Regional Stress Changes Research Articles

A model for the hydrothermal tremor source of the Mefite d’Ansanto (Italy) CO2 non-volcanic emissions in the intermediate frequency band (1–15 Hz)

In the Summer 2021 a seismic passive survey was carried out at Mefite d’Ansanto (Italy), well-known for the cold non-volcanic and lethal CO2 emissions. Mefite is located close to that part of Irpinia region hosting the large historical earthquakes’ faults, that generated the destructive magnitude 6.8 earthquake of 1980 and have been related to the CO2 leakage by the scientific community. The survey was conducted by installing a small short-period seismic array with the purpose of determining the wavefield features and detecting the possible signature of the regional stress variations. By analyzing the acquired data, we have determined the properties of the seismic wavefield associated with the emission vents, in the intermediate frequency band, 1–15 Hz, which was found to be composed of a stationary background component and an intermittent higher energy one. Basing on our results, considerations on the medium properties and the deep source available information, we have depicted a schematic model of the shallow seismic source: the background components are linked to the shallower activity of the hydrothermal system, e.g. the bubbling, while the intermittent ones are likely generated by the passage of the overpressured gas, trapped in the first-tens-meters’ layers, after overcoming the internal cohesion charge. The characteristics of the wavefield that we have defined, refer to a condition in which no regional earthquakes occurred and could be considered as a basis to identify possible significant variations linked to CO2 emissions and to the regional stress changes.

P-wave velocity structure beneath reservoirs and surrounding areas in the lower Jinsha River

The lower reaches of the Jinsha River are rich in hydropower resources because of the high mountains, deep valleys, and swift currents in this area. This region also features complex tectonic structures and frequent earthquakes. After the impoundment of the reservoirs, seismic activity increased significantly. Therefore, it is necessary to study the P-wave velocity structure and earthquake locations in the lower reaches of the Jinsha River and surrounds, thus providing seismological support for subsequent earthquake prevention and disaster reduction work in reservoir areas. In this study, we selected the data of 7,670 seismic events recorded by the seismic networks in Sichuan, Yunnan, and Chongqing and the temporary seismic arrays deployed nearby. We then applied the double-difference tomography method to this data, to obtain the P-wave velocity structure and earthquake locations in the lower reaches of the Jinsha River and surrounds. The results showed that the Jinsha River basin has a complex lateral P-wave velocity structure. Seismic events are mainly distributed in the transition zones between high- and low-velocity anomalies, and seismic events are particularly intense in the Xiluodu and Baihetan reservoir areas. Vertical cross-sections through the Xiangjiaba and Xiluodu reservoir areas revealed an apparent high-velocity anomaly at approximately 6 km depth; this high-velocity anomaly plays a role in stress accumulation, with few earthquakes distributed inside the high-velocity body. After the impoundment of the Baihetan reservoir, the number of earthquakes in the reservoir area increased significantly. The seismic events in the reservoir area north of 27° N were related to the enhanced activity of nearby faults after impoundment; the earthquakes in the reservoir area south of 27° N were probably induced by additional loads (or regional stress changes), and the multiple microseismic events may have been caused by rock rupture near the main faults under high pore pressure.

Regolith and host rock influences on CO2 leakage: Active source seismic profiling across the Little Grand Wash fault, Utah

Understanding carbon dioxide (CO2) reservoir to surface migration is crucial to successful carbon capture and sequestration approaches; especially fault/reservoir interactions under injection pressure. Through seismic imaging, we explore regolith and shallow stratigraphy across the Little Grand Wash fault. The presence of natural CO2 seeps, travertine and tufa deposits confirm modern and ancient fault-controlled CO2 leakage. We consider this an analogue for a long-failed sequestration site. We estimate bulk porosity and fracture density for host rock, regolith, and fault zone from petrophysical relationships. When combined with existing geochemical and geological data, we characterize a 60 m wide damage zone that represents the primary surface delivery channel for CO2 originating from reservoir depths. Within this damage zone, low seismic velocities suggest sediments have formed through host rock chemical dissolution or mechanical weathering. In contrast, velocities within the adjacent host rock are consistent with low fracture density clastic rocks. We measure anomalously high seismic velocities within the fault zone along one profile that best represents a sealed (cemented/plugged) low permeability, relic flow channel. This suggests that shallow fault zone permeability varies along strike. While regional stress changes may account for decadal- to millennial-scale changes in CO2 pathways, we speculate that the total fluid pressure has locally reduced the fault's minimum horizontal effective stress; thereby producing both low- and high-permeability fault segments that either block or promote fluid migration. Studying CO2 migration in this system can inform potential risks to future sequestration projects and guide monitoring efforts.

Complex Coseismic and Postseismic Faulting During the 2021 Northern Thessaly (Greece) Earthquake Sequence Illuminated by InSAR Observations

AbstractThree Mw > 5.5 earthquakes (Tyrnavos, Elassona and Verdikoussa earthquakes) struck the northern Thessaly, Greece, from 3 March to 12 March 2021, filling a significant seismic gap. We exploit Interferometric synthetic aperture radar data to investigate coseismic deformation of these events and the first half‐year postseismic deformation related to the Tyrnavos event. Our coseismic modeling reveals that the seismic sequence activated at least four unmapped low‐angle normal faults. However, postseismic analysis shows that afterslip propagates updip to a high‐angle shallow structure, indicating that the coseismic slip and afterslip jointly activated a ramp‐flat structure. By a comprehensive analysis of the inversions, fault distribution, relocated aftershocks and regional stress changes, we highlight that intersections between the main and secondary faults control the slip extent and rupture termination in each earthquake and that static stress transfers induced by earlier event encourage the rupture reinitiation of the later event.

Raise Caving\u2014A\xa0Hybrid Mining Method Addressing Current Deep Cave Mining Challenges

Cave mining progresses to depths exceeding 1000 m and ore bodies situated in competent and strong rock masses are nowadays extracted by different cave mining methods. Widely applied caving methods in massive deposits are block and panel caving, inclined caving, and sublevel caving. All caving methods have in common that rock mass caves during extraction of an ore body in a controlled way. As a result, regional stress changes occur, considerable abutment stresses form, and large-scale subsidence and significant seismic energy releases occur. Experience shows that these rock mechanics effects become especially critical at great depths, where primary stress magnitudes reach and exceed rock mass strength, as well as in strong competent rock masses, which require large footprints to enable continuous caving. The presented raise caving method addresses previously mentioned rock mechanics issues. Initially, de-stressing slots are developed from raises with a minimum amount of pre-development. Substantial pillars separate neighboring slots in order to control stress magnitudes and seismicity near slots. The slots provide a stress shadow for production infrastructure so that large-scale mineral extraction can take place in de-stressed ground. As mining progresses, pillars are extracted and hanging wall is allowed to cave. Results of a pre-study conducted together with LKAB have highlighted advantages of raise caving from a rock mechanics, safety, and cost point of view.

Geophysical Borehole Observatory at the North Anatolian Fault in the Eastern Sea of Marmara (GONAF): initial results

Given its intense seismic activity and damaging earthquake generation potential, the western part of the North Anatolian Fault constitutes a serious natural hazard. As a result, the fault is monitored with a broad range of seismological and geodetic instrumentation making it a natural laboratory environment for scientific studies. One of the long-term projects in this region is GONAF (Geophysical Borehole Observatory at the North Anatolian Fault) which is the first borehole seismometer network project in Turkey. GONAF is a joint research project that started in 2011 as joint initiative of the Turkish Ministry of Interior, Disaster and Emergency Management Presidency AFAD and GFZ and the German Research Center for Geoscience Helmholtz Center Potsdam. The aim of GONAF is to detect, examine, and monitor the microseismic activity in the region and to observe the physical processes before, during and after a large Marmara earthquake (M > 7.0) that is expected to rupture the western part of the North Anatolian Fault, below the Marmara Sea along the Princes Islands segment or even further to the west. For this purpose, the permanent GONAF observatory was established consisting of 7 borehole seismometer arrays installed down to a depth of 300 m. In this paper, we report on regional stress changes in the western part of the North Anatolian Fault Zone (NAFZ) using instrumental data and the Coulomb stress method. We also present preliminary results of the observation and evaluation of microseismic activity obtained from the GONAF observatory. For the automatic evaluation of real-time data, Seiscomp3, RTQUAKE, and Earthworm Softwares were used. Within the scope of automatic earthquake detection studies, between March, 2016 and November, 2017, a total of 2568 earthquakes were detected using the RTQUAKE software. Of these, 1459 could be analyzed. While the magnitude of the analyzed earthquakes varies between 0.8 and 4.2, the depth of these events ranges from 2 to 30 km.

Deep Mining: A Rock Engineering Challenge

Increasing demand for metals caused by global economic growth and exploitation of shallow mineral deposits forces mineral extraction to go deeper. A direct consequence of this development is an increase in rock pressure-related mining problems. The role of rock engineering in the design and operation of deep mines is discussed in detail. Critical issues are the rock fracturing around mining excavations, the support and control of the fractured rock, and the rock mechanics design of mine infrastructure and extraction (stoping) systems. Progress of the science of rock mechanics in the areas related to these issues is highlighted and critically examined. Specific areas are the prediction and assessment of the mechanical properties of rock mass, the mechanics of controlling fractured rock around deep mining excavations and the resulting demands on support systems. Rock engineering aspects of stoping systems and the regional stress changes resulting from the extraction of large mineral bodies are discussed in detail. The progress in design concepts for open stopes and stopes with caving of the roof strata is illustrated. It is shown that the stress environment in deep mines does not favour the highly productive caving systems of stoping. The value of energy-based design concepts for very deep mines exploiting tabular mineral deposits is shown. Despite the considerable progress that has been made in the science of rock mechanics since the 1950s, progress in applying this knowledge to solve rock pressure problems in deep mines has been rather slow. The tools are available. What is needed is the development of robust design criteria for mine infrastructure, excavations and support systems for dynamic and changing stress environments. The second critical issue is the lack of highly qualified rock engineering personnel on the mines. This has been recognized by the European mining industry through supporting a continued education programme in rock engineering for deep mines.

Cenozoic development of southwestern Malay Basin: new insights from subsidence analysis and thermal history

Subsidence and thermal history analysis are carried out in order to investigate the Cenozoic basin development of the southwestern (Tenggol Arch and basinal side) part of the Malay basin. Structurally, the southwestern part consists of normal faults and horst and graben geometry. Tectonic subsidence curves show that the basinal side is more active than the Tenggol Arch due to movement along normal faults. Cenozoic development initiated with the deposition of sedimentary Units M & L (Oligocene) and the activation of the Tenggol fault on the basinal side. Several periods of accelerated and slow subsidence are observed during the Oligocene to Middle Miocene that could be associated with changes in regional stresses caused by pulsating plate movement. The Malay Basin experienced inversion throughout the Middle to Late Miocene related to mantle induced slab avalanche effect, causing relatively higher tectonic subsidence rates on the Tenggol Arch compared to the basinal side, suggesting that the Tenggol Arch is less affected by inversion than the basinal side. After a period (Late Miocene) of non-deposition, the basin was reactivated (Pliocene to recent) due to thermal relaxation with thick sedimentation. Paleo heat flows estimated utilizing a novel technique introduced in this study and present day heat flow calibrated using BHT data further supports our results, in that increase in heat flow is related to rapid tectonic subsidence. An anomalously high heat pulse affected the basin during inversion and could be the cause of meta-sediment formation whereas present heat flows, although high compared to average basins, shows decreasing trend from the inversion period.

Revisiting the Canterbury earthquake sequence after the 14 February 2016 Mw 5.7 event

AbstractOn 14 February 2016, an Mw 5.7 (GNS Science moment magnitude) earthquake ruptured offshore east of Christchurch, New Zealand. This earthquake occurred in an area that had previously experienced significant seismicity from 2010 to 2012 during the Canterbury earthquake sequence, starting with the 2010 Mw 7.0 Darfield earthquake and including four Mw ~6.0 earthquakes near Christchurch. We determine source parameters for the February 2016 event and its aftershocks, relocate the recent events along with the Canterbury earthquakes, and compute Coulomb stress changes resolved onto the recent events and throughout the greater Christchurch region. Because the February 2016 earthquake occurred close to previous seismicity, the Coulomb stress changes resolved onto its nodal planes are uncertain. However, in the greater Christchurch region, there are areas that remain positively loaded, including beneath the city of Christchurch. The recent earthquake and regional stress changes suggest that faults in these regions may pose a continuing seismic hazard.

Natural Fractures in shale: A review and new observations

Natural fractures have long been suspected as a factor in production from shale reservoirs because gas and oil production commonly exceeds the rates expected from low-porosity and low-permeability shale host rock. Many shale outcrops, cores, and image logs contain fractures or fracture traces, and microseismic event patterns associated with hydraulic-fracture stimulation have been ascribed to natural fracture reactivation. Here we review previous work, and present new core and outcrop data from 18 shale plays that reveal common types of shale fractures and their mineralization, orientation, and size patterns. A wide range of shales have a common suite of types and configurations of fractures: those at high angle to bedding, faults, bed-parallel fractures, early compacted fractures, and fractures associated with concretions. These fractures differ markedly in their prevalence and arrangement within each shale play, however, constituting different fracture stratigraphies—differences that depend on interface and mechanical properties governed by depositional, diagenetic, and structural setting. Several mechanisms may act independently or in combination to cause fracture growth, including differential compaction, local and regional stress changes associated with tectonic events, strain accommodation around large structures, catagenesis, and uplift. Fracture systems in shales are heterogeneous; they can enhance or detract from producibility, augment or reduce rock strength and the propensity to interact with hydraulic-fracture stimulation. Burial history and fracture diagenesis influence fracture attributes and may provide more information for fracture prediction than is commonly appreciated. The role of microfractures in production from shale is currently poorly understood yet potentially critical; we identify a need for further work in this field and on the role of natural fractures generally.

Nonvolcanic Tremor in the Aleutian Arc

Nonvolcanic tremors (NVT) have been observed across the Aleutian arc. The tremorlike events are observed over a large region spanning seismic networks on several volcanoes and are therefore unlikely to be related to any one volcanic complex. Although locating the events is not possible at the present time, we provide general locations based on move-out times and amplitudes across multiple networks. The majority of NVT events are recorded in regions where the subducting Pacific plate is inferred to be locked or transitioning from creeping to locked. One NVT event was recorded during a time of increased seismic and volcanic activity in the area near the Rat and Andreanof Islands. Similar simultaneous increases have been observed in the Aleutians in the past and are hypothesized to be caused by large regional stress changes, possibly caused by slow-slip events. A prominent recent NVT event was recorded on the Korovin and Great Sitkin seismic networks approximately one hour after the onset of the 12 July 2008 eruption of Okmok volcano. NVT does not appear sporadically across the Aleutian arc but seems to concentrate in a few regions. With 2500 km of subduction zone, we have an opportunity to examine these regions and compare them to regions not producing NVT in an attempt to identify factors that may play a role in its creation.

Groundwater changes in relation to seismic activity: a case study from Eskipazar (Karabuk, Turkey)

The reported study contributes to research on earthquake prediction. Between 2007 and 2009, changes were observed in two geothermal and mineral springs located in Eskipazar (∼3–5 km to the north of the North Anatolian Fault Zone) in Turkey, in relation to small-magnitude earthquakes. During pre-seismic and post-seismic activities, variations were observed in the hydrogeological parameters of the spring waters. Temperature increases of 0.4–1°C were measured in one of the springs prior to three different earthquakes. There was a slight increase in the spring discharge with respect to the first earthquake, which occurred closest to the spring. This led to a reduction in electrical conductivity (EC), total dissolved solids (TDS), Ca, HCO3, δ13C, Al, Mn, and Fe concentrations in the spring water, whereas tritium and Se values increased. Several days before the third earthquake, which occurred at a shallower depth, a decrease was observed in the discharge, which led to a reduction in tritium, δ13C and Si concentrations. These variations could be explained by changes in the mixing ratio of waters of different genesis, depending on changes in permeability, pore pressure, and flow paths of the aquifer due to regional stress changes.

Contribution of sagittal orientation of facet joints and disc degeneration to degenerative spondylolisthesis in lumbar spine: A finite element research

目的 探讨小关节矢状化与椎间盘退变间的关系及其对退变性腰椎滑移的作用和意义.方法 采用一种新型CAD方法精确构建65°小关节角、45°小关节角、25°小关节角与正常椎间盘、轻度退变椎间盘、重度退变椎间盘相组配的9种腰椎L4-L5活动节段有限元模型.生理压缩载荷下,分别对9种有限元模型的生物力学参数进行测试.结果 与小关节角45°和25°有限元模型相比,小关节角65°有限元模型的矢状方向椎体前移位增加,关节突、峡部等效应力和关节突水平方向接触力明显增加;小关节角65°有限元模型的终板膨出减小,纤维环基质应力增加.与正常有限元模型相比,椎间盘轻度退变有限元模型刚度下降,小关节突及峡部应力轻度增加.9种有限元模型中,轻度退变椎间盘结合小关节角65°有限元模型的抗前剪力能力最差.结论 小关节角矢状化既是退变性腰椎滑移的原发诱因,又是局部应力变化导致关节突再塑形的继发病理改变,矢状型小关节腰椎活动节段矢状方向内在不稳定性受椎间盘退变程度的影响,椎间盘退变对小关节角矢状化无明显促进作用。

Implications of earthquake focal mechanisms for the frictional strength of the San Andreas fault system

Abstract Analysis of stress orientation data from earthquake focal plane mechanisms adjacent to the San Andreas fault in the San Francisco Bay area and throughout southern California indicates that the San Andreas fault has low frictional strength. In both regions, available stress orientation data indicate low levels of shear stress on planes parallel to the San Andreas fault. In the San Francisco Bay area, focal plane mechanisms from within 5 km of the San Andreas and Calaveras fault zones indicate a direction of maximum horizontal compression nearly orthogonal to both subvertical, right-lateral strike-slip faults, a result consistent with those obtained previously from studies of aftershocks of the 1989 Loma Prieta earthquake. In southern California, the direction of maximum horizontal stress near the San Andreas fault is nearly everywhere at a high angle to it, similarly indicating that the fault has low frictional strength. Thus, along these two major sections of the San Andreas fault (which produced great earthquakes in southern California in 1857 and central and northern California in 1906), the frictional strength of the fault is much lower than expected for virtually any common rock type if near-hydrostatic pore pressure exists at depth, and so low as to produce no discernible shear-heating anomaly. Our findings in southern California are in marked contrast to recent suggestions by Hardebeck & Hauksson that stress orientations rotate systematically within c. 25 km of the fault, which prompted a high frictional strength model of the San Andreas fault. As we utilize the same stress data and inversion technique as Hardebeck & Hauksson, we interpret the difference in our findings as being related to the way in which we group focal plane mechanisms to find the best-fitting stress tensor. We suggest that the Hardebeck & Hauksson gridding scheme may not be consistent with the requisite a priori assumption of stress homogeneity for each set of earthquakes. Finally, we find no evidence of regional stress changes associated with the occurrence of the 1992 M7.4 Landers earthquake, again in apparent contradiction with the findings of Hardebeck & Hauksson.

Regional seismicity triggered by the Hyogo-Ken Nanbu, Japan, M=7.2 earthquake on January 17, 1995

The time and spatial feature of the regional seismicity triggered by the Hyogo-Ken Nanbu, Japan, M=7.2 earthquake on January 17,1995, was studied. The concerned region is about several hundred kilometers in length and breadth surrounding the epicenter (33°–37°N, 133°–138°E). It is divided into 16 subregions. The seismicity of these subregions from January of 1976 to June of 1996 has been analyzed. It is showed that, 1 there were significant seismicity changes in 10 subregions triggered by the Hyogo-Ken Nanbu, Japan, M=7.2 earthquake on January 17, 1995. These changes passed a Z statistic test exceeding 0.95 confidence level and the greatest epicenter distance of these subregions was 280 km; 2 seismicity changes were triggered within 1–5 days in three subregions near the main shock while in other subregions the seismicity changes were triggered within several ten days after the main shock; 3 the greatest triggered event is 5.4, which is about the same size as the greatest aftershock; 4 the regional stress change resulted from the main shock may be the triggered mechanism of the regional seismicity.

Aseismic fault movement before the 1976 Tangshan earthquake detected by levelling: implications for pre-seismic stress localization?

Levelling before the 1976 Tangshan earthquake (ML=7.8, China) shows significant height changes in the Tangshan region. Inversion analysis of the height changes observed during 1971–1972 and 1972–1973 reveals two active segments on the Jiyunhe fault which truncates the Tangshan earthquake fault to the southwest. The active fault segment of 1971–1972 is characterized by left‐lateral strike‐slip (1.2±0.2 m) with a length of 37±3 km, a (down‐dip) width of 7±2 km, a dip angle of 84°±2°, a (bottom) depth of 17±2 km and a strike direction of 65°±2° (NW). The active fault segment of 1972–1973 is characterized by left‐lateral strike‐slip (2.8±0.2 m) with a length of 75±5 km, a (down‐dip) width of 8±2 km, a dip angle of 88°±2°, a (bottom) depth of 30±2 km and a strike direction of 47°±2° (NW). The geometric characteristics of these two active segments are in good agreement with those obtained from geological investigations and source‐mechanism solutions of the 1976 Ninghe earthquake (ML=6.9). No microearthquakes of ML≥3.0 occurred during 1971–1973 in the Tangshan region, suggesting that the activity of the fault segments during this period was largely aseismic. The results demonstrate that aseismic slip at the Jiyunhe fault occurred before the 1976 Tangshan earthquake. Evidence for activity of the Jiyunhe fault also comes from the horizontal deformation observed by triangulation and trilateration between 1960 and 1971, which clearly indicates relative motion between two flanks of the fault. Stress analysis reveals that aseismic slip at the Jiyunhe fault during 1972–1973 induced significant shear stress concentration (at a depth of 20 km) within an area of about 40×40 km close to the conjunction of the Tangshan and Jiyunhe faults, and that the Tangshan earthquake hypocentre received a Coulomb failure stress of about 4.5 bar. Our evidence suggests that the 1976 Tangshan earthquake was triggered or induced by aseismic slip at the Jiyunhe fault. In addition, all of the large aftershocks of ML≥6.0 occurred within areas where the calculated Coulomb failure stress is positive and concentrated, indicating that the aftershocks were possibly triggered by the 1976 Tangshan main shock. This study shows that aseismic fault movement could play an important role in controlling regional stress change and in triggering earthquakes on surrounding faults.

Creep Response of the Hayward Fault to Stress Changes Caused by the Loma Prieta Earthquake

In 1996, an 18-millimeter creep event, the largest ever observed on the Hayward fault, occurred between surveys 63 days apart. This event marked the end of a period of severely reduced creep on the southern part of the fault that began after the 1989 Loma Prieta, California, earthquake. The reduction in creep was consistent with elastic models for earthquake-induced static stress changes on the Hayward fault. These data suggest that creep observations can indicate regional stress changes of about 1 bar or less.

State of stress before and after the 1994 Northridge Earthquake

The state of tectonic stress in the epicentral area of the 17 January 1994, Northridge earthquake (Mw 6.7' is investigated by applying a stress inversion method to P‐wave polarity data from earthquakes in Northridge from July 1981 to January 1994 and from the Northridge aftershocks during January 1994 to December 1995. A 3‐D crustal model is used to trace the rays taking off from the hypocenter, which reduced the effects of large structural heterogeneities on the determination of the stress tensor. We found significant temporal changes of stress orientations induced by the Northridge earthquake. The principal pressure (P' axis is oriented N32°E from 1981 to June 1992, and N30°E from 28 June 1992 to 16 January 1994, suggesting that the stress field in Northridge was not affected by the 1992 Landers earthquake. During two weeks following the Northridge mainshock, the P‐axis is oriented N13°E, which is a significant (17°' change from that before the earthquake (N30°E'. Between February 1994 and August 1995 the P‐axis orientation changes from N18°E to N26°E, and finally ends up at N34° by the end of 1995, which is close to that before the Northridge earthquake. These results suggest that the stresses rotated coseismically, then rotated more slowly back to their original orientation. The aftershocks caused by the mainshock changed the stress distribution in the crust, which showed up as a regional stress change. The stress recovery appears to have completed within two years after the mainshock, which is very short compared to the time scale of the earthquake cycle.

Superposed local and regional paleostresses: Fault‐slip analysis of Neogene extensional faulting near coeval caldera complexes, Yucca Flat, Nevada

Numerous reduced stress tensors are computed by multiple inversions of 906 temporally and spatially partitioned fault‐slip data from the Yucca Flat region in the southwest Nevada volcanic field to constrain the Neogene paleostress and faulting history and to investigate how the regional tectonic stress field was affected by local caldera magmatism. Perturbed, shallow (<400 m), pre‐11 Ma paleostress configurations, determined west and northwest of present (post‐11 Ma) Yucca Flat basin, existed during mild extensional faulting and are attributed to superposition of transient caldera‐magmatic stresses on the regional stress field. Northwest of Yucca Flat a progressive shift in least principal stress (σ3) directions near known calderas located 5–15 km to the west occurred under a normal‐slip stress state during caldera development between about 15 and 13 Ma. A brief (∼0.5 m.y.) change to a strike‐slip stress state occurred at about 13 Ma and was accompanied by small‐offset, quasi‐conjugate strike‐slip faulting. This stress state was most distinct, relative to a normal‐slip state, near calderas where stress solutions and fault relations indicate closer affinities to a reverse‐slip state. Inferred 11.6–11.45 Ma paleostress tensors indicate radial tension associated with either initial caldera collapse or local post‐collapse topographic modification of the stress field. Post‐11 Ma normal‐slip stress tensors are associated with normal‐ and oblique‐slip faults that accommodated subsidence and eastward extension of Yucca Flat basin away from the caldera complexes. These tensors do not indicate stress modifications due to residual caldera‐related effects and thus were used to infer post‐11 Ma regional stress changes. The stress field has rotated as much as 65° clockwise since 11 Ma during extensional development of Yucca Flat basin, with most of the rotation and extension occurring before about 8.5 Ma. Results suggest that shallow magmatism and caldera development can strongly alter extensional tectonic stress fields, fault patterns, and slip directions in the uppermost crust out to distances of roughly two magma chamber radii away from a magma body.

An anomalous radon decrease in groundwater prior to an M6.0 earthquake: A possible precursor?

A. significant decrease in the radon concentration of artesian groundwater was observed at a well‐established observation site in Northeast Japan about two days before an M6.0 earthquake occurred about 200 km from the site. On the basis of a concurrent in situ (−126m) temperature drop of the groundwater, we infer that precursory changes in regional stress were locally amplified near this site and resulted in the temporary mixing of cooler, radon‐poor groundwater into the normal artesian flow.