Comment on “Two Foreshock Sequences Post Gulia and Wiemer (2019)” by Kelian Dascher-Cousineau, Thorne Lay, and Emily E. Brodsky

Dascher-Cousineau et al. (2020) apply the so-called foreshock traffic-light system (FTLS) model proposed by Gulia and Wiemer (2019) to two earthquake sequences that occurred after the submission of the model: the 2019 Ridgecrest (Mw 7.1) and the 2020 Mw 6.4 Puerto Rico earthquakes. We show in this comment that the method applied by Kelian Dascher-Cousineau et al. (2020) deviates in at least six substantial and not well-documented aspects from the original FTLS method. As a consequence, they used for example in the Ridgecrest case only 1% of the data available to estimate b-values and from a small subvolume of the relevant mainshock fault. In the Puerto Rico case, we document here substantial issues with the homogeneity of the magnitude scale that in our assessment make a meaningful analysis of b-values impossible. We conclude that the evaluation by Dascher-Cousineau et al. (2020) is misrepresentative and a not a fair test of the FTLS hypothesis.

Equivalent Hazard Magnitude Scale

Hazard magnitude scales are widely adopted to facilitate communication regarding hazard events and the corresponding decision making for emergency management. A hazard magnitude scale measures the strength of a hazard event considering the natural forcing phenomena and the severity of the event with respect to average entities at risk. However, existing hazard magnitude scales cannot be easily adapted for comparative analysis across different hazard types. Here, we propose an equivalent hazard magnitude scale, called the Gardoni Scale after Professor Paolo Gardoni, to measure hazard strength across multiple types of hazards. Using global historical records of hazard magnitude indicators and impacts of events of 12 hazard types from 1900 to 2020, we demonstrate that an equivalent hazard magnitude on the Gardoni Scale can be derived as correlated with the expectation of an impact metric of hazard event. In this study, we model the impact metric as a function of fatalities, total affected population, and total economic damage. Our results show that hazard magnitudes of events can be evaluated and compared across hazard types. For example, we find that tsunami and drought events tend to have large hazard magnitudes, while tornadoes are relatively small in terms of hazard magnitude. In addition, we demonstrate that the scale can be used to evaluate hazard equivalency of historical events. For example, we show that the hazard magnitude of the February 2021 North American cold wave event affecting the southern states of the United States of America was equivalent to the hazard magnitude of Hurricane Harvey in 2017 or a magnitude 7.5 earthquake. Future work will expand the current study in hazard equivalency to modelling of local intensities of hazard events and hazard conditions within a multi-hazard context.

Earthquake Magnitudes from Dynamic Strain

ABSTRACT Dynamic strains have never played a role in determining local earthquake magnitudes, which are routinely set by displacement waveforms from seismic instrumentation (e.g., ML). We present a magnitude scale for local earthquakes based on broadband dynamic strain waveforms. This scale is derived from the peak root-mean-squared strains (A) in 4589 records of dynamic strain associated with 365 crustal earthquakes and 77 borehole strainmeters along the Pacific-North American plate boundary on the west coast of the United States and Canada. In this data set, catalog moment magnitudes range from 3.5≤Mw≤7.2, and hypocentral distances range from 6≤R≤500 km. The 1D representation of geometrical spreading and attenuation of A common to all strain data is logA0(R)=−0.00072R−1.45log(R). After correcting for instrument gain, site terms, and event terms, the magnitude scale, MDS=logA−logA0(R)−log(3×10−9), scales as ≈0.92Mw with a residual standard deviation of 0.19. This close association with Mw holds for events east of the −124° meridian; west of this boundary, however, a constant correction of 0.41 is needed to adjust for additional along-path attenuation effects. As a check on the accuracy of this magnitude scale, we apply it to dynamic strain records from three strainmeters located in the near field of the 2019 M 6.4 and 7.1 Ridgecrest earthquakes. Results from these six records are in agreement to within 0.5 magnitude units, and five out of six records are in agreement to within 0.34 units.

On the Zero Point Constant of the Bolometric Correction Scale

Arbitrariness attributed to the zero point constant of the V band bolometric corrections (BCV) and its relation to “bolometric magnitude of a star ought to be brighter than its visual magnitude” and “bolometric corrections must always be negative” was investigated. The falsehood of the second assertion became noticeable to us after IAU 2015 General Assembly Resolution B2, where the zero point constant of bolometric magnitude scale was decided to have a definite value CBol(W) = 71.197 425 ... . Since the zero point constant of the BCV scale could be written as C2 = CBol − CV, where CV is the zero point constant of the visual magnitudes in the basic definition BCV = MBol − MV = mbol − mV, and CBol > CV, the zero point constant (C2) of the BCV scale cannot be arbitrary anymore; rather, it must be a definite positive number obtained from the two definite positive numbers. The two conditions C2 > 0 and 0 < BCV < C2 are also sufficient for LV < L, a similar case to negative BCV numbers, which means that “bolometric corrections are not always negative”. In sum it becomes apparent that the first assertion is misleading causing one to understand bolometric corrections must always be negative, which is not necessarily true.

Identification, validation and assessment of Multiple Occurrence Regional Landslide Events (MORLE) in Catalonia (Spain) during the last one hundred years.

<p>Landslides in the Pyrenees cause periodical damage to infrastructure and human lives. The European PyrMove project aims to develop cross-border methodologies to manage and reduce risk associated with these geological hazards. One of its approaches are the study of Multiple-Occurrence Regional Landslide Events (MORLE) generated by episodes of intense rainfalls that affect large areas. To prevent and manage MORLE crisis, an identification and categorization of the geological and meteorological factors determining the MORLEs that occurred in Catalonia during the 20th and 21st century were carried out, with special attention to the last 30 years. These events were contrasted to some relevant landslide events at worldwide scale. A new qualitative scale of magnitude multiple Regional Landslide event (mRL) has been conceived according two variables that provide the best reliability for the historical data: (1) the area of the affected region and (2) the magnitude of the largest inventoried landslide. To determine the magnitude of largest landslide we used the ICGC scale based on its size and the total mobilized energy (M). Finally, two MORLE that occurred in 1982 and 2003 in Catalonia have been studied in detail to collect basic information on geological phenomena. These preliminary works will make possible in the future to estimate the triggering precipitation thresholds that induce MORLE scenarios in Catalonia.</p><p>The magnitude scale of MORLE events allows contextualizing the Catalan MORLE in the World. In this approach, seventeen World’s MORLEs events have been described for this work. The main triggering factor of studied regional events has been earthquakes (56%) and intense rainfall or typhoons (44%). Their extension normally do not exceed 50,000 km2 and the number of landslides exceeds, in some cases, 50,000. MORLE’s magnitudes, are mostly 3 or higher, due to their large extension, and to the magnitude of the largest landslide, which normally reaches over the maximum degree within the established magnitude scale for landslides in Catalonia by ICGC (M). Damages and human losses have been difficult to quantify, however, at worldwide scale, most of the MORLEs recorded human losses (> 600 in some cases). The most catastrophic MORLE was in Wenchuan region, China, in October 2008, with more than 87,000 fatalities, 52,194 landslides and 410,000 km2 of affected regional area.</p><p>In Catalonia, 13 MORLEs have been registered from 1900 to present. Here, the main trigger factor has been intense precipitation and the affected areas usually do not exceed 10,000 km2. However, in some cases such as October 1982, which records the largest number of identified landslides (about 900), reached 20,000 km2. The magnitude of the largest event rarely exceeds category M4 in ICGC scale, being the majority category M3. Damages have been considerable in these events such as the most recent, triggered by Gloria storm in January 2020. For Catalonia, three general characteristics are notable: (1) East storm situations are the main generators of MORLE’s; (2) MORLEs usually reach magnitudes mRL3 o mRL4.</p><p>This work has been supported by the European Commission under the Interreg V-A-POCTEFA programme (grant no. PyrMove - EFA364/19).</p>