Chaman Fault System Research Articles

Tectonic Geomorphology Of The Jhelum Fault Zone And Its Contiguous Regions In Western Himalaya

The western Himalayan syntaxis represents the region where the major Himalayan structures abruptly curve and the cause of the curvature and the tectonic geomorphology of the region has not been fully explored. The lack of detailed structural maps with extensive field-based data are missing, which is mainly because of the political problems related to border sharing between Pakistan, India, and Afghanistan. However, and fortunately, the usage of satellite derived images has overcome such constrains by providing a robust platform to remotely map such regions. Therefore, the present study was aimed to supplement our previous works in the region by exploring the western portions of the Hazara-Kashmir-Syntaxis. We have used Google terrain imagery to map the evidence for active faulting that involves mapping of triangular facets, displaced and/or faulted topographic ridges, river terraces, alluvial fans and so on. The crosscutting relationships are used to date the faulting events where absolute dates are not available. Our results show that active tectonic deformation is not just limited to the previously mapped structures (e.g. the Kalabagh Fault, the Salt Range Thrust, the Mahesian Anticline, and the Jhelum Fault) but occurs on a broader deformation zone that is delimited by the Chaman fault system in the west, and the Jhelum Fault in the east. The deformation zone in 3D resembles a tectonically formed diamond shaped box that has Salt Range Thrust in front and the Main Boundary Fault at the back with sides delimited by the Chaman and the Jhelum fault systems. The earthquake centroid moment tensor data compliments our geomorphological work, and establishes that transpression is a dominant tectonic process that governs the western regions, which is in comparison to the east where transtension is the norm in the interior Himalayan with reverse and thrust faulting dominant in the frontal regions.

Post‐Seismic to Co‐Seismic Moment Ratios for the 2016 Moderate Earthquakes Along Chaman Fault

AbstractPostseismic‐to‐coseismic moment ratios (PCMR) for strong (M > 6) earthquakes typically do not exceed ∼0.3, but there have been limited estimates for moderate‐to‐small earthquakes. Three M5‐class earthquakes and one M4.7 earthquake occurred along the Chaman fault, Pakistan, on May 13 and July 10, 2016. Using Sentinel‐1A InSAR time‐series from October 2014 to August 2018, we estimate spatial‐temporal evolution of the co‐ and post‐seismic fault slip and its velocity. Because of the long‐lasting (>1 year) afterslip, the estimated PCMRs of the M5‐class events and M4.7 event were 0.7 ± 0.2 and 8.3 ± 2.2, respectively, which are both larger than those of M > 6 earthquakes but consistent with previous events that occurred on the Chaman Fault System. Unlike in case of large earthquakes, geodetic inversions are not able to resolve spatial relationships between coseismic slip and afterslip. In case of a non‐complementary slip distribution, one explanation might be heterogeneity of friction properties on the fault surface, while other interpretations are possible.

Spatial variation of b-value, creep rate, and seismic moment release along Chaman fault system

Spatial variation of b-value, creep rate, and seismic moment release along Chaman fault system

Remote Sensing Study of Neotectonics of Katwaz Region in Balochistan-Pakistan

This investigation deals with the delineation of neotectonics and its connection to the regional surfacedeformation in the Katwaz region (KR) located in the north of Quetta along the Pakistan-Afghan border. It consists ofrocks dominated by silicates and carbonates, were uplifted due to regional tectonics. In this study, SRTM DEM (90mspatial resolution) based Surface Dynamics (SDMs) of KR is analyzed to investigate the Isobase level (IBL), Relativerelief (RR), Topographic surface roughness (TSR) and topographic Slope (TS) map to constrain the active surfacedeformation due to regional neotectonics. It gives complete information regarding the lithology based erosional marginsand demarcation of neotectonic scarps and established the relationship between Strahler order streams and neotectonicsettings in an actively deformed KR. This study aims to delineate margins for geomorphological variations and to findthe effects of neotectonics. Another purpose is to examine the geomorphic parameters and to map active tectonicsdevelopment due to transform sinistral movement of the Chaman Fault system (CFS) in the context of the ongoingcollision of India-Eurasia. The resultant and generated surface dynamic maps (SDMs) were due to neotectonicdevelopments, progressions and orientations along with CFS and Suleman fold, and thrust belts (SFTBs). The resultsobtained revealed that IBL values are higher in the NNE, SSW, and central parts along with the CFS, and partially in theNW corner of the KR due to uplifted SFTBs. The active tectonic nature of SFTBs and TSR values are lower in the centraland northern parts of KR. The TSR values are higher along SFTBs and CFS due to greater vertical dissection betweenthese two active faults system. The DEM based SDMs are time-efficient and cost-effective for easy identification anddelineation of neo-tectonically active zones using remote sensing and GIS techniques.

Evaluation of DEM based Neotectonics of Sulaiman range and lobe region (SRLR), Balochistan-Pakistan

The Indian plate’s western boundary is extremely tilted towards the direction of convergence between Asia and India signifying an exceptional example of massive scale oblique continent-continent collision. This transform boundary motion is sinistral in nature and Digital Elevation Model (DEM) based geomorphometry permits the investigation of 3-D characteristics of the regional topography. This technique is specifically useful along inter-plate transform margins where the visual interpretation and topographic assessment is unable to observe the trends of the landforms evolution. In this context, the geomorphometry of the regional surface might be influenced largely by climate change and lower rate of active deformation. The geomorphic indices; Isobase Levels (IBLs), Topographic Asymmetry factor (T-factor) and Hack Gradient Index (HGI) have been executed to analyze fluvial drainage networks in connection to neotectonic movements. The sinistral movement along the Chaman Fault system and relatively less deformed Katawaz Block are the major surface dynamics to compute the style of deformation in the Sulaiman Range and Lobe Region (SRLR). Within the SRLR, the Sulaiman Fold and Thrust Belt (SFTB) is thrusting with a rate of 4–15 mm/yr in a North-South (NS) trend between IndoPak-Eurasian and a shortening of 4–6 mm/yr in East-West (EW) context. Seismicity is typically towards NorthWest-SouthEast (NW-SE) accommodating the moment release in the current century between the IndoPak-Eurasian transform plate boundary in the western most of the western Himalayan Syntaxis. The results obtained from the applied geomorphic indices indicate significant uplift in terms of higher IBL and HGI values all along the SRLR (Harnai, Katwaz, Qilla Saifullah, Zhob, Loralai, Jissa Shareef, Sakhi Sarwar and Fort Munro). The T-factor results also display tilt block neotectonics that are in agreement with the results of IBL and HGI.

Reflection seismic, gravity, magnetic, and InSAR analysis of the Chaman Fault in Pakistan

This study presents new insights on the Chaman Fault System that connects deformation in the Makran and Himalayan collision zones. The Chaman Fault is ~ 850 km long, and shear strain is concentrated in a densely populated region between 29 and 32° N. Three earthquakes with Mw ~ 5 occurred in May, July, and August of 2016 along a segment the of the Chaman Fault at 30.5° N. This study uses Interferometric Synthetic Aperture Radar, reflection seismic, gravity, and magnetic data to understand earthquake distribution along the Chaman Fault and to characterize its subsurface structure. Reflection seismic profiles across the Chaman Fault show that it dips ~ 80° towards the west and has been imaged down to ~ 7 km. Gravity and magnetic data integrated with six seismic profiles document a sedimentary basin up to ~ 5 km deep that contains an intrusive body adjacent to the Chaman Fault. The seismic data also show the southern segment of the Chaman Fault widens at a depth between ~ 2.5 and 4 km that correlates with the fault locking depth. Interferometric Synthetic Aperture Radar results from 2006 to 2011 observes slip rate distributions that explain the increasing frequency of earthquake activity along the locked segment of the Chaman Fault at ~ 30.5° N.

Seismicity and seismic hazard parameters in and around Pakistan

Pakistan holds a key position within the Indian–Eurasian collision zone and is a highly earthquake-prone country of South Asia. Besides the devastating earthquakes of Quetta 1935, Makran 1945 and Pattan 1974, the earthquakes that happened in Bhuj 2001, Kashmir 2005 and Hindu Kush 2015 are three typical examples of the new millennium. In this study, the earthquake probability and the return period for magnitude recurrence hazard have been analysed using a magnitude homogenous earthquake catalogue. In Gumbel’s theory of extreme values, the Gumbel III distribution has a vital role to play as far as seismic hazard analysis of any regional seismicity is concerned. The Gumbel III distribution is used here to estimate the upper bound magnitude (ω value) and other earthquake hazard parameters in and around Pakistan. This method is applied to the whole of Pakistan (23° N–39° N; 59° E–80° E) and shallow (0–70 km) and intermediate (70–300 km) depth earthquakes in the Hindu Kush region across the Afghanistan–Pakistan border over the period 1930–2007. From the entire Pakistan dataset, the ω value is estimated as 8.12 (± 0.59). Furthermore, intermediate earthquakes in Hindu Kush have a ω value of 8.15 (± 0.56) compared with 7.08 (± 0.92) for the shallow earthquakes in the Hindu Kush. From probabilistic analysis of a spatial matrix of cells, results of earthquake expected magnitude parameters are then calculated, mapped, contoured and discussed in the context of recognized seismotectonic zones (Hindu Kush, Karakorum–Himalaya, Chaman fault system and Makran–Kutch) which characterize the seismicity of Pakistan and the surrounding region (Afghanistan, Tajikistan and India).

Over 100 years of faults interaction, stress accumulation, and creeping implications, on Chaman Fault System, Pakistan

The curve-shaped Chaman transform fault between Pakistan and Afghanistan demarks the western boundary of the Indian plate, which is the locus of many catastrophic earthquakes, and this is poorly studied region to understand the earthquake/fault interaction and hazard assessment. Based on earthquake triggering theory, we investigate a sequence of earthquakes comprising five earthquakes that occurred in the Chaman Fault System since 1892. Our results elucidate that three out of four earthquakes are triggered by the preceding earthquakes. The 1935 earthquake of magnitude Mw 7.7 is somewhat an independent earthquake in this sequence, slightly influenced by the 1892 event. The 1935 earthquake significantly loaded either terminus of its rupture with positive stress, which later triggered the 1975 earthquake with 35% of its rupture length where ΔCFS is above triggering threshold value, and 1990 earthquake with 44% of its rupture length in positive stress lobe, on Ghazaband Fault, in the south lobe. Since the 1935 earthquake significantly increased the ΔCFS on either terminus of its rupture, the 2013 earthquake with magnitude Mw 7.7 occurred on Hoshab Fault in the positive stress zone with 23% of its rupture length where ΔCFS is positive. Moreover, the northern locked segment of CF, the southern segment of GBF, southeastern segment of HF and PAF demonstrate a high value of ΔCFS. It could trigger seismicity in these regions, thus requiring arrangements to mitigate any seismic hazards in future.

Active tectonic deformation of the western Indian plate boundary: A case study from the Chaman Fault System

Collision of the Eurasian and Indian plates has resulted in two spatially offset subduction zones, the Makran subduction zone to the south and the Himalayan convergent margin to the north. These zones are linked by a system of left-lateral strike-slip faults known as the Chaman Fault System, ∼1200km, which spans along western Pakistan. Although this is one of the greatest strike-slip faults, yet temporal and spatial variation in displacement has not been adequately defined along this fault system. This study conducted geomorphic and geodetic investigations along the Chaman Fault in a search for evidence of spatial variations in motion. Four study areas were selected over the span of the Chaman Fault: (1) Tarnak-Rud area over the Tarnak-Rud valley, (2) Spinatizha area over the Spinatizha Mountain Range, (3) Nushki area over the Nushki basin, and (4) Kharan area over the northern tip of the Central Makran Mountains. Remote sensing data allowed for in depth mapping of different components and faults within the Kohjak group. Wind and water gap pairs along with offset rivers were identified using high-resolution imagery and digital-elevation models to show displacement for the four study areas. The mountain-front-sinuosity ratio, valley height-to-width-ratio, and the stream-length-gradient index were calculated and used to determine the relative tectonic activity of each area. These geomorphic indices suggest that the Kharan area is the most active and the Tarnak-Rud area is the least active. GPS data were processed into a stable Indian plate reference frame and analyzed. Fault parallel velocity versus fault normal distance yielded a ∼8–10mm/yr displacement rate along the Chaman Fault just north of the Spinatizha area. InSAR data were also integrated to assess displacement rates along the fault system. Geodetic data support that ultra-slow earthquakes similar to those that strike along other major strike-slip faults, such as the San Andreas Fault System, are possible along the northern segments of the Chaman Fault zone. Geomorphic data suggest that the Chaman Fault along southern part is not very active now but may have gone through high tectonic activity in the past.

An Investigation of 2013 M<sub>w</sub> 7.7 Awaran Earthquake, Pakistan

Earthquake is a major disaster responsible for vast losses both life and infrastructure. Pakistan is one of the highly earthquake prone areas in Asia. The present study is carried out to investigate the dynamics of disastrous Awaran earthquake. The 2013 Mw 7.7 Awaran earthquake and its Mw 6.8 aftershock caused numerous loss of lives and vast infrastructure damages. The earthquake triggered along Hoshab fault rupturing 230 km of the fault. The rupture propagated at 3 Km/s on average and was concentrated in top 10 km with no major displacement in the underlying decollement. The rupture released a cumulative moment of 5.4 x 1020 Nm. This study reveals that although Hoshab fault originated as thrust fault, the slip was purely strike slip during the earthquake and that the rigid block rotation of southeastern Makran is responsible for the Awaran earthquake. The study concludes that the earthquake significantly increased the coulomb stress on Makran mega thrust and strike slip faults in Chaman fault system, hence increasing the risk of a major seismic event. Therefore, in order to prevent major loss of lives and infrastructure damages; designing of new building codes, reassessing the seismic hazard of the region and marking of hidden faults is of utmost importance.

An Investigation of 2013 M<sub>w</sub> 7.7 Awaran Earthquake, Pakistan

Earthquake is a major disaster responsible for vast losses both life and infrastructure. Pakistan is one of the highly earthquake prone areas in Asia. The present study is carried out to investigate the dynamics of disastrous Awaran earthquake. The 2013 Mw 7.7 Awaran earthquake and its Mw 6.8 aftershock caused numerous loss of lives and vast infrastructure damages. The earthquake triggered along Hoshab fault rupturing 230 km of the fault. The rupture propagated at 3 Km/s on average and was concentrated in top 10 km with no major displacement in the underlying decollement. The rupture released a cumulative moment of 5.4 x 1020 Nm. This study reveals that although Hoshab fault originated as thrust fault, the slip was purely strike slip during the earthquake and that the rigid block rotation of southeastern Makran is responsible for the Awaran earthquake. The study concludes that the earthquake significantly increased the coulomb stress on Makran mega thrust and strike slip faults in Chaman fault system, hence increasing the risk of a major seismic event. Therefore, in order to prevent major loss of lives and infrastructure damages; designing of new building codes, reassessing the seismic hazard of the region and marking of hidden faults is of utmost importance.

Study of Subsidence and Earthquake Swarms in the Western Pakistan

In recent years, the Quetta Valley and surrounding areas have experienced unprecedented levels of subsidence, which has been attributed mainly to groundwater withdrawal. However, this region is also tectonically active and is home to several regional strike-slip faults, including the north–south striking left-lateral Chaman Fault System. Several large earthquakes have occurred recently in this area, including one deadly Mw 6.4 earthquake that struck on 28 October 2008. This study integrated Interferometric Synthetic Aperture Radar (InSAR) results with GPS, gravity, seismic reflection profiles, and earthquake centroid-moment-tensor (CMT) data to identify the impact of tectonic and anthropogenic processes on subsidence and earthquake patterns in this region. To detect and map the spatial-temporal features of the processes that led to the surface deformation, this study used two Synthetic Aperture Radar (SAR) time series, i.e., 15 Phased Array L-band Synthetic Aperture Radar (PALSAR) images acquired by an Advanced Land Observing Satellite (ALOS) from 2006–2011 and 40 Environmental Satellite (ENVISAT) Advanced Synthetic Aperture Radar (ASAR) images spanning 2003–2010. A Small Baseline Subset (SBAS) technique was used to investigate surface deformation. Five seismic lines totaling ~60 km, acquired in 2003, were used to map the blind thrust faults beneath a Quaternary alluvium layer. The median filtered SBAS-InSAR average velocity profile supports groundwater withdrawal as the dominant source of subsidence, with some contribution from tectonic subsidence in the Quetta Valley. Results of SBAS-InSAR multi-temporal analysis provide a better explanation for the pre-, co-, and post-seismic displacement pattern caused by the 2008 earthquake swarms across two strike-slip faults.

InSAR observations of strain accumulation and fault creep along the Chaman Fault system, Pakistan and Afghanistan

AbstractWe use 2004–2011 Envisat synthetic aperture radar imagery and InSAR time series methods to estimate the contemporary rates of strain accumulation in the Chaman Fault system in Pakistan and Afghanistan. At 29 N we find long‐term slip rates of 16 ± 2.3 mm/yr for the Ghazaband Fault and of 8 ± 3.1 mm/yr for the Chaman Fault. This makes the Ghazaband Fault one of the most hazardous faults of the plate boundary zone. We further identify a 340 km long segment displaying aseismic surface creep along the Chaman Fault, with maximum surface creep rate of 8.1 ± 2 mm/yr. The observation that the Chaman Fault accommodates only 30% of the relative plate motion between India and Eurasia implies that the remainder is accommodated south and east of the Katawaz block microplate.

Ball-and-socket tectonic rotation during the 2013 [formula omitted] Balochistan earthquake

The September 2013 Mw7.7 Balochistan earthquake ruptured a ∼200-km-long segment of the curved Hoshab fault in southern Pakistan with 10±0.2 m of peak sinistral and ∼1.7±0.8 m of dip slip. This rupture is unusual because the fault dips 60±15° towards the focus of a small circle centered in northwest Pakistan, and, despite a 30° increase in obliquity along strike, the ratios of strike and dip slip remain relatively uniform. Surface displacements and geodetic and teleseismic source inversions quantify a bilateral rupture that propagated rapidly at shallow depths from a transtensional jog near the northern end of the rupture. Static friction prior to rupture was unusually weak (μ<0.05), and friction may have approached zero during dynamic rupture. Here we show that the inward-dipping Hoshab fault defines the northern rim of a structural unit in southeast Makran that rotates – akin to a 2-D ball-and-socket joint – counter-clockwise in response to India's penetration into the Eurasian plate. This rotation accounts for complexity in the Chaman fault system and, in principle, reduces seismic potential near Karachi; nonetheless, these findings highlight deficiencies in strong ground motion equations and tectonic models that invoke Anderson–Byerlee faulting predictions.

The 2013 Mw 7.7 Balochistan Earthquake: Seismic Potential of an Accretionary Wedge

Great earthquakes rarely occur within active accretionary prisms, despite the intense long‐term deformation associated with the formation of these geologic structures. This paucity of earthquakes is often attributed to partitioning of deformation across multiple structures as well as aseismic deformation within and at the base of the prism (Davis et al., 1983). We use teleseismic data and satellite optical and radar imaging of the 2013 M_w 7.7 earthquake that occurred on the southeastern edge of the Makran plate boundary zone to study this unexpected earthquake. We first compute a multiple point‐source solution from W‐phase waveforms to estimate fault geometry and rupture duration and timing. We then derive the distribution of subsurface fault slip from geodetic coseismic offsets. We sample for the slip posterior probability density function using a Bayesian approach, including a full description of the data covariance and accounting for errors in the elastic structure of the crust. The rupture nucleated on a subvertical segment, branching out of the Chaman fault system, and grew into a major earthquake along a 50° north‐dipping thrust fault with significant along‐strike curvature. Fault slip propagated at an average speed of 3.0 km/s for about 180 km and is concentrated in the top 10 km with no displacement on the underlying decollement. This earthquake does not exhibit significant slip deficit near the surface, nor is there significant segmentation of the rupture. We propose that complex interaction between the subduction accommodating the Arabia–Eurasia convergence to the south and the Ornach Nal fault plate boundary between India and Eurasia resulted in the significant strain gradient observed prior to this earthquake. Convergence in this region is accommodated both along the subduction megathrust and as internal deformation of the accretionary wedge.

The 2013, Mw 7.7 Balochistan earthquake, energetic strike-slip reactivation of a thrust fault

We analyse the Mw 7.7 Balochistan earthquake of 09/24/2013 based on ground surface deformation measured from sub-pixel correlation of Landsat-8 images, combined with back-projection and finite source modeling of teleseismic waveforms. The earthquake nucleated south of the Chaman strike-slip fault and propagated southwestward along the Hoshab fault at the front of the Kech Band. The rupture was mostly unilateral, propagated at 3 km/s on average and produced a 200 km surface fault trace with purely strike-slip displacement peaking to 10 m and averaging around 6 m. The finite source model shows that slip was maximum near the surface. Although the Hoshab fault is dipping by 45° to the North, in accordance with its origin as a thrust fault within the Makran accretionary prism, slip was nearly purely strike-slip during that earthquake. Large seismic slip on such a non-optimally oriented fault was enhanced possibly due to the influence of the free surface on dynamic stresses or to particular properties of the fault zone allowing for strong dynamic weakening. Strike-slip faulting on thrust fault within the eastern Makran is interpreted as due to eastward extrusion of the accretionary prism as it bulges out over the Indian plate. Portions of the Makran megathrust, some thrust faults in the Kirthar range and strike-slip faults within the Chaman fault system have been brought closer to failure by this earthquake. Aftershocks cluster within the Chaman fault system north of the epicenter, opposite to the direction of rupture propagation. By contrast, few aftershocks were detected in the area of maximum moment release. In this example, aftershocks cannot be used to infer earthquake characteristics.

Interseismic strain accumulation along the western boundary of the Indian subcontinent

Despite an overall sinistral slip rate of ≈3 cm/yr, few major earthquakes have occurred in the past 200 years along the Chaman fault system, the western boundary of the India Plate with the Eurasia Plate. GPS and InSAR data reported here indicate sinistral shear velocities of 8–17 mm/yr across the westernmost branches of the fault system, suggesting that a significant fraction of the plate boundary slip is distributed in the fold and fault belt to the east. At its southernmost on‐land segment (≈26°N), near the triple junction between the Arabia, Eurasia, and India Plates, we find the velocity across the Ornach Nal fault is 15.1+13.4+16.9mm/yr, with a locking depth probably less than 3 km. At latitude 30°N near the town of Chaman, Pakistan, where a M6.5 earthquake occurred in 1892, the velocity is 8.5+6.8+10.3mm/yr and the fault is locked at approximately 3.4 km depth. At latitude 33°N and further north, InSAR data indicate a velocity across the Chaman fault of 16.8 ± 2.7 mm/yr. The width of the plate boundary varies from several km in the south where we observe ≈2 mm/yr of convergence near the westernmost strike‐slip faults, to a few hundreds of km in the north where we observe 6–9 mm/yr of convergence, and where the faulting becomes distinctly transpressional. The shallow locking depth along much of the transform system suggests that earthquakes larger than those that have occurred in the historical record would be unexpected, and that the recurrence interval of those earthquakes that have occurred is of the order of one or two centuries, similar in length to the known historical record.

Earthquake fault superhighways

Motivated by the observation that the rare earthquakes which propagated for significant distances at supershear speeds occurred on very long straight segments of faults, we examine every known major active strike-slip fault system on land worldwide and identify those with long (> 100 km) straight portions capable not only of sustained supershear rupture speeds but having the potential to reach compressional wave speeds over significant distances, and call them “fault superhighways”. The criteria used for identifying these are discussed. These superhighways include portions of the 1000 km long Red River fault in China and Vietnam passing through Hanoi, the 1050 km long San Andreas fault in California passing close to Los Angeles, Santa Barbara and San Francisco, the 1100 km long Chaman fault system in Pakistan north of Karachi, the 700 km long Sagaing fault connecting the first and second cities of Burma, Rangoon and Mandalay, the 1600 km Great Sumatra fault, and the 1000 km Dead Sea fault. Of the 11 faults so classified, nine are in Asia and two in North America, with seven located near areas of very dense populations. Based on the current population distribution within 50 km of each fault superhighway, we find that more than 60 million people today have increased seismic hazards due to them.

Slow earthquake in Afghanistan detected by InSAR

The Chaman fault system forms a prominent ∼900‐km‐long left‐lateral transform plate boundary between the Indian and Eurasian plates in Afghanistan and Pakistan. Here we show satellite radar interferometry data that revealed an afterslip (or slow earthquake) signal following an earthquake of magnitude 5.0. This slow slip episode lasted for more than a year, and accompanied a widespread creep signal that occurred at least ∼50 km along the fault. We detected no surface slip before the earthquake during the 1.5 years sampled by our data. This finding of long‐lasting widespread afterslip demonstrates that the plate motion along the Chaman Fault is accommodated by slow slip episodes following moderate earthquakes, and suggests that a potential for magnitude 7‐class earthquakes was significantly reduced. The duration and moment release of the detected afterslip do not fit the recently proposed scaling law for slow earthquakes.

Earthquakes and Associated Deformation in Northern Baluchistan 1892-2001

We examine a century of seismicity and earthquake-related deformation on the western margin of the Indian plate in the Pakistan region of northern Baluchistan. Several catalogs of earthquakes for this region currently exist, but early catalogs in particular suffer from errors, incompleteness, inhomogeneity, and location bias. We form a new catalog of more than 1000 earthquakes using original sources to confirm macroseismic locations and assign M S magnitudes to earthquakes since 1892. In doing so we reveal a systematic east-northeast bias in locations caused largely by the uneven global distribution of seismic stations used in their determination. An appendix provides narrative accounts and historical references to 34 significant earthquakes in the region. The pattern of seismicity in the past century shows activity over a 700-km-long, 200-km-wide segment of the plate boundary with predominantly strike-slip faulting to the west and thrust faulting to the east. At its narrowest near 29° N, transpression of the plate boundary is partitioned into reverse and strike-slip components separated by approximately 100 km. The M 7.3 1931 Mach earthquake (slip 1.1 m on a 40° east-southeast-dipping reverse fault) released fault-normal stresses that may have “unclamped” the subsequent M 7.7 left-lateral Quetta earthquake 4 years later. The northern Chaman fault system in the past century has been largely inactive, suggesting that this time period is not representative of long-term activity in the region and that up to 4 m of potential slip is currently available to drive one or more future M > 7 earthquakes. Despite triangulation installed in the late nineteenth and early twentieth centuries, no recent geodetic data are available to permit plate boundary velocities to be measured directly.