Distribution Of Microearthquakes Research Articles

Three-Dimensional Modeling of the Xichang Crust in Sichuan, China by Machine Learning

Seismicity and distribution of earthquakes can provide active fault structural information on the crust at a regional scale. The morphology of faults can be derived from the epicentral distribution of micro-earthquakes. In this study, we combined both the relocated earthquake catalogue and related preliminary geophysical information for 3D modeling of the crust in the Xichang area, Sichuan province, China. The fault morphology and deep crustal structure were automatically extracted by the machine learning approach, such as the supervised classification and cluster analysis methods. This new 3D crustal model includes the seismic velocity distribution, fault planes in 3D and 3D seismicity. There are many earthquake clusters located in the folded basement and low-velocity zone. Our model revealed the topological relation between the folded basement and faults. Our work show the crustal model derived is supported by the earthquake clusters which in turn controls the morphological characteristics of the crystalline basement in this area. Our use of machine learning techniques can not only be used to predict the refined fault geometry, but also to combine the seismic velocity structure with the known geological information. This 3D crustal model can also be used for geodynamic analysis and simulation of strong motionseismic waves.

Anomalous distribution of microearthquakes in the Newberry Geothermal Reservoir: Mechanisms and implications

Stimulation of enhanced geothermal system (EGS) reservoirs by fluid injection can enhance the reservoir permeability but may also result in undesired microearthquakes (MEQs). A bimodal depth distribution of fluid-injection-induced MEQs was observed in the 2012 stimulation phase of the Newberry Volcano EGS Demonstration project in Oregon, US. During 7 weeks of hydraulic stimulation of well NWG 55-29, 90% of MEQs occurred in the shallow reservoir (∼500m to ∼1800m), only a few occurred adjacent to the bottom of the open borehole (∼2500 to ∼3000m) while almost no seismicity was observed in the intervening interval (∼1800m to ∼2500m). Our analysis of frictional stability using spatial models for fluid pressure diffusion of injected fluids show that the distribution of MEQs is consistent with observed casing damage, and a possible leak at ∼700m, and is inconsistent with migration of fluids from the casing shoe. The role of fluid injection through the ruptured casing is further supported by the analyses of shear failure and pore-pressure diffusion. Finally, the absence of seismicity at intermediate depths is consistent with our laboratory determinations of frictional stability, showing velocity strengthening frictional behavior for samples from intermediate depths, bracketed by velocity neutral and weakening behavior for samples from shallower and greater depths.

Preliminary crustal deformation model deduced from GPS and earthquakes’ data at Abu-Dabbab area, Eastern Desert, Egypt

A local geodetic network consisting of eleven benchmarks has been established to study the recent crustal deformation in the Abu-Dabbab area. Seven campaigns of GPS measurements have been collected started from October 2008 and ended in March 2012. The collected data were processed using Bernese version 5.0, and the result values were adjusted to get the more accurate positions of the GPS stations. The magnitudes of horizontal displacements are variable from one epoch to another and in the range of 1–3 (±0.2)mm/yr. Due to the differences in rates of the horizontal displacement; the area is divided into two main blocks. The first one, moves to the east direction of about 3mm/yr, while the second block, moves to the SW direction of about 6mm/yr. According to the strain fields that were calculated for the different epochs of measurement, the main force is compression force and is taken the NW–SE to NWW–SEE direction. This force could be because of local and regional tectonic processes affecting on the study area. The maximum values of compression stress are found in the southern central and western part of study area. Estimated accumulation of this strain energy may be considered as an indicator of the possibility of earthquake occurrence.From the seismic tomography study, the 3D Vp and Vp/Vs crustal models indicate high Vp/Vs values forms an elongated anomaly, in the central part of the study area, that extends from a depth of 12km to about 1–2km of depth is obtained. By using this crustal model in relocations all seismicity informed that most of the seismicity strongly tend to occur in a cluster manner exactly above the southern part of the study area. Based on the conducted source mechanism study, it is noticed that shallow earthquakes are associated by a high CLVD ratio (up to 40%). Furthermore, initiation of a high level seismic activity, without a large seismic main shock is observed in the Abu-Dabbab area. The distribution of micro-earthquakes tends to align in an ENE–WSW direction marking a zone of activity verse the Red Sea. The nucleation of the seismic activity beneath the southern part of the Abu-Dabbab crust is more consistent with the obtained crustal deformation result by increasing the crustal movement in the south part than the northern part. Then, based on the obtained results of the above mentioned studies; seismic tomography; source mechanisms, and crustal deformation we conclude that these seismic activities that are associated by crustal deformation are owing to some magma activity beneath the crust of the Abu-Dabbab area.

震度データによる１９１４年秋田仙北地震の短周期地震波発生域と地震規模の推定および１８９６年陸羽地震との比較

The 1914 Akita-Senboku earthquake was investigated using seismic intensity data estimated from damage records to clarify their magnitudes and source areas related to short-period seismic waves. We evaluated the most appropriate fault model of the Akita-Senboku earthquake on the basis of grid search analysis and other existing research works such as hypocenter distribution of micro earthquakes and S-wave velocity perturbations [Okada et al. (2010)]. The seismic intensity inversion analysis for the Akita-Senboku earthquake assuming an east-dipping fault model (depth: 6-13 km) indicated that the short-period radiation zone was located in the north and deep part of the fault plane and there was a low S-wave velocity zone in the lower crust beneath it. The magnitude of 6.5.6.6 yielded the least evaluation error. The magnitude was also estimated from the area of seismic intensity .ve-lower or more using empirical relationship [Muramatsu (1969)]. It resulted in M=6.6.∼6.7 and reinforced the accuracy of the intensity inversion result. It is concluded that the magnitude of the Akita-Senboku earthquake should be considerably less than M=7.1 after Utsu (1979) even if considering the estimation error. The 1896 Rikuu earthquake was also analyzed to be compared with the Akita-Senboku’s results and to consider their dieffrences. The fault location of the Rikuu earthquake was already identi.ed from surface faults. We assumed two conjugate faults for the northern fault zone of the eastern margin of the Mahiru mountains and the northern fault zone of the eastern margin of the Yokote basin. The most appropriate magnitude was estimated to be M=7.1, which is equivalent to that after Utsu (1979). Therefore, our results indicate that the Rikuu earthquake was considerably larger than the Akita-Senboku earthquake. The short-period radiation zone of the Rikuu earthquake, which might be less accurate due to the lack of intensity data compared to the Akita-Senboku earthquake, was located in shallow zone at the intersection of two conjugate faults. This location contrasts with that of the Akita-Senboku earthquake without surface faults.

Three-dimensional structure of Vp, Vs and Vp/Vs in the upper crust of the Marmara region, NW Turkey

AbstractWe applied a 3-D seismic tomography inversion algorithm to arrival-time data obtained, during 18 years from 1985 to 2002, from local seismic networks and aftershock studies in the Marmara region, in order to better understand the upper crustal structure of the complex tectonic region. We integrated all the available data set into a common data set and relocated the events, using a 1-D velocity model. We then selected 3,949 earthquakes and obtained 92355 arrival times, in total, consisting of 59,313P-wave and 33,042S-wave arrival times. In this paper we present detailed crustal structures forV p,V sandV p/V sratios from the surface down to 15 km depth with good resolution in terms of the hit count analysis of seismic rays, the checkerboard and restoring resolution tests for the studied region. The results obtained from the inversion suggest that the western part of the North Anatolian Fault Zone shows strong lateral heterogeneity. We concluded that no clear pattern exists between the distribution of microearthquakes and aftershocks and the velocity perturbations presented in this paper. This is probably due to complex tectonic and geological structures. Large coseismic slip associated with the two recent strong earthquakes was found to correspond to higher velocity anomalies, as was often found recently. On the other hand, low velocity values correspond to the sedimentary units or the alluvium regions, as supported by the low resistivity and gravity values. Thus the results presented in this paper are quite consistent with the other data such as gravity, resistivity and magnetic anomalies, indicating that our model is reliable and efficient and it should be useful for further interpretation of tectonic and geological problems in this region. It should be pointed out, however, that the results ofS-wave perturbations andV p/V sratio could not be discussed in detail because of insufficient quality of theS-wave data and the reliability of the results is not very high. Nonetheless, the fact that recent large Izmit earthquake and moderate aftershocks that occurred in or around the high velocity zones near the low velocity region suggests that high velocity regions found in the area close to Istanbul in the Marmara Sea and also at the Iznik-Mekece fault are potential sites for strain energy accumulation and release.

Aqueous fluids derived from a subducting slab: Observed high 3He emanation and conductive anomaly in a non-volcanic region, Kii Peninsula southwest Japan

It has long been recognized that the Kii Peninsula in the southwest Japan arc is peculiar in a non-volcanic region, indicated by the presence of high temperature hot springs, high terrestrial heat flow and high 3He content in hot spring gases. Geophysical and geochemical studies were carried out to understand the geotectonic environment in the southern part of the Kii Peninsula. Most of the measured 3He / 4He ratios are similar or higher than air, indicating wide spread incorporation of mantle-derived helium into meteoric water. A region with rather high 3He / 4He ratios (> 4 R A) on the west side of the Omine Mountains coincides with the occurrence of high temperature hot springs. A deep crustal resistivity structure across the Omine Mountains was imaged by wide-band magnetotelluric soundings. A 2-D inversion with N–S strike using both TM and TE modes reveals two conductors, one in the upper (3–7 km depth) and the other in lower crust (25–35 km depth) to the west of the Omine Mountains. The distribution of microearthquakes and low-frequency tremors, and the existence of seismic reflectors indicate that the large conductor in the lower crust is related to aqueous fluids derived from the Philippine Sea plate. The upper-crustal conductive zone may also reflect the aqueous fluids trapped in the upper crust, which are presumably derived from the subducting slab. Considering the occurrence of seismic events in the subducting slab beneath the southern Kii Peninsula, the aqueous fluids generated by dehydration of the slab mantle could plausibly include MORB-type helium derived from the residual lithospheric mantle. Therefore, the high temperature hot springs and high 3He emanations in hot spring gases and other geotectonic events in the southern Kii Peninsula may be due to heat flux and mantle-derived helium discharged from aqueous fluid in the upper crust.

Seismic reflector imaging by prestack time migration in the Kakkonda geothermal field, Japan

We show that a prestack migration method improves the S/N ratio of seismic reflection profiling in the Kakkonda geothermal field where seismic reflection data are of poor quality. We use non-iterative prestack time migration (PSTM), which does not require multiple iterations to determine the velocity structure for prestack time migration. The optimum constant migration velocity can be determined at each image point from a migration velocity analysis based on primary diffraction patterns. Our results delineate a strong reflector beneath a zone of high seismicity. According to the correspondence between the fracture distribution, the distribution of microearthquakes, and geothermal structure, this reflector is interpreted to be a zone of low-angle fractures saturated with hydrothermal fluids, and to be strongly controlled by the geothermal structure.

Fractal interpretation of the linear relation between logarithms of maximum amplitude and hypocentral distance

The linear relation between logarithms of maximum amplitude and hypocentral distance are known to hold good observationally. But, the direct evaluation of the Q−1 factor from the linear coefficient has been difficult. Here, supposing a random and fractally homogeneous distribution of absorbers or scatterers with fractal dimension Da in a space of dimension d, we can analytically calculate the relation between amplitude and distance. The commonly accepted exponential decay formula corresponds to a special case of Da=d. The amplitude is written as a negative power of distance in the case of Da=d‐1: the negative power is directly related to the linear coefficient between logarithms of amplitude and distance. If seismic faults correspond to absorbers or scatterers, the equality Da=d‐1 is qualitatively in harmony with the well known fact that the hypocenter distribution of microearthquakes has a fractal dimension being smaller than d.

震源分布から見た四国における中央構造線

震源分布から見た四国における中央構造線

Microearthquakes, geothermal activity, and structure, central North Island, New Zealand

Abstract Microearthquakes are very frequent in the Taupo Fault Belt and very infrequent in the neighbouring Taupo-Reporoa Basin and Kaingaroa Plateau; the distribution of microearthquakes does not correlate with areas of geothermal activity. Larger earthquakes follow much the same distribution pattern. They have similar characteristics to the microearthquakes in other respects, including the prominence of swarm activity in the Taupo Fault Belt. From the seismicity of the region it is inferred that under the Taupo-Reporoa Basin the shear strength is reduced to a very low level by the presence of molten and semi-molten magma extending to within a few kilometres of the ground surface, while under the Taupo Fault Belt there is strong material, interpreted as cooled magma, at about 2–6 km depth.

Microseismicity and Recent Tectonic Activity, Whittier Fault Area, California: ABSTRACT

The Whittier fault is a principal strand of the San Andreas fault system near the southwest edge of the Puente Hills in the northeastern Los Angeles basin. Physiographic evidence indicates recent movement along the Whittier fault and its southeast extension, the Elsinore fault. Exposures in test trenches End_Page 789------------------------------ also indicate recent movement along a fault parallel with the principal trace of the Whittier fault in the Puente Hills. However, no major earthquakes or rupturing of the ground surface have occurred along the Whittier-Elsinore fault in historic time. A destructive earthquake occurred in 1929 along the Norwalk fault 6 mi south of the Whittier fault. Analysis of precise level lines run by the Los Angeles County Engineers since 1951 indicates that the Puente and Montebello Hills area and the Santa Fe Springs-Coyote Hills trend are rising at a rate of 0.01 to 0.04 ft/year relative to the synclinal area between the Puente and Montebello Hills on the north and the Santa Fe Springs-Coyote Hills trend on the south. This relative motion may be caused by tectonic motion, or by withdrawal of ground water and compaction of sediments within the synclinal area. Because of oil production and waterflood activities, motion of the ground over oil fields also was detected. Several oil fields are located along the Whittier fault and on anticlinal structures along the Santa Fe Springs--Coyote Hills trend southwest of the Whittier fault. The Whittier and Coyote (west) oil fields are undergoing extensive waterfloods. The Whittier fault provides the updip closure for oil sands in the Whittier oil field. Thus, the Whittier fault area was considered an ideal test site to search for a relation between subsurface pressure and the distribution and frequency of microearthquakes. A network of portable seismometers was operated in the Whittier fault area between July, 1971 and April, 1972. Because of the background noise, the smallest event that could be reliably located had a magnitude of 1.0. Epicenters of 31 microearthquakes with a maximum magnitude of 3.0 were determined, but no direct evidence could be established for a relation between oil production and waterflood activities and the distribution of microearthquakes. Sufficient data were available to determine hypocentral depths for 17 events. Assuming a range of 60-70° north dip on the Whittier fault, 8 of the 17 hypocenters are on the subsurface projection of the Whittier fault; one hypocenter is on the Norwalk fault. Eight of the hypocenters cannot be related to any known structure. On the basis o the microearthquakes detected during this study, the Whittier fault must be considered active. End_of_Article - Last_Page 790------------

Earthquake distribution and mechanism of faulting in the Rainbow Mountain-Dixie Valley-Fairview Peak Area, central Nevada

The distribution of microearthquakes in west-central Nevada correlates well with fault-plane solutions for this area and defines a zigzag series of crustal fractures that vary in length from a few to several tens of kilometers. The main Fairview fault strikes northwest, and motion on this fault is right lateral oblique slip. In other parts of the active zone northeast-striking faults have mainly dip-slip motion. Focal mechanisms are generally consistent with an interpretation of simple block faulting, faults of different orientation having the same slip direction. For the Rainbow Mountain and Fairview Peak areas, crustal blocks to the east of the fracture zone move down and southeast with respect to blocks on the west side. Faulting in this region appears to be related to regional extension, acting in the direction N60°W–S60°E.