Analysis of Site Amplification and Nonlinear Response Based on Ground Motion Records at MYGH10 Station in Japan

Strong horizontal ground motions with the peak ground acceleration (PGA) larger than 1400 gal were observed at Yamamoto (MYGH10) station during the February 2021 Mj 7.3 off the east coast of Honshu, Japan, Fukushima earthquake. Firstly, in this paper, we discussed and verified the theoretical assumptions of the “Nakamura” method under weak and strong ground motions. The site amplification factor of the MYGH10 station was estimated using the surface horizontal-vertical spectral ratio (HVSR) and the surface-to-borehole spectral ratio (SBSR), and the corrected HVSRC, respectively. Meanwhile, the reasons for underestimating the site amplification factor when using HVSR were explained. The vertical amplification phenomenon of seismic P-wave in the high-frequency band was analysed under weak and strong ground motions. Secondly, we utilized HVSR, SBSR, and theoretical transfer function (TTF) based on the 1D wave propagation theory to study the nonlinear site response of MYGH10 station under the mainshock of the Fukushima earthquake and the historically weak and strong ground motions, respectively. The changes in frequencies and amplitudes of the spectral ratio curves when nonlinearities were occurring at the site were analysed and compared using the spectra ratio curves of weak ground motion records and TTF as references. Finally, the recovery of the site after strong nonlinearity was also evaluated by comparing the spectral ratio curves of aftershocks records. We found that the most significant amplification factor of the site increased from 7 to more than 10, and the predominant frequency decreased from 10 Hz to 3.8 Hz under the mainshock of the Fukushima earthquake. The predominant frequency returned to the previous value within three days after the mainshock, but the amplification factor did not.

Broadband Strong-Motion Prediction for Future Nankai-Trough Earthquakes Using Statistical Green’s Function Method and Subsequent Building Damage Evaluation

We used the Green’s function summation method together with the randomly perturbed asperity sources to sum up broadband statistical Green’s functions of a moderate-size source and predict strong ground motions due to the expected M8.1 to 8.7 Nankai-Trough earthquakes along the southern coast of western Japan. We successfully simulated seismic intensity distributions similar to the past earthquakes and strong ground motions similar to the empirical attenuation relations of peak ground acceleration and velocity. Using these results, we predicted building damage by non-linear response analyses and find that at the regions close to the source, as well as regions with relatively thick, soft sediments such as the shoreline and alluvium valleys along the rivers, there is a possibility of severe damage regardless of the types of buildings. Moreover, the predicted damage ratios for buildings built before 1981 are much higher than those built after because of the significant code modifications in 1981. We also find that the damage ratio is highest for steel buildings, followed by wooden houses, and then reinforced concrete buildings.

The Quaternary Active El Arrayan Fault, Santiago, Chile

Understanding the location and nature of Quaternary active crustal faults is critical to the reduction of both fault rupture and strong ground motions hazards in the built environment. Recent work along the San Ramon Fault in Santiago, Chile demonstrates that crustal seismic sources are important hazards. We present the results of a second likely Quaternary active fault (the El Arrayan Fault, EAF) that runs through the City of Santiago. The EAF was discovered at an outcrop in El Arrayan (Lo Barnechea) with up to the North reverse motion and sinistral (left-lateral) motion clearly visible and coincident with fault rocks (gouge, cataclasite, and breccia) and higher topography (i.e. uplift) in the hanging wall. The EAF is at least 12 km long, strikes North-Northwest to South-Southeast, and is steeply dipping (mean dip 77º NE). Clear geomorphic expression with sinistral displaced streams (up to ~210 m) suggest that this fault is Quaternary active and an important local source of fault rupture and crustal strong ground motions. Because no fault zone avoidance criteria in Chile, there is need for enhanced fault mapping, legislation, implementation of active fault rupture avoidance areas in Chile to reduce the risk posed by active crustal structures.

Study of Rotational Motions Caused by Multiple Mining Blasts Recorded by Different Types of Rotational Seismometers

Digging two vertical shafts with the multiple blasts technique gave the opportunity to measure the induced angular motions in a horizontal plane with well-defined positions of sources. Three kinds of rotation rate sensors, sharing an underground location, were used. Two of them—a Fiber-Optic System for Rotational Events & phenomena Monitoring (FOSREM) and a prototypical seismometer housing the liquid-filled torus—sensed the rotation, while a microarray of two double-pendulum seismometers sensed both the rotation and symmetric strain. The FOSREM was sampled at 656.168 Hz, while all the others were only sampled at 100 Hz. There were considerable differences within the results gathered from the mining blasts, which should be attributed to two causes. The first one is the difference in principles of the operation and sampling rates of the devices used, while the other is the complex and spatially variable character of the studied wave fields. Additionally, we established that the liquid-filled sensor, due to its relatively low sensitivity, proved to be viable only during a registration of strong ground motions. Overall, a comparative study of three different rotational seismometers was performed during mining-induced strong ground motions with well-localized sources.