华北北部三次地震前的应变释放加速特征

强地震发生前,区域地震活动往往出现应变释放加速(AMR)现象。本文以华北地区的1976年唐山7.8级地震、1998年张北6.2级地震、2006年文安5.1级地震作为目标地震,分别分析三次主震前4~8年期间、40~120 km半径范围内的AMR现象及其特征。结果显示,唐山7.8级地震与张北6.2级地震之前均不同程度出现AMR异常,但文安5.1级地震之前并未出现。另外,本研究发现80 km半径可能是研究区寻找AMR异常有效、合理的空间扫描尺度。基于这三个震例研究的认识,本文进一步针对晋冀蒙交界地区和首都圈地区进行分析计算,结果识别出首都圈地区至目前存在AMR异常地区,这可作为判定华北北部未来强震危险性的参考。     

P- and S-Wave Site Response of the Seismic Network RESNOM Determined from Earthquakes of Northern Baja California, Mexico

—We determined the response to P- and S-wave incidence of the permanent stations of the seismic network of Baja California (RESNOM) using two independent methods. We selected 65 events with magnitudes between 2.2 and 4.8 and hypocentral distances ranging between 5 and 330 km. The site response of the ten stations analyzed was first estimated using average spectral ratios between the horizontal and the vertical components of motion (H/V ratios). As a second approach we performed a simultaneous inversion for source and site. In order to invert the spectral records to determine the site response, we made an independent estimate of the attenuation for two different source-station path regions. Then we corrected the spectral records for the attenuation effect before we made the inversion. Although the average H/V ratio of many sites is inside the error bars of the site response estimated with the spectral inversion, the spectral inversion tends to give higher values. For the S wave some sites show similar frequency of predominant peak when comparing the responses obtained with both methods. In contrast, for the P waves the H/V ratios disagree with the results of the inversion. In general, the site response of the stations is strongly frequency dependent for both P and S waves. We also found that the natural frequency of resonance of the sites is near 0.5 Hz for P and near 0.8 Hz for the S waves.     

1989年以来华北地区中强震前地震矩加速释放空间扫描特征的研究

利用地震矩释放程度m值对1989年以来华北地区12次MS≥5.0中强地震前的矩加速释放现象进行了研究。基于删除余震后的地震目录,提取了与矩加速释放相关的特征时—空尺度作为AMR现象在华北地区的扫描模型参数。采用震前1年逐3个月的扫描方式,得到了12次震例在震中附近与时间相依的m值的空间演变图像,并考察了震前震源区附近的Benioff应变释放情况。结果表明,华北地区MS5.0~5.5地震前AMR现象与实际震中和发震时刻的关联性不明显;而MS5.6~6.4地震前67%的震例在震中附近出现稳定和集中分布的矩释放加速区域,说明华北地区MS≥5.5中强震前AMR现象具有一定程度的普遍性,利用AMR现象对未来中强震发震地点的预测有一定的潜能和可挖掘性,但仍需其他方法相互佐证。     

Spatial Separation of Large Earthquakes, Aftershocks, and Background Seismicity: Analysis of Interseismic and Coseismic Seismicity Patterns in Southern California

We associate waveform-relocated background seismicity and aftershocks with the 3-D shapes of late Quaternary fault zones in southern California. Major earthquakes that can slip more than several meters, aftershocks, and near-fault background seismicity mostly rupture different surfaces within these fault zones. Major earthquakes rupture along the mapped traces of the late Quaternary faults, called the principal slip zones (PSZs). Aftershocks occur either on or in the immediate vicinity of the PSZs, typically within zones that are ??2-km wide. In contrast, the near-fault background seismicity is mostly accommodated on a secondary heterogeneous network of small slip surfaces, and forms spatially decaying distributions extending out to distances of ??10?km from the PSZs. We call the regions where the enhanced rate of background seismicity occurs, the seismic damage zones. One possible explanation for the presence of the seismic damage zones and associated seismicity is that the damage develops as faults accommodate bends and geometrical irregularities in the PSZs. The seismic damage zones mature and reach their finite width early in the history of a fault, during the first few kilometers of cumulative offset. Alternatively, the similarity in width of seismic damage zones suggests that most fault zones are of almost equal strength, although the amount of cumulative offset varies widely. It may also depend on the strength of the fault zone, the time since the last major earthquake as well as other parameters. In addition, the seismic productivity appears to be influenced by the crustal structure and heat flow, with more extensive fault networks in regions of thin crust and high heat flow.     

Groundwater recharge by channel infiltration in El Barbon basin,Baja California,Mexico

The amount of groundwater recharge by channel infiltration is estimated for El Barbon basin, in Baja California, Mexico. The basin’s lower portion includes the valleys of Ojos Negros and Real del Castillo Viejo, which are crossed by several ephemeral washes, including the mainstem El Barbon Wash. A distributed catchment model with the capability for nonlinear channel routing and channel abstraction is used to calculate groundwater recharge by channel infiltration for storm events of 2-, 5-, 10-, 25-, 50-, and 100-yr return period. The results confirm that event channel infiltration can be a substantial component of the vertical recharge.     

A Forward Test of the Precursory Decelerating and Accelerating Seismicity Model for California

Accelerating strain energy released by the generation of intermediate magnitude preshocks in a broad (critical) region, and decelerating energy released in a narrower (seismogenic) region, is considered as a distinct premonitory pattern useful in research for intermediate-term earthquake prediction. Accelerating seismicity in the broad region is satisfactorily interpreted by the critical earthquake model and decelerating seismicity in the narrower region is attributed to stress relaxation due to pre-seismic sliding. To facilitate the identification of such patterns an algorithm has been developed on the basis of data concerning accelerating and decelerating preshock sequences of globally distributed already occurred strong mainshocks. This algorithm is applied in the present work to identify regions, which are currently in a state of accelerating seismic deformation and are associated with corresponding narrower regions, which are in a state of decelerating seismic deformation in California. It has been observed that a region which includes known faults in central California is in a state of decelerating seismic strain release, while the surrounding region (south and north California, etc.) is in a state of accelerating seismic strain release. This pattern corresponds to a big probably oncoming mainshock in central California. The epicenter, magnitude and origin time, as well as the corresponding model uncertainties of this probably ensuing big mainshock have been estimated, allowing a forward testing of the model's efficiency for intermediate-term earthquake prediction.     

Shallow Miocene basaltic magma reservoirs in the Bahia de Los Angeles basin, Baja California, Mexico

The basement in the Bahía de Los Angeles basin consists of Paleozoic metamorphic rocks and Cretaceous granitoids. The Neogene stratigraphy overlying the basement is formed, from the base to the top, by andesitic lava flows and plugs, sandstone and conglomeratic horizons, and Miocene pyroclastic flow units and basaltic flows. Basaltic dikes also intrude the whole section. To further define its structure, a detailed gravimetric survey was conducted across the basin about 1 km north of the Sierra Las Flores. In spite of the rough and lineal topography along the foothills of the Sierra La Libertad, we found no evidence for large-scale faulting. Gravity data indicates that the basin has a maximum depth of 120 m in the Valle Las Tinajas and averages 75 m along the gravimetric profile. High density bodies below the northern part of the Sierra Las Flores and Valle Las Tinajas are interpreted to be part of basaltic dikes. The intrusive body located north of the Sierra Las Flores is 2.5 km wide and its top is about 500 m deep. The lava flows of the top of the Sierra Las Flores, together with the distribution of basaltic activity north of this sierra, suggests that this intrusive body continues for 20 km along a NNW-trending strike. Between the sierras Las Flores and Las Animas, a 0.5-km-wide, 300-m-thick intrusive body is interpreted at a depth of about 100 m. This dike could be part of the basaltic activity of the Cerro Las Tinajas and the small mounds along the foothills of western Sierra Las Animas. The observed local normal faulting in the basin is inferred to be mostly associated with the emplacement of the shallow magma reservoirs below Las Flores and Las Tinajas.     

Evolving Towards a Critical Point: A Review of Accelerating Seismic Moment/Energy Release Prior to Large and Great Earthquakes

—There is growing evidence that some proportion of large and great earthquakes are preceded by a period of accelerating seismic activity of moderate-sized earthquakes. These moderate earthquakes occur during the years to decades prior to the occurrence of the large or great event and over a region larger than its rupture zone. The size of the region in which these moderate earthquakes occur scales with the size of the ensuing mainshock, at least in continental regions. A number of numerical simulation studies of faults and fault systems also exhibit similar behavior. The combined observational and simulation evidence suggests that the period of increased moment release in moderate earthquakes signals the establishment of long wavelength correlations in the regional stress field. The central hypothesis in the critical point model for regional seismicity is that it is only during these time periods that a region of the earth’s crust is truly in or near a "self-organized critical" (SOC) state, such that small earthquakes are capable of cascading into much larger events. The occurrence of a large or great earthquake appears to dissipate a sufficient proportion of the accumulated regional strain to destroy these long wavelength stress correlations and bring the region out of a SOC state. Continued tectonic strain accumulation and stress transfer during smaller earthquakes eventually re-establishes the long wavelength stress correlations that allow for the occurrence of larger events. These increases in activity occur over longer periods and larger regions than quiescence, which is usually observed within the rupture zone of a coming large event. The two phenomena appear to have different physical bases and are not incompatible with one another.     

Geochemistry of Cenezoic volcanic rocks, Baja California, Mexico: Implications for the petrogenesis of post-subduction magmas

Late Cenozoic volcanism in Baja California records the effects of cessation of subduction at a previously convergent, plate margin. Prior to 12.5 m.y., when subduction along the margin of Baja ceased, the predominant volcanic activity had a calc-alkaline signature, ranging in composition from basalt to rhyolite. Acidic pyroclastic activity was common, and possibly represented the westermost, distal edge of the Sierra Madre Occidental province. After 12.5 m.y., however, the style and composition of the magmatic products changed dramatically. The dominant rock type within the Jaraguay and San Borja volcanic fields is a magnesian andesite, with up to 8% MgO at 57% SiO₂, low Fe/Mg ratios, and high Na/K ratios. These rocks have unusual trace-element characteristics, with high abundances of Sr (up to 3000 ppm), low contents of Rb; K/Rb ratios are very high (usually over 1000, and up to 2500), and Rb/Sr ratios are low (less than 0.01). Furthermore, La_n/Yb_n ratios are high, consistent with derivation from a mantle source with fractionated REE patterns. ⁸⁷Sr/⁸⁶Sr ratios are less than 0.7048, and usually less than 0.7040, whereas the pre-12.5 m.y. lavas have ⁸⁷Sr/⁸⁶Sr ratios between 0.7038 and 0.7063. We have previously termed these rocks bajaites, in order to distinguish them from other magnesian andesites. Bajaites also occur in southernmost Chile and the Aleutian Islands, areas which also have histories of attempted or successful ridge subduction.It is proposed that the bajaite series is produced during the unusual physico-chemical conditions operating during the subduction of young oceanic lithosphere, or subduction of a spreading centre. During normal subduction, the oceanic crust dehydrates, releasing volatiles (water, Rb and other large-ion lithophile elements) into the overlying wedge. Subduction of younger crust will result in a progressive decrease, and eventual cessation of the transfer of volatiles when subduction stops. Thermal rebound of the mantle may cause the slab to melt, perhaps under eclogitestable conditions. The resulting melt will be heavy-REE-depleted, perhaps dacitic, but will otherwise inherit MORB-like Rb/Sr and K/Rb ratios. The ascending melt will react with the mantle to form the source of the bajaitic rocks. Furthermore, any amphibole in the mantle, stabilised during the higher PH₂O conditions of earlier subduction, will break down and contribute a high-K/Rb ratio component.The implications of this study are that firstly, the subducted slab does not contribute a highly fractionated REE component in most modern arcs (i.e. the slab does not melt); secondly, Rb has a very short residence time in the mantle, and its abundance in arc rocks is a direct reflection of the input from the dehydrating slab; and thirdly, bajaitelike rocks may provide recognition of attempted or successful ridge subduction in the geologic past.     

Spatial Distribution,Scaling and Self-similar Behavior of Fracture Arrays in the Los Planes Fault,Baja California Sur,Mexico

We present a case of detailed analysis of fracture arrays spanning four orders of magnitude in length; all of them measured at a single natural site by acquiring images at progressively larger scales. There is a high dispersion of cumulative-length exponents, box dimensions and fracture densities. However, the fractal analysis supports the fractal nature of fracture arrays. Our data indicate the existence of an upper limit for the density parameters, as similarly reported by other authors. We prove that box dimension is in inverse relation with fracture concentration and in direct relation with fracture density. These relations are also observed in our data and additionally there is an upper limit for the box dimensions. We interpret the dispersion in our results as more fundamental than methodological problems. It represents a truncation in the complete evolution of the fracture systems because in natural cases strain initiates overprinting of previous fracture arrays. Considering that larger fractures accommodate strain more efficiently than small fractures, the generation of small fractures is inhibited in the presence of pre-existing larger fractures. Maximum values of fracture density prevent accommodating an excess of strain in a single or restricted range of scales; we claim this condition produces migration of fracturing to larger scales originating fracture scaling.     

Geology, ore deposits and hydrothermal venting in Bahía Concepción, Baja California Sur, Mexico

Abstract Bahía Concepción is located in the eastern coast of the Baja California peninsula and it is shaped by northwestern–southeastern normal faults. These are associated with a 12–6 Ma rifting episode, although some have been reactivated since the Pliocene. The most abundant rocks correspond to the arc related Comondú Group, Oligocene to Miocene, which forms a mainly calc‐alkaline volcanic and volcaniclastic sequence. There are less extensive outcrops of sedimentary rocks, lava flows, domes and pyroclastic rocks of Pliocene to Quaternary ages. The Neogene volcanism in the area indicates a shift from a subduction regime to an intraplate volcanism related to continental extension and the opening of an oceanic basin. The Bahía Concepción area contains numerous Mn ore deposits, being the biggest at El Gavilán and Guadalupe. The Mn deposits occur as veins, breccias and stockworks, and are composed by Mn oxides (pyrolusite, coronadite, romanechite), dolomite, quartz and barite. The deposits are hosted in volcanic rocks of the Comondú Group and, locally, in Pliocene sedimentary rocks. Thus, the Mn deposits formed between the Middle Miocene and the Pliocene. The mineralized structures are associated with Miocene northwestern–southeastern fault systems, which are analogous to those associated with the Cu‐Co‐Zn‐Mn deposits of El Boleo. The Bahía Concepción area also bears subaerial and submarine hot springs, which are associated with the same fault systems and host rocks. The submarine and subaerial geothermal manifestations south of the bay are possibly related with recent volcanism. The geothermal manifestations within the bay are intertidal hot springs and shallow submarine diffuse venting areas. Around the submarine vents (5–15 m deep, 87°C), Fe‐oxyhydroxide crusts with pyrite and cinnabar precipitate. In the intertidal vents (62°C), aggregates of opal, calcite, barite and Ba‐rich Mn oxides occur covered by silica‐carbonate stromatolitic sinters. Some 10–30 cm thick crustiform veins formed by chalcedony, calcite and barite are also found close to the vents. The hydrothermal fluids exhibit mixed isotopic compositions between δ¹⁸O‐enriched meteoric and local marine water. The precipitation of Ba‐rich Mn oxides around the vent sites could be an active analog for the processes that produced Miocene to Pliocene hydrothermal Mn‐deposits.     

Volcanic Markers of the Post-Subduction Evolution of Baja California and Sonora, Mexico: Slab Tearing Versus Lithospheric Rupture of the Gulf of California

The study of the geochemical compositions and K-Ar or Ar-Ar ages of ca. 350 Neogene and Quaternary lavas from Baja California, the Gulf of California and Sonora allows us to discuss the nature of their mantle or crustal sources, the conditions of their melting and the tectonic regime prevailing during their genesis and emplacement. Nine petrographic/geochemical groups are distinguished: ??regular?? calc-alkaline lavas; adakites; magnesian andesites and related basalts and basaltic andesites; niobium-enriched basalts; alkali basalts and trachybasalts; oceanic (MORB-type) basalts; tholeiitic/transitional basalts and basaltic andesites; peralkaline rhyolites (comendites); and icelandites. We show that the spatial and temporal distribution of these lava types provides constraints on their sources and the geodynamic setting controlling their partial melting. Three successive stages are distinguished. Between 23 and 13 Ma, calc-alkaline lavas linked to the subduction of the Pacific-Farallon plate formed the Comondú and central coast of the Sonora volcanic arc. In the extensional domain of western Sonora, lithospheric mantle-derived tholeiitic to transitional basalts and basaltic andesites were emplaced within the southern extension of the Basin and Range province. The end of the Farallon subduction was marked by the emplacement of much more complex Middle to Late Miocene volcanic associations, between 13 and 7 Ma. Calc-alkaline activity became sporadic and was replaced by unusual post-subduction magma types including adakites, niobium-enriched basalts, magnesian andesites, comendites and icelandites. The spatial and temporal distribution of these lavas is consistent with the development of a slab tear, evolving into a 200-km-wide slab window sub-parallel to the trench, and extending from the Pacific coast of Baja California to coastal Sonora. Tholeiitic, transitional and alkali basalts of subslab origin ascended through this window, and adakites derived from the partial melting of its upper lip, relatively close to the trench. Calc-alkaline lavas, magnesian andesites and niobium-enriched basalts formed from hydrous melting of the supraslab mantle triggered by the uprise of hot Pacific asthenosphere through the window. During the Plio-Quaternary, the ??no-slab?? regime following the sinking of the old part of the Farallon plate within the deep mantle allowed the emplacement of alkali and tholeiitic/transitional basalts of deep asthenospheric origin in Baja California and Sonora. The lithospheric rupture connected with the opening of the Gulf of California generated a high thermal regime associated to asthenospheric uprise and emplaced Quaternary depleted MORB-type tholeiites. This thermal regime also induced partial melting of the thinned lithospheric mantle of the Gulf area, generating calc-alkaline lavas as well as adakites derived from slivers of oceanic crust incorporated within this mantle.     

Structural Geometry of a Sector of the Colorado River Delta,Baja California,Mexico, Based on Seismic Reflections

A structural study in the SW section of the Colorado River delta using seismic reflection data is presented. The study area is located along the Cerro Prieto transform fault, which extends from the northern Gulf of California through the Mexicali Valley and is an active fault within the Pacific-North American plate boundary zone. The research was supported by a database of five seismic profiles with a total length of 215 km, collected in the early 80’s by Petróleos Mexicanos. The results show a high density of faults, most of which are buried by sediments. Within the Cerro Prieto fault zone, several faults were identified, such as: Palmas, Mesa, and Pangas Viejas, until now unknown. In addition, even though the Indiviso fault was investigated and superficially identify prior to this work, herein mapped at depth. West of the Cerro Prieto fault zone lies the Las Tinajas basin, bound by the Dunas and Saldaña faults and by the Montague basin to the southeast. The deformation zone along the plate boundary is 18-km-wide, stretching from the Cerro Prieto fault in the east to the Pangas Viejas fault in the west. The orientations of the faults are NW–SE, and if projected from the southern side of the Sierra Cucapah southward, the faults tend to join the Cerro Prieto fault. In the Las Tinajas basin, the acoustic basement is deeper than 5,000 m. Some of the largest vertical displacements generated by the 2010 7.2-Mw El Mayor-Cucapah earthquake occurred southeast of the epicenter and coincided with the location of the Pangas Viejas Fault, which is buried by sediments. Before this event, seismic activity was very low, and no structures were known in the area. In this paper, we demonstrate that there are at least seven major faults that may now pose a high seismic hazard.     

Flow direction and magnetic mineralogy of lava flows from the central parts of the Peninsula of Baja California,Mexico

Magnetic properties of two apparently unrelated lava suites (one tholeiitic and the other alkalic) coexisting in the central parts of the Peninsula of Baja California, Mexico, were measured in this study. Macroscopic indicators and measurements of anisotropy of magnetic susceptibility (AMS) were combined to infer the flow direction on those lavas. These measurements were used to investigate the likelihood of the existence of an abnormally long tholeiitic lava flow in this part of the Peninsula. The obtained results indicate that the tholeiitic flows in the region constitute regular length flows, and are likely to have been issued through vents located within the area of study. Additionally, comparison of the magnetic signature of tholeiitic and alkalic lavas reveals a systematic difference in some of the magnetic parameters (bulk susceptibility and Curie temperatures). In particular, it is proposed that the narrow range of Curie temperatures characteristic of each lava type can be used as a reliable proxy for the identification of each lava type in the region. All of these findings show that the alkalic and tholeiitic lavas of the region are more closely related to each other than previously suspected.Editorial responsibility: R. Cioni     

The 2010 M w 7.2 El Mayor-Cucapah Earthquake Sequence, Baja California, Mexico and Southernmost California, USA: Active Seismotectonics along the Mexican Pacific Margin

The El Mayor-Cucapah earthquake sequence started with a few foreshocks in March 2010, and a second sequence of 15 foreshocks of M?>?2 (up to M4.4) that occurred during the 24?h preceding the mainshock. The foreshocks occurred along a north?Csouth trend near the mainshock epicenter. The M _w 7.2 mainshock on April 4 exhibited complex faulting, possibly starting with a ~M6 normal faulting event, followed ~15?s later by the main event, which included simultaneous normal and right-lateral strike-slip faulting. The aftershock zone extends for 120?km from the south end of the Elsinore fault zone north of the US?CMexico border almost to the northern tip of the Gulf of California. The waveform-relocated aftershocks form two abutting clusters, each about 50?km long, as well as a 10?km north?Csouth aftershock zone just north of the epicenter of the mainshock. Even though the Baja California data are included, the magnitude of completeness and the hypocentral errors increase gradually with distance south of the international border. The spatial distribution of large aftershocks is asymmetric with five M5+ aftershocks located to the south of the mainshock, and only one M5.7 aftershock, but numerous smaller aftershocks to the north. Further, the northwest aftershock cluster exhibits complex faulting on both northwest and northeast planes. Thus, the aftershocks also express a complex pattern of stress release along strike. The overall rate of decay of the aftershocks is similar to the rate of decay of a generic California aftershock sequence. In addition, some triggered seismicity was recorded along the Elsinore and San Jacinto faults to the north, but significant northward migration of aftershocks has not occurred. The synthesis of the El Mayor-Cucapah sequence reveals transtensional regional tectonics, including the westward growth of the Mexicali Valley and the transfer of Pacific?CNorth America plate motion from the Gulf of California in the south into the southernmost San Andreas fault system to the north. We propose that the location of the 2010 El Mayor-Cucapah, as well as the 1992 Landers and 1999 Hector Mine earthquakes, may have been controlled by the bends in the plate boundary.