Large Distant Earthquakes Research Articles

Temporal changes of seismicity in Salton Sea Geothermal Field due to distant earthquakes and geothermal productions

SUMMARY The Salton Sea Geothermal Field (SSGF) is one of the most seismically active and geothermally productive fields in California. Here we present a detailed analysis of short-term seismicity change in SSGF from 2008 to 2013 during and right following large distant earthquakes, as well as long-term seismicity change due to geothermal productions. We first apply a GPU-based waveform matched-filter technique (WMFT) to the continuous data recorded by the Calenergy Borehole (EN) Network and detect more than 70 000 new micro-earthquakes than listed in the standard Southern California Seismic Network catalogue. We then analyse the seismicity rate changes in the SSGF associated with transient stress fluctuations triggered by regional and large teleseismic earthquakes from 1999 to 2019. We find triggered seismicity in the SSGF following seven regional M > 5.5 earthquakes. In comparison, most teleseismic earthquakes with M > 8.0 did not trigger significant seismicity rate change in the SSGF, likely indicating a frequency dependence in remote dynamic triggering. We further characterize the correlation between the long-term seismicity rate and geothermal production rates, and the temporal and spatial distribution of Guttenberg–Richter b-values inside and outside the SSGF with the newly detected catalogue. The long-term seismicity shows that events with M > 1.5 are likely correlated with net production rates, while smaller events do not show any correlation. The b-values inside the SSGF are higher than those outside the SSGF, and the locations of dynamically triggered events are close to locations with high b-values.

Microgravity surveying before, during and after distant large earthquakes

Microgravity surveying is a widely used geophysical method, especially for detection of deeper features which are difficult to detect with other techniques. However, the accuracy of the readings is strongly affected by the amount of microseism noise, which is typically rejected by using long integration times for each measurement cycle. Large seismic events such as earthquakes create large ground acceleration waves which have been known to affect gravity readings. While previous data which have been collected serendipitously, tend to only record the final readings, unique long-duration datasets during and after three earthquakes including the raw data (sampled at 6–10 Hz) from field gravimeters (Scintrex CG-5 and CG-6) in static locations recorded during surveys are presented. The aim of the current study is to characterise both long and short-term effects of the generated seismic accelerations on measurement accuracy and repeatability, and identify changes to commercial practice to mitigate these effects.During earthquakes, the nature of the microseism noise was fundamentally altered by each of the different associated seismic waves. Minor effects were found for body P- and S-waves, but much larger effects were found for surface waves, especially the Rayleigh waves which gave errors many times those which would occur in normal conditions. These accelerations persisted in the data for several hours or even days after the earthquake affecting instrument performance. The main finding is that the optimum course of action is to identify the earthquake early by analysing the data in the frequency domain through FFT, switch to 60 s cycles and return to the base station until the strongest waves have passed. Surveying on subsequent days was affected by lower frequency free earth oscillations requiring removal of the unwanted signals using linear trends between at least hourly base stations. Using these techniques will both facilitate data collection as well as improve data confidence in these challenging conditions. Instruments operating in a gradiometer configuration were shown to be comparatively unaffected by the increased noise for typical commercial integration times paving the way for the next generation of instruments to operate successfully, even with this challenging environmental noise.

Amplification and Tuning of Ground Motion at the Outer Cascadia Accretionary Prism

AbstractGround‐motion amplification in the outer Cascadia subduction zone accretionary prism has been documented previously by comparing earthquake and microseismic vertical ground‐motion and pressure records from an ONC/NEPTUNE Canada cabled observatory site on the outer prism to those from a site on the flank of the Juan de Fuca Ridge. Since then, four additional instruments became operational, and data, now spanning 10 years, include more than 100 large (Mw > 7) distant earthquakes. Well‐tuned response at the outer prism sites is observed in both vertical and horizontal components, with peaks in the spectral ratio of vertical velocity relative to nearby ocean crustal and continental bedrock sites (V/Vref) at 9 s, and in the intra‐site horizontal to vertical spectral ratio horizontal‐to‐vertical spectral ratio at 14 s, both with band widths of ±10% to 15% at half amplitude. The vertical response is consistent with ¼ wavelength compressional wave reinforcement, while the horizontal motion tuning most likely reflects the effects of the low velocity prism sediments on surface‐wave propagation. At the periods of maximum relative motion, outer prism surface‐wave vertical and horizontal accelerations are enhanced by factors of up to 15 and 25, respectively. Similar behaviour is seen in microseismic records from these sites, and, to a lesser extent, in earthquake records from temporarily deployed Cascadia Initiative seismometers along the outer Cascadia prism to the south. Such tuning and amplification must be accounted for when assessing the dynamic behavior of the prism and its basal fault at the time of large local earthquakes.

Seismic microzoning of Guwahati metropolitan area, Assam, India

Seismic microzonation maps for Guwahati metropolitan area are presented based on the Uniform-Hazard-Spectrum (UHS) methodology. UHS method satisfies the performance-based design (PBD) criteria, which are based on two sets of spectral amplitudes, one for which the structure will remain linear, and the other for which it may undergo nonlinear response. The UHS method also considers contributions from large distant earthquakes and includes simultaneously the effects of site geology and of site soils. The maps presented include in a balanced way the effects of near and distant large earthquakes, spatial distribution of seismic activity, the site geology and site soil properties. The results also show that scaling design spectra by peak acceleration to which a standard spectral shape is anchored is not conservative.

Development of short return period spectra for the regions with high to moderate seismicity: an example in Iran

Response spectrum is considered one of the main tools in seismic design of structural systems. However, for short return period earthquakes, such spectra have not been studied in details. These spectra are needed, particularly, in the construction industry for determination of the required level of strength and stiffness for the structures during construction. In this work, using probabilistic seismic hazard analysis, short return period spectra for two geographical regions with high and moderate level of seismic intensities are developed. According to the results of this study, large distant earthquakes have significant contribution in defining the amplitude and frequency contents of short return period spectra in the regions with moderate seismicity and soft soils. The results also show that, in the regions with moderate seismicity, such spectra are about 30% wider in the frequency range of short and middle height buildings if they compare with those developed for high seismicity regions.

Remote Triggering in the Koyna‐Warna Reservoir‐Induced Seismic Zone, Western India

AbstractDynamic triggering following large distant earthquakes has been observed in many regions globally. In this study, we present evidence for remote dynamic triggering in the Koyna‐Warna region of Western India, which is known to be a premier site of reservoir‐induced seismicity. Using data from a closely spaced broadband network of 11 stations operated in the region since 2005, we conduct a systematic search for dynamic triggering following 20 large distant earthquakes with dynamic stresses of at least 1 kPa in the region. We find that the only positive cases of dynamic triggering occurred during 11 April 2012, Mw8.6 Indian Ocean earthquake and its largest aftershock of Mw8.2. In the first case, microearthquakes started to occur in the first few cycles of the Love waves, and the largest event of magnitude 3.3 occurred during the first few cycles of the Rayleigh waves. The increase of microseismicity lasted for up to five days, including a magnitude 4.8 event occurred approximately three days later. Our results suggest that the Koyna‐Warna region is stress sensitive and susceptible for remote dynamic triggering, although the apparent triggering threshold appears to be slightly higher than other regions.

Testing for the ‘predictability’ of dynamically triggered earthquakes in The Geysers geothermal field

The Geysers geothermal field is well known for being susceptible to dynamic triggering of earthquakes by large distant earthquakes, owing to the introduction of fluids for energy production. Yet, it is unknown if dynamic triggering of earthquakes is ‘predictable’ or whether dynamic triggering could lead to a potential hazard for energy production. In this paper, our goal is to investigate the characteristics of triggering and the physical conditions that promote triggering to determine whether or not triggering is in anyway foreseeable. We find that, at present, triggering in The Geysers is not easily ‘predictable’ in terms of when and where based on observable physical conditions. However, triggered earthquake magnitude positively correlates with peak imparted dynamic stress, and larger dynamic stresses tend to trigger sequences similar to mainshock–aftershock sequences. Thus, we may be able to ‘predict’ what size earthquakes to expect at The Geysers following a large distant earthquake.

Seismic hazard maps in Serbia

Mapping seismic hazard parameters for use in earthquake-resistant design must satisfy the guidelines for performance-based design (PBD). At present, PBD requires specification of two spectral amplitudes, one in which the structure will remain essentially linear, with system period Tlinear, and the other in which it may undergo a nonlinear response and will vibrate with longer-system period Tnonlinear. These requirements cannot be satisfied via one (same) fixed spectral shape, and such spectra cannot be scaled by peak ground acceleration. Another source of difficulty in the selection of the PBD-design amplitudes occurs when the standard spectrum shape is not capable of describing excitation from large distant earthquakes. In this paper we illustrate these problems by examining Serbia seismic hazard maps and show that scaling design ground motion via peak ground acceleration and fixed shape spectra lead to serious underestimation of long-period spectral amplitudes. We show how mapping of seismic hazard based on our method, which uses Uniform Hazard Spectra (UHS) eliminates these difficulties.

Seismic hazard maps in Macedonia

We present seismic hazard maps for Macedonia and show how the Uniform Hazard Spectra (UHS) method eliminates the difficulties associated with the use of the fixed- shape design spectra. The UHS method also eliminates the issues that result from scaling in terms of peak accelerations—when it is necessary to include hazard contributions from large distant earthquakes—as well as the problem of the inability of the fixed-shape spectra to describe frequency-dependent spectral shapes for different probabilities of exceedance. We also show how far south contributions of the Vrancea earthquakes reach as seen in the UHS of strong ground motion in Macedonia.

Detecting remotely triggered microseismicity around Changbaishan Volcano following nuclear explosions in North Korea and large distant earthquakes around the world

AbstractWe conduct a systematic survey on locally triggered earthquakes by large distant earthquakes in Changbaishan Volcano, an active intraplate volcano on the border between China and North Korea. We examine waveforms of distant earthquakes recorded at broadband station Changbaishan (CBS) near the volcano with estimated dynamic stresses over 5 kPa between 2000 and 2016. Out of 26 selected distant earthquakes, three of them show positive evidence of triggering during large‐amplitude surface waves. The earthquakes that had positive or possible evidences of triggering generated larger long‐period surface waves, indicating that they are more efficient in triggering microseismicity. In addition, since 2006 North Korea has conducted five underground nuclear explosion (UNE) tests only ~140 km away from Changbaishan Volcano. By systematically examining waveforms of these UNEs recorded at station CBS, we find that none of them have triggered microearthquakes in Changbaishan Volcano.

Seismic microzoning in Skopje, Macedonia

Seismic microzonation maps for Skopje (Macedonia) are presented based on the Uniform-Hazard-Spectrum (UHS) methodology. UHS satisfies the guidelines for performance-based design (PBD), which require specification of two sets of spectral amplitudes, one for which the structure will remain essentially linear, and the other for which it will undergo a nonlinear response. The UHS method also considers the contributions from large distant earthquakes and includes simultaneous effects of site geology and site soils. The maps presented include the effects of near and distant large earthquakes in a balanced way, spatial distribution of seismic activity, site geology, and site soil properties.

Seismic microzoning of Štip in Macedonia

Seismic microzonation maps for Štip (Macedonia) and its surroundings are presented based on the uniform-hazard-spectrum (UHS) methodology. Such mapping satisfies the guidelines for performance-based design (PBD), which requires specification of two sets of spectral amplitudes—one for which the structure will remain essentially linear, and the other for which it will undergo a nonlinear response. The UHS method also enables us to include contributions from excitation by large distant earthquakes as well as the simultaneous effects of site geology and site soils. Thus, the maps we present include, in a balanced way, the effects of near and distant large earthquakes, spatial distribution of seismic activity, the site geology, and the site soil properties.

Seismic microzoning of Belgrade

Seismic microzonation maps for Belgrade (Serbia) and its surroundings are presented based on the uniform-hazard-spectrum (UHS) methodology. Such mapping must satisfy the guidelines for performance-based design (PBD), which at present requires the specification of two sets of spectral amplitudes, one in which the structure would remain essentially linear, and one in which it would undergo nonlinear response. These requirements cannot be achieved by specifying the design spectra using only one (same) fixed spectral shape, and such spectra cannot be scaled by the peak ground acceleration alone. Another source of difficulties in the selection of the design amplitudes for PBD occurs when the standard spectrum shape is not capable of describing excitation by large, distant earthquakes. Furthermore, scaling the site dependent design ground motion only via soil site classification ignores the effects of site geology and thus leads to biased results. The maps we present in this paper avoid these shortcomings and include the effects of near and distant large earthquakes, spatial distribution of seismic activity, site geology, and site soil properties in a balanced way.

Seismic Evaluation of Tall Buildings Using a Simplified but Accurate Analysis Procedure

A simplified analysis procedure for evaluating the nonlinear seismic responses of tall reinforced concrete (RC) buildings is examined in this study. It is called the Uncoupled Modal Response History Analysis (UMRHA) procedure. It can be viewed as an extended version of the classical modal analysis procedure, where the nonlinear response of each vibration mode is first computed, and they are later on combined into the total response of the structure. The procedure requires the knowledge of the modal hysteretic behavior, which can be obtained from a cyclic modal pushover analysis. The responses of four tall buildings in Bangkok to distant large earthquakes are computed by this procedure and compared with those obtained from the Nonlinear Response History Analysis (NLRHA) procedure. These four buildings have different heights—varying from 20 to 44 stories, different configurations of floor plan, and different arrangement of RC walls. The comparison shows that the UMRHA procedure is able to accurately compute the story shears and story overturning moments, floor accelerations, and inter-story drifts of all these tall buildings. The required computational effort is also extremely low compared to that of the NLRHA procedure. Moreover, since the UMRHA procedure computes the response of each individual vibration mode, it provides more understanding and insight into the complex nonlinear seismic responses of these tall buildings.

Isolated regions of remote triggering in South/Southeast Asia following the 2012 Mw 8.6 Indian Ocean earthquake

AbstractLarge earthquakes are capable of triggering either shallow earthquakes or deep tectonic tremors at long‐range distances. So far, most of remotely triggered tremors were found along major plate boundary faults around the Pacific Rim. Here we conduct a systematic search for remotely triggered earthquakes and tremors in South/Southeast Asia following the 11 April 2012 Mw 8.6 Indian Ocean earthquake. We find additional evidence of triggered tectonic tremors beneath the Java Island and the Sulabes Island in Eastern Indonesia and triggered earthquakes in Vietnam. Tremors mostly occurred during the large‐amplitude Rayleigh waves of the Indian Ocean main shock and were also triggered by several other large distant earthquakes. Although we are unable to locate them, they were recorded at stations close to major tectonic faults, suggesting that they are likely of tectonic origin. However, we find no evidence of triggered tremor/earthquakes along the Sumatra subduction zone and eastern portion of Himalaya frontal thrusts, indicating that remote triggering was not as widespread as previously thought.

Hydrological response to multiple large distant earthquakes in the Mile well, China

AbstractGroundwater level changes of up to 0.716 m and temperature changes of up to 0.0708°C have been observed in the Mile well, Yunnan Province, China, in response to earthquakes with a seismic energy density exceeding 1 × 10−3 J⋅m−3 from 2004 to 2012. Sustained water level changes, however, only occurred in earthquakes with a surface wave magnitude Ms ≥ 8.0. Groundwater temperature and permeability changes also only occurred following such large earthquakes. This indicates that larger earthquakes (with Ms ≥ 8.0) are more effective at triggering hydrologic responses than smaller earthquakes. The amplitudes of changes in groundwater level and groundwater temperature were positively correlated, indicating that temperature changes are possibly caused by advection‐diffusion and mixing following groundwater inflows into the well. We used a tidal response as a proxy to investigate aquifer properties, showing that the changes in groundwater level and temperature correlate well with phase shift and tidal factor. Based on the aquifer parameters calculated from the tidal response using the M2 wave, we found that amplitudes of inferred changes in transmissivity and storage coefficient have a positive relationship with the groundwater level and temperature changes. The scale of transmissivity change was much larger than that of the storage coefficient. We interpret the co‐seismic changes in groundwater level and temperature in the Mile well to be due to changes of permeability in the aquifer due to unclogging of fractures induced by seismic waves.

Model of Deep Nonvolcanic Tremor Part I: Ambient and Triggered Tremor

There is evidence of triggering of tremor by seismic waves emanating from distant large earthquakes. The frequency contents of triggered and ambient tremor are largely identical, suggesting that tremor does not depend directly on the nature of the source. We show here that the model of plate dynamics developed earlier by us is an appropriate tool for describing the onset of tremor. In the framework of this model, tremor is an internal response of a fault to a failure triggered by external disturbances. The model predicts generation of radiation in a frequency range defined by the fault parameters. Other specific features predicted are: the upper limit of the size of the emitting area is a few dozen km; tremor accompanies earthquakes and aseismic slip; the frequency content of tremor depends on the type of failure. The model also explains why a tremor has no clear impulsive phase, in contrast to earthquakes. A comparatively small effective normal stress (hence a high fluid pressure) is required to make the model consistent with observed tremor parameters. Our model indicates that tremor is not necessarily a superposition of low frequency earthquakes, as commonly assumed, although the latter may trigger them. The approach developed complements the conventional viewpoint which assumes that tremor reflects a frictional process with low rupture speed. Essentially our model adds the hypothesis that resonant-type oscillations exist inside a fault. This addition may change our understanding of the nature of tremor in general, and the methods of its identification and location in particular.

Lack of Additional Triggered Tectonic Tremor around the Simi Valley and the San Gabriel Mountain in Southern California

Abstract We conduct a systematic search of tectonic tremor triggered by distant large earthquakes around the Simi Valley (SV) and the San Gabriel Mountain (SGM) in southern California. Out of 59 large earthquakes between 2000 and 2013, only the 2002 M w 7.9 Denali fault earthquake triggered clear tremor in the region. The observed travel times of the triggered tremors are consistent with theoretical predictions from tremor sources that are spatially clustered in the SV, close to the rupture zone of the 1994 M w 6.7 Northridge earthquake. We also estimate the triggering stress threshold as ∼12 kPa from measuring the peak ground velocities near the tremor source. The lack of clear tremor beneath the SGM provides a “negative” example for a region where tremor is expected to occur because of clear evidence of fluid‐rich zones at the middle crust. The results imply that the necessary conditions for tremor to occur are more than fluid‐induced low effective normal stress. Online Material: Figures of additional tremor inspection, tables of earthquake catalogs, and tremor locations.

Computation of ground motion amplification in Kolkata megacity (India) using finite-difference method for seismic microzonation

This paper presents the ground motion amplification scenario along with fundamental frequency (F0) of sedimentary deposit for the seismic microzonation of Kolkata City, situated on the world’s largest delta island with very soft soil deposit. A 4th order accurate SH-wave viscoelastic finite-difference algorithm is used for computation of response of 1D model for each borehole location. Different maps, such as for F0, amplification at F0, average spectral amplification (ASA) in the different frequency bandwidth of earthquake engineering interest are developed for a variety of end-users communities. The obtained ASA of the order of 3–6 at most of the borehole locations in a frequency range of 0.25–10.0 Hz reveals that Kolkata City may suffer severe damage even during a moderate earthquake. Further, unexpected severe damage to collapse of multi-storey buildings may occur in localities near Hoogly River and Salt Lake area due to double resonance effects during distant large earthquakes.

Statistical properties of low-frequency earthquakes triggered by large earthquakes in southern Taiwan

The recent discovery of triggered tremors (TTs) and low-frequency earthquakes (LFEs) in various tectonic environments provides an opportunity for studying the fundamental properties and physical mechanisms of deep tectonic tremor. Here, we quantify the relationship between TTs and LFEs beneath the Central Range in southern Taiwan and their statistical properties during the teleseismic waves of six large distant earthquakes. Using waveforms of 11 LFEs triggered by the 2005 Mw 8.6 Nias earthquake as templates, we scan through 12hours of waveform data around six mainshocks and identify a total of 783 LFEs. The LFEs were mainly located in a compact region between 12 and 36km in depth near the Chaochou–Lishan Fault. Most of LFEs occurred within TT during the passage of large-amplitude surface waves, and the increase of the LFE rate during the surface waves is statistically significant. The LFE rates do not follow an Omori's type decay, but rather abruptly return to the background rate immediately after the surface-wave passage. These findings suggest that LFEs do not trigger any additional LFEs at later times and are primarily driven by an external forcing. Our observations are consistent with the inference that TTs consist of many reoccurring LFEs.