Time Difference Correlation between Seismic Waves and Earthquake Lights

Earthquakes occur due to the sudden release of energy from within the earth causing seismic waves. Earthquake activity with a magnitude ³ 3.5 SR is grouped over 21 years from 2000-2020 to determine the level of seismicity in Seram Island, Buru Island, and surrounding areas. The area of seismic activity is clustered over three blocks. The purpose of this research is to determine the seismicity level and mitigation model in each block in the research area. The results showed that seismic activity in the study area with shallow earthquakes was mostly in Block II as many as 1195 times (40.4%) and Block III as many as 1135 times (38.4%). This block is estimated to have the potential for a tsunami to occur. Mitigation efforts to reduce the risk of earthquakes are by implementing a resilient-elastic building system and the location of settlements from the coastline > 100 m with a topography of at least 25 meters

A study based on earthquake occurrences in the United States shows several zones of continuing seismic activity. Over the past 30 years there is a suggestion of systematic variations in seismic activity following what appears to be a three‐year cycle which is superimposed on a longer period. Areas of activity can not be related to either geologic age or type of structure. In California transcurrent movement on known faults is associated with seismic activity. In the other western states activity is associated with the boundaries of areas of epeirogenic uplift and subsidence and appears to involve predominantly vertical crustal displacement. In the Midcontinent area activity for the most part is associated with positive crystalline rock tectonic elements and their borders. In the Appalachian area activity is associated with the zones of overthrusts and shear. A conspicuous zone of earthquakes paralleling the Appalachians from Missouri to the St. Lawrence River valley while not related to known geologic controls does follow a zone of better‐than‐normal seismic transmissibility. The depth of focus varies from area to area and appears to be related directly to crustal thickness and possibly also changes in crustal rigidity with depth. No direct relation is noted between areas of isostatic unbalance and seismic activity or variations in degree of seismic activity with changes in crustal thickness. For the most part, it appears local stress conditions are dominant in causing earthquakes.

The conventional method for determining horizontal movement in the earth's crust has been to reobserve networks of triangulation and compare the coordinates of the adjusted results. A new method of analysis of reobservations is presented. The changes in the angles in a network indicate the presence of strain or deformation within the crust. This type of analysis will also indicate small displacements which may occur along fault lines in an area of seismic activity. Results of the application of this technique to resurveys along the San Andreas Fault in California are given in graphical form.

Seismic waves are refracted, reflected, or scattered by heterogeneities in the Earth's crust. Small-scale structures less than wave-lengths are difficult to be observed by conventional refraction and/or reflection methods although scattered wave energy is observed as coda waves. In March 1997, a seismic reflection survey was conducted to reveal the active tectonics in the Japan arc across the Northern Fossa Magna in Japan. The profiling revealed a strongly deformed thick sedimentary layer. At the same time, we carried out a seismic array observation using off-line seismic recorders in the same area to image crustal structure. We used four large vibrators as seismic sources with a spacing of 200m. The array consisted of 39 seismometers distributed two dimensionally with a spacing of 100m. We applied the CSP (common scatterer point) stacking method for all the combination of sources and receivers. We assumed 3-D grid points with a spacing of 100m in a studied area. We calculated a travel time field for the 3-D structure by a finite difference method. To investigate whether a scattered energy was really originated from a particular grid point or not, we calculated cross-correlation coefficients between the CSP stacked waveforms and a source waveform. We interpret that the correlation coefficients represent a distribution of point scatterers. The scatterers are distributed near the Earth's surface and at depths of 800m and 1600m. Compared with the reflection profile, scatterers are distributed in severely deformed strata. Since our method strongly depends on the background velocity structure, it is important to use a realistic background velocity structure to estimate scatterer distributions accurately.

Studies of passive seismic data in the frequency range below 20Hz have shown that the frequency content of the ever-present seismic background noise changes above hydrocarbon reservoirs. Different explanations for this observation have been proposed. In this study, the effect of oscillating pore fluids, i.e. oil, on the seismic background noise is investigated. A non-wetting fluid drop entrapped in a pore can oscillate with a characteristic eigenfrequency. Capillary forces act as the restoring force driving the oscillations. A 1D wave equation is coupled with a linear oscillator equation, which represents these pore fluid oscillations. The resulting linear system of equations is solved numerically with explicit finite differences. The most energetic part of the seismic background noise, i.e. frequencies around 0.1-0.3Hz, is used as the external source. This part is presumably related to seismic surface waves generated by ocean waves. It is shown that the resulting elastic wave initiates oscillations of the fluid drops. The oscillatory energy of the pore fluid is transferred continuously to the elastic rock matrix. In consequence, seismic waves in the elastic rock carry a second frequency, the eigenfrequency of the pore fluid oscillations on top of the applied external frequency. Both frequencies can be measured at the earth surface. The presented model is considered as a possible explanation for observed spectral modifications above hydrocarbon reservoirs. Time evolution of the pore fluid oscillations seems to be related to the thickness of the hydrocarbon reservoir.

In the search for precursors to earthquakes, correlation has been found between geochemical characteristics of soil gases and seismic activity. In this paper we present evidence that seismic waves can trigger emission of soil radon (Rn) and carbon dioxide (CO2). An active experiment was performed in two fault zones in China, the Annighe fault in Sichuan province and the Xiadian fault in Heibei province. An active seismic source was used to generate seismic waves at 10 m depth in wells within bedrock. Rn and CO2detectors were placed around the wells at a distance of ∼1 m for observing the effects of the seismic waves on the emission of the gases. The observations confirm that the seismic waves have a significant and direct effect on the concentration and flux of soil radon and carbon dioxide. When the seismic events were triggered, the observed concentrations of Rn and CO2immediately increased and reached peak values within 5–50 min and 30–60 min, with corresponding increases of Rn and CO2concentrations by 10.5%–238.7% and 3.1%–54.1%, respectively. The measured concentrations and flux of CO2and Rn after the passage of the seismic waves showed strong correlation, confirming the suggestion that CO2is the carrier gas for Rn. To the best of our knowledge this is the first direct, in‐situ measurement of gas emission caused by the passage of seismic waves and provides important constraints for better understanding of geochemical earthquake precursors.

In this paper, we present the result of a statistical study performed on the ionospheric ion density variations above areas of seismic activity. The ion density was observed by the low altitude satellite DEMETER between 2004 and 2010. In the statistical analysis a superposed epoch method is used where the observed ionospheric ion density close to the epicenters both in space and in time is compared to background values recorded at the same location and in the same conditions. Data associated with aftershocks have been carefully removed from the database to prevent spurious effects on the statistics. It is shown that, during nighttime, anomalous ionospheric perturbations related to earthquakes with magnitudes larger than 5 are evidenced. At the time of these perturbations the background ion fluctuation departs from a normal distribution. They occur up to 200 km from the epicenters and mainly 5 days before the earthquakes. As expected, an ion density perturbation occurring just after the earthquakes and close to the epicenters is also evidenced.

<abstract> <p>A transverse harmonic seismic wave, which propagates through the upper fractured layer of the Earth towards the Earth's surface, is considered. It is shown that the attenuation of a seismic wave in the Earth's upper crust is due to internal friction, i.e., friction between the sides of micro-fractures. Such a damping mechanism does not work in the deeper layers of the Earth where high lithostatic pressure prevents the movement along fractures. The governing equation for a brittle fractured medium is nonlinear. In the study of wave propagation, an approximate method of harmonic linearization is used. It is shown that internal friction in the upper crust leads to the transformation of a transverse harmonic wave into a shock wave.</p> </abstract>

<p>Seismic instrumentation is critical for instantaneous tsunami early warning systems as well as assessing long-term risk of tsunami activity in areas with high seismic hazard. Ocean Bottom Seismometer (OBS) systems provide real-time data in areas with appropriate infrastructure or batch data from offline temporary autonomous stations.</p><p>OBS systems detect ground motion from seismic waves significantly before detecting any pressure change in the water column from an associated tsunami due to the order of magnitude difference in wave velocity. Güralp’s OBS systems combine seismic and pressure detection in both permanent cabled networks and temporary non-cabled systems utilising near-real-time acoustic transmission. All seismic sensors used in Güralp systems are sensitive to both earthquakes as well as other tsunami-triggering events such as landslides (e.g. Anak Krakatau, 2018) or volcanic eruptions (e.g. Hunga Tonga–Hunga Haʻapai, 2022).</p><p>Cabled systems provide obvious benefits of real-time data, confidence of installation and flexibility to add additional instrumentation without power consideration. For example, Güralp Orcus and Maris cabled OBS systems are both deployed off the western coast of North America monitoring volcanic and tectonically induced earthquakes that have potential to cause tsunamis. Seismometers at these stations coupled with pressure gauges allow for immediate notification of a threat and subsequent refinement of hazard estimates using surrounding assets such as dedicated DART buoys.</p><p>Both Orcus and Maris allow for multiple auxiliary systems to be incorporated into the system while maintaining as well as providing additional installation flexibility for operators. Orcus has facility for both strong & weak motion seismometers in addition to auxiliary sensors while Maris has the unique feature of operating at any angle without the need for a gimbal mechanism, simplifying installation and network design considerations.</p><p>The Güralp Aquarius is the latest generation autonomous OBS for short-to-medium term or rapid response campaigns to monitor areas with increased seismic and tsunami hazard. Aquarius also uses omnidirectional capabilities as well as acoustic communication of seismic data to the surface to improve operator confidence of installation. Acoustic communication also allows for near-real-time communication with land-based warning systems after a significant seismic event in anticipation of a tsunami. This can be verified and communicated after the initial seismic wave using onboard pressure gauges. In areas where surface communication is not required, intelligent battery systems optimise deployment lengths beyond 18 months for maximum data/cost benefit.</p><p>Güralp is also pioneering the use of seismic sensors and auxiliary equipment within Science Monitoring And Reliable Telecommunications (“SMART”) cables which have already been shown to be useful in incorporating pressure gauges to detect tsunami events. These cables utilise regular telecommunication cables making uses of their natural communication and power source qualities to improve sensor network coverage. Güralp is currently manufacturing a demonstration system to be deployed in the Ionian Sea, monitoring seismic and volcanic activity with the aim of indicating practicality and data quality using this installation method.</p>

Radon and mercury concentrations were measured in 10 fault gas profiles in Generalized Haiyuan Fault. This paper aims to predetermine the potential seismic risk in different segments of the fault zone from the perspective of geochemistry. The background value and anomaly threshold were adopted and synthesized using the maximum value method and average method to calculate concentration intensity values of radon and mercury. Fault soil gas mercury and radon concentrations show a decreasing gradient from NW to SE indicating evident segmentation. Higher values are mostly distributed in the Maomao Mountain–Tiger Mountain fault and Jingtai area. Combined with the seismotectonic background of historical and recent earthquakes and the spatial distribution characteristics of b-values, the fault soil gas concentration intensity shows a close correlation with earthquake activity within the fault zone. Concentrations of fault gas are higher and the b-value lower in areas of strong seismic activity, and regions with weak seismic activity correspond to lower fault gas concentrations and higher b-values. It is thus considered that the Jingtai area may be more dangerous than the other areas. This paper could provide vital background information for earthquake prediction in the Generalized Haiyuan Fault Zone.

I present a rigorous unifying interpretation of the well‐documented widespread near‐surface and fault zone damage induced by earthquakes by simulating three‐dimensional dynamic rupture propagation on a vertical strike‐slip fault. Stresses in the crust are depth‐dependent and material response is governed by the Drucker‐Prager yield criterion. I show that material yielding induced by seismic waves under the low confining pressure causes widespread near‐surface damage. Because the confining pressure increases with depth, the yielding zone at depth is narrowly confined near the fault, but its thickness broadens dramatically near the surface, forming a “flower‐like” damage zone. The fault zone damage at depth is induced by large dynamic stresses associated with the rupture front, while is induced by strong seismic waves ahead of the rupture front near the Earth's surface. These results have important implications for the formation and evolution of fault zones and possibly for the dynamic triggering of earthquakes as well.

Megathrust earthquakes exhibit a ubiquitous seismic radiation style: low‐frequency (LF) seismic energy is efficiently emitted from the shallowest portion of the fault, whereas high‐frequency (HF) seismic energy is efficiently emitted from the deepest part of the fault. Although this is observed in many case‐specific studies, we show that it is ubiquitous in global megathrust earthquakes between 1995 and 2021. Previous studies have interpreted this as an effect of systematic depth variation in either the plate interface frictional properties (Lay et al., 2012) or the P wavespeeds (Sallarès & Ranero, 2019). This work suggests an alternative hypothesis: the interaction between waves and ruptures due to the Earth's free surface is the leading mechanism that generates this behavior. Two‐dimensional dynamic rupture simulations of subduction zone earthquakes support this hypothesis. Our simulations show that the interaction between the seismic waves reflected at the Earth's free surface and the updip propagating rupture results in LF radiation at the source. In contrast, the downdip propagation of rupture is less affected by the free surface and is thus dominated by HF radiation typical of buried faults. To a second degree, the presence of a realistic Earth structure derived from P‐wave velocity (VP) tomographic images and realistic VP/VS ratio estimated in boreholes further enhances the contrast in source radiation. We conclude that the Earth's free surface is necessary to explain the observed megathrust earthquake radiation style, and the realistic structure of subduction zone is necessary to better predict earthquake ground motion and tsunami potential.

Fracture connectivity is a key parameter controlling fluid flow throughout the Earth's crust. While some theoretical and numerical studies suggest that seismic waves are sensitive to fracture connectivity, an experimental validation of this critically important phenomenon was so far unavailable. In this study, we present a novel methodology for fabricating synthetic analogs of rock samples containing connected and unconnected fluid‐saturated fractures with well‐constrained geometric characteristics. Using a low‐frequency forced‐oscillation apparatus, we show that the P‐wave velocities are higher in samples with unconnected fractures than in those with connected ones. Complementary numerical simulations corroborate these findings and indicate that the dominant mechanism behind the observed differences is wave‐induced fluid pressure diffusion within connected fractures. Our results provide direct experimental evidence that, for otherwise identical fracture networks, the presence of interconnectivity produces a measurable reduction in P‐wave velocity at seismic frequencies, which is consistent with that previously predicted by corresponding numerical models. This, in turn, opens new and important perspectives for the seismo‐hydraulic characterization of fractured rocks.

Synthetic seismograms produced by the finite difference method are used to study the scattering of elastic and acoustic waves in two‐dimensional media with random spatial variations in seismic velocity. The results of this study provide important insights about the propagation of short‐period (< 1 s) seismic waves in the earth's crust and place significant constraints on the fluctuation spectrum of crustal heterogeneity on length scales from tens of kilometers to tens of meters. The synthetic seismograms are analyzed to determine the variation in travel times and waveforms across arrays of receivers. The apparent attenuation caused by scattering and the time decay and amplitude of the seismic coda are also quantified with the numerical simulations. Random media with Gaussian and exponential correlation functions are considered, as well as a self‐similar medium with equal variations in seismic velocity over a broad range of length scales. These media differ in the spectral falloff of their velocity fluctuations at wavelengths smaller than 2π times the correlation distance a. The synthetic seismograms demonstrate that a random medium with self‐similar velocity fluctuations at length scales less than about 50 km (a ≥ 10 km) can explain both travel time anomalies reported for teleseismic arrivals across large‐scale seismic arrays (e.g., LASA and NORSAR) and the presence of seismic coda at frequencies of 30 Hz and greater commonly observed in microearthquake waveforms. Media with Gaussian and exponential correlation functions in velocity do not account for both sets of observations for reasonable standard deviations in velocity (≤10%). The scattering attenuation (Q−1) observed in the simulations for Gaussian media is peaked at ka between 1 and 2, where k is the seismic wave number. The observed attenuation in exponential media increases with frequency for ka < 1 and remains about constant for 1 ≤ ka ≤ 5.6. At high frequencies (ka > 5), the self‐similar medium is characterized by a scattering Q that is constant with frequency, whereas theory predicts that the apparent Q in an exponential medium is proportional to frequency. These alternative models of crustal heterogeneity can thus be tested by improved measurements of the frequency dependence of crustal Q at frequencies greater than about 1 Hz, assuming that scattering is responsible for most of the attenuation at these frequencies. Measurements of the time decay of the synthetic coda waves clearly show that the single scattering model of coda decay is not appropriate in the presence of moderate amounts of scattering attenuation (scattering Q ≤ 200). In these cases, Q values derived from the coda decay rate using the single scattering theory do not correspond to the transmission Q of the medium. The cross correlation of synthetic waveforms observed for an array of receivers along the free surface is observed to be dependent on the correlation distance of the medium. The self‐similar random medium proposed here for the crust produces waveform variations at high frequencies (15–30 Hz) similar to those reported for actual small‐scale seismic arrays with apertures of hundreds of meters.

Hydroseimicity — New evidence