Well Well Well
Yesterday there was a very interesting magnitude M 6.5 earthquake that ruptured in central Idaho, near the Sawtooth fault.
Idaho lies in the intersection of several different physiographic provinces. Physiographic provinces are areas of Earth that have landforms of similar shape. These landforms are largely caused by tectonics and climate (of course, the climate is controlled largely by tectonics, but there are other factors like the rotation of the planet, convection cells in the atmosphere, etc. well, those convection cells are also controlled by tectonics (i.e. where continents are) too. so, yes, tectonics controls everything (even though it does not).
The two main physiographic provinces (also called geomorphic provinces, after the word “geomorphology” – the shape of the landscape) at play in central Idaho are the Basin and Range and the Rocky Mountains.
- The Rocky Mountains were formed long ago (between 80 and 55 million years ago) and are the result of compressional tectonics that uplifted the continent, forming these mountains. While the compression that formed the Rocky Mtns ceased millions of years ago, the topography remains (e.g. Denver, the mile high city).
- The Basin and Range is a region of the western US and northwestern Mexico that has undergone East-West directed extension since the Miocene (~17 million years ago). This extension forms normal fault bounded basins (valleys), separated by ranges (mountains). These faults generally trend north-south, but there have been several phases of extension in slightly different directions. So, the faults preserve a complicated history of these changes in tectonic regime. Though, the landforms left behind are persistent (the basins and the ranges).
Here is a view of the physiographic provinces in the USA.
There are many different phases of tectonic deformation that formed the geomorphic provinces of North America, so take an historical geology course to learn more!
In northern Idaho, there is additional period of tectonic deformation that left behind geologic structures that appear to be playing a part in the M 6.5 temblor. During the Eocene, there was a period of east-west extension that caused lots of faults to form. These faults have been inactive for a very long time.
However, sometimes there are older inactive faults that are oriented optimally to be reactivated under newer and possibly different tectonic forces. One example of this is in the Gorda plate offshore of northern California. Faults formed along the spreading ridge (the Gorda Rise), initially formed as normal faults, are exposed to north-south oriented compression and reactivate as strike-slip faults.
Here we are, in central Idaho, where there are some Basin and Range faults (generally northwest trending here) that have been responsible for very large historic earthquakes.
- The 1959 Hebgen Lake M 7.3 earthquake in Montana was felt widely, caused surface rupture (where the fault breaks through the ground surface, forming a topographic escarpment called a fault scarp), and triggered many landslides. One of these landslides slipped into a river, blocking the flow of the river, forming a lake. After I defended my Ph.D. I went on a drive about. Beginning at a Geological Society of America meeting in Bozeman (yes, this is what geologists do for their vacation), I drove through Yellowstone and crossed the continental divide to visit friends in Colorado. As I was camping near Yellowstone, I drove to see the scarp from this large earthquake and stopped at the “Earthquake Lake.” Lucky me, it was the opening day of the Earthquake Lake Visitor’s Center (though it turns out it was just a new building, lol). I grabbed an Earthquake Lake coffee mug and went on my way.
- The 1983 magnitude M 6.9 Borah Peak Earthquake ruptured a normal fault about 70 km to the east of yesterday’s M 6.5. That earthquake also caused surface rupture and geologists like Dr. Chris Duross (from the USGS) have been studying that fault to learn about the prehistoric earthquake history.
A recent example of a Basin and Range fault earthquake happened in 2017 in southeastern Idaho, just south of the Snake River Plain (another geomorphic province, formed by the passage of the Yellowstone Hotspot). Here is my report for that earthquake.
The M 6.5 earthquake yesterday happened in an area where a Basin & Range (B&R) fault ends near one of these older Eocene aged faults. Most of us saw the earthquake notification and probably thought that the quake would have been a B&R normal (extensional) fault. However, when the mechanism was posted online, the earthquake mechanism was instead a strike-slip earthquake. This was really interesting. I love when things happen that are unexpected. This is what makes life exciting.
Over the past few years, there has been an increase in the amount of people making observations, looking at the academic and govt literature, and forming hypotheses about these events.It used to be just a few of us, but now the bug has spread and lots of people are part of this educational process. This all is expressed via social media (mostly on twitter), where peoples’ hypotheses are discussed, shot down, or synchronistically further developed to learn something new we were not expecting. I am a coauthor of a forthcoming paper where we discussed some of these events. This is where it happens, online and in real time.
The same was true for this M 6.5 earthquake in Idaho. People started using existing data, using visualizations in Google Earth, and using all the tools we have at our desktop fingertips, to figure out what the heck happened in a remote region of Idaho.
Thanks to the Idaho Geological Survey, I learned of some of the faults in the region. I downloaded their geologic maps and GIS data and started to work.
The main B&R normal fault that may be somehow related to the M 6.5 earthquake is the Sawtooth fault, a northwest trending (striking) fault that Dr. Glenn Thackray (2013) suggested was “Holocene Active.” (This means the last time it had a large earthquake was sometime during the Holocene, or during the last 12,000 years or so.)
Dr. Thackray used newly collected high resolution LiDAR topographic data to identify fault scarps that offset geomorphic features that during Holocene time. If the landforms were created less than 12,000 years ago and the fault cut through these landforms, then the earthquake that cut the landforms happened after the landforms were created (and also less than 12,000 years ago).
Here is a figure from Thackray et al. (2013) that shows the fault they observed (in the inset B, look at the shadow formed by the fault; the arrows are pointing at the fault scarp). This fault is listed as a high priority to be studied, yet there are no published records yet (Crone et al., 2009).
One of the major older faults (Eocene age) that cuts through the center of Idaho is the Trans-Challis fault zones (TCFZ; Bennet, 1986). Based on the work of others (like Kiilsgaard et al., 1986), this fault is thought to be related to the extension from Eocene time and is possibly related to the volcanism and detatchemnt faulting associated with metamorphic core complexes.
Most of the faults in the TCFZ are also normal faults (makes sense since they were formed from extension). However, there are lots of faults of different types as they can form is they are oriented in ways different than the normal faults.
So, at second glance, the M 6.5 event may have been on one of these older faults associated with the TCFZ. Perhaps the pre-existing older fault, which was inactive, was oriented in the correct position to respond to the modern tectonic forces. Thus, this fault would be considered to be reactivated.
At third glance, it is possible that the M 6.5 event happened on a fault not observed at Earth’s surface and could be related to the Sawtooth fault (or some other fault).
The mechanism is not a purely strike-slip earthquake as it is not a 100% double-couple earthquake (a double couple is the type of force that is associated with the crust moving in one direction on one side of the fault and in the other direction on the other side of the fault). Someone has hypothesized that the M 6.5 earthquake may have been complicated and involved both normal and strike-slip faulting. I like this hypothesis as it fits my idea of an older fault being reactivated under a newer (modern = today) tectonic regime.
Something else to note. I took a look at Wells and Coppersmith (1994). These authors use earthquake event data to prepare some empirical relations between earthquakes of various sizes, types,e tc. and the magnitude of those earthquakes. So we can take one parameter and estimate what another parameter may be.
OK, lets look at some eye candy. (sorry for the long introduction)
Below is my interpretive poster for this earthquake
- I plot the seismicity from the past 3 months, with diameter representing magnitude (see legend). I include earthquake epicenters from 1920-2020 with magnitudes M ≥ 6.5.
- 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.
- Some basic fundamentals of earthquake geology and plate tectonics can be found on the Earthquake Plate Tectonic Fundamentals page.
- In the lower left corner is a map of the western USA showing the topography and seismicity for the past 3 months. Note the M 6.5 event in yellow and the recent earthquake in Utah near the Great Salt Lake.
- In the upper left corner I include a map from Bennett (1986) that shows some of the major faults in Idaho. I placed a blue star in the location of the M 6.5 and labeled the Trans-Challis fault zone.
- In the upper right corner I include a map showing the region impacted by this earthquake. The Earthquake Intensity uses the MMI scale (the colors), read more about this here. This map represents an estimate of ground shaking from the M 6.5 based on a statistical model using the results of tens of thousands of earthquakes.
- To the right of the Bennet map is a plot showing how these USGS models “predict” the ground shaking intensity will be relative to distance from the earthquake. These models are represented by the broan and green lines. People can fill out an online form to enter their observations and these “Did You Feel It?” observations are converted into an intensity number and these are plotted as dots in this figure.
- In the lower center is a map from the U.S. Geological Survey National Seismic Hazard Map (Petersen et al., 2019). This map shows the chance that any region may experience strong ground shaking from an earthquake in the next 100 years. The M 6.5 happened in an area thought to have a 36-74% chance of shaking at least MMI VI. Looking at the other plots on this poster, we can see that this map held true. What is the highest MMI in the upper right inset map? What is the highest ground shaking intensity in the plot in the upper center? Most of the observed intensities are less than MMI 6, but there were some.
I include some inset figures. Some of the same figures are located in different places on the larger scale map below.
- After I worked for the day, I thought to put together an updated map with aftershocks plotted, at a larger scale. I had downloaded the 10m digital elevation model data for Idaho about a year ago, so it was easy to load it up as a base map.
- I annotated the Bennet (1986) tectonic map to highlight the different faults (older faults in light orange, younger B&R faults in darker orange). I encircled the area of the M 6.5 sequence.
- These seismicity data are sourced from IRIS’ earthquake browser. The USGS earthquakes website was not working, so I needed to go elsewhere to obtain seismicity data. This has become a problem in the past few years as more and more people find the excellent services from he USGS to be useful to them. This is good and bad. It makes it difficult to get data. Another problem is that the “Did You Feel It” website does not work (the M 7.1 Ridgecrest Earthquake has many fewer DYFI observations due to this problem).
One thing we might do is estimate what the surface rupture length might it take to generate a M 6.5 earthquake. According to the Wells and Coppersmith (1994) empirical relations, there may be a surface rupture length of about 20 km. If we look at the aftershock sequence in the poster below, we might observe that the fault length may be about 24 km. So, while these are not the same thing, they are of about the same scale. (I used the relations in their figure 9)
Earthquake Triggered Landslides
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.
Here is an excellent educational video from IRIS and a variety of organizations. The video helps us learn about how earthquake intensity gets smaller with distance from an earthquake. The concept of liquefaction is reviewed and we learn how different types of bedrock and underlying earth materials can affect the severity of ground shaking in a given location. The intensity map above is based on a model that relates intensity with distance to the earthquake, but does not incorporate changes in material properties as the video below mentions is an important factor that can increase intensity in places.
If we look at the map at the top of this report, we might imagine that because the areas close to the fault shake more strongly, there may be more landslides in those areas. This is probably true at first order, but the variation in material properties and water content also control where landslides might occur.
There are landslide slope stability and liquefaction susceptibility models based on empirical data from past earthquakes. The USGS has recently incorporated these types of analyses into their earthquake event pages. More about these USGS models can be found on this page.
I prepared some maps that compare the USGS landslide probability maps for the 2020 M 6.5 and 1959 M 7.3 Hebgen Lake earthquakes.
- Here is the landslide probability map (Jessee et al., 2018). Below the poster I include the text from the USGS website that describes how this model is prepared.
- Note that they are at different scales.
Nowicki Jessee and others (2018) is the preferred model for earthquake-triggered landslide hazard. Our primary landslide model is the empirical model of Nowicki Jessee and others (2018). The model was developed by relating 23 inventories of landslides triggered by past earthquakes with different combinations of predictor variables using logistic regression. The output resolution is ~250 m. The model inputs are described below. More details about the model can be found in the original publication. We modify the published model by excluding areas with slopes <5° and changing the coefficient for the lithology layer "unconsolidated sediments" from -3.22 to -1.36, the coefficient for "mixed sedimentary rocks" to better reflect that this unit is expected to be weak (more negative coefficient indicates stronger rock).To exclude areas of insignificantly small probabilities in the computation of aggregate statistics for this model, we use a probability threshold of 0.002.
Other Report Pages
Some Relevant Discussion and Figures
- Here is the tectonic map from Bennett (1986). The Challis Volcanics are the stippled areas near the Trans-Challis fault zone.
Trans-Challis fault system and other selected geologic features in Pacific Northwest and southern British Columbia, Canada. Modified from Tipper et al. (1981); strontium data from Armstrong (1979) and Armstrong et al. (1977). Volcanics: 1—McAbee Basin; 2—Tranquille Basin; 3—Monte Lake Volcanics; 4—Torada graben; 5—Republic graben; 6—Kettle graben; 7—Clarno Volcanics; 8—Challis Volcanics; 9—Challis Volcanics in Owyhee County. Core complexes: A—Shuswap Complex; B—Valhalla gneiss dome and Passmore gneiss dome; C— Kcitie yueiss dome; D—Okanogan gneiss dome; E—Selkirk igneous complex (Kaniksu batholith); F—Spokane dome; 6—Boehls Butte Formation; H—Pioneer Mountain core complex; I—House Mountain metamorphic complex. X—Chilly Buttes; Borah Peak earthquake, October 28,1983. Dashdot line = boundary of Basin and Range province in Oregon.
- This is a larger scale map showing some of the detailed fault mapping (Bennett, 1986). The Trans-Challis fault system is the northeast trending faults. Normal (extensional) faults are shown with symbols that look like small balls at the end of tiny sticks. The balls are on the side of the fault that goes down.
- Note the location of Stanley, Idaho. I labeled the location of Stanley in the updated poster above, as well as the landslide probability map.
- The M 6.5 earthquake is to the northwest of Stanley, just to the east of the Knapp Creek graben.
Major geologic features of trans-Challis fault system in central Idaho. Modified from Kiilsgaard et al. (1986).
- This map shows the geologic structures formed at different times since the Jurassic (150-200 million years ago), through the Eocene (56-34 million years ago).
Tectonic map of the western United States, showing the major components of the Cordilleran orogenic belt. The initial Sr ratio line is taken to represent the approximate western edge of North American cratonic basement (Armstrong and others, 1977; Kistler and Peterman, 1978). Abbreviations as follows: CRO, Coast Range ophiolite; LFTB, Luning-Fencemaker thrust belt; CNTB, Central Nevada thrust belt; WH, Wasatch hinge line; UU, Uinta Mountains uplift; CMB, Crazy Mountains basin; PRB, Powder River basin; DB, Denver basin; RB, Raton basin. Precambrian shear zones after Karlstrom and Williams (1998).
- Here is another version of that map. The Idaho Batholith is the plus “+” symbolized ares in central western Idaho, a magmatic arc formed adjacent to an ancient convergent plate boundary.
Simplified version of figure 2, showing some of the major tectonic features in the Cordilleran thrust belt discussed in the text. Abbreviations as follows: LCL, Lewis and Clark line; SWMT, Southwest Montana transverse zone; CC, Cabin culmination; WC, Wasatch culmination; SAC, Santaquin culmination; SC, Sevier culmination; CNTB, Central Nevada thrust belt; LFTB, Luning-Fencemaker thrust belt; WH, Wasatch hinge line. Stippled region represents Cordilleran foreland basin system.
- If one looks at the updated aftershock poster above, or the Bennet (1986) map that shows the B&R faults in dark orange. These are some of the faults in the figure below, from Janecke (1992).
- The fault (thick black line) the is southwest of the Lost River Range and extends southeast of Challis is the Lost River fault zone.
Location map of central Idaho showings elected Cenozoic normal faults. Solid triangles hows location of tilted Tertiary conglomerates in the footwall of the Pass Creek fault system. Widely-spaced diagonal rule shows Trans-Challis zone. Selected Tertiary plutons are cross-hatched. Small dots outline late Cenozoic basin fill. Numerous NE striking normal faults in the central Lost River Range are omitted for clarity. BPH is Borah Peak horst; WKH is White Knob horst; PCWC is Pass Creek-Wet Creek reentrant.
- This is a great cross section to check out the proposed geometry of some of these normal faults (Janecke, 1992),
Northwest-southeast cross section of three NE striking normal faults. Volcanic rocks are stippled. Location of cross section is in above map. Restoration indicates 30% extension during synvolcanic faulting int he area. The Long Lost fault may have been reactivated.
- This is a map that shows the geologic regions of Idaho (Kuntz et al., 1982). The Idaho Batholith is the mapped geologic unit where the M 6.5 earthquake happened.
Generalized map of southern Idaho showing major geologic and physiographic features and locations referred to in the text.
- A recent study of the Lost River fault by DuRoss et al. (2019) has given us an idea about how much that fault slips during earthquakes. This is the fault that ruptured during the Borah Peak earthquake in 1983.
- This is a map showing the part of the fault that they studied.
Surface-rupture extent of the 1983 Mw 6.9 Borah Peak earthquake (red), which ruptured the Thousand Springs and southernmost Warm Springs sections of the Lost River fault zone (LRFZ). The Willow Creek Hills are an area of hanging-wall bedrock and complex surface faulting that form a normal-fault structural barrier between the two sections. Yellow polygons show the extent of digital surface models generated in this study using low-altitude aerial imagery derived from unmanned aircraft systems. Fault traces and time of most recent faulting modified from U.S. Geological Survey (2018). Focal mechanism from Doser and Smith (1985); approximate location is 10 km south of figure extent (Richins et al., 1987). Triangles indicate paleoseismic sites: RC—Rattlesnake Creek; SC—Sheep Creek; PS—Poison Spring; DP—Doublespring Pass; EC—Elkhorn Creek; MC—McGowen Creek. Inset map shows regional context. LFZ—Lemhi fault zone; BFZ—Beaverhead fault zone; ESRP—Eastern Snake River Plain; INL—Idaho National Laboratory. Base maps are National Elevation Data set 10 m and 30 m (inset map) digital elevation models.
- Dr. DuRoss and his colleagues made a series of measurements of the displacement across the fault for for past earthquakes, including a surface measurement from the most recent 1983 earthquake (using a high resolution topographic model they created using aerial images they collected and “structure from motion” computer processing they applied. Using these different measurements, along with radiocarbon ages of the timing of these past earthquakes, we can get an idea about what type of size of an earthquake happens here and how often.
- This is the type of information that is used to create seismic hazard maps. The first figure shows two estimates of slip for the 1983 earthquake along the Warm Springs section of the Lost River fault.. The lower panel shows the slip distribution for the penultimate (PE1) and the ante-penultimate (PE2) earthquakes.
- This second figure shows something similar for the Arentson Gulch fault, a system that crosses the valley in the middle of the valley to the west of the Lost River Mtns.Knowing about how much this fault slips during earthquakes allows us to consider different earthquake models and how these faults interact with each other during earthquakes.
Vertical separation (VS) along the southern 8 km of Warm Springs section. (A) 1983 VS measured in this study (red) compared to those of Crone et al. (1987) (blue) for the 1983 surface rupture. RC shows displacement measured at the Rattlesnake Canyon trench (Schwartz, written communication, 2016). (B) Cumulative VS for prehistoric scarps along the Warm Springs section, showing scarps having VS of ≤2 m (PE1; blue line and shading) and >2 m (PE2; magenta line and shading). Plus signs (1983 rupture) and circles (prehistoric) indicate preferred VS values; vertical lines show min-max VS range based on multiple VS measurement iterations.
Vertical separation (VS) along the 8-km-long Arentson Gulch fault near the northernmost Thousand Springs section. (A) 1983 VS measured in this study (red) compared to those of Crone et al. (1987) (blue) for the 1983 surface rupture. (B) Cumulative VS for prehistoric scarps (squares), including VS for compound (including 1983 and prehistoric displacement) and single-event (prehistoric displacement only) scarps.
- Here is a compilation of all their data for slip along the different faults in their study.
Summary of vertical separation (VS) along the Warm Springs and Thousand Springs sections. (A) Cumulative VS, showing Warm Springs section scarps (magenta and blue) and the 1983 rupture (red). Prehistoric scarps along the northern Thousand Springs section (gray circles; this study) show a pattern of VS decreasing toward the Willow Creek Hills that is similar to the 1983 (red) and prehistoric (green) VS curves for the Arentson Gulch fault. The VS curve for the 1983 rupture of the Thousand Springs section (kilometers 13–34) is fit to data reported in Crone et al. (1987). (B) Per-event vertical displacement based on mean displacement difference curves (see text for discussion). Along the Warm
Springs section, prehistoric ruptures PE2 (magenta) and PE1 (blue) show significantly more displacement than the 1983 rupture (red). Green line shows prehistoric VS along the Arentson Gulch fault. Gray box shows extent of the Willow Creek Hills structure along the Lost River fault zone. Triangles show paleoseismic sites. SC—Sheep Creek; DP—Doublespring Pass.
- 2020.03.18 M 5.7 Salt Lake City, Utah
- 2020.03.31 M 6.5 Idaho
- 2017.09.02 M 5.3 Idaho
- 2016.12.28 M 5.7 swarm Nevada
- 2014.11.05 M 4.6 swarm Nevada
Basin and Range
Poorly constrained, first-motion mechanism is similarhttps://t.co/X6uaWFVKia pic.twitter.com/9MTWWeo3SY
— Anthony Lomax 🌍🇪🇺 (@ALomaxNet) April 1, 2020
#EarthquakeReport for #Idaho #Earthquake near #Stanley and #Challis #IdahoEarthquake
updated @USGSBigQuakes landslide maps@IDGeoSurvey faults@IRIS_EPO sourced #AftershockUpdate!
report w tectonic backgrhttps://t.co/cVonuc98VA pic.twitter.com/fAKT7YGsRA
— Jason "Jay" R. Patton (@patton_cascadia) April 2, 2020
Epicenter of today's #idahoearthquake placed on our Miocene and Younger Fault map (M-8)-ZOOMED IN. It falls close to the trans-Challis faults system and northern end of the Basin and Range. Thanks @cmcfeeney for quick turn around. pic.twitter.com/egOkiwnj3F
— ID Geological Survey (@IDGeoSurvey) April 1, 2020
Mw=6.4, WESTERN IDAHO (Depth: 16 km), 2020/03/31 23:52:31 UTC – Full details here: https://t.co/KoSYYgCGpJ pic.twitter.com/JtS0KbyVxj
— Earthquakes (@geoscope_ipgp) April 1, 2020
See those waves on Lake Okanagan? It's a perfectly still day and no boats are out. That's from the earthquake in Idaho 10 min ago pic.twitter.com/ztFWWBErb8
— Molly Millions (@lilorphanammo) April 1, 2020
Exactly! There are many miocene-Quaternary faults documented by the Idaho Geologic Survey with similar north-northeast orientations pic.twitter.com/8DmoJmmosG
— Colin Chupik (@ChupikColin) April 1, 2020
Geomorphically, there are some compelling uphill-facing scarps along the linear valley where the Idaho epicenter sits, precisely atop the "Pre-Miocene" strike-slip Trans-Challis Fault Zone.
cf. fault maps https://t.co/d28LmscgnFhttps://t.co/HI9CvkxAwshttps://t.co/P3jEoJ2KLM pic.twitter.com/i6IYXgS9xE
— Austin Elliott (@TTremblingEarth) April 1, 2020
Today's quake struck off the end of the Sawtooth Fault, which accommodates E-W stretching of the northern Basin and Range. Could the Sawtooth Fault now unzip? It happened before, in 1983, when the nearby Lost River Fault ruptured in an M 7.3 earthquake. pic.twitter.com/msbHRqg0Px
— temblor (@temblor) April 1, 2020
The USGS ShakeMap of ground motion intensity has been updated since y'day, as new constraints have come in. Main new constraints appear to be the DYFI community responses, as new pockets of MMI V emerged in Snake River Plain towns with clusters of respondents but few seismometers https://t.co/ZPBcCugUBJ pic.twitter.com/iYNuuVpoGD
— Austin Elliott (@TTremblingEarth) April 1, 2020
Watch the waves from the M6.5 Idaho earthquake roll across seismic stations in North America! (THREAD) #IdahoEarthquake #IdahoQuake pic.twitter.com/ZbWYe1svBe
— IRIS Earthquake Sci (@IRIS_EPO) April 1, 2020
… "These earthquakes are caused by tectonic extension of the region and are not related to Yellowstone, nor will they have a significant impact on the Yellowstone system."
Full report in the link below: https://t.co/30Jpx70s0g
— Dr Janine Krippner (@janinekrippner) April 1, 2020
Trying to decompose the moment tensor of M6.5 Idaho earthquake into two double couple mechanisms. It can be decomposed into an Mw6.5 strike-slip event plus an Mw6.1 normal event, which seems consistent with the local tectonics. A first-motion mechanism might offer additional info pic.twitter.com/xP8s5R0waV
— Baoning Wu (@BoilingWoo) April 1, 2020
More Idaho earthquake fun in Jerome Idaho. Video from Tanner Bangerter #idahoquake #earthquake @CNN @BreitbartNews @FoxNews @NBCNews pic.twitter.com/8bgQ8LWS17
— Paul Dickinson (@pdicky77) April 1, 2020
A friend of mine sent these cool pictures of her sand pendulum. It recorded #earthquakeidaho! pic.twitter.com/sE1wCTsmmk
— Geo_Sci_Jerry (@SciJerry) April 1, 2020
Our flight over the #IdahoEarthquake epicenter region north of Stanley was pretty uneventful in terms of observing earthquake effects. pic.twitter.com/M6esihwi2V
— Zach Lifton (@zachlifton) April 2, 2020
Latest #Sentinel1 interferogram for M6.5 Idaho #earthquake; still low coherence from snow/forests but fringes & aftershocks hint at a NNW main fault plane, continuation of Sawtooth FZ? Rupture prob. more complex. Processed with DIAPASON at @esa_gep _gep #idahoearthquake pic.twitter.com/7Lfk0VFCCs
— Sotiris Valkaniotis (@SotisValkan) April 5, 2020
- Frisch, W., Meschede, M., Blakey, R., 2011. Plate Tectonics, Springer-Verlag, London, 213 pp.
- Hayes, G., 2018, Slab2 – A Comprehensive Subduction Zone Geometry Model: U.S. Geological Survey data release, https://doi.org/10.5066/F7PV6JNV.
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