Continental Earthquakes Research Articles

Short- and long-term earthquake triggering along the strike-slip Kunlun fault, China: Insights gained from the Ms 8.1 Kunlun earthquake and other modern large earthquakes

Abstract Following the 2001 Ms8.1 Kunlun earthquake, earthquake records of more than 10 years, in addition to more than one century's records of large earthquakes, provide us with a chance to examine short-term (days to a few years) and long-term (years to decades) seismic triggering following a magnitude ~ 8 continental earthquake along a very long strike-slip fault, the Kunlun fault system, located in northern Tibet, China. Based on the calculations of coseismic Coulomb stress changes (ΔCFS) from the larger earthquake and post-seismic stress changes due to viscoelastic stress relaxation in the lower crust and upper mantle, we examined the temporal evolution of seismic triggering. The ETAS (epidemic type aftershocks sequence) model shows that the seismic rate in the aftershock area over ~ 10 years was higher than the background seismicity before the mainshock. Moreover, we discuss long-term (years to decades) triggering and the evolution of stress changes for the sequence of five large earthquakes of M ≥ 7.0 that ruptured the Kunlun fault system since 1937. All subsequent events of M ≥ 7.0 occurred in the regions of positive accumulated ΔCFS. These results show that short-term (up to 200 days in our case) triggering along the strike-slip Kunlun fault is governed by coseismic stress changes, while long-term triggering is somewhat due to post-seismic Coulomb stress changes resulting from viscoelastic relaxation.

A method for the joint inversion of geodetic and seismic waveform data using ABIC: application to the 1997 Manyi, Tibet, earthquake

Geodetic imaging data and seismicwaveformdata have complementary strengthswhen considering the modelling of earthquakes. The former, particularly modern space geodetic techniques such as Interferometric Synthetic Aperture Radar (InSAR), permit high spatial density of observation and thus fine resolution of the spatial pattern of fault slip; the latter provide precise and accurate timing information, and thus the ability to resolve how that fault slip varies over time. In order to harness these complementary strengths, we propose a method through which the two data types can be combined in a joint inverse model for the evolution of slip on a specified fault geometry. We present here a derivation of Akaike's Bayesian Information Criterion (ABIC) for the joint inversion of multiple data sets that explicitly deals with the problem of objectively estimating the relative weighting between data sets, as well as the optimal influence of model smoothness constraints in space and time. We demonstrate our ABIC inversion scheme by inverting InSAR displacements and teleseismic waveform data for the 1997 Manyi, Tibet, earthquake.We test, using a simplified fault geometry, three cases-InSAR data inverted alone, vertical component teleseismic broad-band waveform data inverted alone and a joint inversion of both data sets. The InSAR-only model and seismic-only model differ significantly in the distribution of slip on the fault plane that they predict. The joint-inversion model, however, has not only a similar distribution of slip and fit to the InSAR data in the InSAR-only model, suggesting that those data provide the stronger control on the pattern of slip, but is also able to fit the seismic data at a minimal degradation of fit when compared with the seismic-only model. The rupture history of the preferred, joint-inversion model, indicates bilateral rupture for the first 20 s of the earthquake, followed by a further 25 s of westward unilateral rupture afterwards, with slip peaking at 7m in the upper 6 km of the fault. This joint-inversion approach is thus shown to be a viable method for the study of large shallow continental earthquakes, and may be of particular benefit in cases where near-field seismic observations are not available. © The Authors 2014. Published by Oxford University Press on behalf of The Royal Astronomical Society.

Journey to Antarctica: Modeling crustal structure with an earthquake and a genetic algorithm

In a previous work, we have used the genetic algorithm NSGA-II to generate a set of solutions to model the receiver functions and dispersion curves of several seismometer stations located in southern Africa. Now in continuation of applying the NSGA-II to seismic problems, we have used it to model the average velocity profiles along two-dimensional paths from a single seismic event to several stations across West Antarctica. The event was a rare continental earthquake of magnitude 5.6 that took place in West Antarctica near the Ross Ice Shelf during the austral winter of 2012. Data were collected from stations in the Global Seismic Network as well as a local network during the 2012–2013 field season. The seismograms were first modeled using a full body wave modeling code that generates synthetics based on a structure composed of layers with user-defined velocities, thicknesses, and densities. Those models then served as the starting models in NSGA-II, which created a set of solutions from which an average structure with error bounds was calculated for each station.

Source Characteristics of the Egyptian Continental Margin Earthquake, 19 October 2012

On 19 October 2012, at 03:35:11.2, an earthquake of m b 5.1 (European Mediterranean Seismological Centre [EMSC]) occurred in the passive continental margin of Egypt within the Nile Cone (Fig. 1a, Table 1). This event is considered the largest offshore earthquake in the last 57 years. Because much high‐quality data are recorded on modern digital instruments, we can make a more detailed source analysis. Figure 1a shows the locations of the seismograph stations in Egypt and the surrounding countries that recorded the earthquake. The same zone was attacked by a significant earthquake with M s 6.7 on 12 September 1955. The 1955 event caused notable damage over the Nile Delta and Alexandria. The focal mechanism solution of Costantinescu et al. (1966) for this event (Fig. 1b, Table 2) indicates a strike‐slip faulting mechanism with a considerable reverse component along a northeast or east‐southeast‐striking plane. The east‐southeast‐striking plane yields a right‐lateral motion whereas the northeast reflects left‐lateral offset. Figure 1. (a) View map of tectonic boundaries of the Eastern Mediterranean region (cited from Hussein et al. , 2013) and the seismic stations that recorded the 19 October 2012 earthquake. Acronyms: ERA, Eratosthenes Seamount; FL, Florence; IB, Ionian basin; MR, Mediterranean Ridge; LEV, Levantine basin; BAR, Bardawil; HA, Hellenic Arc; AS, Aegean Sea. Open triangles, Egyptian National Seismological Network (ENSN) stations; shaded squares, Greek stations; circles, refer …

Earthquake cycle deformation and the Moho: Implications for the rheology of continental lithosphere

article i nfo The last 20 years has seen a dramatic improvement in the quantity and quality of geodetic measurements of the earthquakeloadingcycle.In thispaper we compile andreviewtheseobservations andtest whether crustal thick- ness exerts any control. We found 78 earthquake source mechanisms for continental earthquakes derived from satellite geodesy, 187 estimates of interseismic locking depth, and 23 earthquakes (or sequences) for which postseismic deformation has been observed. Globally we estimate seismogenic thickness to be 14 ± 5 and 14 ± 7 km from coseismic and interseismic observations respectively. We find that there is no global relation- ship between Moho depth and the seismogenic layer thickness determined geodetically. We also found no clear global relationship between seismogenic thickness and proxies for the temperature structure of the crust. Thissuggests that the effectof temperature,so clearin oceaniclithosphere,ismaskedinthe continents by consid- erable variation in lithology, strain-rate, and/or grain size. Elastic thicknesses from Bouguer gravity are systemat- ically larger than the geodetic seismogenic thicknesses but there is no correlation between them. By contrast, elastic thicknesses from free-air methods are typically smaller than the geodetic estimates of seismogenic layer thickness. Postseismic observations show considerable regional variations, but most long-term studies of large earthquakes infer viscoelastic relaxation in the lower crust and/or upper mantle with relaxation times of a few monthstoafewhundredyears.These areinapparentcontradiction withthe higherestimatesof elasticthickness. Our analysis of the geodetic data therefore supports the creme brulee model, in which the strength of the con- tinental lithosphere ispredominantlyinthe upperseismogenic layer. However, the distribution of geodeticobser- vations is biased towards weaker areas, and faults can also modify the local rheology. Postseismic results could therefore be sampling weak regions within an otherwise strong crust or mantle.

Stress rise precursor to earthquakes in the Tibetan Plateau

Earthquake prediction thus far has proven to be a very difficult task, but changes in situ stress appear to offer a viable approach for forecasting large earthquakes in Tibet and perhaps other continental regions. High stress anomalies formed along active faults before large earthquakes and disappeared soon after the earthquakes occurred in the Tibetan Plateau. Principle stress increased up to ~2 - 5 times higher than background stress to form high stress anomalies along causative faults before the Ms 8.1 West Kunlun Pass earthquake in November 2001, Ms 8.0 Wenchuan earthquake in May 2008, Ms 6.6 Nimu earthquake in October 2009, Ms 7.1 Yushu earthquake in April 2010 and the Ms 7.0 Lushan earthquake in April 2013. Stress near the epicenters rapidly increased 0.10 - 0.12 MPa over 45 days, ~8 months before the Ms 6.6 Nimu earthquake occurred. The high principle stress anomalies decreased quickly to the normal stress state in ~8 - 12 months after the Ms 8.1 West Kunlun Pass and the Ms 8.0 Wenchuan earthquakes. These high stress anomalies and their demise appear directly related to the immediate stress rise along a fault prior to the earthquakes and the release during the event. Thus, the stress rise appears to be a viable precursor in prediction of large continental earthquakes as in the Tibetan Plateau.

Perspectives of Cross-Correlation in Seismic Monitoring at the International Data Centre

We demonstrate that several techniques based on waveform cross-correlation are able to significantly reduce the detection threshold of seismic sources worldwide and to improve the reliability of arrivals by a more accurate estimation of their defining parameters. A master event and the events it can find using waveform cross-correlation at array stations of the International Monitoring System (IMS) have to be close. For the purposes of the International Data Centre (IDC), one can use the spatial closeness of the master and slave events in order to construct a new automatic processing pipeline: all qualified arrivals detected using cross-correlation are associated with events matching the current IDC event definition criteria (EDC) in a local association procedure. Considering the repeating character of global seismicity, more than 90 % of events in the reviewed event bulletin (REB) can be built in this automatic processing. Due to the reduced detection threshold, waveform cross-correlation may increase the number of valid REB events by a factor of 1.5–2.0. Therefore, the new pipeline may produce a more comprehensive bulletin than the current pipeline—the goal of seismic monitoring. The analysts’ experience with the cross correlation event list (XSEL) shows that the workload of interactive processing might be reduced by a factor of two or even more. Since cross-correlation produces a comprehensive list of detections for a given master event, no additional arrivals from primary stations are expected to be associated with the XSEL events. The number of false alarms, relative to the number of events rejected from the standard event list 3 (SEL3) in the current interactive processing—can also be reduced by the use of several powerful filters. The principal filter is the difference between the arrival times of the master and newly built events at three or more primary stations, which should lie in a narrow range of a few seconds. In this study, one event at a distance of about 2,000 km from the main shock was formed by three stations, with the stations and both events on the same great circle. Such spurious events are rejected by checking consistency between detections at stations at different back azimuths from the source region. Two additional effective pre-filters are f–k analysis and F prob based on correlation traces instead of original waveforms. Overall, waveform cross-correlation is able to improve the REB completeness, to reduce the workload related to IDC interactive analysis, and to provide a precise tool for quality check for both arrivals and events. Some major improvements in automatic and interactive processing achieved by cross-correlation are illustrated using an aftershock sequence from a large continental earthquake. Exploring this sequence, we describe schematically the next steps for the development of a processing pipeline parallel to the existing IDC one in order to improve the quality of the REB together with the reduction of the magnitude threshold.

Roughness of fault surfaces over nine decades of length scales

We report on the topographic roughness measurements of five exhumed faults and thirteen surface earthquake ruptures over a large range of scales: from 50 μm to 50 km. We used three scanner devices (LiDAR, laser profilometer, white light interferometer), spanning complementary scale ranges from 50 μm to 10 m, to measure the 3‐D topography of the same objects, i.e., five exhumed slip surfaces (Vuache‐Sillingy, Bolu, Corona Heights, Dixie Valley, Magnola). A consistent geometrical property, i.e., self‐affinity, emerges as the morphology of the slip surfaces shows at first order, a linear behavior on a log‐log plot where axes are fault roughness and spatial length scale, covering five decades of length‐scales. The observed fault roughness is scale dependent, with an anisotropic self‐affine behavior described by four parameters: two power law exponents H, constant among all the faults studied but slightly anisotropic (H∥ = 0.58 ± 0.07 in the slip direction and H⊥ = 0.81 ± 0.04 perpendicular to it), and two pre‐factors showing variability over the faults studied. For larger scales between 200 m and 50 km, we have analyzed the 2‐D roughness of the surface rupture of thirteen major continental earthquakes. These ruptures show geometrical properties consistent with the slip‐perpendicular behavior of the smaller‐scale measurements. Our analysis suggests that the inherent non‐alignment between the exposed traces and the along or normal slip direction results in sampling the slip‐perpendicular geometry. Although a data gap exists between the scanned fault scarps and rupture traces, the measurements are consistent within the error bars with a single geometrical description, i.e., consistent dimensionality, over nine decades of length scales.

Accounts of the New Madrid Earthquakes: Personal Narratives across Two Centuries of North American Seismology

ArgumentThe New Madrid earthquakes shook much of North America in the winter of 1811–1812. Accounts of the New Madrid earthquakes originally were collected and employed as scientific evidence in the early nineteenth century. These early accounts were largely ignored when scientific instruments promised more quantitative and exact knowledge. Years later the earthquakes themselves became both more important and less understood because of changes in scientific models. Today, so-called intraplate or stable continental region earthquakes pose a significant problem in seismology. Historical accounts of the New Madrid events offer some of the most significant examples upon which researchers can draw and form the basis for debates over present public policy. The changing function of accounts, from narrative elements of widely-shared scientific discussion, to raw data, and back into wide-ranging conversation once again, demonstrates both deep ruptures and surprising continuities during two centuries of understanding the earth and its movement.

Global compilation of interferometric synthetic aperture radar earthquake source models: 2. Effects of 3-D Earth structure

[1] We carry out long-period surface wave centroid moment tensor (CMT) inversions using various global tomographic models and two different forward modeling techniques for 32 large earthquakes previously studied using interferometric synthetic aperture radar (InSAR) data. Since InSAR methods provide an alternative and independent way of locating and characterizing shallow continental earthquakes, comparisons of our source parameters with those from InSAR are a novel way to assess limitations in the InSAR models as well as the effects of inaccurate wave propagation formulations and/or 3-D Earth structure on earthquake source determinations. We show that comparing InSAR results with our seismic solutions is valuable to identify inaccuracies in the earthquake slip distribution retrieved using InSAR. Moreover, we find that using more accurate formulations, together with the best fitting Earth models, substantially reduces biases and differences between moment magnitude and fault strike determined using InSAR and seismic data. As expected for long-period surface wave source inversions for shallow earthquakes, the fault dip and rake angles are difficult to constrain, but we show that when using the best fitting Earth models, differences to InSAR estimates are reduced. Moreover, spurious deviations from a pure double-couple earthquake mechanism are on average smaller for the best fitting Earth models and the more accurate formulation of wave propagation. There are large differences between InSAR epicentral locations and those obtained in this study and, on average, no clear improvements to the Global CMT locations are achieved. This suggests that higher-resolution Earth models are necessary to further refine long-period CMT epicentral locations.

Seismogenesis of the lower crustal intraplate earthquakes occurring in Kachchh, Gujarat, India

Large intraplate continental earthquakes like the 1811–12 New Madrid (M w ⩾ 8.0) and the 2001 Bhuj (Mw7.7) were highly destructive because they occurred in strong crust, but the mechanisms underlying their seismogenesis are not understood. Here we show, using local earthquake velocity tomography, and joint inversion of receiver functions and surface wave group velocity dispersion that the crust and uppermost mantle beneath the 2001 Bhuj earthquake region of western India is far more complex than hitherto known through previous studies. A new image of the crust and underlying mantle lithosphere indicates the presence of a 18-km thick high velocity (Vp: 7.15–8.11 km/s) differentiated crustal and mantle magmatic layer above a hot and thin lithosphere (only 70 km) in the epicentral region of 2001 Bhuj earthquake. This magmatic layer begins at the depth of 24 km and continues down to 42 km depth. Below this region, brittle-ductile transition reaches as deep as the Moho (∼34 km) due to the possible presence of olivine rich mafic magma. Our 1-D velocity structure envisages an initial phase of plume activity (Deccan plume at 65 m.y. ago) resulting in basaltic magma in the eclogitic layers at sub-lithospheric levels, wherein they were subjected to crystallization under ultra-high pressure conditions. Our study also delineates an updoming of Moho (∼4–7 km) as well as asthenosphere (∼6–10 km) below the Kachchh rift zone relative to surrounding areas, suggesting the presence of a confined body of partial melts below the lithosphere-asthenosphere boundary. Restructuring of this warm and thin lithosphere may have been caused due to rifting (at 184 and 88 m.y. ago) and tholeiitic and alkalic volcanism related to the Deccan Traps K/T boundary event (at 65 m.y. ago). Recent study of isotopic ratios proposed that the alkalic basalts found in Kachchh are generated from a CO 2 rich lherzolite partial melts in the asthenosphere that ascended along deep lithospheric rift faults into the lithosphere. It appears that such kind of crust–mantle structure, deepening of brittle-ductile transition and a high input of volatiles containing CO 2 emanating from mantle control the seismogenesis of lower crustal earthquakes in the Kachchh continental rift zone.

Continental Intraplate Earthquakes: Science, Hazard, and Policy Issues

One of the puzzles of modern seismology is the cause (or causes) of continental intraplate earthquakes such as those in 1811–1812 (M ∼8) associated with the New Madrid seismic zone near Memphis, Tennessee. This is the subject of Continental Intraplate Earthquakes: Science, Hazard, and Policy Issues , a Geological Society of America Special Paper that contains 26 papers, which together concisely cover the science as well as related hazards and public policy. It is well written, and the graphics are abundant and legible. Of the papers presented, perhaps Susan Hough's (U.S. Geological Survey) chapter “Remotely Triggered Earthquake Following Moderate Earthquakes” is one of the most informative papers in the collection, along with Gail Atkinson's (University of Western Ontario) “Challenges in Seismic Hazard Analysis for Continental Interiors.” These two papers make the price of the publication worth every penny at $70. The collection is essentially divided into three sections: The first 13 papers are related to continental intraplate tectonic science; papers 14 through 20 are related to intraplate science and seismic hazard assessment for areas such as northwestern Rhine Valley, the Pannonian Basin, Australia, Antarctica, China, and the Bhuj, India, earthquake; and papers 21, 23, 24, and 26 emphasize seismic hazard analysis, while paper 22 documents horizontal-to-vertical ground-motion velocity ratios through hard rock in eastern Canada in order to estimate horizontal ground motions from vertical data because of the former's greater significance in seismic hazard analysis. Finally, paper 25, “Policy Development and Uncertainty in Earthquake Risk in the New Madrid Seismic Zone,” is the only paper in the volume that addresses policy for seismic hazard assessment and risk reduction as its main theme. The editors and authors make it clear that it is difficult to tease out useful information from continental intraplate earthquakes since so little …

Long aftershock sequences within continents and implications for earthquake hazard assessment

One of the most powerful features of plate tectonics is that the known plate motions give insight into both the locations and average recurrence interval of future large earthquakes on plate boundaries. Plate tectonics gives no insight, however, into where and when earthquakes will occur within plates, because the interiors of ideal plates should not deform. As a result, within plate interiors, assessments of earthquake hazards rely heavily on the assumption that the locations of small earthquakes shown by the short historical record reflect continuing deformation that will cause future large earthquakes. Here, however, we show that many of these recent earthquakes are probably aftershocks of large earthquakes that occurred hundreds of years ago. We present a simple model predicting that the length of aftershock sequences varies inversely with the rate at which faults are loaded. Aftershock sequences within the slowly deforming continents are predicted to be significantly longer than the decade typically observed at rapidly loaded plate boundaries. These predictions are in accord with observations. So the common practice of treating continental earthquakes as steady-state seismicity overestimates the hazard in presently active areas and underestimates it elsewhere.

World's largest coseismic strike‐slip offset: The 1855 rupture of the Wairarapa Fault, New Zealand, and implications for displacement/length scaling of continental earthquakes

We used detailed microtopographic surveys to measure fault offset along the southern trace of the Wairarapa fault, near Wellington, New Zealand, which most recently experienced a Mw> 8.1 earthquake in 1855. Our measurements at 16 localities support the inference that dextral slip in 1855 reached 18.7 m and averaged ∼16 m over the 16 km length that we studied. Five measurements were made where a single active strand comprises the fault zone, yielding “smallest” dextral offsets of 13.0–18.7 m. At Pigeon Bush, sequential beheading of a stream and new14C dating support the interpretation that its 18.7 ± 1.0 m of offset accumulated in 1855. We also measured three “next‐smallest” offsets on single‐strand faults of 26.3–32.7 m, evidence that dextral slip during the previous event was ∼14 m. Eight measurements were made where the Wairarapa fault includes two closely spaced strands, yielding smallest dextral offsets of 12.9–16.0 m. At Tauherenikau River,14C dating of postoffset mud yielded ages indistinguishable from A.D. 1855. Combining all single‐strand and two‐strand (minimum) estimates yields an average dextral slip of 15.5 ± 1.4 m in the study area. Historical observations and our data indicate that vertical slip reached ∼2.5 m. The large displacement and short (∼145 km) strike length yield an unusually high displacement/length ratio for the rupture. As suggested by previous dislocation modeling, we propose that the rupture extended tens of kilometers downdip (W) to merge with the underlying subduction interface. Alternatively, the rupture may have been strongly segmented at depth, yielding an earthquake with an unusually large static stress drop.

Relocation of Early and Late Aftershocks of the 2001 Bhuj Earthquake Using Joint Hypocentral Determination (JHD) Technique: Implication toward the Continued Aftershock Activity for more than Four Years

We employed layered model joint hypocentral determination (JHD) with station corrections to improve location identification for the 26 January, 2001 Mw 7.7 Bhuj early and late aftershock sequence. We relocated 999 early aftershocks using the data from a close combined network (National Geophysical Research Institute, India and Center for Earthquake Research Institute, USA) of 8–18 digital seismographs during 12–28 February, 2001. Additionally, 350 late aftershocks were also relocated using the data from 4–10 digital seismographs/accelerographs during August 2002 to December 2004. These precisely relocated aftershocks (error in the epicentral location<30 meter, error in the focal depth estimation < 50 meter) delineate an east-west trending blind thrust (North Wagad Fault, NWF) dipping (~ 45°) southward, about 25 km north of Kachchh main land fault (KMF), as the causative fault for the 2001 Bhuj earthquake. The aftershock zone is confined to a 60-km long and 40-km wide region lying between the KMF to the south and NWF to the north, extending from 2 to 45 km depth. Estimated focal depths suggest that the aftershock zone became deeper with the passage of time. The P- and S-wave station corrections determined from the JHD technique indicate that the larger values (both +ve and -ve) characterize the central aftershock zone, which is surrounded by the zones of smaller values. The station corrections vary from −0.9 to +1.1 sec for the P waves and from −0.7 to +1.4 sec for the S waves. The b-value and p-value of the whole aftershock (2001–2004) sequences of Mw ≥ 3 are estimated to be 0.77 ± 0.02 and 0.99 ± 0.02, respectively. The p-value indicates a smaller value than the global median of 1.1, suggesting a relatively slow decay of aftershocks, whereas, the relatively lower b-value (less than the average b-value of 1.0 for stable continental region earthquakes of India) suggests a relatively higher probability for larger earthquakes in Kachchh in comparison to other stable continental regions of the Indian Peninsula. Further, based on the b-value, mainshock magnitude and maximum aftershock magnitude, the Bhuj aftershock sequence is categorized as the Mogi's type II sequence, indicating the region to be of intermediate level of stresses and heterogeneous rocks. It is inferred that the decrease in p-value and increase in aftershock zone, both spatially as well as depth over the passage of time, suggests that the decay of aftershocks perhaps could be controlled by visco-elastic creep in the lower crust.

Late Pleistocene and Holocene paleoseismology of an intraplate seismic zone in a large alluvial valley, the New Madrid seismic zone, Central USA

Late Pleistocene and Holocene paleoseismology of an intraplate seismic zone in a large alluvial valley, the New Madrid seismic zone, Central USA

Correlations between earthquakes and anomalous particle bursts from SAMPEX/PET satellite observations

In the last decades, a possible influence of electromagnetic fields of seismic origin in the ionosphere–magnetosphere transition region has been reported in the literature. In recent years, a few space experiments also revealed anomalous bursts of charged particles precipitating from the lower boundary of the inner radiation belt. They were thought to be caused by low-frequency seismo-electromagnetic emissions. A recent study [Aleksandrin et al., 2003. Annales Geophysicae 21, 597–602] seems to confirm that these particle bursts have a short-term preseismic character. The particle longitudinal drift should enhance the detectability of preseismic particle bursts, thus magnifying their importance in earthquake prediction studies. This paper takes into consideration the method introduced by Aleksandrin et al. (2003) to carry out a deeper investigation on the subject. In this sense, a method for the temporal correlation between continental earthquakes with M ⩾ 5.0 and anomalous particle bursts collected by the PET-SAMPEX satellite mission is critically investigated and presented here. Several constraints and cuts have been applied to data in order to exclude, from the correlation, charged particles collected inside the SAA region and/or during ionospheric and magnetospheric perturbations caused by non-seismic sources. After the data have been detrended by these effects, a short-term seismic precursor of ∼4 h is observed in the histogram of the time difference between the time occurrence of earthquakes and that of particle burst events. The best correlation is obtained only when considering high-energy electrons ( E ⩾ 4 MeV ) with pitch angles near the loss cone. Such a result confirms previous ones but also points out the importance of an ad hoc method of analysis. Results suggest the importance of coordinated and simultaneous ground-based and space investigations specifically dedicated to the subject. Also, a deeper investigation based on particle data prepared and analyzed as carefully as possible is requested to understand the physical mechanisms underlying the phenomenon under study.

Deep infrasound radiated by the Sumatra earthquake and tsunami

Infrasound arrays in the Pacific and Indian oceans that are part of the International Monitoring System (IMS) of the Comprehensive Nuclear Test Ban Treaty (CTBT) recorded distinct signatures associated with the 26 December 2004 Sumatra earthquake (M/9, http://earthquake.usgs.gov/) and tsunami. Although the radiation of infrasound from large continental earthquakes is established [e.g., Le Pichon et al., 2003], the results presented in the present article indicate that islands undergoing significant surface displacements from submarine earthquakes can produce infrasound.Far more intriguing is the possibility that the initiation and propagation of a tsunami may produce low‐frequency sound near the source as well as along coastlines and basins. Since distant sound effectively propagates at ˜300 m/s and tsunamis propagate at ˜200 m/s, precursory sound could potentially be used as a discriminant for tsunami genesis.

Preface to the Issue Dedicated to the 2002 Denali Fault Earthquake Sequence

On 3 November 2002, a M w 7.9 earthquake, the largest continental strike-slip earthquake in North America since the 1857 Fort Tejon, California, event, occurred in central Alaska. The earthquake began with reverse faulting on a ∼40-km extent of the previously unknown Susitna Glacier fault, but rupture transferred eastward to the right-lateral Denali fault and continued for over 200 km, finally transferring to rupture ∼70 km of the Totschunda fault. This large, complex event we term the Denali fault earthquake (dfe), after the major crustal fault that carried most of the displacement. The initiation of the rupture, the Susitna Glacier fault, is in a remote region of central Alaska that under normal circumstances is sparsely instrumented. On 23 October of that year, however, a large earthquake of M w 6.7, referred to as the Nenana Mountain earthquake (nme), occurred only 22 km to the west of the dfe epicenter. The nme, in hindsight recognized as a foreshock to the dfe, prompted deployment of a temporary network by the Alaska Earthquake Information Center (aeic). Hence, the area was under significantly enhanced seismic surveillance at the time of the dfe, 10 days later, which was further augmented by the addition of 19 more stations following the dfe mainshock. As a result, high-quality data were available in the near field, providing enhanced coverage for aftershock activity from the Susitna Glacier fault initiation point, along the Denali fault as far as the western portion of the Totschunda fault, to augment regional and teleseismic data for this sequence. As the rupture proceeded eastward, the Richardson Highway, one of the two north–south roads connecting the central and southern parts of the state, was disrupted where it crosses the Denali fault trace. Also significantly displaced was the Trans-Alaska Pipeline, operated by the Alyeska Pipeline …

The 2002 Denali Fault and 2001 Kunlun Fault Earthquakes: Complex Rupture Processes of Two Large Strike-Slip Events

We studied the source processes of two large continental earthquakes, the 3 November 2002 Denali fault earthquake and the 14 November 2001 Kunlun fault earthquake, associated with strike-slip faulting along ancient sutures. We inverted teleseismic P waveforms using a pulse-stripping method for multiple time windows with different focal mechanisms and derived composite source models. According to our results, the 2002 Denali fault earthquake began with initial thrusting ( M w 7.3) along a 40-km-long segment of the north-northwest-dipping Susitna Glacier fault and later ruptured a 300-km-long segment along the Denali and Totschunda faults with a right-lateral strike-slip mechanism ( M w 7.7). In contrast, the 2001 Kunlun fault earthquake nucleated near an extensional step-over with a subevent pair consisting of 30-km-long strike-slip ( M w 6.9) event and 40-km-long normal ( M w 6.8) faulting event and later ruptured a 350-km-long segment along the Kunlun fault with a left-lateral strike-slip mechanism ( M w 7.7). Both earthquakes propagated primarily unilaterally to the east and released most of their energy along slip patches (asperities) far from the hypocenter locations. We find that both the Denali fault and Kunlun fault earthquakes had high-average rupture velocities of 3.2 km/sec and 3.4 km/sec, respectively. We also compared the source properties of these two earthquakes with other strike-slip earthquakes. For scaling purposes, large strike-slip earthquakes were classified as interplate, oceanic intraplate, or continental intraplate events. By using this classification the Denali fault and Kunlun fault earthquakes have an interplate signature that suggests overall weak faulting.