Earthquake Report: Fiji

WOW
We just had a Great Earthquake in the region of the Fiji Islands, in the central-western Pacific. Great Earthquakes are earthquakes with magnitudes M ≥ 8.0.

This earthquake is one of the largest earthquakes recorded historically in this region. I include the other Large and Great Earthquakes in the posters below for some comparisons.

Today’s earthquake has a Moment Magnitude of M = 8.2. The depth is over 550 km, so is very very deep. This region has an historic record of having deep earthquakes here. Here is the USGS website for this M 8.2 earthquake. While I was writing this, there was an M 6.8 deep earthquake to the northeast of the M 8.2. The M 6.8 is much shallower (about 420 km deep) and also a compressional earthquake, in contrast to the extensional M 8.2.

This M 8.2 earthquake occurred along the Tonga subduction zone, which is a convergent plate boundary where the Pacific plate on the east subducts to the west, beneath the Australia plate. This subduction zone forms the Tonga trench.

The subduction zone megathrust fault dips downwards to the west and the location of this “slab” has been evaluated by Hayes et al. (2012). These USGS geologists have updated the global slab model and I will incorporate these new data in upcoming reports. Today’s earthquake hypocenter (the 3-dimensional location of the earthquake) is at 563 km and the slab depth is about 520 km in this location (pretty good match given the range of depths for earthquakes relative to the fault location.

Due to the large depth, this earthquake did not shake very strongly at Earth’s surface. In addition, due to the large depth, a large tsunami is not expected. I checked the UNESCO IOC Sea Level Monitoring Facility, which posts a global set of tide gage data online. Here is their online map interface.

In 1994 there was a deep Great Earthquake (M 8.0) very close to today’s M 8.2 earthquake. One interesting thing is that the 2002 earthquake was compressional (a thrust or reverse fault earthquake) and today’s M 8.2 earthquake is extensional (a normal fault earthquake).
We are still unsure what causes an earthquake at such great a depth. The majority of earthquakes happen at shallower depths, caused largely by the frictional between differently moving plates or crustal blocks (where earth materials like the crust behave with brittle behavior and not elastic behavior). Some of these shallow earthquakes are also due to internal deformation within plates or crustal blocks.

As plates dive into the Earth at subduction zones, they undergo a variety of changes (temperature, pressure, stress). However, because people cannot directly observe what is happening at these depths, we must rely on inferences, laboratory analogs, and other indirect methods to estimate what is going on.
Below is a review of possible explanations as provided by Thorne Lay (UC Santa Cruz) in an interview in response to the 2013 M 8.3 Okhotsk Earthquake.

One option could be “fluid-assisted faulting,” in which water is released from minerals as they change phases during faulting, thus lubricating the plates, Lay says.
But although this is a common mechanism for earthquakes between 70 and 400 kilometers deep, it’s unlikely to be the cause of this quake because the plate is significantly dewatered by the time it reaches 400 kilometers deep. Minerals releasing carbon dioxide as they are compacted could provide an alternative fluid to lubricate the fault, he says, much like water does at shallower depths.
And another possibility is that a transition in mineral form from low-pressure polymorphs (the form in which a mineral is stable at the surface) to high-pressure polymorphs (a denser form of a mineral that is stable at greater depths), gives the fault a start. According to this model, the plate subducts too quickly for the mineral to slowly transition to its denser form. The mineral will reach depths greater than where it is normally stable, and thus the transformation may be a catastrophic process, causing a jolt at 600 kilometers, which would allow for movement along the fault, Lay says.

There have been a number of deep earthquakes globally in the past several years. These include the 2013 M 8.3 in the Sea of Okhtosk, the 2015 M 7.8 along the Izu-Bonin Arc, and several along the central Andes. I present some interpretive posters for these earthquakes below.
In early 2017 there was an M 6.9 earthquake in this region near Fiji. Here is the report for this earthquake.

There are many interesting earthquakes on this map and I will attempt to fill in this report with discussion and figures for some of these earthquakes. For example the 2009 Samoa earthquake, the 2009 Vanuatu doublet earthquakes, and the 1995 : 1998 earthquakes at the southern New Hebrides Trench.

Below is my interpretive poster for this earthquake

I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I include earthquake epicenters from 1918-2018 with magnitudes M ≥ 7.50 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.

I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely.
I also include the shaking intensity contours on the map. These use the Modified Mercalli Intensity Scale (MMI; see the legend on the map). This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here. This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations.
I include the slab contours plotted (Hayes et al., 2012), which are contours that represent the depth to the subduction zone fault. These are mostly based upon seismicity. The depths of the earthquakes have considerable error and do not all occur along the subduction zone faults, so these slab contours are simply the best estimate for the location of the fault.

Magnetic Anomalies

In the map below, I include a transparent overlay of the magnetic anomaly data from EMAG2 (Meyer et al., 2017). As oceanic crust is formed, it inherits the magnetic field at the time. At different points through time, the magnetic polarity (north vs. south) flips, the north pole becomes the south pole. These changes in polarity can be seen when measuring the magnetic field above oceanic plates. This is one of the fundamental evidences for plate spreading at oceanic spreading ridges (like the Gorda rise).
Regions with magnetic fields aligned like today’s magnetic polarity are colored red in the EMAG2 data, while reversed polarity regions are colored blue. Regions of intermediate magnetic field are colored light purple.
Note the magnetic anomalies (alternating bands of red and blue), parallel to the spreading ridges (the green lines with diverging orange arrows in the North Fiji Basin).

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

In the lower left corner is a portion of the map from Benz et al. (2011). This map shows earthquake epicenters (2-D locations) for seismicity from the past century or so. Depth is represented by color and earthquake magnitude is represented by the size of the circle symbols. Seismicity cross sections are located along the green (H-H’) and blue (I-I’) lines. I place a blue star in the general location of today’s M 8.2 earthquake on this map, the H-H’ cross section, as well as the other inset figures.
Cross sections showing earthquake hypocenters along the two profiles (H-H’ and I-I’) are presented above the Benz et al. (2011) map. These seismicity cross section locations are also shown on the main map.
The lower right corner includes a map from de Alteriis et al. (1993) that shows some details of the plate boundaries in this region. Note the subduction zones (New Hebrides Trench and Tonga Trench). Also not some strike-slip fault systems (e.g. the Hunter fracture zone and the North Fiji fracture zone). There is a good example of a strike-slip earthquake along the Hunter fracturezone from 1990.
In the upper right corner is a figure that shows the tectonic development of the region surrounding Fiji (Begg and Gray, 2002). These authors worked on the volcanic and tectonic history of the Fiji Plateau.

Here is the map with a month’s seismicity plotted.

Here is the map with a centuries seismicity plotted with M ≥ 7.5.

Other Report Pages

earthquake report dot com

Some Relevant Discussion and Figures

This is the plate tectonic map from de Alteriis et al. (1993) that shows the major fault systems in the region.

Location map of North Fiji Basin ridge; box indicates full multibeam covered area of Figure 2. Heavy lines denote north-south, N15°, and N160° main segments of ridge axis; dashed lines are pseudofaults indicating double propagation. F. Z.— fracture zone.

Here is a figure from Schellart et al. (2002) that shows their model of tectonic development of the North Fiji Basin. Schellart et al. (2002) include a long list of references for the tectonics in this region here. Below I include the text from the original figure caption in blockquote.

Tectonic reconstruction of the New Hebrides – Tonga region (modified and interpreted from Auzende et al. [1988], Pelletier et al. [1993], Hathway [1993] and Schellart et al.(2002a)) at (a) ~ 13 Ma, (b) ~ 9 Ma, (c) 5 Ma and (d) Present. The Indo-Australian plate is fixed. DER = d’Entrcasteaux Ridge, HFZ = Hunter Fracture Zone, NHT = New Hebrides Trench, TT = Tonga Trench, WTP = West Torres Plateau. Arrows indicate direction of arc migration. During opening of the North Fiji Basin, the New Hebrides block has rotated some 40-50° clockwise [Musgrave and Firth 1999], while the Fiji Plateau has rotated some 70-115° anticlockwise [Malahoff et al. 1982]. During opening of the Lau Basin, the Tonga Ridge has rotated ~ 20° clockwise [Sager et al. 1994]. (Click for enlargement)

This is the plate tectonic history map from Begg and Gray (1993) that shows how they interpret the Fiji Plateau to have formed.

Tectonic setting (Figures 1a–1c) and tectonic reconstructions (Figures 1d and 1e) of the Outer Melanesian region (adapted from Hathway [1993]; reprinted with permission from the Geological Society of London).

(a) Map of the Fiji platform and north end of the Lau Ridge showing the major islands in the Fiji area, the major early Pliocene volcanoes of Viti Levu, the major seafloor fracture zones, and part of the spreading center of the Fiji Basin (adapted from Gill and Whelan [1989]). Shoshonitic volcanoes, including the Tavua Volcano (T), are shown by squares and calc-alkaline volcanoes by circles.

(b) Tectonic features of the northeastern segment of the plate boundary between the Australian and Pacific plates showing the Outer Melanesian Arc of the southwest Pacific, trenches and ridge systems, and oceanic plateaus (adapted from Kroenke [1984]). Fiji, as part of the Fiji Platform, consists of a series of islands at the north end of the Lau Ridge, with the North Fiji Basin formed as part of a spreading center.

(c) Present plate configuration.

(d) Reconstruction at 5.5 Ma.

(e) Reconstruction at 10 Ma. In Figures 1a–1e the Australian plate is fixed and the east-west convergence rate between plates was assumed to be 9–10 cm yr-1. Shading represents submarine depths <2000 m.

Abbreviations are as follows: VT, Vitiaz trench; VAT, Vanuatu trench; LR, Lau Ridge; LB, Lau Basin; TR, Tonga Ridge; FFZ, Fiji Fracture Zone; LL, Lomaiviti lineament; V-BL, Vatulele-Beqa lineament. Long dashes denote southern margin of the Melanesian Border Plateau (MBP). The open square (Figures 1b and 1c) denotes the location of the Tavua Volcano.

Okal (1997) conducted an analysis of seismological records from a deep earthquake that happened in the region of the 2017.01.03 M 6.3 earthquake. This earthquake occurred on 26 May 1932, long before modern seismometers made it to the scene. Okal estimated the magnitude to be similar in size to earthquakes in the mid M 7 range. Here is a figure from Okal (1997) that shows some focal mechanisms for the earthquakes from 1932. Compare the mainshock (the largest focal mechanism) with the moment tensor for the 2016.01.02 M 6.3 earthquake. Below I include the text from the original figure caption in blockquote.
1932.05.26 M 7.6 (USGS)

Focal mechanism of the 1932 earthquake, as determined in this study. We also show CMT solutions in the immediate vicinity of the event, as available from Dziewonski et al. (1983, and subsequent quarterly updates) and Huang et al. (1997). Their spatial distribution is shown in map view. The background map at the upper right sets the study area (shaded) into the familiar bathymetry of the Fiji-Tonga-Kermadec region. The separation of isobaths is 1000 m.

Interestingly, deep focus earthquakes take up ~66% of the deep earthquakes globally. From Yu and Wen (2012), we can see some moment tensors for deep earthquakes in this region. The 1994.07.30 earthquake is just west of the 2017 M 6.3 earthquake and also has a similar moment tensor to the 2017 M 6.3 earthquake.

Regional map of deep-focus similar earthquake pairs and seismicity near the Tonga–Fiji subduction zone. Deep similar earthquake pairs (black stars) and their available Global Centroid Moment Tensor (CMT) (Dziewonski et al., 1981; Ekstrom et al., 2003) are labeled with event date and doublet/cluster ID where applicable. Source parameters of the doublets/clusters are listed in Tables 1, 2. Background deep seismicity is shown as gray dots. Black lines indicate the slab contours below 300 km depth (Gudmundsson and Sambridge, 1998), with an interval of 100 km. Regional map of the Tonga–Fiji–Kermadec subduction zone is shown in the inset, with gray dotted box indicating the region blow-up in the main figure. Black lines are the slab contours below 300 km depth and the Tonga–Kermadec trench (Bird, 2003). The color version of this figure is available only in the electronic edition.

Green (2007) presents a great review about what may control the mechanics of deep earthquakes. I present his abstract in its entirety because it is so well written. Below are a couple supporting figures. Read the paper for more insight.

Abstract: Deep earthquakes have been a paradox since their discovery in the 1920s. The combined increase of pressure and temperature with depth precludes brittle failure or frictional sliding beyond a few tens of kilometers, yet earthquakes occur continually in subduction zones to ≈700 km. The expected healing effects of pressure and temperature and growing amounts of seismic and experimental data suggest that earthquakes at depth probably represent self-organized failure analogous to, but different from, brittle failure. The only high-pressure shearing instabilities identified by experiment require generation in situ of a small fraction of very weak material differing significantly in density from the parent material. This “fluid” spontaneously forms mode I microcracks or microanticracks that self-organize via the elastic strain fields at their tips, leading to shear failure. Growing evidence suggests that the great majority of subduction zone earthquakes shallower than 400 km are initiated by breakdown of hydrous phases and that deeper ones probably initiate as a shearing instability associated with breakdown of metastable olivine to its higher-pressure polymorphs. In either case, fault propagation could be enhanced by shear heating, just as is sometimes the case with frictional sliding in the crust. Extensive seismological interrogation of the region of the Tonga subduction zone in the southwest Pacific Ocean provides evidence suggesting significant metastable olivine, with implication for its presence in other regions of deep seismicity. If metastable olivine is confirmed, either current thermal models of subducting slabs are too warm or published kinetics of olivine breakdown reactions are too fast.

Here is a profile into the Earth that shows depths for various chemical – mechanical process that are thought to control seismicity in various ways (Green, 2007).

Earthquake depth distribution. (a) Semilog plot of global earthquake frequency per 10-km-thick depth interval, showing a bimodal distribution. All earthquakes below 50 km are in subduction zones, the coldest parts of the mantle. The boundary between the mantle transition zone and lower mantle in subduction zones is at 700 km. No earthquake has ever been detected in the lower mantle. Modified from ref. 35. (b) Cartoon of subduction zone and earthquake distribution. Lithosphere (speckled) at right, with uppermost layers altered to antigorite (serpentine), is subducting beneath lithosphere at left. Earthquakes in olivine-dominated upper mantle are shown as red dots in serpentine and white diamonds. In the mantle transition zone, olivine is hypothesized to remain present despite being no longer thermodynamically stable and to slowly react away to spinel (wadsleyite or ringwoodite) during descent, occasionally generating earthquakes (black dots) by the process discussed in the text. Note volume reductions accompanying phase transformations at 410 and 660 km. Modified from ref. 36.

Here is an illustration showing a visualization of the slab associated with the Tonga subduction zone (Green, 2007).

Cartoon showing active Tonga subduction zone and fossil slab floating above it. Original figure is modified after ref. 26. Yellow and orange stars and circles were added in ref. 28.

The Goes et al. (2017) paper presents an excellent review of the various forces and earthquake types along subduction zones globally. This paper is open source and free to download. Below are some summary figures.
This shows the general relations between various forces exerted on a subducting slab.

Schematic diagram showing the main forces that affect how slabs interact with the transition zone. The slab sinks driven by its negative thermal buoyancy (white filled arrows). Sinking is resisted by viscous drag in the mantle (black arrows) and the frictional/viscous coupling between the subducting and upper plate (pink arrows). To be able to sink, the slab must bend at the trench. This bending is resisted by slab strength (curved green arrow). The amount the slab needs to bend depends on whether the trench is able to retreat, a process driven by the downward force of the slab and resisted (double green arrow) by upper-plate strength and mantle drag (black arrows) below the upper plate. At the transition from ringwoodite to the postspinel phases of bridgmanite and magnesiowüstite (rg – bm + mw), which marks the interface between the upper and lower mantle, the slab’s further sinking is hampered by increased viscous resistance (thick black arrows) as well as the deepening of the endothermic phase transition in the cold slab, which adds positive buoyancy (open white arrow) to the slab.
By contrast, the shallowing of exothermic phase transition from olivine to wadsleyite (ol-wd) adds an additional driving force (downward open white arrow), unless it is kinetically delayed in the cold core of the slab (dashed green line), in which case it diminishes the driving force. Phase transitions in the crustal part of the slab (not shown) will additionally affect slab buoyancy. Buckling of the slab in response to the increased sinking resistance at the upper-lower mantle boundary is again resisted by slab strength.

Here is a plot showing their summary of observations for various subduction zones globally.

Summary of morphologies of transition-zone slabs as imaged by tomographic studies and their Benioff stress state. Arrows on the map indicate the approximate locations of the cross sections shown around the map, with their points in downdip direction. Blue shapes are schematic representations of slab morphologies (based on the extent of fast seismic anomalies that were tomographically resolvable from the references listed). Horizontal black lines indicate the base of the transition zone (~660 km depth). For flattened slabs, the approximate length of the flat section is given in white text inside the shapes. For penetrating slabs, the approximate depth to which the slabs are continuous is given in black text next to the slabs. Circles inside the slabs indicate whether the mechanisms of earthquakes at intermediate (100–350 km) and deep (350–700 km) are predominantly downdip extensional (black) or compressional (white). Stress states are from the compilations of Isacks and Molnar (1971), Alpert et al. (2010), Bailey et al. (2012), complemented by Gorbatov et al. (1997) for Kamchatka, Stein et al. (1982) for the Antilles, McCrory et al. (2012) for Cascadia, Papazachos et al. (2000) for the Hellenic zone, and Forsyth (1975) for Scotia. The subduction zones considered are (from left to right and top to bottom): RYU—Ryukyu, IZU—Izu, HON—Honshu, KUR—Kuriles, KAM—Kamchatka, ALE—Aleutians, ALA—Alaska, CAL—Calabria, HEL—Hellenic, IND—India, MAR—Marianas, CAS—Cascadia, FAR—Farallon, SUM—Sumatra, JAV—Java, COC—Cocos, ANT—Antilles, TON—Tonga, KER—Kermadec, CHI—Chile, PER—Peru, SCO—Scotia. Numbers next to the red subduction zone codes refer to the tomographic studies used to define the slab shapes

I include some inset figures in this interpretive poster for the 2017.01.03 M 6.9 Fiji Earthquake.

In the lower left corner I include map that shows the historic seismicity for this region (Martin, 2014). The color shows well how the earthquakes that happen along the Tonga Trench get deeper along with the subducting slab. Shallow earthquakes are generally subduction zone earthquakes and deeper earthquakes are related (generally) to processes happening withing the downgoing slab. The 2017.01.02 M 6.3 earthquake is one of these deep earthquakes. I will briefly compare this M 6.3 earthquake with an earthquake from the region that occurred in 1932 (Okal, 1997).
In the center top I include a figure that shows a small scale map of the southwestern Pacific (a) and a large scale map of the North Fiji Basin (b) from Martin, 2013. The various spreading ridges are indicated as double lines. I present this figure below.
In the upper right corner I include a figure from Schellart et al. (2002) that shows a conceptual model for the development of the North Fiji Basin formed by extension in the plate as the Basin rotated clockwise towards the New Hebrides Trench. I present this below.
In the lower right corner I include a figure from Richards et al. (2011) that shows their model of how the subducting slabs have interacted through time. These authors think that there is a stalled out and torn slab at depth below the North Fiji Basin. The M 7.2 earthquake occurred near the cross section c-c’.

Geologic Fundamentals

For more on the graphical representation of moment tensors and focal mechnisms, check this IRIS video out:

Here is a fantastic infographic from Frisch et al. (2011). This figure shows some examples of earthquakes in different plate tectonic settings, and what their fault plane solutions are. There is a cross section showing these focal mechanisms for a thrust or reverse earthquake. The upper right corner includes my favorite figure of all time. This shows the first motion (up or down) for each of the four quadrants. This figure also shows how the amplitude of the seismic waves are greatest (generally) in the middle of the quadrant and decrease to zero at the nodal planes (the boundary of each quadrant).

Here is another way to look at these beach balls.

The two beach balls show the stike-slip fault motions for the M6.4 (left) and M6.0 (right) earthquakes. Helena Buurman's primer on reading those symbols is here. pic.twitter.com/aWrrb8I9tj

— AK Earthquake Center (@AKearthquake) August 15, 2018

There are three types of earthquakes, strike-slip, compressional (reverse or thrust, depending upon the dip of the fault), and extensional (normal). Here is are some animations of these three types of earthquake faults. The following three animations are from IRIS.

Strike Slip:

Compressional:

Extensional:

This is an image from the USGS that shows how, when an oceanic plate moves over a hotspot, the volcanoes formed over the hotspot form a series of volcanoes that increase in age in the direction of plate motion. The presumption is that the hotspot is stable and stays in one location. Torsvik et al. (2017) use various methods to evaluate why this is a false presumption for the Hawaii Hotspot.

A cutaway view along the Hawaiian island chain showing the inferred mantle plume that has fed the Hawaiian hot spot on the overriding Pacific Plate. The geologic ages of the oldest volcano on each island (Ma = millions of years ago) are progressively older to the northwest, consistent with the hot spot model for the origin of the Hawaiian Ridge-Emperor Seamount Chain. (Modified from image of Joel E. Robinson, USGS, in “This Dynamic Planet” map of Simkin and others, 2006.)

Here is a map from Torsvik et al. (2017) that shows the age of volcanic rocks at different locations along the Hawaii-Emperor Seamount Chain.

Hawaiian-Emperor Chain. White dots are the locations of radiometrically dated seamounts, atolls and islands, based on compilations of Doubrovine et al. and O’Connor et al. Features encircled with larger white circles are discussed in the text and Fig. 2. Marine gravity anomaly map is from Sandwell and Smith.

New Britain | Solomon | Bougainville | New Hebrides | Tonga | Kermadec Earthquake Reports

General Overview

Earthquake Reports

2018.08.18 M 8.2 Fiji
2018.03.26 M 6.9 New Britain
2018.03.26 M 6.6 New Britain
2018.03.08 M 6.8 New Ireland
2018.02.25 M 7.5 Papua New Guinea
2018.02.26 M 7.5 Papua New Guinea Update #1
2017.11.19 M 7.0 Loyalty Islands Update #1
2017.11.07 M 6.5 Papua New Guinea
2017.11.04 M 6.8 Tonga
2017.10.31 M 6.8 Loyalty Islands
2017.08.27 M 6.4 N. Bismarck plate
2017.05.09 M 6.8 Vanuatu
2017.03.19 M 6.0 Solomon Islands
2017.03.05 M 6.5 New Britain
2017.01.22 M 7.9 Bougainville
2017.01.03 M 6.9 Fiji
2016.12.17 M 7.9 Bougainville
2016.12.08 M 7.8 Solomons
2016.10.17 M 6.9 New Britain
2016.10.15 M 6.4 South Bismarck Sea
2016.09.14 M 6.0 Solomon Islands
2016.08.31 M 6.7 New Britain
2016.08.12 M 7.2 New Hebrides Update #2
2016.08.12 M 7.2 New Hebrides Update #1
2016.08.12 M 7.2 New Hebrides
2016.04.06 M 6.9 Vanuatu Update #1
2016.04.03 M 6.9 Vanuatu
2015.03.30 M 7.5 New Britain (Update #5)
2015.03.30 M 7.5 New Britain (Update #4)
2015.03.29 M 7.5 New Britain (Update #3)
2015.03.29 M 7.5 New Britain (Update #2)
2015.03.29 M 7.5 New Britain (Update #1)
2015.03.29 M 7.5 New Britain
2015.11.18 M 6.8 Solomon Islands
2015.05.24 M 6.8, 6.8, 6.9 Santa Cruz Islands
2015.05.05 M 7.5 New Britain

Social Media

Seismogram at Niue Island 850km east of the deep, Fiji #earthquake shows very long period, S and surface waves of 3cm amplitude! These waves may be cause of a small tsunami. https://t.co/oETJBDNe5D https://t.co/KrpIrEeK28 pic.twitter.com/L7brVKc7yA

— Anthony Lomax 🌍🇪🇺 (@ALomaxNet) August 19, 2018

Historical seismicity map for the M8.2 deep (~580km) Tonga earthquake, with a 3D cartoon of the subducting slab from https://t.co/igpBWSeEBN pic.twitter.com/LmM6DxQzAR

— Jascha Polet (@CPPGeophysics) August 19, 2018

Depth map of the recent #Fiji, #Tonga #earthquake and subduction geometry, second strongest ever recorded earthquake in such a depth (after Okhotsk, 2013) pic.twitter.com/TNYV6UvFlG

— CATnews (@CATnewsDE) August 19, 2018

Deep earthquakes, such as today's M8.2 near Fiji at 58- km depth, produce very pretty waveforms, such as this one at PET at about 74 degrees distance. The pP and sP depth phases can help constrain the depth of the earthquake. pic.twitter.com/ctmREkGE5J

— Jascha Polet (@CPPGeophysics) August 19, 2018

This looks like the @raspishake station nearest to the Mag8 Fiji quake. Wow!! pic.twitter.com/vrxysq4HmE

— Mark Tingay (@CriticalStress_) August 19, 2018

DYK that the ground is shaking (ever so slightly) all across #Canada due to the very deep M8.2 earthquake near Fiji?
Shaking at #Victoria, #Toronto, #Ottawa and #Halifax shown.
The P-wave took about 12 minutes to travel to Victoria…https://t.co/bRKdpHuFJH pic.twitter.com/tAAIIZUN7H

— John Cassidy (@earthquakeguy) August 19, 2018

#GeoDato 💡 Eventos a gran profundidad son considerados a más de 300 kilómetros debido al proceso de subducción durante millones de años pueden llegar a introducirse a grandes distancias bajo tierra.

Hipocentro del 8.2Mw en Fiji 🇫🇯 en los puntos azules⬇️ pic.twitter.com/WmH91JVnko

— EarthQuakesTime (@EarthQuakesTime) August 19, 2018

Woah! Mag 8 #earthquake deep under Fiji. Huge response on my @raspishake Shake and Boom, despite being >6000km away. pic.twitter.com/qdZvKqDw1f

— Mark Tingay (@CriticalStress_) August 19, 2018

This looks like the @raspishake station nearest to the Mag8 Fiji quake. Wow!! pic.twitter.com/vrxysq4HmE

— Mark Tingay (@CriticalStress_) August 19, 2018

Deep (Z=558km) #quake M8.2 strikes Fiji Region 28 min ago. Deep #quakes rarely cause damage. https://t.co/ryPVZdgaz7

— EMSC (@LastQuake) August 19, 2018

Mw=8.3, FIJI ISLANDS REGION (Depth: 570 km), 2018/08/19 00:19:37 UTC – Full details here: https://t.co/VbC2e9vb3G pic.twitter.com/2SFhy7HITw

— Earthquakes (@geoscope_ipgp) August 19, 2018

M8.2 Fiji Islands #earthquake 560km deep: very few earthquake this big and this deep since 1900 https://t.co/840PC7Wcd2 pic.twitter.com/aRLnSryG8A

— Anthony Lomax 🌍🇪🇺 (@ALomaxNet) August 19, 2018

The Powerful Earth https://t.co/Iw3kYOm7W5 pic.twitter.com/EPZ9FjcsTB

— TXESP (@KristiFinkTXESP) August 19, 2018

Los grandes sismos muy profundos, como el de hoy a 560 km bajo Fiji, son inofensivos: a pesar de su gran magnitud, no provocan sacudida fuerte ni tsunami. Pero para un sismólogo son una oportunidad única para investigar como funciona la Tierra profunda. https://t.co/hO81v6k8WP

— Pablo Ampuero (@DocTerremoto) August 19, 2018

Cross-section of "historical" (past 50 years) seismicity in area of yesterday's M8.2 & M6.8 Fiji earthquakes, shown with black outline.
(Disclaimer: since these quakes were located 100+ km apart, this shows fairly large region of messy, 3D, slab projected onto 1 plane.) pic.twitter.com/geBIjGeq0E

— Jascha Polet (@CPPGeophysics) August 19, 2018

UPDATE 2018.08.20

Watch the waves from the M8.2 Fiji earthquake roll across the USArray seismic network (https://t.co/RIcNz4bgWq)! Each dot is a seismic station and when the ground moves up it turns red and when it moves down it turns blue. (More info in the thread below). #Fijiearthquake pic.twitter.com/oqRQ5GtNYM

— IRIS Earthquake Sci (@IRIS_EPO) August 20, 2018

This weekend's M8.2 #Fijiearthquake occurred at a depth of more than 500 km! Here is what it looked like at our broadband station BKS: pic.twitter.com/QC8XX9ovin

— Berkeley Seismo Lab (@BerkeleySeismo) August 20, 2018

VIDEO (00:01:58) – M8.2 #Fijiearthquake Simplified animation showing the M8.2 #earthquake between #Fiji and #Tonga. https://t.co/P8fjhEXE8e pic.twitter.com/z4BWc0gyCe

— IRIS Earthquake Sci (@IRIS_EPO) August 20, 2018