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During the COVID-19 outbreak that took place in early 2020, restrictions on human activities were imposed by government-mandated lockdowns and stay-at-home orders. In this study, we analyse the impact of reduced anthropogenic activities on the detection of seismic events. We hypothesise and show that with reduced background noise levels due to the COVID-19 lockdowns, low-magnitude earthquakes are more easily detectable. We investigate the magnitudes of earthquakes recorded at the seismometers before and after COVID-19 lockdowns for two regions — Cascadia Subduction Zone and New Zealand. Gutenberg–Richter law, which gives a relationship between the number of earthquakes and their magnitudes (b-value), was applied in these two areas. Our results point to an increase in detection of smaller-magnitude earthquakes, as observed by an increased b-value during the COVID-19 period compared to those obtained in the pre-COVID time periods. Previous studies have shown that changes in b-value of an area over a sustained period of time affect the short-time probabilistic risk assessment. The variability of b-value also gives useful insights into the prevailing stress state of the region, crustal heterogeneity, pore pressure and tectonic setting of the area.

The potential areas of upcoming earthquakes were investigated along the Northern segment of the Sumatra–Andaman Subduction Zone according to the b-value of the frequency-magnitude distribution. After enhancing the completeness of the earthquake catalogue, two datasets, those recorded during (i) 1980–1994 and (ii) 1980–2003, were tested in order to verify the effective correlation between precursory b-values and the location of subsequent earthquakes. The results confirmed that areas with low b-values agreed well with the locations of the subsequent earthquakes in that region. Accordingly, the present-day dataset from 1980–2010 was carefully evaluated to determine the b-values across the region. Within this spatial investigation, three areas of low b-values and so potential hazards were found. These consisted of the (i) West coast of Myanmar, and (ii) North and (iii) South of the Nicobar Islands. From 2010–2012, a major earthquake with magnitude 7.5 m_b was recorded as being generated in the region South of the Nicobar Islands. Thus, attention should be paid to the remaining two until now quiescent areas, and mitigation plans should be raised for both seismic and tsunami hazards.

In this study, the b-values of frequency-magnitude earthquake distributions were mapped spatially along the sagaing fault zone (SFZ), central Myanmar. Three sub-datasets of the complete earthquake catalogue were tested in order to ensure the applied assumption. Using the present-day dataset (1980–2010), two areas of low b-values, which are prospective potential earthquake sources, were identified at the Naypyidaw-Mandalay and southwestern part of Myitkyina in the central and northern part of the SFZ, respectively. To assess the possible earthquake magnitudes, the b-values were mapped in the cross-section dimension along the SFZ. The obtained areas of low b-values, referred to as the fault asperity regions, conformed to those illustrated in the surface map. The asperity's sizes, examined from specific low b-values of ≤ 0.65 and ≤ 0.060 were quantitatively estimated and empirically converted to the potential earthquake magnitudes. This analysis revealed three prospective areas surrounding the Myitkyina regions capable of generating earthquakes in the future with a possible magnitude of 8.6 Richter. The contribution of effective mitigation plans are, therefore, urgently needed for Myanmar and the adjacent area.

In order to determine the prospective areas of the forthcoming earthquake sources, the $b$-values of the frequency-magnitude earthquake distributions were analyzed spatially and mapped along the strike-slip fault system at the Thailand–Myanmar border. In order to constrain the relationship between the variation of $b$ and the following hazardous earthquake, the completeness of earthquake catalogue was manipulated into two datasets for (i) 1980–2000 and (ii) 1980–2005 and the $b$-values mapped. Utilizing the suitable assumption of 30 fixed earthquake events, the following $M_w\ge 5.0$ earthquakes illustrate a significant relation between their epicenter and the areas showing relatively low $b$-values. By utilizing the most recent earthquake data (1980–2015), five areas exhibiting low $b$-values (implying prospective earthquake sources) can be identified along the strike-slip fault system. Compared with earthquake activities evaluated previously along the strike-slip fault system, the data reveal that these five areas may potentially generate earthquakes up to 7.0$M_w$ within the coming 50 years; the recurrence of the $M_w$-5.0 earthquake is about 10 years and the probabilities of the $M_w$-5.0 earthquake are about 40–95%, respectively. Since these prospective hazardous seismic zones are located close to cities, population centers and hydropower dams, an effective mitigation plan should be developed.

$b$-value of the Gutenberg–Richter relation as an earthquake precursor depends on the tectonic setting features. This paper presents an alternative method to calculate $b$-value in the presence of characteristic earthquakes. The proposed equation is based on the maximum likelihood method applied on the probability density function of the characteristic earthquake model. Data from real and simulated catalogs were used to evaluate the accuracy of the proposed model. For this purpose, 224 seismic event catalogs with various properties including catalogs’ sample size, the ratios of characteristic earthquakes number to catalog’s sample size $(R_{\mathrm{\text{nc}}})$ and different magnitude of characteristic earthquakes were simulated. According to the estimated $b$-values, the earthquake occurrence probability was calculated and discussed. The results indicate that the proposed method of estimation for $b$-value has more adaptable consideration of the characteristic earthquake behavior.

The earthquake potential hazard around the off coast of the West Sumatra-Bengkulu is investigated based on the availability of pre-seismic surface displacement data and the shallow crustal earthquake catalog data of the year 1907 to 2016. Our previous study results of local covariance functions and the correlation dimension (Dc) relationship with the b-value are used to estimate the maximum horizontal crustal strain rate (Sh_max) and Dc around the study area. The Sh_max is calculated using least square prediction based on the horizontal displacement data by employing local covariance functions. The Dc is estimated based on the b-value using the maximum likelihood method with a constant number by assuming that the regional b-value is equal to 1. Furthermore, using the spatial correlation of Sh_max and Dc, the possibility of earthquake potential hazards exist around Batu-Siberut Island can be characterized. The hypothetical source model is defined by referring to the dip and width fault parameter of the 1935 event. Based on the source model, the peak ground acceleration (PGA) at the base rock is estimated using the Ground Motion Prediction Equation (GMPE) referred to in our previous study. To better understand the potential ground shaking, the evaluation of PGA at the surface is then estimated by including the amplification factor. The amplification factor is estimated based on the Horizontal-Vertical Spectral Ratio (HVSR) method of the BMKG data around Pulau Batu and Mentawai station. The PGA estimated at the surface could reach the possible maximum MMI scale. The results obtained in this study might be very beneficial for earthquake mitigation and modeling efforts for the possible potential of the earthquake hazard study and future analysis.

The seismic hazard function analysis around the off coast of Java Island is investigated based on the changes of the b-value using the shallow crustal earthquake catalog data from 1963 to 2016. The study areas took place around M7.8 in 1994 and M7.7 in 2006. The change of the b-value is estimated using the maximum likelihood method with a constant number. First, the b-value surrounding the center area of interest with a radius of about 150 km is calculated based on the earthquake catalog data from 1963 to 2016 (b50). Second, the b-value based on five years with a one-year moving window (b5) is estimated before M7.8 in 1994 and M7.7 in 2006. The b5 is calculated based on the constant number of events of 25, 50, 75, and 100, and we evaluate the mean b-value. Furthermore, the SHF of b50 and b5 are calculated, and then they are compared. The results showed that the Probability of Exceedance (PE) of SHF b5 increased by about five years before the two large earthquake events. Therefore, the results obtained in this study might be very beneficial for earthquake mitigation and modeling efforts for the possible potential of the earthquake hazard study and future analysis.

This paper presents test results of mechanical behavior, failure pattern and b-value of full-scale flat-plate floor subjected to fire. The experimental results show that crack patterns on top surface of the flat-plate floor under fire are consistent with those at ambient temperature. The experimental results also show that concrete spalling has a significant impact on fire resistance of the flat-plate floor. Serious spalling would directly cause failure of concrete structures or members under fire even though they have not reached their fire resistance. On the basis of experimental results, acoustic emission parameters were analysed, which proved that b-value has a close relationship with the damage variation rate of the slab and can be used to estimate the damage process of the flat-plate floor under fire.