I checked this out and found a 20 cm wave height tsunami observed on a tide gage directly east of the earthquake epicenter. This was interesting as the earthquake was an “outer rise” event (seaward of the subduction zone trench, where the Nazca plate flexes downward prior to being subducted.<\/p>\n
As the plate flexes downward, the upper part of the plate gets stretched and extensional faults can form here (or cause pre-existing faults to be reactivated as extensional\/normal faults). For more background about different types of faults, head here: Earthquake Plate Tectonic Fundamentals page<\/a>.<\/p>\n And, this M 6.7 earthquake was a normal fault earthquake (based on the earthquake mechanism). The largest tsunami waves can be generated by landslides or subduction zone faults, but other fault types can generate tsunami too (albeit smaller in size). Interesting indeed (there is more, like it is in a region of a triggered outer rise events following the 1960 M 9.5 Chile earthquake; is this M 6.7 an aftershock?, probably not).<\/p>\n BUT, this earthquake report is about the earthquake in Croatia that Jackie also mentioned in her email. Upon quick review, looking at the USGS PAGER Alert page, I knew that there was a high likelihood for casualties.<\/p>\n https:\/\/earthquake.usgs.gov\/earthquakes\/eventpage\/us6000d3zh\/executive<\/a><\/p>\n \nPAGER<\/a> provides shaking and loss estimates following significant earthquakes anywhere in the world. These estimates are generally available within 30 minutes and are updated as more information becomes available. Rapid estimates include the number of people and names of cities exposed to each shaking intensity level as well as the likely ranges of fatalities and economic losses.\n<\/p><\/blockquote>\n Yesterday’s M 6.4 is a strike-slip earthquake (look at the earthquake mechanism legend on the top center of the poster) and appears to have slipped along the Petrinja fault. This fault has different names in different papers (which is common), but this name comes from the European Database of Seismogenic Faults<\/a>.<\/p>\n According to the database<\/a>, the Petrinja fault is capable of a M 6.5 earthquake.<\/p>\n There are many different ways in which a landslide can be triggered. The first order relations behind slope failure (landslides) is that the \u201cresisting\u201d forces that are preventing slope failure (e.g. the strength of the bedrock or soil) are overcome by the \u201cdriving\u201d forces that are pushing this land downwards (e.g. gravity). The ratio of resisting forces to driving forces is called the Factor of Safety (FOS). We can write this ratio like this:<\/p>\n FOS = Resisting Force \/ Driving Force<\/strong><\/p>\n When FOS > 1, the slope is stable and when FOS < 1, the slope fails and we get a landslide. The illustration below shows these relations. Note how the slope angle \u03b1 can take part in this ratio (the steeper the slope, the greater impact of the mass of the slope can contribute to driving forces). The real world is more complicated than the simplified illustration below. \nLocation of the South Balkan extensional system (SBER) withing the eastern European region. The system today is within the southern Balkan region north of the North Anatolian fault (NAF), shown by the horizontal line patter. Retreating subduction zones and related backarc extensional areas for the Mediterranean region are shown in blue , and advancing subduction zones an related are a of backarc shortening are shown in red). Backarc extensional regions are shown by dotted pattern. KF = Kefalonia fault zone.\n<\/p><\/blockquote>\n<\/ul>\n \nMap of the most important seismogenic faults\n<\/p><\/blockquote>\n<\/ul>\n \nDigital terrain model of the Pannonian basin to show its position within the Alpine mountain belt and the location of different subunits.\n<\/p><\/blockquote>\n<\/ul>\n \nBlock model depicting the present position of the Alcapa and Tisza-Dacia terranes in the Carpathian embayment and the associated lithospheric and asthenosphericprocesses down to the upper mantle transition zone (inspired after Ustaszewski et al., 2008).\n<\/p><\/blockquote>\n<\/ul>\n \nMap of the Neogene Pannonian Basin, showing depocenters of the subbasins. The associated Transylvanian (TR) and Vienna (V) basins are shown. Modified from Horvath (1985a).\n<\/p><\/blockquote>\n<\/ul>\n \nTectonic map of the Pannonian Basin and surrounding regions showing the main extensional faults of Neogene age. After Rumpler and Horvath (1988). Area of Pannonian Basin Tertiary rocks within the Alpine-Carpathian fold belts shown as white.\n<\/p><\/blockquote>\n<\/ul>\n \nModel geometry and boundary conditions used in the finite element procedure. Note that a larger framework was created to minimize edge effects and errors. As a result, the \u2018free\u2019 edges are buffered but can be deformed on a small scale. For further discussion see text. The Adria\u2013Europe rotation pole was taken from Ward (1994).\n<\/p><\/blockquote>\n \nBest-fitting resultant stress pattern reflecting the combined effects of the applied boundary conditions (see insets), changing crustal thickness and two predefined weakness zones. (a), ( b) The edge at the Bohemian Massif is fixed and slightly deforming, respectively. In order to make direct comparison possible, the smoothed (observed) and calculated stress directions are superimposed.\n<\/p><\/blockquote>\n \nCartoon summarizing the main stress sources in the Alpine\u2013Carpathian\u2013Pannonian\u2013Dinaric system applied in our finite element models. Buttresses are rigid crustal blocks indenting or blocking their surroundings. Dashed lines represent faults that were included during modelling. The kinematics of some major faults showing present-day activity are also shown (after Gerner et al. 1997) 1: Molasse belt; 2: Flysch belt; 3: internal units; 4: Neogene and Quaternary #EarthquakeReport<\/a> poster for M6.4 strike-slip #Earthquake<\/a> in #Croatia<\/a> https:\/\/t.co\/nWCoPdKZ68<\/a><\/p>\n high chance for liquefaction<\/p>\n likely ruptured the Petrinja fault, thought to be capable of M6.5 eventshttps:\/\/t.co\/4Agp4cBnrz<\/a> pic.twitter.com\/TLJkADieQZ<\/a><\/p>\n — Jason "Jay" R. Patton (@patton_cascadia) December 30, 2020<\/a><\/p><\/blockquote>\n Below is my interpretive poster for this earthquake<\/font><\/h2>\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>\n
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\n![\"\"](\"http:\/\/earthjay.com\/earthquakes\/20201229_croatia\/20201229_croatia_interpretation.jpg\")
<\/a><\/p>\nUPDATE: 2021.01.03 Aftershocks and Intensity Comparison.<\/font><\/h3>\n
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\n![\"\"](\"http:\/\/earthjay.com\/earthquakes\/20201229_croatia\/20201229_croatia_aftershocks_interpretation.jpg\")
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Shaking Intensity and Potential for Ground Failure<\/font><\/strong><\/h2>\n
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\n![\"\"](\"http:\/\/earthjay.com\/earthquakes\/20180928_indonesia\/force_balance_landscape.jpg\")
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\nLandslide ground shaking can change the Factor of Safety in several ways that might increase the driving force or decrease the resisting force. Keefer (1984) studied a global data set of earthquake triggered landslides and found that larger earthquakes trigger larger and more numerous landslides across a larger area than do smaller earthquakes. Earthquakes can cause landslides because the seismic waves can cause the driving force to increase (the earthquake motions can \u201cpush\u201d the land downwards), leading to a landslide. In addition, ground shaking can change the strength of these earth materials (a form of resisting force) with a process called liquefaction.
\nSediment or soil strength is based upon the ability for sediment particles to push against each other without moving. This is a combination of friction and the forces exerted between these particles. This is loosely what we call the \u201cangle of internal friction.\u201d Liquefaction is a process by which pore pressure increases cause water to push out against the sediment particles so that they are no longer touching.
\nAn 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.
\nBelow 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.
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\n![\"\"](\"http:\/\/earthjay.com\/earthquakes\/20180928_indonesia\/liquefaction_pore_pressure_change_patton.jpg\")
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\nBelow 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.
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\n![\"\"](\"http:\/\/earthjay.com\/earthquakes\/20180928_indonesia\/landslide_terminology_patton.jpg\")
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\n![\"\"](\"http:\/\/earthjay.com\/earthquakes\/20201229_croatia\/20201229_croatia_slope_failure_map.jpg\")
<\/a>\n<\/ul>\nSeismic Hazard and Seismic Risk<\/font><\/strong><\/h2>\n
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\n![\"\"](\"http:\/\/earthjay.com\/earthquakes\/20161026_italy\/Giardini_2013_seismic_hazard.jpg\")
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\n![\"\"](\"http:\/\/earthjay.com\/earthquakes\/20201229_croatia\/seismic_hazard_risk_20201229_croatia.jpg\")
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\n![\"\"](\"http:\/\/earthjay.com\/earthquakes\/20190714_halmahera\/gem_global_seismic_hazard_map_v2018.jpg\")
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\n![\"\"](\"http:\/\/earthjay.com\/earthquakes\/20190714_halmahera\/GEM_hazard_legend.png\")
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\n![\"\"](\"http:\/\/earthjay.com\/earthquakes\/20190714_halmahera\/gem_global_seismic_risk_map_v2018.jpg\")
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\n![\"\"](\"http:\/\/earthjay.com\/earthquakes\/20190714_halmahera\/GEM_risk_legend.png\")
<\/a><\/p>\nHuman Impact<\/font><\/strong><\/h2>\n
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\n![\"\"](\"http:\/\/earthjay.com\/earthquakes\/20201229_croatia\/20201230_DM_Croatia_EQ%20.jpg\")
<\/a>\n<\/ul>\nSome Relevant Discussion and Figures<\/font><\/strong><\/h2>\n
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\n![\"\"](\"http:\/\/earthjay.com\/earthquakes\/20161016_greece\/woudloper_2009_alpide_belt_mediterranean.jpg\")
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\n![\"\"](\"http:\/\/earthjay.com\/earthquakes\/20161016_greece\/burchfiel_etal_2008_balkan_extension_fig_1.jpg\")
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\n![\"\"](\"http:\/\/earthjay.com\/earthquakes\/20201229_croatia\/marku%c5%a1i%c4%87_herak_1998_seismic_hazard_zoning_croatia_fig_03.PNG\")
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\n![\"\"](\"http:\/\/earthjay.com\/earthquakes\/20201229_croatia\/horv%c3%a1th_etal_2015_evolution_pannonian_basin_geothermal_fig_01.PNG\")
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\n![\"\"](\"http:\/\/earthjay.com\/earthquakes\/20201229_croatia\/horv%c3%a1th_etal_2015_evolution_pannonian_basin_geothermal_fig_04.PNG\")
<\/a><\/p>\n\n
\n![\"\"](\"http:\/\/earthjay.com\/earthquakes\/20201229_croatia\/dolton_2006_pannonian_province_petroleum_b2204-b_508_fig_02.PNG\")
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\n![\"\"](\"http:\/\/earthjay.com\/earthquakes\/20201229_croatia\/dolton_2006_pannonian_province_petroleum_b2204-b_508_fig_05.PNG\")
<\/a><\/p>\n\n
\n![\"\"](\"http:\/\/earthjay.com\/earthquakes\/20201229_croatia\/bada_etal_1998_sources_tectonic_stress_pannonian_fig_04.PNG\")
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\n![\"\"](\"http:\/\/earthjay.com\/earthquakes\/20201229_croatia\/bada_etal_1998_sources_tectonic_stress_pannonian_fig_11.PNG\")
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\n![\"\"](\"http:\/\/earthjay.com\/earthquakes\/20201229_croatia\/bada_etal_1998_sources_tectonic_stress_pannonian_fig_12.PNG\")
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\n volcanites; 5: Pieniny Klippen Belt; 6: strike-slip faults; 7: normal faults; 8: thrust faults.\n<\/p><\/blockquote>\n<\/ul>\n<\/ul>\n\n
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Europe<\/font><\/strong><\/h2>\n
General Overview<\/h3>\n
Earthquake Reports<\/h3>\n
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Social Media<\/font><\/strong><\/h2>\n
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