Trigger Processor Job

The TriggerProcessor monitors a single seismic channel (typically the vertical component, EHZ) to detect earthquake events in real-time. It uses the industry-standard STA/LTA (Short-Term Average / Long-Term Average) algorithm to distinguish between background noise and impulsive seismic arrivals.

How it Works: Recursive STA/LTA

The algorithm calculates the ratio between the average signal amplitude in a moving short-term window and a moving long-term window.

• STA (Short-Term Average): Captures the immediate “energy” of the seismic wave (e.g., 0.5 seconds).
• LTA (Long-Term Average): Captures the “background” noise level (e.g., 10 seconds).
• Ratio:

The Hysteresis Principle

To prevent the system from rapidly flickering “ON” and “OFF” during a shaky earthquake, we use Dual Thresholds:

1. Trigger ON: When the Ratio crosses thr_on (e.g., 3.5), the earthquake_event is set.
2. Stay ON: Even if the ratio drops to 2.0, the event remains active.
3. Trigger OFF: Only when the ratio falls below thr_off (e.g., 1.5) is the event cleared.

Why Hysteresis?

Without a lower “OFF” threshold, a single earthquake would trigger dozens of separate “events” as the signal oscillates above and below a single line. Hysteresis ensures one earthquake equals one clean detection.

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Configuration

These parameters are defined in the jobs_settings.trigger section of your config.yml.

Parameter	Default	Description
sta_sec	0.5	Sensitivity to sharp onsets (seconds).
lta_sec	10.0	Background noise averaging window (seconds).
thr_on	3.5	Ratio required to declare an earthquake.
thr_off	1.5	Ratio required to end the event.
trigger_channel	EHZ	The physical channel monitored for triggers.

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Implementation Details

The processor uses a thread-safe deque (Rolling Buffer) to maintain exactly enough data for stable LTA calculation without consuming excessive memory.

Buffer Strategy

• Type: collections.deque(maxlen=2000)
• Size: . For a 10s LTA at 100Hz, this is 2000 samples (~16 KB).
• Processing: The recursive_sta_lta function from ObsPy is called every time a new packet arrives (100 times per second).

Trigger Logic Snippet

# Compute characteristic function
cft = recursive_sta_lta(data_arr, nsta, nlta)
current_ratio = cft[-1]

if current_ratio > thr_on and not last_trigger:
    earthquake_event.set() # Alert other threads!