Northward Rupture Research Articles

GNSS-Constrained Rupture Kinematics of the 2022 Mw 6.7 Luding, China, Earthquake: Directivity Pulse during the Asymmetrical Bilateral Rupture

Abstract Impulse motion characterized by a large amplitude in the fault-normal direction can be observed at near-fault strong motion sites during strike-slip earthquakes. The large pulse, which always causes high intensity and stronger damage to structures close to faults, is usually attributed to the directivity effect of rupture propagating along strike and the proximity to the fault. We present an analysis of such a large directivity pulse captured by the near-fault high-rate Global Navigation Satellite System (GNSS) during the 2022 Mw 6.7 Luding, China, earthquake—the largest event ever observed by space geodesy on the seismically active Xianshuihe fault in the eastern Tibetan Plateau. We invert the displacement waveforms and offsets derived from the continuous and campaign GNSS for the rupture kinematics. The inferred slip model reveals a rupture zone of 30 km in length above 15 km depth along the Moxi segment, yielding a seismic moment of 1.1×1019 N·m and a source duration of 13 s. The high-rate GNSS (hrGNSS) waveforms suggest an asymmetric bilateral rupture: most slips with long rise time are concentrated on the southern part of the ruptured fault, whereas a short-duration pulse-like slip rate with low final slip propagates during the northward rupture. We found that the directivity pulse observed by the nearest hrGNSS site is controlled primarily by the sharp pulse-like slip rate and rapid rupture velocity approximating the local S-wave velocity. Along with additional local amplification, this large directivity pulse may be responsible for the heavy damage in Moxi town close to the northern ruptured fault.

Strong ground motion over a large area in northern Taiwan caused by the northward rupture directivity of the 2019 Hualien earthquake

An Mw 6.2 earthquake struck Hualien on the eastern coast of Taiwan on April 18, 2019. Its focal mechanism was determined to be a thrust fault with a depth of 18.8 km. A wide range of strong ground shaking with an intensity value from 4 (25–80 Gal) to 7 (greater than 400 Gal) occurred in northern Taiwan, which is unusual for earthquakes of this size. Here we show a source model as determined by local seismic waveform inversion. Two asperities were found, the smaller one occurred at the hypocenter and the larger one was found 10 km north of the hypocenter with a depth of 18–25 km. The initial rupture started from the hypocenter and then rapidly propagated all the way to the large asperity in the north. A near supershear speed of close to 4.0 km/s was found to have occurred during this northward rupture process. The inverted source model was evaluated using forward three-dimensional ground motion simulations. The results show strong agreement with island-wide observed displacement waveforms and peak ground acceleration for periods from 3.0 to 20.0 s. The rapid northward rupture caused a strong directivity effect coupled with the specific source radiation pattern, resulting in a large area of strong ground shaking in northern Taiwan, even though the Hualien earthquake was a moderately-sized event.

Seismological and Hydrogeological Controls on New Zealand-Wide Groundwater Level Changes Induced by the 2016 Mw7.8 Kaikōura Earthquake

The 2016 Mw7.8 Kaikōura earthquake induced groundwater level changes throughout New Zealand. Water level changes were recorded at 433 sites in compositionally diverse, young, shallow aquifers, at distances of between 4 and 850 km from the earthquake epicentre. Water level changes are inconsistent with static stress changes but do correlate with peak ground acceleration (PGA). At PGAs exceeding ~2 m/s2, water level changes were predominantly persistent increases. At lower PGAs, there were approximately equal numbers of persistent water level increases and decreases. Shear-induced consolidation is interpreted to be the predominant mechanism causing groundwater changes at accelerations exceeding ~2 m/s2, whereas permeability enhancement is interpreted to predominate at lower levels of ground acceleration. Water level changes occur more frequently north of the epicentre, as a result of the fault’s northward rupture and resulting directivity effects. Local hydrogeological conditions also contributed to the observed responses, with larger water level changes occurring in deeper wells and in well-consolidated rocks at equivalent PGA levels.

Recurrent slow slip events as a barrier to the northward rupture propagation of the 2016 Pedernales earthquake (Central Ecuador)

The northern Ecuador segment of the Nazca/South America subduction zone shows spatially heterogeneous interseismic coupling. Two highly coupled zones (0.4° S–0.35° N and 0.8° N–4.0° N) are separated by a low coupled area, hereafter referred to as the Punta Galera-Mompiche Zone (PGMZ). Large interplate earthquakes repeatedly occurred within the coupled zones in 1958 (Mw 7.7) and 1979 (Mw 8.1) for the northern patch and in 1942 (Mw 7.8) and 2016 (Mw 7.8) for the southern patch, while the whole segment is thought to have rupture during the 1906 Mw 8.4–8.8 great earthquake. We find that during the last decade, the PGMZ has experienced regular and frequent seismic swarms. For the best documented sequence (December 2013–January 2014), a joint seismological and geodetic analysis reveals a six-week-long Slow Slip Event (SSE) associated with a seismic swarm. During this period, the microseismicity is organized into families of similar earthquakes spatially and temporally correlated with the evolution of the aseismic slip. The moment release (3.4×1018Nm, Mw 6.3), over a ~60×40km area, is considerably larger than the moment released by earthquakes (5.8×1015Nm, Mw 4.4) during the same time period. In 2007–2008, a similar seismic-aseismic episode occurred, with higher magnitudes both for the seismic and aseismic processes. Cross-correlation analyses of the seismic waveforms over a 15years-long period further suggest a 2-year repeat time for seismic swarms, which also implies that SSEs recurrently affect this area. Such SSEs contribute to release the accumulated stress, likely explaining why the 2016 Pedernales earthquake did not propagate northward into the PGMZ.

Rupture features of the 2010 Mw 8.8 Chile earthquake extracted from surface waves

This study used the rupture directivity theory to derive the fault parameters of the 2010 Mw 8.8 Chile earthquake on the basis of the azimuth-dependent source duration obtained from the Rayleigh-wave phase velocity. Results revealed that the 2010 Chile earthquake featured asymmetric bilateral faulting. The two rupture directions were N171°E (northward) and N17°E (southward), with rupture lengths of approximately 313 and 118 km, respectively, and were related to the locking degree in the source region. The entire source duration was approximately 187 s. After excluding the rise time from the source duration, the northward rupture velocity was approximately 2.02 km/s, faster than the southward rupture velocity (1.74 km/s). On average, the rupture velocity derived from this study was slower than that estimated from finite-fault inversion; however, several historical earthquakes in the Chile region also showed slow rupture velocity when using low-frequency signals, as surface waves do. Two earlier studies through global-positioning-system data analysis showed that the static stress drop of 50–70 bars for the 2010 Chile earthquake was higher than that for subduction-zone earthquakes. Hence, a remarkable feature was that the 2010 Chile earthquake had a slow rupture velocity and a high static stress drop, which suggested an inverse relationship between rupture velocity and static stress drop.Graphical abstract.

Rapidly Estimated Seismic Source Parameters for the 16 September 2015 Illapel, Chile M w 8.3 Earthquake

On 16 September 2015, a great (M w 8.3) interplate thrust earthquake ruptured offshore Illapel, Chile, producing a 4.7-m local tsunami. The last major rupture in the region was a 1943M S 7.9 event. Seismic methods for rapidly characterizing the source process, of value for tsunami warning, were applied. The source moment tensor could be obtained robustly by W-phase inversion both within minutes (Chilean researchers had a good solution using regional data within 5 min) and within an hour using broadband seismic data. Short-period teleseismic P wave back-projections indicate northward rupture expansion from the hypocenter at a modest rupture expansion velocity of 1.5–2.0 km/s. Finite-fault inversions of teleseismic P and SH waves using that range of rupture velocities and a range of dips from 16°, consistent with the local slab geometry and some moment tensor solutions, to 22°, consistent with long-period moment tensor inversions, indicate a 180- to 240-km bilateral along-strike rupture zone with larger slip northwest to north of the epicenter (with peak slip of 7–10 m). Using a shallower fault model dip shifts slip seaward toward the trench, while a steeper dip moves it closer to the coastline. Slip separates into two patches as assumed rupture velocity increases. In all cases, localized ~5 m slip extends down-dip below the coast north of the epicenter. The seismic moment estimates for the range of faulting parameters considered vary from 3.7 × 1021Nm(dip 16°) to 2.7 × 1021 Nm (dip 22°), the static stress drop estimates range from 2.6 to 3.5 MPa, and the radiated seismic energy, up to 1 Hz, is about 2.2–3.15 × 1016 J.

Numerical earthquake model of the 31 October 2013 Ruisui, Taiwan, earthquake: Source rupture process and seismic wave propagation

We build a numerical earthquake model, including numerical source and wave propagation models, to understand the rupture process and the ground motion time history of the 2013 ML 6.4 Ruisui earthquake in Taiwan. This moderately large event was located in the Longitudinal Valley, a suture zone of the Philippine Sea Plate and the Eurasia Plate. A joint source inversion analysis by using teleseismic body wave, GPS coseismic displacement and near field ground motion data was performed first. The inversion results derived from a western dipping fault plane indicate that the slip occurred in depths between 10 and 20km. The rupture propagated from south to north and two asperities were resolved. The largest one was located approximately 15km north of the epicenter with a maximum slip about 1m. A 3D seismic wave propagation simulation based on the spectral-element method was then carried out by using the inverted source model. A strong rupture directivity effect in the northern area of the Longitudinal Valley was found, which was due to the northward rupture process. Forward synthetic waveforms could explain most of the near-field ground motion data for frequencies between 0.05 and 0.2Hz. This numerical earthquake model not only helps us confirm the detailed rupture processes on the Central Range Fault but also gives contribution to regional seismic hazard mitigation for future large earthquakes.

Coseismic displacement, bilateral rupture, and structural characteristics at the southern end of the 1999 Chi-Chi earthquake rupture, central Taiwan

[1] The 1999 Chi-Chi earthquake (Mw 7.6) was due to slip along the Chelungpu fault: a major N striking thrust fault in the fold-thrust belt of western Taiwan. The surface rupture extends over 100 km in a N-S trend with NW trending horizontal displacement increasing from 2 to 10 m from south to north. The central segment of the Chelungpu fault is characterized by bedding slip along the Pliocene Chinshui Shale, which has resulted in the development of a monoclinal structure on the hanging wall. At the southern end of the Chi-Chi earthquake rupture, however, the Chelungpu fault connects with the NE striking Tachienshan fault and cuts into Miocene strata with complex structures on the hanging wall. In this study, we use digital cadastral data to calculate coseismic horizontal displacement around the Chushan area. Results show the amount and direction of horizontal displacement to be ca. 1.2 to 2.6 m and ca. 220° to 260°, respectively, on the hanging wall and ca. 1.4 m and 105°, respectively, on the footwall. Coseismic displacement and seismological data pertaining to the hanging wall at the southern end of the Chi-Chi earthquake rupture indicate the existence of three distinct blocks. Horizontal displacement indicates the Chushan block's movement trended SW. This result reflects the Chushiang and Luliao faults being reactivated with different faulting mechanisms and slip azimuths. This study considers a complex coseismic displacement field, resulting from a bilateral rupture process whereby a southward rupture with SW trending movement results in right-lateral strike-slip faulting of the Tachienshan fault and thrusting with a right-lateral component on the NS strike of the Chelungpu fault. The northward rupture is associated with NW trending movement that results in activation of the Luliao and Chushiang faults.

Estimation of the rupture velocity and fault length of the 2004 Sumatra–Andaman earthquake using a dense broadband seismic array in Taiwan

The rupture process of the disastrous Sumatra–Andaman earthquake of 26 December 2004 was analyzed by array processes for teleseismic P-waves recorded by a dense broadband seismic array in Taiwan with epicentral distances of close to 31°. The azimuthal variation from the BATS array center to both ends of the rupture fault is approximately 21°, which is larger than that reported previously for seismic arrays used to image the rupture process of this earthquake, thereby providing a high spatial resolution in studying the source rupture behavior. Two array-processing methods were used to analyze teleseismic P-wave trains. Both analyses were based on data recorded by a broadband network, covering a region of 200 × 400 km, with the aim of evaluating the rupture behavior of the earthquake. Consistent results from both analyses indicate that the earthquake had a rupture duration exceeding 500 s, with major asperities encountered at 80, 260, and 330 s after the initiation of rupturing. We traced the ruptured fault for more than 1200 km from the point of initial rupture. The average rupture velocity was approximately 3.0 km/s and the major northward rupture propagation began at 80 s after the initiation of rupturing.

Strong ground motion simulation of the 1999 Chi‐Chi, Taiwan earthquake from a realistic three‐dimensional source and crustal structure

We simulate the strong ground motion of 1999 Chi‐Chi, Taiwan earthquake (Mw = 7.6) by considering a three‐dimensional source rupture model in a full waveform three‐dimensional wave propagation study. The strong ground motion records during the 1999 Chi‐Chi earthquake show various characteristics at different sites in Taiwan. We adopt a three‐dimensional source model derived from an inversion study with identical path effects as considered in this three‐dimensional forward study. Comparisons between the simulation results and observed waveforms from dense island‐wide strong motion stations demonstrate that the fault geometry, lateral velocity variation, and complex source rupture process greatly influence the distribution of strong ground shaking. The simulation has reproduced the heavy damage area that is mainly concentrated in the hanging wall, especially close to the surface break of the Chelungpu fault. The source directivity effect is also reproduced and shows serious shaking along the northward rupture direction. Low‐velocity material in the shallow part of the Western Plain is found to generate significantly amplified ground motions. In the Central Range, the shaking is relatively weak owing to the energy radiation characteristics of a low‐angle thrust of the Chelungpu faulting system. The wavefield is then amplified by a high‐velocity gradient under the Coastal Range. Our simulation results in the frequency range of 0.01–0.5 Hz give good agreement with the extensive strong motion observations of the Chi‐Chi earthquake. We find that adequate source representation, good three‐dimensional crustal velocity structures, and careful numerical work are necessary to make the ground motion prediction feasible.

Strong shaking in Los Angeles expected from southern San Andreas earthquake

The southernmost San Andreas fault has a high probability of rupturing in a large (greater than magnitude 7.5) earthquake sometime during the next few decades. New simulations show that the chain of sedimentary basins between San Bernardino and downtown Los Angeles form an effective waveguide that channels Love waves along the southern edge of the San Bernardino and San Gabriel Mountains. Earthquake scenarios with northward rupture, in which the guided wave is efficiently excited, produce unusually high long‐period ground motions over much of the greater Los Angeles region, including intense, localized amplitude modulations arising from variations in waveguide cross‐section.

Primary Surface Rupture Associated with the Mw 7.1 16 October 1999 Hector Mine Earthquake, San Bernardino County, California

The M w 7.1 Hector Mine earthquake occurred within the Mojave Desert portion of the eastern California shear zone and was accompanied by 48 km of dextral surface rupture. Complex northward rupture began on two branches of the Lavic Lake fault in the northern Bullion Mountains and also propagated southward onto the Bullion fault. Lesser amounts of rupture occurred across two right steps to the south. Surface rupture was mapped using postearthquake, 1:10,000-scale aerial photography. Field mapping provided additional detail and more than 400 fault-rupture observations; of these, approximately 300 measurements were used to characterize the slip distribution. En echelon surface rupture predominated in areas of thick alluvium, whereas in the bedrock areas, rupture was more continuous and focused within a narrower zone. Measured dextral offsets were relatively symmetrical about the epicentral region, with a maximum displacement of 5.25 ± 0.85 m.[*][1] Vertical slip was a secondary component and was variable, with minor west-side-down displacements predominating in the Bullion Mountains. Field and aerial photographic evidence indicates that most of the faults that ruptured in 1999 had had prior late-Quaternary displacement, although only limited sections of the rupture show evidence for prior Holocene displacement. [1]: #fn-1

Rupture history of the Great Bolivian Earthquake: Slab interaction with the 660‐km discontinuity?

Teleseismic body waves of the great Bolivian earthquake of June 9, 1994 are analyzed to determine the focal parameters and rupture history. Broadband seismograms reveal a complex rupture process: A small initial event (Mw 7.2) was followed about 10 s later by a large moment release pulse of about 40 s duration. Focal mechanisms determined for the mainshock indicate normal faulting with one very shallow NE dipping plane. Azimuthal variation in body‐wave displacement pulse widths suggest northward rupture. From master event and body‐wave inversion, the main moment release is located 25–50 km NE of the initiation point at about 650‐km depth with only small depth variations between the initial and main event. This suggests that rupture was N directed on the near‐horizontal plane. Because the slab along other parts of the Andean arc at about 600‐km depth dips steeply, a sub‐horizontal plane may imply shearing perpendicular to slab dip. Downdip compression on a sub‐horizontal plane would then imply that the slab does not penetrate the 660‐km seismic discontinuity, but rather, is being sheared out to the NE. This interpretation is not unique as other scenarios are also possible. Such an event immediately above the 660‐km discontinuity suggests massive deformation above the discontinuity with no smooth slab penetration into the lower mantle.

Coseismic fault slip associated with the 1992 Mw 6.1 Joshua Tree, California, earthquake: Implications for the Joshua Tree‐Landers earthquake sequence

Coseismic surface deformation associated with the Mw 6.1, April 23, 1992, Joshua Tree earthquake is well represented by estimates of geodetic monument displacements at 20 locations independently derived from Global Positioning System and trilateration measurements. The rms signal to noise ratio for these inferred displacements is 1.8 with near‐fault displacement estimates exceeding 40 mm. In order to determine the long‐wavelength distribution of slip over the plane of rupture, a Tikhonov regularization operator is applied to these estimates which minimizes stress variability subject to purely right‐lateral slip and zero surface slip constraints. The resulting slip distribution yields a geodetic moment estimate of 1.7 × 1018 N m with corresponding maximum slip around 0.8 m and compares well with independent and complementary information including seismic moment and source time function estimates and main shock and aftershock locations. From empirical Green's function analyses, a rupture duration of 5 s is obtained which implies a rupture radius of 6–8 km. Most of the inferred slip lies to the north of the hypocenter, consistent with northward rupture propagation. Stress drop estimates are in the range of 2–4 MPa. In addition, predicted Coulomb stress increases correlate remarkably well with the distribution of aftershock hypocenters; most of the aftershocks occur in areas for which the mainshock rupture produced stress increases larger than about 0.1 MPa. In contrast, predicted stress changes are near zero at the hypocenter of the Mw 7.3, June 28, 1992, Landers earthquake which nucleated about 20 km beyond the northernmost edge of the Joshua Tree rupture. Based on aftershock migrations and the predicted static stress field, we speculate that redistribution of Joshua Tree‐induced stress perturbations played a role in the spatio‐temporal development of the earthquake sequence culminating in the Landers event.

A new look back at the 1969 Santa Rosa, California, earthquakes

On 1 October 1969, two damaging earthquakes of Richter magnitude ( ML ) 5.6 and 5.7 occurred near the city of Santa Rosa followed by at least 200 aftershocks. These are the two largest events to occur in the northern San Francisco Bay area since the great 1906 San Francisco earthquake and its aftershocks. The epicentral area coincides with a 1-km right step-over from the northwest-striking Rodgers Creek fault to the Healdsburg fault zone and a 6-km right step-over to the Maacama fault. The 1969 sequence and its relationship to the Healdsburg and Rodgers Creek faults has taken on greater significance because of the potential for a moment magnitude ( Mw ) 7 earthquake occurring on the latter, possibly in the next several decades. We have relocated the five largest events in the 1969 sequence using the largest aftershock recorded by a temporary network operated by the U.S. Geological Survey (USGS) as the master event. The locations and focal mechanisms of the principal events, in addition to many of the aftershocks, are consistent with right-lateral strike-slip faulting on the northwest-striking Healdsburg fault. An interesting feature of the aftershocks is that they exhibit a cruciform pattern suggesting possible activity on a northeast-striking cross-fault located within the step-over. The significance of such a cross structure is that it may act as a barrier to northward rupture from a large earthquake on the Rodgers Creek fault. Seismicity since 1969 continues to image an active southern Healdsburg fault surrounding a quiescent rupture zone of the 1969 earthquakes and the possible cross-fault. The Rodgers Creek fault is generally aseismic during this period.