Well, it has been a busy couple of weeks.
- On 18 June, here was a M 7.4 earthquake in the Pacific plate along the Kermadec trench north of New Zealand which generated a small tsunami, even though it was a strike-slip earthquake (hopefully I can get to write that up, people are often surprised by tsunami generated by strike-slip earthquakes, but they are not that uncommon).
- Then, on 19 June, there was a M 4.6 event as part of the Monte Cristo Earthquake Sequence. I put together a poster, but no report. See more on this sequence here. My coworker is developing an earthquake Quick Reporting program to provide earthquake information to the state geologist (Steven Bohlen), the director of the Department of Conservation (David Shabazian), and the Secretary of the Natural Resources Agency (Wade Crowfoot). Cindy has been doing some of this in various roles for many years, but we are formalizing the process. I have been supporting Cindy by preparing maps. Even though this event was in Nevada, it satisfied the [draft] minimum criteria to prepare a Quick Report.
- Then, on 23 June (Monday my time), there was a M 4.6 south of Lone Pine, CA. This happened late in the day, but Cindy prepared a Quick Report, along with my map and other information from the Strong Motion Instrument Program (seismometers in CA) run by Hamid Haddadi.
- Then, on 23 June (Tuesday my time), right before work started, there was an earthquake in Oaxaca, Mexico. The Tsunami Unit at CGS was having our meeting and we all made observations and interpreted the earthquake and tsunami in real time. This is what I am writing about here.
- Today, on 24 June (Wednesday my time), the CGS was having an all staff meeting. During the meeting, there was a M 5.8 earthquake near where the M 4.6 happened, near Lone Pine and Keeler. I will write about that earthquake next.
The west coast coastline of southern Mexico, Central America, and South America is formed by a convergent plate boundary where oceanic tectonic plates dive eastwards beneath the continents. The fault formed at this plate boundary is called a subduction zone and the dynamics of subduction zones form deep sea trenches. I spend a few paragraphs discussing the different faults that form at different plate boundaries here.
Offshore of southern Mexico the Middle America trench shows us the location of the subduction zone megathrust fault. This fault system has a long history of damaging earthquakes, including some events that affect areas hundreds of kilometers from the source earthquake (e.g. the 1985 magnitude M 8 Mexico City earthquake).
In the past few years, evidences this megathrust is active continue to present themselves. There is a list of some earthquake reports at the bottom of this page.
The M 7.4 Oaxaca, Mexico Earthquake occurred along the megathrust fault interface (an “interplate” earthquake) based on our knowledge of the location of the fault, our calculation of the earthquake location, and the earthquake mechanisms prepared by seismologists (i.e. focal mechanisms or moment tensors).
The earthquake generated seismic waves that travelled around the world, including some that caused strong shaking in Mexico City. Mexico City was built where the Aztec Civilization had once constructed a great city. This city was built next to a lake where the Aztec constructed floating gardens. Eventually, these gardens filled the lake and the lake filled with sediment (I am simplifying what happened over a long time).
So, Mexico City is built in a sedimentary basin. Sedimentary basins can amplify shaking from seismic waves. These basins can also focus seismic waves and these waves can resonate within the basin, causing further amplification. This is why there was so much damage in Mexico City from the 1985 subduction zone earthquake.
The same thing happened a couple years ago for a recent earthquake there.
Well, when subduction zone earthquakes happen, the crust around the fault can flex like the elastic on one’s waist band. As the crust moves, if that crust is beneath the water, this crust motion moves the water causing a tsunami.
There are a number of organizations that monitor the Earth for earthquakes that may cause tsunami. These organizations alert officials in regions where these tsunami may inundate so that residents and visitors to the coast can take action (e.g. head to high ground). These programs save lives.
This M 7.4 earthquake generated a tsunami that was recorded along the coastline, but not at all tide gage stations. The Salina Cruz station has a great record of this tsunami and is located >80 km from the epicenter. The Acapulco station also recorded a tsunami, but those data were not uploaded to the IOC website (they are working this out now). It appeared that the Acapulco data were being streamed in real time, but I noticed that they were the same data as posted for the Salina Cruz station.
Here I plot the water surface elevations observed at the Salina Cruz tide gage. I mark the earthquake event time and the tsunami arrival time, then calculate the tsunami travel time.
I noticed that there is a down-first wave prior to the tsunami. This was observed at both stations (Acapulco and Salina Cruz). Dr. Costas Synolakis (USC) informed me that this is a well known phenomena called a “Leading Depression N-wave.” I mark the location of the Salna Cruz gage on the interpretive poster below.
The Wave Height of the tsunami is the vertical distance measured between the peak and the trough. These data show a Maximum Wave Height of 1.4 meters.
The strong ground shaking from an earthquake can also cause landslides and liquefaction. I discuss these further down in this report and include maps in the poster.
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 ≥ 7.0.
- 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. In the map below I include the magnetic anomaly data, also explained on this web page.
- Some basic fundamentals of earthquake geology and plate tectonics can be found on the Earthquake Plate Tectonic Fundamentals page.
- In the upper left corner is a global scale map showing the major plate boundary faults and arrows show relative plate motions for these fault systems. The spreading ridge (orange arrows) between the Pacific and Cocos plates interacts with a flipping magnetic pole to form these magnetic anomalies. Because of this, they form parallel to the spreading ridge. Note how the anomalies are parallel to the East Pacific Rise spreading center (not labelled on this map, but look at figures lower in this report).
- In the lower right corner is a map showing the earthquake intensity using the Modified Mercalli Intensity Scale (MMI) as modeled by the California Integrated Seismic Network (CISN). I also include observations from the USGS “Did You Feel It?” observations (these are from reports from the public).
- To the left of this map is a plot showing how shaking intensity lowers with distance from the earthquake. The models that were used to produce the Earthquake Intensity map to the right are the same model results represented by the orange and green lines. However, on this plot, there are also observations from real people! The USGS Did You Feel It? questionnaire lets people report their observations from the earthquake and these data are plotted here. We can then compare the model with the observations.
- In the upper and center right are maps that shows the liquefaction susceptibility and landslide probability models from the USGS. These are models and not based on direct observation, however, they could be used to help direct field teams to search for this type of effect.
- To the left of these slope failure maps is a map and cross section from Benz et al. (2011). The circles represent earthquake locations and the diameter represents earthquake magnitude. The cross section B-B’ location in shown on this inset map and the main map as blue lines.
- To the right of the main legend is the tide gage record from Salina Cruz.
I include some inset figures. Some of the same figures are located in different places on the larger scale map below.
Other Report Pages
Some Relevant Discussion and Figures
- Here is tectonic map from Franco et al. (2012).
Tectonic setting of the Caribbean Plate. Grey rectangle shows study area of Fig. 2. Faults are mostly from Feuillet et al. (2002). PMF, Polochic–Motagua faults; EF, Enriquillo Fault; TD, Trinidad Fault; GB, Guatemala Basin. Topography and bathymetry are from Shuttle Radar Topography Mission (Farr&Kobrick 2000) and Smith & Sandwell (1997), respectively. Plate velocities relative to Caribbean Plate are from Nuvel1 (DeMets et al. 1990) for Cocos Plate, DeMets et al. (2000) for North America Plate and Weber et al. (2001) for South America Plate.
- These figures are from the USGS publication (Benz et al., 2011) that presents an educational poster about the historic seismicity and seismic hazard along the Middle America Trench.
- First is a map showing earthquake depth as color (green depth > red). Seismicity cross section B-B’ is shown on the map. Today’s M=6.6 quake is nearest this section.
- Here are some figures from Manea et al. (2013). First are the map and low angle oblique view of the Cocos plate.
- Here is the figure that shows how the upper and lower plate structures interplay.
A. Geodynamic and tectonic setting alongMiddle America Subduction Zone. JB: Jalisco Block; Ch. Rift—Chapala rift; Co. rift—Colima rift; EGG—El Gordo Graben; EPR: East Pacific Rise; MCVA: Modern Chiapanecan Volcanic Arc; PMFS: Polochic–Motagua Fault System; CR—Cocos Ridge. Themain Quaternary volcanic centers of the TransMexican Volcanic Belt (TMVB) and the Central American Volcanic Arc (CAVA) are shown as blue and red dots, respectively. B. 3-D view of the Pacific, Rivera and Cocos plates’ bathymetrywith geometry of the subducted slab and contours of the depth to theWadati–Benioff zone (every 20 km). Grey arrows are vectors of the present plate convergence along theMAT. The red layer beneath the subducting plate represents the sub-slab asthenosphere.
Kinematic model (mantle reference frame) of the subducting Cocos slab along the MAT in the vicinity of Cocos–Caribbe–North America triple junction since Early Miocene. The evolution of Caribbean–North America tectonic contact is based on the model of Witt et al. (2012). The blue strips represent markers on the Cocos plate. Note how trench roll forward is associated with steep slab in Central America, whereas trench roll back is associated with flat slab in Mexico.
- Here is a map showing the spreading ridge features, along with the plate boundary faults (Mann, 2007). This is similar to the inset map in the interpretive poster.
Marine magnetic anomalies and fracture zones that constrain tectonic reconstructions such as those shown in Figure 4 (ages of anomalies are keyed to colors as explained in the legend; all anomalies shown are from University of Texas Institute for Geophysics PLATES  database): (1) Boxed area in solid blue line is area of anomaly and fracture zone picks by Leroy et al. (2000) and Rosencrantz (1994); (2) boxed area in dashed purple line shows anomalies and fracture zones of Barckhausen et al. (2001) for the Cocos plate; (3) boxed area in dashed green line shows anomalies and fracture zones from Wilson and Hey (1995); and (4) boxed area in red shows anomalies and fracture zones from Wilson (1996). Onland outcrops in green are either the obducted Cretaceous Caribbean large igneous province, including the Siuna belt, or obducted ophiolites unrelated to the large igneous province (Motagua ophiolites). The magnetic anomalies and fracture zones record the Cenozoic relative motions of all divergent plate pairs infl uencing the Central American subduction zone (Caribbean, Nazca, Cocos, North America, and South America). When incorporated into a plate model, these anomalies and fracture zones provide important constraints on the age and thickness of subducted crust, incidence angle of subduction, and rate of subduction for the Central American region. MCSC—Mid-Cayman Spreading Center.
- Here are 2 different figures from Mann (2007). First we see a map that shows the structures in the Cocos plate. Note the 3 profile locations labeled 1, 2, and 3. These coincide with the profiles in the lower panel.
- Here are 2 different views of the slabs in the region. These were modeled using seismic tomography (like a CT scan, but using seismic waves instead of X-Rays). The upper maps show the slabs in map-view at 3 different depths. The lower panels are cross sections 1, 2, and 3. Today’s M=6.6 earthquake happened between sections 1 & 2.
Present setting of Central America showing plates, Cocos crust produced at East Pacifi c Rise (EPR), and Cocos-Nazca spreading center (CNS), triple-junction trace (heavy dotted line), volcanoes (open triangles), Middle America Trench (MAT), and rates of relative plate motion (DeMets et al., 2000; DeMets, 2001). East Pacifi c Rise half spreading rates from Wilson (1996) and Barckhausen et al. (2001). Lines 1, 2, and 3 are locations of topographic and tomographic profi les in Figure 6.
(A) Tomographic slices of the P-wave velocity of the mantle at depths of 100, 300, and 500 km beneath Central America. (B) Upper panels show cross sections of topography and bathymetry. Lower panels: tomographic profi les showing Cocos slab detached below northern Central America, upper Cocos slab continuous with subducted plate at Middle America Trench (MAT), and slab gap between 200 and 500 km. Shading indicates anomalies in seismic wave speed as a ±0.8% deviation from average mantle velocities. Darker shading indicates colder, subducted slab material of Cocos plate. Circles are earthquake hypocenters. Grid sizes on profi les correspond to quantity of ray-path data within that cell of model; smaller boxes indicate regions of increased data density. CT—Cayman trough; SL—sea level (modifi ed from Rogers et al., 2002).
- Below is a video that explains seismic tomography from IRIS.
- Here is the McCann et al. (1979) summary figure, showing the earthquake history of the region.
Rupture zones (ellipses) and epicenters (triangles and circles) of large shallow earthquakes (after KELLEHER et al., 1973) and bathymetry (CHASE et al., 1970) along the Middle America arc. Note that six gaps which have earthquake histories have not ruptured for 40 years or more. In contrast, the gap near the intersection of the Tehuantepec ridge has no known history of large shocks. Contours are in fathoms.
- This is a more updated figure from Franco et al. (2005) showing the seismic gap.
- Here is a map from Franco et al. (2015) that shows the rupture patches for historic earthquakes in this region.
The study area encompasses Guerrero and Oaxaca states of Mexico. Shaded ellipse-like areas annotated with the years are rupture areas of the most recent major thrust earthquakes (M≥6.5) in the Mexican subduction zone. Triangles show locations of permanent GPS stations. Small hexagons indicate campaign GPS sites. Arrows are the Cocos-North America convergence vectors from NUVEL-1A model (DeMets et al., 1994). Double head arrow shows the extent of the Guerrero seismic gap. Solid and dashed curves annotated with negative numbers show the depth in km down to the surface of subducting Cocos plate (modified from Pardo and Su´arez, 1995, using the plate interface configuration model for the Central Oaxaca from this study, the model for Guerrero from Kostoglodov et al. (1996), and the last seismological estimates in Chiapas by Bravo et al. (2004). MAT, Middle America trench.
- Here are some figures that show how the subduction zone varies across the Tehuantepec Ridge. More about this in my initial report, as well as in my reports for the M 8.1 earthquake.
- This is a figure showing the location of the Tehuantepec Ridge (Quzman-Speziale and Zunia, 2015).
- Here is another figure, showing seismicity for this region (Quzman-Speziale and Zunia, 2015).
- This shows the location of the cross sections. The cross sections show how the CP changes dip along strike (from north to south) (Quzman-Speziale and Zunia, 2015).
- Here are the cross sections showing the seismicity associated with the downgoing CP (Quzman-Speziale and Zunia, 2015).
- This figure shows thrust and normal earthquakes for three ranges of depth (Quzman-Speziale and Zunia, 2015).
Tectonic framework of the Cocos plate convergent margin. Top- General view. Yellow arrows indicate direction and speed (in cm/yr) of plate convergence, calculated from the Euler poles given by DeMets et al. (2010) for CocoeNoam (first three arrows, from left to right), and CocoeCarb (last four arrows). Length of arrow is proportional to speed. Red arrow shows location of the 96 longitude. Box indicates location of lower panel. Bottom- Location of features and places mentioned in text. Triangles indicate volcanoes of the Central American Volcanic Arc (CAVA) with known Holocene eruption (Siebert and Simkin, 2002).
Seismicity along the convergent margin. Top: Map view. Blue circles are shallow (z < 60 km) hypocenters; orange, intermediate-depth (60 < z < 100 km); yellow, deep (z > 100 km). Next three panels: Earthquakes as a function of longitude and magnitude for shallow (blue dots), intermediate (orange), and deep (yellow) hypocenters. Numbers indicate number of events on each convergent margin, with average magnitude in parenthesis. Gray line in this and subsequent figures mark the 96 deg longitude.
Location of hypocentral cross-sections. Hypocentral depths are keyed as in previous figures.
Hypocentral cross-sections. Depths are color-coded as in previous figures. Dashed lines indicate the 60-km and 100-km depths. Tick marks are at 100-km intervals, as shown on the sections. There is no vertical exaggeration and Earth’s curvature is taken into account. Number of sections refers to location on Fig. 3.
Earthquake fault-plane solutions from CMT data. a. Shallow (z < 60 km), thrust-faulting mechanisms. b. Intermediate-depth (60 < z < 100 km) thrust-faulting events. c. Deep (z > 100 km), thrust-faulting earthquakes. d. to f. Normal-faulting events, in same layout as for thrust-faulting events.
- Here is an educational animation from IRIS that helps us learn about how different earth materials can lead to different amounts of amplification of seismic waves. Recall that Mexico City is underlain by lake sediments with varying amounts of water (groundwater) in the sediments.
- Here is an educational video from IRIS that helps us learn about resonant frequency and how buildings can be susceptible to ground motions with particular periodicity, relative to the building size.
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.
- 2020.06.23 M 7.4 Mexico
- 2019.02.01 M 6.6 Guatemala & Mexico
- 2018.02.16 M 7.2 Oaxaca, Mexico
- 2018.01.19 M 6.3 Gulf of California
- 2017.09.19 M 7.1 Puebla, Mexico
- 2017.09.19 M 7.1 Puebla, Mexico Update #1
- 2017.09.08 M 8.1 Chiapas, Mexico
- 2017.09.08 M 8.1 Chiapas, Mexico Update #1
- 2017.09.23 M 8.1 Chiapas, Mexico Update #2
- 2017.06.22 M 6.8 Guatemala
- 2017.06.14 M 6.9 Guatemala
- 2017.05.12 M 6.2 El Salvador
- 2017.03.29 M 5.7 Gulf of California
- 2016.11.24 M 7.0 El Salvador
- 2016.04.29 M 6.6 East Pacific Rise / MAT
- 2016.01.21 M 6.6 Mexico
- 2015.09.13 M 6.6 Gulf California
- 2015.09.13 M 6.6 Gulf California Update #1
- 2015.01.07 M 6.6 Panama
- 2015.01.31 M 5.5 Panama Update #1
- 2014.05.13 M 6.5 Panama
- 2014.05.13 M 6.5 Panama Update #1
- 2014.10.14 M 7.3 El Salvador
- 2013.10.20 M 6.4 Gulf California
Mexico | Central America
be safe everyonehttps://t.co/jTfZEjQ0H0
— Jason "Jay" R. Patton (@patton_cascadia) June 23, 2020
shaking felt in #MexicoCity
— Jason "Jay" R. Patton (@patton_cascadia) June 25, 2020
Watch the waves from the M7.4 Mexico earthquake roll across seismic stations in North America! (THREAD) pic.twitter.com/7dpPr1Ueuy
— IRIS Earthquake Sci (@IRIS_EPO) June 23, 2020
— Nick Graehl (@nickgraehl) June 23, 2020
#Terremoto en México: en estas impactantes imágenes, tomadas en Bosques de las Lomas (#CDMX), podemos observar como ambos lados de una grieta se mueven ligeramente durante el seísmo.pic.twitter.com/u50mCx1XFS
— eSPAINews (@eSPAINews) June 23, 2020
Heres my first cut at estimating offsets for the event today in Mexico. I will add more sites as they become available. The mechanism is USGS W-phase. pic.twitter.com/PomjZF0JKL
— Brendan Crowell (@bwcphd) June 23, 2020
Así se sintió el sismo en el nivel 56 de la Torre Mitikah en la alcaldía Benito Juárez, se trata de un inmueble de 267.3m de altura. pic.twitter.com/J6cpAZfPIU#Mexico #earthquake #alert #warning #mitigation #tsunami
— Desianto F. Wibisono (@TDesiantoFW) June 23, 2020
— Manuel Bojorquez (@BojorquezCBS) June 23, 2020
"Nearly" automatic forward model based on the USGS focal mechanism for the M 7.4 #Mexico #earthquake (June 22, 15:29 UTC). And expected InSAR fringe pattern from C-band ascending orbit. Waiting, as usual, for real InSAR data.
Calculated with @antandre71 pic.twitter.com/6hQFMV55EX
— Simone Atzori (@SimoneAtzori73) June 23, 2020
Cerrada la Carretera Oaxaca-Istmo por Derrumbe en las Cercanías de la Comunidad del Camarón, evite Viajar más información en un momento. pic.twitter.com/B3G5b17YZv
— Domingo 7 (@domingo7th) June 23, 2020
— DailyNewsf (@DailyNe25683877) June 23, 2020
— RedTvo Television (@redtvo) June 23, 2020
— Diario El Fortín (@diarioelfortin) June 23, 2020
— Desde Peninsula (@DesdePeninsula) June 23, 2020
Playa Riscalillo, mar se aleja unos cien metros.
— Periodistas Oaxaca (@PeriodistasOax) June 23, 2020
OXUM roughly 75 km away to the west had pretty sizable vertical motions. pic.twitter.com/YIltT3uBE4
— Brendan Crowell (@bwcphd) June 23, 2020
Así se sintió el sismo de 7.5 está mañana en Xochimilco pic.twitter.com/yK0CCKwGNt
— Salvador García Soto (@SGarciaSoto) June 23, 2020
Earthquake in Mexico City- still occurring – not sure where epicenter is nor if there is any real damage pic.twitter.com/KDAou9YhPG
— (@Andalalucha) June 23, 2020
First rough model of the tsunami from today's M7.5 earthquake in Mexico using the USGS finite fault as input. Clear edgewaves and shelf resonance in the Tehuantepec gulf. https://t.co/2FAV5m81ZX
— Diego Melgar (@geosmx) June 23, 2020
Today's Oaxaca tsunami in the Pacific. So far, only on 43413… pic.twitter.com/hDqG2ip3jI
— Amir Salaree (@amirsalaree) June 23, 2020
El 23 de junio del 2020, un terremoto M7.4 cerca de Oaxaca, México, ocurrió como resultado de una falla inversa en o cerca del límite de la placa que está entre las placas de Cocos y América del Norte. (Enlaces y detalles: https://t.co/qvcse5eYQe) pic.twitter.com/53IxlOr0VG
— USGS (@USGS) June 23, 2020
A section from today's M7.4 earthquake near Santa María Zapotitlán at 2020-06-23 15:29:05 UTC recorded on the worldwide @raspishake network and processed with a wider frequency range than I normally use. See: https://t.co/44McZsCCpE. Uses @obspy & @matplotlib software. pic.twitter.com/kItaJfsJ4W
— Mark Vanstone (@wmvanstone) June 23, 2020
— IRIS Earthquake Sci (@IRIS_EPO) June 23, 2020
How big was that magnitude 7.4 earthquake?
According to my calculations (see below), it's as if about 4 trillion college students jumped about 1 ft. And, that's about 500 times the population of planet Earth! (Peer review of my calculations is welcome…) pic.twitter.com/TlrmXhQSLj
— Alan Kafka (@Weston_Quakes) June 24, 2020
#Mexico M7.4 #earthquake
Surface projection of the slip distribution by @USGS superimposed on the quickly forward model on #ArcGis platform; white lines indicate #Slab2 depth boundaries. The seismogenic fault is also shown in 3D view map (magenta plain). pic.twitter.com/sm62EHfF8Z
— Vincenzo De Novellis (@VDN75) June 24, 2020
— Copernicus EMS (@CopernicusEMS) June 24, 2020
I find it mightily impressive that the low-cost geophone sensors inside @raspishake seismometers were able to capture the long-period surface wave signals, as well as higher-freq body waves, from yesterday's M7.4 Mexico earthquake.
This one below is a recording from SE England pic.twitter.com/6RRzPsHAGK
— Stephen Hicks (@seismo_steve) June 24, 2020
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Basic & General References
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