Earthquake Location And Detection

Earthquake location and detection are the fundamental problems in seismology, while accurate determination of earthquake location and small earthquake detection are still the immediate issues today. In this thesis, we introduce several studies focusing on eartqhuake location and detection.1) We develop a new method for earthquake depth determination by stacking multiple-station autocorrelograms.2) We develop an effective method for detecting and locating small earthquake at the same time.3) We locate North Korea’s2013nuclear test in high-precision by applying cross-correlation based double difference location method.4) By applying the newly developed event detection method, we detect and locate a low-yield nuclear test conducted by North Korea in2010,5) and detect earthquakes before the volcanic eruptions of Japan’s Mount Ontake in2007and2014.Accurate determination of earthquake depth is important, but particularly challenging. We develop a new method to determine the earthquake source depth by stacking multiple-station autocorrelograms (SMAC) of seismic data. The basic concept of SMAC method is to enhance the coherent surface reflected energy by autocorrelation and stacking, and uses the surface reflected energy to determine the source depth. Autocorrelation effectively enhances the energy of the seismic phases related to the source depth, while stacking the autocorrelograms of array data further improves the signal-noise ratio of the energy. The procedures are applied using both the main SH waves and the coda. Using coda waves, the method extracts the two-way travel time between the source and the surface by stacking the coda wave autocorrelations over an array of seismic observations. Using the main SH waves, SMAC explores the interference of the down-going Moho reflected SmS wave and up-going surface bounced Moho reflected sSmS wave by autocorrelation. The autocorrelograms are then stacked along theoretical differential sSmS-SmS travel time predicted for all potential source depths, and the source depth is determined to be the one that produces the maximum stacking energy. Synthetic tests demonstrate the validity of the SMAC procedures. As an example of application, we apply the SMAC method to determine the source depth of an earthquake occurring in Japan Island. The procedures of using both main SH waves and their coda waves yield robust surface reflected energy and a consistent source depth. The error of the depth estimation is less than1km assuming an uncertainty of10%in the averaged crust velocity.Detection of low magnitude event is critical and challenging in seismology. We develop a new method, named the Match and Locate (M&L) method, for small event detection. The M&L method employs some template events and detects small events through stacking cross-correlograms between waveforms of the template events and potential small event signals in the continuous waveforms over multiple stations and components, but the stacking is performed after making relative travel time corrections based on the relative locations of the template event and potential small event scanning through a3-D region around the template. Compared to the current methods of small event detection, the M&L method places event detection to a lower magnitude level and extends the capability of detecting small events that have large distance separations from the template. The method has little dependence on the accuracy of the velocity models used, and, at the same time, provides high-precision location information of the detected small-magnitude events. We demonstrate the effectiveness of the M&L method and its advantage over the matched filter method using examples of scaled-down earthquakes occurring in the Japan Island and foreshock detection before the2011Mw9.0Tohoku earthquake. In the foreshock detection, the M&L method detects four times more events (1427) than the templates and9%(134) more than the matched filter under the same detection threshold. Up to41%(580) of the detected events are not located at the template locations with the largest separation of9.4km. Based on the identified foreshocks, we observe five sequences of foreshock migration along the trench-parallel direction toward the epicenter of the Mw9.0mainshock.Using North Korea’s2009nuclear test as reference and satellite imagery, we show that the location and yield of North Korea’s2013nuclear test can be quickly and accurately determined based on seismic data. North Korea’s2013nuclear test site is pinpointed by deriving relative location of North Korea’s2009and2013nuclear tests and using the previously determined location of2009nuclear test, while its yield is estimated based on the relative amplitude ratios of the Lg waves recorded for both events, the previously determined Lg-magnitude of2009nuclear test and burial depth inferred from satellite imagery. North Korea’s2013test site is determined to be located at (41.2908°N,129.0763°E), about345m south and453m west of its2009nuclear test site, with a geographic precision of94m. Its yield is estimated to be12.2±3.8kt. We detect and locate a low-yield nuclear test conducted on12May2010by North Korea using the M&L method. North Korea’s2010test is determined to be occurring at00:08:45.067UTC,12May2010, and located at (41.2863°N,129.0790°E) with a geographic precision of350m. Pg/Lg spectral ratios of the seismic data further indicate that it is of explosion in nature. Its yield is estimated to be2.9±0.8t based on the relative amplitude ratios of Lg waves recorded for the2009and2010events and burial depth inferred from satellite imagery. Our study provides seismological evidence for a low-yield North Korea’s nuclear test on12May2010supporting the radionuclide isotope observations, and demonstrates the scientific capability of monitoring low-yield nuclear tests by combining seismic and radionuclide isotope data.We also detect the earthquakes before the volcanic eruption of Japan’s Mount Ontake in2014, and compare the seismicity and earthquake characteristic with another small eruption in2007to investigate their difference and eruption precursor. We detect37times and30times more earthquakes than the Japan Meteorological Agency (JMA) catalog during volcanic activities in2007and2014, respectively. The number of earthquakes in2014increase and decrease with time regularly, which is not observed in the2007case. Then we classify them as volcano-tectonic earthquakes and long-period events according to their frequency components. Some long-period events with single frequency occur before both eruptions, which could be related to the volcanic interior activity and could be used to predict the eruption of Japan’s Mount Ontake in the future.