Coseismic Coulomb Stress Research Articles

Detailed microseismicity study in the area of Florina (Greece): Evidence for fluid driven seismicity

A local seismic network was installed and operated in the area of Florina, northern Greece, during July 2013 – January 2014 for studying the high microseismic activity following the occurrence of an Mw=4.1 event on 17 February 2013. The recordings of the local network along with the ones of the Hellenic Unified Seismological Network (HUSN) are used for obtaining accurate locations and defining the characteristics of the seismic activity. A new velocity model is calculated for a broader area using the recordings of the HUSN stations. Relocation is performed for 1330 events recorded by the local network and 423 events recorded by HUSN using the double difference technique and cross correlation measurements. Fault plane solutions are determined for the two largest events (Mw=3.6 and Mw=4.1) using waveform inversion technique. The causative fault of the largest event (Mw=4.1) is striking almost E-W and dipping to the north. However, most of the activity is concentrated in its south and forms an almost vertical, south dipping plane, striking almost E-W. This southern cluster consists of multiplets and is located in an area of positive Coulomb stress changes, due to the coseismic slip of the Mw=4.1 earthquake. The spatio-temporal evolution of microseismicity is in most cases well explained by diffusion curves, a signature of fluid induced seismicity. However, there are cases where a triggering front could not describe the evolution of the activity resulting to an earthquake - earthquake triggering. The dependent or independent (Poisson process) earthquake occurrence was examined by searching their interevent time distribution. These characteristics are interpreted as the consequence of CO2 emission through faults which are used as pathways in an area enhanced by the coseismic Coulomb stress changes.

Study on Coulomb Stress Triggering of the April 2015 M7.8 Nepal Earthquake Sequence

In April 2015, a M7.8 earthquake occurred less than one month before a M7.3 earthquake near Kodari, Nepal. The Nepal earthquake sequences also include four larger (M > 6) aftershocks. To reveal the interrelation between the main shock and the aftershocks, we check the role of coseismic coulomb stress triggering on aftershocks that follow the M7.8 main shock. Based on the focal mechanisms of the aftershocks and source models of the main shock, the coulomb failure stress changes on both of the focal mechanism nodal planes are calculated. In addition, the coulomb stress changes on the focal sources of each aftershock are also calculated. A large proportion of the M > 6 aftershocks occurred in positive coulomb stress areas triggered by the M7.8 main shock. The secondary triggering effect of the M7.3 aftershock is also found in this paper. More specifically, the M7.3 aftershock promoted failure on the rupture plane of the M6.3 aftershock. Therefore, we may conclude that the majority of larger aftershocks, which accumulated positive coulomb stress changes during the sequence, were promoted or triggered by the main shock failure. It suggests that coulomb stress triggering contributed to the evolution of the Nepal M7.8 earthquake sequence.

Coseismic Coulomb stress changes caused by the Mw6.9 Yutian earthquake in 2014 and its correlation to the 2008 Mw7.2 Yutian earthquake

A three-dimensional viscoelastic finite element model of the Qinghai–Tibet Plateau and its adjacent regions was used to explore the relationship between the Mw7.2 Yutian earthquake in 2008 and the 2014 Mw6.9 Yutian earthquake. We further analyzed the Coulomb failure stress change caused by the Yutian Mw6.9 earthquake on faults surrounding the Bayan Har block and discussed the relationship between the Coulomb stress change and aftershock distribution. The preliminary results showed that: (1) The Coulomb failure stress change caused by the Mw7.2 Yutian earthquake in 2008, which was projected on the slip direction of the 2014 Yutian Mw6.9 earthquake, exceeded the earthquake triggering threshold of 0.01MPa, implying an apparent triggering effect. Specifically, the coseismic Coulomb failure stress changes were 0.0167MPa and 0.0170MPa when assuming apparent friction coefficients of 0.4 and 0.6, respectively. The Coulomb failure stress changes, including viscoelastic relaxation effects, were 0.0187MPa and 0.0194MPa respectively. Combined with the tectonic stressing rate, the Mw6.9 Yutian earthquake in 2014 was advanced 21.4–24.9years by the 2008 Mw7.2 Yutian earthquake; (2) The stress changes derived from the viscoelastic relaxation effect were much less than that from the coseismic effect, which was possibly interpreted as the shorter elapsed time between the two earthquakes relative to the characteristic time; (3) The Coulomb failure stresses increased on the mid-northern segment of the Altyn Tagh fault, the middle segment of the Mani-Yushu fault and the western segment of the eastern Kunlun fault, which demonstrated increasing seismic potential. In particular, the Coulomb failure stress on the middle segment of the Altyn Tagh fault increased by 2.8×103Pa, the most significant increase. In contrast, the Coulomb failure stress decreased on the western segment of the Mani-Yushu and Minjiang faults, with the western segment of the Mani-Yushu fault dropping by 3.6×103Pa. The Coulomb stress changes on most faults surrounding the Bayan Har block were calculated to have increased, while the Minjiang fault on the eastern boundary was inhabited due to the enhancement of coseismic compressive stresses; (4) Combining the relocation of the aftershock sequence with the coseismic failure stress change, we concluded that the Mw6.9 Yutian main-shock triggered the aftershock sequences.

Observations of static Coulomb stress triggering of the November 2011M5.7 Oklahoma earthquake sequence

AbstractIn November 2011, aM5.0 earthquake occurred less than a day before aM5.7 earthquake near Prague, Oklahoma, which may have promoted failure of the mainshock and thousands of aftershocks along the Wilzetta fault, including aM5.0 aftershock. TheM5.0 foreshock occurred in close proximity to active fluid injection wells; fluid injection can cause a buildup of pore fluid pressure, decrease the fault strength, and may induce earthquakes.Keranen et al. [] links theM5.0 foreshock with fluid injection, but the relationship between the foreshock and successive events has not been investigated. Here we examine the role of coseismic Coulomb stress transfer on earthquakes that follow theM5.0 foreshock, including theM5.7 mainshock. We resolve the static Coulomb stress change onto the focal mechanism nodal plane that is most consistent with the rupture geometry of the threeM ≥ 5.0 earthquakes, as well as specified receiver fault planes that reflect the regional stress orientation. We find that Coulomb stress is increased, e.g., fault failure is promoted, on the nodal planes of ~60% of the events that have focal mechanism solutions, and more specifically, that theM5.0 foreshock promoted failure on the rupture plane of theM5.7 mainshock. We test our results over a range of effective coefficient of friction values. Hence, we argue that theM5.0 foreshock, induced by fluid injection, potentially triggered a cascading failure of earthquakes along the complex Wilzetta fault system.

Short- and long-term earthquake triggering along the strike-slip Kunlun fault, China: Insights gained from the Ms 8.1 Kunlun earthquake and other modern large earthquakes

Abstract Following the 2001 Ms8.1 Kunlun earthquake, earthquake records of more than 10 years, in addition to more than one century's records of large earthquakes, provide us with a chance to examine short-term (days to a few years) and long-term (years to decades) seismic triggering following a magnitude ~ 8 continental earthquake along a very long strike-slip fault, the Kunlun fault system, located in northern Tibet, China. Based on the calculations of coseismic Coulomb stress changes (ΔCFS) from the larger earthquake and post-seismic stress changes due to viscoelastic stress relaxation in the lower crust and upper mantle, we examined the temporal evolution of seismic triggering. The ETAS (epidemic type aftershocks sequence) model shows that the seismic rate in the aftershock area over ~ 10 years was higher than the background seismicity before the mainshock. Moreover, we discuss long-term (years to decades) triggering and the evolution of stress changes for the sequence of five large earthquakes of M ≥ 7.0 that ruptured the Kunlun fault system since 1937. All subsequent events of M ≥ 7.0 occurred in the regions of positive accumulated ΔCFS. These results show that short-term (up to 200 days in our case) triggering along the strike-slip Kunlun fault is governed by coseismic stress changes, while long-term triggering is somewhat due to post-seismic Coulomb stress changes resulting from viscoelastic relaxation.

Coulomb static stress changes before and after the 23 October 2011 Van, eastern Turkey, earthquake (<i>M</i><sub>W</sub>= 7.1): implications for the earthquake hazard mitigation

Abstract. Coulomb stress changes before and after the 23 October 2011 Van, eastern Turkey, earthquake have been analysed using available data related to the background and the aftershock seismicity and the source faults. The coseismic stress changes of the background seismicity had slightly promoted stress over the rupture plane of the 2011 Van earthquake, while it yielded a stress shadow over the Gürpı nar Fault which has been argued to have produced the 7 April 1646 Van earthquake. The stress shadow over the Gürp\\i nar fault has become more pronounced following the occurrence of the 2011 Van earthquake, meaning that the repetition of the 1646 Van earthquake has been further suppressed. Spatial distribution and source mechanisms of the 2011 Van earthquake's aftershocks have been utilised to define four clusters with regard to their relative location to the mainshock rupture. In addition, the aftershock sequence covers a much broader area toward the northeast. Correlations between the observed spatial patterns of the aftershocks and the coseismic Coulomb stress changes caused by the mainshock are determined by calculating the stress changes over both optimally oriented and specified fault planes. It is shown here that there is an apparent correlation between the mainshock stress changes and the observed spatial pattern of the aftershock occurrence, demonstrating the usefulness of the stress maps in constraining the likely locations of the upcoming aftershocks and mitigating earthquake hazard.

Lushan M S7.0 earthquake: A special earthquake occurs on curved fault

Relocation result shows that the aftershocks of the Lushan M S7.0 earthquake spatially distribute in a shape like “half bowl”, indicating that the rupture structure of the mainshock is a highly curved surface. Kinematic analysis reveals that a laterally varied dislocation pattern occurs on this curved fault even though a single relative horizontal movement controls slip on this fault. Reverse slip prevails on curved fault. However, significant normal slip is predicted near the edge of north flank. Moreover, the north flank features left-lateral slip while the south flank contrarily features right-lateral slip. The relative scope of aftershock distribution implies inadequate breaking of the curved fault during the mainshock, calling for the attention to potential earthquake risk on the neighboring portions of the coseismic rupture due to significant increase of the coseismic Coulomb stress. Coseismic stress modeling also reveals that it is unnecessary for the stress on ruptured part to be unloaded following the earthquakes on the curved fault. The coseismic stress loading on ruptured elements unveils the specialty of faulting for the Lushan earthquake and we conclude that this specialty is due to the highly curved fault geometry.

Seismogenesis and earthquake triggering during the Van (Turkey) 2011 seismic sequence

article i nfo Article history: A unique and very interesting earthquake of magnitude Mw 7.2 occurred in the Van region of Turkey on October 23, 2011 that caused a heavy loss of human lives and properties. The earthquake occurred on a blind oblique thrust fault oriented towards the NE-SW direction and dipping towards NW as evidenced by focal mechanism solution and aftershock distribution. In this study, we analyzed the seismogenesis and earthquake triggering during this sequence with the help of estimated seismological parameters (b-value of frequency-magnitude relation, p-value of aftershocks temporal decay and D-value of fractal dimension), 2D mapping of b- and p-values, 3D mapping of b-value and coseismic Coulomb stress modeling. The estimat- ed seismic b-value equal to 0.89 reveals that the mainshock occurred in a highly stressed region and sequence comprised larger magnitude aftershocks due to the presence of large size asperities within the rupture zone. The normal estimate of p-value (0.98) suggests a tectonic genesis of the aftershocks sequence. The estimated D-value equal to 1.80 reveals that rupture propagated in a two-dimensional plane filled up by fractures. The spatial 2D and 3D mapping of seismic b-value suggests that the Van earthquake originated in a highly hetero- geneous fractured rock matrix with fluid intrusions into it at deeper depth beneath the mainshock hypocen- ter region. The estimated coseismic Coulomb stress using the variable slip model for depth range 0-30 km exhibits a 'butterfly' pattern and most of the aftershocks fall (90%) in the region of enhanced Coulomb stress. This suggests that most of the aftershock activities have been triggered by transfer of positive Coulomb stress due to coseismic slip of the mainshock. The results estimated in the present study have potential useful im-

1954年山丹7 1/4 级地震的孕震构造和发震机制探讨

Located in the front margin of the northeastern Tibetan Plateau, the Longshoushan fault zone is one of the boundary faults between the Hexi corridor and the Alxa block. Although the resolution of observation is limited, the 1954 M7(1/4) earthquake on the Longshoushan fault zone is a rare big event in this zone with modern seismological observation and documentation. During this earthquake, one continuous dextral-normal rupture occurred with a length of only 7 km along a transfer fault connecting two secondary faults of the Longshoushan fault zone. However, the dextral-normal sense of this surface rupture is contrary to the tectonic kinematics in this area. Based on recent field investigation and mapping, two phenomena were discovered. Firstly, except the one appeared on the transform fault, no surface rupture zone is found on the branches of the Longshoushn fault zone. Secondly, the main damage caused by earthquake is distributed on the region between the northern and the southern branches of the Longshoushan fault zone. These two findings indicate that the 1954 earthquake was caused by the main faults rather than the transfer fault of the Longshoushan fault zone, and the co-seismic surface rupture on the transfer fault is only a form of stress release at local portion. According to calculation of static co-seismic Coulomb stress change, the surface rupture could be triggered by an earthquake on the southern branch of the Longshoushan fault zone. Furthermore, in consideration of the possibility of dynamic triggering along the northern branch of the Longshoushan fault zone, the surface rupture should be the co-seismic response to the earthquake occurred on the main branches of the Longshoushan fault zone. Relocation of the small earthquakes also shows that a narrow inverted north-extending triangle about 10 km wide was formed between the northern and the southern faults of the Longshoushan fault zone.

Fault geometry, coseismic-slip distribution and Coulomb stress change associated with the 2009 April 6, Mw 6.3, L’Aquila earthquake from inversion of GPS displacements

The 2009 April 6, Mw= 6.3 L’Aquila earthquake occurred within a complex system of NW-SE trending normal faults in the Abruzzi Central Apennines (Italy). We analyse the coseismic deformation as measured by >70 global positioning system (GPS) stations, both from continuous and survey-mode networks, providing unprecedented details for a moderate normal faulting earthquake in Italy from GPS measurements. We use rectangular, uniform-slip, dislocations embedded in an elastic, homogeneous and isotropic half-space and a constrained, non-linear optimization algorithm, to solve for the best-fitting rectangular dislocation geometry and coseismic-slip distribution. We use a bootstrap approach to investigate uncertainties in the model parameters and define confidence bounds for all the inverted parameters. The rupture occurred on a N129°E striking and 50° southwestward dipping normal fault, in agreement with geological observations of surface breaks along the Paganica fault. Our distributed slip model exhibits a zone of relatively higher slip (>60 cm) between ~1.5 and ~11 km depth, along a roughly downdip, NW-SE elongated patch, confined within the fault plane inverted assuming uniform-slip. The highest slip, of the order of ~1 m, occurred on a ~16 km2 area located at ~5 km depth, SE of the mainshock epicentre. The analysis of model resolution suggests that slip at depth below ~5 km can be resolved only at a spatial scale larger than 2 km, so a finer discretization of different asperities within the main patch of coseismic-slip is not allowed by GPS data. We compute the coseismic Coulomb stress changes in the crustal volume affected by the major aftershocks, and compare the results obtained from the uniform-slip and the heterogeneous-slip models. We find that most of the large aftershocks occurred in areas of Coulomb stress increase of 0.2-13 bar and that a deepening of the slip distribution down to a depth greater than 6 km in the SE part of the fault plane, in agreement with the inverted slip model, can explain the deepest, April 7, Mw 5.3 aftershock.

Tectonic implications and seismicity triggering during the 2008 Baluchistan, Pakistan earthquake sequence

A damaging and widely felt moderate earthquake (Mw 6.4) hit the rural, mountainous region of southwestern Pakistan on October 28, 2008. The main shock was followed by another earthquake of identical magnitude (Mw 6.4) on the next day. The spatial distribution of aftershocks and focal mechanism revealed a NW–SE striking rupture with right-lateral strike-slip motion which is sympathetic to the NNW–SSE striking active mapped Urghargai Fault. The occurrence of strike-slip earthquakes suggests that along with the thrust faults, strike slip faults too are present beneath the fold-and-thrust belt of Sulaiman–Kirthar ranges and accommodates some of the relative motion of the Indian and Eurasian plates. To assess the characteristics of this sequence, the statistical parameters like aftershocks temporal decay, b-value of G–R relationship, partitioning of radiated seismic energy due to aftershocks, and spatial fractal dimension (D-value) have been examined. The b-value is estimated as 1.03 ± 0.42 and suggests the tectonic genesis of the sequence and crustal heterogeneity within rock mass. The low p-value of 0.89 ± 0.07 implies slow decay of aftershocks activity which is probably an evidence for low surface heat flow. A value of spatial fractal dimension of 2.08 ± 0.02 indicates random spatial distribution and that the source is a two-dimensional plane filled-up by fractures. The static coseismic Coulomb stress changes due to the foreshock (Mw 5.3) were found to increase stress by more than 0.04 bars at the hypocenter of the main shock, thus promoting the failure. The cumulative coseismic Coulomb stress changes due to the foreshock and mainshocks suggest that most of the aftershocks occurred in the region of increased Coulomb stress, and to the SE to the mainshock rupture.

The co-seismic Coulomb stress change and expected seismicity rate caused by 14 April 2010 Ms = 7.1 Yushu, China, earthquake

A moderate and widely felt earthquake of Ms7.1 (Mw6.9) occurred on April 14, 2010 in Yushu district of Qinghai province (China) which destroyed a large number of buildings and killed more than two thousand people. The distribution of the aftershock sequence is sparse, but with relatively large magnitudes (Ms6.3 and Ms5.7 aftershocks were recorded), strong aftershocks are still highly potential to occur in the near future. Therefore, it is critical to delineate areas where potential aftershocks could occur. Based on static stress triggering theory, we calculated the coseismic stress changes induced by the mainshock in Yushu County and its surrounding regions by adopting elastic dislocation theory and a multilayered crustal model. According to the rate- and state-variable friction law, we calculated the expected seismicity rate and probability of occurrences of M ≥ 5.0 earthquake in the next ten years. It is observed that the Coulomb stress changes increase apparently on the segment of the Wudaoliang-Qumalai Fault, the Batang-Luoxu and Dangjiang-Longbao segments of the Ganzi-Yushu Fault and the whole Wulanwula Lake-Yushu Fault. Based on the theory of Coulomb stress triggering, the probability of earthquake occurrence on these faults would increase in the near future. Considering the historical seismicity, the strain accumulation and the Coulomb stress changes, the probability of M ≥ 5 earthquake and M ≥ 6 earthquake occurrence in the entire study area in the next 10 years are 79–87% and 26.7–33.6%, respectively. Actually, a moderate earthquake of Ms5.3 occurred to the northwest of the Nangqian County on June 26, 2011.

Rate/state Coulomb stress transfer model for the CSEP Japan seismicity forecast

Numerous studies retrospectively found that seismicity rate jumps (drops) by coseismic Coulomb stress increase (decrease). The Collaboratory for the Study of Earthquake Prediction (CSEP) instead provides us an opportunity for prospective testing of the Coulomb hypothesis. Here we adapt our stress transfer model incorporating rate and state dependent friction law to the CSEP Japan seismicity forecast. We demonstrate how to compute the forecast rates of large shocks in 2009 using the large earthquakes during the past 120 years. The time dependent impact of the coseismic stress perturbations explains qualitatively well the occurrence of the recent moderate size shocks. Such ability is partly similar to that of statistical earthquake clustering models. However, our model differs from them as follows: the off-fault aftershock zones can be simulated using finite fault sources; the regional areal patterns of triggered seismicity are modified by the dominant mechanisms of the potential sources; the imparted stresses due to large earthquakes produce stress shadows that lead to a reduction of the forecasted number of earthquakes. Although the model relies on several unknown parameters, it is the first physics based model submitted to the CSEP Japan test center and has the potential to be tuned for short-term earthquake forecasts.

Interaction between adjacent left-lateral strike-slip faults and thrust faults: the 1976 Songpan earthquake sequence

Based on the published focal mechanisms we have built the fault model of the main shocks of the 1976 Songpan earthquake sequence and calculated the coseismic Coulomb stress changes in the region. The results show that most of the aftershocks had occurred in the region where the Coulomb stresses had been increased, indicating a triggering relationship between the main shocks and the aftershocks. We also show that the first main shock (M s = 7.2), which is a left-lateral slip event, had increased the Coulomb stresses by 5×105 Pa at the second main shock (a thrust event with M s = 6.7). Therefore, we conclude that the first main shock had triggered the second main shock. The third main shock is also a left-lateral event, however, the triggering relationship between the third main shock and the previous two events is less obvious. General model calculations show that there is a good triggering relationship between adjacent left-lateral slip fault and thrust fault, but triggering between parallel slip faults is rather weak.

Poroelastic relaxation and aftershocks of the 2001 Bhuj earthquake, India

We analyse aftershocks of the 26 January 2001 Bhuj earthquake, India, that were recorded for 10 weeks following the mainshock. We calculate undrained or instantaneous pore pressure and change in Coulomb stress due to the earthquake and their poroelastic relaxation in the following 10 weeks period. Almost all aftershocks occurred in the region of coseismic dilatation. In the subsequent period, pore pressure increased through relaxation in the dilatation region which further modified coseismic Coulomb stress. Maximum increase in pore pressure is estimated to be about 0.7 MPa in 60 days time following the mainshock. Correlation between the zones of increased pore pressure and postseismic Coulomb stress with that of aftershocks, suggests a definite role of fluid diffusion in their delayed triggering.

Evolution of stress in Southern California for the past 200 years from coseismic, postseismic and interseismic stress changes

SummaryThe seismicity of southern California results from stresses that arise from the relative motion of the Pacific and North American Plates being accommodated along the San Andreas Fault (SAF) system and the Eastern California Shear Zone (ECSZ). Here we calculate how the stress field in southern California has evolved over the past two centuries due to interseismic loading, as inferred from current GPS observations of surface velocities, from redistributions of static stress induced by large (Mw ≥ 6.5) earthquakes since the 1812 Wrightwood quake, and postseismic viscoelastic relaxation associated with these events that serves to transfer coseismic stresses from the deep, warm, lower crust and upper mantle to the overlying seismogenic upper crust. We calculate Coulomb stress changes on vertical strike-slip faults striking parallel to the SAF and at the hypocenters on the rupture planes of all Mw ≥ 6 events over the past two centuries. Our results suggest that the 1857 Mw = 8.2 Fort Tejon earthquake, by far the largest event to have occurred in the region over the past two centuries, had a profound influence on the state of stress in Southern California during the 19th century, inducing significant stress increases to the north (Parkfield region and adjoining creeping SAF) and south (southern SAF and San Jacinto fault), and stress relief across the southern ECSZ. These stress changes were then greatly magnified by postseismic relaxation through the early part of the 20th century. Slow interseismic build-up of stress further loads all major strike-slip faults and works to reload the areas of the ECSZ where stress was relieved by the 1857 quake. Our calculations suggest that only 56% of hypocenters were pushed closer to failure by preceding coseismic stress changes, suggesting that the occurrence of large earthquakes is not strongly determined by coseismic Coulomb stress changes. This percentage rises to 70% when postseismic stress changes are also considered. Our calculations demonstrate the importance of postseismic viscoelastic relaxation in the redistribution of stress following large earthquakes. We find, however, that postseismic processes associated with events more than about a decade old are near completion and thus do not significantly influence the regional velocity field presently observed in southern California.

Space-Time Clustering of Large Thrust Earthquakes along the Mexican Subduction Zone: An Evidence of Source Stress Interaction

The spatiotemporal plot of epicenters of large ( M s ≥6.9) subduction earthquakes in Mexico (1900 to present day) suggests that these earthquakes cluster in space and time. In this work we test the hypothesis that the coseismic stress transfer may lead to this clustering. For the analysis we estimate the spatial extent of the coseismic Coulomb stress change for these large events and then perform a statistical analysis using the χ 2 goodness-of-fit test for the interevent time intervals. We find that there are, at least, two groups of time intervals where the observed frequencies are much higher than that expected from a Poisson model, indicating a bimodal pattern. For the first mode, the observed frequencies for the 0- to 5-year interval becomes about 2.1 times the expectation, with a probability of occurrence of about 30%. These results show that large thrust Mexican earthquakes between 1900 and 2003 are clustered in space and time probably due to stress interactions among them. The second mode includes the time interval of 30–50 years. In the interval of 30–40 years, the observed frequencies become about 1.7 times the expectation and about 1.2 times the expectation for the 40- to 50-year interval. This second mode could be associated with a reloading interval of tectonic stress due to the plate convergence and appears consistent with the long-term recurrence periods of large thrust earthquakes in the Mexican subduction zone.

Earthquake triggering and delaying caused by fault interaction on Xianshuihe fault belt, southwestern China

The coseismic Coulomb stress change caused by fault interaction and its influences on the triggering and delaying of earthquake are briefly discussed. The Xianshuihe fault belt consists of Luhuo, Daofu, Kangding, Qianning and Ganzi fault. Luohuo (MS=7.6, 1973)-Kangding (MS=6.2, 1975)-Daofu (MS=6.9, 1981)-Ganzi (MS=6.0, 1982) earthquake is a seismic sequence continuous on the time axis with magnitude greater than 6.0. They occurred on the Luhuo, Kangding, Daofu and Ganzi fault, respectively. The coseismic Coulomb stress changes caused by each earthquake on its surrounding major faults and microcracks are calculated, and their effects on the triggering and delaying of the next earthquake and aftershocks are analyzed. It is shown that each earthquake of the sequence occurred on the fault segment with coseismic Coulomb stress increases caused by its predecessors, and most aftershocks are distributed along the microcracks with relatively larger coseismic Coulomb stress increases resulted from the main shock. With the fault interaction considered, the seismic potential of each segment along Xianshuihe fault belt is reassessed, and contrasted with those predicted results ignoring coseismic Coulomb stress change, the significance of fault interaction and its effect on triggering and delaying of earthquake are emphasized. It is concluded that fault interaction plays a very important role on seismic potential of Xianshuihe fault belt, and the maximal change of future earthquake probability on fault segment is up to 30.5%.

Aftershocks and pore fluid diffusion following the 1992 Landers earthquake

We model the evolution of regional stress following the 1992 Landers earthquake in order to test the importance of pore fluid flow in producing aftershocks. Rising fluid pressure due to pore fluid flow and the resulting Coulomb stress changes were found to be strongly correlated with the time and location of aftershock events. Regional aftershock frequencies computed by integrating pore pressure decay rates also agreed quite well with aftershock data. Calculations show that regions of rising postseismic poroelastic Coulomb stress overlap considerably with regions of positive coseismic Coulomb stress. Thus pore fluid flow, which affects pore pressure within faults and causes regional poroelastic stress evolution following earthquakes, gradually evolves the initial coseismic stress changes. Together these changes provide a reasonable physical mechanism for aftershock triggering which agrees with data for the 1992 Landers earthquake.