The Case of Western Corinth Gulf (Greece): A Detachment Zone or Seismic – Aseismic Transition?

Abstract:

Corinth Gulf is one the most seismically active rifts worldwide, with several low in magnitude earthquakes as well as a few stronger ones (M>6), especially in its western part. This study focuses on the spatiotemporal properties of the seismic activity occurred between 2008-2014, when the national seismological network was denser. In this respect, a highly accurate earthquake catalog consisting of ~22,000 events was compiled using double difference technique and differential times from both phase picked data and cross correlation measurements. The locations showed the existence of a very shallow north dipping structure in the western part of Corinth Gulf, which is void of spatiotemporal clusters, whereas seismic excitations are placed above that zone in shallower depths. The waveform database was searched for repeating events (i.e., events with identical waveforms) and the repeaters were classified into multiplets of repeating sequences. These repeaters revealed two patterns of activity namely continuous type and burst-like repeaters. Continuous type repeaters last the entire study period, have low slip rates and are located on the shallow north dipping zone, which was found void of spatiotemporal clusters. On the other hand, burst-like repeaters are located above the shallow north dipping zone, in areas where seismic excitations occur and evidence was found that their occurrence can be related to fluid intrusion. The major finding of this study is that the spatial distribution of the relocated seismicity revealed two patterns of activity in the western subarea, namely, strongly clustered seismicity in both space and time in depths shallower than 10 km and below that activity a very narrow shallow north dipping zone which consists of continuous type repeaters. Based on the properties of the continuous type repeaters, the aseismic slip along the shallow dipping zone, was calculated.

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Cascadia Onshore-Offshore Site Response, Submarine Sediment Mobilization, and Earthquake Recurrence

Abstract:

Local geologic structure and topography may modify arriving seismic waves. The consequent variation in shaking, or ‘site-response’, may affect the distribution of slope-failures and redistribution of submarine sediments. I used seafloor seismic data from the 2011-2015 Cascadia Initiative and permanent onshore seismic networks to derive estimates of site-response, denoted Sn, in low- and high-frequency (0.02-1 and 1-10 Hz) passbands. Three shaking metrics (peak velocity, peak acceleration, and energy density) Sn vary similarly throughout the study region (onshore and offshore) and change primarily in the convergence direction, roughly east-west. In the two passbands, Sn patterns offshore are nearly opposite one another and range over an order of magnitude or more across Cascadia. Sn patterns may be attributed broadly to sediment resonance and attenuation. These findings, and an abrupt step in the east-west trend of Sn suggest that changes in topography and structure at the edge of the continental shelf significantly impact shaking. The variations in Sn also correlate with the edges of gravity lows diagnostic of marginal basins and with methane plumes channeled within shelf-bounding faults. The offshore Sn exceeds the onshore Sn in both passbands. The relatively greatest and smallest Sn estimates at low- and high-frequencies, respectively, coincide with the steepest slopes and the shelf. These results should be considered in submarine shaking-triggered slope-stability failure studies. Significant north-south Sn variations are not apparent from the sparse sampling, but do not permit rejection of the hypothesis that the southerly decrease in intervals between shaking-triggered turbidites and inferred great earthquakes inferred by Goldfinger et al. [2012; 2013; 2016] and Priest et al. [2017] may be due to inherently stronger shaking southward.

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Run-up Estimation Using Non-Uniform Stochastic Sources: The South American Subduction Zone

Abstract:

Throughout history, megathrust earthquakes have produced large tsunamis that have devastated coastal cities in the near and far field. South America hosts one of the largest subduction zones in the world and it is important to study tsunamigenic earthquakes here to forecast and mitigate future catastrophes. We estimate the maximum magnitude of possible earthquakes along the South American subduction interface using scaling laws, subducting seafloor features, seismic-geodetic coupling and seismic history. We use the Slab2 subduction zone geometry model from the United States Geological Survey (USGS) to constrain the geometry of the interface. Then, we estimate tsunami run-up using numerical modeling for 100 non-uniform stochastic k² sources in each targeted area. Our results show great variability in run-up distribution along the Nazca-South America subduction zone. The most vulnerable areas are: Valparaíso in Chile, with a most likely scenario of 20 m run-up and a maximum of 33 m, and Lima in Perú, with a most likely scenario exceeding 25 meters of run-up and a maximum of 40 meters. Similar results are obtained in Huasco, Chile, and Iquique, Chile, and other areas along the Pacific Coast of South America. We conclude that tsunami hazard remains high along South America, even in areas where megathrust earthquakes have recently occurred.

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The mblg 3.9 September 13, 2017, Earthquake on the Virginia-West Virginia Border: A Significant Shock in the Giles County Seismic Zone

Abstract:

On September 13, 2017 the USGS NEIC reported a duration magnitude MD 3.2 earthquake at 37.473N 80.703W, depth 18 km near Lindside, West Virginia, close to the Virginia-West Virginia border. The earthquake was felt primarily in Monroe, Mercer and Summers counties, West Virginia and in Giles, Montgomery, Pulaski and Bland counties, Virginia. The maximum intensity reported to the USGS Did You Feel It? program was IV MM. The earthquake occurred in an area of moderate seismicity known as the Giles County Seismic Zone (GCSZ). The largest shock in the GCSZ occurred in 1897 near Pearisburg, VA, with mblg magnitude estimated from the felt area at 5.8. We relocated the hypocenter of the September, 2017 earthquake using a locally specific velocity model, at 37.4775N, 80.7035W, depth 21 km. We estimated the mblg magnitude at 3.90 +/- 0.26 using 26 stations at regional distances, and determined a duration magnitude MD of 3.71 +/- 0.17, using 33 stations. The duration magnitude is based on a correlation between the log of short-period signal duration and mblg. We determined a focal mechanism using 27 P polarities, 12 SH polarities and 16 SH/P amplitude ratios. The nodal planes with least rms amplitude ratio error are: strike N91E, dip 69 deg., rake -22 deg.; auxiliary plane strike N189E, dip 69 deg., rake -158 deg. This event is notable because it is the largest shock in the GCSZ since May, 1974 (mblg 3.7). This recent shock, like many others in the GCSZ, shares characteristics with those in the Eastern Tennessee Seismic Zone (ETSZ), which is also in the Appalachian Valley and Ridge province. The 2017 GCSZ focal mechanism is mostly strike-slip with a small normal component, on steeply dipping nodal planes trending approximately N-S and E-W. This type of mechanism is dominant in the ETSZ. Also, in both areas, focal depths tend to be greater than 12 km, unlike shocks to the east in the Blue Ridge, Piedmont and Coastal Plain provinces which tend to occur at shallower depths.

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Measurement and Modeling of Ground Motions in Myanmar for Seismic Hazard Assessment

Abstract:

An important first step in assessing the seismic hazard for any region is the selection or development of appropriate ground-motion prediction equations (GMPEs) that adequately describe the expected shaking from a specified earthquake. In this study, we apply the log-likelihood method of Scherbaum et al. (2004) to analyze residuals between predictions from a suite of GMPEs developed for tectonically active regions and ground-motion observations recorded by the Myanmar National Seismic Network (MNSN). Following major upgrades beginning in January 2016 (Hrin Nei Thiam et al., 2017), the MNSN now comprises >12 real-time broadband and/or strong-motion stations distributed throughout the country. During its first two years of operation, the upgraded MNSN recorded multiple M≥4 events per month, including two deep M6.8 earthquakes near Chauk and Kale in 2016, and the M5.1 Taikkyi earthquake in 2017. We will briefly discuss ongoing work to evaluate the suitability of low-cost, low-power Raspberry Shake instruments to augment monitoring by the MNSN. We measure peak ground acceleration (PGA) for M≥3.5 events within Myanmar and within 300 km of MNSN stations. We compare these PGA measurements with predicted ground motions from GMPEs developed for tectonically active regions as part of the NGA West and NGA West-2 model suite, including Chiou and Youngs (2008), among others. VS30 for MNSN stations is estimated from topographic slope using the proxy method of Wald and Allen (2007), and site amplification factors are calculated using the relationships of Seyhan and Stewart (2014). The results of this analysis will be used to inform decisions regarding the configuration of real-time ShakeMaps, produced by the Myanmar Department of Meteorology and Hydrology for rapid earthquake response.

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The 1 May 2017 British Columbia Earthquake Doublet and Implications for Complexity near the Southern End of the Denali Fault System

Abstract:

On May 1, 2017, two M6+ earthquakes occurred in northwestern British Columbia separated by about 2 hours. Despite their close distance about 10 km, the two events have different focal mechanisms, with first earthquake featuring a thrust focal mechanism and the second strike slip. Both focal plane solutions are inconsistent with slip on the nearby southeastern Denali fault system. To resolve their ruptured fault planes, we invert for the point source parameters, and analyze rupture directivity via waveform modeling. The results indicate that the first event ruptured upwards along a steep SW-dipping fault (strike 153°/dip 61°/rake 113°) with rupture length about 8 km, and the second event ruptured to the ESE along a left-lateral fault (strike 292°/dip 36°/rake 54°) with rupture length about 6 km. We infer that the earthquake doublet is related to slip on the active Duke River fault, and the involved faults could be associated with the transpression caused by collision of the Yakutat block.

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