A New P-Wave Detector via Moving Empirical Cumulative Distribution Function

Kwon, Junhyeon,Heo, Taemin,Kim, Jae Kwan,Oh, Hee-Seok SEISMOLOGICAL SOCIETY OF AMERICA 2018 Bulletin of the Seismological Society of America Vol.108 No.4

<P>Fast and accurate alarm of a seismic event is one of the most important tasks to reduce damage from earthquakes. This article proposes a new method for detection of P-wave arrival that might be useful for a better earthquake early warning (EEW) system. As an existing method, short-term average to long-term average (STA/LTA) ratio is widely used to identify the onset time of earthquake using moving time windows. It calculates the averages of the absolute values of seismic wave in short and long windows and detects the outbreak of the seismic event when a ratio of them exceeds a predetermined thresholding value. The proposed method is based on the shape of moving empirical cumulative distribution function (MECDF) of absolute-valued signal in a moving window. It requires one less window than the STA/LTA method, which alleviates the burden of optimizing window lengths while keeping similar performance. Furthermore, the MECDF method holds less parametersensitive behavior in detection when the window length is not set correctly compared with the STA/LTA method. Hence, the proposed method is an alternative algorithm that facilitates the station-wise parameter setting to detect P wave.</P>

Successive Groundwater Level Changes on Jeju Island due to the <i>M</i>_w 9.0 Off the Pacific Coast of Tohoku Earthquake

Lee, Soo‐,Hyoung,Ha, Kyoochul,Soo Shin, Jin,Ko, Kyung‐,Seok,Hamm, Se‐,Yeong Seismological Society of America 2013 Bulletin of the Seismological Society of America Vol.103 No.2

<P>Changes in groundwater levels on Jeju Island, which is ∼1500  km from the epicenter of the 2011 <I>M</I><SUB>w</SUB> 9.0 Off the Pacific Coast of Tohoku earthquake in Japan, were analyzed. The results show a series of water level fluctuations related to the foreshock (<I>M</I><SUB>w</SUB> 7.3 and <I>M</I><SUB>w</SUB> 6.1) and aftershock (<I>M</I><SUB>w</SUB> 7.9) in addition to the mainshock (<I>M</I><SUB>w</SUB> 9.0). The groundwater‐level changes in response to the earthquake were oscillatory, and the groundwater levels at some wells had an irregular pattern after the <I>M</I><SUB>w</SUB>  9.0 earthquake, although they recovered in five to seven days. This phenomenon may reflect the unstable elastic aquifer properties that are present for a period of time after a large earthquake. In addition, the successive groundwater‐level change was different for each magnitude and well location. The magnitude increases the groundwater‐level change, but the response amplitude is also dependent on the hydrogeological characteristics at the well. On Jeju Island, the groundwater‐level changes due to the earthquake generally increased where there was more volcanic hard rock and more permeable layers, but these changes were inversely correlated with the presence of sedimentary deposits with less permeability and less restrictive characteristics.</P>

Relocation of Earthquakes in the New Madrid Seismic Zone: Estimation of 1D Velocity Structure and Geometry of a Seismogenic Fault

Park, Junghyun,Lee, Jung Mo,Chiu, Jer‐,Ming,Kim, Woohan,Kim, Won‐,Young Seismological Society of America 2015 Bulletin of the Seismological Society of America Vol.105 No.2

<P>Determination of reliable hypocenters of earthquakes is crucial to earthquake seismology and to evaluate hazards associated with earthquakes. There are many associated computer codes for this purpose; however, most of the location algorithms are designed to determine hypocentral parameters based on previously determined velocity models. In contrast, we employed a location method that is independent of the initial velocity model, using a genetic algorithm (GA) to determine an optimal 1D velocity model and the locations of earthquakes. Using this GA, we relocated earthquakes that occurred in the New Madrid Seismic Zone (NMSZ) in the central United States between October 1989 and August 1992. The goal of this work was to delineate the possible fault planes by reliable relocation of those earthquakes and to determine a 1D velocity structure for the NMSZ. A total of 502 earthquakes recorded by 37 Portable Array for Numerical Data Acquisition (PANDA) stations were used in the relocation study. In the relocation process, the root mean square travel‐time residuals were reduced by ∼35%, corresponding to an average of 2.3 km deeper in depth, 0.7 km shift in latitude, and 0.8 km shift in longitude compared with those in the initial catalog locations. The hypocenters of the earthquakes can be subdivided into four groups based on their spatial distributions. The group that corresponds to the Cottonwood Grove fault (CGF) in the southwestern NMSZ represents a very steep plane, whereas the other three groups fall into Reelfoot fault (RF). We inverted <I>P</I>‐ and <I>S</I>‐wave travel times from the new hypocentral parameters to determine 1D velocity models. The resulting eight‐layered velocity models consist of a 2 km thick surface layer followed by seven 2 km thick layers, with <I>V</I><SUB><I>P</I></SUB> ranges from 5.36 to 6.74  km/s and <I>V</I><SUB><I>S</I></SUB> ranges from 2.83 to 3.90  km/s for both CGF and RF regions.</P><P><I>Online Material: </I>Interactive visualizations of hypocentral distributions.</P>

Separation of Intrinsic and Scattering Attenuation Using Single Event Source in South Korea

Rachman, Asep Nur,Chung, Tae Woong,Yoshimoto, Kazuo,Son, Busoon Seismological Society of America 2015 Bulletin of the Seismological Society of America Vol.105 No.2

<P>Simultaneous use of numerous events in the multiple lapse time window analysis (MLTWA) method is the most effective way to obtain [Formula] and [Formula] values. Such simultaneous use of numerous events normally shows a large observational scatter, which may be caused by different radiation pattern and focal depth of each earthquake, and regional alteration of the local structure. To avoid these scattering factors, the current study attempted to use single event source as the input data in MLTWA for South Korea. The direct simulation Monte Carlo (DSMC) method was used to simulate the observations, which showed realistic results when the number of recorded stations was eight or more. The obtained values of seismic albedo (<I>B</I><SUB>0</SUB>) were less than 0.5, which were in accordance with the same in the previous results. Three envelopes constructed by combination of three regional values of [Formula] at 1 Hz produced regional variations, of which the ones at high frequencies of [Formula] and at low frequencies of [Formula] were remarkable. This result, showing the local variation indicated an advantage of the study using single source event over the previous studies in South Korea that involved extensively distributed data. Because DSMC was known to be applicable to 3D structure due to its simple algorithm, the current study also tried to correct radiation pattern of the observations by inserting focal mechanism algorithm to the code of the DSMC method. Such insertion, however, generated extremely low values of [Formula] because the geometrically shrunk receiver could not capture most of the scattered particles. This receiver problem is most likely to be a major obstacle of the 3D study of DSMC. In addition, the significant depth effect of <I>Q</I><SUP>−1</SUP> was identified for the prior MLTWA studies resulting from different measurement range and focal depth.</P>

The Effect of Fracture Energy on Earthquake Source Correlation Statistics

Seismological Society of America 2015 Bulletin of the Seismological Society of America Vol.105 No.2

<P>Complex earthquake rupture processes can affect ground‐motion characteristics significantly, particularly in the near‐source region for large events. Correlations between earthquake source parameters are efficient metrics in quantifying earthquake source characteristics and consequently in understanding the effect of the earthquake source on ground‐motion characteristics. I investigate the effect of fracture energy on source correlation statistics by performing a set of spontaneous dynamic rupture model simulations, each with a different strategy in generating input fracture energy distributions. I find that source parameter correlation structures can be significantly affected by the input fracture energy distribution because it plays an important role in controlling the temporal evolution of the rupture process. This study implies that it is important to stably constrain the fracture energy by observational data, or at least to test a wide range of possibilities in generating fracture energy distributions in dynamic rupture modeling for source correlation studies.</P><P><I>Online Material: </I>Figures showing distributions of input dynamic parameters for positive and negative correlations.</P>

Regional Source Scaling of the 9 October 2006 Underground Nuclear Explosion in North Korea

Hong, T.-K.,Rhie, J. Seismological Society 2009 Bulletin of the Seismological Society of America Vol.99 No.4

Rapid Assessment of Epicentral Distance Using the Initial P Waves Observed in South Korea

Jung, Seula,Rhee, Hyun-Me,Sheen, Dong-Hoon SEISMOLOGICAL SOCIETY OF AMERICA 2018 Bulletin of the Seismological Society of America Vol.108 No.1

Spatiotemporal Distribution of Events during the First Three Months of the 2016 Gyeongju, Korea, Earthquake Sequence

Son, Minkyung,Cho, Chang Soo,Shin, Jin Soo,Rhee, Hyun-Me,Sheen, Dong-Hoon SEISMOLOGICAL SOCIETY OF AMERICA 2018 Bulletin of the Seismological Society of America Vol.108 No.1

Improving Global Radial Anisotropy Tomography: The Importance of Simultaneously Inverting for Crustal and Mantle Structure

Chang, Sung‐,Joon,Ferreira, Ana M. G. Seismological Society of America 2017 Bulletin of the Seismological Society of America Vol.107 No.2

<P>Observed seismic anisotropy gives the most direct information on mantle flow, but it is challenging to image it robustly at global scales. Difficulties in separating crustal from mantle structures in particular can have a strong influence on the imaging. Here we carry out several resolution tests using both real and synthetic data, which show that unconstrained crustal structure can strongly contaminate retrieved radial anisotropy at 100–150 km depth. To efficiently reduce crustal effects, we perform whole‐mantle radially anisotropic tomographic inversions including crustal thickness perturbations as model parameters. Our data set includes short‐period group velocity data, which are sensitive to shallow structure. We perform a series of tests that highlight the advantages of our approach and show that to properly constrain thin oceanic crust in global radially anisotropic inversions, group velocity data with wave periods of at least <I>T</I>∼20  s or shorter are required. Our Moho perturbation model shows thicker crust along subduction zones and beneath the Ontong Java plateau in the southwestern Pacific than in the global crustal model CRUST2.0. These features agree well with other crustal models as well as with refraction survey data and tectonic features in these regions.</P>

Earthquake Source Mechanism and Rupture Directivity of the 12 September 2016 Mw 5.5 Gyeongju, South Korea, Earthquake

Kim, YoungHee,He, XiaoHui,Ni, SiDao,Lim, Hobin,Park, Sun‐,Cheon Seismological Society 2017 Bulletin of the Seismological Society of America Vol.107 No.5

<P>Two earthquakes (M-w 5.1 and 5.5) ruptured branches of the Yangsan fault system in Gyeongju, South Korea, on 12 September 2016. Aftershocks, including a notable M-w 4.3 earthquake on 19 September 2016, were clustered around the epicenters of the first two events. The M-w 5.5 earthquake is considered the largest earthquake in South Korea to have occurred during the modern instrumental recording period since 1978. Although there is no apparent surface rupture, these earthquakes have greatly shaken South Korea, leaving both physical and societal impacts. In this study, we determine the source mechanism and rupture directivity using regional seismic-waveform data to understand the earthquake source processes. Based on the waveform inversion, we report that the mainshock (M-w 5.5 event) is a strike-slip event with two nodal planes 117 degrees/84 degrees/21 degrees and 24 degrees/69 degrees/173 degrees at a depth of 14 km. The inversion also demonstrates that the mainshock event ruptured against the 24 degrees seismogenic fault plane to the south-southwest, with a rupture length of similar to 4.3 km. This rupture propagation direction agrees well with the spatial distribution of relocated aftershock events and reported seismic intensities.</P>