.
    • Each arrow orientation represents the relative motion direction and the length of the arrow (making it a vector) represents the rate of motion (see the scale, in mm per year).

    • GPS velocities. Red lines are traces of model faults. The origin of the coordinate system is located at 22N, 120E. (a) Horizontal. Ellipses are 2s confidence regions. White dashed boxes indicate locations of two profiles in Figure 3. Only 54 sites out of 75 are shown for legibility. (b) Vertical. Circles are 2s confidence regions.

    • These are the GPS/GNSS motion rates from sites in the two white-outlined profiles in the above map.
    • The upper two panels are the relative horizontal motion and the lower panel shows the vertical rates.
    • The authors note the location of the two main fault traces (the CRF and LVF).
    • Note how the rates change near the fault traces. Can you tell which fault(s) are controlling the motion of the Earth here? In what way?

    • Profiles of GPS velocities. Dotted lines bound Longitudinal Valley (LV). (a) Northern profile. The gradient of velocities is distinct across the LV. (b) Southern profile. A step-like change in velocity appears at the surface trace of the Longitudinal Valley fault.

    • These authors use their geodetic data to constrain fault modeling. This figure shows the results of their modeling.
    • These two figures show the two faults (LVF above and CRF below).
    • The color of the fault represents the slip rate for that part of the fault. Which fault has a higher slip rate?
    • The higher the slip rate, the more tectonic strain accumulates each year.

    • Estimated Holocene slip rate vectors on faults using two-fault lithospheric block model.
      (a) Longitudinal Valley fault (LVF). (b) Central Range fault (CRF).

    • Here is a figure from Tang et al. (2023) where they use GPS/GNSS displacements to constrain fault modeling.
    • While the above geodetic analyses were for long term motion between earthquakes (the interseismic part of the earthquake cycle), this analysis is from the relative motion during an earthquake (called the coseismic period).
    • These authors conducted their earthquake modeling for an earthquake sequence from September, 2022.

    • Surface displacements of the September 2022 Chihshang earthquake sequence. a Tectonic setting of the Longitudinal Valley suture zone (LVS). CeR: Central Range; CoR: Coastal Range. b Cross section view showing the distribution of background seismicity between 1990 and 2020 (light gray dots)55. Relocated aftershocks prior to November 17 are in black dots (catalog from Taiwan Geophysical Database Management System, Taiwan GDMS, See (d) for profile location. c Coseismic interferogram and GNSS horizontal displacements for the foreshock56. Traces of the Central Range fault and the Longitudinal Valley fault (LVF) are based on the Taiwan Earthquake Model31. Traces of the 1951 surface ruptures on the Yuli fault (YLF) and the LVF are based on previous field surveys28,29. LOS: radar line-of-sight direction. d Coseismic interferogram and GNSS horizontal displacements for the mainshock. e Cumulative N–S offsets from the foreshock to the mainshock. Vectors represent the GNSS vertical coseismic displacement for the mainshock.

    • Here is a figure from Tang et al. (2023) where they show the coseismic offsets in the geodetic data, the relative offsets as vectors, and their subsurface fault model.

    • Field photos and geodetic observations of the ruptures along the CRF and LVF. a Map showing locations of the surface ruptures and field photos along (b, c) the CRF and (d, e) the LVF. f Coseismic GNSS displacements for the mainshock near Chihshang. g North and vertical component of the GNSS position time series at station TAPE (30m west of the LVF), TAPO (540m east of the LVF) and CE2A (2.3 km east of the LVF)56. FS foreshock, MS mainshock. The northward and upward coseismic motion of TAPO during the mainshock deviates from the motions of other stations in the hanging wall of the CRF (with southwestward and upward coseismic motion), as well as CE2A located further east (with northwestward and downward coseismic motion). This anomalous motion suggests the near-field interference from the LVF, most likely a coseismic shallow slip.

  • Here is a figure from Tang et al. (2023) where they show the coseismic fault model.
  • The warmer the color, the more that the fault moved.
  • On the right are time steps, showing how the fault slip migrated (moved) during the earthquake.

  • Coseismic slip during the September 2022 Chihshang earthquake sequence. a Overview of the kinematic model constrained from satellite imagery and GNSS coseismic displacements56. Brown contours and colored patches represent the slip during the foreshock and mainshock, respectively. Gray circles are aftershocks between the foreshock and mainshock. YL: Yuli. CS: Chihshang. b Source time functions obtained from the rupture process modeling constrained by HR-GNSS data. Gray and black plots represent the foreshock and the mainshock on the CRF, respectively, whereas orange curve shows contributions of the LVF during the mainshock. Records from two of the HR-GNSS sites (KUA2 and FUDN) are shown in (a). Black and magenta curves are observed and modeled records. c, d Rupture process for the foreshock. Hatched areas indicate the fault plane excluded from the inversion. e–g Rupture process for the mainshock.

Potential for Ground Failure

Luckily I updated this page because I noticed that the interpretive figure below was incorrect (it was for a different earthquake).

  • Below are a series of maps that show the potential for landslides and liquefaction. These are all USGS data products.
    There are many different ways in which a landslide can be triggered. The first order relations behind slope failure (landslides) is that the “resisting” forces that are preventing slope failure (e.g. the strength of the bedrock or soil) are overcome by the “driving” forces that are pushing this land downwards (e.g. gravity). The ratio of resisting forces to driving forces is called the Factor of Safety (FOS). We can write this ratio like this:

    FOS = Resisting Force / Driving Force

  • When FOS > 1, the slope is stable and when FOS < 1, the slope fails and we get a landslide. The illustration below shows these relations. Note how the slope angle α can take part in this ratio (the steeper the slope, the greater impact of the mass of the slope can contribute to driving forces). The real world is more complicated than the simplified illustration below.

  • Landslide ground shaking can change the Factor of Safety in several ways that might increase the driving force or decrease the resisting force. Keefer (1984) studied a global data set of earthquake triggered landslides and found that larger earthquakes trigger larger and more numerous landslides across a larger area than do smaller earthquakes. Earthquakes can cause landslides because the seismic waves can cause the driving force to increase (the earthquake motions can “push” the land downwards), leading to a landslide. In addition, ground shaking can change the strength of these earth materials (a form of resisting force) with a process called liquefaction.
  • Sediment or soil strength is based upon the ability for sediment particles to push against each other without moving. This is a combination of friction and the forces exerted between these particles. This is loosely what we call the “angle of internal friction.” Liquefaction is a process by which pore pressure increases cause water to push out against the sediment particles so that they are no longer touching.
  • An analogy that some may be familiar with relates to a visit to the beach. When one is walking on the wet sand near the shoreline, the sand may hold the weight of our body generally pretty well. However, if we stop and vibrate our feet back and forth, this causes pore pressure to increase and we sink into the sand as the sand liquefies. Or, at least our feet sink into the sand.
  • Below is a diagram showing how an increase in pore pressure can push against the sediment particles so that they are not touching any more. This allows the particles to move around and this is why our feet sink in the sand in the analogy above. This is also what changes the strength of earth materials such that a landslide can be triggered.

  • Below is a diagram based upon a publication designed to educate the public about landslides and the processes that trigger them (USGS, 2004). Additional background information about landslide types can be found in Highland et al. (2008). There was a variety of landslide types that can be observed surrounding the earthquake region. So, this illustration can help people when they observing the landscape response to the earthquake whether they are using aerial imagery, photos in newspaper or website articles, or videos on social media. Will you be able to locate a landslide scarp or the toe of a landslide? This figure shows a rotational landslide, one where the land rotates along a curvilinear failure surface.

  • Below is the liquefaction susceptibility and landslide probability map (Jessee et al., 2017; Zhu et al., 2017). Please head over to that report for more information about the USGS Ground Failure products (landslides and liquefaction). Basically, earthquakes shake the ground and this ground shaking can cause landslides.
  • I use the same color scheme that the USGS uses on their website. Note how the areas that are more likely to have experienced earthquake induced liquefaction are in the valleys. Learn more about how the USGS prepares these model results here.
  • As for the intensity figures, below the upper figure is for the M 7.4 and the lower figure is for the M 6.4.

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