Application of the Foreshock Traffic Light System to the 2019 Ridgecrest sequence

Mapping Intimacies ◽

10.5194/egusphere-egu21-8220 ◽

2021 ◽

Author(s):

Laura Gulia ◽

Stefan Wiemer ◽

Gianfranco Vannucci

Keyword(s):

Size Distribution ◽

Reference Value ◽

B Value ◽

Aftershock Sequence ◽

Traffic Light ◽

Natural Systems ◽

Background Value ◽

Light System ◽

The North ◽

Average Size

The relative earthquake size distribution, or b-value of the Gutenberg and Richter relationship, can act as an indirect stress meter in the earth crust, a finding confirmed in numerous laboratory studies but also in diverse natural systems. &#160;In 2018, we analysed the average size-distribution of about 60 well-monitored earthquakes sequences showing that, after a mainshock with M>=6, the b-value increases by about 20% respect to the background reference value.In 2019, based on such result, we hypothesized and demonstrated that it may be possible, under specific circumstances, to discriminate if an ongoing sequence is representing a typically decaying aftershock sequence or rather foreshocks to an upcoming larger event. &#160;We proposed a simple traffic light classification to assess in near real-time the level of concern for subsequent larger event, and tested it against 58 sequences, reaching a classification accuracy of 95%.The Foreshock Traffic Light System (FTLS) has been implemented in a pseudo-prospective test to the 2019 Ridgecrest sequence. Results are fully in line with the hypothesis: in this Ridgecrest case study, after analyzing carefully the magnitude of completeness of the sequences, we find that in the hours after the first Mw6.4 Ridgecrest event, the b-value drops by 23% on average, when compared to the background value, resulting in a red foreshock traffic light. Spatially mapping the changes in b, we identify an area to the north of the rupture plane as the most likely location of a subsequent event. After the second, magnitude-7.1 mainshock, which did occur in the low b-value region, the b-value subsequently increased by 26% over the background value, triggering a green traffic light setting. Here we will report on these findings, discuss additional case studies, criticisms raised and discuss physics-based mechanics that may allow us to understand and model the observations.

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Real-time discrimination of earthquake foreshocks and aftershocks

10.5194/egusphere-egu2020-7590 ◽

2020 ◽

Author(s):

Laura Gulia ◽

Stefan Wiemer

Keyword(s):

Real Time ◽

Large Earthquake ◽

B Value ◽

Aftershock Sequence ◽

Main Question ◽

Traffic Light ◽

Background Value ◽

The North ◽

The Hours ◽

Average Size

Immediately after a large earthquake, the main question asked by the public and decision-makers is whether it was the mainshock or a foreshock to an even stronger event yet to come. So far, scientists can only offer empirical evidence from statistical compilations of past sequences, arguing that normally the aftershock sequence will decay gradually whereas the occurrence of a forthcoming larger event has a probability of a few per cent.We analyse the average size distribution of aftershocks of the 2016 Amatrice&#8211;Norcia (Italy) and Kumamoto (Japan) earthquake sequences and we suggest that in many cases it may be possible to discriminate whether an ongoing sequence represents a decaying aftershock sequence or foreshocks to an upcoming large event.We propose a simple traffic light classification (FTLS, Foreshock Traffic Light System) to assess in real time the level of concern about a subsequent larger event and test it against 58 sequences, achieving a classification accuracy of 95 per cent.We finally test, in near-real-time, the performance of the FTLS to the 2019 Ridgecrest sequence, California: a Mw6.4 followed, about 2 days later, by a Mw7.1. We find that in the hours after the first Ridgecrest event (Mw 6.4, the b-value drops by 23% on average, when compared to the background value, resulting in a &#8216;red&#8217; foreshock traffic light.Mapping in space the changes in b, we identify an area to the north of the rupture plane as the most likely location of a subsequent event. The second mainshock of magnitude 7.1 then indeed occurred in this location and after this event, the b-value increased by 26 percent over the background value, resulting in a green traffic light state.

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Pseudoprospective Evaluation of the Foreshock Traffic-Light System in Ridgecrest and Implications for Aftershock Hazard Assessment

Seismological Research Letters ◽

10.1785/0220190307 ◽

2020 ◽

Vol 91 (5) ◽

pp. 2828-2842 ◽

Cited By ~ 3

Author(s):

Laura Gulia ◽

Stefan Wiemer ◽

Gianfranco Vannucci

Keyword(s):

Real Time ◽

Hazard Assessment ◽

Monitoring Network ◽

B Value ◽

Traffic Light ◽

B Values ◽

Background Value ◽

Light System ◽

Aftershock Sequences ◽

Aftershock Hazard

Abstract The Mw 7.1 Ridgecrest earthquake sequence in California in July 2019 offered an opportunity to evaluate in near-real time the temporal and spatial variations in the average earthquake size distribution (the b-value) and the performance of the newly introduced foreshock traffic-light system. In normally decaying aftershock sequences, in the past studies, the b-value of the aftershocks was found, on average, to be 10%–30% higher than the background b-value. A drop of 10% or more in “aftershock” b-values was postulated to indicate that the region is still highly stressed and that a subsequent larger event is likely. In this Ridgecrest case study, after analyzing the magnitude of completeness of the sequences, we find that the quality of the monitoring network is excellent, which allows us to determine reliable b-values over a large range of magnitudes within hours of the two mainshocks. We then find that in the hours after the first Mw 6.4 Ridgecrest event, the b-value drops by 23% on average, compared to the background value, triggering a red foreshock traffic light. Spatially mapping the changes in b values, we identify an area to the north of the rupture plane as the most likely location of a subsequent event. After the second, magnitude 7.1 mainshock, which did occur in that location as anticipated, the b-value increased by 26% over the background value, triggering a green traffic light. Finally, comparing the 2019 sequence with the Mw 5.8 sequence in 1995, in which no mainshock followed, we find a b-value increase of 29% after the mainshock. Our results suggest that the real-time monitoring of b-values is feasible in California and may add important information for aftershock hazard assessment.

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Observations of the Coyote Lake, California earthquake sequence of August 6, 1979

Bulletin of the Seismological Society of America ◽

10.1785/bssa0700020559 ◽

1980 ◽

Vol 70 (2) ◽

pp. 559-570 ◽

Cited By ~ 1

Author(s):

R. A. Uhrhammer

Keyword(s):

San Francisco ◽

Fault Zone ◽

Strong Earthquake ◽

Main Shock ◽

B Value ◽

Aftershock Sequence ◽

The South ◽

The North ◽

Temporal And Spatial Characteristics ◽

Largest Aftershock

abstract At 1705 UTC on August 6, 1979, a strong earthquake (ML = 5.9) occurred along the Calaveras fault zone south of Coyote Lake about 110 km southeast of San Francisco. This strong earthquake had an aftershock sequence of 31 events (2.4 ≦ ML ≦ 4.4) during August 1979. No foreshocks (ML ≧ 1.5) were observed in the 3 months prior to the main shock. The local magnitude (ML = 5.9) and the seismic moment (Mo = 6 × 1024 dyne-cm from the SH pulse) for the main shock were determined from the 100 × torsion and 3-component ultra-long period seismographs located at Berkeley. Local magnitudes are determined for the aftershocks from the maximum trace amplitudes on the Wood-Anderson torsion seismograms recorded at Berkeley (Δ ≊ 110 km). Temporal and spatial characteristics of the aftershock sequence are presented and discussed. Some key observations are: (1) the first six aftershocks (ML ≧ 2.4) proceed along the fault zone progressively to the south of the main shock; (2) all of the aftershocks (ML ≧ 2.4) to the south of the largest aftershock (ML = 4.4) have a different focal mechanism than the aftershocks to the north; (3) no aftershocks (ML ≧ 2.4) were observed significantly to the north of the main shock for the first 5 days of the sequence; and (4) the b-value (0.70 ± 0.17) for the aftershock sequence is not significantly different from the average b-value (0.88 ± 0.08) calculated for the Calaveras fault zone from 16 yr of data.

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Two Foreshock Sequences Post Gulia and Wiemer (2019)

Seismological Research Letters ◽

10.1785/0220200082 ◽

2020 ◽

Vol 91 (5) ◽

pp. 2843-2850 ◽

Cited By ~ 3

Author(s):

Kelian Dascher-Cousineau ◽

Thorne Lay ◽

Emily E. Brodsky

Keyword(s):

Puerto Rico ◽

Real Time ◽

Exact Result ◽

Monte Carlo Sampling ◽

B Value ◽

Aftershock Sequence ◽

Traffic Light ◽

B Values ◽

Aftershock Sequences ◽

Abrupt Changes

Abstract Recognizing earthquakes as foreshocks in real time would provide a valuable forecasting capability. In a recent study, Gulia and Wiemer (2019) proposed a traffic-light system that relies on abrupt changes in b-values relative to background values. The approach utilizes high-resolution earthquake catalogs to monitor localized regions around the largest events and distinguish foreshock sequences (reduced b-values) from aftershock sequences (increased b-values). The recent well-recorded earthquake foreshock sequences in Ridgecrest, California, and Maria Antonia, Puerto Rico, provide an opportunity to test the procedure. For Ridgecrest, our b-value time series indicates an elevated risk of a larger impending earthquake during the Mw 6.4 foreshock sequence and provides an ambiguous identification of the onset of the Mw 7.1 aftershock sequence. However, the exact result depends strongly on expert judgment. Monte Carlo sampling across a range of reasonable decisions most often results in ambiguous warning levels. In the case of the Puerto Rico sequence, we record significant drops in b-value prior to and following the largest event (Mw 6.4) in the sequence. The b-value has still not returned to background levels (12 February 2020). The Ridgecrest sequence roughly conforms to expectations; the Puerto Rico sequence will only do so if a larger event occurs in the future with an ensuing b-value increase. Any real-time implementation of this approach will require dense instrumentation, consistent (versioned) low completeness catalogs, well-calibrated maps of regionalized background b-values, systematic real-time catalog production, and robust decision making about the event source volumes to analyze.

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The 1991 Sierra Madre earthquake sequence in southern California: Seismological and tectonic analysis

Bulletin of the Seismological Society of America ◽

10.1785/bssa0840041058 ◽

1994 ◽

Vol 84 (4) ◽

pp. 1058-1074 ◽

Cited By ~ 2

Author(s):

Egill Hauksson

Keyword(s):

Los Angeles ◽

Fault Plane ◽

B Value ◽

Tectonic Stress ◽

Aftershock Sequence ◽

Strike Slip ◽

Focal Mechanisms ◽

Stress Axis ◽

The North ◽

Sierra Madre

Abstract The (ML 5.8) Sierra Madre earthquake of 28 June 1991 occurred at a depth of 12 km under the San Gabriel Mountains of the central Transverse Ranges. Since at least 1932 this region had been quiescent for M ≧ 3. The mainshock focal mechanism derived from first-motion polarities exhibited almost pure thrust faulting, with a rake of 82° on a plane striking N62°E and dipping 50° to the north. The event appears to have occurred on the Clamshell-Sawpit fault, a splay of the Sierra Madre fault zone. The aftershock sequence following the mainshock occurred at a depth of 9 to 14 km and was deficient in small earthquakes, having a b value of 0.6. Twenty nine single-event focal mechanisms were determined for aftershocks of M > 1.5. The 4-km-long segment of the Clamshell-Sawpit fault that may have ruptured in the mainshock is outlined by several thrust focal mechanisms with an east-northeast-striking fault plane dipping to the north. To the west, several thrust aftershocks with east-striking nodal planes suggest some complexity in the aftershock faulting, such as a curved rupture surface. In addition, several strike-slip and normal faulting events occurred along the edges of the mainshock fault plane, indicating secondary tear faulting. The tectonic stress field driving the coexisting left-lateral strike-slip and thrust faults in the northern Los Angeles basin is north-south horizontal compression with vertical intermediate or minimum principal stress axis.

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The aftershock sequence of the north-west Kashmir earthquake of September 3, 1972

Annals of Geophysics ◽

10.4401/ag-4896 ◽

2010 ◽

Vol 28 (2-3) ◽

Author(s):

V. K. SRIVASTAVA ◽

R. K. S. CHOUHAN ◽

R. NIGAM

Keyword(s):

Upper Mantle ◽

B Value ◽

Aftershock Sequence ◽

Creep Recovery ◽

Average Strain ◽

Shear Creep ◽

Kashmir Earthquake ◽

North West ◽

The North ◽

Total Average

This paper is an attempt to study the aftershock sequence of the Northwest Kashmir earthquake of September 3, 1972. b value of the sequence is 1.59. The area of active strain zone is approximately 2.9 • ]013 sq. cm. The total average strain, average elastic energy and average stress of the rock before slip are 3.3 • 10~5, 3.2 ergs/cm3 and 19.8 kg/cm2. The strain rebound curve of the sequence has been constructed which shows a dual type of recovery where the compressional elastic creep is followed by the shear creep recovery. The relaxation time of the sequence is about 0.7 day, showing the Kelvin body like behaviour of the upper mantle.

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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

Seismological Research Letters ◽

10.1785/0220210059 ◽

2021 ◽

Author(s):

Kelian Dascher-Cousineau ◽

Thorne Lay ◽

Emily E. Brodsky

Keyword(s):

Puerto Rico ◽

Real Time ◽

Monitoring System ◽

Expert Judgment ◽

Aftershock Sequence ◽

Real Time Monitoring ◽

Traffic Light ◽

Light System ◽

System Output ◽

Earthquake Sequences

Abstract 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.

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Variation in size and growth of West Greenland capelin (Mallotus villosus) along latitudinal gradients

ICES Journal of Marine Science ◽

10.1093/icesjms/fsq024 ◽

2010 ◽

Vol 67 (6) ◽

pp. 1128-1137 ◽

Cited By ~ 17

Author(s):

R. Hedeholm ◽

P. Grønkjær ◽

A. Rosing-Asvid ◽

S. Rysgaard

Keyword(s):

Size Distribution ◽

Somatic Growth ◽

Latitudinal Gradients ◽

Mallotus Villosus ◽

West Greenland ◽

The North ◽

Growth Difference ◽

Capelin Mallotus Villosus ◽

The Difference ◽

Average Size

Abstract Hedeholm, R., Grønkjær, P., Rosing-Asvid, A., and Rysgaard, S. 2010. Variation in size and growth of West Greenland capelin (Mallotus villosus) along latitudinal gradients. – ICES Journal of Marine Science, 67: 1128–1137. Capelin (Mallotus villosus) is the dominant pelagic species in the West Greenland ecosystem. Historical data are re-examined and new data are presented on the size distribution and the growth pattern of capelin along latitudinal gradients of temperature and other growth factors extending over a distance of 1300 km. The average size of capelin increases with latitude. Fish 3 years old were 48 mm larger (54%) at the secondmost northern station compared with the most southern one. Otolith analysis revealed a significant effect of age as well as area on back-calculated growth. Hence, at all ages, fish grew faster in the north than in the south. Average somatic growth across areas in the first and fourth years was 56 and 15 mm, respectively. Temperature data suggest a 2°C difference in temperature between areas, temperatures being warmest in the north. Hence, the increase in temperature with latitude explains much of the latitudinal growth gradient and suggests a growth difference in West Greenland capelin of 0.4–0.6 cm °C−1 year−1. However, the difference in growth and size distribution between northern and southernmost stations, where temperatures are similar, suggests that other factors, such as differences in available prey and diet composition, also contribute to the differences.

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Grain Refinement in Titanium-Erbium Alloys

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100052626 ◽

1974 ◽

Vol 32 ◽

pp. 522-523

Author(s):

B. B. Rath ◽

J. E. O'Neal ◽

R. J. Lederich

Keyword(s):

Electron Microscopy ◽

Size Distribution ◽

Grain Refinement ◽

Grain Growth ◽

Volume Fraction ◽

Pure Titanium ◽

Systematic Investigation ◽

Second Phase ◽

Titanium Matrix ◽

Average Size

Addition of small amounts of erbium has a profound effect on recrystallization and grain growth in titanium. Erbium, because of its negligible solubility in titanium, precipitates in the titanium matrix as a finely dispersed second phase. The presence of this phase, depending on its average size, distribution, and volume fraction in titanium, strongly inhibits the migration of grain boundaries during recrystallization and grain growth, and thus produces ultimate grains of sub-micrometer dimensions. A systematic investigation has been conducted to study the isothermal grain growth in electrolytically pure titanium and titanium-erbium alloys (Er concentration ranging from 0-0.3 at.%) over the temperature range of 450 to 850°C by electron microscopy.

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