Earthquakes Ripple Through 3D Printed Models of Los Angeles

Using stainless steel models, researchers find that high-frequency seismic waves—the most damaging to buildings—are attenuated in the Los Angeles sedimentary basin.

Overview of collapsed buildings in Mexico City after the 19 September 2017 (Mw7.1) earthquake

An intraslab normal-faulting earthquake struck the central region of Mexico on 19 September 2017, leading to the collapse of 44 buildings in Mexico City. After the earthquake, the authors collected information in situ and through social media about the collapsed buildings, which was statistically processed to identify the causes of their collapse. This article presents the main collapse statistics, which revealed that 64% of the collapsed buildings had between 1 and 5 stories, 61% had a seismic-force-resisting system based on reinforced concrete columns with flat slabs, 57% experienced a soft-story mechanism, 91% were built before 1985, 43% were located at the corner blocks, and 10% exhibited pounding with neighboring buildings. The spatial distribution of the collapsed buildings and the recorded ground motion features suggest that short- and medium-period buildings having well-known vulnerabilities were particularly prone to collapse under amplified high-frequency seismic waves typical of intraslab normal-faulting earthquakes, such as the 2017 Puebla–Morelos earthquake.

Global quieting of high-frequency seismic noise due to COVID-19 pandemic lockdown measures

Human activity causes vibrations that propagate into the ground as high-frequency seismic waves. Measures to mitigate the coronavirus disease 2019 (COVID-19) pandemic caused widespread changes in human activity, leading to a months-long reduction in seismic noise of up to 50%. The 2020 seismic noise quiet period is the longest and most prominent global anthropogenic seismic noise reduction on record. Although the reduction is strongest at surface seismometers in populated areas, this seismic quiescence extends for many kilometers radially and hundreds of meters in depth. This quiet period provides an opportunity to detect subtle signals from subsurface seismic sources that would have been concealed in noisier times and to benchmark sources of anthropogenic noise. A strong correlation between seismic noise and independent measurements of human mobility suggests that seismology provides an absolute, real-time estimate of human activities.

Landslide Characteristics Revealed by High-Frequency Seismic Waves from the 2017 Landslide in Central Japan

The recent development of advanced seismograph networks offers us a chance to remotely detect landslide occurrences with high-frequency (>∼1 Hz) components. This study examined a landslide in central Japan that produced clearly detectable seismic signals at multiple seismic stations in a permanent network. Wave packets propagated with a group velocity of 3 km/s from the landslide area. Using a source location determination method with amplitude information from the high-frequency component, the source location of the wave packets was shown to be in the vicinity of the landslide with an error of 5 km. Moreover, seismograms specific to this landslide also contained a distinct impulsive phase with a source located in the vicinity of the landslide. The study demonstrated that seismic waves with a high-frequency component from landslides can be used to estimate their mechanisms as well as their locations when they are recognized by a routine seismic network.

From fault creep to slow and fast earthquakes in carbonates

A major part of the seismicity striking the Mediterranean area and other regions worldwide is hosted in carbonate rocks. Recent examples are the destructive earthquakes of L’Aquila (Mw 6.1) in 2009 and Norcia (Mw 6.5) in 2016 in central Italy. Surprisingly, within this region, fast (≈3 km/s) and destructive seismic ruptures coexist with slow (≤10 m/s) and nondestructive rupture phenomena. Despite its relevance for seismic hazard studies, the transition from fault creep to slow and fast seismic rupture propagation is still poorly constrained by seismological and laboratory observations. Here, we reproduced in the laboratory the complete spectrum of natural faulting on samples of dolostones representative of the seismogenic layer in the region. The transitions from fault creep to slow ruptures and from slow to fast ruptures were obtained by increasing both confining pressure (P) and temperature (T) up to conditions encountered at 3–5 km depth (i.e., P = 100 MPa and T = 100 °C), which corresponds to the hypocentral location of slow earthquake swarms and the onset of seismicity in central Italy. The transition from slow to fast rupture is explained by an increase in the ambient temperature, which enhances the elastic loading stiffness of the fault, i.e., the slip velocities during nucleation, allowing flash weakening and, in turn, the propagation of fast ruptures radiating intense high-frequency seismic waves.