Low Stress Drop Research Articles

Magma intrusion process during pre-magmatic period (2010−2013) of Sinabung volcano as revealed by seismicity of volcano-tectonic and hybrid earthquakes

Sinabung volcano in Indonesia reactivated, with a series of phreatic eruptions, in August and September 2010. These eruptions were followed by various types of magmatic eruption at the summit, including lava dome growth, pyroclastic density currents, lava flow, and vulcanian eruptions, starting in December 2013. Prior to the magmatic eruptions, Sinabung exhibited two sequences of phreatic eruption and an increase in the seismicity of volcanic earthquakes. This study clarifies the progress of magma intrusion during the pre-magmatic period based on the hypocenter distribution, waveform similarity, seismic moment, rupture length, and stress drop for volcano-tectonic (VT) and hybrid earthquakes. It was found that the hypocenters of VT earthquakes are distributed north and northwest of the summit at a depth range of 1–10 km below sea level. Deep seismicity (4–10 km) alternated with shallow seismicity (2–4 km). The last shallow seismicity, from July to mid-December 2013, was different from previous seismicity in terms of higher intensity, as determined from the seismic moment, a temporary increase in the rupture length, and the migration of hypocenters towards the summit. The seismicity of VT earthquakes was altered by a swarm of hybrid earthquakes in mid-December. The hybrid earthquakes were smaller, in terms of seismic moment (mostly <3 × 1010 Nm), than the VT earthquakes. Their hypocenters were concentrated in the shallowest depth range (−0.5 to 1.5 km below sea level) directly below the summit, their source process was repeatable (they could be grouped into six earthquake families), their dominant peak was in a low-frequency range (2.5–4.5 Hz), and they had a relatively low stress drop (<0.15 MPa). This suggests that the swarm of hybrid earthquakes was induced by a frequent repeated fracture of fluid-filled cracks due to the intrusion of magma up to directly below the summit. The transition of the earthquake family and the change in its source parameters, namely an increase in the stress drop prior to the appearance of a lava dome and then a slight decrease, may reflect a gradual change in internal pressure in the hypocentral zone through the magma intrusion process.

On the Unloading‐Induced Fault Reactivation: The Effect of Stress Path on Failure Criterion and Rupture Dynamics

AbstractFault reactivations induced by deep excavation can pose significant challenges to underground construction or resource extraction. Laboratory experiments on rock faults demonstrate that unloading‐induced fault reactivations obey the Coulomb failure criterion derived from loading‐induced events. However, the effect of stress path during unloading on the failure criterion and rupture dynamics of fault reactivations remains poorly understood. Here, we present findings from a series of laboratory experiments aimed at elucidating the effect of the unloading path on the failure criterion and rupture dynamics of fault reactivations. We conducted experiments under various stress conditions, examining two cases of unloading paths. In Case I, we unloaded the minimum principal stress, while in Case II, the maximum principal stress was unloaded. Strain gauges and high‐speed photography were employed to capture the transient dynamic rupture process. Our investigations have yielded new insights into the effect of unloading path on the rupture dynamics when the fault is reactivated. In Case I, we observed fault reactivations resembling those loading‐induced events characterized by forward sliding. Conversely, in Case II, fault reactivations associated with stress reversal produce mild reversed sliding with lower stress drop and rupture velocity. Furthermore, we find that there is a remarkable reduction in static friction for reversed sliding, indicating that the failure criterion for fault reactivation is influenced by the stress path. We demonstrate that enhanced stress heterogeneity, caused by stress reversal, serves as a mechanism for reduced static friction. These findings contribute to our understanding of the mechanisms underlying fault reactivations, particularly those involving reversed sliding.

Research on The Source Parameters and Scaling Relations of Earthquakes in Weihe Basin and Its Adjacent Area

By using the data recorded in Shaanxi seismic network, seismic source parameters were determined through spectrum analyzing basing on Brune’s source model for 125 small to moderate earthquakes (1.1 ≤ ML ≤ 4.8) from 1997 to 2016 in Weihe basin and its adjacent area. Seismic radiated energies were also obtained through integral of observed velocity spectra. The relations between stress drop, scaled energy E s / M 0 and Zúñiga parameter ε with seismic moment are analyzed. The results show that the earthquakes are with low stress drop in this area, the mean value is 0.16 MPa. More than 90% of the events in the region observed the ε value is less than 1. The low stress drop and low ε value indicate that earthquakes in this area can be explained by fractional stress drop mechanism or low effective stress model. These may be the result of mature fault activity, however, it cannot be ruled out effect of underground fluid. The scaled energy of earthquakes (1.3 ≤ MW ≤ 3.6) in this area is less than that of larger earthquakes (3.6 ≤ MW ≤ 6.0) in other areas. The earthquakes in this area are fairly small, so the heat energy generated in the rupture process is small, which is not enough to produce melting along the fault plane. With large dynamic friction force, it is not easy to produce rapid brittle rupture, resulting in low scalsed energy.

Source characteristics of earthquakes in Delhi and its vicinity: Implications for seismogenesis in the stable continental region of India

Delhi and its surrounding region are among the important intraplate seismically active regions of India. According to the Bureau of Indian Standards, it lies in seismic zone IV and is capable of generating small to moderate earthquakes. This region has also experienced moderate to large earthquakes in the past due to its proximity to the Himalayan arc and its own tectonics. There is an urgent need to assess the source characteristics of earthquakes and their scaling relationships for a reliable and accurate estimation of seismic hazard. Source parameters and their scaling relations have been estimated for this region using well-located small to moderate earthquakes (Mw 2.3–5.1) that occurred during the period from 2000 to 2020. For this purpose, we investigated three components of earthquake waveform data using the ω−2 source model. The outcomes show the occurrence of low static stress drop (0.5–52.87 ​bars) in the Delhi region indicating approximately 57% of the events exhibit a stress drop < 10 ​bars, 27 % of events fall in the range of 10–20 ​bars, and only 16 % events exhibit stress drop > 20 ​bars. This observation suggests that the shallow subsurface of the study region may have low-strength material characteristics and a heterogeneous nature. In addition, the Zuniga parameter (ε) is estimated less than 1.0 by analyzing static and apparent stress drops, which infers the partial stress drop model fits very well in the study region. The seismic moment varies from 1.02×1012 to 1.03×1016 ​N.m. for P-wave and 6.46×1011 Nm to 7.54×1015 Nm for S-wave. The average seismic source radius lies in the range of 0.1–3.05 ​km with a ratio {r(p)/r(s)} of 1.5 ​km in the study region. The estimated values of corner frequency are comparatively lower for the S-wave (1.5–18.2 ​Hz) than for the P-wave (1.88–19.3 ​Hz) suggesting the ‘shifting properties’ of the corner frequency corroborated with the theoretical agreement. The seismic energy (E) is estimated using both P- and S-wave separately and its average value varies from 4.28×106 to 6.22×1011 ​J. The estimated stress drop and seismic moment demonstrate no correlation with each other. Therefore small to moderate-size earthquakes inherently follow the self-similarity behavior. The obtained scaling relationship between Seismic Moment and Corner Frequency is Mo=7.94∗1015fc−2.97. The derived scaling relations and source parameters are expected to provide a priori information for the assessment of seismic hazards and are useful in the simulation of strong ground motion characteristics in the region.

Fan‐hinged shear instead of frictional stick–slip as the main and most dangerous mechanism of natural, induced, and volcanic earthquakes in the earth's crust

AbstractFrictional stick–slip instability along pre‐existing faults has been accepted as the main mechanism of earthquakes for about 60 years, since it is believed that fracture of intact rocks cannot reflect such features inherent in earthquakes as low shear stresses activating instability, low stress drop, repetitive dynamic instability, and connection with pre‐existing faults. This paper demonstrates that all these features can be induced by a recently discovered shear rupture mechanism (fan‐hinged), which creates dynamic ruptures in intact rocks under stress conditions corresponding to seismogenic depths. The key element of this mechanism is the fan‐shaped structure of the head of extreme ruptures, which is formed as a result of an intense tensile cracking process, with the creation of inter‐crack slabs that act as hinges between the shearing rupture faces. The preference of the fan mechanism over the stick–slip mechanism is clear due to the extraordinary properties of the fan structure, which include the ability to generate new faults in intact dry rocks even at shear stresses that are an order of magnitude lower than the frictional strength; to provide shear resistance close to zero and abnormally large energy release; to cause a low stress drop; to use a new physics of energy supply to the rupture tip, providing supersonic rupture velocity; and to provide a previously unknown interrelation between earthquakes and volcanoes. All these properties make the fan mechanism the most dangerous rupture mechanism at the seismogenic depths of the earth's crust, generating the vast majority of earthquakes. The detailed analysis of the fan mechanism is presented in the companion paper “New physics of supersonic ruptures” published in DUSE. Further study of this subject is a major challenge for deep underground science, earthquake and fracture mechanics, volcanoes, physics, and tribology.

Spiral grass inspired eco-friendly zein fibrous membrane for multi-efficient air purification

Air pollution, one of the severest threats to public health, may lead to cardiovascular and respiratory illnesses. In order to cope with the deteriorating air pollutant, there is an increasing demand for filters with high purification efficiency, but it's tough to strike a balance between efficiency and resistance. Fabricating an eco-friendly fibrous filter which can capture both PM2.5 and gaseous chemical hazards with high efficiency but under ultra-low resistance is a long-term challenge. Herein, inspired by the interesting ribbon shape of spiral grass, a green and robust 3D nonwoven membrane with controllable hierarchical structure made of self-curved zein nanofibers modified by zeolitic imidazolate framework-8 (ZIF-8) via bi-solvent electrospinning and fumigation welding method was fabricated. The obtained ZIF-8 modified zein membranes showed extraordinary overall performance with high PM2.5 removal efficiency (99.04 %) at a low stress drop (54.87 Pa), first-rate formaldehyde removal efficiency (98.8 %) and excellent photocatalytic antibacterial. In addition, the relatively weak mechanical properties of zein fibrous membranes have been improved via solvent fumigation welding of the joint zein fibers. This study provides a green and convenient insight to the manufacturing of environmentally-friendly zein fibrous membranes with high filtration efficiency, low air resistance and high formaldehyde removal for sustainable air remediation.

Rupture Directivity of the 25 October 2022 Mw 5.1 Alum Rock Earthquake

Abstract The 25 October 2022 Mw 5.1 Alum Rock earthquake shows strong evidence for southeast rupture directivity along the central Calaveras fault (CCF), as indicated by observed ground motions and simulated kinematic ruptures. Peak ground accelerations (PGAs) and peak ground velocities (PGVs) are notably higher to the southeast, with an order of magnitude difference for stations at the same distance but different azimuths. In addition, PGAs are lower than that predicted by ground-motion models by a factor of 3 on average in all the directions, indicating a low stress drop (∼1.57 MPa). Directivity function modeling and ground-motion simulations both indicate rupture propagation to the southeast with rupture velocity between 2.3 and 2.5 km/s. We suggest that the southward rupture propagation and relatively low stress drop may be typical of M ∼5 earthquakes on this portion of the CCF.

Ultralow frictional healing explains recurring slow slip events.

Plate motion on shallow subduction megathrusts is accommodated by a spectrum of tectonic slip modes. However, the frictional properties and conditions that sustain these diverse slip behaviors remain enigmatic. Frictional healing is one such property, which describes the degree of fault restrengthening between earthquakes. We show that the frictional healing rate of materials entrained along the megathrust at the northern Hikurangi margin, which hosts well-characterized recurring shallow slow slip events (SSEs), is nearly zero (<0.0001 per decade). These low healing rates provide a mechanism for the low stress drops (<50 kilopascals) and short recurrence times (1 to 2 years) characteristic of shallow SSEs at Hikurangi and other subduction margins. We suggest that near-zero frictional healing rates, associated with weak phyllosilicates that are common in subduction zones, may promote frequent, small-stress-drop, slow ruptures near the trench.

Identification of fracturing behavior in thermally cracked granite using the frequency spectral characteristics of acoustic emission

The frequency characteristics of acoustic emission (AE) during triaxial compression of thermally cracked and unheated (“fresh”) granite samples were investigated with the aim of understanding the influence of pre-existing cracks on precursor information regarding macroscopic failure. The peak frequency during the damage process was the same for thermally cracked and fresh granites. Analysis of AE signals showed that signals with low peak frequency appeared before failure of the sample, implying the initiation of macrocracks with progressive growth of cracks. The peak amplitude of the frequency spectrum recorded in the thermally cracked samples was much lower than that in the fresh samples. This result suggests two reasons for the difference in peak amplitude: reduction in shear modulus and the attenuation filtering phenomenon caused by thermal cracks (i.e., the differences in overall bulk properties). In particular, the maximum value of peak amplitude in the low-frequency band for the thermally cracked samples was smaller than that for fresh samples. This characteristic can be related to the properties of source of AE signals, such as the stress drop and crack size. Assuming that pre-existing thermal cracks grow during the pre-failure stage, the events with low peak frequency and low peak amplitude in the heat-treated samples are interpreted as exhibiting a low stress drop for individual events. Therefore, although AE signals with low frequency can be considered as precursors to rock failure, cracking behavior suggested by events with low frequency depends on the initial damage condition of the rock sample.

EASOt-AP: An open-source MATLAB package to estimate the seismic moment, rupture radius, and stress-drop of earthquakes from time-dependent P-wave displacements

We developed a MATLAB package, to rapidly obtain the apparent EArthquake SOurce time function using the Average of near-source P-wave displacement records (EASOt-AP) and to estimate the source parameters of the seismic moment, the rupture radius, and the average static stress drop. The algorithm implemented in this package is based on a rapid and straightforward methodology that is recently developed by Zollo et al. (2021). To this purpose, EASOt-AP models a LPDT curve, which is the average P-wave displacement versus time in the logarithm scale. In this paper, the performance of the EASOt-AP has been shown by evaluating the strong motion data recorded in the near-source range of small magnitude events (2 = Mw ≤ 3) that occurred in the Irpinia region, southern Italy. The source parameters follow a self-similar, constant-stress-drop scaling with a relatively low average stress drop of about 01-0.2 MPa. This Tool is designed for an easy use, and all steps of data processing are performed completely within the numerical environment of MATLAB.

Stress dissipation and seismic potential in the central seismic gap of the north-west Himalaya

• One-third of the earthquakes in CSG show stress drop <1 bar. • High stress drop events are absent in the upper 10 km of the crust. • Source parameter values are lower in Kumaon than CSG. • Kangra and Kumaon segments can be tipped to host a future great earthquake. The central seismic gap (CSG) of the Himalaya continues to pose the ever more intriguing question of whether it has the potential to host a devastating earthquake. To address the issue, we examine the distribution and dissipation of post-seismic stress in the northwest segment of the CSG. This study computes the source parameters of 47 local earthquake events (3.5 ≤ M L ≤ 5.5) in the Garhwal-Kumaon Himalaya during the period 2016–2018 using P-wave spectral analysis. In addition, we compile the stress drop results for 326 earthquake events from numerous studies carried out across the seismic gap, to estimate the source parameter variation. The estimated seismic moments (10 12 ≤ M o ≤ 10 16 N-m), source radii (0.3 ≤ r ≤ 1.3 km) and stress drops (0.1 ≤ σ ≤ 40.6 bar) for Kumaon are observed to be significantly lower than the overall values for the CSG (10 10 ≤ Mo ≤ 10 20 N-m; 0.1 ≤ r ≤ 13.2 km; 0.01 ≤ σ ≤ 77 bar), showing incomplete dissipation of the accumulated stress. About one-third of the earthquakes in the CSG show very low stress drop (σ < 1 bar). The high stress drop events are predominant along the Main Himalayan Thrust at mid-crustal depths, with the upper ∼ 10 km of the brittle crust rarely hosting any big event. Based on the analysis, the scaling relation M o f c 3.677 = 6E + 15 N-m/s 3 has been proposed for the Kumaon region. Other scaling relations have been developed between important source parameters (M L , M w , M o , and σ) that may serve as valuable inputs for assessing the earthquake hazard in the region. The estimated b-value for the Kumaon Himalaya estimated from the local earthquake catalog is 0.64 ± 0.08, which is low compared to the Garhwal and rest of the NW Himalaya. The overall b-value of the CSG region computed from the ISC catalog is 0.62 ± 0.06. From the stress drop and b-value variation in the CSG, the Kangra and the Kumaon segments are identified to be potentially hazardous zones for a future great earthquake.

Across‐Slab Propagation and Low Stress Drops of Deep Earthquakes in the Kuril Subduction Zone

AbstractDeep earthquakes occur down to 700 km depth where pressure is up to two orders of magnitude greater than in the crust. Rupture characteristics and propagation mechanisms under those extreme conditions are still poorly constrained. We invert seismic waveforms for the spatial dimensions, duration and stress drop of deep‐focus earthquakes (Mw6.7–7.7) in the Kuril subduction zone. We find stress drops of ∼1–10 MPa and rigidity‐corrected spatial dimensions and durations similar to crustal earthquakes. Radiated efficiency &gt;1 is observed, suggesting that undershooting is prevalent in deep earthquakes, consistent with laboratory derived weakening mechanisms. Comparisons with subduction models suggest across‐slab propagation within regions with temperaturesT &lt; ∼ 1,000°C, similar to shallow events. Hence, despite different triggering mechanisms, the same physics seems to control the rupture propagation of both shallow and deep earthquakes.

Radiated seismic energy from the 2021 ML 5.8 and ML 6.2 Shoufeng (Hualien), Taiwan, earthquakes and their aftershocks

We calculated radiated seismic energy (ES), seismic moment (M0), and moment magnitude (MW) and then determined the ES–ML, ES–M0, M0–ML, and MW–ML relations for the 2021 Shoufeng earthquake sequence (2.5 < ML < 6.3), where ML is the local magnitude. Notably, a crossover magnitude was detected as ML = 4.0 for the earthquake sequence. For ML < 4.0, we obtained logM0 ∝ ML, MW ∝ 0.67ML, and a low ES/M0, indicating a low average stress drop; for ML > 4.0, logM0 ∝ 0.67ML, MW ∝ ML, and a high ES/M0 were present, and then there was a high average stress drop. These derived relations implied that source duration (T) is independent of M0 for ML < 4.0. Moreover, the M0 ∝ T3 relation seemed able to interpret those relations for ML > 4.0. Nevertheless, the ES–ML relation remains logES ∝ 2.0ML for 2.5 < ML < 6.3. From this study, the derived relations could not predict the source parameters for ML > 6.3 events. This might indicate that ML saturates beyond ML 6.3. Through such analyses, we not only established the relations among source parameters but also elucidated the basic physics of the earthquake sequence.

The Role of Fault Rock Fabric in the Dynamics of Laboratory Faults

AbstractFault stability is inherently linked to the frictional and healing properties of fault rocks and associated fabrics. Their complex interaction controls how the stored elastic energy is dissipated, that is, through creep or seismic motion. In this work, we focus on the relevance of fault fabrics in controlling the reactivation and slip behavior of dolomite‐anhydrite analog faults. We designed a set of laboratory experiments where we first develop fault rocks characterized by different grain size reduction and localization at normal stresses of σN = 15, 35, 60, and 100 MPa and second, we reload and reactivate these fault rocks at the frictional stability transition, achieved at σN = 35 MPa by reducing the machine stiffness. If normal stress is lowered this way, reactivation occurs with relatively large stress drops and large peak‐slip velocities. Subsequent unstable behavior produces slow stick‐slip events with low stress drop and with either asymmetric or Gaussian slip velocity function depending on the inherited fault fabric. If normal stress is raised, deformation is accommodated within angular cataclasites promoting stable slip. The integration of microstructural data (showing brittle reworking of preexisting textures) with mechanical data (documenting restrengthening and dilation upon reactivation) suggests that frictional and chemically assisted healing, which is common in natural faults during the interseismic phase, can be a relevant process in developing large instabilities. We also conclude that fault rock heterogeneity (fault fabric) modulates the slip velocity function and thus the dynamics of repeating stick‐slip cycles.

Stress Drop Variations of Induced Earthquakes near the Dallas–Fort Worth Airport, Texas

Abstract We estimate stress drops for injection-induced earthquakes near the Dallas–Fort Worth Airport in the Fort Worth basin (FWB), Texas, to investigate source properties in response to fluid injection. The stress drops for the Airport sequence show three unique characteristics compared to those estimated for other earthquake sequences in the FWB: (1) stress drops have lower mean and median values; (2) stress drops increase with moment magnitude; and (3) stress drops increase in size over the first 1.5 km in radial distance from the injection point. The low stress drop Airport events occurred shortly after the initiation of injection near a fault within hundreds of meters of the well. Pore pressure perturbations in the Airport area are 1 order of magnitude lower than those from the other sequences, suggesting that absolute pore pressure changes may not be the main factors of stress drop variations. We suggest that the low stress drop events may be related to transition from aseismic slip to seismic rupture previously observed in laboratory and field experiments.

Occurrences of low-stress drop earthquakes in the eastern Ladakh-Karakoram zone of the Trans Himalaya and their tectonic implications

The Ladakh-Karakoram zone is a unique testing ground for understanding the geodynamic evolution of the Himalaya-Karakoram orogeny. Despite the accumulation of a large amount of strain energy that originated due to the India-Asia collision along the Karakoram fault, earthquakes of M ≥ 7.0 are considerably less in the Karakoram Fault Zone (LKZ) compared to the Himalayan seismic belt in the northwest Himalaya. The spectral analysis of microearthquakes data (1.9 < ML < 4.3) recorded during 2009–2012 by 10 seismological stations of the LKZ reveals that the region produces low-stress drop earthquakes. Analysis of selected 51 earthquakes shows the seismic moment (Mo) within the range of 1.2 × 1012–4.3 × 1015 Nm with stress drop values varying from ∼ 0.06 to 64.36 bar. The low stress-drop earthquakes have significant implications in understanding seismogenesis. The earthquake cluster near the Tso-Morari Crystallines is associated with the brittle shear failure on the faults (e.g. Zildat, and Karzok Faults), and the low-stress drop of the earthquakes are explained by the partial stress drop model. The low effective fault strength due to fault weakening mechanisms in the studied segment of the Karakoram Fault zone promotes aseismic creeping patches producing low-stress drop earthquakes at the expense of generating large earthquakes. This study provides the scaling relations between important source parameters for the first time in the region which can serve as useful inputs for the assessment of earthquake hazards.

Emergence of Low-Frequency Aftershocks of the 2019 Ridgecrest Earthquake Sequence

ABSTRACTLow-frequency earthquakes (LFEs) generally have relatively stronger spectral components in the lower frequency range compared with what is expected for regular earthquakes based on their magnitude. LFEs generally occur in volcanic systems or deep (&amp;gt;∼15 km) in plate boundary fault zones; however, LFEs have also been observed in nonvolcanic, upper crustal settings. Because there are few studies that explore the spatiotemporal behaviors of LFEs in the shallow crust, it remains unclear whether the shallow-crustal LFEs reflect local attenuation in their immediate vicinity or differences in their source mechanism. Therefore, it is important to identify shallow-crustal LFEs and to characterize their spatiotemporal activity, which may also improve our understanding of LFEs. In this study, we focus on detecting shallow-crustal LFEs and explore the possible generation mechanisms. We analyze 29,646 aftershocks in the 2019 Ridgecrest, California, earthquake sequence, by measuring the frequency index (FI) to identify candidate low-frequency aftershocks (LFAs), while accounting for the magnitude dependency of the FI. Using small earthquakes (ML 1–3) recorded in the borehole stations to minimize the attenuation effects in near-surface layers, we identify 68 clear LFAs in total. Based on their distribution and comparisons with other seismic parameters measured by Trugman (2020), the LFAs possess distinct features from regular events in the same depths range, including low corner frequencies and low stress drops. Events in the close vicinity of LFAs exhibit lower average FI values than regular aftershocks, particularly if the hypocentral distance between an LFA and its neighbors is less than 1 km. Our results suggest that LFAs are related to local heterogeneity or a highly fractured fault zone correlated with an abundance of cross faults induced by the aftershock sequence at shallow depths. Zones of high pore-fluid pressure in intensely fractured fault zones could cause the bandlimited nature of LFAs and LFEs in general.

A source and ground motion study of earthquakes in and near Delhi (the National Capital Region), India.

We study source parameters of 10 local earthquakes (2.7 ≤ M_{w} ≤ 4.5) that have occurred in the National Capital Region (NCR) since 2001 and the ground motions produced by these events. Moment rate spectra of the earthquakes retrieved from the recordings at hard sites after applying corrections for geometrical spreading (1/R, R ≤ 100 km), anelastic attenuation (Q = 253f0.8) and cutoff frequency (fm = 35 Hz) are reasonably well fit by the Brune ω2-source model with stress drop ranging between 0.9 and 13 MPa. Neglecting the outlier low-stress drop value, the average stress drop is 6 MPa. We apply a modified standard spectral ratio technique to estimate site effect at 38 soft sites in the NCR as well as the geometrical mean site effect with respect to a reference hard site. Application of the stochastic method, with source characterized by the Brune ω2- model with stress drop of 6 MPa and the mean site effect for soft sites, yields peak horizontal ground acceleration and velocity curves that are in good agreement with the observed values. These results provide the parameters needed for the application of the stochastic method to predict ground motions at hard and soft sites in the NCR during postulated M_{w} ≤ 5.5 earthquakes.

Earthquake Rupture and Tsunami Generation of the 2015 Mw 5.9 Bonin Event Revealed by In Situ Pressure Gauge Array Observations and Integrated Seismic and Tsunami Wave Simulation

AbstractOn September 1, 2015, an Mw 5.9 interplate earthquake occurred near the Bonin Trench. An array of in situ ocean‐bottom absolute pressure gauges (APGs) observed its tsunami generation field consisting of static and dynamic pressure changes due to tsunami and crustal deformation and due to seismic motion, respectively, under much higher station density than ever reported. We propose an approach to synthesize the pressure change inside the focal area, which reproduces the APG waveforms well. We further successfully estimate the finite fault model of the earthquake and constrain the rupture duration only from the APG data. The relatively low stress drop and seismic wave radiation from the fault model may suggest the nature of a tsunami earthquake. The in situ APGs have a large potential to reveal the broadband source process of the earthquake, which is essential to understand the seismotectonics in the subduction zone.

The Role of Rock Matrix Permeability in Controlling Hydraulic Fracturing in Sandstones

A concomitant effect of a hydraulic fracturing experimenting is frequently fluid permeation into the rock matrix, with the injected fluid permeating through the porous rock matrix (leak-off) rather than contributing to the buildup of borehole pressure, thereby slowing down or impeding the hydro-fracturing process. Different parameters, such as low fluid viscosity, low injection rate and high rock permeability, contribute to fluid permeation. This effect is particularly prominent in highly permeable materials, therefore, making sleeve fracturing tests (where an internal jacket separates the injected fluid in the borehole from the porous rock matrix) necessary to generate hydraulic fractures. The side effect, however, is an increase in pressure breakdown, which results in higher volume of injected fluid and in higher seismic activity. To better understand this phenomenon, we report data from a new comparative study from a suite of micro-hydraulic fracturing experiments on highly permeable and on low-permeability rock samples. Experiments were conducted in both sleeve fracture and direct fluid fracture modes using two different injection rates. Consistent with previous studies, our results show that hydraulic fracturing occurred only with low permeation, either due to the intrinsic low permeability or due to the presence of an inner silicon rubber sleeve. In particular, due to the presence of quasi-impermeable inner sleeve or borehole skin in the sleeve fracturing experiment, fracturing occurs, with the breakdown pressure supporting the linear elastic approach considering poroelastic effects, therefore, with low stress drop and consequently low microseismicity. Rock matrix permeability also controls the presence of precursory Acoustic Emission activity, as this is linked to the infiltration of fluids and consequent expansion of the pore space. Finally, permeability is shown to mainly control fracturing speed, because the permeation of fluid into the newly created fracture via the highly permeable rock matrix slows down its full development. The application of these results to the field may help to reduce induced seismicity and to conduct well stimulation in a more efficient way.