SUMMARY
We examine localization processes of low magnitude seismicity in relation to the occurrence of large earthquakes using three complementary analyses: (i) estimated production of rock damage by ...background events, (ii) evolving occupied fractional area of background seismicity and (iii) progressive coalescence of individual earthquakes into clusters. The different techniques provide information on different time scales and on the spatial extent of weakened damaged regions. Techniques (i) and (ii) use declustered catalogues to avoid the occasional strong fluctuations associated with aftershock sequences, while technique (iii) examines developing clusters in entire catalogue data. We analyse primarily earthquakes around large faults that are locked in the interseismic periods, and examine also as a contrasting example seismicity from the creeping Parkfield section of the San Andreas fault. Results of analysis (i) show that the M > 7 Landers 1992, Hector Mine 1999, El Mayor-Cucapah 2010 and Ridgecrest 2019 main shocks in Southern and Baja California were preceded in the previous decades by generation of rock damage around the eventual rupture zones. Analysis (ii) reveals localization (reduced fractional area) 2–3 yr before these main shocks and before the M > 7 Düzce 1999 earthquake in Turkey. Results with technique (iii) indicate that individual events tend to coalesce rapidly to clusters in the final 1–2 yr before the main shocks. Corresponding analyses of data from the Parkfield region show opposite delocalization patterns and decreasing clustering before the 2004 M6 earthquake. Continuing studies with these techniques, combined with analysis of geodetic data and insights from laboratory experiments and model simulations, might improve the ability to track preparation processes leading to large earthquakes.
We document space-dependent clustering properties of earthquakes with m ≥ 4 in the 1975–2015 worldwide seismic catalogue of the Northern California Earthquake Data Center. Earthquake clusters are ...identified using a nearest-neighbour distance in time–space–magnitude domain. Multiple cluster characteristics are compared with the heat flow level and type of deformation defined by parameters of the strain rate tensor. The analysis suggests that the dominant type of seismicity clusters in a region depends strongly on the heat flow, while the deformation style and intensity play a secondary role. The results show that there are two dominant types of global clustering: burst-like clusters that represent brittle fracture in relatively cold lithosphere (e.g. shallow events in subduction zones) and swarm-like clusters that represent brittle–ductile deformation in relatively hot lithosphere (e.g. mid-oceanic ridges). The global results are consistent with theoretical expectations and previous analyses of earthquake clustering in southern California based on higher quality catalogues. The observed region-specific deviations from average universal description of seismicity provide important constraints on the physics governing earthquakes and can be used to improve local seismic hazard assessments.
We perform a comparative spatial analysis of inter-seismic earthquake production of rupture area and volume in southern California using observed seismicity and basic scaling relations from ...earthquake phenomenology and fracture mechanics. The analysis employs background events from a declustered catalog in the magnitude range 2≤M<4 to get temporally stable results representing activity during a typical inter-seismic period on all faults. Regions of high relative inter-seismic damage production include the San Jacinto fault, South Central Transverse Ranges especially near major fault junctions (Cajon Pass and San Gorgonio Pass), Eastern CA Shear Zone (ECSZ) and the Imperial Valley – Brawley seismic zone area. These regions are correlated with low velocity zones in detailed tomography studies. A quasi-linear zone with ongoing damage production extends between the Imperial fault and ECSZ and may indicate a possible future location of the main plate boundary in the area. The regions around the 1992 M6.1 Joshua Tree, M7.3 Landers and M6.3 Big Bear earthquakes have background seismic activity before 1990. This may represent a regional weakening process by damage production in future rupture zones. The depth of background seismicity and damage production decreases steadily from SW of the coastline to NE of the San Andreas fault, and also to the SE near the US–Mexico border. The seismicity and rock damage become more pronounced and continuous along-strike of main faults with increasing depth.
•We analyze inter-seismic earthquake production of rock damage in southern California.•Regions with ongoing activity beyond fluctuations associated with large events are highlighted.•The results can help separating rock damage and composition in seismic imaging studies.•Regions around several M>6 earthquakes have background damage production before their occurrence.
We extract significant spatially coherent strain variations from horizontal seasonal Global Positioning System (GPS) displacements in the American Southwest. The dilatational strain is largest in ...northern California with maximum margin‐normal contraction and extension in spring and fall, respectively, consistent with the Earth's surface going down and up at those times. The northern California signal has a phase shift with respect to that in southern California and the Great Basin. For northern and southern California the proportion of larger earthquakes are in‐phase and the aftershock productivity out of phase with the inferred Coulomb stress on the San Andreas fault system. The intensity of mainshocks is in‐phase in the north as well but not in the south. This suggests that a seasonal increase in fault‐normal extension may or may not trigger mainshocks, but when an earthquake happens at those times, they grow larger than they otherwise would, which would cause a larger stress reduction and result in fewer aftershocks.
Plain Language Summary
The changing amount of water and snow mass that lays on top of the Earth's surface is one possible explanation for observed seasonal variations in seismicity. This hydrological loading would change the state of stress inside the crust minutely with the seasons. We image the seasonal stress variation by using the horizontal seasonal displacements of GPS monuments in the southwestern United States. This reveals large‐scale seasonal patterns of the crust contracting and extending in‐phase with the Earth's surface going down and up, respectively, particularly in northern California which experiences a large excess of water and snow in late winter. The seasonal variations in horizontal deformation there correspond to variations in the number of mainshocks, with more earthquakes occurring when the crust is under extension. In southern California, we see no correlation with the number of mainshocks. In both regions, seasonal deformation correlates with the proportion of large earthquakes and shows an anticorrelation with the aftershock production. So even though seasonal deformation may not directly trigger earthquakes, if an earthquake happens during the right season, it seems to be able to grow a little larger, releasing a little more stress than it otherwise would and reducing the need for (more) aftershocks.
Key Points
We convert 1,202 horizontal seasonal GPS displacements into a strain field for California and surroundings
Seasonal variations in dilatational strain vary regionally and, at least in northern California, are related to vertical displacements
Seasonal strain may facilitate mainshock occurrence and causes an increase in earthquake magnitude and decrease in aftershock production
We introduce
generalized dynamical pruning
on rooted binary trees with edge lengths that encompasses a number of discrete and continuous pruning operations, including the tree erasure and Horton ...pruning. The pruning removes parts of a tree
T
, starting from the leaves, according to a pruning function defined on descendant subtrees within
T
. We prove the invariance of critical binary Galton–Watson tree with exponential edge lengths with respect to the generalized dynamical pruning for an arbitrary admissible pruning function. These results facilitate analysis of the continuum 1-D ballistic annihilation model
A
+
A
→
∅
for a constant particle density and initial velocity that alternates between the values of
±
1
. We show that the model’s shock wave is isometric to the level set tree of the potential function, and the model evolution is equivalent to the generalized dynamical pruning of the shock wave tree.
We use recent results on statistical analysis of seismicity to present a robust method for comprehensive detection and analysis of earthquake clusters. The method is based on nearest‐neighbor ...distances of events in space‐time‐energy domain. The method is applied to a 1981–2011 relocated seismicity catalog of southern California having 111,981 events with magnitudes m ≥ 2 and corresponding synthetic catalogs produced by the Epidemic Type Aftershock Sequence (ETAS) model. Analysis of the ETAS model demonstrates that the cluster detection results are accurate and stable with respect to (1) three numerical parameters of the method, (2) variations of the minimal reported magnitude, (3) catalog incompleteness, and (4) location errors. Application of the method to the observed catalog separates the 111,981 examined earthquakes into 41,393 statistically significant clusters comprised of foreshocks, mainshocks, and aftershocks. The results reproduce the essential known statistical properties of earthquake clusters, which provide overall support for the proposed technique. In addition, systematic analysis with our method allows us to detect several new features of seismicity that include (1) existence of a significant population of single‐event clusters, (2) existence of foreshock activity in natural seismicity that exceeds expectation based on the ETAS model, and (3) dependence of all cluster properties, except area, on the magnitude difference of events from mainshocks but not on their absolute values. The classification of detected clusters into several major types, generally corresponding to singles, burst‐like and swarm‐like sequences, and correlations between different cluster types and geographic locations is addressed in a companion paper.
Key Points
Earthquake clusters are identified in southern California
Accuracy and stability of detection is tested using ETAS model
Several new cluster features are reported
We introduce an algorithm for declustering earthquake catalogs based on the nearest‐neighbor analysis of seismicity. The algorithm discriminates between background and clustered events by random ...thinning that removes events according to a space‐varying threshold. The threshold is estimated using randomized‐reshuffled catalogs that are stationary, have independent space and time components, and preserve the space distribution of the original catalog. Analysis of catalog produced by the Epidemic Type Aftershock Sequence model demonstrates that the algorithm correctly classifies over 80% of background and clustered events, correctly reconstructs the stationary and space‐dependent background intensity, and shows high stability with respect to random realizations (over 75% of events have the same estimated type in over 90% of random realizations). The declustering algorithm is applied to the global Northern California Earthquake Data Center catalog with magnitudes m ≥ 4 during 2000–2015; a Southern California catalog with m ≥ 2.5, 3.5 during 1981–2017; an area around the 1992 Landers rupture zone with m ≥ 0.0 during 1981–2015; and the Parkfield segment of San Andreas fault with m ≥ 1.0 during 1984–2014. The null hypotheses of stationarity and space‐time independence are not rejected by several tests applied to the estimated background events of the global and Southern California catalogs with magnitude ranges Δm < 4. However, both hypotheses are rejected for catalogs with larger range of magnitudes Δm > 4. The deviations from the nulls are mainly due to local temporal fluctuations of seismicity and activity switching among subregions; they can be traced back to the original catalogs and represent genuine features of background seismicity.
Key Points
A new declustering method is proposed based on nearest‐neighbor analysis of earthquakes in time‐space‐magnitude domain
Declustering the examined catalogs with magnitude range Δm< 4 leads to a stationary field with independent space‐time components
Declustering data with Δm> 4 reveal nonstationary patterns attributed to the original catalog rather than the method
We introduce a statistical methodology for clustering analysis of seismicity in the time-space-energy domain and use it to establish the existence of two statistically distinct populations of ...earthquakes: clustered and nonclustered. This result can be used, in particular, for nonparametric aftershock identification. The proposed approach expands the analysis of Baiesi and Paczuski Phys. Rev. E 69, 066106 (2004)10.1103/PhysRevE.69.066106 based on the space-time-magnitude nearest-neighbor distance eta between earthquakes. We show that for a homogeneous Poisson marked point field with exponential marks, the distance eta has the Weibull distribution, which bridges our results with classical correlation analysis for point fields. The joint 2D distribution of spatial and temporal components of eta is used to identify the clustered part of a point field. The proposed technique is applied to several seismicity models and to the observed seismicity of southern California.
SUMMARY
We examine the relations between spatial symmetry properties of earthquake patterns along faults in California (CA) and local velocity structure images to test the hypothesis that ruptures on ...bimaterial faults have statistically preferred propagation directions. The analysis employs seismic catalogues for 25 fault zones in CA. We distinguish between clustered and homogeneous parts of each catalogue, using a recently introduced earthquake cluster analysis, and examine asymmetry of offspring with respect to parent events within the clustered portion of each catalogue. The results indicate strong asymmetric patterns along large faults with prominent bimaterial interfaces (e.g. sections of the San Andreas Fault), with enhanced activities in the directions predicted for the local velocity contrasts, and absence of significant asymmetry along most other faults. Assuming the observed asymmetric properties of seismicity reflect the properties of the parent earthquake ruptures, the discussed methodology and results can be used to develop refined estimates of seismic shaking hazard associated with individual fault zones.
This is a second paper in a study of statistical identification and classification of earthquake clusters using a relocated catalog of 1981–2011 seismicity in southern California and synthetic ...catalogs produced by the Epidemic Type Aftershock Sequence model. Here we focus on classification of event families—statistically significant clusters composed of foreshocks, mainshocks, and aftershocks—that are detected with the methodology discussed in part I of the study. The families are analyzed using their representation as time oriented tree graphs. The results (1) demonstrate that the clustering associated with the largest earthquakes, m > 7, is statistically different from that of small‐to‐medium earthquakes; (2) establish the existence of two dominant types of small‐to‐medium magnitude earthquake families—burst‐like and swarm‐like sequences—and a variety of intermediate cluster forms obtained as a mixture of the two dominant types; (3) suggest a simple new quantitative measure for identifying the cluster type based on its topological structure; (4) demonstrate systematic spatial variability of the cluster characteristics on a scale of tens of kilometers in relation to heat flow and other properties governing the effective viscosity of a region; and (5) establish correlation between the family topological structure and a dozen of metric properties traditionally considered in the literature (number of aftershocks, duration, spatial properties, b‐value, parameters of Omori‐Utsu and Båth law, etc.). The burst‐like clusters likely reflect highly brittle failures in relatively cold regions, while the swarm‐like clusters are likely associated with mixed brittle‐ductile failures in regions with relatively high temperature and/or fluid content. The results of this and paper I may be used to develop improved region‐specific hazard estimates and earthquake forecasts.
Key Points
Four main types of earthquake clusters in southern California are detected.
Earthquake cluster type is defined via topological characterization.
Earthquake cluster type is related to the physical properties of the crust.
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