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- 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
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- Here is the map with a month’s seismicity plotted.<\/li>\n
\n![\"\"](\"https:\/\/earthjay.com\/earthquakes\/20260102_mexico\/20260102_mexico_interpretation.jpg\")
<\/a><\/p>\n<\/ul>\nOther Report Pages<\/font><\/strong><\/H2><\/p>\n
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- 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\/20250328_burma\/20250328_burma_intensity_interpretation_20250404.jpg\")
<\/a><\/p>\n<\/ul>\nPotential for Ground Failure<\/font><\/strong><\/h2>\n
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- 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![\"\"](\"https:\/\/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![\"\"](\"https:\/\/earthjay.com\/earthquakes\/20180928_indonesia\/liquefaction_pore_pressure_change_patton.jpg\")
<\/a><\/p>\n- Here Dr. Bohon demonstrates the phenomena of liquefaction.<\/li>\n
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@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 - 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
Shaking Intensity<\/font><\/strong><\/h2>\n
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- 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
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<\/ul>\n
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- Some basic fundamentals of earthquake geology and plate tectonics can be found on the Earthquake Plate Tectonic Fundamentals page<\/a>.<\/li>\n<\/ul>\n