Distant Earthquakes Research Articles

Sustained water‐level changes caused by damage and compaction induced by teleseismic earthquakes

AbstractSustained water‐level increase and decrease induced by distant earthquakes were observed in two wells, Gomè 1 and Meizar 1 in Israel. The Gomè 1 well is located within a damage zone of a major fault zone, and Meizar 1 is relatively far from a fault. The monitored pressure change in both wells shows significant water‐level oscillations and sustained water‐level changes in response to the passage of the seismic waves. The sustained water‐level changes include short‐term (minutes) undrained behavior and longer‐period (hours and days) drained behavior associated with groundwater flow. We model the short‐term undrained response of water pressure oscillations and sustained change to the distant 2013 Mw 7.7 Balochistan earthquake by nonlinear elastic behavior of damaged rocks, accounting for small wave‐induced compaction and damage accumulation. We suggest that the rocks are close to failure in both locations and strain oscillations produced by the passing seismic waves periodically push the rock above the yield cap, creating compaction when volumetric strain increases and damage when shear strain increases. Compaction increases pore pressure, whereas damage accumulation decreases pore pressure by fracture dilation. The dominant process depends on the properties of the rock. For highly damaged rocks, dilatancy is dominant and a sustained pressure decrease is expected. For low‐damage rocks, compaction is the dominant process creating sustained water‐level increase. We calculate damage and porosity changes associated to the Balochistan earthquake in both wells and quantify damage accumulation and compaction during the passage of the seismic waves.

Teleseismic imaging of the mantle beneath southernmost China: New insights into the Hainan plume

Intraplate volcanism during the Late Cenozoic in the Leiqiong area of southernmost China, with basaltic lava flows covering a total of more than 7000km2, has been attributed to an underlying Hainan plume. To clarify detailed features of the Hainan plume, such as the morphology of its magmatic conduits, the depth of its magmatic pool in the upper mantle and the pattern of mantle upwelling, we determined tomographic images of the mantle down to a depth of 1100km beneath southern China using 18,503 high-quality arrival-time data of 392 distant earthquakes recorded by a dense seismic array. Our results show a mushroom-like continuous low-velocity anomaly characterized by a columnar tail with a diameter of 200–300km extending down to the lower mantle beneath north of the Hainan hotspot and a head spreading laterally in and around the mantle transition zone, indicating a magmatic pool in the upper mantle. Further upward, the plume head is decomposed into smaller patches, and when reaching the base of the lithosphere, a pancake-like anomaly has formed to feed the Hainan hotspot. This result challenges the classical model of a fixed thermal plume that rises vertically to the surface. Hence we propose a new layering-style model for the magmatic upwelling of the Hainan plume. Our results indicate spatial complexities and variations of mantle plumes probably due to heterogeneous compositions and thermochemical structures of the deep mantle.

Tsunami hazard potential for the equatorial southwestern Pacific atolls of Tokelau from scenario-based simulations

Abstract. Devastating tsunami over the last decade have significantly heightened awareness of the potential consequences and vulnerability of low-lying Pacific islands and coastal regions. Our appraisal of the potential tsunami hazard for the atolls of the Tokelau Islands is based on a tsunami source–propagation–inundation model using Gerris Flow Solver, adapted from the companion study by Lamarche et al. (2015) for the islands of Wallis and Futuna. We assess whether there is potential for tsunami flooding on any of the village islets from a selection of 14 earthquake-source experiments. These earthquake sources are primarily based on the largest Pacific earthquakes of Mw ≥ 8.1 since 1950 and other large credible sources of tsunami that may impact Tokelau. Earthquake-source location and moment magnitude are related to tsunami-wave amplitudes and tsunami flood depths simulated for each of the three atolls of Tokelau. This approach yields instructive results for a community advisory but is not intended to be fully deterministic. Rather, the underlying aim is to identify credible sources that present the greatest potential to trigger an emergency response. Results from our modelling show that wave fields are channelled by the bathymetry of the Pacific basin in such a way that the swathes of the highest waves sweep immediately northeast of the Tokelau Islands. Our limited simulations suggest that trans-Pacific tsunami from distant earthquake sources to the north of Tokelau pose the most significant inundation threat. In particular, our assumed worst-case scenario for the Kuril Trench generated maximum modelled-wave amplitudes in excess of 1 m, which may last a few hours and include several wave trains. Other sources can impact specific sectors of the atolls, particularly distant earthquakes from Chile and Peru, and regional earthquake sources to the south. Flooding is dependent on the wave orientation and direct alignment to the incoming tsunami. Our "worst-case" tsunami simulations of the Tokelau Islands suggest that dry areas remain around the villages, which are typically built on a high islet. Consistent with the oral history of little or no perceived tsunami threat, simulations from the recent Tohoku and Chile earthquake sources suggest only limited flooding around low-lying islets of the atoll. Where potential tsunami flooding is inferred from the modelling, recommended minimum evacuation heights above local sea level are compiled, with particular attention paid to variations in tsunami flood depth around the atolls, subdivided into directional quadrants around each atoll. However, complex wave behaviours around the atolls, islets, tidal channels and within the lagoons are also observed in our simulations. Wave amplitudes within the lagoons may exceed 50 cm, increasing any inundation and potential hazards on the inner shoreline of the atolls, which in turn may influence evacuation strategies. Our study shows that indicative simulation studies can be achieved even with only basic field information. In part, this is due to the spatially and vertically limited topography of the atoll, short reef flat and steep seaward bathymetry, and the simple depth profile of the lagoon bathymetry.

A new classification of earthquake-induced landslide event sizes based on seismotectonic, topographic, climatic and geologic factors

This paper reviews the classical and some particular factors contributing to earthquake-triggered landslide activity. This analysis should help predict more accurately landslide event sizes, both in terms of potential numbers and affected area. It also highlights that some occurrences, especially those very far from the hypocentre/activated fault, cannot be predicted by state-of-the-art methods. Particular attention will be paid to the effects of deep focal earthquakes in Central Asia and to other extremely distant landslide activations in other regions of the world (e.g. Saguenay earthquake 1988, Canada). The classification of seismically induced landslides and the related ‘event sizes’ is based on five main factors: ‘Intensity’, ‘Fault factor’, ‘Topographic energy’, ‘Climatic background conditions’, ‘Lithological factor’. Most of these data were extracted from papers, but topographic inputs were checked by analyzing the affected region in Google Earth. The combination and relative weight of the factors was tested through comparison with well documented events and complemented by our studies of earthquake-triggered landslides in Central Asia. The highest relative weight (6) was attributed to the ‘Fault factor’; the other factors all received a smaller relative weight (2–4). The high weight of the ‘Fault factor’ (based on the location in/outside the mountain range, the fault type and length) is strongly constrained by the importance of the Wenchuan earthquake that, for example, triggered far more landslides in 2008 than the Nepal earthquake in 2015: the main difference is that the fault activated by the Wenchuan earthquake created an extensive surface rupture within the Longmenshan Range marked by a very high topographic energy while the one activated by the Nepal earthquake ruptured the surface in the frontal part of the Himalayas where the slopes are less steep and high. Finally, the calibrated factor combination was applied to almost 100 other earthquake events for which some landslide information was available. This comparison revealed the ability of the classification to provide a reasonable estimate of the number of triggered landslides and of the size of the affected area. According to this prediction, the most severe earthquake-triggered landslide event of the last one hundred years would actually be the Wenchuan earthquake in 2008 followed by the 1950 Assam earthquake in India – considering that the dominating role of the Wenchuan earthquake data (including the availability of a complete landslide inventory) for the weighting of the factors strongly influences and may even bias this result. The strongest landslide impacts on human life in recent history were caused by the Haiyuan-Gansu earthquake in 1920 – ranked as third most severe event according to our classification: its size is due to a combination of high shaking intensity, an important ‘Fault factor’ and the extreme susceptibility of the regional loess cover to slope failure, while the surface morphology of the affected area is much smoother than the one affected by the Wenchuan 2008 or the Nepal 2015 earthquakes. The main goal of the classification of earthquake-triggered landslide events is to help improve total seismic hazard assessment over short and longer terms. Considering the general performance of the classification-prediction, it can be seen that the prediction either fits or overestimates the known/observed number of triggered landslides for a series of earthquakes, while it often underestimates the size of the affected area. For several events (especially the older ones), the overestimation of the number of landslides can be partly explained by the incompleteness of the published catalogues. The underestimation of the extension of the area, however, is real – as some particularities cannot be taken into account by such a general approach: notably, we used the same seismic intensity attenuation for all events, while attenuation laws are dependent on regional tectonic and geological conditions. In this regard, it is likely that the far-distant triggering of landslides, e.g., by the 1988 Saguenay earthquake (and the related extreme extension of affected area) is due to a very low attenuation of seismic energy within the North American plate. Far-distant triggering of landslides in Central Asia can be explained by the susceptibility of slopes covered by thick soft soils to failure under the effect of low-frequency shaking induced by distant earthquakes, especially by the deep focal earthquakes in the Pamir – Hindukush seismic region. Such deep focal and high magnitude (> > 7) earthquakes are also found in Europe, first of all in the Vrancea region (Romania). For this area as well as for the South Tien Shan we computed possible landslide event sizes related to some future earthquake scenarios.

Co-seismic water level changes in response to multiple large earthquakes at the LGH well in Sichuan, China

We examined the water level data at the LGH well in Sichuan, China, from December 2007 to July 2015 and their responses to multiple large earthquakes with seismic energy densities greater than 10−4J/m3. Co-seismic water level declines were observed in response to eleven earthquakes out of twelve in the farfield, and co-seismic water level increase was observed in one nearfield case. The water level declines in the farfield showed a linear relation with the common logarithm of the seismic energy densities, whereas the water level increase in the nearfield fell away from this relation, indicating that the farfield responses and the nearfield response were produced by distinct mechanisms. We used the phase shift of tidal responses as a proxy for permeability and found that permeability enhancements were observed both in the farfield and nearfield. The co-seismic water level declines in response to the distant earthquakes could be explained by permeability enhancements caused by the passage of seismic waves through the mobilization of colloidal particles; the co-seismic water level increase in response to the nearfield case could be caused both by the compression of the static stress and by the seismic waves.

Molten rock underlies North Korean volcano

Geological Sciences Mt. Paektu is a volcano responsible for one of the largest eruptions in the past few thousand years. However, its location on the North Korea-China border means that little is known about its history and underlying structure. In a North Korea/United Kingdom/United States collaboration, Song et al. used distant earthquakes recorded on seismometers in North Korea to look inside the volcano. They found evidence of molten rock within the volcano that may provide a source of the magma involved in past eruptions and recent volcanic unrest. Sci. Adv. 2 , 10.1126.sciadv.01513 (2016)

Calibration of a local magnitude relationship for microseismic events using earthquake data

Calibration of local seismic arrays is crucial to quantify the magnitude of local microseismic events. This is traditionally done by performing a site survey consisting of calibration shots at known locations with known strength. However, when a calibration survey cannot be conducted, alternative calibration methods are required. We have demonstrated the feasibility of magnitude estimation of microseismic events recorded at the [Formula: see text] storage site at Ketzin (Germany), by analyzing earthquake signals that have been recorded on the noncalibrated geophone array and on standardized seismometers of a regional seismological network. We estimated a linear calibration relation by fitting three different signal attributes for 10 distant earthquakes. Through extrapolation of the linear fit toward lower magnitudes, we estimated the local magnitude of weak local seismic events. Reported magnitudes of the distant earthquakes show significant variation because there are different methods and constants used to calculate a magnitude estimate from recordings on multiple seismometers. As a consequence, there was a considerable spread in our magnitude estimates for the Ketzin events. We found magnitude values in the range between [Formula: see text] and [Formula: see text] for two local events recorded at the Ketzin site. We expect that by incorporating additional distant earthquake data, the uncertainty in this estimate can be further reduced.

A Study on the Correlation of Milk Yield of Cows with Seismicity and ULF Magnetic Field Variations

A statistical study on the basis of one-year data of 2014 has been performed in order to find whether abnormal animal behavior is related with seismic activity and also whether the ULF (Ultra Low Frequency) electromagnetic radiation might be a possible sensory mechanism of abnormal animal behavior. Abnormal animal behavior has been studied with the use of digitally recorded milk yield of cows at Ibaraki Prefecture Livestock Station, and the ULF magnetic field changes have been studied with the data at a magnetic observatory of Kakioka. As the result of correlation analyses, clear responses are observed for both the milk yield of cows and ULF magnetic field changes (both ULF radiation (ULF emissions from the lithosphere) and ULF depression (as an indicator of lower ionospheric perturbations)) for most powerful and not distant earthquakes (EQs) with magnitude > 6, that is, the milk yield of cows is found to exhibit a conspicuous depletion about 17 - 18 days before an EQ, though the correlation coefficient is not so big. Another important objective in this paper is to identify that ULF radiation is the main agent of abnormal behavior so that we have compared the temporal evolutions of milk yield of cows, ULF radiation and ULF depression for three major EQs. As a result, it is found that ULF radiation happens, at least, during the periods of abnormal depletion of milk yield of cows.

Subsurface fluid distribution and possible seismic precursory signal at the Salse di Nirano mud volcanic field, Italy

Mud volcanoes are geological systems often characterized by elevated fluid pressures at depth deviating from hydrostatic conditions. This near-critical state makes mud volcanoes particularly sensitive to external forcing induced by natural or man-made perturbations. We used the Nirano mud volcanic field as a natural laboratory to test pre- and post-seismic effects generated by distant earthquakes. We first characterized the subsurface structure of the Nirano mud volcanic field with a geoelectrical study. Next, we deployed a broad-band seismic station in the area to understand the typical seismic signal generated by the mud volcano. Seismic records show a background noise below 2 s, sometimes interrupted by pulses of drumbeat-like high-frequency signals lasting from several minutes to hours. To date this is the first observation of drumbeat signal observed in mud volcanoes. In 2013 June we recorded a M4.7 earthquake, that occurred approximately 60 km far from our seismic station. According to empirical estimations the Nirano mud volcanic field should not have been affected by the M4.7 earthquake. Yet, before the seismic event we recorded an increasing amplitude of the signal in the 10–20 Hz frequency band. The signal emerged approximately two hours before the earthquake and lasted for about three hours. Our statistical analysis suggests the presence of a possible precursory signal about 10 min before the earthquake.

Co-seismic changes of well water level and volume strain meter in capital area and its vicinity, due to the Nov.14, 2001 Ms8.1 Kunlun Mountain earthquake, China

The Kunlunshan Mountain Ms8.1 earthquake, occurred in Nov.14, 2001, is the first event with magnitude more than 8 in the China earthquake monitoring history, specifically at the beginning of digital techniques in precursor monitoring networks. Any investigation of recorded data on this earthquake is very important for testing the operation of the digital monitoring networks and understanding the preparation, occurrence, and adjustment of stress/strain of strong continental earthquakes. In this paper we investigated the co-seismic response changes of well water level of groundwater and volume strain meter of bore hole in digital earthquake monitoring network of Capital area and its vicinity, due to the Nov.14, 2001 Ms8.1 Kunlun Mountain earthquake. The responding time, shapes or manners, amplitudes, and lasting time of well water level and strain-meters to seismic wave are studied in comparison. Then we discussed the possibility that the response changes of groundwater to strong distant earthquakes can be understood as one kind of observing evidence of stress/strain changes induced by distant earthquake.

Mantle dynamics and Cretaceous magmatism in east-central China: Insight from teleseismic tomograms

Both the rich mineralization in the Lower Yangtze Block (LYB) and the post-collisional mafic rocks in the Dabie Orogen (DBO) are closely related to the Cretaceous magmatism in east-central China. Various geodynamic models have been proposed for explaining the mechanism of the Cretaceous magmatism, but these models are controversial and even contradictory with each other, especially on the mechanism of adakites. A unified geodynamic model is required for explaining the magmatism in east-central China, in particular, the spatial and temporal correlations of magmatic activity in the DBO and that in the LYB. For this purpose, we apply teleseismic tomography to study P-wave velocity structure down to 800km depth beneath east-central China. A modified multiple-channel cross-correlation method is used to collect 28,805 high-quality P-wave arrival-time data from seismograms of distant earthquakes recorded by permanent seismic stations and our temporary stations in the study region. To remove the influence of crustal heterogeneity on the mantle tomography, we used the CRUST1.0 model to correct the teleseismic relative residuals. Our tomography revealed distinct high-velocity (high-V) anomalies beneath the DBO and two flanks of the LYB, and low-velocity (low-V) anomalies above the high-V zones. Combining our tomographic images with previous geological, geochemical and geophysical results, we infer that these high-V and low-V anomalies reflect the detached lithosphere and upwelling asthenospheric materials, respectively, which are associated with the Late Mesozoic dynamic process and the Cretaceous magmatism. We propose a double-slab subduction model that a ridge subduction yielded the adakitic rocks in the LYB during 150–135Ma and the subsequent Pacific Plate subduction played a crucial role in not only the formation of igneous rocks in the LYB but also remelting of the subducted South China Block beneath the DBO during 135–101Ma.

Delayed dynamic triggering of deep tremor along the Parkfield‐Cholame section of the San Andreas Fault following the 2014 M6.0 South Napa earthquake

AbstractLarge, distant earthquakes are known to trigger deep tectonic tremor along the San Andreas Fault and in subduction zones. However, there are relatively few observations of triggering from regional distance earthquakes. Here we show that a small tremor episode about 12–18 km NW of Parkfield was triggered during and immediately following the passage of surface waves from the 2014 Mw 6.0 South Napa main shock. More notably, a major tremor episode followed, beginning about 12 h later, and centered SE of Parkfield near Cholame. This major episode is one of the largest seen over the past several years, containing intense activity for ~3 days and taking more than 3 weeks to return to background levels. This episode showed systematic along‐strike migration at ~5 km/d, suggesting that it was driven by a slow‐slip event. Our results suggest that moderate‐size earthquakes are capable of triggering major tremor and deep slow slip at regional distances.

Seismic monitoring in the oceans by autonomous floats.

Our understanding of the internal dynamics of the Earth is largely based on images of seismic velocity variations in the mantle obtained with global tomography. However, our ability to image the mantle is severely hampered by a lack of seismic data collected in marine areas. Here we report observations made under different noise conditions (in the Mediterranean Sea, the Indian and Pacific Oceans) by a submarine floating seismograph, and show that such floats are able to fill the oceanic data gap. Depending on the ambient noise level, the floats can record between 35 and 63% of distant earthquakes with a moment magnitude M≥6.5. Even magnitudes <6.0 can be successfully observed under favourable noise conditions. The serendipitous recording of an earthquake swarm near the Indian Ocean triple junction enabled us to establish a threshold magnitude between 2.7 and 3.4 for local earthquakes in the noisiest of the three environments.

Assessment of tsunami hazard due to regional and remote sources: The Coast of the Sea of Okhotsk

The tsunami hazard for the coast of the Sea of Okhotsk requires a careful analysis, because this sea will be a zone of responsibility for the Tsunami Warning Service for the Far East coast of Russia. While it is not subject to such hazards on the part of seismogenic zones that can produce dangerous tsunamis, nevertheless the Sea of Okhotsk is open for penetration of tsunamis that can be produced by sources in other tsunamigenerating zones of the Kuril–Kamchatka region, as well as those of the entire Pacific Ocean. The tsunami hazard for the coast of the Sea of Okhotsk is examined here on the basis of historical observations and the results of numerical simulation for tsunami propagation from hypothetical rupture zones of near and distant earthquakes. It is shown that the real tsunami hazard can only emanate from those regional earthquakes with magnitudes 8.5 or greater that occur in the Kuril–Kamchatka seismogenic zone. Among the remote tsunamigenerating zones in the Pacific, the most dangerous locations are the rupture zones of mega-earthquakes of the class M9 that come from the South America zone and from Papua–New Guinea. These can produce water waves with amplitudes as great as 5 m along the entire coast of the Sea of Okhotsk.

Site response assessment at the city of Al Khobar, eastern Saudi Arabia, from microtremor and borehole data

Al Khobar City is affected by distant earthquakes from the Zagros fold-fault belt, which is part of the subduction zone between the Arabian and Eurasian plates. These earthquakes have generated substantial site effects on the sedimentary layers, which in turn significantly influence earthquake ground motions in the area. Mapping of site response using microtremor measurements compared with geological and borehole data from Al Khobar City is the main objective of this work. The resonance frequency and the corresponding horizontal-to-vertical (H/V) ratio values have been calculated using the Nakamura technique on data from seismograph instruments deployed at 113 sites in Al Khobar City for different time periods. The recording length was about 1 h with a sampling frequency of 100 Hz. Most of the measured sites present three peaks of resonance frequency; the first peak ranges from 0.33 to 1.03 Hz, the second peak ranges from 1.03 to 1.23 Hz, while the third one ranges from 1.23 to 1.73 Hz. Tests have been conducted to ensure that these peaks are of natural origin The northern zones of Al Khobar City have lower resonance frequency values, indicating great sediment thickness. In contrast, the southern parts of the city have higher resonance frequency values, suggesting shallow bedrock depths. Furthermore, 29 geotechnical boreholes have been drilled to different depths in Al Khobar City. Standard penetration test (SPT) data has been corrected and used to calculate the resonance frequency at their locations. The borehole results showed that the resonance frequency values range from 0.27 to 1.95 Hz. These results are well correlated with those from the microtremor measurements. Accordingly, the first peak has been interpreted as being due to the impedance contrast between the limestone bedrock and the overlying sediments, while the third peak originates from a boundary between the uppermost surface layer and the underlying sediments.

Estimation of the maximum credible hazard in Kuala Lumpur and Singapore due to gigantic Sumatran megathrust earthquakes: based on a comparative study on attenuation laws

This paper has classified nine attenuation laws derived for subduction interface earthquakes through a comparative study based on the peak ground accelerations (PGAs) recorded in Peninsular Malaysia and Singapore. The goal of classification was to identify the attenuation laws that were the most compatible with the region. The PGAs predicted by the relations provided by Nabilah and Balendra (Earthq Eng 16:1076–1094, 2012) and Zhao et al. (Bull Seismol Soc Am 96:898–913, 2006) were found to correlate well with the recorded PGAs. The maximum credible hazard in Kuala Lumpur and Singapore was obtained by calculating the acceleration response spectra caused by three great Sumatran megathrust earthquakes (the worst possible earthquake scenarios) using Zhao et al. spectral attenuation law. Current building code in the region requires that buildings should be capable of resisting a notional ultimate lateral design load equals to 1.5 % of the characteristic dead weight, simultaneously applied at each floor (i.e., the minimum design level of buildings equals to ~0.15 m/s2). The results show that the risk level for structures of the two cities with natural period Tn = 1–1.5 s corresponding to 10- and 15-story buildings located on hard soil sites [National Earthquake Hazards Reduction Program (NEHRP) site class C] is high as the demand spectrum due to a worst possible earthquake scenario with moment magnitude (MW) 9.5 and an epicentral distance (Repi) of 600 km from Singapore was noticeably higher than their design level. The results of the present study are applicable in seismic hazard assessment projects in Peninsular Malaysia and Singapore. The most interesting conclusion from the earthquake engineering perspective, however, is that the design spectral shapes specified in IBC 2012 and Eurocode 8 (type 2) codes may not be particularly appropriate for sites that could be affected by large-magnitude, distant earthquakes.

Triggered Seismic Events along the Eastern Denali Fault in Northwest Canada Following the 2012 Mw 7.8 Haida Gwaii, 2013 Mw 7.5 Craig, and Two Mw&gt;8.5 Teleseismic Earthquakes

We conduct a systematic search for remotely triggered seismic activity along the eastern Denali fault (EDF) in northwest Canada, an intraplate strike‐slip region. We examine 19 distant earthquakes recorded by nine broadband stations in the Canadian National Seismograph Network and find that the 2012 M w 7.8 Haida Gwaii and 2013 M w 7.5 Craig, Alaska, earthquakes triggered long duration (>10 s), emergent tremor‐like signals near the southeastern portion of the EDF. In both cases, tremor coincides with the peak transverse velocities, consistent with Love‐wave triggering on right‐lateral strike‐slip faults. The 2011 M w 9.0 Tohoku‐Oki and 2012 M w 8.6 Indian Ocean earthquakes possibly triggered tremor signals, although we were unable to locate those sources. In addition, we also identify many short‐duration (<5 s) bursts that were repeatedly triggered by the Rayleigh waves of the 2012 M w 7.8 Haida Gwaii earthquake. Although we were unable to precisely locate the short‐duration (<5 s) events, they appear to be radiating from the direction of the Klutlan Glacier and from a belt of shallow historical seismicity at the eastern flank of the Wrangell–St. Elias mountain range. The fact that these events were triggered solely by the Rayleigh waves suggests a different source mechanism as compared with triggered tremor observed along the EDF and other plate boundary regions. Online Material: Description of Rayleigh‐wave polarization detection; figures of spectograms, S ‐wave amplitudes, polarization analysis, and static stress changes; tables of earthquake catalogs and velocity model; and animations of tremors and bursts.

Rare dynamic triggering of remote M ≥ 5.5 earthquakes from global catalog analysis

AbstractProbing the effects of a transient stress on the timing of an earthquake occurrence is necessary for understanding the remote interaction of large‐magnitude events. Global catalog data containing 35 years of M ≥ 5.5 earthquakes allow us to explore for periods of enhanced or suppressed seismic activity. We consider 113 M ≥ 7.5 main shocks between 1977 and 2012 and focus on seismic activity on time scales from seconds to days following these main shocks. We search for evidence of dynamic triggering of large‐magnitude events similar to the previously observed global increase during the first few days following the 2012 M8.6 Indian Ocean main shock. We restrict the analysis to regions of elevated strain during the passage of surface waves. Using a threshold of 0.1 microstrain (~3 kPa) and a temporal window of ±1 year, we stack daily seismicity rate curves using the exclusion‐zone declustered M ≥ 5.5 catalog events in order to resolve deviations from the background rate. Our results do not indicate a significant change in activity for at least 10 days when considering the collective set of 113 main shocks and subsets at M8.0 and M8.5 thresholds. The results also do not indicate immediate triggering of M ≥ 5.5 events. We do find two instances of increased seismicity in the elevated strain region within 10 days. These increases are subsequent to two main shocks, the 1977 M8.3 and 2012 M8.6, both located in the Indian Ocean. We conclude that a global change in M ≥ 5.5 earthquake rates following a transient stress from distant earthquakes is a rare occurrence.

Conditional Mean Spectra in Site-Specific Seismic Hazard Evaluation for a Major River Crossing in the Central United States

Current seismic design practice often relies on the use of the uniform hazard response spectrum (UHRS), which implicitly includes motions from multiple earthquake sources and envelops possible spectra, yet does not represent a single event. Seismic hazard analyses at the site of a major Mississippi River crossing near St. Louis, Missouri, showed bimodal seismic hazard dominated by small, nearby earthquakes at short periods and large, distant earthquakes in the New Madrid seismic zone at long periods. UHRS motions resulted in large seismic demands and predictions of pervasive liquefaction that were inconsistent with historical and geologic records. UHRS-compatible conditional mean spectra (CMS) were developed to bridge deterministic and probabilistic seismic hazard evaluations, and used to evaluate liquefaction, lateral spreading, and settlement potential. The computed response was consistent with the historical and geologic record. CMSs offer hazard-compatible alternatives to the UHRS and result in seismic demand consistent with historical and geologic evidence.

What Lies Deep in the Mantle Below?

For decades, scientists have probed Earth's remote mantle by analyzing how seismic waves of distant earthquakes pass through it. But we are still challenged by the technique's limitations.