An attenuation study of body waves in the south‐central region of the Gulf of California, México

Author Posting. © Seismological Society of America, 2014. This article is posted here by permission of Seismological Society of America for personal use, not for redistribution. The definitive version was published in Bulletin of the Seismological Society of America 104 (2014): 2027-2042, doi:10.1785/0120140015.We studied the seismic attenuation of body waves in the south‐central region of the Gulf of California (GoC) with records from the Network of Autonomously Recording Seismographs of Baja California (NARS‐Baja), from the Centro de Investigación Científica y de Educación Superior de Ensenada’s Broadband Seismological Network of the GoC (RESBAN), and from the ocean‐bottom seismographs (OBS) deployed as part of the Sea of Cortez Ocean Bottom Array experiment (SCOOBA). We examine 27 well‐located earthquakes reported in Sumy et al. (2013) that occurred from October 2005 to October 2006 with magnitudes (Mw) between 3.5 and 4.8. We estimated S‐wave site effects by calculating horizontal‐to‐vertical spectral ratios and determined attenuation functions with a nonparametric model by inverting the observed spectral amplitudes of 21 frequencies between 0.13 and 12.59 Hz for the SCOOBA (OBS) stations and 19 frequencies between 0.16 and 7.94 Hz for NARS‐Baja and RESBAN stations. We calculated the geometrical spreading and the attenuation (1/Q) factors for two distance intervals (10–120 and 120–220 km, respectively) for each frequency considered. The estimates of Q obtained with the SCOOBA (OBS) records for the interval 10–120 km indicate that the P waves attenuate more than S waves (QP=34±1.2f 0.82±0.10, QS=59±1.1f 0.90±0.03) for frequencies between 0.6 and 12.6 Hz; whereas for the 120–220 km interval, where ray paths travel deeper, S waves attenuate more than P waves (QP=117±1.3f 0.44±0.19, QS=51±1.2f 1.12±0.11). The estimates of Q obtained using NARS‐Baja and RESBAN records, within 10–120 km, indicate that P waves attenuate more than S waves (QP=69±1.2f 0.87±0.16, QS=176±1.4f 0.61±0.26) at frequencies between 0.3 and 6.3 Hz; whereas at the 120–220 km distance interval S waves attenuate slightly more than P waves (QP=39±1.1f 0.64±0.06, QS=48±1.1f 0.37±0.07) at high frequencies (f>3  Hz). These results, based on a unique OBS dataset, provide an indirect mean to constrain future models of the thermal structure beneath the GoC.The operation of the RESBAN network has been possible thanks to the financial support of the Mexican National Council for Science and Technology (CONACYT; projects CB-2011-01-165401[C0C059], G33102-T, and 59216). The first author benefited from a fellowship provided by CONACYT between August 2011 and August 2013. We also thank the financial support given by the Earth Science Division of Centro de Investigación Científica y de Educación Superior de Ensenada (CICESE) to write this paper.2015-07-0

Latency and geofence testing of wireless emergency alerts intended for the ShakeAlert® earthquake early warning system for the West Coast of the United States of America

ShakeAlert, the earthquake early warning (EEW) system for the West Coast of the United States, attempts to provides crucial warnings before strong shaking occurs. However, because the alerts are triggered only when an earthquake is already in progress, and the alert latencies and delivery times are platform dependent, the time between these warnings and the arrival of shaking is variable. The ShakeAlert system uses, among other public alerting platforms like a mobile phone operating system, smartphone apps, and the Federal Emergency Management Agency Integrated Public Alert & Warning System (IPAWS). IPAWS sends Wireless Emergency Alerts (WEAs) informing people via their smartphones and other mobile devices about various events, such as natural hazards, child abductions, or public health information about COVID-19. However, little is known about the IPAWS delivery latencies. Given that people may have only a few seconds of notice after they receive an alert to take a protective action before they feel earthquake shaking, quantifying latencies is critical to understanding whether the IPAWS system is useful for EEW. In this study, we developed new methods to test the IPAWS distribution system's performance, both with devices in a controlled environment and as well as with a 2019 community-based feedback form, in Oakland and San Diego County, California, respectively. The controlled environment test used mobile phones (including smart and non-smart phones) and associated devices to determine alert receipt times; the community research form had participants self-report their receipt times. By triangulating the data between the controlled test environment and the community research, we determined the latency statistics as well as whether the geofence (the geographic area where the alert was intended to be sent) held broadly. We found that the latencies were similar between the two tests despite the large differences in population sizes. WEA messages were received within a median time frame of 6–12 s, and the geofence held with only a few exceptions. We use this latency to assess how the system would have performed in two large earthquakes, the 1989 M6.9 Loma Prieta and 2019 M7.1 Ridgecrest earthquakes, which both occurred near our WEA test locations. Our analysis revealed that had IPAWS been available during those earthquakes, particularly Loma Prieta, it would have provided crucial seconds of notice that damaging shaking was imminent in some locations relatively far from the epicenter. Further, we find affordable non-smart phones can receive WEAs as fast as smartphones. Finally, our new method can be used for latency and geospatial testing going forward for IPAWS and other similar alerting systems.ISSN:0925-753

Strong-Motion Observations of the M 7.8 Gorkha, Nepal, Earthquake Sequence and Development of the N-SHAKE Strong-Motion Network

We present and describe strong-motion data observations from the 2015 M 7.8 Gorkha, Nepal, earthquake sequence collected using existing and new Quake-Catcher Network (QCN) and U.S. Geological Survey NetQuakes sensors located in the Kathmandu Valley. A comparison of QCN data with waveforms recorded by a conventional strong-motion (NetQuakes) instrument validates the QCN data. We present preliminary analysis of spectral accelerations, and peak ground acceleration and velocity for earthquakes up to M 7.3 from the QCN stations, as well as preliminary analysis of the mainshock recording from the NetQuakes station. We show that mainshock peak accelerations were lower than expected and conclude the Kathmandu Valley experienced a pervasively nonlinear response during the mainshock. Phase picks from the QCN and NetQuakes data are also used to improve aftershock locations. This study confirms the utility of QCN instruments to contribute to ground-motion investigations and aftershock response in regions where conventional instrumentation and open-access seismic data are limited. Initial pilot installations of QCN instruments in 2014 are now being expanded to create the Nepal–Shaking Hazard Assessment for Kathmandu and its Environment (N-SHAKE) network.Published versio