Development of a Global Spatio-Temporal Seismicity Model and Its Application to the Vrancea Seismic Zone, Romania

<p>This study investigates the temporal behaviour of major earthquakes in the Vrancea Seismic Zone (VSZ)in Romania. I used the Romplus catalogue, which is a compilation of several sources and spans the time from 984 AD to the year 2005 and in which the data are of different quality. This catalogue contains only Vrancean earthquakes and consists of more than 8000 events. Qualities 'A', 'B' and 'C' were used to model the data. 'D' and '=' were found as too unreliable for modeling. Using the b-value, I concluded that 3.5 is the correct cut-off magnitude for earthquakes after 1980 and at depths of 60 km and greater. Thereby I detected an increase in the b-value after 1986 of about 0.2 units. The reason for this increase could not be found. Plotting the Gutenberg-Richter relation for several time and depth intervals, it was found that at larger depths than 60 km, there are too many M7 earthquakes as compared to small shocks. The shape of the Gutenberg-Richter relation is similar as to the one expected by the characteristic earthquake model (Schwarz and Coppersmith, 1984; Wesnousky, 1994). A strike of 53 degree was found and the earthquake coordinates were rotated correspondingly. The resulting view on the slab showed the confined volume in which the earthquakes happen and well as the 'aseismic part' of the slab between 40 km and 60 km of depth. The seismicity seems to reach a depth of 180 km. Only the earthquakes in the slab, below a depth of 60 km, show clustering behaviour. Furthermore, the M7 earthquakes all happened in the slab. Thus, a depth limit of 60 km was introduced for modeling. In order to find aftershocks in the catalogue, the temporal behaviour of the Vrancea earthquakes was examined. The mean magnitude increases after each major earthquake, indicating an aftershock process. This was confirmed by the rate of occurrence, which showed an increase in rate after the 1990 earthquakes. The rate of occurrence is too low for the first 580 days after 1980, possibly due to insufficient earthquake detection in this period of time. All the damaging M7 earthquakes all happened in the slab. Thus, shallow earthquakes had to be considered separately. A depth limit of 60 km was introduced and earthquake in shallower and deeper depths were considered separately. For the shallow earthquakes there was a sharp increase in the apparent b-value below the cut-off magnitude of 3.5. After reaching a value of 2.4, the b-value starts to fall steeply. This was attributed to biases in the magnitude calculation. I used the rounded value of 3.5 as a cut-off magnitude for the shallow earthquakes. Having found the magnitude cut-off, depth and time limit, modeling could be started. The model gives two important parameters: the proportion of aftershock and the time to the next earthquake. Using the Maximum Likelihood Method, a best fit was found for a data set starting at 1980 and consisting of earthquakes with a cut-off magnitude of 3.5 and a depth equal and greater than 60 km. According to the model, this data set consists of 13 plus or minus 5% aftershocks and has an inter-event time for new earthquakes of 13 plus or minus 1 days. Using several cut-off magnitudes, it was found that the calculated inter-event time for these earthquakes is consistent with the Gutenberg-Richter law. In contrast, the predicted value for the interevent time of M7 earthquakes does not match the one found in the catalogue. While the Maximum Likelihood Method leads to 814 years as recurrence time, the data shows a recurrence time of only 23 years. The model fits the data set of the 1990 aftershocks very well, too, leading to a aftershock proportion of 58 plus or minus 15%. The data set for the 1986 did not lead to good results, probably due to missing aftershocks shortly after the main shock. Comparing model and data with a pure Poisson model I could see that earthquakes tend to cluster in the first days after the major event. Several days later, their behaviour changes and then is similar to the one proposed by the seismic gap model. Looking at the ratio between the probabilities of the model of Smith and Christophersen and of the Poisson model, a clustering behaviour in the first 24 hours after the main shock was found, followed by a decreased seismicity, which reverts to be Poissonian after 100 days. Thus, I concluded that aftershock behaviour is only relevant after the first 24 hours following a major earthquake. After 24 hours, seismic hazard decreases to be less than as expected by the Poisson model in the following 100 days, until seismicity returns to be Poissonian again. Additionally, I suggest that the 1990 earthquake and its aftershocks should be considered as a 'model earthquake' for future earthquakes as it seems to be representative for earthquake behaviour in the VSZ.</p>

Development of a Global Spatio-Temporal Seismicity Model and Its Application to the Vrancea Seismic Zone, Romania

<p>This study investigates the temporal behaviour of major earthquakes in the Vrancea Seismic Zone (VSZ)in Romania. I used the Romplus catalogue, which is a compilation of several sources and spans the time from 984 AD to the year 2005 and in which the data are of different quality. This catalogue contains only Vrancean earthquakes and consists of more than 8000 events. Qualities 'A', 'B' and 'C' were used to model the data. 'D' and '=' were found as too unreliable for modeling. Using the b-value, I concluded that 3.5 is the correct cut-off magnitude for earthquakes after 1980 and at depths of 60 km and greater. Thereby I detected an increase in the b-value after 1986 of about 0.2 units. The reason for this increase could not be found. Plotting the Gutenberg-Richter relation for several time and depth intervals, it was found that at larger depths than 60 km, there are too many M7 earthquakes as compared to small shocks. The shape of the Gutenberg-Richter relation is similar as to the one expected by the characteristic earthquake model (Schwarz and Coppersmith, 1984; Wesnousky, 1994). A strike of 53 degree was found and the earthquake coordinates were rotated correspondingly. The resulting view on the slab showed the confined volume in which the earthquakes happen and well as the 'aseismic part' of the slab between 40 km and 60 km of depth. The seismicity seems to reach a depth of 180 km. Only the earthquakes in the slab, below a depth of 60 km, show clustering behaviour. Furthermore, the M7 earthquakes all happened in the slab. Thus, a depth limit of 60 km was introduced for modeling. In order to find aftershocks in the catalogue, the temporal behaviour of the Vrancea earthquakes was examined. The mean magnitude increases after each major earthquake, indicating an aftershock process. This was confirmed by the rate of occurrence, which showed an increase in rate after the 1990 earthquakes. The rate of occurrence is too low for the first 580 days after 1980, possibly due to insufficient earthquake detection in this period of time. All the damaging M7 earthquakes all happened in the slab. Thus, shallow earthquakes had to be considered separately. A depth limit of 60 km was introduced and earthquake in shallower and deeper depths were considered separately. For the shallow earthquakes there was a sharp increase in the apparent b-value below the cut-off magnitude of 3.5. After reaching a value of 2.4, the b-value starts to fall steeply. This was attributed to biases in the magnitude calculation. I used the rounded value of 3.5 as a cut-off magnitude for the shallow earthquakes. Having found the magnitude cut-off, depth and time limit, modeling could be started. The model gives two important parameters: the proportion of aftershock and the time to the next earthquake. Using the Maximum Likelihood Method, a best fit was found for a data set starting at 1980 and consisting of earthquakes with a cut-off magnitude of 3.5 and a depth equal and greater than 60 km. According to the model, this data set consists of 13 plus or minus 5% aftershocks and has an inter-event time for new earthquakes of 13 plus or minus 1 days. Using several cut-off magnitudes, it was found that the calculated inter-event time for these earthquakes is consistent with the Gutenberg-Richter law. In contrast, the predicted value for the interevent time of M7 earthquakes does not match the one found in the catalogue. While the Maximum Likelihood Method leads to 814 years as recurrence time, the data shows a recurrence time of only 23 years. The model fits the data set of the 1990 aftershocks very well, too, leading to a aftershock proportion of 58 plus or minus 15%. The data set for the 1986 did not lead to good results, probably due to missing aftershocks shortly after the main shock. Comparing model and data with a pure Poisson model I could see that earthquakes tend to cluster in the first days after the major event. Several days later, their behaviour changes and then is similar to the one proposed by the seismic gap model. Looking at the ratio between the probabilities of the model of Smith and Christophersen and of the Poisson model, a clustering behaviour in the first 24 hours after the main shock was found, followed by a decreased seismicity, which reverts to be Poissonian after 100 days. Thus, I concluded that aftershock behaviour is only relevant after the first 24 hours following a major earthquake. After 24 hours, seismic hazard decreases to be less than as expected by the Poisson model in the following 100 days, until seismicity returns to be Poissonian again. Additionally, I suggest that the 1990 earthquake and its aftershocks should be considered as a 'model earthquake' for future earthquakes as it seems to be representative for earthquake behaviour in the VSZ.</p>

Temporal Changes in Seismic Anisotropy as a New Eruption Forecasting Tool

<p>The orientation of crustal anisotropy changed by ~80 degrees in association with the 1995/96 eruption of Mt. Ruapehu volcano, New Zealand. This change occurred with a confidence level of more than 99.9%, and affects an area with a radius of at least 5 km around the summit. It provides the basis for a new monitoring technique and possibly for future mid-term eruption forecasting at volcanoes. Three deployments of seismometers were conducted on Mt. Ruapehu in 1994, 1998 and 2002. The fast anisotropic direction was measured by a semi-automatic algorithm, using the method of shear wave splitting. Prior to the eruption, a strong trend for the fast anisotropic direction was found to be around NW-SE, which is approximately perpendicular to the regional main stress direction. This deployment was followed by a moderate phreatomagmatic eruption in 1995/96, which ejected material with an overall volume of around 0.02-0.05 km3. Splitting results from a deployment after the eruption (1998) suggested that the fast anisotropic direction for deep earthquakes (>55 km) has changed by around 80 degrees, becoming parallel to the regional stress field. Shallow earthquakes (<35 km) also show this behaviour, but with more scatter of the fast directions. Another deployment (2002) covered the exact station locations of both the 1994 and the 1998 deployments and indicates further changes. Fast directions of deep events remain rotated by 80 degrees compared to the pre-eruption direction, whereas a realignment of the shallow events towards the pre-eruption direction is observed. The interpretation is that prior to the eruption, a pressurised magma dike system overprinted the regional stress field, generating a local stress field and therefore altering the fast anisotropic direction via preferred crack alignment. Numerical modelling suggests that the stress drop during the eruption was sufficient to change the local stress direction back to the regional trend, which was then observed in the 1998 experiment. A refilling and pressurising magma dike system is responsible for the newly observed realignment of the fast directions for the shallow events, but is not yet strong enough to rotate the deeper events with their longer delay times and lower frequencies. These effects provide a new method for volcano monitoring at Mt. Ruapehu and possibly at other volcanoes on Earth. They might, after further work, serve as a tool for eruption forecasting at Mt. Ruapehu or elsewhere. It is therefore proposed that changes in anisotropy around other volcanoes be investigated.</p>

Temporal Changes in Seismic Anisotropy as a New Eruption Forecasting Tool

<p>The orientation of crustal anisotropy changed by ~80 degrees in association with the 1995/96 eruption of Mt. Ruapehu volcano, New Zealand. This change occurred with a confidence level of more than 99.9%, and affects an area with a radius of at least 5 km around the summit. It provides the basis for a new monitoring technique and possibly for future mid-term eruption forecasting at volcanoes. Three deployments of seismometers were conducted on Mt. Ruapehu in 1994, 1998 and 2002. The fast anisotropic direction was measured by a semi-automatic algorithm, using the method of shear wave splitting. Prior to the eruption, a strong trend for the fast anisotropic direction was found to be around NW-SE, which is approximately perpendicular to the regional main stress direction. This deployment was followed by a moderate phreatomagmatic eruption in 1995/96, which ejected material with an overall volume of around 0.02-0.05 km3. Splitting results from a deployment after the eruption (1998) suggested that the fast anisotropic direction for deep earthquakes (>55 km) has changed by around 80 degrees, becoming parallel to the regional stress field. Shallow earthquakes (<35 km) also show this behaviour, but with more scatter of the fast directions. Another deployment (2002) covered the exact station locations of both the 1994 and the 1998 deployments and indicates further changes. Fast directions of deep events remain rotated by 80 degrees compared to the pre-eruption direction, whereas a realignment of the shallow events towards the pre-eruption direction is observed. The interpretation is that prior to the eruption, a pressurised magma dike system overprinted the regional stress field, generating a local stress field and therefore altering the fast anisotropic direction via preferred crack alignment. Numerical modelling suggests that the stress drop during the eruption was sufficient to change the local stress direction back to the regional trend, which was then observed in the 1998 experiment. A refilling and pressurising magma dike system is responsible for the newly observed realignment of the fast directions for the shallow events, but is not yet strong enough to rotate the deeper events with their longer delay times and lower frequencies. These effects provide a new method for volcano monitoring at Mt. Ruapehu and possibly at other volcanoes on Earth. They might, after further work, serve as a tool for eruption forecasting at Mt. Ruapehu or elsewhere. It is therefore proposed that changes in anisotropy around other volcanoes be investigated.</p>

100+ years of recomputed surface wave magnitude of shallow earthquakes

Among the multitude of magnitude scales developed to measure the size of an earthquake, the surface wave magnitude MS is the only magnitude type that can be computed since the dawn of modern observational seismology (beginning of the 20th century) for most shallow earthquakes worldwide. This is possible thanks to the work of station operators, analysts and researchers that performed measurements of surface wave amplitudes and periods on analogue instruments well before the development of recent digital seismological practice. As a result of a monumental undertaking to digitize such pre-1971 measurements from printed bulletins and integrate them in parametric data form into the database of the International Seismo- logical Centre (ISC, www.isc.ac.uk, last access: August 2021), we are able to recompute MS using a large set of stations and obtain it for the first time for several hundred earthquakes. We summarize the work started at the ISC in 2010 which aims to provide the seismological and broader geoscience community with a revised MS dataset (i.e., catalogue as well as the underlying station data) starting from December 1904 up to the last complete year reviewed by the ISC (currently 2018). This MS dataset is available at the ISC Dataset Repository at https://doi.org/10.31905/0N4HOS2D.

Identifying Direct SP-Converted Waves Constrains Local Induced Earthquake Depths

Seismicity in southern Kansas and northern Oklahoma in the past decade has been associated with fluid injections. In southcentral Kansas, the Wellington earthquake catalog is primarily composed of local, low-magnitude events. Approximately 22% of recorded earthquakes over a 2.5 yr period exhibit a seismic phase arriving between the direct P phase and direct S phase with particle motion similar to the P wave. This intermediate phase was identified as an S to P conversion (SP phase) occurring in the sedimentary rocks instead of the hypothesized basement to sedimentary section transition. We exploit the SP-converted phases to improve the depth accuracy of shallow earthquakes and to constrain VP/VS. The revised depth calculations further confirm that these local induced earthquakes are occurring in the shallow crystalline basement, below the sedimentary section in which fluids are injected.

Intraplate earthquake occurrence and distribution in Peninsular Malaysia over the past 100 years

Peninsular Malaysia is tectonically situated on a stable craton (intraplate) and so far experiences relatively little earthquake activities, thus considered as a region with low seismicity. This study uses earthquake data from 59 events obtained from various sources in the period 1922 to 2020. The overall seismicity in the study area is low as expected due to the general intraplate setting. Earthquakes occurred onshore and offshore of Peninsular Malaysia between latitudes 1° and 7° N and longitudes 99° and 105° E. The seismicity pattern shows that the epicenters are distributed spatially in some parts of the peninsula and in the Malacca Strait with several epicenter zones. Most of earthquakes are associated with several preexisting faults and fault zones indicating that they are the major contributor to the local seismicity. Meanwhile, some further earthquakes were caused by activities related to reservoirs. Magnitudes are ranging from Mw 0.7 to 5.4 with the majority is Mw 1.0 + and 2.0 +. Hypocenters are located in between 1 and 167 km deep (shallow to intermediate earthquakes) with the majority being shallow earthquakes (1–70 km). The deepest earthquake located in the Straits of Malacca can be associated with a slab detachment broken off from the Sumatran Subduction Zone. Finally, this study contributes to the understanding of the intraplate seismicity of Peninsular Malaysia as a basis for seismic hazard and risk assessment.Article Highlights Earthquake assessment over the last 100 year reveals low but clear seismicity with an associated seismic hazard and risk for certain areas. Shallow, low-magnitude earthquakes associated with reservoir activities and preexisting faults reactivated by the nearby subduction zone. A deeper, low-magnitude earthquake can be related to slab detachment from the Sumatran subduction zone toward the east.

Intensity of Induced Earthquakes in Northeast British Columbia, Canada

The damage potential of induced earthquakes associated with fluid injection is a major concern in hydrocarbon resource development. An important source of data for the assessment of damage is macroseismic intensity perceived by people and structures. In the Western Canada Sedimentary Basin (WCSB) where the occurrence of seismicity is mostly related to oil and gas activities, the collection of intensity data is incomplete. In this study, we present a comprehensive dataset gathered by the BC Oil and Gas Commission in the period 2016–2020. We assign intensities to individual felt reports according to the modified Mercalli intensity (MMI) scale and associate each MMI value to an earthquake. The isoseismal map of the largest earthquake in the Septimus region of northeast British Columbia is also provided using the compiled intensity dataset complemented with data from the U.S. Geological Survey and Natural Resources Canada “Did You Feel It?” systems along with the intensities converted from ground-motion amplitudes. We consider an approximate 10 km radius around the mainshock of 30 November 2018 earthquake with moment magnitude of 4.6 to be the meizoseismal area based on maximum intensities of 4–5. We also investigate the distance decay of intensity for shallow induced earthquakes in comparison with deeper natural events with the same magnitudes. Although intensities from shallow earthquakes (depth≤5 km) can be higher than deep events (depth≥10 km) at close distances (10–15 km), they tend to decrease abruptly at greater distances to become lower than deep events. The localization of large intensities from induced earthquakes within the meizoseismal area warrants special attention in future resource developments and call for systematic intensity data collection in the WCSB.

Synchronized and asynchronous modulation of seismicity by hydrological loading: A case study in Taiwan

Delineation of physical factors that contribute to earthquake triggering is a challenging issue in seismology. We analyze hydrological modulation of seismicity in Taiwan using groundwater level data and GNSS time series. In western Taiwan, the seismicity rate reaches peak levels in February to April and drops to its lowest values in July to September, exhibiting a direct correlation with annual water unloading. The elastic hydrological load cycle may be the primary driving mechanism for the observed synchronized modulation of earthquakes, as also evidenced by deep earthquakes in eastern Taiwan. However, shallow earthquakes in eastern Taiwan (<18 km) are anticorrelated with water unloading, which is not well explained by either hydrological loading, fluid transport, or pore pressure changes and suggests other time-dependent processes. The moderate correlation between stacked monthly trends of large historic earthquakes and present-day seismicity implies a modestly higher seismic hazard during the time of low annual hydrological loading.