This earthquake has a normal (extensional) mechanism (moment tensor).<\/p>\n
I will next briefly describe the regional tectonic setting that drives the development of tectonic faults. I next present the interpretive poster, followed by a review of the literature.<\/p>\n
Tectonic Background<\/font><\/h2>\nCentral China and Tibet here, part of the Tibetan Plateau, is dominated by the plate tectonics and climate.<\/p>\n
To the south is the India plate that is moving northwards, pummeling into the southern part of the Eurasia plate, at a rate of about 25 to 50 mm per year, depending upon the reference frame (Pusok and Stegman, 2020). The India plate began moving northward from Antarctica since before 80 million years ago.<\/p>\n
The details of the story has changed as more geological information is interpreted. But the general story is that the India plate moved away from Antarctica as an oceanic spreading center formed between these plates. The India plate moved towards Asia.<\/p>\n
Prior to about 45-50 Ma, there was oceanic crust between India and Eurasia. But at this time, the continental crust of the India and Eurasia plates collided. This collision would eventually cause uplift of the Himalaya mountain range and the Tibetan Plateau to the north of the Himalaya.<\/p>\n
There are marine fossils on the top of the mountains in the Himalaya! (this is how we know there was ocean between these plates in the past)<\/p>\n
Here is a time series showing the convergence of these two plates modified from Pusok & Stegman (2020) and the USGS.<\/p>\n
\n![\"\"](\"http:\/\/earthjay.com\/earthquakes\/20210521_china\/india_eurasia_convergence.jpg\")
<\/a><\/p>\nA part of the tectonic story is told by one of the rock stars of plate tectonics, Dr. Paul Tapponier. Tapponier conducted experiments that showed how north-south convergence, like that of India and Asia, coupled with a backstop (something that is more difficult to move) from the west, would lead to some crust to squish out to the east.<\/p>\n
This is called extrusion tectonics as the crust of eastern Asia is being extruded to the east, like a watermelon seed is extruded from between one’s fingers when they squeeze on the wet seed.<\/p>\n
Below is a color version of the results from Tapponier’s experiment. Compare this with maps showing the GPS motion of the crust in this region.<\/p>\n
\n![\"\"](\"http:\/\/earthjay.com\/earthquakes\/20210521_china\/tapponier_etal_1982_extrusion.png\")
<\/a><\/p>\nNote that the plastic has numerous faults develop as part of the extrusion. We can see how the blue and yellow lines show lateral offset along these faults.<\/p>\n
Many of these faults are left-lateral strike-slip faults. Strike-slip means that the curst moves side-by-side when looking down on the crust from outer space (or an airplane, or Google Earth).<\/p>\n
Left-lateral means that, when standing on one side of the fault, looking across the fault at something, that thin one is looking at is moving to the left during an earthquake. More about tectonic fundamentals here<\/a>.<\/p>\nIn the map on the right ^^^ we can see that these left-lateral strike-slip faults that are mapped in the region are just like the faults in the blue-yellow plastic.<\/p>\n
One of the major left-lateral strike-slip faults along the Tibetan Plateau is the Kunlun fault, which has been well studied in places. A tectonic history of the region, and how the Kunlun fault fits into this history, is presented by Staisch et al., 2020.<\/p>\n
The M 7.0 earthquake appears to have slipped on a previously unmapped faults (see aftershock inset map in the interpretive poster). In this area, there is a major left-lateral strike-slip fault. There are a series of faults that splay from this main fault.<\/p>\n
These splayed faults appear to form horsts and grabens, much like in the basin and range geomorphic province in the western North America (e.g., throughout most of Nevada).<\/p>\n
These are normal faults (they probably have some portion of strike-slip motion but I don’t know) formed from east-west extension in the crust. This extension is part of the extrusion tectonic model.<\/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 1926-2026 with magnitudes M \u2265 6.0 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
\nI 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 plate tectonic map that shows the major faults, plates, and plate boundaries. The Gray arrows show the ways the plates are moving (compare with the Extrusion Tectonics figure).<\/li>\n
- To the right of that tectonic map is the photo of the Tapponier et al. (1982) experiment on the left and a map showing the faults that really exist that match faults from the model.<\/li>\n
- On the right of the Tapponier et al. (1982) experiment is a diagram that shows the evolution of faulting in the region that are the result of changes in driving forces (e.g. broad scale tectonic plate motions and how their relative motions cause different types of faults at different times; also that these older faults remain and may be reactivated as different types of faults later (Staisch et al., 2020).<\/li>\n
- I also plot the aftershocks here to show how they relate to the mapped fault on the left. I present an hypothetical fault responsible for the M7.0 in the context of the other mapped faults. The aftershocks span a distance of 70km.<\/li>\n
- In the lower left corner is the US Geological Survey finite fault slip model<\/a>. This graphic shows their estimate of how much the fault slipped during this earthquake, and where it slipped.<\/li>\n
- In the upper right corner are maps that display data from the USGS event page. On the left is the modeled landslide probability for this earthquake. On the right is a map that shows the susceptibility (chance of) liquefaction from this M 7.3 earthquake. The headwaters of the Yellow River feed the lakes to the west of the earthquake and this river valley is where the Maduo Grande fault is mapped.<\/li>\n
- In the lower right corner is a map that shows the earthquake shaking intensity modeled for this earthquake. The colors use the Modified Mercalli Intensity scale (MMI), which is listed in the upper right margin (compare the numbers on the map with the data in the table). <\/li>\n
- In the lower left center is a map that has red and blue arrows (Taylor and Lin, 2009). These arrows represent places that are moving as measured by GPS observations. The orientation of the arrow shows the direction the plate is moving relative to “stable” Eurasia (data are from Zhang et al., 2004). I place a yellow star in the location of the M 7.0 epicenter.<\/li>\n<\/ul>\n
\n- Here is the map with a month’s seismicity plotted.<\/li>\n
\n![\"\"](\"https:\/\/earthjay.com\/earthquakes\/20261022_china\/20261022_tibet_interpretation.jpg\")
<\/a><\/p>\n<\/ul>\nOther Report Pages<\/font><\/strong><\/H2><\/p>\n\n<\/ul>\nShaking Intensity<\/font><\/strong><\/h2>\n\n- 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<\/a>. 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).<\/li>\n
- 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).<\/li>\n
- 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).<\/li>\n
- Below the map and 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).<\/li>\n
\n![\"\"](\"https:\/\/earthjay.com\/earthquakes\/20261022_china\/20261022_tibet_intensity_interpretation.jpg\")
<\/a><\/p>\n<\/ul>\nPotential for Ground Failure<\/font><\/strong><\/h2>\n\n- Below are a series of maps that show the potential for landslides and liquefaction. These are all USGS data products.
\nThere are many different ways in which a landslide can be triggered. The first order relations behind slope failure (landslides) is that the \u201cresisting\u201d forces that are preventing slope failure (e.g. the strength of the bedrock or soil) are overcome by the \u201cdriving\u201d 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:<\/p>\nFOS = Resisting Force \/ Driving Force<\/strong><\/p>\n<\/li>\n- 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 \u03b1 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.<\/li>\n
\n![\"\"](\"http:\/\/earthjay.com\/earthquakes\/20180928_indonesia\/force_balance_landscape.jpg\")
<\/a><\/p>\n- 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 \u201cpush\u201d 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.<\/li>\n
- 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 \u201cangle of internal friction.\u201d 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.<\/li>\n
- 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.<\/li>\n
- 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.<\/li>\n
\n![\"\"](\"http:\/\/earthjay.com\/earthquakes\/20180928_indonesia\/liquefaction_pore_pressure_change_patton.jpg\")
<\/a><\/p>\n- Here Dr. Bohon demonstrates the phenomena of liquefaction.<\/li>\n
\n @drwendyrocksit<\/a> #Liquefaction<\/a> is a process by which water-saturated sediment temporarily loses strength and acts as a fluid. This can happen during #earthquake<\/a> shaking. #geophysics<\/a> #geology<\/a> \u266c Quicksand – Hatchie<\/a> <\/section>\n<\/blockquote>\n
\n
<\/a><\/p>\nA part of the tectonic story is told by one of the rock stars of plate tectonics, Dr. Paul Tapponier. Tapponier conducted experiments that showed how north-south convergence, like that of India and Asia, coupled with a backstop (something that is more difficult to move) from the west, would lead to some crust to squish out to the east.<\/p>\n
This is called extrusion tectonics as the crust of eastern Asia is being extruded to the east, like a watermelon seed is extruded from between one’s fingers when they squeeze on the wet seed.<\/p>\n
Below is a color version of the results from Tapponier’s experiment. Compare this with maps showing the GPS motion of the crust in this region.<\/p>\n
Note that the plastic has numerous faults develop as part of the extrusion. We can see how the blue and yellow lines show lateral offset along these faults.<\/p>\n Many of these faults are left-lateral strike-slip faults. Strike-slip means that the curst moves side-by-side when looking down on the crust from outer space (or an airplane, or Google Earth).<\/p>\n Left-lateral means that, when standing on one side of the fault, looking across the fault at something, that thin one is looking at is moving to the left during an earthquake. More about tectonic fundamentals here<\/a>.<\/p>\n In the map on the right ^^^ we can see that these left-lateral strike-slip faults that are mapped in the region are just like the faults in the blue-yellow plastic.<\/p>\n One of the major left-lateral strike-slip faults along the Tibetan Plateau is the Kunlun fault, which has been well studied in places. A tectonic history of the region, and how the Kunlun fault fits into this history, is presented by Staisch et al., 2020.<\/p>\n The M 7.0 earthquake appears to have slipped on a previously unmapped faults (see aftershock inset map in the interpretive poster). In this area, there is a major left-lateral strike-slip fault. There are a series of faults that splay from this main fault.<\/p>\n These splayed faults appear to form horsts and grabens, much like in the basin and range geomorphic province in the western North America (e.g., throughout most of Nevada).<\/p>\n These are normal faults (they probably have some portion of strike-slip motion but I don’t know) formed from east-west extension in the crust. This extension is part of the extrusion tectonic model.<\/p>\n FOS = Resisting Force \/ Driving Force<\/strong><\/p>\n<\/li>\n
\n<\/a><\/p>\nBelow is my interpretive poster for this earthquake<\/font><\/H2><\/p>\n
\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
\n
\n<\/a><\/p>\n<\/ul>\nOther Report Pages<\/font><\/strong><\/H2><\/p>\n
\n<\/ul>\n
Shaking Intensity<\/font><\/strong><\/h2>\n
\n
\n<\/a><\/p>\n<\/ul>\nPotential for Ground Failure<\/font><\/strong><\/h2>\n
\n
\nThere are many different ways in which a landslide can be triggered. The first order relations behind slope failure (landslides) is that the \u201cresisting\u201d forces that are preventing slope failure (e.g. the strength of the bedrock or soil) are overcome by the \u201cdriving\u201d 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:<\/p>\n
\n<\/a><\/p>\n
\n<\/a><\/p>\n\n