Recent earthquakes in Chile, 2014, Mw 8.2 Iquique, 2015, Mw 8.3 Illapel and 2016, Mw 7.6 Chiloé have put in evidence some problems with the straightforward application of ideas about seismic gaps, ...earthquake periodicity and the general forecast of large megathrust earthquakes. In northern Chile, before the 2014 Iquique earthquake 4 large earthquakes were reported in written chronicles, 1877, 1786, 1615 and 1543; in North-Central Chile, before the 2015 Illapel event, 3 large earthquakes 1943, 1880, 1730 were reported; and the 2016 Chiloé earthquake occurred in the southern zone of the 1960 Valdivia megathrust rupture, where other large earthquakes occurred in 1575, 1737 and 1837. The periodicity of these events has been proposed as a good long-term forecasting. However, the seismological aspects of historical Chilean earthquakes were inferred mainly from old chronicles written before subduction in Chile was discovered. Here we use the original description of earthquakes to re-analyze the historical archives. Our interpretation shows that a-priori ideas, like seismic gaps and characteristic earthquakes, influenced the estimation of magnitude, location and rupture area of the older Chilean events. On the other hand, the advance in the characterization of the rheological aspects that controlled the contact between Nazca and South-American plate and the study of tsunami effects provide better estimations of the location of historical earthquakes along the seismogenic plate interface. Our re-interpretation of historical earthquakes shows a large diversity of earthquakes types; there is a major difference between giant earthquakes that break the entire plate interface and those of Mw ~ 8.0 that only break a portion of it.
•Recently 4 large earthquakes occurred in Chile, inside active seismic zones.•Mostly of Chilean Eqs Mw ~8.0 break the middle or the bottom of the plate interface.•Chile seismicity is characterized by short time sequences of Eqs and super-cycles.
We observe the nucleation phase of in‐plane ruptures in the laboratory. We show that the nucleation is composed of two distinct phases, a quasi‐static and an acceleration stage, followed by dynamic ...propagation. We propose an empirical model which describes the rupture length evolution: The quasi‐static phase is described by an exponential growth while the acceleration phase is described by an inverse power law of time. The transition from quasi‐static to accelerating rupture is related to the critical nucleation length, which scales inversely with normal stress in accordance with theoretical predictions, and to a critical surfacic power, which may be an intrinsic property of the interface. Finally, we discuss these results in the frame of previous studies and propose a scaling up to natural earthquake dimensions.
Key Points
Nucleation evolves in 2 phases, 1:exponantial, 2:inverse power function of time
Transition occurs at critical length and velocity that scale inversely to stress
A typical time is scaled to earthquakes to get the duration of their nucleation
The Valparaiso 2017 sequence occurred in the Central Chile megathrust, an active zone where the last mega‐earthquake occurred in 1730. Intense seismicity started 2 days before the Mw 6.9 mainshock, a ...slow trenchward movement was observed in the coastal GPS antennas and was accompanied by foreshocks and repeater‐type seismicity. To characterize the rupture process of the mainshock, we perform a dynamic inversion using the strong‐motion records and an elliptical patch approach. We suggest that a slow slip event preceded and triggered the Mw 6.9 earthquake, which ruptured an elliptical asperity (semiaxis of 10 km and 5 km, with a subshear rupture, stress drop of 11.71 MPa, yield stress of 17.21 MPa, slip weakening of 0.65 m, and kappa value of 1.98). This earthquake could be the beginning of a long‐term nucleation phase to a major rupture, within the highly coupled Central Chile zone where a megathrust earthquake like 1730 is expected.
Key Points
Valparaiso 2017 Mw 6.9 was preceded by repeaters and a slow slip event
An intense seismicity was recorded before the mainshock
The mainshock broke a small asperity of few kilometers
We monitor dynamic rupture propagation during laboratory stick‐slip experiments performed on saw‐cut Westerly granite under upper crustal conditions (10–90 MPa). Spectral analysis of high‐frequency ...acoustic waveforms provided evidence that energy radiation is enhanced with stress conditions and rupture velocity. Using acoustic recordings band‐pass filtered to 400–800 kHz (7–14 mm wavelength) and high‐pass filtered above 800 kHz, we back projected high‐frequency energy generated during rupture propagation. Our results show that the high‐frequency radiation originates behind the rupture front during propagation and propagates at a speed close to that obtained by our rupture velocity inversion. From scaling arguments, we suggest that the origin of high‐frequency radiation lies in the fast dynamic stress‐drop in the breakdown zone together with off‐fault coseismic damage propagating behind the rupture tip. The application of the back‐projection method at the laboratory scale provides new ways to locally investigate physical mechanisms that control high‐frequency radiation.
Plain Language Summary
Over geological time scales, partially or fully locked tectonic plates accumulate stress and strain. The stress and the strain build up on discontinuities that we call “faults.” Natural faults exist either inside a tectonic plate or at the boundary between two distinct tectonic plates. When the stress accumulated on a fault exceeds the strength of the fault, the accumulated stress and strain, which can be interpreted in term of accumulated energy, are suddenly released. This natural phenomenon is called an “earthquake.” During an earthquake, part of the energy is released in the form of seismic waves. Those seismic waves are responsible for the ground shaking. High‐frequency waves usually cause most of the damage. To better understand the physical parameters that influence the generation of high‐frequency waves, we experimentally reproduced microearthquakes and used them as a proxy to study real earthquakes. Our results showed that the higher the pressure acting on the fault when an earthquake is generated, the higher the amount of high‐frequency radiations. Moreover, our observations underlined that, during an earthquake, high‐frequency waves are released in specific areas on the fault. Thus, these results might be of relevance to improve seismic hazard assessment.
Key Points
High‐frequency radiation is enhanced with both confining pressure and rupture velocity
Acoustic sensors can be used as an array to track high‐frequency sources during rupture propagation
High‐frequency radiation sources propagate consistently with the rupture front and are located behind it
Fast and reliable characterization of earthquakes can provide vital information to the population, even reducing the effects of strong shaking produced by them. In this study, we explore the minimum ...time required to estimate the magnitude for subduction earthquakes. Using traditional P wave earthquake early warning parameters and considering a progressively increasing time window, we are able to estimate magnitude for subduction earthquakes ~30 s from the origin time (with an average residual of 0.01 ± 0.28). However, estimations for larger events (Mw ≥ 7.5) present larger errors (average residual of −0.70 ± 0.30). We complement our data with Global Navigational Satellite System observations for these events, enabling magnitude estimations ~70 s from the origin time (average residual of −0.42 ± 0.41). We propose that rapid estimations of magnitude should consider, initially, P waves in a progressively increasing time window, and complemented with GNSS data, for large events.
Plain Language Summary
Fast and reliable magnitude estimation of earthquakes enables the preparation of the public to reduce its impact. Here we test known methods to rapidly estimate the magnitude of subduction earthquakes. We found encouraging results, taking a few tens of seconds to provide reliable values. However, results for larger events tend to underestimate the real magnitude. Hence, we propose the combination with other sources of information, such as Global Positioning System, that are able to resolve these larger events.
Key Points
We calibrate coefficients from earthquake early warning methodologies to do a fast estimation of magnitude for subduction earthquakes
These methodologies are able to robustly estimate the magnitude for small‐to‐moderate events (Mw ≤ 7.0) ~30 from origin time
Large events (Mw ≥ 7.5) required data from GNSS to perform the magnitude estimation ~70 s from origin time
The Concepción–Constitución area 35–37°S in South Central Chile is very likely a mature seismic gap, since no large subduction earthquake has occurred there since 1835. Three campaigns of global ...positioning system (GPS) measurements were carried out in this area in 1996, 1999 and 2002. We observed a network of about 40 sites, including two east–west transects ranging from the coastal area to the Argentina border and one north–south profile along the coast. Our measurements are consistent with the Nazca/South America relative angular velocity (55.9°N, 95.2°W, 0.610°/Ma) discussed by Vigny et al. (2008, this issue) which predicts a convergence of 68
mm/year oriented 79°N at the Chilean trench near 36°S. With respect to stable South America, horizontal velocities decrease from 45
mm/year on the coast to 10
mm/year in the Cordillera. Vertical velocities exhibit a coherent pattern with negative values of about 10
mm/year on the coast and slightly positive or near zero in the Central Valley or the Cordillera. Horizontal velocities have formal uncertainties in the range of 1–3
mm/year and vertical velocities around 3–6
mm/year. Surface deformation in this area of South Central Chile is consistent with a fully coupled elastic loading on the subduction interface at depth. The best fit to our data is obtained with a dip of 16
±
3°, a locking depth of 55
±
5
km and a dislocation corresponding to 67
mm/year oriented 78°N. However in the northern area of our network the fit is improved locally by using a lower dip around 13°. Finally a convergence motion of about 68
mm/year represents more than 10
m of displacement accumulated since the last big interplate subduction event in this area over 170 years ago (1835 earthquake described by Darwin). Therefore, in a worst case scenario, the area already has a potential for an earthquake of magnitude as large as 8–8.5, should it happen in the near future.
On 2012 August 11, an earthquake doublet (Mw6.5 and Mw6.3), separated in time by 11 min, occur in the northwest of Iran. The hypocentres of these earthquakes are close (∼6 km) and located near the ...cities of Ahar and Varzaghan. The rupture process of both main shocks is retrieved by inverting the near-field strong motions data and using the elliptical subfault approximation method. Our calculations show that the two earthquakes are occurring on two distinct fault planes: the first main shock (M1) has nucleated at a depth of ∼8.5 km, and is located ∼4 km east of the eastern termination of the E-W trending surface rupture. The slip reaches the ground surface west of the hypocentre on an E-W striking fault (N88°E) that dips almost vertically (80°S). This earthquake exhibits a right-lateral strike-slip mechanism. The entire slip is imaged on a single patch that ruptures with an average speed of 2.4 km s−1. The rupture duration is ∼5.6 s and the earthquake releases a seismic moment of ∼8.41E + 18 N·m. The slip reaches the surface with a right-lateral dislocation value of ∼1 m, which is consistent with the observed surface rupture. About 11 min later, the second main shock (M2) nucleates ∼5 km to the west and 4 km to the north with respect to the hypocentre of the M1, and at a depth of ∼16.5 km. The M2 rupture evolves toward shallower depths and to the west on an ENE-WSW oriented fault plane (strike ∼256°) with a dip of ∼60° northward. The slip is essentially distributed on two distinct patches with strike-slip and reverse mechanisms, respectively. The first patch has a pure right-lateral strike-slip mechanism, and ruptures at a relatively fast speed of over 2.8 km s−1, and last for about 2.6 s until it reaches the second patch. The latter has a reverse mechanism (rake∼112°) and extends the rupture toward shallow depths, and to the west at a speed of ∼2.5 km s−1, and its rupture lasts for ∼2.5 s. The top of the slip distribution of M2 stops at a depth of ∼8 km. We observe that aftershocks surround the M1 and most of the M2 slip models. They are not distributed in the region of high slip (∼3.1 m) of M1. We show that the rupture of M2 is controlled by the static Coulomb stress changes caused by M1, with the maximum slip of M2 located in the positive Coulomb stress caused by M1. The M2 rupture stops where it reaches the area of high negative Coulomb stress change (over −10 bars). The cumulative Coulomb stress fields of both main shocks show a transfer of positive static Coulomb stress change of >0.1 bars on the eastern segment of the North Tabriz Fault. This segment did not rupture since the 1721 M∼7.6–7.7 event that has destroyed the city of Tabriz, and that currently hosts 2 million people. The occurrence of this earthquake doublet with different mechanisms reveals the slip partitioning of the oblique convergence regime of NW Iran on the Ahar–Varzaghan complex fault system
SUMMARY
We study the scaling of spectral properties of a set of 68 aftershocks of the 2007 November 14 Tocopilla (M 7.8) earthquake in northern Chile. These are all subduction events with similar ...reverse faulting focal mechanism that were recorded by a homogenous network of continuously recording strong motion instruments. The seismic moment and the corner frequency are obtained assuming that the aftershocks satisfy an inverse omega‐square spectral decay; radiated energy is computed integrating the square velocity spectrum corrected for attenuation at high frequencies and for the finite bandwidth effect. Using a graphical approach, we test the scaling of seismic spectrum, and the scale invariance of the apparent stress drop with the earthquake size. To test whether the Tocopilla aftershocks scale with a single parameter, we introduce a non‐dimensional number, , that should be constant if earthquakes are self‐similar. For the Tocopilla aftershocks, Cr varies by a factor of 2. More interestingly, Cr for the aftershocks is close to 2, the value that is expected for events that are approximately modelled by a circular crack. Thus, in spite of obvious differences in waveforms, the aftershocks of the Tocopilla earthquake are self‐similar. The main shock is different because its records contain large near‐field waves. Finally, we investigate the scaling of energy release rate, Gc, with the slip. We estimated Gc from our previous estimates of the source parameters, assuming a simple circular crack model. We find that Gc values scale with the slip, and are in good agreement with those found by Abercrombie and Rice for the Northridge aftershocks.
Large earthquakes produce crustal deformation that can be quantified by geodetic measurements, allowing for the determination of the slip distribution on the fault. We used data from Global ...Positioning System (GPS) networks in Central Chile to infer the static deformation and the kinematics of the 2010 moment magnitude (M(w)) 8.8 Maule megathrust earthquake. From elastic modeling, we found a total rupture length of ~500 kilometers where slip (up to 15 meters) concentrated on two main asperities situated on both sides of the epicenter. We found that rupture reached shallow depths, probably extending up to the trench. Resolvable afterslip occurred in regions of low coseismic slip. The low-frequency hypocenter is relocated 40 kilometers southwest of initial estimates. Rupture propagated bilaterally at about 3.1 kilometers per second, with possible but not fully resolved velocity variations.
On 25 December 2016, the Mw 7.6 Chiloé earthquake broke a plate boundary asperity in south central Chile near the center of the rupture zone of the Mw 9.5 Valdivia earthquake of 1960. To gain insight ...on decadal‐scale deformation trends and their relation with the Chiloé earthquake, we combine geodetic, teleseismic, and regional seismological data. GPS velocities increased at continental scale after the 2010 Maule earthquake, probably due to a readjustment in the mantle flow and an apparently abrupt end of the viscoelastic mantle relaxation following the 1960 Valdivia earthquake. It also produced an increase in the degree of plate locking. The Chiloé earthquake occurred within the region of increased locking, breaking a circular patch of ~15 km radius at ~30 km depth, located near the bottom of the seismogenic zone. We propose that the Chiloé earthquake is a first sign of the seismic reawakening of the Valdivia segment, in response to the interaction between postseismic viscoelastic relaxation and changes of interseismic locking between Nazca and South America.
Key Points
The 2016 Chiloe earthquake is a sign of the seismic reactivation of the south central Chile megathrust after the Mw 9.5 earthquake of 1960
South central Chile has been affected by postseismic viscoelastic relaxation and superinterseismic locking after the 2010 Maule earthquake
The Chiloe earthquake broke an ~15 km radius area at ~30 km depth, near the bottom of the seismogenic zone
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