Quantitative Analysis of Shallow Earthquake Sequences and Regional Earthquake Behavior: Implications for Earthquake Forecasting

<p>This study is a quantitative investigation and characterization of earthquake sequences in the Central Volcanic Region (CVR) of New Zealand, and several regions in New Zealand and Southern California. We introduce CURATE, a new declustering algorithm that uses rate as the primary indicator of an earthquake sequence, and we show it has appreciable utility for analyzing seismicity. The algorithm is applied to the CVR and other regions around New Zealand. These regions are also compared with the Southern California earthquake catalogue. There is a variety of behavior within these regions, with areas that experience larger mainshock-aftershock (MS-AS) sequences having distinctly different general sequence parameters than those of more swarm dominated regions. The analysis of the declustered catalog shows that Lake Taupo and at least three other North Island regions have correlated variations in rate over periods of ~5 years. These increases in rate are not due to individual large sequences, but are instead caused by a general increase in earthquake and sequence occurrence. The most obvious increase in rate across the four North Island subsets follows the 1995-1996 magmatic eruption at Ruapehu volcano. The fact that these increases are geographically widespread and occur over years at a time suggests that the variations may reflect changes in the subduction system or a broad tectonic process. We examine basic sequence parameters of swarms and MS-AS sequences to provide better information for earthquake forecasting models. Like MS-AS sequences, swarm sequences contain a large amount of decay (decreasing rate) throughout their duration. We have tested this decay and found that 89% of MS-AS sequences and 55% of swarm sequences are better fit with an Omori's law decay than a linear rate. This result will be important to future efforts to forecast lower magnitude ranges or swarm prone areas like the CVR. To look at what types of process may drive individual sequences and may be associated with the rate changes, we examined a series of swarms that occurred to the South of Lake Taupo in 2009. We relocated these earthquakes using double-difference method, hypoDD, to obtain more accurate relative locations and depths. These swarms occur in an area about 20x20 km. They do not show systematic migration between sequences. The last swarm in the series is located in the most resistive area of the Tokaanu geothermal region and had two M =4.4 earthquakes within just four hours of each other. The earthquakes in this swarm have an accelerating rate of occurrence leading up to the first M = 4.4 earthquakes, which migrate upward in depth. The locations of earthquakes following the M = 4.4 event expand away from it at a rate consistent with fluid diffusion. Our statistical investigation of triggering due to large global (M ≥ 7) and regional earthquakes (M ≥ 6) concludes that more detailed (waveform level) investigation of individual sequences will be necessary to conclusively identify triggering, but sequence catalogs may be useful in identifying potential targets for those investigations. We also analyzed the probability that a series of swarms in the central Southern Alps were triggered by the 2009 Dusky Sound Mw = 7.8 and the 2010 Darfield Mw = 7.1 earthquake. There is less than a one-percent chance that the observed sequences occurred randomly in time. The triggered swarms do not show a significant difference to the swarms occurring in that region at other times in the 1.5-year catalog. Waveform cross-correlation was performed on this central Southern Alps earthquake catalog by a fellow PhD student Carolin Boese, and reveals that individual swarms are often composed of a single waveform family or multiple waveform families in addition to earthquakes that did not show waveform similarities. The existence of earthquakes that do not share waveform similarity in the same swarm (2.5 km radius) as a waveform family indicates that similar waveform groups may be unique in their location, but do not necessarily necessitate a unique trigger or driver. In addition to these triggered swarms in the Southern Alps we have also identified two swarms that are potentially triggered by slow-slip earthquakes along the Hikurangi margin in 2009 and 2010. The sequence catalogs generated by the CURATE method may be an ideal tool for searching for earthquake sequences triggered by slow-slip.</p>

Quantitative Analysis of Shallow Earthquake Sequences and Regional Earthquake Behavior: Implications for Earthquake Forecasting

<p>This study is a quantitative investigation and characterization of earthquake sequences in the Central Volcanic Region (CVR) of New Zealand, and several regions in New Zealand and Southern California. We introduce CURATE, a new declustering algorithm that uses rate as the primary indicator of an earthquake sequence, and we show it has appreciable utility for analyzing seismicity. The algorithm is applied to the CVR and other regions around New Zealand. These regions are also compared with the Southern California earthquake catalogue. There is a variety of behavior within these regions, with areas that experience larger mainshock-aftershock (MS-AS) sequences having distinctly different general sequence parameters than those of more swarm dominated regions. The analysis of the declustered catalog shows that Lake Taupo and at least three other North Island regions have correlated variations in rate over periods of ~5 years. These increases in rate are not due to individual large sequences, but are instead caused by a general increase in earthquake and sequence occurrence. The most obvious increase in rate across the four North Island subsets follows the 1995-1996 magmatic eruption at Ruapehu volcano. The fact that these increases are geographically widespread and occur over years at a time suggests that the variations may reflect changes in the subduction system or a broad tectonic process. We examine basic sequence parameters of swarms and MS-AS sequences to provide better information for earthquake forecasting models. Like MS-AS sequences, swarm sequences contain a large amount of decay (decreasing rate) throughout their duration. We have tested this decay and found that 89% of MS-AS sequences and 55% of swarm sequences are better fit with an Omori's law decay than a linear rate. This result will be important to future efforts to forecast lower magnitude ranges or swarm prone areas like the CVR. To look at what types of process may drive individual sequences and may be associated with the rate changes, we examined a series of swarms that occurred to the South of Lake Taupo in 2009. We relocated these earthquakes using double-difference method, hypoDD, to obtain more accurate relative locations and depths. These swarms occur in an area about 20x20 km. They do not show systematic migration between sequences. The last swarm in the series is located in the most resistive area of the Tokaanu geothermal region and had two M =4.4 earthquakes within just four hours of each other. The earthquakes in this swarm have an accelerating rate of occurrence leading up to the first M = 4.4 earthquakes, which migrate upward in depth. The locations of earthquakes following the M = 4.4 event expand away from it at a rate consistent with fluid diffusion. Our statistical investigation of triggering due to large global (M ≥ 7) and regional earthquakes (M ≥ 6) concludes that more detailed (waveform level) investigation of individual sequences will be necessary to conclusively identify triggering, but sequence catalogs may be useful in identifying potential targets for those investigations. We also analyzed the probability that a series of swarms in the central Southern Alps were triggered by the 2009 Dusky Sound Mw = 7.8 and the 2010 Darfield Mw = 7.1 earthquake. There is less than a one-percent chance that the observed sequences occurred randomly in time. The triggered swarms do not show a significant difference to the swarms occurring in that region at other times in the 1.5-year catalog. Waveform cross-correlation was performed on this central Southern Alps earthquake catalog by a fellow PhD student Carolin Boese, and reveals that individual swarms are often composed of a single waveform family or multiple waveform families in addition to earthquakes that did not show waveform similarities. The existence of earthquakes that do not share waveform similarity in the same swarm (2.5 km radius) as a waveform family indicates that similar waveform groups may be unique in their location, but do not necessarily necessitate a unique trigger or driver. In addition to these triggered swarms in the Southern Alps we have also identified two swarms that are potentially triggered by slow-slip earthquakes along the Hikurangi margin in 2009 and 2010. The sequence catalogs generated by the CURATE method may be an ideal tool for searching for earthquake sequences triggered by slow-slip.</p>

An Operational Earthquake Forecasting Experiment for Israel: Preliminary Results

Operational Earthquake Forecasting (OEF) aims to deliver timely and reliable forecasts that may help to mitigate seismic risk during earthquake sequences. In this paper, we build the first OEF system for the State of Israel, and we evaluate its reliability. This first version of the OEF system is composed of one forecasting model, which is based on a stochastic clustering Epidemic Type Earthquake Sequence (ETES) model. For every day of the forecasting time period, January 1, 2016 - November 15, 2020, the OEF-Israel system produces a weekly forecast for target earthquakes with local magnitudes greater than 4.0 and 5.5 in the entire State of Israel. Specifically, it provides space-time-dependent seismic maps of the weekly probabilities, obtained by using a fixed set of the model’s parameters, which are estimated through the maximum likelihood technique based on a learning period of about 32 years (1983–2015). According to the guidance proposed by the Collaboratory for the Study of Earthquake Predictability (CSEP), we also perform the N- and S-statistical tests to verify the reliability of the forecasts. Results show that the OEF system forecasts a number of events comparable to the observed one, and also captures quite well the spatial distribution of the real catalog with the exception of two target events that occurred in low seismicity regions.

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.

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

Gulia and Wiemer (2019; hereafter, GW2019) proposed a near-real-time monitoring system to discriminate between foreshocks and aftershocks. Our analysis (Dascher-Cousineau et al., 2020; hereinater, DC2020) tested the sensitivity of the proposed Foreshock Traffic-Light System output to parameter choices left to expert judgment for the 2019 Ridgecrest Mw 7.1 and 2020 Puerto Rico Mw 6.4 earthquake sequences. In the accompanying comment, Gulia and Wiemer (2021) suggest that at least six different methodological deviations lead to different pseudoprospective warning levels, particularly for the Ridgecrest aftershock sequence which they had separately evaluated. Here, we show that for four of the six claimed deviations, we conformed to the criteria outlined in GW2019. Two true deviations from the defined procedure are clarified and justified here. We conclude as we did originally, by emphasizing the influence of expert judgment on the outcome in the analysis.

Earthquake doublet revealed by multiple pulses in lacustrine seismo-turbidites

Earthquake doublets have been described in fault systems around the world but have not yet been confidently resolved in paleoseismic records. Our current knowledge is limited to historical occurrences, preventing researchers from uncovering potential patterns or recognizing common fault behavior. Identification of prehistoric doublets is thus of crucial importance for adequate seismic hazard assessment and risk mitigation. We developed a new methodology to reveal the sedimentary imprint of earthquake doublets in lacustrine paleoseismic records based on flow direction analysis in multipulsed turbidites, because the delayed arrival of turbidity currents originating from the same source location demonstrates the occurrence of individual triggering mechanisms. As grains tend to align in the presence of a flow, we analyzed flow directions by determining the dominant orientation of elongated grains using a combination of grain size, paleomagnetism, and high-resolution X-ray computed tomography. This methodology was applied to a turbidite deposited by the 2007 CE earthquakes in West Sumatra (Mw 6.4 and 6.3, 2 h apart), and it provides the first unmistakable sedimentary evidence for an earthquake doublet. We argue that this methodology has great potential to be applied to multipulsed turbidites in various subaquatic paleoseismic records and can reveal the occurrence of unknown earthquake sequences.

Stability of the Fault Systems That Host-Induced Earthquakes in the Delaware Basin of West Texas and Southeast New Mexico

The Delaware basin of west Texas and southeast New Mexico has experienced elevated earthquake rates linked spatiotemporally to unconventional petroleum operations. Limited knowledge of subsurface faults, the in situ geomechanical state, and the exact way in which petroleum operations have affected pore pressure (Pp) and stress state at depth makes causative assessment difficult, and the actions required for mitigation uncertain. To advance both goals, we integrate comprehensive regional fault interpretations, deterministic fault-slip potential (DFSP), and multiple earthquake catalogs to assess specifically how faults of two systems—deeper basement-rooted (BR) and shallow normal (SN)—can be made to slip as Pp is elevated. In their natural state, the overall population faults in both the systems have relatively stable DFSP, which explains the low earthquake rate prior to human inducement. BR faults with naturally unstable DFSP and associated earthquake sequences are few but include the Culberson–Mentone earthquake zone, which is near areas of wastewater injection into strata above basement. As a system, the SN faults in the southcentral Delaware basin are uniformly susceptible to slip with small increases in Pp. Many earthquakes sequences have occurred along these shallow faults in association with elevated Pp from shallow wastewater injection and hydraulic fracturing. Our new maps and methods can be used to better plan and regulate petroleum operations to avoid fault rupture.

Effect of Aftershock Characteristics on the Fragility Curve of Post-Mainshock RC Frames

Buildings constructed in seismic zones are not only damaged by mainshocks but may also be damaged by the impact of aftershocks and cause them to collapse. Therefore, studying the behavior of the damaged structures due to the mainshock and aftershock helps in post-mainshock decision making and also in the selection of suitable aftershock records for seismic assessing of the structure under earthquake sequences. This paper presents the effects of aftershock ground motion on the collapse capacity of post-mainshock buildings. The mean period (Tm), predominant velocity period (Tg), frequency bandwidth (Ω), the 5%-95% significant duration (Ds) and seismic records of different sites were selected to evaluate the effect of its characteristics on the collapse capacity of buildings. The intensity of the ground motions was determined by the first-mode spectral acceleration with 5% damping. Collapse capacities of two non-ductile reinforced concrete (RC) frames with 3 and 6 stories were evaluated using a set of 62 aftershock records with a wide range of characteristics. Box plot collapse diagrams and fragility curves have been developed by applying the incremental dynamic analysis (IDA). The results show that in the frequency content with a longer period, the probability of its collapse is higher. In addition, the high significant duration of aftershocks increases the collapse probability of structures. Also, the evaluation of the site characteristics shows differences in collapse capacities of the same frames in varying sites. Therefore, the effect of aftershock characteristics on the capacity of the structures is significant and it is necessary to carefully determine the seismic sequences’ recordings for the evaluation of the seismic behavior of the structures.