Interplate Earthquakes Research Articles

Ground‐Motion Prediction Model for Vertical Response Spectra from Mexican Interplate Earthquakes

Abstract We present ground‐motion prediction equations (GMPEs) for vertical response spectra from Mexican interplate earthquakes at rock sites (National Earthquake Hazards Reduction Program B class). Two options to obtain the vertical response spectrum are presented: (1) directly from the vertical‐attenuation model and (2) by multiplying the horizontal spectral ordinates by the vertical‐to‐horizontal ratios. The equations were built as functions of magnitude and distance to the fault surface of the earthquake, using 40 interplate earthquakes. We compared our results with previous attenuation models for inslab and interplate earthquakes in Mexico, and other vertical‐attenuation models in the world. The predicted intensities from the model were found to be in broad agreement with models derived with other models worldwide. Not many vertical and/or vertical‐to‐horizontal attenuation models were found in the literature, although the vertical response of structures is used in practice and included in design codes. Nevertheless, it seems to be a trend to pay more attention to this model. There was also found to be a lack of GMPEs for vertical response spectra applicable to subduction interplate earthquakes for a large range of distances and the region under study. Therefore, an attenuation model for interplate earthquakes was developed for distances up to around 400 km and M w from ∼5 to 8. The model could be useful for probabilistic seismic‐hazard assessment and as an aid for code writers.

Seafloor observations indicate spatial separation of coseismic and postseismic slips in the 2011 Tohoku earthquake

Large interplate earthquakes are often followed by postseismic slip that is considered to occur in areas surrounding the coseismic ruptures. Such spatial separation is expected from the difference in frictional and material properties in and around the faults. However, even though the 2011 Tohoku Earthquake ruptured a vast area on the plate interface, the estimation of high-resolution slip is usually difficult because of the lack of seafloor geodetic data. Here using the seafloor and terrestrial geodetic data, we investigated the postseismic slip to examine whether it was spatially separated with the coseismic slip by applying a comprehensive finite-element method model to subtract the viscoelastic components from the observed postseismic displacements. The high-resolution co- and postseismic slip distributions clarified the spatial separation, which also agreed with the activities of interplate and repeating earthquakes. These findings suggest that the conventional frictional property model is valid for the source region of gigantic earthquakes.

Coseismic slip model of offshore moderate interplate earthquakes on March 9, 2011 in Tohoku using tsunami waveforms

We estimated the coseismic slip distribution associated with the Mw 7.2 and 6.5 foreshocks of the 2011 Tohoku-Oki earthquake based on analysis of the tsunami waveform records obtained just above their focal areas. The results show that the main rupture areas of each of the foreshocks do not overlap with each other, and show a distribution that is complementary to the postseismic slip area of the first Mw 7.2 foreshock as well as to the epicenters of smaller earthquakes during foreshock activity. After the second largest foreshock, seismicity increased in the area between the rupture area of the second largest foreshock and the mainshock epicenter, suggesting propagation of aseismic slip towards the mainshock epicenter. The calculated stress drop of the second largest foreshock was smaller than the largest one, implying strength reduction during the postseismic period of the largest foreshock. Based on a comparison of coastal tsunami records, it is suggested that the asperity ruptured in the M 7.0 earthquake in 1981 ruptured again during the largest foreshock in 2011, but it expanded to the updip side of the 1981 rupture area and became larger in magnitude, exemplifying the irregularity of earthquake recurrence in the area.

Where giant earthquakes may come

AbstractGiant earthquakes (Mw≥8.5) usually occur on the boundary between subducting and overriding plates of converging margins, but it is not yet clear which (if any) subduction zones are more prone to produce such a kind of events. Here we show that subduction zones may have different capabilities to produce giant earthquakes. We analyze the frequency‐magnitude distribution of the interplate earthquakes at subduction zones that occurred during 1976–2007 and calculate the propensity (defined as the average annual rate) of giant events for about half of the subduction zones. We find that the b value of interplate earthquakes is significantly different among the subduction zones, and out‐of‐sample giant earthquakes (before 1976 and after 2007) have occurred preferentially in high‐propensity areas. Besides the importance for seismic hazard assessment and risk mitigation, our results seem to indicate that a higher seismicity rate does not necessarily imply a higher likelihood to generate giant earthquakes, and the way in which the stress is released at a subduction interface does not change significantly after the occurrence of such events.

Slab segmentation controls the interplate slip motion in the SW Hellenic subduction: New insight from the 2008Mw 6.8 Methoni interplate earthquake

AbstractWe present an integrated approach of the seismic structure and activity along the offshore SW Hellenic subduction from combined observations of marine and land seismic stations. Our imaging of the slab top topography from teleseismic receiver function analysis at ocean bottom seismometers supports a trenchward continuation of the along‐dip slab faults beneath the Peloponnesus. We further show that their morphostructural control accounts for the backstepping of the thrust contact of the Mediterranean Ridge accretionary wedge over the upper plate. Local seismic activity offshore SW Peloponnesus constrained by ocean bottom seismometer observations reveals a correlation with specific features of the forearc: the Matapan Troughs. We study theMw 6.8 14.02.2008 interplate earthquake offshore SW Peloponnesus and show that its nucleation, rupture zone, and aftershocks sequence are confined to one slab panel between two adjacent along‐dip faults and are thus controlled by not only the offshore slab top segmentation but also the upper plate sea‐bottom morphology.

Structure of the tsunamigenic plate boundary and low-frequency earthquakes in the southern Ryukyu Trench.

It has been recognized that even weakly coupled subduction zones may cause large interplate earthquakes leading to destructive tsunamis. The Ryukyu Trench is one of the best fields to study this phenomenon, since various slow earthquakes and tsunamis have occurred; yet the fault structure and seismic activity there are poorly constrained. Here we present seismological evidence from marine observation for megathrust faults and low-frequency earthquakes (LFEs). On the basis of passive observation we find LFEs occur at 15–18 km depths along the plate interface and their distribution seems to bridge the gap between the shallow tsunamigenic zone and the deep slow slip region. This suggests that the southern Ryukyu Trench is dominated by slow earthquakes at any depths and lacks a typical locked zone. The plate interface is overlaid by a low-velocity wedge and is accompanied by polarity reversals of seismic reflections, indicating fluids exist at various depths along the plate interface.

Seismic Source Parameters of Normal-Faulting Inslab Earthquakes in Central Mexico

We studied 62 normal-faulting inslab earthquakes in the Mexican subduction zone with magnitudes in the range of 3.6 ≤ M w ≤ 7.3 and hypocentral depths of 30 ≤ Z ≤ 108 km. We used different methods to estimate source parameters to observe differences in stress drop, corner frequencies, source dimensions, source duration, energy-to-moment ratio, radiated efficiency, and radiated seismic energy. The behavior of these parameters is derived. We found that normal-faulting inslab events have higher radiated seismic energy, energy-to-moment ratio, and stress drop than interplate earthquakes as expected. This may be explained by the mechanism dependence of radiated seismic energy and apparent stress reported in previous source parameter studies. The energy-to-moment ratio data showed large scatter and no trend with seismic moment. The stress drop showed no trend with seismic moment, but an increment with depth. The radiated seismic efficiencies showed similar values to those obtained from interplate events, but higher than near-trench events. We found that the source duration is independent of the depth. We also derived source scaling relationships for the mentioned parameters. The low level of uncertainties for the seismic source parameters and scaling relationships showed that the obtained parameters are robust. Therefore, reliable source parameter estimation can be carried out using the obtained scaling relationships. We also studied regional stress field of normal-faulting inslab events. Heterogeneity exists in the regional stress field, as indicated by individual stress tensor inversions conducted for two different depth intervals (Z 40 km, respectively). While the maximum stress axis (σ 1) appears to be consistent and stable, the orientations of the intermediate and minimum stresses (σ 2 and σ 3) vary over the depth intervals. The stress inversion results showed that the tensional axes are parallel to the dip direction of the subducted plate. At depths greater than 40 km, the maximum horizontal stresses are oblique to the dip direction following the general trend of the slab geometry.

Moment Magnitude Estimation of Large Earthquakes Based on Long-Period Ground Motion Prediction Equations and Preassumed Fault Models

We proposed a simple method of moment magnitude estimation based on long-period (5–30[Formula: see text]s) ground motion prediction equations (GMPEs). We constructed empirical fault models for different trial magnitudes. The source-to-site distance from the preassumed fault models to seismic stations was estimated. The best magnitude estimate was determined with reference to the minimum residual between the magnitude estimated from the GMPEs and the trial magnitude used to define the fault model. Using this method, the moment magnitudes of six large interplate earthquakes that occurred around the Japan Islands were estimated. The estimated magnitudes obtained in this study are in good agreement with those calculated from the global centroid moment tensor project. The method has potential to be used for rapid moment magnitude estimation of large earthquakes.

Possible scenarios for occurrence of M ~ 7 interplate earthquakes prior to and following the 2011 Tohoku-Oki earthquake based on numerical simulation.

We show possible scenarios for the occurrence of M ~ 7 interplate earthquakes prior to and following the M ~ 9 earthquake along the Japan Trench, such as the 2011 Tohoku-Oki earthquake. One such M ~ 7 earthquake is so-called the Miyagi-ken-Oki earthquake, for which we conducted numerical simulations of earthquake generation cycles by using realistic three-dimensional (3D) geometry of the subducting Pacific Plate. In a number of scenarios, the time interval between the M ~ 9 earthquake and the subsequent Miyagi-ken-Oki earthquake was equal to or shorter than the average recurrence interval during the later stage of the M ~ 9 earthquake cycle. The scenarios successfully reproduced important characteristics such as the recurrence of M ~ 7 earthquakes, coseismic slip distribution, afterslip distribution, the largest foreshock, and the largest aftershock of the 2011 earthquake. Thus, these results suggest that we should prepare for future M ~ 7 earthquakes in the Miyagi-ken-Oki segment even though this segment recently experienced large coseismic slip in 2011.

Spatial distribution of the faulting types of small earthquakes around the 2011 Tohoku‐oki earthquake: A comprehensive search using template events

AbstractWe developed a new method to classify the faulting types of small earthquakes using interevent waveform similarity and applied it to earthquakes in the northeast Japan subduction zone. In the method, we used separate time windows for P and S waves and established a relationship between waveform similarity and differences in focal mechanisms from event pairs whose focal mechanisms we know. Then we applied the relationship to many pairs of such focal‐mechanism‐known events (5607 from the moment tensor catalogue and 3623 events from the interplate repeating earthquakes catalogue) and focal‐mechanism‐unknown events for the period from 1984 to 2013. As a result, 8984 earthquakes were newly classified into interplate (N = 5401), noninterplate thrust (N = 631), normal (N = 1070), and strike‐slip faulting earthquakes (N = 165). From the new data set, which doubles the number of mechanism types, we confirmed that there have been almost no interplate earthquakes in the area of large coseismic slip of the Tohoku‐oki earthquake since that event. We also saw that this trend continued until at least the end of 2013, suggesting a nearly complete stress release and slow interplate stress recovery. The abundant interplate aftershocks also indicate the precise spatial extent of postsesimic slip, which is usually difficult to obtain from land‐based geodetic data. The postseismic slip also suggests stress concentration at the asperities of the 1968 Tokachi‐oki (M7.9) and 1994 Sanriku‐oki (M7.6) earthquakes. The present‐day faulting types offshore Tohoku correlated well with the static‐stress change from the Tohoku‐oki earthquake, suggesting a stress state change during the earthquake cycle of megathrust earthquakes.

Comparison Between Tsunami Signals Generated by Different Source Models and the Observed Data of the Illapel 2015 Earthquake

A major interplate earthquake occurred on September 16th, 2015, near Illapel, central Chile. This event generated a tsunami of moderate height, however, one which caused significant near field damage. In this study, we model the tsunami produced by some rapid and preliminary fault models with the potential to be calculated within tens of minutes of the event origin time. We simulate tsunami signals from two different heterogeneous slip models, a homogeneous source based on parameters from the global CMT Project, and furthermore we used plate coupling data from GPS observations to construct a heterogeneous fault based on a priori knowledge of the subduction zone. We compare the simulated signals with the observed tsunami at tide gauges located along the Chilean coast and at offshore DART buoys. For this event, concerning rapid response, the homogeneous source and coupling model represent the tsunami at least as well as the heterogeneous sources. We suggest that the initial heterogeneous fault models could be better constrained with continuous GPS measurements in the rupture area, and additionally DART records directly in front of the rupture area, to improve the tsunami simulation based on quickly calculated models for near coastal areas. Additionally, in terms of tsunami modeling, the source estimated from prior plate coupling information in this case is representative of the event that later occurs; placing further importance on the need to monitor subduction zones with GPS.

The source model and recurrence interval of Genroku-type Kanto earthquakes estimated from paleo-shoreline data

In the southern Kanto region of Japan, where the Philippine Sea plate is descending at the Sagami trough, two different types of large interplate earthquakes have occurred repeatedly. The 1923 (Taisho) and 1703 (Genroku) Kanto earthquakes characterize the first and second types, respectively. A reliable source model has been obtained for the 1923 event from seismological and geodetical data, but not for the 1703 event because we have only historical records and paleo-shoreline data about it. We developed an inversion method to estimate fault slip distribution of interplate repeating earthquakes from paleo-shoreline data on the idea of crustal deformation cycles associated with subduction-zone earthquakes. By applying the inversion method to the present heights of the Genroku and Holocene marine terraces developed along the coasts of the southern Boso and Miura peninsulas, we estimated the fault slip distribution of the 1703 Genroku earthquake as follows. The source region extends along the Sagami trough from the Miura peninsula to the offing of the southern Boso peninsula, which covers the southern two thirds of the source region of the 1923 Kanto earthquake. The coseismic slip takes the maximum of 20 m at the southern tip of the Boso peninsula, and the moment magnitude (Mw) is calculated as 8.2. From the interseismic slip-deficit rates at the plate interface obtained by GPS data inversion, assuming that the total slip deficit is compensated by coseismic slip, we can roughly estimate the average recurrence interval as 350 years for large interplate events of any type and 1400 years for the Genroku-type events.

New Evaluation Method for Liquefaction of Ground Using Dynamic Liquefaction Analysis Method and Its Application

AbstractNankai megathrust earthquakes are expected to occur with a return period of approximately 90–200 years along the fault of the Nankai Trough. Because they are interplate earthquakes, it is predicted that they will cause heavy liquefaction damage as a result of the long duration of their ground motion, especially around bay areas with soft deposits. In this study, a numerical analysis was performed for the ground in Osaka City with a hypothetical earthquake in the Nankai Trough region. In the numerical simulation of the dynamic behavior, a liquefaction analysis program was used. To quantify the degree of liquefaction at certain points, an index, named the liquefaction risk index (LRI), was defined and obtained by the weighted integration of the effective stress-decreasing ratio (ESDR) with respect to depth. Moreover, the LRI values were evaluated by data from the 1995 Hyogoken-Nanbu earthquake. It was found that the obtained LRI values were greater than 6.0 for the points at which heavy liquefaction...

太平洋プレートの海溝型地震の広帯域震源モデルに基づく微視的断層パラメータの特性

It has been pointed out that large amounts of slip areas for large interplate earthquakes are different from strong motion generation areas SMGAs. It has been also shown that the combined areas of asperities which are defined as large slip areas extracted from slip models by waveform inversion estimated from records in the period longer than about 2 s are larger than the combined areas of SMGAs estimated from strong motion records in the period range of about 0.1 to 10 s. However, the strong motion recipe for interplate earthquakes is established mainly based on asperities of previous large earthquakes. In addition, the characteristics of each SMGA as well as total SMGAs is important to strong motion prediction since SMGAs of large interplate earthquakes are located at spatially separated positions and ruptured with time intervals. Therefore we examine the characteristics of inner fault parameters based on 19 previous broadband source models composed of SMGAs estimated by the empirical or stochastic Green's function method for five interplate and five intraplate earthquakes from MW 6.8 to 9.0 in Japan. Nine earthquakes of the Pacific plate from MW 7.0 to 9.0 out of ten are used for the development of scaling relations. In order to examine the time-dependent source characteristics of each SMGA we define acceleration envelope source time functions of SMGA as AENVSMGA by applying the scaling relation of the rms value of acceleration in the source region by Hanks (1979) and the envelope model by Boore (1983) to element faults of SMGAs. For interplate earthquakes the power coefficient for seismic moment M0 is less than 1/3 for the maximum value of A of SMGAs because the number of SMGAs increases with M0. The rupture starting points of all SMGAs are located at deeper portion than the hypocenter and the ruptures tend to progress to deeper zone. The maximum value of AENVSMGA for each earthquake is 13.5×1010[Nm/s2] on average with the standard deviation of a factor of 1.26 and independent of MW from 7.2 to 9.0. The maximum values of each AENVSMGA are ranging from 4×1010～20×1010[Nm/s2]. These features are qualitatively consistent with the saturation effects of peak ground accelerations to MW pointed out by the attenuation characteristics of interplate earthquakes. The maximum value of AENVSMGA for the first ruptured SMGA tends to be a little larger than those of the other SMGAs located at deeper portion. For intraplate earthquake, the maximum value of AENVSMGA for the first ruptured SMGA tends to be smaller than those of the other SMGAs. We found that the combined area of SMGAs for both interplate and intraplate earthquakes can be modeled by the same scaling relation to M02/3. The combined area of SMGAs is 0.20 times of combined area of asperities by Murotani et al. (2013) for interplate earthquakes and 0.54 times of those by Iwata and Asano (2011) for intraplate earthquakes. The difference between the combined areas of SMGAs and asperities for subduction-zone earthquakes is larger than crustal earthquakes, that is, 0.8 times (Satoh, 2010). The duration Td from the origin time to the end of the last ruptured SMGA for both interplate and intraplate earthquakes is also modeled by the same scaling relation to M01/3 and the centroid depth of SMGAs. The duration Td decreases with the centroid depth. The ratio of combined duration of SMGAs to Td is larger than one for intraplate earthquakes and less than one for interplate earthquakes. This result means that SMGAs for interplate earthquakes rupture with longer time intervals than intraplate earthquakes.

Evaluation of strain accumulation in global subduction zones from seismicity data

We compared the cumulative seismic slip of interplate earthquakes (≥M5.5) with relative plate motion at subduction zones. By assuming that each interplate earthquake occurred on a stick–slip patch, we used the slip history of each patch to calculate the interplate slip in the surrounding area. We considered that areas in which interplate earthquakes occurred but that had small cumulative slips compared with relative plate motion were accumulating strain, and we calculated the size of these areas. We first used this method to test the rupture areas of six M9-class interplate earthquakes that have occurred during the past 100 years. The cumulative seismic slip preceding and following the six earthquakes was smaller than the relative plate motion in the rupture areas of the earthquakes. We interpret the areas of slip-deficient stick–slip patches to be the rupture areas of future huge earthquakes. We applied the same procedure to global subduction zones and found that slip-deficient stick–slip patches with large spatial extents (equivalent to the rupture area of M9-class earthquakes) occur in an additional 25 locations. Considering that six M9-class earthquakes have occurred in the past 110 years and that the recurrence interval in each case is probably between a few hundred and a thousand years, it is not surprising that 25 regions globally are capable of producing M9-class earthquakes. These regions may be the most likely candidates for the rupture areas of future M9-class interplate earthquakes.

Geological and historical evidence of irregular recurrent earthquakes in Japan.

Great (M∼8) earthquakes repeatedly occur along the subduction zones around Japan and cause fault slip of a few to several metres releasing strains accumulated from decades to centuries of plate motions. Assuming a simple 'characteristic earthquake' model that similar earthquakes repeat at regular intervals, probabilities of future earthquake occurrence have been calculated by a government committee. However, recent studies on past earthquakes including geological traces from giant (M∼9) earthquakes indicate a variety of size and recurrence interval of interplate earthquakes. Along the Kuril Trench off Hokkaido, limited historical records indicate that average recurrence interval of great earthquakes is approximately 100 years, but the tsunami deposits show that giant earthquakes occurred at a much longer interval of approximately 400 years. Along the Japan Trench off northern Honshu, recurrence of giant earthquakes similar to the 2011 Tohoku earthquake with an interval of approximately 600 years is inferred from historical records and tsunami deposits. Along the Sagami Trough near Tokyo, two types of Kanto earthquakes with recurrence interval of a few hundred years and a few thousand years had been recognized, but studies show that the recent three Kanto earthquakes had different source extents. Along the Nankai Trough off western Japan, recurrence of great earthquakes with an interval of approximately 100 years has been identified from historical literature, but tsunami deposits indicate that the sizes of the recurrent earthquakes are variable. Such variability makes it difficult to apply a simple 'characteristic earthquake' model for the long-term forecast, and several attempts such as use of geological data for the evaluation of future earthquake probabilities or the estimation of maximum earthquake size in each subduction zone are being conducted by government committees.

Tsunami potential from local seismic sources along the southern Middle America Trench

The Middle America convergent margin is capable of generating tsunamigenic earthquakes as portrayed through historical data. However, historical tsunamigenic sources appear to be unevenly distributed along the Middle America convergent margin. The last significant tsunami generated along the southern segments of this convergent margin occurred in 1992 causing 10-m run-up in Nicaragua. In particular, the Costa Rican Pacific coast has not clear evidences of significant wave heights or local tsunami events. Partly this could be explained by the lack of historical data since paleotsunami information is not available; thus, no tsunami data beyond 500 years could be found. Nevertheless, large rupture areas or shallow ruptures offshore Costa Rica cannot be neglected. We hereby present the results of numerical simulations aimed to assess the tsunamigenic potential of seismic sources offshore southern Central America. Our approach follows the deterministic method where earthquake maximum credible scenarios are assessed. We assumed hundreds of fault ruptures to obtain surface displacement and tsunami wave heights at the shoreline to assess the tsunami threat along south Nicaragua and Costa Rica. These fault scenarios are based on historical earthquakes as well as on the newest geodetic studies that provided slip deficit. Shallow part of locked segments in this region could release accumulated stress in the near future and may generate tsunamis similar to the 1992 Nicaragua M w 7.6 and 2012 El Salvador M w 7.4 tsunami earthquakes. The numerical simulations presented in this study show that maximum wave heights up to 2–6 m could affect the southern Pacific coast of Central America even when moderate-to-large M w 7.0–8.0 interplate or outer rise earthquakes occur.

Seismic hazard estimation: directly using observations versus applying seismic hazard model

Seismic hazard evaluation for a site can be carried out based on direct observations such as ground motion records at the site or based on an adopted seismic hazard model, including the distribution of earthquake intensity, the spatio-temporal occurrence model of earthquakes in a region, and the ground motion prediction equations. The former is adequate if sufficient records at the site are available, while the latter is capable to incorporate information from similar tectonic regions. This study compares the seismic hazard estimated using these two approaches for a recording site in Mexico City. The comparison is carried out in terms of uniform hazard spectra by considering interplate earthquakes alone, inslab earthquakes alone, and both interplate and inslab earthquakes. Up to date ground motion records obtained at the site are employed for the analysis. The results showed quantitatively the differences between the uniform hazard spectrum (UHS) obtained by the two approaches and the differences between the estimated UHS and the design spectrum, which is important for seismic risk assessment and design code making. The results are also used to identify the differences between the standard deviation of the residuals for GMPEs developed using records from a single station and that obtained considering a broad network of stations.

Systematic Changes of Earthquake Rupture with Depth: A Case Study from the 2010Mw 8.8 Maule, Chile, Earthquake Aftershock Sequence

The very shallow part of subduction megathrusts occasionally hosts tsunami earthquakes, with unusually slow rupture propagation. The aftershock sequence of the 2010 Mw 8.8 Maule earthquake, offshore Chile, provides us with the opportunity to study systematic changes in source properties for smaller earthquakes within a single segment of a subduction zone. We invert amplitude spectra for double-couple moment tensors and centroid depths of 71 aftershocks of the Maule earthquake down to magnitudes Mw 4.0. In addition, we also derive average source durations.We find that shallower earthquakes tend to have longer normalized source durations on average, similar to the pattern observed previously for larger magnitude events. This depth dependence is observable for thrust and normal earthquakes. The normalized source durations of normal- faulting earthquakes are at the lower end of those for thrust earthquakes, probably because of the higher stress drops of intraplate earthquakes compared to interplate earthquakes. We suggest from the similarity of the depth dependence of normal and thrust events and between smaller and larger magnitude earthquakes that the depth-dependent variation of rigidity, rather than frictional conditional stability at the plate interface, is primarily responsible for the observed pattern. Tsunami earthquakes probably require both low rigidity and conditionally stable frictional conditions; the presence of longduration moderate-magnitude events is therefore a helpful but not sufficient indicator for areas at risk of tsunami earthquakes.

Mw dependence of the preseismic ionospheric electron enhancements

AbstractIonospheric electron enhancement was reported to have occurred ~40 min before the 2011 Tohoku‐oki (Mw9.0) earthquake, Japan, by observing total electron content (TEC) with Global Navigation Satellite Systems receivers. Their reality has been repeatedly questioned due mainly to the ambiguity in the derivation of the reference TEC curves from which anomalies are defined. Here we propose a numerical approach, based on Akaike's information criterion, to detect positive breaks (sudden increase of TEC rate) in the vertical TEC time series without using reference curves. We demonstrate that such breaks are detected 25–80 min before the eight recent large earthquakes with moment magnitudes (Mw) of 8.2–9.2. The amounts of precursory rate changes were found to depend upon background TEC as well as Mw. The precursor times also showed Mw dependence, and the precursors of intraplate earthquakes tend to start earlier than interplate earthquakes. We also performed the same analyses during periods without earthquakes to evaluate the usefulness of TEC observations for short‐term earthquake prediction.