Understanding PGA: Reading Earthquake Sensor Data

You've built your home earthquake sensor, and it's streaming data. But what do those numbers actually mean? How do you distinguish a passing truck from a real earthquake? And how does your sensor's data relate to what you feel during shaking?

The answer lies in understanding Peak Ground Acceleration (PGA) — the most widely used metric for quantifying earthquake shaking intensity at a specific location.

What Is PGA?

Peak Ground Acceleration is the maximum acceleration of the ground at your sensor's location during an earthquake. It's measured in two units:

g (gravitational acceleration units) — where 1g = 9.81 m/s²
Gal (cm/s²) — where 1 Gal = 1 cm/s² = 0.00102 g

When you're standing still, gravity pulls you downward at 1g. During an earthquake, the ground accelerates horizontally and sometimes vertically — and PGA measures the peak value of that acceleration.

PGA vs. Magnitude: Not the Same Thing

This is the most common confusion in earthquake communication:

Magnitude (Richter scale, Moment Magnitude) measures the total energy released at the earthquake's source
PGA measures the actual ground shaking at YOUR location

A M7.0 earthquake 500 km away might produce 0.01g PGA at your location (barely noticeable), while a M4.5 earthquake 10 km away might produce 0.1g PGA (objects falling off shelves).

PGA depends on three factors:

Earthquake magnitude — larger earthquakes produce stronger waves
Distance — seismic waves lose energy as they travel (attenuation)
Local soil conditions — soft soils amplify shaking; bedrock dampens it

The PGA Intensity Scale

PGA (g)	PGA (Gal)	Human Perception	Typical Effects	Color Code
< 0.001	< 1	Not felt	None — only instruments detect it	🟢 Normal
0.001–0.01	1–10	Barely felt	Light rattling of objects	🟢 Normal
0.01–0.05	10–50	Clearly felt	Hanging objects swing, dishes rattle	🟡 Warning
0.05–0.1	50–100	Strong shaking	Difficult to stand, furniture moves	🟠 Danger
0.1–0.3	100–300	Violent shaking	Damage to vulnerable buildings	🔴 Critical
0.3–0.6	300–600	Severe	Significant structural damage	🔴 Critical
> 0.6	> 600	Extreme	Heavy damage, potential collapse	🔴 Critical

GeoShake uses this color-coded threshold system in its app, so you can instantly understand the severity of detected ground motion.

Reading Your Sensor Data

Three Axes of Motion

Your accelerometer measures acceleration along three perpendicular axes:

X-axis — horizontal (typically East-West)
Y-axis — horizontal (typically North-South)
Z-axis — vertical (Up-Down)

At rest, your sensor should read approximately:

X ≈ 0g
Y ≈ 0g
Z ≈ 1g (gravity pulling downward)

During an earthquake:

X and Y values spike (horizontal S-wave shaking)
Z may also change (P-waves produce vertical motion)
The peak value across all three axes is your PGA

ENU Convention

Seismologists use the ENU (East-North-Up) convention:

E (East) = positive X-axis pointing East
N (North) = positive Y-axis pointing North
U (Up) = positive Z-axis pointing upward

GeoShake's app displays data in ENU format, with separate charts for each axis so you can see the directional components of ground motion.

Common Non-Earthquake Signals

Your sensor will detect many things besides earthquakes. Learning to distinguish them is essential:

Footsteps

Pattern: Regular, rhythmic pulses on the Z-axis
PGA: 0.001–0.01g
Duration: Continuous while walking
Distinguishing feature: Very local (only detected by one sensor), rhythmic

Traffic

Pattern: Low-frequency rumble, predominantly on horizontal axes
PGA: 0.001–0.05g (heavy trucks can reach 0.02g)
Duration: Seconds to minutes
Distinguishing feature: Gradual onset and decay, correlated with traffic times

Construction/Demolition

Pattern: Irregular spikes, often with sustained vibration
PGA: 0.01–0.1g (pile driving can exceed 0.05g)
Duration: Varies
Distinguishing feature: Only during work hours, very local

Wind

Pattern: Low-frequency building sway, primarily on horizontal axes
PGA: 0.001–0.005g
Duration: Sustained for hours
Distinguishing feature: Correlates with weather data

Earthquake

Pattern: P-wave arrival (sharp vertical pulse) followed 2–10 seconds later by S-wave (strong horizontal shaking)
PGA: 0.01g+ (for events you can feel)
Duration: 10 seconds to several minutes
Distinguishing feature: Detected by multiple sensors simultaneously, matches P-wave then S-wave pattern, not correlated with time-of-day or weather

Multi-Node Validation

This is why community networks like GeoShake use multi-node validation: an earthquake produces similar signals across multiple sensors simultaneously. A truck passing your house only affects YOUR sensor. By requiring agreement from multiple sensors, the network filters out local noise and confidently identifies real seismic events.

Building PGA Intuition

Historical PGA Values

Earthquake	Location	PGA Recorded
2011 Tōhoku, Japan (M9.1)	Miyagi Prefecture	2.99g (among highest ever recorded)
2023 Kahramanmaraş, Turkey (M7.8)	Near epicenter	~1.0g
1994 Northridge, USA (M6.7)	Tarzana, CA	1.78g
2010 Canterbury, NZ (M7.1)	Christchurch area	0.82g
2009 L'Aquila, Italy (M6.3)	City center	0.35g

What PGA Thresholds Mean for Buildings

PGA Range	Effect on Well-Built Structures	Effect on Vulnerable Structures
0.05–0.1g	No damage, cosmetic cracking	Minor cracking, items fall
0.1–0.2g	Minor non-structural damage	Moderate cracking, some failures
0.2–0.4g	Some structural damage, repairable	Significant damage, partial collapse
0.4g+	Structural damage in modern buildings	Collapse risk in vulnerable structures

Practical Tips for Sensor Operation

Calibration

Run a calibration routine when you first install your sensor and after any repositioning. A well-calibrated sensor should read exactly 1g on the Z-axis at rest. Deviations indicate miscalibration or sensor drift.

Firmware Settings for Earthquake Detection

Set accelerometer range to ±2g for maximum sensitivity to small events
Sample at 200+ Hz to capture the full frequency range of seismic waves (0.1–20 Hz for earthquakes, but higher sampling prevents aliasing)
Log continuously — don't use trigger-only recording, as you'll miss the P-wave onset

Data Storage

Stream to cloud in real-time (MQTT recommended)
Also maintain local buffer (at least 5 minutes of ring buffer) in case of network outage
Timestamp every sample with NTP-synchronized time

How GeoShake Visualizes PGA

The GeoShake mobile app provides real-time PGA visualization for your sensor:

PGA Charts — see Peak Ground Acceleration over configurable time ranges (1 hour to 30 days)
ENU Axis Graphs — monitor East-North-Up displacement in real time
Threshold Indicators — color-coded bands show Normal (🟢), Warning (🟡), Danger (🟠), and Critical (🔴) levels
Event Detection — when PGA exceeds thresholds, events are recorded with timestamps and peak values

📱 See your sensor's PGA data in real time. Download GeoShake — free on iOS and Android.

Related Articles:

Understanding PGA: What Your Earthquake Sensor Data Actually Means