Search

Earth's crust is continuously subjected to oscillatory stress perturbations due to the solid Earth and ocean tides. The seismic response to such stress modulations carries information on earthquake physics and crustal properties. Experimental and observational studies suggested but could not demonstrate that the strength of tidal modulation of seismicity increases before large earthquakes. We tested this hypothesis by (a) developing a new, comprehensive 10‐year long earthquake catalog preceding the 6 July 2019 magnitude 7.1 Ridgecrest, CA earthquake, and (b) applied our novel method for extracting a statistical signal of tidal modulation. Our results show enhanced tidal sensitivity of seismicity along the fault starting about 1.5 years before the mainshock, corroborating the hypothesis. This observation suggests that small magnitude earthquakes may be used to gain insight into subtle changes in fault conditions, bringing new promise for studying the earthquake preparation process. Plain Language Summary: The solid Earth experiences tides, like the ocean, it deforms under the gravitational attraction of the Sun and Moon. This deformation induces an oscillatory stress change in the crust with small peak‐to‐peak amplitudes of the order of 1 kPa (about 1% of the atmospheric pressure). Tidal stresses weakly influence the rate of earthquake occurrence and the characteristics of this modulation carry information on earthquake physics and crustal properties. On the basis of experimental and field observation studies, it has been proposed that the modulation of seismicity by the tides increases before a large earthquake. We tested this hypothesis by analyzing 10 years of seismicity before the M7.1 2019 Ridgecrest, CA earthquake. We first built a new, comprehensive earthquake catalog with our automated method and used our novel method to extract the signal of tidal modulation throughout the study period. We found that seismicity became strongly modulated by the tides about 2 years before the mainshock, thus corroborating the hypothesis. Key Points: Seismicity in the Ridgecrest area is modulated by the solid Earth tidesPeak seismicity occurs when tidal Coulomb stress or stress rate favors ruptureStrength of tidal triggering is gradually increasing in the rupture area about 2 years before the Mw 7.1 2019 mainshock [ABSTRACT FROM AUTHOR]

On June 14, 2020, Mw 5.9 Kaynarpınar earthquake occurred in the easternmost part of the North Anatolian Fault Zone, along the Yedisu Seismic Gap near the Karlıova Triple Junction. Following the event, various agencies reported different epicenter locations. This issue and the complex fault pattern around the triple junction caused uncertainties about the ruptured fault and the rupture mechanism. To define the geometry of the ruptured fault, we (1) held a field survey within a week of the mainshock, (2) correlated our field data with the multitrack InSAR data, (3) relocated the epicenter of the mainshock, and its aftershocks, and (4) implemented moment tensor inversion analyses to the earthquakes larger than M 3.3. We defined a ~ N70E-striking surface rupture and measured the average co-seismic dextral slip magnitude as ~ 16 ± 1 cm in the field which is compatible with the horizontal displacement magnitude measured by InSAR (13–15 cm). Since no ~ N70E-striking faults were reported in the existing active fault database, we examined the prominent geomorphological structures along the rupture area. We determined two dextral fault families along the eastern part of the Yedisu Seismic Gap; (1) NW–SE-striking faults of the North Anatolian Fault Zone, and (2) N70E-striking faults forming the Kaynarpınar–Yuvaklı Fault Zone. This fault zone, defined and named in this study, is a part of the N70E-striking faults of the Turkish–Iranian High Plateau and the North Anatolian Fault Zone. The fault zone must have carried some, if not most, of the dextral sense-of-slip in the easternmost part of the North Anatolian Fault Zone during the Quaternary. [ABSTRACT FROM AUTHOR]

The novel technique of distributed acoustic sensing (DAS) holds great potential for underwater seismology by transforming standard telecommunication cables, such as those currently traversing various regions of the world's oceans, into dense arrays of seismo‐acoustic sensors. To harness these measurements for seismic monitoring, the ability to record transient ground deformations is investigated by analyzing ambient noise, earthquakes, and their associated phase velocities, on DAS records from three dark fibers in the Mediterranean Sea. Recording quality varies dramatically along the fibers and is strongly correlated with the bathymetry and the apparent phase velocities of recorded waves. Apparent velocities are determined for several well‐recorded earthquakes and used to convert DAS S‐wave strain spectra to ground motion spectra. Excellent agreement is found between the spectra of nearby underwater and on‐land seismometers and DAS converted spectra, when the latter are corrected for site effects. Apparent velocities greatly affect the ability to detect seismic deformations: for the same ground motions, slower waves induce higher strains and thus are more favorably detected than fast waves. The effect of apparent velocity on the ability to detect seismic phases, quantified by expected signal‐to‐noise ratios, is investigated by comparing signal amplitudes predicted by an earthquake model to recorded noise levels. DAS detection capabilities on underwater fibers are found to be similar to those of nearby broadband sensors, and superior to those of on‐land fiber segments, owing to lower velocities at the ocean‐bottom. The results demonstrate the great potential of underwater DAS for seismic monitoring and earthquake early warning. Key Points: The noise content of underwater distributed acoustic sensing (DAS) along three different telecommunication cables is quantified and compared to adjacent broadband stationsEarthquake detection capabilities using DAS are similar to those of broadband instrumentsDetection capabilities are mainly a function of the recorded noise, cable response, and apparent velocity [ABSTRACT FROM AUTHOR]

Unlike the other Himalayan plate boundary segments, the eastern Nepal to Bhutan Himalayan region is not known to have generated prominent shallow thrust faulting earthquakes, typical of the ongoing convergence. This region features strike‐slip earthquakes over the depth ranges of 40–120 km, indicating intraslab deformation. Here we present for the first time a slip distribution model for the largest recorded intraslab strike‐slip earthquake in this region, the Mw 6.9 Sikkim event that occurred on 18 September 2011. Relying on kinematic source process modeling, our results indicate a NE‐SW trending, steeply dipping sinistral source zone within the underthrusting Indian slab. The rupture propagated radially, with a low rupture velocity of 1.7 km/s, breaking a large asperity of 20 × 20 km2 with a maximum slip of 1.6 m. The rupture nucleated at a depth of 45 km and reached upper mantle depths. The computed coseismic stress drop value is 13.6 MPa. We suggest that most of the aftershocks occurred on the conjugate plane, possibly due to stress triggering. Stress inversion of focal mechanisms indicates a transpressive stress regime throughout the crust and pure strike‐slip regime in the upper mantle. We observed a unimodal distribution of earthquakes beneath the Higher Himalaya. This indicates a strong, brittle Indian slab and unravels a scenario of an eventual breakup of the lithosphere; the key trigger might be variation in the convergence rates along the Himalayan arc. Key Points: The 2011 Sikkim earthquake ruptured a NE‐SW plane with maximum slip of 1.6 m with a rupture velocity of 1.7 km/s and stress drop of 13.6 MPaThe state of stress is transpressive in the lower crust and pure strike slip in the upper mantle beneath the eastern Nepal‐Bhutan regionThe unimodal seismicity distribution beneath the Higher Himalaya could be indicative of a strong crust and upper mantle [ABSTRACT FROM AUTHOR]

Abstract: We present a catalog of offshore seismicity generated from Cascadia Initiative (CI) ocean‐bottom seismometer data. Earthquakes were detected within the CI data using a short‐time‐average/long‐time‐average trigger and located using 1‐D velocity models developed from seismic reflection/refraction surveys. The catalog, which contains 271 earthquakes with magnitude 0.4–4.0 along the coasts of Vancouver Island, Washington, Oregon, and Northern California, spans all 4 years of the ocean bottom seismometer deployment and shows distinct along‐strike variations in seismicity consistent with structural observations from recent active source seismic reflection/refraction studies. Seismicity is sparse off Vancouver Island and Washington (49°N–46°N) but increases off northern and central Oregon, corresponding to a roughened, more deformed subducting slab. Widespread earthquakes are observed at near‐interface depths between 46°N and 45°N, though an increase in underthrust sediment thickness between 45°N and 43°N likely restricts seismicity to scattered asperities on the plate interface. South of 43°N, where both the overriding and subducting plates are severely deformed approaching the Mendocino triple junction, seismicity is abundant. We locate an additional 440 events in the Juan de Fuca plate seaward of the deformation front. The higher seismicity south of 46°N is consistent with more extensive intraplate deformation. Along with the complex stress field induced by the Mendocino triple junction, our observations imply that the smoothness and degree of hydration of the incoming plate, which are linked to the amount of underthrust sediment and extent of intraplate deformation, are major contributing factors to the distribution of microseismicity in the Cascadia subduction zone. [ABSTRACT FROM AUTHOR]

We present results for joint kinematic inversion of high-rate GPS, strong motion and InSAR data for the 2016 M7.6 Melinka earthquake. We show that the source is a compact 35 s long rupture with 5.0 ± 0.15 m of peak slip. We find the Melinka earthquake occurs inside the slip region for the 1960 M9.5 Valdivia earthquake and within the highly locked portion of the megathrust inferred from inter-seismic velocity analysis. We show that there is very modest post-seismic deformation at a nearby GPS site QLLN and argue that this indicates the Melinka earthquake ruptures within the intermediate portion of the megathrust and is an isolated asperity surrounded by locked velocity weakening material. Further we find that the peak slip observed during this earthquake is larger than what has been accumulated in the intervening 57 years since the 1960 rupture and conclude that, at least in the area of the Melinka earthquake, this is indicates that the 1960 Valdivia event might not have used all the slip deficit available on the megathrust. [ABSTRACT FROM AUTHOR]

A major difficulty in inverting geodetic data for fault slip distribution is that measurement errors are mapped from the data space onto the solution space. The amplitude of this mapping is sensitive to the condition number of the inverse problem, i.e., the ratio between the largest and smallest singular value of the forward matrix. Thus, unless the problem is well-conditioned, slip inversions cannot reveal the actual fault slip distribution. In this study, we describe a new iterative algorithm that optimizes the condition of the slip inversion through discretization of InSAR data. We present a numerical example that demonstrates the effectiveness of our approach. We show that the condition number of the reconditioned data sets are not only much smaller than those of uniformly spaced data sets with the same dimension but are also much smaller than non-uniformly spaced data sets, with data density that increases towards the model fault. [ABSTRACT FROM AUTHOR]

The existence of earthquakes within continental lithospheric mantle remains a highly controversial topic. Here, we present a detailed set of seismological analyses confirming the occurrence of a mantle earthquake beneath the Wind River Range of central Wyoming. Combining regional waveform inversion with the analysis of the delay and relative amplitudes of teleseismically-observed depth phases, we demonstrate that the 2013 Wind River earthquake – a M W 4.7 highly-oblique thrust-faulting event – occurred at 75 ± 8 km , well beneath the base of the crust. The magnitude, mechanism, and location of this earthquake suggest that it represents simple brittle failure at relatively high temperatures within the mantle lithosphere, as a result of tectonic, rather than magmatic, processes. [ABSTRACT FROM AUTHOR]

Abstract: Tsunami earthquakes generate much larger tsunami than their surface wave magnitude would suggest and are a problem for tsunami warning systems. They are often not accompanied by intense or even strong ground shaking and hence do not provide a natural warning for self-evacuation. The lesser-known 1947 Offshore Poverty Bay and Tolaga Bay earthquakes along the east coast of the North Island, New Zealand share many characteristics with other well-known tsunami earthquakes (including low amplitude shaking, long durations and anomalously large tsunami), however these two New Zealand events are rare in that their source area has been imaged directly by long-offset 2D seismic reflection profiles. In this contribution we propose a source model for the 1947 Offshore Poverty Bay tsunami earthquake, recognising that the hypocentre occurs in a region where seismic reflection and magnetic data support the existence of a shallow (<10 km) subducted seamount updip of an area that experiences slow slip events. We propose a fault source model for the 1947 Offshore Poverty Bay event with two potential slip scenarios: i) uniform slip of 2.6 m across the fault; or ii) variable slip with slip of up to 5–6 m in the region of a more strongly geodetically coupled subducted seamount. Both the uniform and variable slip models require an unusually low rupture velocity of 150–300 m/s in order to model regional and teleseismic seismograms. Tsunami modelling shows that tsunami run-up heights are more than doubled when low rupture speeds of 150–300 m/s are employed, rather than assuming instantaneous rupture. This study suggests that subducted topography can cause the nucleation of up to earthquakes with complex, low velocity rupture scenarios that enhance tsunami waves, and their role in seismic hazard should not be under-estimated. [Copyright &y& Elsevier]

• Earthquakes in cratonic Australia are confirmed to be very shallow (<4 km). • They release large stresses that support large gravity anomalies. • They represent an exfoliation of the elastic layer in response to local loads. • A simple model can relate the earthquake characteristics to the gravity anomalies. The cratonic shield system of central and western Australia, with its lithosphere up to 200 km thick, is geologically similar to other ancient, stable continental interiors. But since 1968 it has experienced a number of moderate-sized (M w 5.0–6.6) earthquakes characterised by the extreme shallowness of their sources (the deepest is 8 km and most are shallower than 4 km). At least 11 of these have produced co-seismic faulting, often very long compared to their depth, with typically no evidence of previous movement on those faults in either the local geomorphology or paleoseismological trenching. Other earthquakes show that cratonic Australia, like other shield regions, has a seismogenic layer about 30–40 km thick, but the intense very shallow seismicity in the region of thickest lithosphere stands out and is unusual. A clue to the origin of these shallow earthquakes lies in their association with some of the largest continental gravity anomalies outside the forelands of young orogenic belts, yet in essentially flat topography. The wavelength of the gravity anomalies (∼240 km) is large compared with the seismogenic thickness (∼30 km) of the lithosphere, and their amplitude is ∼50 mGals. These anomalies need stresses to support them, which can be estimated by a simple model of a flexed elastic plate that reproduces the essential features of the earthquakes, including their focal mechanisms and shallow depth limit. The model shows that the maxima of the compressive stress occur beneath the maxima and minima of the gravity, on the upper and lower boundaries of the layer respectively. Perhaps surprisingly, the magnitude of such stresses is considerably greater than most estimates of the regional stress within plates. The maxima of the shear stress occur on planes with dips of 45°. The locations and mechanisms of the earthquakes show the same features. We conclude that the earthquakes release stored elastic stresses in an exfoliation process, perhaps activated by a reduction in strength through weathering, erosion or some other process. [ABSTRACT FROM AUTHOR]