Going Beyond Rate Changes as the Sole Indicator for Dynamic Triggering of Earthquakes (original) (raw)
Related papers
Journal of Geophysical Research, 2007
1] Various studies have examined remote earthquake triggering in geothermal areas, but few studies have investigated triggering in nongeothermal areas. We search the ANZA (southern California) network catalog for evidence of remote triggering. Using three statistical tests (binomial, Kolmogorov-Smirnov, and Wilcoxon rank sum), we determine the significance of the rates and timing of earthquakes in southern California following large teleseismic events. To validate our statistical tests, we identify 20 local main shocks (M L ! 3.1) with obvious aftershock sequences and 22 local main shocks (M L ! 3.0) that lack obvious aftershock sequences. Our statistical tests quantify the ability of these local main shocks to trigger aftershocks. Assuming that the same triggering characteristic (i.e., a particular seismic wave amplitude, perhaps in a specific frequency band) is evident for both local and remote main shocks, we apply the same tests to 60 remote main shocks (m b ! 7.0) and assess the ability of these events to trigger seismicity in southern California. We find no obvious signature of remote triggering. We find minimal differences between the spectral amplitudes and maximum ground velocities of the local triggering and nontriggering earthquakes. Similar analysis of a select few of our remote earthquakes shows that the related ground motion regularly exceeds that of local earthquakes both at low frequencies and in maximum velocity. This evidence weakly suggests that triggering requires larger amplitudes at high frequencies and that a maximum ground velocity alone is not the primary factor in remote triggering. Our results are complex, suggesting that a triggering threshold, if it exists, may depend on several factors.
Remote Triggering Not Evident Near Epicenters of Impending Great Earthquakes
Bulletin of the Seismological Society of America, 2013
Recently, there have been numerous great (M w ≥ 8), devastating earthquakes, with a rate in the last seven years that is 260% of the average rate during the 111-year seismological history. Each great earthquake presents an opportunity to study a major fault at the very beginning and end of the inferred seismic cycle. In this work, we use these events as both targets and sources to probe susceptibility to dynamic triggering in the epicentral region before and after a large earthquake. This study also carefully addresses the possibility that large earthquakes interact in a cascade of remotely triggered sequences that culminates in further large earthquakes. We seek evidence of triggering associated with the 16 great M w ≥ 8 events that occurred between 1998 and 2011, using regional and global earthquake catalogs to measure changes in interevent time statistics. Statistical significance is calculated with respect to a nonstationary reference model that includes mainshock-aftershock clustering. We find limited evidence that a few great earthquakes triggered an increase in seismicity at the site of the next great earthquake in the sequence. However, this evidence is not corroborated by all statistical tests nor all earthquake catalogs. Systematic triggered rate changes in the years to decades before each great earthquake are less than 19% at the 95% confidence level, too small to explain the observed rate increase. The catalogs are insufficient for the purpose of resolving more moderate triggering expected from previous studies. We calculate that an improvement in completeness magnitude from 3.7 to 3.5 could resolve the expected triggering signal in the International Seismic Center (ISC) catalog taken as a whole, but an improvement to M 2.0 would be needed to consistently resolve triggering on a regional basis.
Geophysical Journal International, 2015
The recent spate of mega-earthquakes since 2004 has led to speculation of an underlying change in the global 'background' rate of large events. At a regional scale, detecting changes in background rate is also an important practical problem for operational forecasting and risk calculation, for example due to volcanic processes, seismicity induced by fluid injection or withdrawal, or due to redistribution of Coulomb stress after natural large events. Here we examine the general problem of detecting changes in background rate in earthquake catalogues with and without correlated events, for the first time using the Bayes factor as a discriminant for models of varying complexity. First we use synthetic Poisson (purely random) and Epidemic-Type Aftershock Sequence (ETAS) models (which also allow for earthquake triggering) to test the effectiveness of many standard methods of addressing this question. These fall into two classes: those that evaluate the relative likelihood of different models, for example using Information Criteria or the Bayes Factor; and those that evaluate the probability of the observations (including extreme events or clusters of events) under a single null hypothesis, for example by applying the Kolmogorov-Smirnov and 'runs' tests, and a variety of Z-score tests. The results demonstrate that the effectiveness among these tests varies widely. Information Criteria worked at least as well as the more computationally expensive Bayes factor method, and the Kolmogorov-Smirnov and runs tests proved to be the relatively ineffective in reliably detecting a change point. We then apply the methods tested to events at different thresholds above magnitude M ≥ 7 in the global earthquake catalogue since 1918, after first declustering the catalogue. This is most effectively done by removing likely correlated events using a much lower magnitude threshold (M ≥ 5), where triggering is much more obvious. We find no strong evidence that the background rate of large events worldwide has increased in recent years.
Absence of remotely triggered large earthquakes beyond the mainshock region
Large earthquakes are known to trigger earthquakes elsewhere. Damaging large aftershocks occur close to the mainshock and microearthquakes are triggered by passing seismic waves at significant distances from the mainshock 1-6. It is unclear, however, whether bigger, more damaging earthquakes are routinely triggered at distances far from the mainshock, heightening the global seismic hazard after every large earthquake. Here we assemble a catalogue of all possible earthquakes greater than M 5 that might have been triggered by every M 7 or larger mainshock during the past 30 years. We compare the timing of earthquakes greater than M 5 with the temporal and spatial passage of surface waves generated by large earthquakes using a complete worldwide catalogue. Whereas small earthquakes are triggered immediately during the passage of surface waves at all spatial ranges, we find no significant temporal association between surface-wave arrivals and larger earthquakes. We observe a significant increase in the rate of seismic activity at distances confined to within two to three rupture lengths of the mainshock. Thus, we conclude that the regional hazard of larger earthquakes is increased after a mainshock, but the global hazard is not. Surface waves are usually the largest-amplitude arrivals on a seismogram, and they produce transient strain as they travel within Earth's crustal waveguide. Large (M ≥ 7) earthquakes are known to trigger earthquakes 1-6 and other phenomena, such as non-volcanic tremor 7-11 , at remote distances. Although remote earthquake triggering is seen in all tectonic settings 12 , the mechanism of triggered earthquake failure remains unsolved. Thus far, the remotely triggered earthquakes we have associated with the onset of passing seismic waves have been small-magnitude (M < 5) events. However, what if each large mainshock raises the global rate of other large earthquakes? Should there be a worldwide alarm period of heightened earthquake probability? We turn to the 30-yr global catalogue to search for high-magnitude (M > 5) triggered earthquakes at all offsets following large (M ≥ 7), shallow (Z ≤ 50 km) earthquakes to see whether there are significant rate increases. Our global earthquake catalogue is compiled from the Advanced National Seismic System and Global Seismograph Network. We find the minimum magnitude of completeness to range between coda magnitude (M c) = 4.7 and M c = 5.1, depending on the methods applied (Supplementary Fig. S1). We thus investigate M > 5 events throughout this study, which we define as earthquakes with catalogue listings of M ≥ 5.1. As will be shown, we conducted tests with M c ≥ 5.5 and M c ≥ 6.0 without substantive change in result. We identify links among large earthquakes by calculating earthquake density (number km −2) for 5 < M < 7 events in concentric radii measured from 205 M ≥ 7 global earthquakes (1979-2009). We calculate earthquake density to normalize results 1 US Geological Survey, MS-999, calculated over the larger areas caused by increasing radii. We isolate the largest events (M ≥ 7) for study as triggering mainshocks, leaving 25,222 potentially triggered 5 < M < 7 events. We compute relative origin times and ranges of every 5 < M < 7 catalogue event to each of the 205 M ≥ 7 mainshocks. We then calculate before and after 5 < M < 7 earthquake density (number km −2) in bins ranging from 20 to 200 km width, and over 100 time intervals ranging from 30 s to 1 day (Fig. 1; see Methods). We determine the significance of observed rate changes by establishing the global background rate of 5 < M < 7 earthquakes over the time and distance ranges used in the study. The question we want to answer in establishing significance is, what are the mean and confidence bounds on the expected steady-state density of 5 < M < 7 earthquakes as a function of distance from the M ≥ 7 triggering event locations used in the study? This allows us to recognize anomalous rate changes at any distance range. For example, if we know the mean background rate in a given window of time as a function of distance from sources, we can compare it with the observed rate versus distance. Wherever or whenever the observed density is significantly higher than background, then we suspect triggering is happening. We examine many periods at random times throughout the 30-year catalogue to establish mean rates and confidence bounds (see Methods). We search for triggered earthquakes that lie within and after time intervals containing surface-wave arrivals (Fig. 2), but find no significant 5 < M < 7 earthquake rate increase coincident with surface-wave arrivals at any distance range on Earth in the past 30 years. This result is surprising because past studies 12 , using just 15 mainshocks and spatially limited detection, identified ∼1,500 M ≤ 3 events that occurred within 15 min of the first surface-wave arrivals. Extrapolating with the Gutenberg-Richter relation between magnitude and frequency (logN = a − bM , where N is the number of earthquakes and a and b are constants defining intercept and linear slope), we expect a minimum of ∼70 M > 5.0 and ∼25 M > 6.0 triggered earthquakes to have occurred above background rates within 15 min of surface-wave arrivals after 205 mainshocks over 30 yr. We calculate the Gutenberg-Richter relation using the number of triggered detections for 15 large (M > 7.0) earthquakes 12. If we assume that all of the detections recorded in the first 15 min were triggered events (∼1,500) with maximum magnitude M ≤ 3.0 and a b value of 1.0, we expect at least five M > 5.0 and two M > 6.0 triggered earthquakes should have occurred. If we assume M < 2.0 for all triggered events, the number of triggered earthquakes decreases to approximately two for M > 5.0 and 0.2 for M > 6.0 in 14 years previously studied 12. In our case, we analyse data from a 30-year span, indicating that we should at minimum expect 4-10 M > 5.0 triggered events and 0-2 M > 6.0 triggered events. This is a lower bound estimate, because it is based only on 15 M > 7.0 earthquakes, and detections were spatially limited to events very 312 NATURE GEOSCIENCE | VOL 4 | MAY 2011 | www.nature.com/naturegeoscience
Evidence of Systematic Triggering at Teleseismic Distances Following Large Earthquakes
Scientific Reports, 2018
Earthquakes are part of a cycle of tectonic stress buildup and release. As fault zones near the end of this seismic cycle, tipping points may be reached whereby triggering occurs and small forces result in cascading failures. The extent of this effect on global seismicity is currently unknown. Here we present evidence of ongoing triggering of earthquakes at remote distances following large source events. The earthquakes used in this study had magnitudes ≥M5.0 and the time period analyzed following large events spans three days. Earthquake occurrences display increases over baseline rates as a function of arc distance away from the epicenters. The p-values deviate from a uniform distribution, with values for collective features commonly below 0.01. An average global forcing function of increased short term seismic risk is obtained along with an upper bound response. The highest magnitude source events trigger more events, and the average global response indicates initial increased ea...
Do earthquakes talk to each other? Triggering and interaction of repeating sequences at Parkfield
J. Geophys. Res., 2013
1] Knowledge of what governs the timing of earthquakes is essential to understanding the nature of the earthquake cycle and to determining earthquake hazard, yet the variability and controls of earthquake recurrences are not well established. The large population of small, characteristically repeating earthquakes at Parkfield provides a unique opportunity to study how the interaction of earthquakes affects their recurrence properties. We analyze 112 M À0.4~3.0 repeating earthquake sequences (RESs) to examine the triggering effect from nearby microseismicity. We find that the repeating events with a smaller number of neighboring earthquakes in their immediate vicinity tend to recur in a more periodic manner (i.e., the coefficient of variation in recurrence intervals is less than 0.3). The total static stress perturbation from close-by earthquakes, however, does not seem to strongly influence RES regularity. The uneven distribution of stress changes in time has a modest but significant impact on recurrence intervals. A significant reduction of recurrence intervals occurs in the case of very high-stress changes from neighboring events. Close-by events influence RES timing in a matter of several days or less by short-term triggering. Events that occurred within less than 1 day of an RES often imposed or experienced high-stress changes. A static stress increment of~30 kPa can be enough to produce such short-term triggering. We find that the triggered repeating events are often near the end of their average earthquake cycle, but some events occur following a substantially shortened interval. When comparing the accelerated occurrence at the time of RES events following neighboring events with varying magnitudes, we find that the distance of short-term triggering increases from <1 km to 4 km for M1 to M4 events.
Dynamics of the Markov Time Scale of Seismic Activity May Provide a Short-Term Alert for Earthquakes
2005
We propose a novel method for analyzing precursory seismic data before an earthquake that treats them as a Markov process and distinguishes the background noise from real fluctuations due to an earthquake. A short time (on the order of several hours) before an earthquake the Markov time scale tM increases sharply, hence providing an alarm for an impending earthquake. To distinguish a false alarm from a reliable one, we compute a second quantity, T1, based on the concept of extended self-similarity of the data. T1 also changes strongly before an earthquake occurs. An alarm is accepted if both tM and T1 indicate it simultaneously. Calibrating the method with the data for one region provides a tool for predicting an impending earthquake within that region. Our analysis of the data for a large number of earthquakes indicate an essentially zero rate of failure for the method.
Spatial-temporal variation of low-frequency earthquake bursts near Parkfield, California
Geophysical Journal International, 2015
Tectonic tremor (TT) and low-frequency earthquakes (LFEs) have been found in the deeper crust of various tectonic environments globally in the last decade. The spatial-temporal behaviour of LFEs provides insight into deep fault zone processes. In this study, we examine recurrence times from a 12-yr catalogue of 88 LFE families with ∼730 000 LFEs in the vicinity of the Parkfield section of the San Andreas Fault (SAF) in central California. We apply an automatic burst detection algorithm to the LFE recurrence times to identify the clustering behaviour of LFEs (LFE bursts) in each family. We find that the burst behaviours in the northern and southern LFE groups differ. Generally, the northern group has longer burst duration but fewer LFEs per burst, while the southern group has shorter burst duration but more LFEs per burst. The southern group LFE bursts are generally more correlated than the northern group, suggesting more coherent deep fault slip and relatively simpler deep fault structure beneath the locked section of SAF. We also found that the 2004 Parkfield earthquake clearly increased the number of LFEs per burst and average burst duration for both the northern and the southern groups, with a relatively larger effect on the northern group. This could be due to the weakness of northern part of the fault, or the northwesterly rupture direction of the Parkfield earthquake.
Bulletin of the Seismological Society of America, 2016
Technological advances in combination with the onslaught of data availability allow for large seismic data streams to automatically and systematically be recorded, processed, and stored. Here, we develop an automated approach to identify small, local earthquakes within these large continuous seismic data records. Our aim is to automate the process of detecting small seismic events triggered by a distant large earthquake, recorded at a single station. Specifically, we apply time-domain shortterm average (STA) to long-term average (LTA) ratio algorithms to three-component data to create a catalog of detections. We remove some of the false detections by requiring the detection be recorded on a minimum of two channels. To calibrate the algorithm, we compare our automatic detection catalog to a set of analyst-derived P-wave arrival times for a subset of small earthquakes occurring in the December 2008 Yellowstone swarm. Of the four STA/LTA algorithms we test (1 s/10 s; 4 s/40 s; 8 s/80 s; 16 s/160 s), the 1 s/10 s and 4 s/40 s detectors proved most effective at identifying the majority of events in the swarm. We apply these detectors to 45 hrs and 5 hrs of USArray data from the 2011 Japan M 9.0 and the 2010 Chile M 8.8 earthquakes, respectively. Using time-of-day versus number of detection relationships, we identify 38 of the 728 available stations that exhibit strong anthropogenic noise following the 2011 Japan earthquake. Our detection algorithm identified three regional earthquakes concurrent with the passage of the Sand surface waves of the Chile mainshock at USArray station R11A that locate in the Coso region of California, as well as events in Texas following the Japan earthquake.
Increased earthquake rate prior to mainshocks
arXiv (Cornell University), 2023
According to the Omori-Utsu law, the rate of aftershocks after a mainshock decays as a power law with an exponent close to 1. This well-established law was intensively used in the past to study and model the statistical properties of earthquakes. Moreover, according to the socalled inverse Omori law, the rate of earthquakes should also increase prior to a mainshock-this law has received much less attention due to its large uncertainty. Here, we mainly study the inverse Omori law based on a highly detailed Southern California earthquake catalog, which is complete for magnitudes larger than m c ≥ 0.3. First, we propose a technique to identify mainshocks, foreshocks, and aftershocks. We then find, based on a statistical procedure we developed, that the rate of earthquakes is higher a few days prior to a mainshock. We find that this increase is much smaller for a catalog with a magnitude threshold of m c ≥ 2.5 and for the Epidemic-Type Aftershocks Sequence (ETAS) model catalogs, even when used with a small magnitude threshold. We also analyze the rate of aftershocks after mainshocks and find that the Omori-Utsu law does not hold for many mainshocks and that it may be valid only statistically when considering many mainshocks. Yet, analysis of the ETAS model that is based on the Omori-Utsu law exhibits similar behavior as for the real catalogs, indicating the validity of this law.