Ionospheric Disturbances Observed Following the Ridgecrest Earthquake of 4 July 2019 in California, USA

Earthquakes are known to generate disturbances in the ionosphere. Such disturbances, referred to as co-seismic ionospheric disturbances, or ionoquakes, were previously reported for large earthquakes with magnitudes Mw≥ 6.6. This paper reports ionoquakes associated with the Ridgecrest earthquakes of magnitude (Mw=6.4), that occurred on 4 July 2019 in California, USA. The ionoquakes manifested in total electron content (TEC) in the form of traveling ionospheric disturbances (TIDs) within 1 h from the mainshock onset. These seismic-origin TIDs have unique wave characteristics that distinguish them from TIDs of non-seismic origin arising from a moderate geomagnetic activity on the same day. Moreover, in the space-time domain of the detection of seismic-origin TIDs, TIDs are absent on the day before and day after the earthquake day. Their spectral characteristics relate them to the Earth’s normal modes and atmospheric resonance modes. We found the ground velocity associated with the mainshock, rather than the ground displacement, satisfies the threshold criteria for detectable ionoquakes in TEC measurements. Numerical simulation suggested that the coupled seismo–atmosphere–ionosphere (SAI) dynamics energized by the atmospheric waves are responsible for the generation of ionoquakes. This study’s findings demonstrate the potential of using TEC measurement to detect the ionospheric counterparts of moderate earthquakes.

A new approach to constrain the seismic origin for prehistoric turbidites as applied to the Dead Sea Basin

<p>Seismogenic turbidites are widely used for geohazard assessment. The use of turbidites as an earthquake indicator requires a clear demonstration that an earthquake, rather than non-seismic factors, is the most plausible trigger. The seismic origin is normally verified either by correlating the turbidites to historic earthquakes, or by demonstrating synchronous deposition over large areas of a basin. Correlating historic earthquakes could potentially constrain the seismic intensities necessary for triggering turbidites, however this method is not applicable to prehistoric events. In addition, the synchronous deposition of turbidites cannot be verified for a single core record.</p><p>Here, we propose a new approach to establish the seismic origin of prehistoric turbidites that involves analyzing in situ deformation that underlies each turbidite, as recorded in a 457 m-long core from the Dead Sea depocenter. These in situ deformations have been previously verified as seismites and could thus authenticate the trigger for each overlying turbidite. We also constrain the seismic intensities that triggered prehistoric turbidites by analyzing the degree of in situ deformation underlying each turbidite. Moreover, our high-resolution chemical and sedimentological data validate a long-lasting hypothesis that soft-sediment deformation in the Dead Sea formed at the sediment-water interface. In addition, we use our results to propose seven basic earthquake-related depositional scenarios preserved in depocenters located in tectonically active regions like the Dead Sea. These techniques and findings permit a more confident geohazard assessment in the region and act as a model for other similar tectonic settings, by improving the completeness of a paleoseismic archive.</p>

The Marsquake Catalogue from InSight, Sols 0-478

The InSight (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) mission began collecting high quality seismic data on Mars from February 2019. This manuscript documents the seismicity observed by SEIS, InSight’s seismometer, since this time until the end of March, 2020. Within the InSight project, the Marsquake Service (MQS) is responsible for prompt review of all seismic data collected by InSight, detection of events that are likely to be of seismic origin, and curation and release of seismic catalogues. In the first year of data collection, MQS have identified 465 seismic events that we interpret to be from regional and teleseismic marsquakes. Seismic events are grouped into 2 different event families: the low frequency family are dominated by energy at long period below 1 second, and the high frequency family primarily include energy at and above 2.4~Hz. Event magnitudes, from Mars-specific scales, range from 1.3 to 3.7. A third class of events with very short duration but high frequency bursts have been observed 712 times. These are likely associated with a local source driven by thermal stresses. This paper describes the data collected so far in the mission and the procedures under which MQS operates; summarises the content of the current MQS seismic catalogue; and presents the key features of the events we have observed so far, using the largest events as examples.

Analysis of the Influence of Structural Geology on the Massive Seismic Slope Failure Potential Supported by Numerical Modelling

The stability of rock slopes is often guided significantly by the structural geology of the rocks composing the slope. In this work, we analysed the influences of structural characteristics, and of their seismic responses, on large and deep-seated rock slope failure development. The study was focused on the Tamins and Fernpass rockslides in the European Alps and on the Balta and Eagle’s Lake rockslides in the southeastern Carpathians. These case studies were compared with catastrophic rock slope failures with ascertained or very likely seismic origin in the Tien Shan Mountains. The main goals was to identify indicators for seismically-induced rock slope failures based on the source zone rock structures and failure scar geometry. We present examples of failures in anti-dip slopes and along-strike rock structures that were potentially (or partially) caused by seismic triggering, and we also considered a series of mixed structural types, which are more difficult to interpret conclusively. Our morpho-structural study was supported by distinct element numerical modelling that showed that seismic shaking typically induces deep-seated deformation in initially “stable” rock slopes. In addition, for failures partially triggered by dynamic shaking, these studies can help identify the contribution of the seismic factor to slope instability. The identification of the partial seismic origin on the basis of the dynamic response of rock structures can be particularly interesting for case histories in less seismically active mountain regions (in comparison with the Andes, Tien Shan, Pamirs), such as in the European Alps and the Carpathian Mountains.

Structural geology of large (ancient) rockslides - an indicator for a seismic or climatic origin?

<p>The stability of rock slopes is often guided by the structural geology of the rocks composing the slope. Geological structures, such as ductile folds, discontinuities as well as brittle faults and fractures, are known factors contributing to a decrease in slope stability according to their orientation in space - with respect to the general orientation of the main slope and its (seismo-) tectonic damage history. Additionally, a rock slope may undergo many forms of gravitationallyinduced, erosional and/or weathering-induced destabilisation.</p><p>Rock slope failures may be classified and described according to several factors, such as their volume, displacement mechanisms and velocity. In this work, especially deep-seated and very large failures (with a volume of >10<sup>7</sup> m<sup>3</sup>) are analyzed with regard to their structural characteristics.</p><p>Giant rockslides originate as planar, rotational, wedge, compound, or irregular slope failures. Most of them convert into flow-like rock avalanches during emplacement. Here, we will not detail the evolution of rock slope failures but rather focus on their origin. The main goal is to identify features allowing to distinguish seismic trigger modes from climatic ones, notably on the basis of the source zone rock structures. We will present examples of classical anti-dip slope (and along-strike) rock structures that hint at a seismic origin, but we will also consider a series of mixed structural types, which are more difficult to interprete. This morpho-structural study is supported by numerical modelling results showing that seismic shaking typically induces deeper seated deformation in initially ‘stable’ rockslopes.</p><p>For failures only partially triggered by dynamic shaking, these study results could help to identify the seismic factor in slope evolution. Especially in less seismically active mountain regions, such as the Alps and the Carpathian Mountains, these analyses can be used for paleoseismic studies – provided that dating the seismic initiation of mass movement is possible. For instance, we will show that the “Tamins” and the “Fernpass” rockslides in the Alps present structural and morphological features hinting at a partly seismic origin. Furthermore, we present study cases of ancient rockslides in the SE Carpathians (“Balta” and “Eagle’s Lake”), where a pure seismic origin is most probable and currently under discussion (supported by numerical analyses).</p>

Seismogenic Disturbances of the Ionosphere During High Geomagnetic Activity

Herein, we analyze the variations in the ionosphere for the period of two weeks before the M6.7 earthquake in India on January 3, 2016. The earthquake occurred after a series of magnetic substorms on December 31, 2015 and January 1, 2016. The relative total electron content (TEC) disturbances have been estimated using global TEC maps and calculated numerically using the 3D global first-principle Upper Atmosphere Model (UAM) for the whole period including the days before, during, and after the substorms. Numerical simulations were repeated with the seismogenic vertical electric currents switched on at the earthquake epicenter. The UAM calculations have reproduced the general behavior of the ionosphere after the main phase of the geomagnetic storm on January 1, 2016 in the form of negative TEC disturbances propagating from high latitudes, being especially strong in the Southern (summer condition) Hemisphere. It was shown that the local ionospheric effects of seismic origin can be identified in the background of the global geomagnetic disturbances. The seismo-ionospheric effects are visible in the nighttime regions with the additional negative TEC disturbances extending from the eastern side of the epicenter meridian to the western side, both in the observations and in the UAM simulations. It was found that the vertical electric field and corresponding westward component of the electromagnetic [E × B] drift played a decisive role in the formation of the ionospheric precursors of this earthquake.