Shortly after that, there were then two M7.3 earthquakes. One was located east of the trench (an earthquake within the Pacific plate, much like the March 2023 M 7.3 earthquake<\/a>, which was also in a similar location). The other earthquake was located west of the trench and had a depth that suggested it was a megathrust subduction zone earthquake. <\/p>\n Because these earthquakes happened at nearly the same time and had the same magnitude, I suspected that they were actually the same earthquake but had been automatically located in two locations (possibly due to something about the seismic waves that complicated the automatic location algorithm). In a few minutes, this was all worked out and the two earthquake pages began to show the same information, a single M 7.3 that was a subduction zone interface earthquake (an earthquake that slipped the megathrust fault). <\/p>\n Within a few more minutes, the magnitude was revised to be M 7.1. This is a much smaller earthquake than a M 7.3 but still quite significant. People on Raoul Island, about 75 km from the epicenter, reported strong ground shaking (intensity MMI 8, though initially reported as MMI 9).<\/p>\n After a few tweets, I went over to the tide gage websites that I monitor when there are subduction zone earthquakes. I often look at the UNESCO Sea Level Monitoring Facility website<\/a> first. There is a map and one may click on the dots that represent most of the tide gages around the globe. <\/p>\n This page provides basic information about water surface elevations. One may take a quick look to see if there are excursions in the sea level data, possibly related to tsunami.<\/p>\n Then, when I am ready to download some data so that I may plot these data I head over to the European Commission World Sea Levels website<\/a>. This is also a map interface and it takes a little more effort to learn how to operate the website to obtain the data one likes. These data are in a better format than the UNESCO site since they provide the observations, the tide prediction, and the excursion (i.e., the tsunami with the tide data removed).<\/p>\n I prefer to prepare my own plots so that I can control their graphical composition, these organizations create plots automatically and they are not always the best looking; I download these data, open them in excel, plot, then place them in adobe illustrator so that I can annotate them.<\/p>\n OK, back to the earthquake.<\/p>\n There was a magnitude M8.1 subduction zone earthquake in this area on 4 March 2021. Here is my poster for that earthquake<\/a>, where I show that several large earthquakes happened closely in space and in time. It was phenomenal that these 3 earthquakes also generated 3 tsunami that showed up on tide gages across the south Pacific. <\/p>\n Yesterday’s M 7.1 happened within the area of aftershocks from the M 8.1. So, I interpret this to be an aftershock of the M8.1. (Though I could easily be convinced that it was instead simply a triggered earthquake; it also followed the 15 March 2023 M 7.0 earthquake which was directly east of yesterday’s M 7.1<\/a>.<\/p>\n The earlier M 8.1 and yesterday’s M 7.1 earthquakes were along the subduction zone, where the Pacific plate subducts beneath the Australia plate. This subduction zone is quite active with many analogical historical earthquakes of similar magnitude in this area and also further to the north and to the south.<\/p>\n We may recall the 15 January 2022 Hunga Tonga eruption that generated a large trans-Pacific tsunami. Here is my report on that event<\/a>. Here is a web page that I put together for the California Geological Survey where I serve the public in the Seismic Hazards Program and Tsunami Unit (actually, I cannot share that page as it does not work outside of the USA, sadly; I will add a link once I am back home).<\/p>\n At the bottom of this report are a series of tweets that include some additional educational material. Check out the EarthScope Consortium tweets!<\/p>\n \nBathymetric map of the Tonga\u2013Kermadec arc system. Map showing the depth of the subducted slab beneath the Tonga\u2013Kermadec arc system. Louisville seamount ages are after Koppers et al.49 ELSC, eastern Lau-spreading centre; DSDP, Deep Sea Drilling Programme; NHT, Northern Havre Trough; OT, Osbourn Trough; VFR, Valu Fa Ridge. Arrows mark total convergence rates.\n<\/p><\/blockquote>\n<\/ul>\n \nMap of the Southwest Pacific Ocean showing the regional tectonic setting and location of the two dredged profiles. Depth contours in kilometres. The presently active arcs comprise New Zealand\u2013Kermadec Ridge\u2013Tonga Ridge, linked with Vanuatu by transforms associated with the North Fiji Basin. Colville Ridge\u2013Lau Ridge is the remnant arc. Havre Trough\u2013Lau Basin is the active backarc basin. Kermadec\u2013Tonga Trench marks the site of subduction of Pacific lithosphere westward beneath Australian plate lithosphere. North and South Fiji Basins are marginal basins of late Neogene and probable Oligocene age, respectively. 5.4sK\u2013Ar date of dredged basalt sample (Adams et al., 1994).\n<\/p><\/blockquote>\n<\/ul>\n \nLarge subduction-zone interplate earthquakes (large open gray stars) labeled with event date, Mw, GCMT focal mechanisms, and GPS velocity vectors (gray arrows and black triangles labeled with station name). GPS velocities are listed in Table 3. Black lines indicate the Tonga\u2013Kermadec and Vanuatu trenches. Note that the 2009\/09\/29 Samoa\u2013Tonga outer trench-slope event (Mw 8.1) triggered large interplate doublets (both of Mw 7.8; Lay et al., 2010). The Pacific plate subducts westward beneath the Australian plate along the Tonga\u2013Kermadec trench, whereas the Australian plate subducts eastward beneath the Vanuatu arc and North Fiji basin. The opposite orientation between the Tonga\u2013Kermadec and Vanuatu subduction systems is due to complex and broad back-arc extension in the Lau and North Fiji basins (Pelletier et al., 1998).\n<\/p><\/blockquote>\n \nRegional map of moderate-sized (mb > 4:7) shallow-focus repeating earthquakes and background seismicity along the (a) Tonga\u2013Kermadec and (b) Vanuatu (former New Hebrides) subduction zones. Shallow repeating earthquakes (black stars) and their available Global Centroid Moment Tensor (GCMT; Dziewo\u0144ski et al., 1981; Ekstr\u00f6m et al., 2003) are labeled with event date and doublet\/cluster id where applicable. Colors of GCMT are used to distinguish nearby different repeaters. Source parameters for the clusters and doublets are listed in Tables 1 and 2. Background seismicity is shown as gray dots and large interplate earthquakes (moment magnitude, Mw > 7:3) since 1976 are shown as large open gray stars. Black lines indicate the trench (Bird, 2003) and slab contour at 50-km depth (Gudmundsson and Sambridge, 1998). Repeating earthquake clusters in the (a) T1 and T2 plate-interface regions in Tonga and (b) V3 plate-interface region in Vanuatu are used to study the fault-slip rate ( _d). A regional map of the Tonga\u2013Kermadec\u2013Vanuatu subduction zones isBelow is my interpretive poster for this earthquake<\/font><\/H2><\/p>\n
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I include some inset figures. Some of the same figures are located in different places on the larger scale map below.<\/font><\/H3><\/p>\n
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\n![\"\"](\"https:\/\/earthjay.com\/earthquakes\/20230424_kermadec\/20230424_kermadec_interpretation.jpg\")
<\/a><\/p>\n<\/ul>\nTsunami<\/font><\/strong><\/H2><\/p>\n
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\n![\"\"](\"https:\/\/earthjay.com\/earthquakes\/20230424_kermadec\/20232424_kermadec_tide_gage_plots_raoul_island_fishing_dock.jpg\")
<\/a><\/p>\n
\n![\"\"](\"https:\/\/earthjay.com\/earthquakes\/20230424_kermadec\/20232424_kermadec_tide_gage_plots_raoul_island_boat_cove.jpg\")
<\/a><\/p>\n<\/ul>\n\n
Some Relevant Discussion and Figures<\/font><\/strong><\/H2><\/p>\n
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\n![\"\"](\"https:\/\/earthjay.com\/earthquakes\/20221111_tonga\/timm_etal_2013_louisville_seamount_subduction_fig_01.PNG\")
<\/a><\/p>\n\n
\n![\"\"](\"http:\/\/earthjay.com\/earthquakes\/20180910_kermadec\/ballance_etal_1999_history_kermadec_fig_01.JPG\")
<\/a><\/p>\n\n
\n![\"\"](\"https:\/\/earthjay.com\/earthquakes\/20180910_kermadec\/yu_2013_shallow_repeating_earthquakes_tonga_kermadec_vanuatu_fig_03.PNG\")
<\/a><\/p>\n
\n![\"\"](\"https:\/\/earthjay.com\/earthquakes\/20180910_kermadec\/yu_2013_shallow_repeating_earthquakes_tonga_kermadec_vanuatu_fig_02.PNG\")
<\/a><\/p>\n
\n shown in the inset figure, with the gray dotted box indicating the expanded region in the main figure.\n<\/p><\/blockquote>\n<\/ul>\n\n