Interplay of seismic and a-seismic deformation during the 2020 sequence of Atacama, Chile.

An earthquake sequence occurred in the Atacama region of Chile throughout September 2020. The sequence initiated by a mainshock of magnitude Mw6.9, followed 17 hours later by a Mw6.4 aftershock. The sequence lasted several weeks, during which more than a thousand events larger than Ml 1 occurred, including several larger earthquakes of magnitudes between 5.5 and 6.4. Using a dense network that includes broad-band, strong motion and GPS sites, we study in details the seismic sources of the mainshock and its largest aftershock, the afterslip they generate and their aftershock, shedding light of the spatial temporal evolution of seismic and aseismic slip during the sequence. Dynamic inversions show that the two largest earthquakes are located on the subduction interface and have a stress drop and rupture times which are characteristics of subduction earthquakes. The mainshock and the aftershocks, localised in a 3D velocity model, occur in a narrow region of interseismic coupling (ranging 40%-80%), i.e. between two large highly coupled areas, North and South of the sequence, both ruptured by the great Mw~8.5 1922 megathrust earthquake. High rate GPS data (1 Hz) allow to determine instantaneous coseismic displacements and to infer coseismic slip models, not contaminated by early afterslip. We find that the total slip over 24 hours inferred from precise daily solutions is larger than the sum of the two instantaneous coseismic slip models. Differencing the two models indicates that rapid aseismic slip developed up-dip the mainshock rupture area and down-dip of the largest aftershock. During the 17 hours separating the two earthquakes, micro-seismicity migrated from the mainshock rupture area up-dip towards the epicenter of the Mw6.4 aftershocks and continued to propagate upwards at ~0.7 km/day. The bulk of the afterslip is located up-dip the mainshock and down-dip the largest aftershock, and is accompanied with the migration of seismicity, from the mainshock rupture to the aftershock area, suggesting that this aseismic slip triggered the Mw6.4 aftershock. Unusually large post-seismic slip, equivalent to Mw6.8 developed during three weeks to the North, in low coupling areas located both up-dip and downdip the narrow strip of higher coupling, and possibly connecting to the area of the deep Slow Sleep Event detected in the Copiapo area in 2014. The sequence highlights how seismic and aseismic slip interacted and witness short scale lateral variations of friction properties at the megathrust.

Seismotectonic Analysis of the 2017 Moiyabana Earthquake (MW 6.5; Botswana), Insights from field investigations, aftershock and InSAR studies

<p>The 3 April 2017 M<sub>W</sub> 6.5, Moiyabana (Botswana) earthquake occurred in the continental interior of the African plate and in a seismogenic region previously considered as stable. We analyse the mainshock and aftershock sequence based on a local seismic network and local seismotectonic characteristics. The earthquake rupture geometry is constrained with more than 1,000 aftershocks recorded over a period of three months and from the InSAR analysis of Sentinel-1 images (ascending orbit). The mainshock (25.134 E, 22.565 S; depth 22 ± 3 km) was followed by more than 500 events of magnitude M ≥ 0.8 recorded in April 2017 including the largest aftershock (M<sub>W</sub> 4.6 on the 5 April 2017). Focal mechanism solutions of the mainshock and aftershocks display predominance of NW-SE trending and NE dipping normal faulting. Stress inversion of focal mechanisms obtained from the mainshock and aftershock database are compatible with a NE-SW extension under normal faulting regime. The InSAR study shows fringes with two lobes with 4 to 6 cm coseismic slip on a NW-SE elongated and 30-km-long surface deformation consistent with the mainshock location and normal faulting mechanism. The modelling of surface deformation provides the earthquake rupture dimension at depth with ~ 1 m maximum slip on a fault plane striking 315°, dipping 45°, -80° rake and with M<sub>o</sub> 7.12 10<sup>18</sup> Nm Although the seismic strain rate is of low level, the occurrence of the 2017 Moiyabana earthquake, followed by an aftershock sequence in the central Limpopo belt classifies the intraplate region as an active plate interior. </p>

Felt reports and impact of the November 25, 1988, magnitude 5.9 Saguenay, Quebec, earthquake sequence

The November 25, 1988, moment magnitude 5.9 (Mw) Saguenay earthquake is one of the largest eastern Canadian earthquakes of the 20th century. It was preceded by a magnitude (MN) 4.7 foreshock and followed by very few aftershocks considering the magnitude of the main shock. The largest aftershock was a magnitude (MN) 4.3 event. This Open File (OF) Report presents a variety of documents (including original and interpreted felt information, images, newspaper clippings, various engineering reports on the damage, mass movements). This OF updates the report of Cajka and Drysdale (1994) with additional material, including descriptions of the foreshock and largest aftershock. Most of the felt report information come from replies of a questionnaire sent to postmasters in more than 2000 localities in Canada and in the United States. Images of the original felt reports from Canada are included. The OF also includes information gathered in damage assessments and newspaper accounts. For each locality, the interpreted information is presented in a digital table. The fields include the name, latitude and longitude of the municipality and the interpreted intensity on the Modified Mercalli Intensity (MMI) scale (most of which are the interpretations of Cajka and Drysdale, 1996). When available or significant, excerpts of the felt reports are added. This OF Report also includes images from contemporary newspapers that describe the impact. In addition, information contained in post-earthquake reports are discussed together with pictures of damage and mass movements. Finally, a GoogleEarth kmz file is added for viewing the felt information reports within a spatial tool.

Macroseismic information for the 1935 moment magnitude 6.1 earthquake, near Témiscamingue, Quebec

The November 1st, 1935, Témiscaming earthquake occurred within 20 km of the town of Témiscaming, Quebec. This earthquake was felt west to Fort William (now part of Thunder Bay), Ontario, east to Saint John, New Brunswick, and south to Kentucky and Virginia. Damaged chimneys were reported in Témiscaming, Quebec, and North Bay and Mattawa, Ontario. In the epicentral region, rockfalls were observed as well as cracks in gravel and sand along the shores of islands and lakes. Some 350 km away from the epicentre, near Parent, Quebec, earthquake vibrations triggered a 30 metre slide of railroad embankment. Numerous aftershocks were felt in Témiscaming and Kipawa during the following months, the largest rated as magnitude ML 5.4 (or mN 4.9). For the main shock and its largest aftershock, this Open File Report provides the available macroseismic information interpreted on the Modified Mercalli Intensity Scale using newspaper accounts as the main source of information for Canada. Macroseismic information from total of 126 localities in Canada and nearly 900 communities in the US (from the NOAA database of intensities) are tabulated in a Microsoft Excel spreadsheet. When available, newspaper clippings are included, together with some original damage accounts, photographs and scientific reports. The Open File also includes a Google Earth kmz file that allows the felt information reports to be viewed in a spatial tool.

EARTHQUAKES FELT IN MOLDOVA 2014: March, 29 with КP=12.5, Mw=4.7, September, 10 with КP=12.4, Mw=4.5 and November 22 with КP=14.3, Mw=5.8 (Romania–Moldova)

All earthquakes felt in 2014 on the territory of Moldova occurred outside its borders, in the Vrancea and Pre-Carpathian regions (Romania). In 2014, the population of Moldova felt 13 earthquakes. The article discusses in detail the most powerful events, occurred on March 29, September 6, and November 22. The March 29 earthquake, Mw=4.7, hрР=136 km was felt in the eastern and southern counties of Romania (in 41 settlements), in the Odessa region of Ukraine, and also in the central and southern regions of the Republic of Moldova (22 points). The epicenter was situated in a bend of the Vrancea mountains. The earthquake on Sep-tember 10, Mw=4.5, hрР=108 km was felt in the eastern and southern counties of Romania (in 27 settlements), in the central and southern parts of Moldova (22 points), in the north of Bulgaria and in the Odessa region of Ukraine. Both earthquakes, March 29 and September 10, occurred under the action of prevailing near-horizontal compressive stress. The November 22 earthquake, Mw=5.8, hрР=37 km occurred in the southwestern part of Romania and turned out to be the most significant crust event for the instrumental observation period. Movement in the source occurred under the action of tensile stresses. Earthquakes in this zone continued until January 19, 2015. The largest aftershock was on December 7 with МwMED=4.4. Foci are associated with the activation of the Peceneaga-Camena fault. The main shock was felt in Romania (in 66 settlements) and neighboring countries: Bulgaria, Moldova (23 settlements), Ukraine (18 settlements). The isoseismal maps were constructed for all three earthquakes considered in detail in this work. The intensity at the epicenter of the November 22 earthquake reached I0=6, for other two events I0=5.

Characteristics of seismic activity before and after the 2018 M6.7 Hokkaido Eastern Iburi earthquake

We applied the epidemic type aftershock sequence (ETAS) model, the two-stage ETAS model and the non-stationary ETAS model to investigate the detailed features of the series of earthquake occurrences before and after the M6.7 Hokkaido Eastern Iburi earthquake on 6 September 2018, based on earthquake data from October 1997. First, after the 2003 M8.0 Tokachi-Oki earthquake, seismic activity in the Eastern Iburi region reduced relative to the ETAS model. During this period, the depth ranges of the seismicity were migrating towards shallow depths, where a swarm cluster, including a M5.1 earthquake, finally occurred in the deepest part of the range. This swarm activity was well described by the non-stationary ETAS model until the M6.7 main shock. The aftershocks of the M6.7 earthquake obeyed the ETAS model until the M5.8 largest aftershock, except for a period of several days when small, swarm-like activity was found at the southern end of the aftershock region. However, when we focus on the medium and larger aftershocks, we observed quiescence relative to the ETAS model from 8.6 days after the main shock until the M5.8 largest aftershock. For micro-earthquakes, we further studied the separated aftershock sequences in the naturally divided aftershock volumes. We found that the temporal changes in the background rate and triggering coefficient (aftershock productivity) in respective sub-volumes were in contrast with each other. In particular, relative quiescence was seen in the northern deep zones that includes the M5.8 largest aftershock. Furthermore, changes in the b-values of the whole aftershock activity showed an increasing trend with respect to the logarithm of elapsed time during the entire aftershock period, which is ultimately explained by the spatially different characteristics of the aftershocks.

Gravity and magnetic field interpretation to detect deep buried paleobasinal fault lines contributing to intraplate earthquakes: a case study from Pohang Basin, SE Korea

SUMMARY Potential field interpretation is a powerful method to locate deep buried tectonic fault lines that contribute to intraplate earthquakes. A magnitude 5.4 earthquake (2017, M5.4_PO) occurred in the Middle-Miocene Pohang Basin (PB), SE of the Korean Peninsula on 15 November 2017, in the area where no fault lines appear on geological and tectonic maps. To constrain fault locations, we calculate the gravity effect of the current basin fill with a gravity stripping method and used curvature analysis to identify former geological and tectonic structures, assumed formed in the Early-Miocene. The Early-Miocene PB is divided into two subregions (northern- and southern sub-basins) by a modelled NW–SE fault line, similar to the other Early-Miocene basins (e.g. Eoil basin). Fault line trends are NE–SW in the northern sub-basin, and NNE–SSW in the southern sub-basin. 2017M5.4_PO arose from a tectonic movement along the eastern boundary of the northern sub-basin, the cross-over area from the isolated high-magnetic/low-gravity region to low-magnetic/high-gravity region. The largest aftershock of the 2017M5.4_PO occurred along the NW–SE fault line bordering the northern- and southern sub-basin.

Topographic characteristics of landslides induced by the 2015 Gorkha earthquake, Nepal

The 2015 Gorkha earthquake and its aftershocks induced landslides in central Nepal. In this study, field surveys were conducted, and Google Earth satellite images were analysed for pre- and post-mainshock and aftershock scenarios to clarify the distribution of landslides. A total of 13,097 new landslides and 750 enlarged landslides were identified and mapped as polygon-based data over an area of 7.8 × 103 km2 between the epicenters of the main shock and the largest aftershock at the mountainous southern margin of the High Himalayas. Shallow-disrupted landslides were the most common type of mass movement. The areas of individual landslides ranged from 10 to 3.2× 105 m2, covering a cumulative area of 5.4 × 107 m2 or 0.7% of the study area. The landslide density was high in the Gorkha, Rasuwa, and Sindhupalchok districts, indicating that these areas suffered greater damage. Landslides occurred mainly on steep slopes (>35°) in V-shaped inner gorges, on geologically controlled steep slopes such as the scarp slopes (infacing slopes) of mountain ridges, and on terrace scarps. The results suggest that earthquake-induced landslides occur on slopes preconditioned by topographic and litho-structural factors. Based on our observations, recommendations for the mitigation of future landslide disasters are provided.