Wenchuan Mainshock Research Articles

Analysis of seismic strain release related to the tidal stress preceding the 2008 Wenchuan earthquake

Abstract. Tidal stresses could load or unload focal media and trigger small to moderate earthquakes in and around the focal region before a large or great earthquake. Based on the preliminary reference Earth model, we calculated the time series of tidal Coulomb failure stress (TCFS) acting on the focal fault plane of the Wenchuan earthquake. For earthquakes (2.5≤ML≤4.0) that occurred in and around the focal region from January 1990 to April 2008, we calculated the time rate of TCFS, ΔTCFS, at the occurrence time of each earthquake. These earthquakes were divided into two categories on the basis of the signs of ΔTCFS: one is positive earthquakes (PEQs) occurring at times of ΔTCFS > 0 and the other negative earthquakes (NEQs) occurring at times of ΔTCFS < 0. First, we obtained cumulative seismic strain release (CSSR) curves of NEQs and PEQs and found that the two curves nearly overlapped prior to September 2004 and then began to separate increasingly with time. We used a parameter Rp, the proportion of seismic strain release of PEQs, to investigate the effect of TCFS on earthquake occurrence and found that Rp was significantly higher than 0.5 about 6 months before the Wenchuan event at a 99 % confidence level, indicating a significant correlation between earthquake occurrence and increasing TCFS. Furthermore, we calculated the slope k (time rate) of the CSSR curve vs. time for PEQs and NEQs separately. It was observed in the pre-event period that the seismic strain release accelerated when TCFS increased but decelerated when TCFS decreased. The difference in the time rate of seismic strain release between PEQs and NEQs was quantified using Rk, the ratio of k for PEQs to that for NEQs. We discovered stable Rk values (around 1.0) until they began to rise rapidly with time in early 2005, reaching their highest value of 2.7 just before the Wenchuan event. Rk could reveal the promoting and inhibiting effects of tidal stress on seismic strain release. When kp increases alone or kn decreases alone, Rk will increase. Thus, an increase in Rk corresponds to a promoting effect during times of increasing TCFS and an inhibiting one during that of decreasing TCFS. Both effects were observed in the focal region prior to the Wenchuan mainshock. The b value in the Gutenberg–Richter relationship decreases as the tectonic stress in the crust increases. We also calculated the temporal evolution of the b value in the study region. It was observed that after 2.5 years of increasing tectonic stress, the focal region became unstable, and the tidal stress began to take effect. The effects of the tidal stress were gradually enhanced as the tectonic stress increased further. The increase in the tidal Coulomb failure stress may have aided the occurrence of earthquakes, whereas the decrease had the opposite effect. This observation may shed light on the seismogenic processes that led to the Wenchuan earthquake and its precursors.

Temporal variations in coda attenuation associated with the 2008 Wenchuan (MW7.9) Earthquake in SW, China

In this study, waveform data obtained from Western Sichuan Seismic Array (WSSA) in China was utilized to reveal the temporal variations in coda attenuation around the eastern Tibet Plateau and Western Sichuan. Based on the single-scattering model, coda attenuation factor Qc−1 is calculated in narrower overlapping frequency bands at 1.0–24 Hz by measuring the coda decay rates for local earthquakes before and after the Wenchuan mainshock. The temporal variations in coda attenuation are investigated within three periods. The period I lasted from January 2007 to the end of 2007. The period II lasted from January 2008 till Wenchuan earthquake. The period III lasted from the mainshock to the end of 2008. The resulted temporal variations demonstrate an increase in average Qc−1 by approximately 35%–45% in the vicinity of Longmenshan and 30%–35% in Sichuan Basin after the Wenchuan earthquake in lower frequency bands within 1.25–8 Hz. On the contrary, the average Qc−1 is shown to decrease by approximately 10%–18% in southern segment of Longmenshan, 15%–38% in the Chuan-Dian block, and 10–12% in the South China block. These results are confirmed by a statistical t-test at 99.9% confidence level. No statistically significant change in Qc−1 (<10%) is found in the Songpan-Ganzi block after the mainshock. Temporal variations of coda attenuation differ significantly in individual blocks after the mainshock. The coda attenuation is proposed to be a beacon to tectonic static stress changes associated with the Wenchuan earthquake.

Easy detection for the high-pass filter cut-off frequency of digital ground motion record based on STA/LTA method: a case study in the 2008 Wenchuan mainshock

Easy detection for the high-pass filter cut-off frequency of digital ground motion record based on STA/LTA method: a case study in the 2008 Wenchuan mainshock

Eikonal Equation-Based Seismic Tomography of the Source Areas of the 2008 Mw 7.9 Wenchuan Earthquake and the 2013 Mw 6.6 Lushan Earthquake

ABSTRACT We use the eikonal equation-based seismic travel-time tomography method to image the source areas of the 2008 Wenchuan earthquake and the 2013 Lushan earthquake in the Longmenshan fault zone. High-resolution VP and VS models are obtained by inverting 75,686 P-wave and 74,552 S-wave travel times of local earthquakes during the period from 2009 to 2018. The tomographic models reveal strong crustal velocity heterogeneities in the study area. A significant velocity contrast exists across the Longmenshan fault zone: The western Songpan–Ganzi block is a high-velocity body, whereas the eastern Sichuan basin is a low-velocity anomaly. The hypocenter of the 2008 Wenchuan earthquake is between a high-velocity and a low-velocity anomaly. Beneath the Wenchuan mainshock, there is a significant low-velocity structure in the lower crust. The 2013 Lushan earthquake occurred in rocks associated with a high-velocity anomaly. A distinct low-velocity zone with low seismicity is imaged between the 2008 Wenchuan earthquake and the 2013 Lushan earthquake, where the crustal ductile deformation is likely to occur. The Baoxing complex to the northwest of the Lushan hypocenter exhibits as a high-velocity anomaly, which may be a carrier of stress accumulation and more prone to seismic activities in the future.

Spatial and temporal b-value precursors preceding the 2008 Wenchuan, China, earthquake (Mw = 7.9): implications for earthquake prediction

The sudden and unexpected Wenchuan earthquake (Mw = 7.9) occurred on the Longmen Shan Fault without any definite precursors. Therefore, a new computing strategy of b-values was applied in the study area around the epicenter of the Wenchuan mainshock, which is divided into nine circular sub-regions with a radius of 50 km uniformly distributed in space, ignoring geological structures. Then the temporal mapping of b-values is performed at each sub-region. The simulated results show that, at one sub-region where the Wenchuan shock is located, the b-value increased to ∼1.25 in 2002 and was almost unchanged for over five years until 2007. But, the b-value dropped to ∼0.5 in Nov. 2007 and remained the same for about three months. Then, b-value increased again since March 2008, and on May 12, 2008, the Wenchuan mainshock occurred. Indeed, according to previous results, this pattern of b-variations is a typical precursor for major earthquakes, which has not been discovered by many retrospect studies for the Wenchuan earthquake. Therefore, this study, which may capture subtle precursors on b-variations for major earthquakes, is significant for earthquake prediction and seismic hazard assessment.

Did the core phase ScS of the Wenchuan earthquake trigger its first &amp;lt;italic&amp;gt;M&amp;lt;/italic&amp;gt;6 aftershock?

The temporal pattern and generation mechanism of strong aftershocks are important for seismic hazard mitigation purposes, as aftershocks usually hamper rescue efforts and cause further damage to ground and buildings that have been weakened by mainshocks. Previous studies have demonstrated that both static stress change and dynamic stress can trigger aftershocks. Surface waves are thought to be effective in triggering seismicity, and there are limited studies of the triggering capability of far field body waves. About 15.5 min after the 2008 M 7.8 Wenchuan earthquake, an M 6 aftershock occurred, and it has been proposed that it was triggered by the core reflected ScS phase of the mainshock. It is important to study this proposal in detail because it is the first case of a strong aftershock triggered by core reflected body waves. In this paper, we investigate the effects of ScS triggering with ray-base theory. We calculate the geometrical spreading factor of ScS at short distances and derive equations relating moment tensor, focal depth and displacement of ScS. For very short epicentral distances, theoretical analysis demonstrates that the displacement of ScS is almost zero on the vertical component, while the ScS east-west and north-south components are in proportion to the M ze and M zn of the moment tensor respectively. This prediction is consistent with observations of ScS at station ENH. However, the purported ScS signal at station CD2 is very strong on the vertical component, which is inconsistent with the theoretical prediction. From the moment tensor of the Wenchuan mainshock from GCMT, we estimate that the ScS displacement is about 1 mm and particle velocity about 0.01 mm/s, consistent with the observations at station ENH. We also calculate synthetic seismograms of ScS and compare them with observations, and confirm that ScS is observed at station ENH. But the long period signal at station CD2 does not agree with the theoretical ScS waveforms; this could have resulted from the filtering of clipped waveforms of the M 6 aftershock. The M 6 event was relocated by modeling the P wave polarity and the differential travel time between P and S waves. Broadband synthetic seismograms of the M 6 event recorded at CD2 were also computed, assuming a focal depth of 14 km and strike 90 ° , dip 25 ° and rake 110 ° . The synthetic seismograms agree well with the observations in amplitude, polarity and timing of P and S waves, suggesting that the source parameters of this event are reliable. Furthermore, we find that the M 6 aftershock actually occurred before the ScS arrival, thus invalidating the hypothesis that the mainshock ScS triggered the M 6 aftershock. The stress due to ScS for receivers at different periods from 10−50 s was also calculated, and the stress change was found to be less than 1 kPa for periods longer than 30 s. The relatively weak dynamic stress due to ScS results from the free surface boundary condition, which requires that some components of the stress must approach zero for shallow receivers. Further, the boundary condition leads to a difference in the dynamic triggering capability of horizontally propagating surface waves and almost vertically propagating body waves, because the latter involves of direct body waves from deep Earth and surface-reflected waves, which interact to reduce the amplitude of the dynamic stress. Since previous studies demonstrated that seismic waves with periods longer than 30 s are more effective in triggering seismicity, it is probable that the dynamic stress caused by ScS is too weak to trigger aftershocks. In summary, we propose that ScS from the Wenchuan earthquake did not trigger the strong aftershock, and this has been confirmed with analysis of the 2005 Kashmior and the 2015 Nepal earthquakes in the Tibetan Plateau. The core reflected phase ScS of great continental earthquakes may have low potential to trigger strong aftershocks. Further studies of the triggering potential of ScS for subduction earthquakes are needed, as statistics on lag time of strong aftershocks suggest a peak around the ScS arrival time, though the case of ScS triggering for Wenchuan earthquake did not hold to be viable.

Evolution and Distribution of the Early Aftershocks Following the 2008 Mw 7.9 Wenchuan Earthquake in Sichuan, China

AbstractUsing seismic data recorded by a dense temporary seismic array, we apply a waveform matched‐filter technique and obtain an early aftershock catalog around the mainshock rupture zone of the 2008 Wenchuan, Sichuan, China, earthquake. Specifically, we use 1,273 events in a relocated aftershock catalog as templates and scan through the waveforms within 7 days following the mainshock. We obtain up to ∼5.3 and ∼10 times more aftershocks than the standard and relocated catalogs, respectively. We find that early aftershocks mostly occurred below 10 km depth, downdip to large coseismic slip areas. Early aftershocks also show minor along‐strike migration with time since the mainshock, indicating triggering of afterslip following the mainshock. From the epicentral region to Beichuan where the mainshock slip was primarily reverse faulting, early aftershocks illustrated the Yingxiu‐Beichuan and Guanxian‐Jiangyou faults as high‐angle listric feature and rooted to a gentle dipping plane at the depth of ∼20 km. The aftershocks in the northeast segment of the Longmen Shan faults mostly occurred along a near‐vertical dipping fault, consistent with the primary strike‐slip component of mainshock slip in this region. In addition, differences in early and long‐term aftershock distribution indicate significant postseismic deformation along the conjugate Xiaoyudong fault. Our results help to illuminate the deep geometry of the Longmen Shan fault zone and the early evolution after the Wenchuan mainshock, which provides additional constraints to the mechanism of plateau uplifting.

Earth tidal stress as an earthquake trigger prior to the Wenchuan earthquake, Sichuan, China

Cyclic stress variations in the crust of the earth are caused by the earth tide. Although these stress variations are small in comparison to tectonic stress, tidal stress rates are often larger than those of tectonic stress accumulation. Thus, tidal stress could be considered as one of the possible triggers for an earthquake. Many scientific studies have been conducted to examine the correlation between the earth tide and earthquake occurrence. In this study, the shear and normal stresses caused by the earth tide on the focal fault plane of the Wenchuan earthquake, Sichuan, China, are calculated using the focal mechanism solution published by the Global Centroid Moment Tensor catalog and the Preliminary Reference Earth Model. We analyzed how the tidal stresses on the fault plane changed when the Wenchuan earthquake occurred. The Schuster’s test was applied to define correlations between earthquakes occurring around the epicenter area before the Wenchuan earthquake and stresses caused by the daily earth tide. A P -value was calculated and used to quantify the statistical significance of the evidence. Earthquake data used in this study were obtained from the China Earthquake Networks Center, China Earthquake Administration. The temporal variation and spatial distribution of the P -value, indicating the significance of the test, were investigated. The obtained results show that the daily tidal stress was in favor of slip of the focal fault when the Wenchuan earthquake occurred. For earthquakes ( M L≥2.7) that occurred in the study area, the P -value began to drop in June 2007, approaching the lowest value (0.85%) very close to the occurrence time of the M S8.0 Wenchuan main shock. Most of low- P -value ( P -value situated in the near northeast of the epicenter can be found prior to the Wenchuan earthquake from the spatial distribution of P -value in the Longmenshan Fault. It can be concluded from the above-mentioned results that significant correlations between earthquakes occurring around the epicenter area and the daily tide existed prior to the Wenchuan earthquake. These tide-triggered earthquakes were in the south part of the rupture zone of the Wenchuan earthquake, northeast of the epicenter. Therefore, tidally triggered earthquakes occurred around the epicenter area prior to the Wenchuan earthquake.

Deep deformation of the Longmenshan fault zone related to the 2008 Wenchuan earthquake

This paper reviews studies of the past ten year related to the crustal deformation of the Longmenshan fault zone (LMSFZ) which was struck by the M 8.0 Wenchuan earthquake on May 12, 2008. The tectonics of the LMSFZ located at the eastern margin of the Tibetan Plateau is complex, with complicated fault geometry and a heterogeneous velocity structure. The Wenchuan earthquake rupture extended for about 300 km along the middle and northern segments of the LMSFZ, with a very complicated rupture process involving multiple sub-events. The Wenchuan slip distribution is characterized by two major slip patches near Hongkou/Yingxiu and Beichuan with peak slip of 5.0–15.5 m and extended from near the surface to below 20 km depth. Prior to the Wenchuan earthquake, the LMSFZ had been seismically quiet for several centuries and there were no hints that suggested that such an M 8.0 earthquake might strike the area. The long-term geological investigations and short-term geodetic measurements before the Wenchuan earthquake generally agree that the horizontal slip rate along the LMSFZ is no more than 3 mm per year. The low slip rate observed at the surface around the LMSFZ may not reveal the real state of accumulated strain at depth where the devastating Wenchuan earthquake nucleated. Rates of aseismic slip at depth derived from seismological investigation of repeating microearthquakes were found to be approximately twice as large as the interseismic rates inferred from surface GPS and geological data. Most of the clusters of repeating microearthquakes are located at the edge of locked areas where large coseismic slips were observed during the Wenchuan earthquake, suggesting a close relationship between microearthquakes and impending large earthquakes. A two-dimensional viscoelastic finite-element model produces a depth-related slip rate pattern around the LMSFZ that is consistent with that revealed by the seismological observation of repeating earthquakes. The measured in situ deep slip rates increase with depth and vary from 3.5 to 9.6 mm/a over a depth range of 4–18 km.The seismological observations of deep slip rates and microseismicity in the three decades before the Wenchuan earthquake reveal that the LMSFZ is indeed not as “quiet” as traditionally assumed in comparison with its neighboring fault systems. Considering the deep slip rates (3.5−9.6 mm/a) from the repeating microearthquakes and coseismic peak offsets of 5.0–15.5 m, the recurrence interval of Wenchuan-like events is estimated to be about 500−4500 years. The estimated recurrence interval based on the deep slip rates is much smaller than those estimates using the same coseismic displacements divided by GPS-derived or geological slip rates. Slip rate increases with depth were also recognized in the Parkfield section of the San Andreas fault zone and in the northeastern Japan subduction zone before the ruptures of the 2004 M w6.0 Parkfield earthquake and the 2011 M w9.0 Tohoku-oki earthquake, respectively. Accelerated slip is thought to have preceded a number of recent large subduction zone earthquakes and the 2008 Wenchuan earthquake, and repeating earthquakes may document short-term precursory slip at depth. Alternatively, the rapid slip rates indicated by the repeating microearthquakes may represent transiently accelerated slip preceding the Wenchuan mainshock. We suggest that slip rates at seismogenic depths are of critical importance in seismic hazard analysis. Repeating earthquakes can be regarded as “deep creepmeters” that measure the in - situ deep slip rate on otherwise aseismically slipping faults. For less well defined and widespread faults within the continents, it is essential to reveal a fault’s or region’s seismic history over different time scale. Combining a better understanding of earthquake diversity with modern technology is the key to effective and comprehensive hazard mitigation practices. The potential earthquake hazard of locked faults with unusually high inferred deep slow slip rates should be paid more attention. The 2008 Wenchuan earthquake is the best-studied continental earthquake to date with a large number of scientific publications enabled by the vast collected data sets. The research community efforts have provided first-order information about this unexpected event regarding its coseismic slip distribution and fault geometry. However, there are many remaining questions about the basic nature of this earthquake to be further constrained with multiple data sets, including the relationship of its rupture with prior coupling and interseismic creep, the varying dynamical rupture processes with depth, the frequency dependence of seismic radiation across the fault zone, and the role of multiple postseismic deformation processes.

Why did the most severe seismic hazard occur in the Beichuan area in the 2008 Wenchuan earthquake, China? Insight from finite element modelling

The Beichuan area suffered the most serious seismic damage in the 2008 Wenchuan earthquake although the Beichuan is over 100 km away from the instrumental epicenter of the mainshock. The mechanism for this peculiar phenomenon remains unclear even though 10 years has passed since the Wenchuan event. For this purpose, we construct a spontaneous rupture model in which Gaochuan right bend (GRB) is included in the middle of Yingxiu-Beichuan fault, a major seismogenic fault for the Wenchuan event. The simulated results show that the complex geometry of the GRB played a controlling role in the rupture propagation. Once the rupture was initiated at the epicenter of the Wenchuan mainshock, it propagated spontaneously, and the rupture speed on the first segment of the fault is ∼2.79 km/s, slower than the shear wave speed of local medium. When the rupture front spread near the end of the Yingxiu-Gaochuan fault, a new rupture was re-nucleated at the curve section of the Gaochuan bend, and propagated in the northeast direction with the speed of 5.02 km/s, greater than the S wave velocity. In addition, this rupture speed transition from subshear to supershear does not need any time delay, much distinct from the case of fault stepover in which delay is needed in supershear transition. The result also demonstrates that the relation between the high values of peak ground acceleration (PGA) and the supershear rupture is strong. The large area with high values of PGA distributed in the Beichuan directly led to grave seismic catastrophe there. Therefore, this work may give some insight into why the most serious seismic damage occurred in the Beichuan area, and may have important implications for understanding earthquake dynamics and assessing seismic hazards.

Is the 2013 Lushan earthquake (Mw = 6.6) a strong aftershock of the 2008 Wenchuan, China mainshock (Mw = 7.9)?

Following the devastating 2008 Wenchuan earthquake (Mw=7.9), the 2013 Lushan strong shock (Mw=6.6) occurred again on the Longmen Shan thrust fault. Ever since the earthquake occurred, it has been a topic of hot debate on whether it is a strong aftershock of the 2008 Wenchuan earthquake or a new independent one. To this purpose, we investigated the seismogenic fault for the Lushan earthquake, and found that it is situated on the same fault system as that for the Wenchuan earthquake, and the Lushan shock ruptured one of the asperities which were left unbroken in the 2008 Wenchuan earthquake. In addition, we analyzed the statistical properties of the Wenchuan-Lushan earthquake sequence (WLES) using three established empirical laws on aftershocks. The modelling results showed that the WLES, in which the Lushan event is regarded as one of aftershocks of the Wenchuan mainshock, well satisfied three accepted statistical laws, namely the Gutenberg-Richter relation, modified Omori-Utsu law and Båth’s law. At the same time, we observed that the weak and ductile materials between the hypocenters of the Wenchuan and Lushan shocks resulted in seismic gap in between, and the strong materials in the source region and special curved geometry of seismogenic fault for the Lushan event may be responsible for the approximately 5-year delay of the occurrence. In particular, the focal mechanism of the Lushan event, like most of the other strong aftershocks of the Wenchuan mainshock, is nearly thrust-type fault, suggesting the Lushan earthquake, together with other strong aftershocks, is controlled by the same stress environment. Most of all, we noticed that the Lushan event was triggered by the 2008 Wenhcuan main shock. Taken together, it is reasonable to infer that the 2013 Lushan event is a strong aftershock, possibly being the largest one, of the Wenchuan earthquake. Meanwhile, we propose in the paper that it is an important criterion for identification of aftershocks by means of judging whether they are triggered by the main shock or not. Identifying and forecasting strong aftershocks shall be crucial in evaluating the seismic hazard for large earthquakes.

Monitoring shallow resistivity changes prior to the 12 May 2008 M 8.0 Wenchuan earthquake on the Longmen Shan tectonic zone, China

An active source measurement of shallow resistivity using fixed-electrode quasi-Schlumberger arrays has been conducted at Pixian, Jiangyou and Wudu stations on the Longmen Shan tectonic zone in western China, with the hope of detecting earthquake-associated changes. For the duration of the monitoring experiment, a gradual decrease of apparent resistivity of up to 6.7% several years prior to the 12 May 2008 M 8.0 Wenchuan earthquake had been recorded clearly at Pixian station, approximately 35km from the epicenter. The change of apparent resistivity was monitored with a fixed Schlumberger array of AB/MN spacings of 736m/226m in the direction of N57.5°E, giving precisions in measured daily averages of 0.16% or less. A coseismic resistivity drop of up to 5.3% was observed at Jiangyou station, using a Schlumberger array of AB/MN spacings of 710m/90m in the direction of N10°E. No fluctuation of resistivity was detected at Wudu station at the time of the Wenchuan mainshock. While the focus of this paper is on monitoring or tracking resistivity variations prior to, during, and after the Wenchuan earthquake, we also aim to compare resistivity records of the Wenchuan earthquake to those of the M 7.8 Tangshan and M 7.2 Songpan earthquakes of 1976. Attempts to explain the observed resistivity variations have been made. The results show that the resistivity variations observed at all three stations are in approximate agreement with resistivity–stress behavior deduced from in situ experiments, focal mechanisms, a simplified dynamical model, static stress analyses, and field investigations from along the Longmen Shan fault zone.

Signature of Fault Healing in an Aftershock Sequence? The 2008 Wenchuan Earthquake

We analyzed the aftershock sequence of the 2008 Wenchuan earthquake from May 12, 2008 to May 12, 2013 using the earthquake catalog of the China Earthquake Networks Center (CENC). In the analysis performed, we took under consideration the temporary variation in the completeness of the earthquake catalog just after the Wenchuan mainshock. The cutoff completeness magnitude from May 12 to June 27, 2008 was above 3.0 due to the impact of the earthquake sequence on the seismological observatory practice. It was observed that the b value has an increasing trend from June 27, 2008 to late April 2009, while since May 2009, the b value has remained stable. If these characteristics were associated with the possible signature of fault healing, the ‘apparent healing time’ could be pinpointed by this measure as around 1 year. Due to two strong asperities present on the rupture of the Wenchuan mainshock, the aftershock zone can be divided into two segments, namely the north and the south segment. The b values of the two segments seem to show different trends of temporal variation. The main contribution of the increasing trend comes from the south segment, or the ‘initiation segment’ of the main rupture.

Seismic velocity changes in the epicentral region of the 2008 Wenchuan earthquake measured from three‐component ambient noise correlation techniques

AbstractWe investigate temporal changes of seismic velocity in the epicentral region of the 12 May 2008 Mw 7.9 Wenchuan earthquake using three‐component continuous waveforms recorded by a seven‐station small‐aperture array. We use an ambient noise cross‐correlation technique to compute the empirical Green's function between station pairs from August 2004 to September 2011. Our results show no obvious precursory change immediately before the main shock, clear coseismic reduction of seismic velocity of up to 0.2%, and initial postseismic recovery followed by a long‐lived velocity reduction. The coseismic and postseismic velocity changes are most prominent in the period band of 2–4 s (approximate depth of 1–4 km), and the velocity changes are smaller in other period bands. The seismic velocity in the period band of 1–2 s (i.e., top 2 km) correlates well with the water level change of the Zipingpu Reservoir. The observed temporal changes likely reflect damage and healing processes with possible permanent deformation in the upper crust associated with the Wenchuan main shock.

Stress, Distance, Magnitude, and Clustering Influences on the Success or Failure of an Aftershock Forecast: The 2013 M 6.6 Lushan Earthquake and Other Examples

Immediately after the 12 May 2008 M 7.9 Wenchuan earthquake, we began calculating the static stress change on major faults surrounding the rupture zone (Parsons et al. , 2008), and Toda et al. (2008) mapped out regional Coulomb stress changes (Fig. 1). The purpose was twofold: (1) to identify the most likely locations (stress increases) of dangerous aftershocks, and (2) to conduct a prospective test of stress mapping as a rapid‐response forecast tool. The occurrence of the 20 April 2013 M 6.6 Lushan earthquake in the Longmen fault zone near Ya’an was consistent with the prospective static stress forecast; for example, press coverage in 2008 (Kuang, 2008) stated According to their results, published in Nature on 6 July, Ya’an Thrust, Xiong Po Thrust, and Xianshuihe Fault, all near Longmen Fault where the earthquake occurred, show a notable increase in static stress changes, and therefore face a high risk of aftershocks. “The 12 May earthquake … caused grievous losses, yet its legacy includes possible large aftershocks in the near future because it increased failure stress on important faults within and around the Sichuan basin,” write the authors. Figure 1. Earthquake activity ( M ≥2) following the 12 May 2008 M 7.9 Wenchuan mainshock in Sichuan Province, China. Epicenters are superimposed on static stress‐change calculations made shortly after the mainshock in 2008 on mapped faults (red and blue lines) and optimally oriented faults (Parsons et al. , 2008). It is evident that many aftershocks occurred in regions and faults with calculated stress decreases, which led Parsons et al. (2012) to conclude that the forecast was a failure. However, a formal evaluation of the post‐Wenchuan forecast performance by Parsons et al. (2012) was not favorable, as …we show two fully prospective static stress forecasts that have failed to reproduce spatial patterns of microseismicity … over a span of 1 to 4 years we …

Tidal response variation and recovery following the Wenchuan earthquake from water level data of multiple wells in the nearfield

Tidal response variation and recovery following the Wenchuan earthquake from water level data of multiple wells in the nearfield

Kappa (k) derived from accelerograms recorded in the 2008 Wenchuan mainshock, Sichuan, China

High-frequency spectral decay factor, kappa (k), in the accelerograms of the Wenchuan mainshock was measured using strong motion data from 52 stations within 311km of the epicenter. The derived k range from 0.0034s to 0.0468s. The correlation of k versus fault distance was given, which is k=0.01288+5.9068×10–5R for the N-S component, k=0.01881+1.4219×10–5R for the E-W component, and k=0.00855+5.6086×10–5R for the U-D component. The analysis on the spatial variation of k demonstrates that k relates to source effect and propagation effect besides local site effect. Ground motions for the 52 stations were simulated using derived k and compared to actual recordings in terms of waveforms, amplitude spectra and response spectra. The results show agreement at shorter periods (<1s), but a slight overestimation at longer periods (1–7s).

Influence of the 2008 Wenchuan earthquake (Mw 7.9) on the occurrence probability of future earthquakes on neighboring faults

The Longquan–Shan fault and the Huya fault are two major neighboring faults of the Longmen–Shan fault zone where the 12 May 2008 Wenchuan earthquake (Mw 7.9) occurred. To study the influence of the Wenchuan event on these two active faults, we calculate changes of Coulomb stress on the Longquan–Shan fault and the Huya fault caused by the Wenchuan mainshock. Our results indicate that the Coulomb stress in the northern section (Zone A) of the Longquan–Shan fault is increased by 0.07–0.10bars, that in the middle section (Zone B) by 0.04–0.11bars, and that in the southern section (Zone C) shows almost no change. For the Huya fault, the Coulomb stress is decreased by 0.01–0.03bars in the northern section (Zone A), 0.10–0.35bars in the middle section (Zone B), and nearly 0.5bars in the southern section (Zone C). The epicenter distribution of small earthquakes (ML⩾1.5) on the Longquan–Shan fault and the Huya fault after the Wenchuan earthquake is consistent with the distribution of the Coulomb stress change. This implies that the Wenchuan earthquake may have triggered small events on the Longquan–Shan fault, but inhibited those on the Huya fault. We then use the rate/state friction law to calculate the occurrence probability of future earthquakes in the study region for the next decade. They include the distribution of b-values, magnitude of completeness (Mc), the background seismicity rate, a value of Aσn and the duration for the transient effect (ta) in the study region. We also estimate the earthquake occurrence probabilities on the neighboring faults after the Wenchuan earthquake. Our results show that, the occurrence probability of future earthquakes in the Longquan–Shan has a slight increase, being 7% for M⩾5.0 shocks during the next decade, but the earthquake probability in the Huya region is reduced obviously, being 5–20%, 7–26% and 3–9% for M⩾5.0 shocks during the next decade in sections A, B and C of the Huya fault, respectively.

The Precursory Significance of Tectonic Stress Field Transformation Before the Wenchuan MW7.9 Earthquake and the Chi‐Chi MW7.6 Earthquake

AbstractThe 2008 Wenchuan earthquake and the 1999 Chi‐chi earthquake are both intraplate thrust fault earthquakes occurred under the tectonic setting of thrust‐nappe. Through comparative analysis on CMT solutions, focal mechanism solutions of small and moderate earthquakes and the tectonic stress field derived from focal mechanisms near the mainshcok region, we found that the thrust type focal mechanism of the Chi‐Chi mainshock coincides with the tectonic stress field from the CMT solutions of earthquakes occurred in Taiwan central and western inland; but for the small and moderate earthquakes randomly occurred near mainshock region, the strike slip type focal mechanism of regional stress field is not consistent with the tectonic stress field before the Chi‐Chi mainshock; the Chi‐Chi mainshcok occurred a few months after the local stress field became consistent with the tectonic stress field. Similarly, the compressive stress axis of the thrust fault type focal mechanism of Wenchuan mainshock coincides with the compressive stress axis of tectonic stress field determined by the CMT solutions of earthquake occurred in the eastern part of Qinghai‐Tibet Plateau, the local stress field from the focal mechanisms of small and moderate earthquake near Wenchuan mainshock is consistent with the tectonic stress field. When the local stress field is consistent with the thrust fault type stress field, the Wenchuan mainshock occurred immediately. With the stress accumulation near the source region, there are similar local stress field transformation phenomena before the thrust fault mainshock. When the local stress state transformed to the stress state of mainshock, the stress level is up to very high near the source region. It may be a precursor of large earthquake and must arouse further concerns.

A shallow aftershock sequence in the north-eastern end of the Wenchuan earthquake aftershock zone

Previous studies show that mature faults are filled with fault gouge in the shallow part and thus cannot accumulate enough strain energy for earthquakes. Therefore most earthquakes are deeper than 5 km, except those events occurring on new faults or in intact rocks. From field observation, Wenchuan earthquake is found to rupture the free surface about 200 km, but the rupture may extend underground much further from teleseismic body waves inversion and aftershocks distribution. In the northeastern end of the rupture zone, deep rupture may induce stress increase near the free surface, and trigger shallow earthquakes. An Ms 5.7 aftershock occurred at Qingchuan, northeast end of Wenchuan earthquake fault on July 24, 2008, featuring thrust mechanism with a 3 km source centroid depth. The shallow focal depth is confirmed with the sPL phase recorded at station L0205. As Rayleigh wave is well only developed for source depth less than 1/5 of epicentral distance, the observed large amplitude of Rg at a distance of 15 km implied depth of 3 km or less. Dozens of aftershocks’ sPL waveforms are also analyzed to confirm the source depths less than 3 km. On the other hand, no surface ruptures are found by geological survey or InSAR studies. It is strongly suggested that these aftershock sequences initiate fresh rupture in intact rocks triggered by stress increase from the deep co-seismic rupture of the Wenchuan mainshock.