The relationship between earthquake swarms and magma transport: Kilauea volcano,Hawaii

A detailed investigation of earthquake locations and focal mechanisms for swarms associated with intrusive events at Kilauea volcano, Hawaii, further illuminates the relationships among stress state, faulting, and magma transport. We determine the earthquake locations and mechanisms using a three-dimensional crustal model to improve their accuracy and consistency. Swarms in Kilauea's upper east and southwest rift zones, from the years 1980 through 1982, provide clear evidence for the propagation and/or dilation of dikes. Focal mechanisms are predominantly strike-slip, and the faulting and inferred dike orientations can be interpreted quite consistently in terms of the model ofHill (1977). Stresses induced by the summit magma reservoir system strongly control faulting and magma transport in the rift zones close to the summit.     

Nature of the magma conduit under the east rift zone of Kilauea Volcano,Hawaii

From a combination of results of gravity, magnetic and seismic refraction surveys, the dike complex under the east rift zone of Kilauea Volcano in Hawaii was found to extend for 110 km from the summit area of the volcano to a point 60 km at sea beyond the eastern tip of the island. Near the summit the complex is 20 km wide, and at about 40 km distance from the summit, the complex narrows to 12 km wide. The main body of the dike complex is 2.3 km deep, but some parts are as shallow as 1 km. From extrapolation of temperature data of a deep well and from analysis of magnetic data, it was inferred that temperature of the dike complex is above the Curic point of 540°C. The internal part of the complex can approach the melting point of 1060°C. The dike complex was formed by numerous excursions of magma from the holding reservoir under the volcano summit. The theory of forceful intrusion of magma into rift zones accounts for the magma excursions and migration of the passageways. Gravity and seismic velocity data indicate that density of the material left in the dike complex is 3.1 g/cm³. In the light of recent density determinations of Hawaiian rocks under high pressure and temperature, it is concluded that during Hawaiian volcanic activity, less dense components of the parent magma crupt through surface vents while the more dense components remain trapped below. Samples of the dense material from the dike complex are required before we can have a complete picture of the parent magma of Hawaiian volcanoes. The dike complex is the source of thermal energy for a commercial quality geothermal reservoir that was found by drilling.     

汶川M_W7.9地震起始破裂断层几何结构

本文收集使用紫坪铺水库台网记录到的汶川地震主震P波波形资料,利用P波反投影叠加法获取了2008年5月12日汶川M_W7.9地震起始破裂的时空演化过程.通过分析本次大地震起始破裂阶段(0～1s)破裂点在三维空间内的分布特征,确定了本次大地震起始破裂位置及起始破裂断层几何结构模型.得到以下结果:汶川地震起始破裂点位于31.013±0.002°N、103.392±0.002°E,震源深度为8.2±0.4km,发震时刻为2008年5月12日14∶27∶58.80±0.4.汶川地震起始破裂的最佳断层面走向为NE48°,倾向NW35°,起始阶段破裂的深度范围为地下7.5～9km.     

On the Application of Mwp in the Near Field and the March 11, 2011 Tohoku Earthquake

Tsunami Warning Centers issue rapid and accurate tsunami warnings to coastal populations by estimating the location and size of the causative earthquake as soon as possible after rupture initiation. Both US Tsunami Warning Centers have therefore been using Mwp to issue Tsunami Warnings 5–10 min after Earthquake origin time since 2002. However, because Mwp (Tsuboi et al., Bulletin of the Seismological society of America 85:606–613, 1995) is based on the far-field approximation to the P-wave displacement due to a double couple point source, we should only very carefully apply Mwp to data obtained in the near field, at distances of less than a few wavelengths from the fault. On the other hand, the surface waves from Great Earthquakes, including those that occur just offshore of populated areas, such as the 2011 Tohoku earthquake, clip seismographs located near the fault. Because the first arriving P-waves from such large events are often on scale, Mwp should provide useful information, even for these Great Earthquakes. We therefore calculate Mwp from 18 unclipped STS-1 broadband P-wave seismograms, recorded at 2–15° distance from the Tohoku epicenter to determine if Mwp can usefully estimate Mw for this earthquake, using data obtained close to the epicenter. In this case there should be a good chance to get reliable Mwp values for stations at epicentral distances of 9–10°, since the source duration for the Tohoku earthquake is less than 200 s and the time window used to estimate Mwp is 120 s in duration. Our analysis indicates that Mwp does indeed give reliable results (Mw ~ 9.1) beginning at about 11° distance from the epicenter. The values of Mwp from seismic waveforms obtained at 11–15° epicentral distance from the Mw 9.1 off the east coast of Tohuku earthquake of March 11, 2011 fell within the range 9.1–9.3, and were available within 4–5 min after origin time. Even the Mwp values of 7.7–8.4, obtained at less than 5° epicentral distance, exceed the PTWC’s threshold of Mw 7.6 for issuing a regional tsunami warning to coastal populations within 1,000 km of the epicenter, and of Mw 6.9 for issuing a local tsunami warning to the coastal populations of Hawaii.     

Mechanism and tectonic implications of the great Banda Sea earthquake of November 4, 1963

The Banda Sea earthquake of November 4, 1963 (h = 100 km, m_B = 7.8) is probably one of the largest intermediate-depth shocks to have occurred worldwide this century. The mechanism of this earthquake is studied in detail on the basis of P-wave first motion, surface wave and aftershock data. From the analysis of long-period multiple surface waves, a seismic moment of 3.1 × 10²⁸ dyn-cm is obtained, which is the largest reported so far for any intermediate or deep focus shock. This value, together with the estimated fault area of 90 × 70 km², gives an average dislocation of 7.0 m and a stress drop of 120 bar. This event represents an oblique thrust movement on a plane with dip direction N170°E, dip 48° and rake 52°. A geometrical consideration for the fault plane and the configuration of the inclined seismic zone beneath the Banda arc suggests, almost definitely, that the large-scale faulting took place within the subducted plate and offset it. Further repetition of such large-scale faulting might eventually break the subducted plate. The 1963 Banda Sea earthquake thus represents a seismological manifestation of the large-scale deformation of the subducted plate in the mantle.     

Nemuro-Oki earthquake of June 17, 1973: A lithospheric rebound at the upper half of the interface

P-wave first motions, radiation patterns and amplitudes of long-period surface waves, relocated aftershock distributions, leveling and tsunami data indicate that the 1973 Nemuro-Oki earthquake is caused by a low-angle thrust-faulting, representing a rebound at the upper 50 km of the interface between the continental and oceanic lithospheres. Rebound, most likely aseismic, at depths below 50 km, is suggested to take place in the near future from a comparison of recent geologic crustal deformation with pre-seismic and co-seismic data. The estimated seismic moment is about $\text{1}\text{3}\text{–}\text{1}\text{4}$ of that for the neighboring great earthquakes. The macro-seismic data suggest that the 1973 earthquake is smaller than the 1894 Nemuro-Oki earthquake, the last great earthquake in this region.The 1973 earthquake had been predicted on the basis of a seismic gap. Although the prediction was successful as to the location and nature of the faulting and partly as to the occurrence time, it is smaller than the predicted one. A part of the seismic gap may still remain. The difference between the observed seismic slip (1.6 m) and that predicted on the basis of the pre-seismic crustal deformation (3.0 m) indicates either (1) the 1973 earthquake relieved only a part of the strain accumulated in the upper 50 km, or (2) a significant amount of aseismic slip took place on the seismic fault and completely relieved the accumulated strain in the focal region of the 1973 earthquake. If the former is the case, the remaining strain, not only in the focal region, but also in the remaining seismic gap adjoining it, may be relieved in a larger earthquake in the future.The source parameters obtained are as follows: fault plane, dip direction = N40°W, dip angle = 27°; seismic moment = 6.7 · 10²⁷ dyn cm; average slip dislocation, 1.6 m in N63°W direction; stress drop = 35 bars. In these calculations, the fault dimension and the rigidity are assumed to be 100 · 60 km² and 7.0 · 10¹¹ dyn/cm², respectively.     

A comparison of rift-zone tectonics in Iceland and Hawaii

Observations of eroded volcanic rift zones indicate that dikes in Iceland are typically several times thicker than those in Hawaii. Geodetic and seismic observations of active rifts, however, suggest that dike heights in the two regions are similar. Provided the elastic properties of the rift zones are the same, this implies that dikes are intruded with higher driving pressures (magma pressure minus compressive stress perpendicular to the dike plane) in Iceland than Hawaii. A second major difference between the two regions is the greater prevalence of large normal fault scarps in rift zones in Iceland. From this it can be infered that a lower percentage of dikes breach the surface in Iceland than in Hawaii. Thus, although dikes in Iceland are intruded with higher driving pressures, they possess lower absolute magma pressures than in Hawaii. These differences can be interpreted in terms of the tectonic settings in the two regions. In Iceland, a steady remote extension reduces the horizontal stress perpendicular to the rift zone, allowing dikes to be intruded with low absolute pressures but high driving pressures when magma becomes available. In Hawaii, a more continuous magma supply on the timescale over which the dike-induced stresses are relaxed, and perhaps a greater role for intrusions in driving long-term rift extension, ensure that the rift-compressive stress is not relaxed significantly before the next dike is intruded. Thus the magma pressure must be nearly sufficient for eruption in order for intrusion to occur. If the mechanism for relaxing the rift-compressive stress were less efficient still, then an even higher percentage of dikes would erupt, and at times the rift zone trend could become an unfavorable orientation for dike intrusion. Such might be the case at Mauna Loa, which lacks large rift-zone faults and fissures and possesses numerous radial vents outside its two main rift zones.     

Focal mechanism and seismogenic structure of the Shiqu MS4.4 earthquake

The focal mechanism solution of the Shiqu M_S 4.4 earthquake occurred on May 16^th, 2017 in Sichuan Province is studied by the gCAP method using the waveform data from the regional seismic networks in Sichuan, Qinghai, Tibet and Gansu provinces. The strike/dip/dipping angle of the first nodal plane are 214°/80°/167° and those of the second nodal plane are 306°/77°/10°, the optimal centroid depth is 7.3 ​± ​0.6 ​km and the moment magnitude is M_W 4.5. Furthermore, the study investigates the robustness of the results against the error of crustal velocity structure, location, data quality and difference of seismic parameters, subsequently obtaining a stable resolved focal mechanism. According to the geological structure in the seismogenic area, spatial distribution of aftershock sequenceof the regional tectonic stress field, and the focal mechanism of the main shock, we suggest that the Shiqu earthquake is induced by a left-lateral strike-slip mechanism and the second nodal plane is inferred to be the seismogenic fault, consistent with the geometry of the Changshagongma fault which is the secondary fault of the northwest part of the Xianshuihe fault zone.     

Structure and evolution of the volcanic rift zone at Ponta de São Lourenço,eastern Madeira

Ponta de São Lourenço is the deeply eroded eastern end of Madeira’s east–west trending rift zone, located near the geometric intersection of the Madeira rift axis with that of the Desertas Islands to the southeast. It dominantly consists of basaltic pyroclastic deposits from Strombolian and phreatomagmatic eruptions, lava flows, and a dike swarm. Main differences compared to highly productive rift zones such as in Hawai’i are a lower dike intensity (50–60 dikes/km) and the lack of a shallow magma reservoir or summit caldera. ⁴⁰Ar/³⁹Ar age determinations show that volcanic activity at Ponta de São Lourenço lasted from >5.2 to 4 Ma (early Madeira rift phase) and from 2.4 to 0.9 Ma (late Madeira rift phase), with a hiatus dividing the stratigraphy into lower and upper units. Toward the east, the distribution of eruptive centers becomes diffuse, and the rift axis bends to parallel the Desertas ridge. The bending may have resulted from mutual gravitational influence of the Madeira and Desertas volcanic edifices. We propose that Ponta de São Lourenço represents a type example for the interior of a fading rift arm on oceanic volcanoes, with modern analogues being the terminations of the rift zones at La Palma and El Hierro (Canary Islands). There is no evidence for Ponta de São Lourenço representing a former central volcano that interconnected and fed the Madeira and Desertas rifts. Our results suggest a subdivision of volcanic rift zones into (1) a highly productive endmember characterized by a central volcano with a shallow magma chamber feeding one or more rift arms, and (2) a less productive endmember characterized by rifts fed from deep-seated magma reservoirs rather than from a central volcano, as is the case for Ponta de São Lourenço.     

Tsunami generation of the 1993 Hokkaido Nansei-Oki earthquake

Heterogeneous fault motion of the 1993 Hokkaido Nansei-Oki earthquake is studied by using seismic, geodetic and tsunami data, and the tsunami generation from the fault model is examined. Seismological analyses indicate that the focal mechanism of the first 10 s, when about a third of the total moment was released, is different from the overall focal mechanism. A joint inversion of geodetic data on Okushiri Island and the tide gauge records in Japan and Korea indicates that the largest slip, about 6 m, occurred in a small area just south of the epicenter. This corresponds to the initial rupture on a fault plane dipping shallowly to the west. The slip on the northernmost subfault, which is dipping to the east, is about 2 m, while the slips on the southern subfaults, which are steeply dipping to the west, are more than 3 m. Tsunami heights around Okushiri Island are calculated from the heterogeneous fault model using different grid sizes. Computation on the smaller grids produces large tsunami height that are closer to the observed tsunami runup heights. Tsunami propagation in the nearly closed Japan Sea is examined as the free oscillation of the Japan Sea. The excitation of the free oscillation by this earthquake is smaller than that by the 1964 Niigata or 1983 Japan Sea earthquake.     

Re-evaluation of the large deep earthquake of January 21, 1906

The large deep earthquake of January 21, 1906 is re-evaluated using old seismogram data and updated analysis techniques. From the P and pP-P time data the hypocentre parameters are determined as follows: origin time, 13h 49min 35s; latitude, 33.8°N; longitude, 137.5°E; depth, 340 km. The body-wave magnitude m_B is re-evaluated from the amplitude and periods of P, PP and S waves. The average value of 7.4 is obtained. This value is the smallest among any values assigned previously to this shock, and it is denied that the earthquake is the world's largest deep shock in this century. The focal mechanism is estimated from the P-wave first motions and amplitude distribution of P and S waves. Synthetic body waves are used to constrain the mechanism and to determine the seismic moment. The mechanism solution suggests the down-dip compression typical of this region. A seismic moment of 1.5 × 10²⁷ dyn · cm is obtained. This value and the re-evaluated value of m_B are consistent with the moment-_B relation obtained for other deep earthquakes.     

琼北火山群形成的动力学机制及地震现象的新认识

分析世界火山分布图发现琼北火山群分布在一南北向的火山带上,应用有限元方法模拟计算了双俯冲作用下海南岛所在雷琼\|越东火山带的形成机制,结合海南岛精确定位的地震数据和形变观测结果,认为琼北地区可能存在岩墙侵入或张性断裂膨胀,并根据地震数据模拟分析了岩墙侵入对区域应力场及形变的影响.琼北地区精确地震（2000~2006）定位结果表明地震主要集中在一个垂直面上,并且地震带两端有分叉现象.通过地震时空分布特征推测存在岩墙侵入,并通过数值模拟很好地解释了琼北地区地震的分布特征（狗骨头状）以及地表垂向形变东升西降的特征.     

Storage, migration, and eruption of magma at Kilauea volcano, Hawaii, 1971–1972

The magmatic plumbing system of Kilauea Volcano consists of a broad region of magma generation in the upper mantle, a steeply inclined zone through which magma rises to an intravolcano reservoir located about 2 to 6 km beneath the summit of the volcano, and a network of conduits that carry magma from this reservoir to sites of eruption within the caldera and along east and southwest rift zones. The functioning of most parts of this system was illustrated by activity during 1971 and 1972. When a 29-month-long eruption at Mauna Ulu on the east rift zone began to wane in 1971, the summit region of the volcano began to inflate rapidly; apparently, blockage of the feeder conduit to Mauna Ulu diverted a continuing supply of mantle-derived magma to prolonged storage in the summit reservoir. Rapid inflation of the summit area persisted at a nearly constant rate from June 1971 to February 1972, when a conduit to Mauna Ulu was reopened. The cadence of inflation was twice interrupted briefly, first by a 10-hour eruption in Kilauea Caldera on 14 August, and later by an eruption that began in the caldera and migrated 12 km down the southwest rift zone between 24 and 29 September. The 14 August and 24–29 September eruptions added about 10⁷ m³ and 8 × 10⁶ m³, respectively, of new lava to the surface of Kilauea. These volumes, combined with the volume increase represented by inflation of the volcanic edifice itself, account for an approximately 6 × 10⁶ m³/month rate of growth between June 1971 and January 1972, essentially the same rate at which mantle-derived magma was supplied to Kilauea between 1952 and the end of the Mauna Ulu eruption in 1971.The August and September 1971 lavas are tholeiitic basalts of similar major-element chemical composition. The compositions can be reproduced by mixing various proportions of chemically distinct variants of lava that erupted during the preceding activity at Mauna Ulu. Thus, part of the magma rising from the mantle to feed the Mauna Ulu eruption may have been stored within the summit reservoir from 4 to 20 months before it was erupted in the summit caldera and along the southwest rift zone in August and September.The September 1971 activity was only the fourth eruption on the southwest rift zone during Kilauea's 200 years of recorded history, in contrast to more than 20 eruptions on the east rift zone. Order-of-magnitude differences in topographic and geophysical expression indicate greatly disparate eruption rates for far more than historic time and thus suggest a considerably larger dike swarm within the east rift zone than within the southwest rift zone. Characteristics of the historic eruptions on the southwest rift zone suggest that magma may be fed directly from active lava lakes in Kilauea Caldera or from shallow cupolas at the top of the summit magma reservoir, through fissures that propagate down rift from the caldera itself at the onset of eruption. Moreover, emplacement of this magma into the southwest rift zone may be possible only when compressive stress across the rift is reduced by some unknown critical amount owing either to seaward displacement of the terrane south-southeast of the rift zone or to a deflated condition of Mauna Loa Volcano adjacent to the northwest, or both. The former condition arises when the forceful emplacement of dikes into the east rift zone wedges the south flank of Kilauea seaward. Such controls on the potential for eruption along the southwest rift zone may be related to the topographic and geophysical constrasts between the two rift zones.     

Analysis of seismological and tsunami data from the 1993 Guam earthquake

The fault parameters of the Guam earthquake of August 8, 1993 are estimated from seismological analyses, and the possibility of identifying the actual fault plane from tsunami waveforms is tested. The Centroid Moment Tensor solution of long-period surface waves shows one nodal plane shallowly dipping to the north and the other nodal plane steeply dipping to the south. The seismic moment is 3.5×10²⁰ Nm and the corresponding moment magnitude is 7.7. The Moment Tensor Rate Function inversion ofP waves also yields a similar focal mechanism and seismic moment. The point source depth is estimated as 40–50 km.This earthquake generated tsunamis that propagated toward the Japanese coast along the Izu-Bonin-Mariana ridge system. The tsunamis are recorded on ocean bottom pressure gauges and tide gauges. Numerical computation of tsunamis shows that the computed waveforms from the two possible fault planes match well with the observed tsunami waveforms. The numerical computation also shows that the tsunami waveforms at Guam Island, just above the fault, should contain useful information regarding the identification of the actual fault plane. However, the current sampling rate of the tide gauges is so small that the records cannot help the identification.     

The 1957 great Aleutian earthquake

The 9 March 1957 Aleutian earthquake has been estimated as the third largest earthquake this century and has the longest aftershock zone of any earthquake ever recorded—1200 km. However, due to a lack of high-quality seismic data, the actual source parameters for this earthquake have been poorly determined. We have examined all the available waveform data to determine the seismic moment, rupture area, and slip distribution. These data include body, surface and tsunami waves. Using body waves, we have estimated the duration of significant moment release as 4 min. From surface wave analysis, we have determined that significant moment release occurred only in the western half of the aftershock zone and that the best estimate for the seismic moment is 50–100×10²⁰ Nm. Using the tsunami waveforms, we estimated the source area of the 1957 tsunami by backward propagation. The tsunami source area is smaller than the aftershock zone and is about 850 km long. This does not include the Unalaska Island area in the eastern end of the aftershock zone, making this area a possible seismic gap and a possible site of a future large or great earthquake. We also inverted the tsunami waveforms for the slip distribution. Slip on the 1957 rupture zone was highest in the western half near the epicenter. Little slip occurred in the eastern half. The moment is estimated as 88×10²⁰ Nm, orM _w =8.6, making it the seventh largest earthquake during the period 1900 to 1993. We also compare the 1957 earthquake to the 1986 Andreanof Islands earthquake, which occurred within a segment of the 1957 rupture area. The 1986 earthquake represents a rerupturing of the major 1957 asperity.     

2014年温州珊溪M4.2水库地震研究

2014年8~11月,浙江省地震台网及水库监测台网利用高密度、高分辨率监测台站完整记录到温州珊溪水库地震序列活动3000多次,最大震级为M4.2。本文从3D震中定位、震源机制解并结合库区地质构造特征等对此地震序列进行了探讨。Hypo SAT地震定位法进行的定位结果线性拟合显示,主震发生在NW向断层,地震序列沿着活动断层密集成带,走向305°,倾向SW,倾角85°;采用P波初动符号方法得到了此次地震序列M3.5以上地震的平均震源机制解,节面B走向308°,倾角84°;野外地质调查研究表明,节面B的走向、倾角、倾向与双溪-焦溪断层产状基本一致;综合分析认为,NW向双溪-焦溪断层为此次地震的发震构造。     

The Campania-Lucania (southern Italy) earthquake of 23 November 1980

We present the main seismological results of our study of the Campania-Lucania earthquake of 23 November 1980. A complete set of far field and local data has been analysed. From long-period body waves data we determine the fault plane solution (φ₁ = 140°,δ₁ = 60°,φ₂ = 75°,δ₂ = 54°), a depth of 15 km and calculate a seismic moment of 6 × 10²⁵ dyne cm and a source duration of 6 s. From data of a local network deployed immediately after the event we determine aftershock locations: they are aligned in a direction NW-SE that fit extremely well with the focal solution determined above. We can choose as fault plane the plane striking 140° and dipping at 60° and the event is a normal event with a large component of left-lateral strike slip. The source area evaluated from this aftershock distribution 14 km × 40 km is quite suitable for an earthquake of a seismic moment of 6 × 10²⁵ dyne cm.     

Spacio-temporal history of the deep Colombia earthquake of 1970

The deep Colombia earthquake of July 31, 1970, showed a very high multiple-shock activity. The hypocentres of the seven events forming the multiple shock were determined by a relative location method. They cluster on a plane which coincides fairly well with the westerly-dipping nodal plane and show a southwards and upwards migration with time. The hypocentre distribution suggests that the earthquake occurred in the form of shear faulting. The primary P waveforms are computed from the long-period P-wave signals by eliminating the responses of the instrument, crust and mantle. The fault process is inferred from the spacio-temporal relation of the seven events and from the primary P waveforms: the rupture initiated in the lower part of the source region and spread out southwards and upwards. The whole process was almost finished when the intense multiple-shock activity terminated (about 50 s). The mechanism change of this earthquake can be described mainly by the change of the orientation of the fault plane rather than by slip vector change. The strike of the fault plane rotated clockwise about 10° during the events which occurred in the lower part of the source region. The further rotation of about 10° occurred at the subsequent stage. The rupture spread out with a jerk over a curved surface. The difference between the rupture processes in shallow and deep large earthquakes is discussed.     

2018年11月26日台湾海峡M_S6.2地震发震构造研究

本文利用福建省地震台网、广东省地震台网和台湾"中央"气象局17个台的宽频带记录,使用CAP方法反演了2018年11月26日台湾海峡M_S6.2地震震源机制解,得到节面1走向/倾角/滑动角为89°/82°/-173°,节面2走向/倾角/滑动角为358°/84°/-7°,最佳拟合深度14km,矩震级5.8.使用双差定位获取了94个M_L2.0以上地震的精定位结果,结果显示,主震位于北纬23.36°,东经118.62°,震源深度10.43km.根据小震分布和构造应力场反演得到余震断层面走向和倾角分别为88°和60°.研究认为,台湾海峡6.2级地震发震构造为近EW向的台湾浅滩断裂,受南海板块张裂拉伸发育而成,孕震过程中有东山隆起东缘断裂的参与,推测在菲律宾板块对欧亚板块NW-SE向挤压碰撞背景下,近EW向的台湾浅滩断裂与近NS向的东山隆起东缘断裂交接部位属于强度薄弱区,最终产生高倾角右旋走滑错动而引发地震,余震主要沿台湾浅滩断裂分布.     

Spatial variation of crustal strain in the Kachchh region,India: Implication on the Bhuj earthquake of 2001

The Kachchh province of Western India is a major seismic domain in an intraplate set-up. This seismic zone is located in a rift basin, which was developed during the early Jurassic break-up of the Gondwanaland. The crustal strain determined from the GPS velocity data of post-seismic time period following the 2001 Bhuj earthquake indicates a maximum strain rate of ∼266 × 10⁻⁹ per year along N013°. Focal mechanism solutions of the main event of 26 January 2001 and the aftershocks show that the maximum principal stress axis is close to this high strain direction. Maximum shear strain rate determined from the GPS data of the area has similar orientation. The unusually high strain rate is comparable in magnitude to the continental rift systems. The partitioning of the regional NE–SW horizontal stress (S_Hmax) by the pre-existing EW-striking boundary fault developed the strike–slip components parallel to the regional faults, the normal components perpendicular to the faults, NE-striking conjugate Riedel shear fractures and tension fractures. The partitioned normal component of the stress is considered to be the major cause for compression across the regional EW faults and development of the second-order conjugate shear fractures striking NE–SW and NW–SE. The NE-striking transverse faults parallel to the anti-Riedel shear planes have become critical under these conditions. These anti-Riedel planes are interpreted to be critical for the seismicity of the Kachchh region. The high strain rate in this area of low to moderate surface heat flow is responsible for deeper position of the brittle–ductile transition and development of deep seated seismic events in this intraplate region.