Earthquake Clusters Research Articles

Seafloor fault ruptures along the North Anatolia Fault in the Marmara Sea, Turkey: Link with the adjacent basin turbidite record

Abstract The relation between seafloor fault ruptures and the generation of turbidity currents was investigated to better understand the structural growth of tectonic basins with direct implications for earthquake hazard assessment. This study focuses on the Holocene earthquake record of transtensional basins in the Marmara Sea, Turkey, that are associated with the North Anatolian Fault system. The physical and chemical composition of three 10 m-long cores recovered from the Central Basin was studied at high-resolution and turbidite–homogenite units were identified. Turbidite–homogenite units (T–H units) are complex deposits that consist of a sharp basal contact and multiple fining upward beds of sand to coarse silt, above. All are capped by a 25 cm to 75 cm thick layer of medium to fine silt. A chronology developed from radiocarbon and short-lived radioisotopes allowed the correlation of these T–H units to the historical record of earthquakes that in Turkey goes back 2000 years. We found that the best location to recover the most complete sedimentation record is in the deepest part of a basin or “depocenter” where T–H units constitute ~ 80% of the sediments. A very good correlation was established between T–H units in Central Basin and proximal inferred historic epicentres along the central Marmara segment of the North Anatolia Fault that occurred in 1343, 860, 740, and 557 AD, and two more distal earthquakes that occurred in 268 and 1963 (or possibly1964). These sedimentation events can then be referred to as “seismo-turbidites”. The results when compared to findings from other transform basins in Marmara Sea reveal a very good correlation between T–H units and historic ruptures. Most importantly, there is a strong correlation between the inferred locations of historical earthquakes and the preservation of turbidite–homogenite units in the basin adjacent to the inferred rupture. The 740 AD earthquake correlates with T–H units in Izmit Gulf and Central Basin and could represent a multi-segment rupture of the NAF. Generally, T–H units appear to be clustered through the Holocene sections, suggesting temporal earthquake clustering in the Marmara Sea region. Such clustering may account for the lack of T–H units and hence large ruptures through the Central Basin since 1343.

The global aftershock zone

Abstract The aftershock zone of each large (M ≥ 7) earthquake extends throughout the shallows of planet Earth. Most aftershocks cluster near the mainshock rupture, but earthquakes send out shivers in the form of seismic waves, and these temporary distortions are large enough to trigger other earthquakes at global range. The aftershocks that happen at great distance from their mainshock are often superposed onto already seismically active regions, making them difficult to detect and understand. From a hazard perspective we are concerned that this dynamic process might encourage other high magnitude earthquakes, and wonder if a global alarm state is warranted after every large mainshock. From an earthquake process perspective we are curious about the physics of earthquake triggering across the magnitude spectrum. In this review we build upon past studies that examined the combined global response to mainshocks. Such compilations demonstrate significant rate increases during, and immediately after (~ 45 min) M > 7.0 mainshocks in all tectonic settings and ranges. However, it is difficult to find strong evidence for M > 5 rate increases during the passage of surface waves in combined global catalogs. On the other hand, recently published studies of individual large mainshocks associate M > 5 triggering at global range that is delayed by hours to days after surface wave arrivals. The longer the delay between mainshock and global aftershock, the more difficult it is to establish causation. To address these questions, we review the response to 260 M ≥ 7.0 shallow (Z ≤ 50 km) mainshocks in 21 global regions with local seismograph networks. In this way we can examine the detailed temporal and spatial response, or lack thereof, during passing seismic waves, and over the 24 h period after their passing. We see an array of responses that can involve immediate and widespread seismicity outbreaks, delayed and localized earthquake clusters, to no response at all. About 50% of the catalogs that we studied showed possible (localized delayed) remote triggering, and ~ 20% showed probable (instantaneous broadly distributed) remote triggering. However, in any given region, at most only about 2–3% of global mainshocks caused significant local earthquake rate increases. These rate increases are mostly composed of small magnitude events, and we do not find significant evidence of dynamically triggered M > 5 earthquakes. If we assume that the few observed M > 5 events are triggered, we find that they are not directly associated with surface wave passage, with first incidences being 9–10 h later. We note that mainshock magnitude, relative proximity, amplitude spectra, peak ground motion, and mainshock focal mechanisms are not reliable determining factors as to whether a mainshock will cause remote triggering. By elimination, azimuth, and polarization of surface waves with respect to receiver faults may be more important factors.

Crustal earthquake triggering by pre-historic great earthquakes on subduction zone thrusts

Triggering of earthquakes on upper plate faults during and shortly after recent great (M > 8.0) subduction thrust earthquakes raises concerns about earthquake triggering following Cascadia subduction zone earthquakes. Of particular regard to Cascadia was the previously noted, but only qualitatively identified, clustering of M > ~6.5 crustal earthquakes in the Puget Sound region between about 1200–900 cal years B.P. and the possibility that this was triggered by a great Cascadia thrust subduction thrust earthquake, and therefore portends future such clusters. We confirm quantitatively the extraordinary nature of the Puget Sound region crustal earthquake clustering between 1200–900 cal years B.P., at least over the last 16,000. We conclude that this cluster was not triggered by the penultimate, and possibly full-margin, great Cascadia subduction thrust earthquake. However, we also show that the paleoseismic record for Cascadia is consistent with conclusions of our companion study of the global modern record outside Cascadia, that M > 8.6 subduction thrust events have a high probability of triggering at least one or more M > ~6.5 crustal earthquakes.

Seismic structure of subducted oceanic crust near the slow-earthquake source region in the southern Ryukyu arc

Seismic tomography and receiver function analysis were carried out to investigate the relation between the slab structure in the southern Ryukyu arc region and the occurrence of slow-slip events that repeat biannually. For calculation of the receiver function, 212 teleseismic earthquakes with magnitudes larger than 6.0 were selected, and teleseismic waveforms were observed using two short-period seismometers and one broadband seismometer. Assuming that each later phase in a receiver function was a wave converted from P to S at depth, the time-domain receiver function was transformed into depth domain along each ray path using a reference velocity model. P- and S-wave arrival times, selected manually by the Japan Meteorological Agency (JMA), were used for the analysis of seismic tomography. In all, 6,750 earthquakes from January 2002 to March 2014 were used. The results showed that a cluster of slab earthquakes is distributed about 10 km below the plate interface. This suggests that the slab earthquakes occur in the lower part of the oceanic crust near the oceanic Moho within the slab. Moreover, the fault of the slow-slip events corresponds with the plate interface. The results also showed that a low Vp and high Vp/Vs area is distributed within the subducted oceanic crust underlying the fault associated with the slow-slip events. These structures are similar to those in the Tokai district, which suggests that the thermal condition of the southern Ryukyu arc is similar to the case of a hot-slab area where slow-slip events occur.

Paleoseismic records of large earthquakes on the cross-basin fault in the Ganyanchi pull-apart basin, Haiyuan fault, northeastern Tibetan Plateau

Field observations and analog models show that cross-basin faults play a key role in the evolution of pull-apart basins and dominate the distribution of earthquake rupture in basin areas. We studied the long-term history of large earthquakes on a cross-basin fault to reveal its behavior in response to propagating earthquake rupture and gain insight into the evolution of the pull-apart basin. A number of pull-apart basins have developed along the Haiyuan fault in the northeastern Tibetan Plateau, the largest being the Ganyanchi pull-apart basin. The surface rupture associated with the 1920 M 8.5 earthquake shows that a cross-basin fault developed in the basin and that the basin is now going through the late stage of its evolution. We excavated two trenches and drilled four cores across the cross-basin fault in the basin and found abundant evidence of paleoseismic events. Seven events were identified and 14C-dated. The two youngest events are associated with the historical records of 1092 AD and 1920 AD, respectively. The paleoseismic sequence shows the recurrence of earthquakes characterized by earthquake clusters alternating with a single event. Comparing these with previous paleoseismic results, all the major earthquake events seem to be associated with cascade events that ruptured multi-fault segments, suggesting that only an earthquake of this scale (likely M > 8) can produce obvious surface rupture along the cross-basin fault. We propose that the fault has a long tectonic history, with a series of cascade rupture events that could play an important part in the evolution of the pull-apart basin.

Stability of earthquake clustering models: Criticality and branching ratios

We study the stability conditions of a class of branching processes prominent in the analysis and modeling of seismicity. This class includes the epidemic-type aftershock sequence (ETAS) model as a special case, but more generally comprises models in which the magnitude distribution of direct offspring depends on the magnitude of the progenitor, such as the branching aftershock sequence (BASS) model and another recently proposed branching model based on a dynamic scaling hypothesis. These stability conditions are closely related to the concepts of the criticality parameter and the branching ratio. The criticality parameter summarizes the asymptotic behavior of the population after sufficiently many generations, determined by the maximum eigenvalue of the transition equations. The branching ratio is defined by the proportion of triggered events in all the events. Based on the results for the generalized case, we show that the branching ratio of the ETAS model is identical to its criticality parameter because its magnitude density is separable from the full intensity. More generally, however, these two values differ and thus place separate conditions on model stability. As an illustration of the difference and of the importance of the stability conditions, we employ a version of the BASS model, reformulated to ensure the possibility of stationarity. In addition, we analyze the magnitude distributions of successive generations of the BASS model via analytical and numerical methods, and find that the compound density differs substantially from a Gutenberg-Richter distribution, unless the process is essentially subcritical (branching ratio less than 1) or the magnitude dependence between the parent event and the direct offspring is weak.

Holocene coastal notches in the Mediterranean region: Indicators of palaeoseismic clustering?

Marine tidal notches are developed by bioerosion in the intertidal zones of rocky coasts, but a combination of sea-level change and crustal movements can result in them being raised above or submerged below the water line. For that reason, the present-day elevation of these former shorelines relative to the mean sea level has long been used to quantify relative coastal uplift and subsidence in tectonically active areas, assuming that the sea-level (eustatic) change component is known. Along the microtidal Mediterranean littoral, it is generally assumed that notches develop during relative stillstands of sea level, when tectonic and eustatic trends are in unison, and that discrete notch levels record abrupt shoreline changes caused by local seismic displacements. Recently, however, a climatic model for notch formation has been proposed, in which stable periods of Holocene climate favour enhanced erosion; in this competing model, the rate of sea-level rise is lower than the tectonic uplift rate and individual notches are not specific seismic indicators. Because marine notches are widely used as geomorphic markers of tectonic, and in some cases palaeoseismic, movements, a reappraisal of the geological significance of these strandlines is warranted. In this paper, we explore the two conflicting notch models using a database of Eastern Mediterranean palaeoshorelines. Although we conclude that the spatial and temporal distribution of the notches supports a dominantly tectonic control on notch genesis as a result of earthquake clustering, we highlight how the diachronous timing of notch development tempers their value as tectonic markers.

The Beni-Ilmane (North-Central Algeria) Earthquake Sequence of May 2010

Starting on 14 May 2010 and lasting several months, the village of Beni-Ilmane (Msila District, North-Central Algeria) and its surroundings were struck by an important seismic crisis marked by three successive moderate shocks (5.0 ≤ M d ≤ 5.2). This sequence of events caused severe damage in the Beni-Ilmane village and in the epicentral area. The poor quality of masonry construction and the cumulative effects of the large number of aftershock events played a key role in the destruction. To follow this earthquake sequence, 11 temporary seismic stations, in addition to the permanent stations of the Algerian seismic network, were deployed in the region. A representative set of well located aftershocks in the period of maximum activity (lasting 18 days) were selected. The horizontal distribution of the aftershocks shows two main earthquake clusters located near Beni-Ilmane village, one cluster oriented E–W and the other oriented NNE–SSW, crossing the first cluster at its eastern tip. The aftershocks distribution suggests that the three main shocks ruptured two distinct and adjacent fault segments of about 8 km length. The focal mechanisms of the first and third events, located in the NNE–SSW cluster, show near-vertical left-lateral strike-slip fault planes. In the second cluster, oriented E–W, focal mechanisms show a high-angle reverse fault. A field survey, initiated immediately after the first main shock, identified surface fissures generated by the three largest events in the sequence. The fissures, concentrated in a narrow area at the western termination of the NE–SW Jebel Choukchot anticline (location of Beni-Ilmane village), showed several orientations which were mainly related to gravity instabilities. The 2010 Beni-Ilmane earthquake sequence, located in the Bibans–Hodna Mountains transition zone, demonstrates that the Tellian Atlas–High Plateaus border region is an active seismic zone marked by moderate and possibly strong earthquakes; thus, a reevaluation of the seismic hazard in the region is needed.

Relocating a Cluster of Earthquakes Using a Single Seismic Station

Coda waves arise from scattering to form the later arriving components of a seismogram. Coda‐wave interferometry (CWI) is an emerging tool for constraining earthquake source properties from the interference pattern of coda waves between nearby events. A new earthquake location algorithm is derived which relies on coda‐wave‐based probabilistic estimates of earthquake separation. The algorithm can be used with coda waves alone or in tandem with arrival‐time data. Synthetic examples (2D and 3D) and real earthquakes on the Calaveras fault, California, are used to demonstrate the potential of coda waves for locating poorly recorded earthquakes. It is demonstrated that CWI: (a) outperforms traditional earthquake location techniques when the number of stations is small; (b) is self‐consistent across a broad range of station situations; and (c) can be used with a single station to locate earthquakes.

Natural and induced seismic activity in Kuzbass

Based on the plotted times of occurrence of seismic events, their timing within a work week and location of epicenters, it is illustrated that since the 1960s the seismicity in the Kuzbass territory has been a complicated natural and mining-induced phenomenon. Since the late 1980s, the natural and mining-induced seismicity has entered the second stage of development, with clustering of low-energy seismic events and strong shallow earthquakes in the areas under heavy mining and, in the first instance, at deep open pit mines. The representative event of this kind is the magnitude 5.2 earthquake at the Bachatsky Coal Open Pit Mine on June 19, 2013. It has been found that the natural and induced seismic activity is associated with deep faults that weakly show themselves in the upper layer of the earth crust but appear in the local relief, which is an evidence of their live emergence onto the daylight surface due to the mining impact. The article proposes the comprehensive research program for the natural and mining-induced seismic activity in the Kuzbass area and the development of early identification of seismically active zones.

Statistical Analyses of Great Earthquake Recurrence along the Cascadia Subduction Zone

Goldfinger et al. (2012) interpreted a 10,000 year old sequence of deep sea turbidites at the Cascadia subduction zone (CSZ) as a record of clusters of plate-boundary great earthquakes separated by gaps of many hundreds of years. We performed statistical analyses on this inferred earthquake record to test the temporal clustering model and to calculate time-dependent recurrence intervals and probabil- ities. We used a Monte Carlo simulation to determine if the turbidite recurrence in- tervals follow an exponential distribution consistent with a Poisson (memoryless) process. The latter was rejected at a statistical significance level of 0.05. We performed a cluster analysis on 20 randomly simulated catalogs of 18 events (event T2 excluded), using ages with uncertainties from the turbidite dataset. Results indicate that 13 cata- logs exhibit statistically significant clustering behavior, yielding a probability of clus- tering of 13=20 or 0.65. Most (70%) of the 20 catalogs contain two or three closed clusters (a sequence that contains the same or nearly the same number of events) and the current cluster T1-T5 appears consistently in all catalogs. Analysis of the 13 cat- alogs that manifest clustering indicates that the probability that at least one more event will occur in the current cluster is 0.82. Given that the current cluster may not be closed yet, the probabilities of an M 9 earthquake during the next 50 and 100 years were estimated to be 0.17 and 0.25, respectively. We also analyzed the sensitivity of results to including event T2, whose status as a full-length rupture event is in doubt. The inclusion of T2 did not change the probability of clustering behavior in the CSZ turbidite data, but did significantly reduce the probability that the current cluster would extend to one more event. Based on the statistical analysis, time-independent and time-dependent recurrence intervals were calculated.

Paleoseismology of the southern Panamint Valley fault: Implications for regional earthquake occurrence and seismic hazard in southern California

AbstractPaleoseismologic data from the southern Panamint Valley fault (PVF) reveal evidence of at least four surface ruptures during late Holocene time (0.33–0.48 ka, 0.9–3.0 ka, 3.3–3.6 ka, and >4.1 ka). These paleo‐earthquake ages indicate that the southern PVF has ruptured at least once and possibly twice during the ongoing (≤1.5 ka) seismic cluster in the Mojave section of the eastern California shear zone (ECSZ). The most recent event (MRE) on the PVF is also similar in age to the 1872 Owens Valley earthquake and the geomorphically youthful MRE on the Death Valley fault. The timing of the three oldest events at our site shows that the PVF ruptured at least once and possibly thrice during the well‐defined 2–5 ka seismic lull in the Mojave section of the ECSZ. Interestingly, the 3.3–3.6 ka age of Event 3 overlaps with the 3.3–3.8 ka age of the penultimate (i.e., pre‐1872) rupture on the central Owens Valley fault. These new PVF data support the notion that earthquake occurrence in the ECSZ may be spatially and temporally complex, with earthquake clusters occurring in different regions at different times. Coulomb failure function modeling of the Panamint Valley and Garlock faults reveals significant stress interactions between these two faults that may influence future earthquake occurrence. Specifically, our models suggest a possible rupture sequence whereby an event on the southern Panamint Valley fault can lead to the potential triggering of an event on the Garlock fault, which in turn could trigger the Mojave section of the San Andreas Fault.

The role of aeromagnetic data analysis (using 3D Euler deconvolution) in delineating active subsurface structures in the west central Arabian shield and the central Red Sea, Saudi Arabia

In this paper, we present a case study of structural mapping by applying the 3D Euler method to the high-resolution aeromagnetic data that was collected in the west central Arabian Shield region and the coastal region of the central Red Sea in Saudi Arabia. We show the 3D Euler deconvolution algorithm and apply it to magnetic potential field data from the west Central Arabian Shield and the Central Red Sea. The solution obtained with 3D Euler deconvolution gives better-focused depth estimates, which are closer to the real position of sources; the results presented here can be used to constrain depth to active crustal structures (volcanisms) for the study area. The results indicated that the area was affected by sets of fault systems, which primarily trended in the NNW–SSE, NW–SE, EW, and NE–SW directions. Moreover, estimated Euler solution map from aeromagnetic data delineated also the boundaries of shallow, small, and confined magnetic bodies on the offshore section of the study area. These nearly exposed basement intrusions are most likely related to the Red Sea Rift and may be associated with structures higher up in the sedimentary section. These volcanic bodies are extended to the continental part (onshore) of the west central Arabian Shield, particularly beneath both sides of the Ad Damm fault zone. This extension verifies that the fault was largely contemporaneous with a major period during the extension of the Red Sea Basin. Moreover, according to the distribution of circular magmatic-source bodies (circular-shaped ring dikes) that resulted from this study, we can state that the clustering of most earthquakes along this fault may most likely be attributed to the active mantle upwelling (volcanic earthquakes), which are ultimately related to volcanic processes. Furthermore, the oceanic crustal structures near and in the Red Sea offshore regions were also estimated and discussed according to the ophiolite occurrences and further opening of the Red Sea. Our results are largely comparable with studies of previous crustal sections, which were performed along the Red Sea Rift and the Arabian Shield. As a result, the areas above these anomalies are highly recommended for further geothermal study. This example illustrates that high-resolution aeromagnetic surveys can greatly help delineating the subsurface active structures in the west central Arabian Shield and the middle coastal region of the Red Sea of Saudi Arabia.

Earthquake clustering in the tectonic pattern and volcanism of the Andaman Sea region

Seismicity pattern in the Andaman Sea region has been analysed to interpret recent dynamics of regional submarine volcanic provinces. Hypocentral determinations and focal mechanisms of available global seismological data have been used. Frequent occurrence of earthquake swarms proved to be an important characteristic of seismicity pattern in the investigated region. Epicenters of the earthquake swarms are situated almost exclusively along a narrow belt forming the easternmost limit of the epicentral area of the Andaman Sea region. The southern zone of the belt between 6° and 10.5° N is trench parallel and coincides with the northward prolongation of the Sumatra volcanic arc. The northern zone of the belt between 10.5° and 13° N is deflected by about 45° northeast and precisely follows the complicated zigzag pattern of the rift system in the middle of the Andaman Basin. Earthquake occurrence in these two zones, the northern and the southern, differs by several aspects — by shape of the epicentral zones of individual swarms, by focal mechanisms and by response to the 2004 Sumatra–Andaman great earthquake. These differences, together with available information on composition of magmas found at the seafloor, lead us to the conclusion that the swarms of the southern zone are induced by intrusions of subduction-generated calc-alkaline magmas whereas the swarms in the northern zone by intrusions of basaltic magmas associated with the seafloor spreading. Earthquake swarm occurrence defines a brittle, seismogenic layer at depths between 9 and 35 km, excludes the existence of large magma reservoirs in respective depth interval and puts their hypothetical location to a greater depth. Episodes of magma ascent from deeper reservoir to shallow magma chamber (depth < 9 km) or up to the surface induce earthquake swarms. The region deserves attention of volcano seismologists through its almost continuous activity and represents an attractive area for on-site geophysical monitoring and direct seafloor observations.

Evidence of Nonextensive Statistical Physics behavior of the Hellenic Subduction Zone seismicity

Abstract The Hellenic Subduction Zone (HSZ) is the most seismically active region in Europe. Many destructive earthquakes have taken place along the HSZ in the past. In this study we investigate the seismicity of the HSZ based on the science of complex systems. The spatiotemporal distributions of seismicity as well as the magnitude distribution are studied using the concept of Nonextensive Statistical Physics (NESP). Defining five seismic zones and forming an earthquake dataset that covers the period 1976–2009, we apply the NESP ideas to formulate the cumulative distribution functions of the inter-event times and distances and the magnitude distribution along the HSZ. Our results indicate that the nonextensive parameter qT, which is related with the inter-event time distribution, presents almost similar values in each of the seismic zones and reflects the long term scale of the seismicity evolution in the HSZ. The qD parameter, which is related with the inter-event distance distribution, presents a significant variation along the seismic zones. This variation is related with the different degree of spatial earthquake clustering in each of the seismic zones of the HSZ. Moreover, in the framework of the fragment–asperity model, the thermostatistical parameter qM, which is related with the frequency–magnitude distribution, could be used as an additional index to inform us about the physical state of each seismic zone along the HSZ. The variations of the qM parameter are related with the energy release rate in each seismic zone. The models used, fit rather well to the observed distributions, implying the usefulness of NESP in investigating such phenomena exhibiting scale-free nature and long range memory effects.

Triggering of microearthquakes in Iceland by volatiles released from a dyke intrusion

We suggest that carbon dioxide exsolved from a mid-crustal basaltic dyke intrusion in Iceland migrated upwards and triggered shallow seismicity by allowing failure on pre-existing fractures under the relatively low elastic stresses (100–200 kPa; 1–2 bar) generated by the dyke inflation. Intense swarms of microseismicity accompanied magmatic intrusion into a dyke at depths of 13–19 km in the crust of Iceland's Northern Volcanic Rift Zone during 2007–2008. Contemporaneously, a series of small normal earthquakes, probably triggered by elastic stresses imposed by the dyke intrusion, occurred in the uppermost 4 km of crust: fault plane solutions from these are consistent with failure along the extensional fabric and surface fissure directions mapped in the area, suggesting that the faults failed along existing rift zone fabric even though the mid-crustal dyke is highly oblique to it. Several months after the melt froze in the mid-crust and seismicity associated with the intrusion had ceased, an upsurge in shallow microseismicity began in the updip projection of the dyke near the brittle–ductile transition at 6–7 km depth below sea level. This seismicity is caused by failure on right-lateral strike-slip faults, with fault planes orientated 23 ± 3°, which are identical with the 24 ± 2° orientation in this area of surface fractures and fissures caused by plate spreading and extension of the volcanic rift zone. However, these earthquakes have T-axes approximately aligned with the opening direction of the dyke, and the right-lateral sense of failure is opposite that of regional strike-slip faults. We suggest that the fractures occurred along pre-existing weaknesses generated by the pervasive fabric of the rift zone, but that the dyke opening in the mid-crust beneath it caused right-lateral failure. The seismicity commenced after a temporal delay of several months and has persisted for over 3 yr. We propose that fluids exsolved from the magma in the dyke, primarily carbon dioxide, percolated updip and to shallower depths predominantly along pre-existing fractures. Increased pore pressure from the volatiles reduced the effective normal compressive stress on faults, increasing the likelihood of failure and allowing the modest stress changes generated by the intrusion to cause failure. Propagation of volatiles through the crust would also account for the observed time delay between the intrusion at depth and the shallow earthquake clusters. A further short-lived cluster of earthquakes at 2–4 km depth beneath the surface exhibits left-lateral strike-slip faulting with epicentres well orientated along a lineation which is identical with other subparallel strike-slip faults in the area that transfer motion between two adjacent spreading segments. These shallow earthquakes lie beyond lobes of significant positive Coulomb stress change caused by the intrusion, implying minimal modifications to the stress field in their vicinity; hence, they continue to respond to the regional stress field rather than the local stress field generated by the dyke intrusion.

Earthquake clusters in southern California I: Identification and stability

AbstractWe 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.

Earthquake clusters in southern California II: Classification and relation to physical properties of the crust

AbstractThis 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 &gt; 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.

Spatial relationship between earthquakes and volcanic vents in the central-northern Main Ethiopian Rift

During the breakup of continents extension is commonly accommodated by connected networks of fluid filled fractures (dykes) and by faults. Despite the importance of these two extension mechanisms their spatial relationship in three dimensions is poorly understood primarily because it is difficult to quantify the subsurface distribution of faulting and intrusion. In order to address this problem, we conduct a quantitative analysis of the spatial distribution and clustering of earthquakes and volcanic vents in the Main Ethiopian Rift in East Africa in order to understand how extension by faulting and intrusion is distributed throughout the volcanic rift. We use fractal analysis of earthquake epicentres in order to infer the 2D characteristics of the subsurface fault network, and directly test our model results against the 3D distribution of earthquake hypocentres. Our results show that fractal analysis of these features is a reliable means to characterise the 3D properties of the fault network. In addition, the strong similarity between the properties of the fault network derived from earthquakes and properties of the magma-filled fracture network derived from fractal analysis of volcanic vents strongly suggests that these are genetically linked. We then explore their spatial link using computation of earthquake and vent density, which shows that the zone of seismicity is generally around 20–30-km-wide, while the zone of vents is narrower and centred within the zone of seismicity. This spatial relationship suggests that the faults, which form rift axial grabens, are induced above a narrower and central zone of diking. We also demonstrate significant along-rift variation in degree of magmatism and faulting with regions of increased degree of diking inferred from a higher cone density characterised by reduced degree of faulting.

Comparison of seismicity declustering methods using a probabilistic measure of clustering

We present a new measure of earthquake clustering and explore its use for comparing the performance of three different declustering methods. The advantage of this new clustering measure over existing techniques is that it can be used for non-Poissonian background seismicity and, in particular, to compare the results of declustering algorithms where different background models are used. We use our approach to study inter-event times between successive earthquakes using earthquake catalog data from Japan and southern California. A measure of the extent of clustering is introduced by comparing the inter-event time distributions of the background seismicity to that of the whole observed seismicity. Theoretical aspects of the clustering measure are then discussed with respect to the Poissonian and Weibull models for the background inter-event time distribution. In the case of a Poissonian background, the obtained clustering measure shows a decrease followed by an increase, defining a V-shaped trend, which can be explained by the presence of short- and long-range correlation in the inter-event time series. Three previously proposed declustering methods (i.e., the methods of Gardner and Knopoff, Reasenberg, and Zhuang et al.) are used to obtain an approximation of the residual “background” inter-event time distribution in order to apply our clustering measure to real seismicity. The clustering measure is then estimated for different values of magnitude cutoffs and time periods, taking into account the completeness of each catalog. Plots of the clustering measure are presented as clustering attenuation curves (CACs), showing how the correlation decreases when inter-event times increase. The CACs demonstrate strong clustering at short inter-event time ranges and weak clustering at long time ranges. When the algorithm of Gardner and Knopoff is used, the CACs show strong correlation with a weak background at the short inter-event time ranges. The fit of the CACs using the Poissonian background model is successful at short and intermediate inter-event time ranges, but deviates at long ranges. The observed deviation shows that the residual catalog obtained after declustering remains non-Poissonian at long time ranges. The apparent background fraction can be estimated directly from the CAC fit. The CACs using the algorithms of Reasenberg and Zhuang et al. show a relatively similar behavior, with a time correlation decreasing more rapidly than the CACs of Gardner and Knopoff for shorter time ranges. This study offers a novel approach for the study of different types of clustering produced as a result of various hypotheses used to account for different backgrounds.