Efficiency of earthquake forecast models based on earth tidal correlation with background seismicity along the Tonga–Kermadec trench

The correlation between Earth’s tides and background seismicity has been suggested to become stronger before great earthquakes and weaker after. However, previous studies have only retrospectively analyzed this correlation after individual large earthquakes; it thus remains vague (i) whether such variations might be expected preceding future large earthquakes, and (ii) the strength of the tidal correlation during interseismic periods. Therefore, we retrospectively investigated whether significant temporal variations of the tidal correlation precede large interplate earthquakes along the Tonga–Kermadec trench, where Mw 7-class earthquakes frequently occurred from 1977 to 31 December 2020. We evaluated a forecast model based on the temporal variations of the tidal correlation via Molchan’s error diagram, using the tidal correlation value itself as well as its rate of change as threshold values. For Mw ≥ 7.0 earthquakes, this model was as ineffective as random guessing. For Mw ≥ 6.5, 6.0, or 5.5 earthquakes, the forecast model performed better than random guessing in some cases, but even the best forecast only had a probability gain of about 1.7. Therefore, the practicality of this model alone is poor, at least in this region. These results suggest that changes of the tidal correlation are not reliable indicators of large earthquakes along the Tonga–Kermadec trench. Graphical Abstract

Preliminary study of stress changes evolution on central part of Sumatran Fault influenced by large interplate earthquakes and tectonic stress rates

The inland seismic activity in Great Sumatran Fault (GSF) has significantly increased over the past several decades after the occurrence of historical large interplate earthquakes along the plate boundary. This condition led to some occurrences of historical intraplate earthquakes along Sumatran fault. To quantitatively examine the physical mechanisms between intraplate earthquakes and interplate earthquakes, we estimated the static coseismic stress changes of Coulomb failure function (ΔCFF) using receiver fault approach from large historical-recorded interplate earthquakes and the increase in tectonic stress rates. We examined this research in the central part of GSF since this zone is assumed to have the most heterogeneous stress field and thus became our focus study area. The cumulative ΔCFF models showed almost all segments in the central part of GSF suffered negative changes (<-0.1 MPa) which assumed to be unlikely to rupture in short time. However, the preliminary analysis of the increase in tectonic stress rate indicated that large intraplate earthquakes occurred on Angkola and Siulak segments were dominantly influenced by the increase in interseismic stress rate just after the series of large subduction earthquake occurrences, apart from the decreased stress changes from those major interplate earthquakes.

Stagnant forearc mantle wedge inferred from mapping of shear-wave anisotropy using S-net seafloor seismometers

Shear-wave anisotropy in Earth’s mantle helps constrain the lattice-preferred orientation of anisotropic minerals due to viscous flow. Previous studies at the Japan Trench subduction zone using land-based seismic networks identified strong anisotropy in the mantle wedge, reflecting viscous flow induced by the subducting slab. Here we map anisotropy in the previously uninvestigated offshore region by analyzing shear waves from interplate earthquakes that are recorded by a new seafloor network (the S-net). The newly detected anisotropy is not in the mantle wedge but only in the overlying crust (∼0.1 s time delay and trench-parallel fast direction). The distinct lack of anisotropy indicates that the forearc mantle wedge offshore is decoupled from the slab and does not participate in the viscous flow, in sharp contrast with the rest of the mantle wedge. A stagnant forearc mantle wedge provides a stable and cold tectonic environment that is important for the petrological evolution and earthquake processes of subduction zones.

Low-Frequency Tremors Immediately After the 2011 Tohoku-Oki Earthquake Detected by Near-Trench OBS Observations

We used temporal seismic observation using pop-up type ocean-bottom seismometers to detect a number of low-frequency tremors (LFTs) immediately after the 2011 Tohoku-Oki earthquake in the northern periphery of its aftershock area. The near-field observation clearly distinguished LFTs from regular earthquakes based on their spectral shape in the frequency band of 1–4 Hz. In addition to the LFTs accompanied by known very low frequency earthquakes (VLFEs), more than 130 LFTs without known VLFE activity were detected during April–October, 2011. The newly detected LFTs were in the vicinity of a sequence of small repeating earthquakes indicating mixed distribution of LFTs and regular interplate earthquakes in the region. The LFTs and repeating earthquake activities show a periodicity of 60–100 days, which is similar to that of the LFT activity in the later period (2016–2018). This suggests that the LFT activity is modulated by sustained background aseismic slip events throughout the postseismic period of the 2011 mainshock.

Análise simplificada do efeito de ações sísmicas em edifícios de concreto armado dimensionados pela norma brasileira

<p class="Normal1">Earthquakes are natural events, caused mainly due to the relative movement between tectonic plates (interplate earthquakes) and in faults between rocky blocks (intra-plate earthquakes), or induced by human activity. It is possible to observe a relation between the regions located in areas with greater seismicity and the areas that are close to several intraplate failures and shale gas reserves in Brazil. According to NBR 15421/2006, this condition results in a map of seismic accelerations characteristic of the design. Such accelerations can be used to estimate equivalent horizontal loads. However, it is not usual to consider these effects in the design of reinforced concrete buildings. Therefore, this work aims to evaluate the global stability of reinforced concrete buildings with different bracing systems, subject to seismic accelerations provided for in NBR 15421/2006. Initially, the global stability was verified considering the wind and vertical actions by the γz coefficient. Therefore, all the frames analysed in this paper behaved as non-sway structures (γz ≤ 1,10). Then the wind actions were replaced by seismic ones since it is improbable that both phenomena occur simultaneously. Finally, global stability was re-evaluated by means of the γz coefficient. Therewith, it was observed that all the buildings analysed started to behave as structures of mobile nodes (γz &gt; 1,10), that is, susceptible to the second-order global effects.</p>

Tsunami source consideration of the 1662 Hyuga-nada earthquake occurred off Miyazaki Prefecture, Japan

&lt;p&gt;Hyuga-nada region is located at the south-western part of Nankai Trough, in the Pacific Ocean. M7-class interplate earthquakes are repeatedly occurred by the subducting Philippine Sea plate beneath the Eurasian plate. The largest earthquake in this area was the 1662 Hyuga-nada earthquake (M=7.6) which occurred off Miyazaki Prefecture, south-eastern area of Kyushu region, Japan, and generated tsunami (after called the 1662 tsunami). Strong ground motion hit and many structures were broken near the coast of Miyazaki Prefecture. The tsunami heights were estimated at least 4-5 m along the coast of Miyazaki city, and more than 200 people died by the earthquake and tsunami by historical records. This region is also active area of the shallow slow earthquakes. The 1662 tsunami was much larger than tsunamis generated by usual M7-class interplate earthquakes. It is known by the 2011 Tohoku earthquake that focal area of shallow slow earthquakes also become a tsunami source area. So, we hypothesized that the 1662 unusual large tsunami was caused by the coseismically slipping of focal area of shallow slow earthquakes. We firstly constructed the fault model of the 1662 earthquake based on the recent result of geophysical observation. To examine the tsunami source of the 1662 earthquake, we surveyed the 1662 tsunami deposits in the lowland along the coast of south-eastern Kyushu region. As a result, sandy event deposits interbedded with clay (organic clay) were recognized at several surveyed points. Based on facies features, these event deposits were possibly formed by the 1662 tsunami. Numerical simulation of the tsunami was carried out using the constructed fault model. Calculated tsunami inundation area can explain distribution of the likely tsunami event deposits at Komei, Miyazaki Prefecture. Furthermore, this study compares calculated tsunami inundation areas, distribution of other surveyed tsunami deposits and tsunami heights of historical records. Tsunami source of the 1662 earthquake proposed by our study could better explain geophysical, geological and historical records.&lt;/p&gt;

The URUP EARTHQUAKE SEQUENCE 2012–2013 with magnitude up to Mwreg=7.0 (Kuril Islands)

In July 2012 – November 2013, a series of strong earthquakes occurred near Urup Island (Kuril Islands, Russia). The article analyzes the position of earthquake foci, focal mechanisms, and macroseismic information according to the SF FRC UGS RAS and IMGiG FEB RAS. The entire set of events according to the epicenter and the mechanisms of the source is divided into a swarm of interplate earthquakes in the immersion zone of the Pacific lithospheric plate with a magnitude up to Mw=6.0 and a stronger and deeper event inside the submerged plate with a magnitude Mw=7.0.