Differences in the rupture process for very deep earthquakes at the Peru-Brazil and Peru-Bolivia borders

<p>We analyze the differences in the rupture process for twelve very deep earthquakes (h>500 km) at the Peruvian-Brazilian subduction zone. These earthquakes are produced by the contact between the Nazca and the South America Plates. We have estimated the focal mechanism from teleseismic waveforms, using the slip inversion over the rupture plane, testing rupture velocities ranging from 2.5 km/s to 4.5 km/s, and analyzing the slip distribution for each  rupture velocity. The selected 12 earthquakes have occurred in the period 1994- 2016, with magnitudes between 5.9 and 8.2 and focal depth 500- 700 km. They can be separated in two groups attending to their epicentral location. The first group is formed by 9 events located, in the Peruvian-Brazil border, with epicenters following a NNW-SSE alignment, parallel to the trench. Their focal mechanisms present solutions of normal faulting with planes oriented in NS direction, dipping about 45 degrees and with vertical pressure axis. The second group is formed by three earthquakes located to the south of the first group in northern Bolivia. Their mechanisms show dip-slip motion with a near vertical plane, oriented in NW-SE direction and the pressure axis dipping 45º to the NE. The moment rate functions correspond to single ruptures with time durations from 6s to 12s, with the exception of the large 1994 Bolivian earthquake (Mw = 8.2) which presents a complex and longer STF. The different mechanisms for the two groups of earthquakes confirm the different dip of the subducting Nazca plate at the two areas, with the steeper part at the southern one.  </p>

Induced Seismicity in the Delaware Basin, West Texas, is Caused by Hydraulic Fracturing and Wastewater Disposal

ABSTRACT Most current seismicity in the southern U.S. midcontinent is related to oil and gas operations (O&G Ops). In Texas, although recorded earthquakes are of low-to-moderate magnitude, the rate of seismicity has been increasing since 2009. Because of the newly developed Texas Seismological Network, in most parts of Texas, recent seismicity is reported on a daily basis with a magnitude of completeness of ML 1.5. Also, funded research has allowed the collection of O&G Op information that can be associated with seismicity. Although in the Dallas–Fort Worth area, recent seismicity has been associated mostly with saltwater disposal (SWD), in the South Delaware Basin, West Texas, both hydraulic fracturing (HF) and SWD have been found to be causal factors. We have begun to establish an O&G Op database using four different sources—IHS, FracFocus, B3, and the Railroad Commission of Texas—with which we can associate recent seismicity to HF and SWD. Our approach is based on time and epicentral location of seismic events and time, location of HF, and SWD. Most seismicity occurs in areas of dense HF and SWD-well activity overlapping in time, making association of seismicity with a specific well type impossible. However, through examination of clustered seismicity in space and time, along with isolated clusters of spatiotemporal association between seismicity and O&G Ops, we are able to show that a causation between HF and seismicity may be favored over causation with SWD wells in areas of spatially isolated earthquake clusters (Toyah South, Reeves West, Jeff Davis Northeast, and Jeff Davis East). Causality between SWD and seismicity may be inferred for isolated cases in Reeves South and Grisham West.

2 February 1816, an Overlooked North Atlantic M 8 Earthquake

The 2 February 1816 North Atlantic earthquake is virtually unknown to the international scientific community, and the few existing studies—solely based on two or three macroseismic intensities—pointed to a magnitude near 7 and a location at the eastern end of the Gloria fault in the Azores-Gibraltar Fracture Zone (AGFZ). Through careful search, we discovered more than 40 independent macroseismic observations and were able to estimate a total of 26 values of intensity, covering a wide geographical area (Iberian Peninsula, Madeira, and Azores). To apply the Bakun and Wentworth (B&W) method to the macroseismic dataset, we also deduced intensity–distance attenuation equations for the three different Atlantic coasts. The B&W procedure enabled us to conclude that the 2 February 1816 earthquake had a moment magnitude of 8.6±0.3 at the 95% confidence level and an epicentral location of 37.8° N and 19.8° W, near the central part of the Gloria fault. These results place the event as the greatest known earthquake in the Gloria fault domain and as one of the greatest ever seismic events along the AGFZ, probably only surpassed by the 1755 Lisbon earthquake.

The 2020 M 5.9 Falam Earthquake the Subduction-Induced Strike-Slip Earthquake, West Myanmar

An earthquake with magnitude 5.9 occurred in the east of Falam on 16th April, 2020 at 11:45:23 (UTC). The epicenter is situated at latitude 22.789°N, longitude 94.025°E, 38 km ESE of Falam, at the depth of 10 km. Focal mechanism solution for this event is normal faulting (USGS). The epicentral location is in the Kabaw Valley along which Kabaw fault runs through in N-S direction. The Kabaw fault is situated in forearc region at the eastern base of N-S trending Rakhine Western Ranges under which the India oceanic plate is obliquely subducting beneath the Burma continental plate. The 2020 M 5.9 Falam earthquake occurred along two closely linked tectonic settings: north-eastward oblique subduction zone and north-south trending Kabaw fault zone system in the forearc region.The Falam earthquake ruptured the Tripura segment, one of the segments of India subduction zone, located approximately between latitude 22°-24°N according to the geographical location. This event is a rare intraplate earthquake and a subduction-induced strike-slip earthquake that ever occurred for the recent time in Myanmar. The shock was felt by cities of Gangaw, Kalemyo, Kalewa, Mandalay, Kyauk-se, Monywa. This earthquake was preceded by a loud sound and shaking lasts 1 minute. A few aftershocks of magnitude >3.5 followed the main shock in the vicinity of the epicenter. The vibration spread a wide area along Rakhine Yoma and Myanmar lowland area. The investigation of field survey from social media was found that the event reaches Modified Mercalli Intensity scale VIII based on people’s perception, indoor effects and damaged buildings. Damage is severe in some poorly built structures and upper parts of stupa and pagodas.

A Web Tool for Interactive Generation and Visualization of Synthetic ShakeMaps

<p>Synthetic shaking-intensity maps provide the necessary information about the detailed shaking distribution for scenario-based seismic risk assessment as well as post-disaster rapid loss estimates. These ShakeMaps allow to identify areas heavily affected by an earthquakes and are becoming, combined with an exposure/vulnerability model, the underlying data for a risk or loss model. Such computations deliver decision makers the data for informed policy decisions for precautionary measures for increasing resilience, or, in case of post-disaster analyses, rapid estimates for disaster mitigation.</p><p><br>We present a new web engine for synthetic ShakeMaps harnessing the OpenQuake engine of the Global Earthquake Model (GEM) foundation. The back-end asynchronously digests requests parameterizing earthquake sources in terms of source depth, epicentral location, moment magnitude and focal mechanism. The back-end returns shaking in user definable ground-motion measures (e.g. PGA or IMS) and can be retrieved in various formats such as ASCII, GeoJSON, among others. This tool implements an open and documented API that users and other services can query systematically and automatically. It integrates into the LEXIS framework, a Horizon 2020 funded project aiming at improving rapid loss assessments and emergency decision support systems.<br>An interactive interface allows to explore the expected shaking in the spatial domain by selecting locations of interest on a map and defining the earthquake source interactively within a web browser. Besides the interactive mode, this service now provides, through HTTP requests, a simple interface for any type of ShakeMap to be used in automated systems that require rapid ShakeMap computations without the need to run local instances of OpenQuake.</p>

A Reassessment of the Magnitude of the 1755 Lisbon Earthquake

ABSTRACT The 1755 Lisbon earthquake is still poorly understood due to its offshore location and complex macroseismic intensity pattern. Gutenberg and Richter (1949) tentatively assigned a magnitude between 8¾ and 9 judging from an estimated perceptibility radius of 2500 km. More recent attempts to estimate the magnitude from isoseismal areas led to results in the 8.5–8.7 range. These values have been adopted in several studies of the seismic hazard of southwest Iberia. In this article, I use the available macroseismic data—a total of 1206 data points from Portugal, Spain, and Morocco—to reassess the magnitude of the 1755 Lisbon earthquake. Because a training set of instrumental earthquakes was not available, I apply the technique of Bakun and Wentworth (1997) in conjunction with the ground-motion model of Atkinson and Wald (2007), which was selected through comparison with the 1969 M 7.8 Gorringe bank earthquake data. I obtain a moment magnitude of 7.7±0.5, significantly lower than previous results. The epicentral location obtained with this analysis is offshore southwest Iberia—as expected given the tsunami effects—but poorly constrained. Based on the pattern of intensity data outliers, I suggest that the source was complex and spatially distributed, with part of the rupture taking place onshore or inshore. I propose an explanation for the large tsunamigenic power of the earthquake, which invokes the basal rupture of the Gulf of Cadiz accretionary prism. Finally, I discuss the implications for hazard assessment of the type of complex rupture proposed.

Differences in Epicentral Location of Mexican Earthquakes between Local and Global Catalogs: An update

Differences in epicentral locations between local and global catalogs for earthquakes in the Mexican subduction zone were first observed to be biased in the 1980s, based on a few well studied events. In this study we compare locations between two local catalogs; (1) a recent high precision catalog of events in the state of Guerrero and (2) the catalog of the Servicio Sismológico Nacional (SSN), to the global catalog of the United States Geological Service (USGS). We find that on average epicentral locations in the global catalog of earthquakes larger than M 5 in the Mexican subduction zone are 26 km towards N54°E of those in the local catalogs. We investigate how the errors vary for different types of earthquakes in Guerrero, and how they vary along the trench, from the state of Jalisco to the state of Chiapas. The average differences are largest for thrust events occurring close to the trench, and for events in Michoacán. The differences are greater on average for large earthquakes than for small. There is a trade-off between the distance from the trench and timing, suggesting a poor resolution of these parameters, due to the lack of stations the Pacific Ocean. We attribute the differences in locations to systematic patterns in the velocity structure of the mantle, with consistently fast paths to the northeast and relatively slow paths towards the southwest.

Spatiotemporal variations of seismicity before major earthquakes in the Japanese area and their relation with the epicentral locations

Using the Japan Meteorological Agency earthquake catalog, we investigate the seismicity variations before major earthquakes in the Japanese region. We apply natural time, the new time frame, for calculating the fluctuations, termed β, of a certain parameter of seismicity, termed κ1. In an earlier study, we found that β calculated for the entire Japanese region showed a minimum a few months before the shallow major earthquakes (magnitude larger than 7.6) that occurred in the region during the period from 1 January 1984 to 11 March 2011. In this study, by dividing the Japanese region into small areas, we carry out the β calculation on them. It was found that some small areas show β minimum almost simultaneously with the large area and such small areas clustered within a few hundred kilometers from the actual epicenter of the related main shocks. These results suggest that the present approach may help estimation of the epicentral location of forthcoming major earthquakes.