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Examination of Event Dependent Latency Values for the Earthquakes Recorded by the Northern California Earthquake Data Center for Use in Shakealert

Session: Earthquake Early Warning Live in California! Current Status and Challenges [Poster]

Type: Poster

Date: 4/23/2021

Presentation Time: 11:30 AM Pacific

Description: 

Realtime seismic waveform data from some SCSN stations for the mainshock of the Ridgecrest Earthquake sequence were delayed by varying amounts, causing delays to both ShakeAlert earthquake early warning (EEW) alerts and production of earthquake information. Detailed investigation found that two major factors contributing to the latency of data arriving at the data processing centers were 1) data tunneling and 2) passage of data through telemetry hubs. The former describes a situation in which the amount of data being transferred was greater than normal due to reduced compression. In the latter case data from many stations were telemetered to a single hub, and then all transported together through one cell modem.

L1Z, a latency measure, recorded for several years in the SCSN, was instrumental in investigating the problems. This parameter is the time it takes after a data packet is created in the data logger to enter the EEW processing system, and is symptomatic of the telemetry. It is now being calculated at all ShakeAlert data centers, including in the NCSS. L1Z is measured on the data of the high sample rate, causally filtered data from the vertical channel of the accelerometer (typically HNZ) at each station and is stored at one sample per second, in units of milliseconds. It allows the latency of data for each station contributing to ShakeAlert to be monitored continuously. We select a suite of earthquakes in Northern California with different magnitudes and examine the continuous L1Z data values for stations at various distances and with a variety of telemetry paths and types of equipment, to investigate which factors influence latency in these situations. We also evaluate sizes of data packets and compression rates to examine how variations in these parameters affect latency. Minimizing latency in ShakeAlert is important, particularly for large earthquakes, in reducing the time to produce EEW alerts.

Presenting Author: Fabia Terra

Student Presenter: No

Authors